-------
How to Obtain Copies
You may electronically download the document referenced above on the U.S. EPA's homepage at
http://www.epa.gov/globalwarming/emissions/national. To obtain additional copies of this report, call the National Center for
Environmental Publications and Information (NCDPI) at (800) 490-9198.
For Further Information
Contact Mr. Wiley Barbour, Environmental Protection Agency, Office of Atmospheric Programs, (202) 260-6972,
barbour.wiley@epa.gov.
For more information regarding climate change and greenhouse gas emissions see EPA web side at
http:/Avww.epa.gov/globalwarming/emissions/national. '
Released for printing: April 2000
High GWP Gas Emissions from Industrial Processes
The processes and applications pictured on the front and back cover of this report can lead to anthropogenic emissions of long-lived
fluorinated compounds, including hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) .and sulfur hexafluoride (SF6). HFCs, PFCs
and SFfi are not harmful to the stratospheric ozpng layer, but they are powerful greenhouse gases.,that -can be thousands of times more
potent than CO2 and may have extremely long atmospheric lifetimes. -
Semiconductors: The semiconductor industry uses a variety of long-lived fluorinated gases (PFCs, HFC-23,
SFg and NF3) in dry etching and cleaning chemical vapor deposition tool chambers. Dry etching using fluo-
rinated gases in a plasma provides pathways to electrically connect individual circuit components in the sili-
con. Chemical vapor deposition chambers, used for depositing insulating and conducting materials, are
cleaned periodically using PFCs and other fluorinated gases.
Refrigerant (grocery): Cold food found in supermarkets, convenience stores, restaurants-and other food serv-
ice establishments are typically displayed in refrigeration units that may use HFC-134a, blends of HCFCs and
HFCs, or other refrigerants. This type of equipment can range in size from small reach-in refrigerators and
freezers, to refrigerated display cases, to walk-in coolers and freezers. Supermarkets usually employ large
systems that contain many display cases connected by means of extensive piping. Because this piping may
be miles long, the amount of refrigerant in these units can be very high.
HCFC-22: HCFC-22 is primarily used in refrigeration and air conditioning systems and as a chemical feed-
stock for manufacturing synthetic polymers. HFC-23, which has a global warming potential 11,700 times that
of CO2, is a by-product of HCFC-22 manufacture. Once separated from HCFC-22, the HFC-23 is generally
vented to the atmosphere or may be captured for use in a limited number of applications. Because HCFC-22
depletes stratospheric ozone, HCFC-22 production for non-feedstock uses is scheduled to be phased out by
2020 under the U.S. Clean Air Act. Feedstock production is permitted to continue indefinitely.
Electrical Transmission and Distribution: The largest use for sulfur hexafluoride (SF6) is as an electrical insu-
lator in equipment that transmits and distributes electricity. Many gas-insulated substations, circuit breakers
and other switchgear contain SFg because of its dielectric strength and arc-quenching characteristics. Fugitive
emissions of SFg can escape from this equipment through seals, especially from older equipment, or when the
equipment is opened for servicing.
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\
a
UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C. 20460
April 2000
OFFICE OF
AIR AND RADIATION
The U.S. Environmental Protection Agency, in cooperation with other federal agencies,
has prepared the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998. The
estimates of emissions and sequestration contained in this report, along with future updates, will
be used to monitor and track the progress of the United States in meeting our commitments under
the United Nations Framework Convention on Climate Change (UNFCCC).
In accordance with a decision of the Conference of Parties to the UNFCCC (Decision
3/CP.5), this inventory complies with the UNFCCC Reporting Guidelines on Annual Inventories
(FCCC/CP/1999/7). Adherence to these guidelines ensures that national inventories are well
documented, consistently prepared, and as accurate and complete as possible. The assumptions
and methodologies used in this report have been clearly explained, and are based on the Revised
1996IPCC Guidelines for National Greenhouse Gas Inventories.
Each year, advances in scientific understanding and availability of underlying data allow
us to improve the quality and comprehensiveness of the inventory. A section entitled "Changes
in this Year's U.S. Greenhouse Gas Inventory Report," guides readers to specific areas where the
EPA has expanded coverage, improved methods, or added additional data since the previous
inventory publication.
Some of this year's advances include improved estimates for agricultural soil
management, petroleum systems, international bunker fuel use, emissions from military
operations, and new data on substitution rates for ozone depleting substances. Also included for
the first time this year are new data on agricultural soil carbon fluxes. A glossary of terms, unit
definitions, and conversion tables has also been added to help make this information more
accessible.
We hope that these improvements make this document more useful, and appreciate the
comments and suggestions we have received from numerous reviewers in both the scientific
community and the general public.
Sincerely,
Robert Perciasepe
Assistant Administrator
Internet Address (URL) • http://www.epa.gov
Recycled/Recyclable • Printed with Vegetable Oil Based Inks on Recycled Paper (Minimum 25% Postconsumer)
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Inventory of U.S. Greenhouse
Gas Emissions and Sinks:
199O - 1998
April 15, 2000
U.S. Environmental Protection Agency
Office of Policy
401 M St., SW
Washington, D.C. 20460
U.S.A.
-------
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Acknowledgments
The Environmental Protection Agency would like to acknowledge the many individual and organizational
contributors to this document, without whose efforts this report would not be complete. Although the list of research-
ers, government employees, and consultants who have provided technical and editorial support is too long to list
here, we would like to thank some key contributors whose work has significantly improved this year's report. In
particular, we wish to acknowledge the efforts of the Energy Information Administration and the Department of
Energy for providing detailed statistics and insightful analysis on numerous energy-related topics; the U.S. Forest
Service for preparing the forest carbon inventory, and the Department of Agriculture's Agricultural Research Service
for their work on nitrous oxide emissions from soils.
Within the EPA, many Offices contributed data, analysis and technical review for this report. The EPA Office of
Atmospheric Programs developed methodologies and provided detailed emission estimates for numerous source
categories, particularly for methane, hydrofiuorocarbons, perfluorocarbons, and sulfur hexafluoride. The Office of
Mobile Sources and the Office of Air Quality Planning and Standards provided analysis and review for several of the
source categories addressed in this report. The Office of Solid Waste and the Office of Research and Development also
contributed analysis and research.
Other government agencies have contributed data as well, including the U.S. Geological Survey, the Federal
Highway Administration, the Department of Transportation, the Bureau of Transportation Statistics, the Department
of Commerce, and the Federal Aviation Administration.
We would especially like to thank the staff of the Global Environmental Issues Group at ICF Consulting for
synthesizing this report and preparing many of the individual analyses.
1 See http://www.epa.gov/globalwarming/inventory
-------
The United States Environmental Protection Agency (EPA) prepares the official U.S. Inventory of Greenhouse
Gas Emissions and Sinks to comply with existing commitments under the United Nations Framework Convention on
Climate Change (UNFCCC)1. Under a decision of the UNFCCC Conference of the Parties, national inventories for
most UNFCCC Annex I parties should be provided to the UNFCCC Secretariat each year by April 15.
In an effort to engage the public and researchers across the country, the EPA has instituted an annual public
review and comment process for this document. The availability of the draft document is announced via Federal
Register Notice and is posted on the EPA web page.2 Copies are also mailed upon request. The public comment
period is generally limited to 30 days; however, comments received after the closure of the public comment period
are accepted and considered for the next edition of this annual report. The EPA's policy is to allow at least 60 days for
public review and comment when proposing new regulations or documents supporting regulatory development—
unless statutory or judicial deadlines make a shorter time necessary—and 30 days for non-regulatory documents of
an informational nature such as the Inventory document.
1 See http://www.unfccc.de
2 See hUpr//www.epa.gov/globalwarming/emissions/national
ii inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
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Table of Contents
Acknowledgments j
Preface N
Table of Contents iii
List of Tables, Figures, and Boxes vi
Tables vj
Figures xj
Boxes "xii
Executive Summary ES-1
Recent Trends in U.S. Greenhouse Gas Emissions ES-2
Global Warming Potentials ES-7
Carbon Dioxide Emissions ES-9
Methane Emissions ,_ gc 14
Nitrous Oxide Emissions ES-17
HFCs, PFCs and SF6 Emissions ES-19
Criteria Pollutant Emissions ES-22
1. Introduction ,„„ -j.-j
What is Climate Change? j_2
Greenhouse Gases j_2
Global Warming Potentials i_g
Recent Trends in U.S. Greenhouse Gas Emissions 1_7
Methodology and Data Sources 1-13
Uncertainty in and Limitations of Emission Estimates 1_14
Organization of Report \_yj
Changes in This Year's U.S. Greenhouse Gas Inventory Report 1-17
2. Energy 2-1
Carbon Dioxide Emissions from Fossil Fuel Combustion 2-3
Stationary Combustion (excluding CO2) 2-17
Mobile Combustion (excluding CO2) 2-21
Coal Mining 2-26
Natural Gas Systems 2-29
Petroleum Systems 2-31
Natural Gas Flaring and Criteria Pollutant Emissions from Oil and Gas Activities 2-34
International Bunker Fuels 2-35
Wood Biomass and Ethanol Consumption 2-40
3. Industrial Processes 3-1
Cement Manufacture 3.4
Lime Manufacture 3.5
Limestone and Dolomite Use 3_g
Soda Ash Manufacture and Consumption 3-10
Carbon Dioxide Consumption ; 3_12
Iron and Steel Production 3_13
Ammonia Manufacture 3-14
Ferroalloy Production 3_15
Petrochemical Production 3_lg
III
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Silicon Carbide Production 3"17
Adipic Acid Production 3-18
Nitric Acid Production 3~20
Substitution of Ozone Depleting Substances 3-21
Aluminum Production 3~2^
HCFC-22 Production 3-28
Semiconductor Manufacture 3~29
Electrical Transmission and Distribution 3~31
Magnesium Production and Processing 3'32
Industrial Sources of Criteria Pollutants 3"3^
4. Solvent Use 4'1
5. Agriculture 5'1
Enteric Fermentation ^~2
Manure Management ->-4
Rice Cultivation 5~8
Agricultural Soil Management 5~12
Agricultural Residue Burning 5~2^
6. Land-Use Change and Forestry 6-1
Changes in Forest Carbon Stocks 6-2
Changes in Non-Forest Soil Carbon Stocks 6-7
Changes in Non-Forest Carbon Stocks in Landfills 6-11
7. Waste 7'1
Landfills 1~l
Waste Combustion • 7~^
Wastewater Treatment 7"8
Human Sewage ••— 7~9
Waste Sources of Criteria Pollutants 7-11
References 8'1
Executive Summary 8-1
Introduction 8-1
Energy 8'3
Industrial Processes °"°
Solvent Use 8'12
Agriculture 8~12
Land-Use Change and Forestry 8"18
Waste • 8'20
Annexes
List of Annexes A-1
ANNEX A: Methodology for Estimating Emissions of CO2 from Fossil Fuel Combustion B-l
ANNEX B: Methodology for Estimating Emissions of CH4, N2O, and Criteria Pollutants from Stationary
Combustion C~l
ANNEX C: Methodology for Estimating Emissions of CH4, N2O, and Criteria Pollutants from Mobile
Combustion D-l
ANNEX D: Methodology for Estimating Methane Emissions from Coal Mining E-l
ANNEX E: Methodology for Estimating Methane Emissions from Natural Gas Systems F-l
ANNEX F: Methodology for Estimating Methane Emissions from Petroleum Systems G-l
ANNEX G: Methodology for Estimating Emissions from International Bunker Fuels used by the U.S.
Military H~*
ANNEX H: Methodology for Estimating Methane Emissions from Enteric Fermentation 1-1
ANNEX I: Methodology for Estimating Methane Emissions from Manure Management....... J-l
iv Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
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ANNEX J: Methodology for Estimating Methane Emissions from Landfills K-l
ANNEX K: Global Warming Potential Values L-l
ANNEX L: Ozone Depleting Substance Emissions M-l
ANNEX M: Sulfur Dioxide Emissions N-l
ANNEX N: Complete List of Sources O-l
ANNEX O: IPCC Reference Approach for Estimating CO2 Emissions from Fossil Fuel Combustion P-l
ANNEX P: Sources of Greenhouse Gas Emissions Excluded Q-l
ANNEX Q: Constants, Units, and Conversions R-l
ANNEX R: Abbreviations S-l
ANNEX S: Chemical Symbols T-l
ANNEX T: Glossary U-l
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List of Tables,
Figures, and Boxes
Tables
Table ES-1: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (MMTCE) ES-4
Table ES-2: Annual Change in CO2 Emissions from Fossil Fuel Combustion for Selected Fuels and Sectors
(MMTCE and Percent) ES-5
Table ES-3: Recent Trends in Various U.S. Data (Index 1990 = 100) ES-6
Table ES-4: Transportation-Related Greenhouse Gas Emissions (MMTCE) ES-8
Table ES-5: Electric Utility-Related Greenhouse Gas Emissions (MMTCE) ES-9
Table ES-6: Global Warming Potentials (100 Year Time Horizon) ES-9
Table ES-7: U.S. Sources of CO2 Emissions and Sinks (MMTCE) ES-11
Table ES-8: CO2 Emissions from Fossil Fuel Combustion by End-Use Sector (MMTCE) ES-12
Table ES-9: U.S. Sources of Methane Emissions (MMTCE) ES-15
Table ES-10: U.S. Sources of Nitrous Oxide Emissions (MMTCE) ES-18
Table ES-11: Emissions of HFCs.PFCs, and SF6 (MMTCE) ES-20
Table ES-12: Emissions of Ozone Depleting Substances (Gg) ES-21
Table ES-13: Emissions of NOX, CO, NMVOCs, and SO2 (Gg) ES-23
Table 1-1: Global Warming Potentials and Atmospheric Lifetimes (Years) 1-7
Table 1-2: Annual Change in CO2 Emissions from Fossil Fuel Combustion for Selected Fuels and Sectors
(MMTCE and Percent) 1-9
Table 1-3: Recent Trends in Various U.S. Data (Index 1990 = 100) , 1-10
Table 1-4: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (MMTCE) 1-11
Table 1-5: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (Gg) 1-12
Table 1-6: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks by Chapter/IPCC Sector
(MMTCE) 1-13
Table 1-7: Transportation-Related Greenhouse Gas Emissions (MMTCE) 1-15
Table 1-8: Electric Utility-Related Greenhouse Gas Emissions (MMTCE) 1-16
Table 1-9: IPCC Sector Descriptions l-l7
Table 1-10: List of Annexes !-18
Table 2-1-.Emissions from Energy (MMTCE) 2-2
Table 2-2: Emissions from Energy (Tg) 2-2
Table 2-3: CO2 Emissions from Fossil Fuel Combustion by Fuel Type and Sector (MMTCE) 2-4
Table 2-4: CO2 Emissions from Fossil Fuel Combustion by Fuel Type and Sector (Tg) 2-5
Table 2-5: Fossil Fuel Carbon in Products and CO2 Emissions from International Bunker Fuel Combustion
(MMTCE) 2-6
Table 2-6: Fossil Fuel Carbon in Products and CO2 Emissions from International Bunker Fuel Combustion
(TgC02) 2-6
Table 2-7: CO2 Emissions from Fossil Fuel Combustion by End-Use Sector (MMTCE) 2-7
Table 2-8: CO2 Emissions from Fossil Fuel Combustion in Transportation End-Use Sector (MMTCE) ... 2-10
Table 2-9: Carbon Intensity from Direct Fossil Fuel Combustion by Sector (MMTCE/EJ) 2-12
Table 2-10: Carbon Intensity from Energy Consumption by Sector (MMTCE/EJ) 2-13
Table 2-11: Change in CO2 Emissions from Direct Fossil Fuel Combustion Since 1990 (MMTCE) 2-14
Table 2-12: CH4 Emissions from Stationary Combustion (MMTCE) 2-18
Table 2-13: N2O Emissions from Stationary Combustion (MMTCE) 2-18
Table 2-14: CH4 Emissions from Stationary Combustion (Gg) 2-19
vi Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
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Table 2-15: N2O Emissions from Stationary Combustion (Gg) 2-19
Table 2-16: NOX, CO, and NMVOC Emissions from Stationary Combustion in 1998 (Gg) 2-20
Table 2-17: CH4 Emissions from Mobile Combustion (MMTCE) 2-22
Table 2-18: N2O Emissions from Mobile Combustion (MMTCE) 2-22
Table 2-19: CH4 Emissions from Mobile Combustion (Gg) 2-23
Table 2-20: N2O Emissions from Mobile Combustion (Gg) 2-23
Table 2-21: NOX, CO, and NMVOC Emissions from Mobile Combustion in 1998 (Gg) 2-24
Table 2-22: CH4 Emissions from Coal Mining (MMTCE) 2-27
Table 2-23: CH4 Emissions from Coal Mining (Gg) 2-27
Table 2-24: Coal Production (Thousand Metric Tons) 2-29
Table 2-25: CH4 Emissions from Natural Gas Systems (MMTCE) 2-30
Table 2-26: CH4 Emissions from Natural Gas Systems (Gg) 2-30
Table 2-27: CH4 Emissions from Petroleum Systems (MMTCE) 2-32
Table 2-28: CH4 Emissions from Petroleum Systems (Gg) : 2-32
Table 2-29: Uncertainty in CH4 Emissions from Production Field Operations (Gg) 2-33
Table 2-30: CO2 Emissions from Natural Gas Flaring 2-35
Table 2-31: NOX, NMVOCs, and CO Emissions from Oil and Gas Activities (Gg) 2-35
Table 2-32: Total Natural Gas Reported Vented and Flared (Million Ft3) and Thermal Conversion
Factor (Btu/Ft3). 2-35
Table 2-33: Emissions from International Bunker Fuels (MMTCE) 2-37
Table 2-34: Emissions from International Bunker Fuels (Gg) 2-37
Table 2-35: Aviation Jet Fuel Consumption for International Transport (Million Gallons) 2-38
Table 2-36: Marine Fuel Consumption for International Transport (Million Gallons) 2-39
Table 2-37: CO2 Emissions from Wood Consumption by End-Use Sector (MMTCE) 2-40
Table 2-38: CO2 Emissions from Wood Consumption by End-Use Sector (Gg) 2-40
Table 2-39: CO2 Emissions from Ethanol Consumption 2-41
Table 2-40: Woody Biomass Consumption by Sector (Trillion Btu) 2-41
Table 2-41: Ethanol Consumption 2-41
Table 3-1: Emissions from Industrial Processes (MMTCE) 3-2
Table 3-2: Emissions from Industrial Processes (Gg) 3-3
Table 3-3: CO2 Emissions from Cement Production 3-4
Table 3-4: Cement Production (Thousand Metric Tons) 3-5
Table 3-5: Net CO2 Emissions from Lime Manufacture 3-6
Table 3-6: CO2 Emissions from Lime Manufacture (Gg) 3-6
Table 3-7: Lime Production and Lime Use for Sugar Refining and PCC (Thousand Metric Tons) 3-7
Table 3-8: CO2 Emissions from Limestone & Dolomite Use (MMTCE) 3-8
Table 3-9: CO2 Emissions from Limestone & Dolomite Use (Gg) 3-8
Table 3-10: Limestone & Dolomite Consumption (Thousand Metric Tons) 3-9
Table 3-11: CO2 Emissions from Soda Ash Manufacture and Consumption 3-10
Table 3-12: CO2 Emissions from Soda Ash Manufacture and Consumption (Gg) 3-10
Table 3-13: Soda Ash Manufacture and Consumption (Thousand Metric Tons) 3-11
Table 3-14: CO2 Emissions from Carbon Dioxide Consumption 3-12
Table 3-15: Carbon Dioxide Consumption 3-12
Table 3-16: CO2 Emissions from Iron and Steel Production 3-13
Table 3-17: Pig Iron Production , 3-14
Table 3-18: CO2 Emissions from Ammonia Manufacture 3-14
Table 3-19: Ammonia Manufacture 3-15
Table 3-20: CO2 Emissions from Ferroalloy Production'. 3-16
Table 3-21: Production of Ferroalloys (Metric Tons) 3-16
Table 3-22: CH4 Emissions from Petrochemical Production 3-17
Table 3-23: Production of Selected Petrochemicals (Thousand Metric Tons) 3-17
Table 3-24: CH4 Emissions from Silicon Carbide Production 3-18
Table 3-25: Production of Silicon Carbide 3-18
VII
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Table 3-26: N2O Emissions from Adipic Acid Production 3-19
Table 3-27: Adipic Acid Production 3-19
Table 3-28: N2O Emissions from Nitric Acid Production 3-20
Table 3-29: Nitric Acid Production 3-21
Table 3-30: Emissions of HFCs and PFCs from ODS Substitution (MMTCE) 3-21
Table 3-31: Emissions of HFCs and PFCs from ODS Substitution (Mg) 3-22
Table 3-32: CO2 Emissions from Aluminum Production 3-26
Table 3-33: PFC Emissions from Aluminum Production (MMTCE) 3-26
Table 3-34: PFC Emissions from Aluminum Production (Gg) 3-26
Table 3-35: Production of Primary Aluminum 3-27
Table 3-36: HFC-23 Emissions from HCFC-22 Production 3-29
Table 3-37: Emissions of Fluorinated Greenhouse Gases from Semiconductor Manufacture 3-30
Table 3-38: SF6 Emissions from Electrical Transmission and Distribution 3-31
Table 3-39: SF6 Emissions from Magnesium Production and Processing 3-32
Table 3-40: 1998 Potential and Actual Emissions of HFCs, PFCs, and SF6 from Selected Sources
(MMTCE) 3-33
Table 3-41: NOX, CO, and NMVOC Emissions from Industrial Processes (Gg) 3-35
Table 4-1: Emissions of NOX, CO, and NMVOC from Solvent Use (Gg) 4-2
Table 5-1: Emissions from Agriculture (MMTCE) 5-2
Table 5-2: Emissions from Agriculture (Gg) 5-2
Table 5-3: CH4 Emissions from Enteric Fermentation (MMTCE) 5-3
Table 5-4: CH4 Emissions from Enteric Fermentation (Gg) 5-3
Table 5-5: Cow Populations (Thousands) and Milk Production (Million Kilograms) 5-4
Table 5-6: CH4 and N2O Emissions from Manure Management (MMTCE) 5-6
Table 5-7: CH4 Emissions from Manure Management (Gg) 5-6
Table 5-8: N2O Emissions from Manure Management (Gg) 5-6
Table 5-9: CH4 Emissions from Rice Cultivation (MMTCE) 5-9
Table 5-10: CH4 Emissions from Rice Cultivation (Gg) 5-9
Table 5-11: Rice Areas Harvested (Hectares) • 5-11
Table 5-12: Rice Flooding Season Lengths (Days) 5-11
Table 5-13: N2O Emissions from Agricultural Soil Management (MMTCE) 5-14
Table 5-14: N2O Emissions from Agricultural Soil Management (Gg) 5-14
Table 5-15: Commercial Fertilizer Consumption & Land Application of Sewage Sludge (Thousand
Metric Tons of Nitrogen) 5-15
Table 5-16: Animal Excretion from Livestock and Poultry (Thousand Metric Tons of Nitrogen) 5-15
Table 5-17: Nitrogen Fixing Crop Production (Thousand Metric Tons of Product) 5-16
Table 5-18: Corn and Wheat Production (Thousand Metric Tons of Product) 5-16
Table 5-19: Histosol Area Cultivated (Thousand Hectares) 5-17
Table 5-20: Direct N2O Emissions from Agricultural Cropping Practices (MMTCE) 5-17
Table 5-21: Direct N2O Emissions from Pasture, Range, and Paddock Animals (MMTCE).: 5-19
Table 5-22: Indirect N2O Emissions (MMTCE) 5-19
Table 5-23: Emissions from Agricultural Residue Burning (MMTCE) 5-20
Table 5-24: Emissions from Agricultural Residue Burning (Gg) 5-21
Table 5-25: Agricultural Crop Production (Thousand Metric Tons of Product) 5-22
Table 5-26: Percentage of Rice Area Burned By State 5-22
Table 5-27: Percentage of Rice Area Burned - 5-23
Table 5-28: Key Assumptions for Estimating Emissions from Agricultural Residue Burning 5-23
Table 5-29: Greenhouse Gas Emission Ratios 5-23
viii Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
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Table 6-1: Net CO2 Flux from Land-Use Change and Forestry (MMTCE) 6-2
Table 6-2: Net CO2 Flux from Land-Use Change and Forestry (Gg) 6-2
Table 6-3: Net CO2 Flux from U.S. Forests (MMTCE) 6-4
Table 6-4: Net CO2 Flux from U.S. Forests (Gg) 6-4
Table 6-5: U.S. Forest Carbon Stock Estimates (Gg) 6-7
Table 6-6: Net CO2 Flux From Non-Forest Soils (MMTCE) 6-9
Table 6-7: Net CO2 Flux From Non-Forest Soils (Gg) 6-9
Table 6-8: Quantities of Applied Minerals (Thousand Metric Tons). 6-10
Table 6-9: Net CO2 Flux from Non-Forest Carbon Stocks in Landfills 6-11
Table 6-10: Composition of Yard Trimmings (%) in MSW and Carbon Storage Factor (Gg Carbon/Gg
Yard Trimmings) 6-12
Table 6-11: Yard Trimmings Disposal to Landfills 6-12
Table 7-1: Emissions from Waste (MMTCE) 7-2
Table 7-2: Emissions from Waste (Gg) 7-2
Table 7-3: CH4 Emissions from Landfills (MMTCE) 7-4
Table 7-4: CH4 Emissions from Landfills (Gg) 7-4
Table 7-5: CO2 and N2O Emissions from Waste Combustion (MMTCE) 7-6
Table 7-6: CO2 and N2O Emissions from Waste Combustion (Gg) 7-6
Table 7-7: 1997 Plastics in the Municipal Solid Waste Stream by Resin (Thousand Metric Tons) 7-6
Table 7-8: 1997 Factors for Calculating CO2 Emissions from Combusted Plastics 7-7
Table 7-9: Municipal Solid Waste Generation (Metric Tons) and Percent Combusted 7-7
Table 7-10: U.S. Municipal Solid Waste Combusted by Data Source (Metric Tons) 7-8
Table 7-11: CH4 Emissions from Domestic Wastewater Treatment 7-8
Table 7-12: U.S. Population (Millions) and Wastewater BOD Produced (Gg) 7-9
Table 7-13: N2O Emissions from Human Sewage 7-10
Table 7-14: U.S. Population (Millions) and Average Protein Intake (kg/Person/Year) 7-10
Table 7-15: Emissions of NOX, CO, and NMVOC from Waste (Gg) 7-11
Table A-l: 1998 Energy Consumption Data and CO2 Emissions from Fossil Fuel Combustion by Fuel
Type A-4
Table A-2: 1997 Energy Consumption Data and CO2 Emissions from Fossil Fuel Combustion by Fuel
Type A-5
Table A-3: 1996 Energy Consumption Data and CO2 Emissions from Fossil Fuel Combustion by Fuel
Type A-6
Table A-4: 1995 Energy Consumption Data and CO2 Emissions from Fossil Fuel Combustion by Fuel
Type A-7
Table A-5: 1994 Energy Consumption Data and CO2 Emissions from Fossil Fuel Combustion by Fuel
Type A-8
Table A-6: 1993 Energy Consumption Data and CO2 Emissions from Fossil Fuel Combustion by Fuel
Type A-9
Table A-7: 1992 Energy Consumption Data and CO2 Emissions from Fossil Fuel Combustion by Fuel
Type A-10
Table A-8: 1991 Energy Consumption Data and CO2 Emissions from Fossil Fuel Combustion by Fuel
Type A-ll
Table A-9: 1990 Energy Consumption Data and CO2 Emissions from Fossil Fuel Combustion by Fuel
Type A-12
Table A-10: 1998 Emissions From International Bunker Fuel Consumption A-13
Table A-ll: 1998 Carbon In Non-Energy Products A-13
Table A-12: Key Assumptions for Estimating Carbon Dioxide Emissions A-14
ix
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Table A-13: Annually Variable Carbon Content Coefficients by Year (MMTCE/QBtu) A-15
Table A-14: Electricity Consumption by End-Use Sector (BiUion Kilowatt-Hours) A-15
Table B-l: Fuel Consumption by Stationary Combustion for Calculating CH4 and N2O Emissions (Tbtu)B-2
Table B-2: CH4 and N2O Emission Factors by Fuel Type and Sector (g/GJ) B-3
Table B-3: NOX Emissions from Stationary Combustion (Gg) B-4
Table B-4: CO Emissions from Stationary Combustion (Gg) B-5
Table B-5: NMVOC Emissions from Stationary Combustion (Gg) B-6
Table C-l: Vehicle Miles Traveled for Gasoline Highway Vehicles (109 Miles) C-3
Table C-2: Vehicle Miles Traveled for Diesel Highway Vehicles (109 Miles) C-4
Table C-3: VMT Profile by Vehicle Age (Years) and Vehicle/Fuel Type for Highway Vehicles (Percent
ofVMT) C-4
Table C-4: Fuel Consumption for Non-Highway Vehicles by Fuel Type (U.S. Gallons) C-5
Table C-5: Control Technology Assignments for Gasoline Passenger Cars (Percent of VMT) C-6
Table C-6: Control Technology Assignments for Gasoline Light-Duty Trucks (Percent of VMT) C-6
Table C-7: Control Technology Assignments for California Gasoline Passenger Cars and Light-Duty
Trucks (Percent of VMT) C-6
Table C-8: Control Technology Assignments for Gasoline Heavy-Duty Vehicles (Percent of VMT) C-7
Table C-9: Control Technology Assignments for Diesel Highway VMT C-7
Table C-10: Emission Factors (g/km) for CH4 and N2O and "Fuel Economy" (g CO2/km)c for Highway
Mobile Combustion C-8
Table C-ll: Emission Factors for CH4 and N2O Emissions from Non-Highway Mobile Combustion
(g/kgFuel) : C-9
Table C-12: NOX Emissions from Mobile Combustion, 1990-1998 (Gg) C-9
Table C-13: CO Emissions from Mobile Combustion, 1990-1998 (Gg) C-10
Table C-14: NMVOCs Emissions from Mobile Combustion, 1990-1998 (Gg) C-10
Table D-l: Mine-Specific Data Used to Estimate Ventilation Emissions D-2
Table D-2: Coal Basin Definitions by Basin and by State D-4
Table D-3: Annual Coal Production (Thousand Short Tons) D-5
Table D-4: Coal Surface and Post-Mining Methane Emission Factors (ft3 Per Short Ton) D-6
Table D- 5: Underground Coal Mining Methane Emissions (Billion Cubic Feet) D-6
Table D-6: Total Coal Mining Methane Emissions (Billion Cubic Feet) D-6
Table D-7: Total Coal Mining Methane Emissions by State (Million Cubic Feet) D-7
Table E-l: 1992 Data and Emissions (Mg) for Venting and Flaring from Natural Gas Field
Production Stage E~2
Table E-2: Activity Factors for Key Drivers • E-3
Table E-3: CH4 Emission Estimates for Venting and Flaring from the Field Production Stage (Mg) E-4
Table F- 1: CH4 Emissions from Petroleum Production Field Operations F-2
Table F-2: 1998 CH4 Emissions from Petroleum Transportation F-3
Table F- 3: CH4 Emissions from Petroleum Refining F-4
Table F- 4: Summary of CH4 Emissions from Petroleum Systems (Gg) F-4
Table G-l: Transportation Fuels (Gallons) from Domestic Fuel Deliveries G-3
Table G-2: Total U.S. DoD Aviation Bunker Fuel (Million Gallons) G-4
Table G-3: Total U.S. DoD Maritime Bunker Fuel (Million Gallons) G-4
Table H-l: Livestock Population (Thousand Head) H-2
Table H-2: Dairy Cow CH4 Emission Factors and Milk Production Per Cow H-2
Table H-3: CH4 Emission Factors Beef Cows and Replacements (kg/Head/Year) H-2
Table H-4: Methane Emissions from Livestock Enteric Fermentation (Gg) H-3
Table H-5: Enteric Fermentation CH4 Emission Factors H-3
x Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Table 1-1: Livestock Population (1,000 Head) I_3
Table 1-2: Dairy Cow Weighted MCF Values '"."".'.'.'. 1-4
Table 1-3: Swine Weighted MCF Values "^'"^'"""'^. 1-5
Table 1-4: Dairy Cow and Swine Constants j.g
Table 1-5: CH4 Emissions from Livestock Manure Management (Gg) 1-6
Table J- 1: Municipal Solid Waste (MSW) Contributing to Methane Emissions (Tg) J-2
Table J-2: Methane Emissions from Landfills (Gg) j-3
Table J-3: Municipal Solid Waste Landfill Size Definitions (Gg) j-3
Table K-l: Global Warming Potentials and Atmospheric Lifetimes (Years) K-l
Table L-l: Emissions of Ozone Depleting Substances (Gg) L-2
Table M-l: SO2 Emissions (Gg) M-2
Table M-2: SO2 Emissions from Electric Utilities (Gg) M-2
Table O-1: 1998 U.S. Energy Statistics (Physical Units) O-5
Table O-2: Conversion Factors to Energy Units (Heat Equivalents) O-6
Table O-3: 1998 Apparent Consumption of Fossil Fuels (TBtu) ,. Q-7
Table O-4: 1998 Potential Carbon Emissions O-8
Table O-5: 1998 Non-Energy Carbon Stored in Products O-9
Table O-6: Reference Approach CO2 Emissions from Fossil Fuel Consumption (MMTCE unless otherwise
noted) O-9
Table O-7: 1998 Energy Consumption in the United States: Sectoral vs. Reference Approaches (Tbtu).... O-9
Table O-8: 1998 CO2 Emissions from Fossil Fuel Combustion by Estimating Approach (MMTCE) O-9
Table O-9: 1997 Energy Consumption in the United States: Sectoral vs. Reference Approaches (TBtu) .O-10
Table O-10: 1997 CO2 Emissions from Fossil Fuel Combustion by Estimating Approach (MMTCE) O-10
Table O-ll: 1996 Energy Consumption in the United States: Sectoral vs. Reference Approaches (Tbtu) O-10
Table O-12: 1996 CO2 Emissions from Fossil Fuel Combustion by Estimating Approach (MMTCE) O-10
Table O-13: 1995 Energy Consumption in the United States: Sectoral vs. Reference Approaches (Tbtu) O-ll
Table O-14: 1995 CO2 Emissions from Fossil Fuel Combustion by Estimating Approach (MMTCE) O-ll
Table Q-l: Guide to Metric Unit Prefixes Q-l
Table Q-2: Conversion Factors to Energy Units (Heat Equivalents) Q-3
Table S-l: Guide to Chemical Symbols S-l
Figures
Figure ES-l: U.S. GHG Emissions by Gas ES-2
Figure ES-2: Annual Percent Change in U.S. GHG Emissions ES-2
Figure ES-3: Absolute Change in U.S. GHG Emissions Since 1990 ES-3
Figure ES-4: 1998 Greenhouse Gas Emissions by Gas ES-3
Figure ES-5: U.S. Greenhouse Gas Emissions Per Capita and Per Dollar of Gross Domestic Product ES-6
Figure ES-6: 1998 Sources of CO2 ES-10
Figure ES-7: 1998 U.S. Energy Consumption by Energy Source ES-10
Figure ES-8: U.S. Energy Consumption (Quadrillion Btu) ES-10
Figure ES-9: 1998 CO2 Emissions from Fossil Fuel Combustion by Sector and Fuel Type ES-12
Figure ES-10: 1998 End-Use Sector Emissions of CO2 from Fossil Fuel Combustion ES-12
Figure ES-l 1: 1998 Sources of CH4 ES-15
Figure ES-12: 1998 Sources of N2O ES-18
Figure ES-13: 1998 Sources of HFCs, PFCs, and SF6 , ES-20
xi
-------
Figure 1-1: U.S. GHG Emissions by Gas 1-7
Figure 1-2: Annual Percent Change in U.S. GHG Emissions 1-8
Figure 1-3: Absolute Change in U.S. GHG Emissions Since 1990 1-8
Figure 1-4: U.S. Greenhouse Gas Emissions Per Capita and Per Dollar of Gross Domestic Product 1-10
Figure 1-5: U.S. GHG Emissions by Chapter/IPCC Sector 1-13
Figure 2-1: 1998 Energy Chapter GHG Sources 2-1
Figure 2-2: 1998 U.S. Energy Consumption by Energy Source 2-3
Figure 2-3: U.S. Energy Consumption (Quadrillion Btu) 2-3
Figure 2-4: 1998 CO2 Emissions from Fossil Fuel Combustion by Sector and Fuel Type 2-4
Figure 2-5: Fossil Fuel Production Prices 2-5
Figure 2-6: 1998 End-Use Sector Emissions of CO2 from Fossil Fuel Combustion 2-7
Figure 2-7: Motor Gasoline Retail Prices (Real) 2-8
Figure 2-8: Motor Vehicle Fuel Efficiency 2-9
Figure 2-9: Heating Degree Days 2-9
Figure 2-10: Cooling Degree Days 2-11
Figure 2-11: Electric Utility Retail Sales by End-Use Sector 2-11
Figure 2-12: Change in CO2 Emissions from Fossil Fuel Combustion Since 1990 by End-Use Sector 2-13
Figure 2-13: Mobile Source CH4 and N2O Emissions 2-21
Figure 3-1:1998 Industrial Processes Chapter GHG Sources 3-1
Figure 5-1: 1998 Agriculture Chapter GHG Sources 5-1
Figure 5-2: Sources of N2O Emissions from Agricultural Soils 5-13
Figure 7-1: 1998 Waste Chapter GHG Sources 7-1
Boxes
BoxES-l: Recent Trends in Various U.S. Greenhouse Gas Emissions-Related Data ES-6
Box ES-2: Greenhouse Gas Emissions from Transportation Activities ES-7
Box ES-3: Greenhouse Gas Emissions from Electric Utilities ES-9
Box ES-4: Emissions of Ozone Depleting Substances ES-21
Box ES-5: Sources and Effects of Sulfur Dioxide ES-23
Box 1-1: Recent Trends in Various U.S. Greenhouse Gas Emissions-Related Data 1-10
Box 1-2: Greenhouse Gas Emissions from Transportation Activities 1-14
Box 1-3: Greenhouse Gas Emissions from Electric Utilities 1-15
Box 2-1: Sectoral Carbon Intensity Trends Related to Fossil Fuel and Overall Energy Consumption 2-12
Box 3-1: Potential Emission Estimates of HFCs, PFCs, and SF6 3-33
Box 7-1: Biogenic Emissions and Sinks of Carbon 7-3
Box 7-2: Recycling and Greenhouse Gas Emissions and Sinks 7-3
xii Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Box ES- 3: Greenhouse Gas Emissions from Electric Utilities
t,- Like transportation, activities related to the generation, transmission, and distribution of electricity in the United States result in
^significant greenhouse gas emissions. Table ES-5 presents greenhouse gas emissions from electric utility-related activities. Aggregate
Remissions from electric utilities of all greenhouse gases increased by 16 percent from 1990 to 1998, and accounted for a relatively
^constant 29 percent of U.S. greenhouse emissions during the same period.7 The majority of these emissions resulted from the
^combustion of coal in boilers to produce steam that is passed through a turbine to generate electricity. Overall, the generation of
^electricity results in a larger portion of total U.S. greenhouse gas emissions than any other activity.
Table ES-5: Electric Utility-Related Greenhouse Gas Emissions (MMTCE)
'Gas/Fuel Type or Source
1990 1991 1992 1993 1994 1995 1996 1997 1998
|co2
P Coal
£ Natural Gas
I.: Petroleum
it." Geothermal
|QH4
f -.--...Stationary Combustion
|N20
H _ Stationary Combustion
I*'SF6
'f~- Electrical Transmission
t - --- --
ITotal
476.6 473.2
409.0 407.2
41.2
26.4
0.1
0.1
(Utilities) 0.1
2.0
(Utilities) 2.0
5.6
and Distribution 5.6
484.3 481
41.1
24.9
0.1
0.1
0.1
2.0
2.0
5.9
5.9
.2
472.7
411.8
40.7
20.2
0.1
0.1
0.1
2.0
2.0
6.2
6.2
481.0
490.5
428,7
39.5
22.3
0.1
0.1
0.1
2.1
2.1
6.4
6.4
499.1
493.9
429.5
44.0
20.5
+
0.1
0.1
2.1
2.1
6.7
6.7
502.9
494.0
433.0
47.2
13.9
+
0.1
0.1
2.1
2.1
7.0
7.0
503.2
513.0
457.5
40.3
15.3
+
0.1
0.1
2.2
2.2
7.0
7.0
522.4
532.8
471.8
43.6
17.5
0.1
0.1
2.3
2.3
7.0
7.0
542.2
549.9
477.3
47.8
24.8
0.1
0.1
2.3
2.3
7.0
7.0
559.3
f+ Does not exceed 0.05 MMTCE
fjjlote: Totals may not sum due to independent rounding.
Table ES-6: Global Warming Potentials
(100 Year Time Horizon)
i— -
'£-_..
I. : '
|-
f
k- '•'
!r
j£ "
^~.
jfe>
£
P
L^,__
^
F
fc
jr^--
ift •
1: -- - .
Gas
Carbon dioxide (C02)
Methane (CH4)*
Nitrous oxide (N20)
HFC-23
• HFC-125
HFC-134a
HFC-143a
HFC-152a
HFC-227ea
HFC-236fa
HFC-4310mee
CF4
C2F6
£4^10
CeF-14
SF6
GWP
1
21
310
11,700
2,800
1,300
3,800
140
2,900
6,300
1,300
6,500
9,200
7,000
7,400
23,900
jFSource: (IPCC 1996)
i£*: The methane GWP includes the direct effects and those
pndjrect effects due to the production of tropospheric ozone and
^stratospheric water vapor. The indirect effect due to the
f^production of C02 is not included.
Carbon Dioxide Emissions
The global carbon cycle is made up of large car-
bon flows and reservoirs. Hundreds of billions of tons of
carbon in the form of CO2 are absorbed by oceans and
living biomass (sinks) and are emitted to the atmosphere
annually through natural processes (sources). When in
equilibrium, carbon fluxes among these various reser-
voirs are roughly balanced.
Since the Industrial Revolution, this equilibrium
of atmospheric carbon has been altered. Atmospheric
concentrations of CO2 have risen about 28 percent (IPCC
1996), principally because of fossil fuel combustion,
which accounted for 98 percent of total U.S. CO2 emis-
sions in 1998. Changes in land use and forestry prac-
tices can also emit CO2 (e.g., through conversion of for-
est land to agricultural or urban use) or can act as a sink
for CO2 (e.g., through net additions to forest biomass).
7 Emissions from nonutility generators are not included in these estimates. Nonutilties were estimated to produce about 10 percent of
the electricity generated in the United States in 1998 (DOE and EPA 1999).
Executive Summary ES-9
-------
Figure ES-6
Fossil Fuel Combustion
Cement Manufacture
Natural Gas Flaring
Lime Manufacture
Limestone and •
Dolomite Use Hi
Soda Ash Manufacture Bj
and Consumption •
Carbon Dioxide I
Manufacture
024 6 8 10 12
MMTCE
Figure ES-6 and Table ES-7 summarize U.S. sources
and sinks of CO2. The remainder of this section then
discusses CO2 emission trends in greater detail.
Energy
Energy-related activities accounted for almost all
U.S. CO2 emissions for the period of 1990 through 1998.
Carbon dioxide from fossil fuel combustion was the domi-
nant contributor. In 1998, approximately 85 percent of
the energy consumed in the United States was produced
through the combustion of fossil fuels. The remaining
15 percent came from other energy sources such as hy-
dropower, biomass, nuclear, wind, and solar (see Figure
ES-7 and Figure ES-8). A discussion of specific trends
related to CO2 emissions from energy consumption is
presented below.
Fossil Fuel Combustion
As fossil fuels are combusted, the carbon stored in
them is almost entirely emitted as CO2. The amount of
carbon in fuels per unit of energy content varies signifi-
cantly by fuel type. For example, coal contains the high-
est amount of carbon per unit of energy, while petroleum
has about 25 percent less carbon than coal, and natural
gas about 45 percent less. From 1990 through 1998, pe-
troleum supplied the largest share of U.S. energy de-
mands, accounting for an average of 39 percent of total
energy consumption. Natural gas and coal followed in
order of importance, accounting for an average of 24
and 22 percent of total energy consumption, respectively.
Most petroleum was consumed in the transportation sec-
tor, while the vast majority of coal was used by electric
utilities, and natural gas was consumed largely in the
industrial and residential sectors.
Emissions of CO2 from fossil fuel combustion in-
creased at an average annual rate of 1.3 percent from
1990 to 1998. The fundamental factors behind this trend
include (1) a robust domestic economy, (2) relatively
low energy prices, and (3) fuel switching by electric utili-
ties. After 1990, when CO2 emissions from fossil fuel
combustion were 1,320.1 MMTCE, there was a slight
decline in emissions in 1991, due in large part to an
economic recession, followed by a relatively steady in-
Flgute ES-7
Figure ES-8
v .,„," JL ''e »•. ,^'., !iu_.,,;..i .iiijn *.:!!, li.ii! JiM pi iiiiilltfMiiiiii ilP; -JT1' «!" v:" • 'mi a^^iiw-^iii' jt" 'CMiillii.iilli!!
1998 U.S. Energy Consumption by Energy Source
;««!:«••;•••• SfSSmv^A»fv- i Hwi£Mfln«iti mrviafe i aaam
7.5% Renewable
7.6% Nuclear
22.9% Coal
23.2% Natural Gas
38.8% Petroleum
Source: DOE/EIA-0384(99), Annual Energy Review 1998,
Table 1.3, July 1999
1990 1991 1992 1993 1994 1995 1996 1997 1998
Note: Expressed as gross Calorific values
Source: DOE/EIA-0384(99), Annual Energy Review 1998,
Table 1.3, July 1999
ES-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Table ES-7: U.S. Sources of C02 Emissions and Sinks (MMTCE)
Source
| _
I Fossil Fuel Combustion
t Cement Manufacture
pfatural Gas Flaring
fLime Manufacture
f Waste Combustion
SJJmestone and Dolomite Use
1990
1,320.1
9.1
2.5
3.0
2.8
1.4
tSoda Ash Manufacture and Consumption 1.1
f Carbon Dioxide Consumption
s Land-Use Change and Forestry (Sink)a
E International Bunker Fuelsb
Total Emissions
: Net Emissions (Sources and
Sinks)
0.2
(316.4)
32.2
1,340.3
1,023.9
1991
1,305.8
8.9
2.8
3.0
3.0
1.3
1.
0,
,1
,2
(316.3)
32,
1,326.
1,009.
,7
,1
8
1992
1,330.1
8.9
2.8
3.1
3.0
1.2
1.1
0.2
(316.2)
30.0
1,350.4
1,034.2
1993
1,361.5
9.4
3.7
3.1
3.1
1.1
1.1
0.2
(212.7)
27.2
1,383.3
1,170.6
1994
1,382.0
9.8
3.8
3.2
3.1
1.5
1.1
0.2
(212.3)
26.7
1,404.8
1,192.5
1995
1,392.0
10.0
4.7
3.4
3.0
1.9
1.2
0.3
(211.8)
27.5
1,416.5
1,204.7
1996
1,441.3
10.1
4.5
3.6
3.1
2.0
1.2
0.3
(211.3)
27.9
1,466.2
1,254.9
1997
1,460.7
10.5
4.2
3.7
3.4
2.3
1.2
0.4
(211.1)
29.9
1,486.4
1,275.3
1998
1,468.2
10.7
3.9
3.7
3.5
2.4
1.2
0.4
(210.8)
31.3
1,494.0
1,283.2
g3 Sinks are only included in net emissions total. Estimates of net carbon sequestration due to land-use change and forestry activities exclude
f non-forest soils, and are based partially upon projections of forest carbon stocks.
p" Emissions from International Bunker Fuels are not included in totals.
| Note: Totals may not sum due to independent rounding.
crease to 1,468.2 MMTCE in 1998. Overall, CO2 emis-
sions from fossil fuel combustion increased by 11 per-
cent over the nine year period and rose by 0.5 percent in
the final year.
In 1998, mild weather and low petroleum prices
comprised the major forces affecting emission trends. A
very mild winter more than offset the effects of a slightly
hotter summer, resulting in significantly lower fuel con-
sumption for residential and commercial heating com-
pared to previous years. Emissions from the combustion
of petroleum products grew the most (11.5 MMTCE or
1.9 percent) due in large part to low prices. Alone, emis-
sions from the combustion of petroleum by electric utili-
ties increased by 7.3 MMTCE (42 percent) from 1997 to
1998. Emissions from the combustion of coal in 1998
increased by 5.5 MMTCE (1 percent) from the previous
year, driven almost entirely by increased emissions by
electric utilities. These increases were offset by a de-
crease in natural gas combustion emissions in every sec-
tor (9.1 MMTCE or 3 percent).
The four end-use sectors contributing to CO2 emis-
sions from fossil fuel combustion include: industrial,
transportation, residential, and commercial. Electric utili-
ties also emit CO2, although these emissions are pro-
duced as they consume fossil fuel to provide electricity
to one of the four end-use sectors. For the discussion
below, electric utility emissions have been distributed
to each end-use sector based upon their fraction of ag-
gregate electricity consumption. This method of distrib-
uting emissions assumes that each end-use sector con-
sumes electricity that is generated with the national av-
erage mix of fuels according to their carbon intensity. In
reality, sources of electricity vary widely in carbon in-
tensity. By giving equal carbon-intensity weight to each
sector's electricity consumption, for example, emissions
attributed to the residential sector may be overestimated,
while emissions attributed to the industrial sector may
be underestimated. Emissions from electric utilities are
addressed separately after the end-use sectors have been
discussed. Emissions from U.S. territories are also calcu-
lated separately due to a lack of end-use-specific con-
sumption data. Table ES-8, Figure ES-9, and Figure ES-
10 summarize CO2 emissions from fossil fuel combus-
tion by end-use sector.
Industrial End-Use Sector. Industrial CO2 emis-
sions resulting from direct fossil fuel combustion and
from the generation of electricity consumed by the sec-
tor accounted for 33 percent of U.S. emissions from fos-
sil fuel combustion in 1998. About two-thirds of these
emissions resulted from producing steam and process
heat from fossil fuel combustion, while the remaining
third resulted from consuming electricity for powering
motors, electric furnaces, ovens, and lighting.
Transportation End-Use Sector. Transportation
Executive Summary ES-11
-------
Table ES-8: C02 Emissions from Fossil Fuel Combustion by End-Use Sector (MMTCE)*
= End-Use Sector
= Residential
^Commercial
Industrial
Transportation
U.S. Territories
: Total
1990
252.9
206.7
451.7
399.6
9.2
1,320.1
1991
257.0
206.3
440.3
391.5
10.7
1,305.8
1992
255.8
205.4
458.0
401.1
9.8
1,330.1
1993
271.6
212.0
458.0
409.1
10.7
1,361.5
1994
268.2
213.8
466.2
422.3
11.5
1,382.0
1995
269.8
218.3
464.4
427.7
11.8
1,392.0
1996
285.4
225.9
477.3
441.7
11.0
1,441.3
1997
284.7
238.0
482.5
443.4
12.0
1,460.7
1998 ;
286.8 . • - i
239.3 ;
478.9
450.3
13.0
1,468.2
" * Emissions from fossil fuel combustion by electric utilities are allocated based on electricity consumption by each end-use sector.
"--. Note: Totals may not sum due to independent rounding.
activities excluding international bunker fuels ac-
counted for 31 percent of CO2 emissions from fossil
fuel combustion in 1998.8 Virtually all of the energy
consumed in this end-use sector came from petroleum
products. Two thirds of the emissions resulted from gaso-
line consumption in motor vehicles. The remaining
emissions came from other transportation activities, in-
cluding the combustion of diesel fuel in heavy-duty
vehicles and jet fuel in aircraft.
Residential and Commercial End-Use Sectors.
The residential and commercial end-use sectors ac-
counted for 20 and 16 percent, respectively, of CO2
emissions from fossil fuel consumption in 1998. Both
sectors relied heavily on electricity for meeting energy
needs, with 67 and 75 percent, respectively, of their
emissions attributable to electricity consumption for
lighting, heating, cooling, and operating appliances.
The remaining emissions were largely due to the con-
sumption of natural gas and petroleum, primarily for
meeting heating and cooking needs.
Electric Utilities. The United States relies on elec-
tricity to meet a significant portion of its energy de-
mands, especially for lighting, electric motors, heat-
ing, and air conditioning. Electric utilities are respon-
sible for consuming 29 percent of U.S. energy from
fossil fuels and emitted 37 percent of the CO2 from
fossil fuel combustion in 1998. The type of fuel com-
busted by utilities has a significant effect on their emis-
sions. For example, some electricity is generated with
low CO2 emitting energy technologies, particularly
non-fossil options such as nuclear, hydroelectric, or
geothermal energy. However, electric utilities rely on
Figure ES-9
Figure ES-10
• Natural Gas
• Petroleum
• Coal
End-Use Sector Emissions o
from Fossil Fuel combustion
I From Electricity
500 " Consumption
8 If emissions from international bunker fuels are included, the transportation end-use sector accounted for 33 percent of U.S. emissions
from fossil fuel combustion in 1998.
ES-12 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
coal for over half of their total energy requirements and
accounted for 88 percent of all coal consumed in the
United States in 1998. Consequently, changes in elec-
tricity demand have a significant impact on coal con-
sumption and associated CO2 emissions.
Natural Gas Flaring
Carbon dioxide is produced when natural gas from
oil wells is flared (i.e., combusted) to relieve rising pres-
sure or to dispose of small quantities of gas that are not
commercially marketable. In 1998, flaring activities
emitted approximately 3.9 MMTCE, or about 0.2 per-
cent of U.S. CO2 emissions.
Biomass Combustion
Biomass in the form of fuel wood and wood waste
was used primarily by the industrial end-use sector,
while the transportation end-use sector was the pre-
dominant user of biomass-based fuels, such as ethanol
from coni and woody crops. Ethanol and ethanol blends,
such as gasohol, are typically used to fuel public trans-
port vehicles.
Although these fuels do emit CO2, in the long run
the CO2 emitted from biofuel consumption does not
increase atmospheric CO2 concentrations if the biogenic
carbon emitted is offset by the growth of new biomass.
For example, fuel wood burned one year but re-grown
the next only recycles carbon, rather than creating a
net increase in total atmospheric carbon. Net carbon
fluxes from changes in biogenic carbon reservoirs in
wooded or crop lands are accounted for under Land-
Use Change and Forestry.
Gross CO2 emissions from biomass combustion
were 66.2 MMTCE, with the industrial sector account-
ing for 81 percent of the emissions, and the residential
sector 15 percent. Ethanol consumption by the trans-
portation sector accounted for only 3 percent of CO2
emissions from biomass combustion.
Industrial Processes
Emissions are often produced as a by-product of
various non-energy-related activities. For example, in-
dustrial processes can chemically transform raw mate-
rials. This transformation often releases greenhouse
gases such as CO2. The production processes that emit
CO2 include cement manufacture, lime manufacture,
limestone and dolomite use (e.g., in iron and steel mak-
ing), soda ash manufacture and consumption, and CO2
consumption. Total CO2 emissions from these sources
were approximately 18.4 MMTCE in 1998, account-
ing for about 1 percent of total CO2 emissions. Since
1990, emissions from each of these sources increased,
except for emissions from soda ash manufacture and
consumption, which remained relatively constant.
Cement Manufacture (10.7 MMTCE)
Carbon dioxide is produced primarily during the
production of clinker, an intermediate product from which
finished Portland and masonry cement are made. Spe-
cifically, CO2 is created when calcium carbonate (CaCO3)
is heated in a cement kiln to form lime and CO2. This
lime combines with other materials to produce clinker,
while the CO2 is released into the atmosphere.
Lime Manufacture (3.7 MMTCE)
Lime is used in steel making, construction, pulp
and paper manufacturing, and water and sewage treat-
ment. It is manufactured by heating limestone (mostly
calcium carbonate, CaCO3) in a kiln, creating calcium
oxide (quicklime) and CO2, which is normally emitted
to the atmosphere.
Limestone and Dolomite Use (2.4 MMTCE)
Limestone (CaCO3) and dolomite (CaCO3MgCO3)
are basic raw materials used by a wide variety of indus-
tries, including the construction, agriculture, chemi-
cal, and metallurgical industries. For example, lime-
stone can be used as a purifier in refining metals. In the
case of iron ore, limestone heated in a blast furnace
reacts with impurities in the iron ore and fuels, generat-
ing CO2 as a by-product. Limestone is also used in flue
gas desulfurization systems to remove sulfur dioxide
from the exhaust gases.
Soda Ash Manufacture and
Consumption (1.2 MMTCE)
Commercial soda ash (sodium carbonate, Na2CO3)
Executive Summary ES-13
-------
is used in many consumer products, such as glass, soap
and detergents, paper, textiles, and food. During the
manufacturing of soda ash, some natural sources of so-
dium carbonate are heated and transformed into a crude
soda ash, in which CO2 is generated as a by-product. In
addition, CO2 is often released when the soda ash is
consumed.
Carbon Dioxide Consumption (0.4 MMTCE)
Carbon dioxide is used directly in many segments
of the economy, including food processing, beverage
manufacturing, chemical processing, and a host of in-
dustrial and other miscellaneous applications. For the
most part, the CO2 used in these applications is eventu-
ally released to the atmosphere.
Land-Use Change and Forestry
When humans alter the biosphere through changes
in land-use and forest management practices, they alter
the natural carbon flux between biomass, soils, and the
atmosphere. Improved forest management practices and
the regeneration of previously cleared forest areas have
resulted in a net uptake (sequestration) of carbon in U.S.
forest lands, which cover about 298 million hectares (737
million acres) (Powell et al. 1993). This uptake is an
ongoing result of land-use changes in previous decades.
For example, because of improved agricultural produc-
tivity and the widespread use of tractors, the rate of clear-
ing forest land for crop cultivation and pasture slowed
greatly in the late 19th century, and by 1920 this prac-
tice had all but ceased. As farming expanded in the Mid-
west and West, large areas of previously cultivated land
in the East were brought out of crop production, prima-
rily between 1920 and 1950, and were allowed to revert
to forest land or were actively reforested.
Since the early 1950s, the managed growth of pri-
vate forest land in the East has nearly doubled the biom-
ass density there. The 1970s and 1980s saw a resurgence
of federally sponsored tree-planting programs (e.g., the
Forestry Incentive Program) and soil conservation pro-
grams (e.g., the Conservation Reserve Program), which
have focused on reforesting previously harvested lands,
improving timber-management, combating soil erosion,
and converting marginal cropland to forests.
In 1998, the CO2 flux from land-use change and
forestry activities was estimated to have been a net up-
take of 210.8 MMTCE. This carbon was sequestered in
trees, understory, litter, soils in forests, wood products,
and wood in landfills. This net carbon uptake represents
an offset of about 14 percent of the CO2 emissions from
fossil fuel combustion in 1998. The amount of carbon
sequestered through U.S. forestry and land-use practices
is estimated to have declined by about a third between
1990 and 1998, largely due to the maturation of exist-
ing forests and the slowed expansion of Eastern forest
cover and a gradual decrease in the rate of yard trim-
mings disposed in landfills. Due to the lack of a national
survey of land use and management more recent than
1992, carbon flux estimates for non-forest mineral and
organic soils were not calculated for the 1993 through
1998 period. Therefore, carbon flux estimates from non-
forest soils are not included in the total fluxes reported.
Waste
Waste Combustion (3.5 MMTCE)
Waste combustion involves the burning of garbage
and non-hazardous solids, referred to as municipal solid
waste (MSW). In 1996, there were approximately 137
municipal waste combustion plants in operation within
the United States (EPA 1998a). Most of the organic (i.e.,
carbon) materials in MSW are of biogenic origin. There-
fore, the CO2 emissions from their combustion are re-
ported under the Land Use Change and Forestry Chap-
ter. However, one component plastics is of fossil fuel
origin, and is included as a source of CO2 emissions.
Methane Emissions
Atmospheric methane (CH4) is an integral compo-
nent of the greenhouse effect, second only to CO2 as a
contributor to anthropogenic greenhouse gas emissions.
Methane's overall contribution to global warming is sig-
nificant because it is estimated to be 21 times more ef-
fective at trapping heat in the atmosphere than CO2 (i.e.,
the GWP value of methane is 21). Over the last two cen-
turies, methane's concentration in the atmosphere has
more than doubled (IPCC 1996). Experts believe these
ES-14 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
atmospheric increases were due largely to increasing
emissions from anthropogenic sources, such as landfills,
natural gas and petroleum systems, agricultural activi-
ties, coal mining, stationary and mobile combustion,
wastewater treatment, and certain industrial processes
(see Figure ES-11 and Table ES-9).
Landfills
Figure ES-11
Landfills
Enteric Fermentation
Natural Gas Systems
Manure Management
Coal Mining
Petroleum Systems
Rice Cultivation
Stationary Sources
Mobile Sources
Wastewater Treatment
Petrochemical Production
Agricultural Residue Burning
Silicon Carbide Production
Portion of All
Emissions
<0.05
0 10 20 30 40 50 60
MMTCE
Landfills are the largest single anthropogenic
source of methane emissions in the United States. In an
environment where the oxygen content is low or nonex-
istent, organic materials, such as yard waste, household
waste, food waste, and paper, can be decomposed by
bacteria, resulting in the generation of methane and bio-
genie CO2. Methane emissions from landfills are affected
by site-specific factors such as waste composition, mois-
ture, and landfill size.
Methane emissions from U.S. landfills in 1998 were
58.8 MMTCE, only a 1 percent increase since 1990. The
relatively constant emission estimates are a result of two
offsetting trends: (1) the amount of MSW in landfills
contributing to methane emissions has increased (thereby
increasing the potential for emissions); and (2) the amount
of landfill gas collected and combusted by landfill op-
erators has also increased (thereby reducing emissions).
Emissions from U.S. municipal solid waste landfills,
which received about 61 percent of the municipal solid
waste generated in the United States, accounted for 93
percent of total landfill emissions, while industrial land-
- fills accounted for the remainder. Approximately 26 per-
cent of the methane generated in U.S. landfills in 1998
was recovered and combusted, often for energy.
Table ES-9: U.S. Sources of Methane Emissions (MMTCE)
^Source
it Landfills
I Enteric Fermentation
j; Natural Gas Systems
.; Manure Management
r Coal Mining
I Petroleum Systems
;, Rice Cultivation
^Stationary Sources
ii Mobile Sources
it Wastewater Treatment
f Petrochemical Production
f> Agricultural Residue Burning
t Silicon Carbide Production
; International Bunker Fuels*
t Total*
£-+ Does not exceed 0.05 MMTCE
1990
58.2
32.7
33.0
15.0
24.0
7.4
2.4
2.3
1.5
0.9
0.3
0.2
+
+
177.9
£•' * Emissions from International Bunker Fuels are not
I Note: Totals may not sum due to
1991
58.1
32.8
33.4
15.5
22.8
7.5
2.3
2.4
1.5
0.9
0.3
0.2
+
+
177.7
included
1992
59.1
33.2
33.9
16.0
22.0
7.2
2.6
2.4
1.5
0.9
0.3
0.2
-1-
+
179.4
in totals.
1993
59.6
33.7
34.6
17.1
19.2
6.9
2.4
2.4
1.5
0.9
0.4
0.1
+
H-
178.7
1994
59.9
34.5
34.3
18.8
19.4
6.7
2.7
2.4
1.5
0.9
0.4
0.2
+
181.6
1995
60.5
34.9
34.0
19.7
20.3
6.7
2.6
2.5
1.4
0.9
0.4
0.2
. +
184.1
1996
60.2
345
34.6
20.4
189
65
24
2.6
1 4
0.9
0.4
0.2
+
183.1
1997
602
342
341
22.1
188
65
26
23
1 4
09
0.4
02
+
183.8
1998
58 8
33 7
336
22.9
178
6 3
2 7
23
1 3 !
0 9
04
02
+
180.9
independent rounding.
Executive Summary ES-15
-------
A regulation promulgated in March 1996 requires
the largest U.S. landfills to begin collecting and com-
busting their landfill gas to reduce emissions of
NMVOCs. It is estimated that by the year 2000, this regu-
lation will have reduced landfill methane emissions by
more than 50 percent.
Natural Gas and Petroleum Systems
Methane is the major component of natural gas.
During the production, processing, transmission, and dis-
tribution of natural gas, fugitive emissions of methane
often occur. Because natural gas is often found in con-
junction with petroleum deposits, leakage from petro-
leum systems is also a source of emissions. Emissions
vary greatly from facility to facility and are largely a
function of operation and maintenance procedures and
equipment conditions. In 1998, methane emissions from
U.S. natural gas systems were estimated to be 33.6
MMTCE, accounting for approximately 19 percent of
U.S. methane emissions.
Petroleum is found in the same geological struc-
tures as natural gas, and the two are retrieved together.
Methane is also saturated in crude oil, and volatilizes as
the oil is exposed to the atmosphere at various points
along the system. Methane emissions from the compo-
nents of petroleum systems including crude oil produc-
tion, crude oil refining, transportation, and distribution
generally occur as a result of system leaks, disruptions,
and routine maintenance. In 1998, emissions from pe-
troleum systems were estimated to be 6.3 MMTCE, or
3.5 percent of U.S. methane emissions.
From 1990 to 1998, combined methane emissions
from natural gas and petroleum systems decreased by
about 1 percent. Emissions from natural gas systems have
remained fairly constant, while emissions from petro-
leum systems have declined gradually since 1990 pri-
marily due to production declines.
Coal Mining
Produced millions of years ago during the forma-
tion of coal, methane trapped within coal seams and sur-
rounding rock strata is released when the coal is mined.
The quantity of methane released to the atmosphere dur-
ing coal mining operations depends primarily upon the
depth and type of the coal that is mined.
Methane from surface mines is emitted directly to
the atmosphere as the rock strata overlying the coal seam
are removed. Because methane in underground mines is
explosive at concentrations of 5 to 15 percent in air,
most active underground mines are required to vent this
methane, typically to the atmosphere. At some mines,
methane-recovery systems may supplement these venti-
lation systems. U.S. recovery of methane has been in-
creasing in recent years. During 1998, coal mining ac-
tivities emitted 17.8 MMTCE of methane, or 10 percent
of U.S. methane emissions. From 1990 to 1998, emis-
sions from this source decreased by 26 percent due to
increased use of the methane collected by mine
degasification systems.
Agriculture
Agriculture accounted for 33 percent of U.S. meth-
ane emissions in 1998, with enteric fermentation in do-
mestic livestock and manure management accounting
for the majority. Other agricultural activities contribut-
ing directly to methane emissions included rice cultiva-
tion and agricultural waste burning.
Enteric Fermentation (33.7 MMTCE)
During animal digestion, methane is produced
through the process of enteric fermentation, in which
microbes residing in animal digestive systems break down
the feed consumed by the animal. Ruminants, which in-
clude cattle, buffalo, sheep, and goats, have the highest
methane emissions among all animal types because they
have a rumen, or large fore-stomach, in which methane-
producing fermentation occurs. Non-ruminant domestic
animals, such as pigs and horses, have much lower meth-
ane emissions. In 1998, enteric fermentation was the
source of about 19 percent of U.S. methane emissions,
and more than half of the methane emissions from agri-
culture. From 1990 to 1998, emissions from this source
increased by 3 percent. Emissions from enteric fermenta-
tion have been decreasing since 1995, primarily due to
declining dairy cow and beef cattle populations.
ES-16 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Executive Summary
Central to any study of climate change is the development of an emissions inventory that identifies and
juantifies a country's primary anthropogenic1 sources and sinks of greenhouse gas emissions. This
inventory adheres to both (1) a comprehensive and detailed methodology for estimating sources and sinks of anthro-
pogenic greenhouse gases, and (2) a common and consistent mechanism that enables signatory countries to the
United Nations Framework Convention on Climate Change (UNFCCC) to compare the relative contribution of
different emission sources and greenhouse gases to climate change. Moreover, systematically and consistently
estimating national and international emissions is a prerequisite for accounting for reductions and evaluating miti-
gation strategies.
In June of 1992, the United States signed the UNFCCC. The objective of the UNFCCC is "to
achieve...stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous
anthropogenic interference with the climate system."2
Parties to the Convention, by signing, make commitments "to develop, periodically update, publish and make
available...national inventories of anthropogenic emissions by sources and removals by sinks of all greenhouse
gases not controlled by the Montreal Protocol, using comparable methodologies..."3 The United States views this
report as an opportunity to fulfill this commitment under UNFCCC.
This chapter summarizes the latest information on U.S. anthropogenic greenhouse gas emission trends from
1990 through 1998. To ensure that the U.S. emissions inventory is comparable to those of other UNFCCC signatory
countries, the estimates presented here were calculated using methodologies consistent with those recommended in
the Revised 1996IPCC Guidelines for National Greenhouse Gas Inventories (IPCC/UNEP/OECD/IEA 1997). For
most source categories, the IPCC default methodologies were expanded, resulting in a more comprehensive and
detailed estimate of emissions.
Naturally occurring greenhouse gases include water vapor, carbon dioxide (CO2), methane (CH4), nitrous oxide
(N2O), and ozone (O3). Se.veral classes of halogenated substances that contain fluorine, chlorine, or bromine are also
greenhouse gases, but they are, for the most part, solely a product of industrial activities. Chlorofluorocarbons
(CFCs) and hydrochlorofluorocarbons (HCFCs) are halocarbons that contain chlorine, while halocarbons that con-
1 The term "anthropogenic", in this context, refers to greenhouse gas emissions and removals that are a direct result of human activities
or are the result of natural processes that have been affected by human activities (IPCC/UNEP/OECD/IEA 1997).
2 Article 2 of the Framework Convention on Climate Change published by the UNEP/WMO Information Unit on Climate Change. See
.
3 Article 4 of the Framework Convention on Climate Change published by the UNEP/WMO Information Unit on Climate Change (also
identified in Article 12). See .
Executive Summary ES-1
-------
tain bromine are referred to as halons. CFCs, HCFCs,
and halons are stratospheric ozone depleting substances
and are covered under the Montreal Protocol. Other fluo-
rine containing halogenated substances include
hydrofluorocarbons (HFCs), perfluorocarbons (PFCs),
and sulfur hexafluoride (SF6). HFCs, PFCs, and SF6 do
not deplete stratospheric ozone.
There are also several gases that do not have a
direct global warming effect but indirectly affect terres-
trial radiation absorption by influencing the formation
and destruction of tropospheric and stratospheric ozone.
These gases referred to as ozone precursors include car-
bon monoxide (CO), oxides of nitrogen (NOX), and
nonmethane volatile organic compounds (NMVOCs).4
Aerosols extremely small particles or liquid droplets of-
ten produced by emissions of sulfur dioxide (SO2) can
also affect the absorptive characteristics of the atmo-
sphere.
Although CO2, CH4, and N2O occur naturally in
the atmosphere, their atmospheric' concentrations have
been affected by human activities. Since pre-industrial
time (i.e., since about 1750), concentrations of these
greenhouse gases have increased by 28, 145, and 13
percent, respectively (IPCC 1996). This build-up has
altered the composition of the earth's atmosphere, and
affects the global climate system.
Beginning in the 1950s, the use of CFCs and other
stratospheric ozone depleting substances (ODSs) in-
creased by nearly 10 percent per year until the mid-1980s,
when international concern about ozone depletion led
to the signing of the Montreal Protocol on Substances
that Deplete the Ozone Layer. Since then, the consump-
tion of ODSs has been undergoing a phase-out. In con-
trast, use of ODS substitutes such as HFCs, PFCs, and
SF6 has grown significantly during this time.
Recent Trends in
U.S. Greenhouse Gas Emissions
Total U.S. greenhouse gas emissions rose in 1998
to 1,834.6 million metric tons of carbon equivalents
(MMTCE)5 (11.2 percent above 1990 baseline). The
single year increase in emissions from 1997 to 1998 was
0.4 percent (6.8 MMTCE), less than the average annual
rate of increase for 1990 through 1998 (1.2 percent).
Figure ES-1 through Figure ES-3 illustrate the overall
trends in total U.S. emissions by gas, annual changes,
and absolute change since 1990! Table ES-1 provides a
Figure ES-1
U.S. GHG JEmissioH by GUt
HFCs, PFCs, & SF6
Nitrous Oxide
1750.
1500.
1250
O 1000.
1 75°
500.
250
0 J
1,650 1|636 1,687 1JO01.733
• Methane
• Carbon Dioxide
1,8041,8281,835
1990 1991 1992 1993 1994 1995 1996 1997 1998
Figure ES-2
Annul! Percenrahalg^
3.2%
3% -
2% -
-1%
1991 1992 1993 1994 1995 1996 1997 1998
4 Also referred to in the U.S. Clean Air Act as "criteria pollutants."
5 Estimates are presented in units of millions of metric tons of carbon equivalents (MMTCE), which weights each gas by its GWP value,
or Global Warming Potential (see following section).
ES-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
detailed summary of U.S. greenhouse gas emissions and
sinks for 1990 through 1998.
Figure ES-4 illustrates the relative contribution of
the direct greenhouse gases to total U.S. emissions in
1998. The primary greenhouse gas emitted by human
activities was CO2. The largest source of CO2 and of
overall greenhouse gas emissions in the United States
was fossil fuel combustion. Methane emissions resulted
primarily from decomposition of wastes in landfills,
manure and enteric fermentation associated with domes-
tic livestock, natural gas systems, and coal mining. Emis-
sions of N2O were dominated by agricultural soil man-
agement and mobile source fossil fuel combustion. The
substitution of ozone depleting substances and emis-
sions of HFC-23 during the production of HCFC-22 were
the primary contributors to aggregate HFC emissions.
PFC emissions came mainly from primary aluminum pro-
duction, while electrical transmission and distribution
systems emitted the majority of SF6.
As the largest source of U.S. greenhouse gas emis-
sions, CO2 from fossil fuel combustion, accounted for
80 percent of weighted emissions in 1998. Emissions
from this source grew by 11 percent (148.1 MMTCE)
from 1990 to 1998 and were also responsible for over 80
percent of the increase in national emissions during this
period. The annual increase in CO2 emissions from this
source was only 0.5 percent in 1998 lower than the
source's average annual rate of 1.3 percent during the
1990s despite a strong 3.9 percent increase in U.S. gross
domestic product.
Figure ES-3
Figure ES-4
1991 1992 1993 1994 1995 1996 1997 1998
2.2% MFCs, PFCs & SF6
6.5% N2O
9.9% CH4
81.4%C02
In addition to economic growth, changes in CO2
emission from fossil fuel combustion are also correlated
with energy prices and seasonal temperatures. Excep-
tionally mild winter conditions in 1998 moderated
growth in CO2 emissions from fossil fuel combustion
below what would have been expected given the strength
of the economy and continued low fuel prices. Table ES-
2 shows annual changes in emissions during the last few
years of the 1990s for particular fuel types and sectors.
Carbon dioxide emissions from fossil fuel com-
bustion increased dramatically in 1996, due primarily
to two factors: 1) fuel switching by electric utilities from
natural gas to more carbon intensive coal as to colder
winter conditions and the associated rise in demand for
natural gas from residential, commercial and industrial
customers for heating caused gas prices to rise sharply;
and 2) higher consumption of petroleum fuels for trans-
portation. Milder weather conditions in summer and win-
ter moderated the growth in emissions in 1997; how-
ever, the shut-down of several nuclear power plants lead
electric utilities to increase their consumption of coal to
offset the lost capacity. In 1998, weather conditions were
a dominant factor in slowing the growth in emissions.
Warm winter temperatures resulted in a significant drop
in residential, commercial, and industrial natural gas
consumption. This drop in emissions from natural gas
used for heating was primarily offset by two factors: 1)
electric utility emissions, which increased in part due to
a hot summer and its associated air conditioning de-
Executive Summary ES-3
-------
Table ES-1: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (MMTCE)
Gas/Source
C02 1
Fossil Fuel Combustion 1
Cement Manufacture
Natural Gas Flaring
Lime Manufacture
Waste Combustion
Limestone and Dolomite Use
Soda Ash Manufacture and
Consumption
Carbon Dioxide Consumption
Land-Use Change and
Forestry (Sink)*
International Bunker Fuels6
CH4
Landfills
Enteric Fermentation
Natural Gas Systems
Manure Management
Coal Mining
Petroleum Systems
Rice Cultivation
Stationary Sources
Mobile Sources
WastewaterTreatment
Petrochemical Production
Agricultural Residue Burning
Silicon Carbide Production
International Bunker Fuels"
N20
Agricultural Soil Management
Mobile Sources
Nitric Acid
Stationary Sources
Manure Management
Human Sewage
Adipic Acid
Agricultural Residue Burning
Waste Combustion
International Bunker Fuels"
HFCs, PFCs, and SF6
Substitution of Ozone Depleting
Substances
HCFC-22 Production
Electrical Transmission
and Distribution
Magnesium Production and
Processing
Aluminum Production
Semiconductor Manufacture
Total Emissions
Net Emission (Sources and Sinks)
1990
,340.3
,320.1
9.1
2.5
3.0
2.8
1.4
1.1
0.2
(316.4)
32.2
177.9
58.2
32.7
33.0
15.0
24.0
7.4
2.4
2.3
1.5
0.9
0.3
0.2
+
+
108.2
75.3
13.8
4.9
3.8
3.4
2.0
5.0
0.1
0.1
0.3
23.3
0.3
9.5
5.6
1.7
5.4
0.8
1,649.7
1,333.3
1991
1,326.1
1,305.8
8.9
2.8
3.0
3.0
1.3
1.1
0.2
(316.3)
32.7
177.7
58.1
32.8
33.4
15.5
22.8
7.5
2.3
2.4
1.5
0.9
0.3
0.2
+
+
110.5
76.3
14.6
4.9
3.8
3.6
2.0
5.2
0.1
0.1
0.3
22.0
0.2
8.4
5.9
2.0
4.7
0.8
1,636.2
1,320.0
1992
1,350.4
1,330.1
8.9
2.8
3.1
3.0
1.2
1.1
0.2
(316.2)
30.0
179.4
59.1
33.2
33.9
16.0
22.0
7.2
2.6
2.4
1.5
0.9
0.3
0.2
+
+
113.3
78.2
15.7
5.0
3.9
3.5
2.0
4.8
0.1
0.1
0.3
23.5
0.4
9.5
6.2
2.2
4.4
0.8
1,666.6
1,350.5
1993
1,383.3
1,361.5
9.4
3.7
3.1
3.1
1.1
1.1
0.2
(212.7)
27.2
178.7
59.6
33.7
34.6
17.1
19.2
6.9
2.4
2.4
1.5
0.9
0.4
0.1
+
+
113.8
77.3
16.5
5.1
3.9
3.7
2.0
5.2
0.1
0.1
0.2
23.8
1.4
8.7
6.4
2.5
3.8
1.0
1,699.7
1,487.0
1994
1,404.8
1,382.0
9.8
3.8
3.2
3.1
1.5
1.1
0.2
(212.3)
26.7
181.6
59.9
34.5
34.3
18.8
19.4
6.7
2.7
2.4
1.5
0.9
0.4
0.2
+
+
121.5
83.5
17.1
5.3
4.0
3.8
2.1
5.5
0.1
0.1
0.2
25.1
2.7
8.6
6.7
2.7
3.2
1.1
1,733.0
1,520.7
1995
1,416.5
1,392.0
10.0
4.7
3.4
3.0
1.9
1.2
0.3
(211.8)
27.5
184.1
60.5
34.9
34.0
19.7
20.3
6.7
2.6
2.5
1.4
0.9
0.4
0.2
+
+
118.8
80.4
17.4
5.4
4.0
3.7
2.1
5.5
0.1
0.1
0.2
29.0
7.0
7.4
7.0
3.0
3.1
1.5
1,748.5
1,536.6
1996
1,466.2
1,441.3
10.1
4.5
3.6
3.1
2.0
1.2
0.3
(211.3)
27.9
183.1
60.2
34.5
34.6
20.4
18.9
6.5
2.4
2.6
1.4
0.9
0.4
0.2
+
+
121.5
82.4
17.5
5.6
.4.2
3.8
2.1
5.7
0.1
0.1
0.2
33.5
9.9
8.5
7.0
3.0
3.2
1.9
1,804.4
1,593.1
1997
1,486.4
1,460.7
10.5
4.2
3.7
3.4
2.3
1.2
0.4
(211.1)
29.9
183.8
60.2
34.2
34.1
22.1
18.8
6.5
2.6
2.3
1.4
0.9
0.4
0.2
+
+
122.4
84.2
17.3
5.8
4.2
3.9
2.1
4.7
0.1
0.1
0.3
35.3
12.3
8.2
7.0
3.0
3.0
1.9
1,827.9
1,616.8
1998
1,494.0
1,468.2
10.7
3.9
3.7
3.5
2.4
1.2
0.4
(210.8)
31.3
180.9
58.8
33.7
33.6
22.9
17.8
6.3
2.7
2.3
1.3
0.9
0.4
0.2
+
+
119.4
83.9
17.2
5.8
4.3
4.0
2.2
2.0
0.1
0.1
0.3
40.3
14.5
10.9
7.0
3.0
2.8
2.1
1,834.6
1,623.8
+ Does not exceed 0.05 MMTCE
1 Sinks are only included in net emissions total. Estimates of net carbon sequestration due to land-use change and forestry activities exclude
non-forest soils, and are based partially upon projections of forest carbon stocks.
" Emissions from International Bunker Fuels are not included in totals.
Note: Totals may not sum due to independent rounding.
ES-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Table ES-2: Annual Change in C02 Emissions from Fossil Fuel Combustion
for Selected Fuels and Sectors (MMTCE and Percent)
Sector
Electric Utility
Electric Utility
Electric Utility
Transportation3
Residential
Commercial
Industrial
All Sectors'1
Fuel Type
Coal
Petroleum
Natural Gas
Petroleum
Natural Gas
Natural Gas
Natural Gas
All Fuels"
1995 to 1996
24.5
1.4
(6.9)
13.8
5.8
1.9
4.7
49.4
5.7%
10.0%
(14.6%)
3.3%
8.1%
4.2%
3.4%
3.5%
1996
14.3
2.2
3.3
1.1
(3.8)
0.9
(1.4)
19.4
to 1997
3.1%
14.4%
8.1%
0.2%
(4.9%)
1.9%
(1.0%)
1.3%
1997 to 1998
5.5
7.3
4.2
7.2
(7.4)
(2.7)
(2.9)
7.5
1.2%
41.6%
9.8%
1.7%
(10.0%)
(5.7%)
(2.0%)
0.5%
a Excludes emissions from International Bunker Fuels.
b Includes fuels and sectors not shown in table.
mand; and 2) increased motor gasoline consumption for
transportation.
Overall, from 1990 to 1998, total emissions of
CO2, CH4, andN2O increased by 153.7 (11 percent), 3.1
(2 percent), and 11.1 MMTCE (10 percent), respec-
tively. During the same period, weighted emissions of
MFCs, PFCs, and SF6 rose by 17.0 MMTCE (73 per-
cent). Despite being emitted in smaller quantities rela-
tive to the other principle greenhouse gases, emissions
of MFCs, PFCs, and SF6 are significant because of their
extremely high Global Warming Potentials and, in the
cases of PFCs and SF6, long atmospheric lifetimes. Con-
versely, U.S. greenhouse gas emissions were partly off-
set by carbon sequestration in forests and in landfilled
carbon, which were estimated to be 12 percent of total
emissions in 1998.
Other significant trends in emissions from addi-
tional source categories over the nine year period from
1990 through 1998 included the following:
• Aggregate HFC and PFC emissions resulting from
the substitution of ozone depleting substances (e.g.,
CFCs) increased by 14.2 MMTCE. This increase
was partly offset, however, by reductions in PFC
emissions from aluminum production by 2.6
MMTCE (48 percent), which were the result of both
voluntary industry emission reduction efforts and
lower domestic aluminum production.
• Combined N2O and CH4 emissions from mobile
combustion rose by 3.3 MMTCE (22 percent), pri-
marily due to increased rates of N2O generation in
highway vehicles.
Methane emissions from the manure management
activities have increased by 7.9 MMTCE (53 per-
cent) as the composition of the swine and dairy in-
dustries shift toward larger facilities. An increased
number of large facilities leads to an increased use
of liquid systems, which translates into increased
methane production.
Methane emissions from coal mining dropped by
6.2 MMTCE (26 percent) as a result of the mining of
less gassy coal from underground mines and the in-
creased use of methane from degasification systems.
Nitrous oxide emissions from agricultural soil man-
agement increased by 8.5 MMTCE (11 percent) as
fertilizer consumption and cultivation of nitrogen
fixing crops rose.
By 1998, all of the three major adipic acid produc-
ing plants had voluntarily implemented N2O abate-
ment technology; as a result, emissions fell by 3.0
MMTCE (60 percent). The majority of this decline
occurred from 1997 to 1998, despite increased pro-
duction.
The following sections describe the concept of Glo-
bal Warming Potentials (GWPs), present the anthro-
pogenic sources and sinks of greenhouse gas emis-
sions in the United States, briefly discuss emission
pathways, further summarize the emission estimates,
and explain the relative importance of emissions
from each source category.
Executive Summary ES-5
-------
Box ES-1: Recent Trends in Various U.S. Greenhouse Gas Emissions-Related Data
There are several ways to assess a nation's greenhouse gas emitting intensity. These measures of intensity could be based on
aggregate energy consumption because energy-related activities6 are the largest sources of emissions, on fossil fuel consumption only
because almost all energy-related emissions involve the combustion of fossil fuels, on electricity consumption because electric utilities
were the largest sources of U.S. greenhouse gas emissions in 1998, on total gross domestic product as a measure of national
economic activity, or on a per capita basis. Depending upon which of these measures is used, the United States could appear to have
reduced or increased its national greenhouse gas intensity. Table ES-3 provides data on various statistics related to U.S. greenhouse
gas emissions normalized to 1990 as a baseline year. Greenhouse gas emissions in the U.S. have grown at an average annual rate of
1.3 percent since 1990. This rate is slightly slower than that for total energy or fossil fuel consumption thereby indicating an improved
or lower greenhouse gas emitting intensity and much slower than that for either electricity consumption or overall gross domestic
product. Emissions, however, are growing faster than national population, thereby indicating a worsening or higher greenhouse gas
emitting intensity on a per capita basis (see Figure ES-5). Overall, atmospheric C02 concentrations a function of many complex
anthropogenic and natural processes are increasing at 0.4 percent per year.
Table ES-3: Recent Trends in Various U.S. Data (Index 1990 = 100)
Variable
1991 1992 1993 1994 1995 1996 1997 1998 Growth Rate'
GHG Emissions'
Energy Consumption11
Fossil Fuel Consumption1"
Electricity Consumption15
GDPC
Population11
Atmospheric C02 Concentration6
99
100
99
102
99
101
100
101
101
101
102
102
102
101
103
104
103
105
104
103
101
105
106
105
108
108
104
101
106
108
106
111
110
105
102
109
112
110
114
114
106
102
111
112
111
116
118
107
103 .
111
112
111
119
123
108
104
1.3%
1.4%
1.4%
2.2%
2.6%
1.0%
0.4%
9 GWP weighted values
b Energy content weighted values. (DOE/EIA)
c Gross Domestic Product in chained 1992 dollars (BEA 1999)
" (U.S. Census Bureau 1999)
e Mauna Loa Observatory, Hawaii (Keeling and Whorf 1999)
' Average annual growth rate
Figure ES-5
,, I
-f v A---
ef Dollar of Gross Domestic Product
106
102
98 -
94
Emissions
per $GDP
90 J
1990 1991 1992 1993 1994 1995 1996 1997 1998
* Energy-related activities are those that involve fossil fuel combustion (industrial, transportation, residential, and commercial end-use
sectors), and the production, transmission, storage, and distribution of fossil fuels.
ES-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
pathways further summarize the emission estimates, and
explain the relative importance of emissions from each
source category.
Global Warming Potentials
Gases in the atmosphere can contribute to the
greenhouse effect both directly and indirectly. Direct
effects occur when the gas itself is a greenhouse gas;
indirect radiative forcing occurs when chemical trans-
formations of the original gas produce a gas or gases that
are greenhouse gases, or when a gas influences the atmo-
spheric lifetimes of other gases. The concept of a Global
Warming Potential (GWP) has been developed to com-
pare the ability of each greenhouse gas to trap heat in
the atmosphere relative to another gas. Carbon dioxide
was chosen as the reference gas to be consistent with
IPCC guidelines.
Global Warming Potentials are not provided for
the criteria pollutants CO, NOX, NMVOCs, and SO2 be-
cause there is no agreed upon method to estimate the
contribution of gases that have only indirect effects on
radiative forcing (IPCC 1996).
All gases in this executive summary are presented
in units of million metric tons of carbon equivalents
(MMTCE). Carbon comprises- 12/44ths of carbon diox-
ide by weight. The relationship between gigagrams (Gg)
of a gas and MMTCE can be expressed as follows:
MMTCE = (Ggof gas)xf-^L]x(GWP)xfll
^1,000 Ggj ^44
The GWP of a greenhouse gas is the ratio of global
warming, or radiative forcing—both direct and indirect—
from one unit mass of a greenhouse gas to that of one
unit mass of carbon dioxide over a period of time. While
any time period can be selected, the 100 year GWPs
recommended by the IPCC and employed by the United
States for policy making and reporting purposes were
used in this report (IPCC 1996). GWP values are listed
below in Table ES-6.
Box ES-2: Greenhouse Gas Emissions from Transportation Activities
r Motor vehicle usage is increasing all over the world, including in the United States. Since the 1970s, the number of highway
I- vehicles registered in the United States has increased faster than the overall population, according to the Federal Highway Administra-
tion (FHWA). Likewise, the number of miles driven up 21 percent from 1990 to 1998 and gallons of gasoline consumed each year in
I ..the United States have increased relatively steadily since the 1980s, according to the FHWA and Energy Information Administration,
^respectively. These increases in motor vehicle usage are the result of a confluence of factors including population growth, economic
ji growth, increasing urban sprawl, and low fuel prices.
;; One of the unintended consequences of these changes is a slowing of progress toward cleaner air in both urban and rural parts
| of the country. Passenger cars, trucks, motorcycles, and buses emit significant quantities of air pollutants with local, regional, and
I global effects. Motor vehicles are major sources of carbon monoxide (CO), carbon dioxide (C02), methane (CH4), nonmethane volatile
;-organic compounds (NMVOCs), nitrogen oxides (NO*), nitrous oxide (N20), and hydrofluorocarbons (HFCs). Motor vehicles are also
|; important contributors to many serious air pollution problems, including ground-level ozone (i.e., smog), acid rain, fine paniculate
: matter, and global warming. Within the United States and abroad, government agencies have taken actions to reduce these emissions.
i Since the 1970s, the EPA has required the reduction of lead in gasoline, developed strict emission standards for new passenger cars
and trucks, directed states to enact comprehensive motor vehicle emission control programs, required inspection and maintenance
; programs, and more recently, introduced the use of reformulated gasoline. New vehicles are now equipped with advanced emissions
, controls, which are designed to reduce emissions of nitrogen oxides, hydrocarbons, and carbon monoxide.
Table ES-4 summarizes greenhouse gas emissions from all transportation-related activities. Overall, transportation activities
excluding international bunker fuels accounted for an almost constant 26 percent of total U.S. greenhouse gas emissions from 1990
to 1998. These emissions were primarily C02 from fuel combustion, which increased by 11 percent from 1990 to 1998. However,
because of larger increases in N20 and HFC emissions during this period, overall emissions from transportation activities actually
^increased by 13 percent.
Executive Summary ES-7
-------
Table ES-4: Transportation-Related Greenhouse Gas Emissions (MMTCE)
Gas/Vehicle Type
C02
Passenger Cars
Light-Duty Trucks
Other Trucks
Buses
Aircraft8
Boats and Vessels
Locomotives
Other1"
International Bunker Fuels0
CH
Passenger Cars
Light-Duty Trucks
Other Trucks and Buses
1990
399.6
169.1
77.4
56.3
2.7
48.2
15.1
7.3
23.6
32.2
1.5
0.7
0.5
0.1
1991
391.5
167.6
77.1
54.2
2.8
46.1
14.4
6.8
22.3
32.7
1.5
0.7
0.5
0.1
1992
401.1
171.7
77.1
55.9
2.9
45.5
18.5
7.3
22.3
30.0
1.5
0.6
0.6
0.1
1993
409.1
173.3
80.4
59.1
3.0
45.8
17.3
6.7
23.6
27.2
1.5
0.6
0.6
0.2
1994
422.3
172.2
87.1
62.1
3.3
48.0
17.0
7.9
24.8
26.7
1.5
0.6
0.6
0.2
1995
427.7
175.0
88.9
63.6
3.5
46.8
17.0
8.1
24.8
27.5
1.4
0.6
0.6
0.2
1996
441.7
178.5
91.1
67.7
3.0
49.1
18.1
8.7
25.4
27.9
1.4
0.6
0.5
0.2
1997
443.4
180.0
92.1
70.1
3.2
48.8
13.7
9.0
26.5
29.9
1.4
0.5
0.5
0.2
1998
450.3
185.1
94.6
70.3
3.2
49.4
12.5
9.0
26.3
31.3
1.3
0.5
0.5
0.2
Aircraft
Boats and Vessels
Locomotives
Other11
In4nrnotinnql Diml/or FitalcC
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
llllWl tlUUUIIUI LSUIIIWl 1 UVIW
N20
Passenger Cars
Light-Duty Trucks
Other Trucks and Buses
Aircraft11
Boats and Vessels
Locomotives
Other11
international Bunker Fuels0
HFCs
Mobile Air Conditioners6
Total0
13.8
8.1
4.2
0.6
0.5
0.1
0.1
0.2
0.3
+
+
414.8
14.6
8.0
5.1
0.7
0.5
0.1
0.1
0.2
0.3
+
+
407.5
15.7
8.4
5.8
0.7
0.5
0.1
0.1
0.2
0.3
0.2
0.2
418.4
16.5
8.6
6.4
0.7
0.5
0.1
0.1
0.2
0.2
0.7
0.7
427.8
17.1
8.8
6.6
0.8
0.5
0.1
0.1
0.2
0.2
1.8
1.8
442.7
17.4
8.9
6.8
0.8
0.5
0.1
0.1
0.2
0.2
2.6
2.6
449.2
17.5
8.9
6.8
0.9
0.5
0.1
0.1
0.2
0.2
3.7
3.7
464.3
17.3
8.7
6.8
0.9
0.5
0.1
0.1
0.2
0.3
4.7
4.7
466.8
17.2
8.6
6.8
1.0
0.5
0.1
0.1
0.2
0.3
4.7
4.7
473.5
+ Does not exceed 0.05 MMTCE
Note: Totals may not sum due to independent rounding.
: a Aircraft emissions consist of emissions from all jet fuel (less bunker fuels) and aviation gas consumption.
b "Other" CO, emissions include motorcycles, construction equipment, agricultural machinery, pipelines, and lubricants.
c Emissions from International Bunker Fuels include emissions from both civilian and military activities, but are not included in totals.
d "Other" CHi and N20 emissions include motorcycles, construction equipment, agricultural machinery, gasoline-powered recreational,
Industrial, lawn and garden, light commercial, logging, airport service, other equipment; and diesel-powered recreational, industrial, lawn and
garden, light construction, airport service.
«Includes primarily HFC-134a
ES-8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Manure Management (22.9 MMTCE)
The decomposition of organic animal waste in an
anaerobic environment produces methane. The most
important factor affecting the amount of methane pro-
duced is how the manure is managed, because certain
types of storage and treatment systems promote an oxy-
gen-free environment. In particular, liquid systems tend
to encourage anaerobic conditions and produce sig-
nificant quantities of methane, whereas solid waste man-
agement approaches produce little or no methane.
Higher temperatures and moist climatic conditions also
promote methane production.
Emissions from manure management were about
13 percent of U.S. methane emissions in 1998, and 38
percent of the methane emissions from agriculture. From
1990 to 1998, emissions from this source increased by
53 percent—the largest increase of all the methane
source categories. The bulk of this increase was from
swine and dairy cow manure, and is attributed to the
shift in the composition of the swine and dairy indus-
tries towards larger facilities. Larger swine and dairy
farms tend to use liquid management systems. Thus the
shift towards larger facilities is translated into an in-
creasing use of liquid systems, which in turn translates
to increased methane production.
Rice Cultivation (2.7 MMTCE)
Most of the world's rice, and all of the rice in the
United States, is grown on flooded fields. When fields
are flooded, anaerobic conditions develop and the or-
ganic matter in the soil decomposes, releasing methane
to the atmosphere, primarily through the rice plants. In
1998, rice cultivation was the source of 1.5 percent of
U.S. methane emissions, and about 5 percent of U.S.
methane emissions from agriculture. Emission estimates
from this source have increased about 15 percent since
1990, due primarily to an increase in the area harvested.
Agricultural Residue Burning (0.2 MMTCE)
Burning crop residue releases a number of green-
house gases, including methane. Agricultural residue
burning is considered to be a net source of methane emis-
sions because, unlike CO2, methane released during burn-
ing is not reabsorbed by crop regrowth during the next
growing season. Because field burning is not common
in the United States, it was responsible for only 0.1 per-
cent of U.S. methane emissions in 1998.
Other Sources
Methane is also produced from several other
sources in the United States, including fuel combustion,
wastewater treatment, and some industrial processes. Sta-
tionary and mobile combustion were responsible for
methane emissions of 2.3 and 1.3 MMTCE, respectively,
in 1998. The majority of emissions from stationary com-
bustion resulted from the burning of wood in the resi-
dential sector. The combustion of gasoline in highway
vehicles was responsible for the majority of the methane
emitted from mobile combustion. Wastewater treatment
was a smaller source of methane, emitting 0.9 MMTCE
in 1998. Methane emissions from two industrial sources
petrochemical and silicon carbide production were also
estimated, totaling 0.4 MMTCE.
Nitrous Oxide Emissions
Nitrous oxide (N2O) is a greenhouse gas that is
produced both naturally—from a wide variety of bio-
logical sources in soil and water—and anthropogenically
by a variety of agricultural, energy-related, industrial,
and waste management activities. While N2O emissions
are much lower than CO2 emissions, N2O is approxi-
mately 310 times more powerful than CO2 at trapping
heat in the atmosphere (TPCC 1996). During the past two
centuries, atmospheric concentrations of N2O have risen
by approximately 13 percent. The main anthropogenic
activities producing N2O in the United States were agri-
cultural soil management, fuel combustion in motor ve-
hicles, and adipic and nitric acid production (see Figure
ES-12 and Table ES-10).
Executive Summary ES-17
-------
Figure ES-12
Agricultural
Soil Management
Mobile Sources
Nitric Acid
Stationary Sources
Manure Management
Human Sewage
AdipicAcid
Agricultural ,
Residue Burning I °-1
Waste Combustion |0.1
I
I
I
Portion of All
Emissions
10 20 30 40 50 60 70
MMTCE
Agricultural Soil Management
Nitrous oxide (N2O) is produced naturally in soils
through microbial processes of nitrification and denitri-
fication. A number of anthropogenic activities add to
the amount of nitrogen available to be emitted as N2O
by these microbial processes. Direct additions of nitro-
gen occur through the application of synthetic and or-
ganic fertilizers, cultivation of nitrogen-fixing crops,
cultivation of high-organic-content soils, the applica-
tion of livestock manure on croplands and pasture, the
incorporation of crop residues in soils, and direct excre-
tion by animals onto soil. Indirect emissions result from
volatilization and subsequent atmospheric deposition
of ammonia (NH3) and oxides of nitrogen (NOX) and from
leaching and surface run-off. These indirect emissions
originate from nitrogen applied to soils as fertilizer and
from managed and unmanaged livestock wastes.
In 1998, agricultural soil management accounted
for 83.9 MMTCE, or 70 percent of U.S. N2O emissions.
From 1990 to 1998, emissions from this source increased
by 11 percent as fertilizer consumption and cultivation
of nitrogen fixing crops rose.
Fuel Combustion
Nitrous oxide is a product of the reaction that oc-
curs between nitrogen and oxygen during fuel combus-
tion. Both mobile and stationary combustion emit N2O,
and the volume emitted varies according to the type of
fuel, technology, and pollution control device used, as
well as maintenance and operating practices. For ex-
ample, catalytic converters installed to reduce highway
vehicle pollution can result in the formation of N2O.
In 1998, N2O emissions from mobile combustion
totaled 17.2 MMTCE, or 14 percent of U.S. N2O emis-
sions. Emissions of N2O from stationary combustion
were 4.3 MMTCE, or 4 percent of U.S. N2O emissions.
From 1990 to 1998, combined N2O emissions from sta-
tionary and mobile combustion increased by 21 per-
cent, primarily due to increased rates of N2O generation
in motor vehicles.
Table ES-10: U.S. Sources of Nitrous Oxide Emissions (MMTCE)
' Source
Agricultural Soil Management
-Mobile Sources
Nitric Acid
Stationary Sources
Manure Management
Human Sewage
^Adipic Acid
International Bunker Fuels*
Agricultural Residue Burning
I Waste Combustion
r Total*
1990
75.3
13.8
4.9
3.8
3.4
2.0
5.0
0.3
0.1
0.1
108.2
1991
76.3
14.6
4.9
3.8
3.6
2.0
5.2
0.3
0.1
0.1
110.5
1992
78.2
15.7
5.0
3.9
3.5
2.0
4.8
0.3
0.1
0.1
113.3
1993
77.3
16.5
5.1
3.9
3.7
2.0
5.2
0.2
0.1
0.1
113.8
1994
83.5
17.1
5.3
4.0
3.8
2.1
5.5
0.2
0.1
0.1
121.5
1995
80.4
17.4
5.4
4.0
3.7
2.1
5.5
0.2
0.1
0.1
118.8
1996
82.4
17.5
5.6
4.2
3.8
2.1
5.7
0.2
0.1
0.1
121.5
1997
84.2
17.3
5.8
4.2
3.9
2.1
4.7
0.3
0.1
0.1
122.4
1998
83.9
17.2
5.8
4.3
4.0
2.2
2.0
0.3
0.1
0,1
119.4
* Emissions from International Bunker Fuels are not included in totals.
' Note: Totals may not sum due to independent rounding.
ES-18 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Adipic Acid Production
The majority of the adipic acid produced in the
United States is used to manufacture nylon 6,6. Adipic
acid is also used to produce some low-temperature lubri-
cants, and to add a "tangy" flavor to foods. Nitrous ox-
ide is emitted as a by-product of the chemical synthesis
of adipic acid.
In 1998, U.S. adipic acid plants emitted 2.0
MMTCE of N2O, or 2 percent of U.S. N2O emissions.
Since 1990, even though adipic acid production in-
creased, by 1998, all of the three major adipic acid plants
in the United States had voluntarily implemented N2O
abatement technology. As a result, emissions in 1998
decreased by 58 percent relative to the previous year.
Nitric Acid Production
Nitric acid production is another industrial source
of N2O emissions. Used primarily to make synthetic com-
mercial fertilizer, this raw material is also a major com-
ponent in the production of adipic acid and explosives.
Virtually all of the nitric acid manufactured in the
United States is produced by the oxidation of ammonia,
during which N2O is formed and emitted to the atmo-
sphere. In 1998, N2O emissions from nitric acid produc-
tion were 5.8 MMTCE, or 5 percent of U.S. N2O eipis-
sions. From 1990 to 1998, emissions from this source
increased by 18 percent as nitric acid production grew.
Manure Management
Nitrous oxide is produced as part of microbial ni-
trification and denitrification processes in managed and
unmanaged manure, the latter of which is addressed un-
der agricultural soil management. Total N2O emissions
from managed manure systems in 1998 were 4.0 MMTCE,
accounting for 3 percent of U.S. N2O emissions. Emis-
sions increased by 19 percent from 1990 to 1998.
Other Sources
Other sources of N2O included agricultural resi-
due burning, waste combustion, and human sewage in
wastewater treatment systems. In 1998, agricultural resi-
due burning and municipal solid waste combustion each
emitted approximately 0.1 MMTCE of N2O. Although
N2O emissions from wastewater treatment were not fully
estimated because of insufficient data availability, the
human sewage component of domestic wastewater re-
sulted in emissions of 2.2 MMTCE in 1998.
MFCs, PFCs and SF6 Emissions
Hydrofluorocarbons (HFCs) and perfluorocarbons
(PFCs) are categories of synthetic chemicals that are be-
ing used as alternatives to the ozone depleting substances
(ODSs), which are being phased out under the Montreal
Protocol and Clean Air Act Amendments of 1990. Be-
cause HFCs and PFCs do not directly deplete the strato-
spheric ozone layer, they are not controlled by the
Montreal Protocol.
These compounds, however, along with sulfur
hexafluoride (SF6), are potent greenhouse gases. In ad-
dition to having high global warming potentials, SF6
and many HFCs and PFCs have extremely long atmo-
spheric lifetimes, resulting in their essentially irrevers-
ible accumulation in the atmosphere. Sulfur hexafluo-
ride, itself, is the most potent greenhouse gas the IPCC
has evaluated.
In addition to their use as substitutes for ozone
depleting substances, the other emissive sources of these
gases are aluminum production, HCFC-22 production,
semiconductor manufacturing, electrical transmission
and distribution, and magnesium production and pro-
cessing. Figure ES-13 and Table ES-11 present emission
estimates for HFCs, PFCs, and SF6, which totaled 40.3
MMTCE in 1998.
Substitution of Ozone
Depleting Substances
The use and subsequent emissions of HFCs and
PFCs as ODS substitutes increased dramatically from
small amounts in 1990 to 14.5 MMTCE in 1998. This
increase was the result of efforts to phase-out CFCs and
other ODSs in the United States, especially the introduc-
tion of HFC-134a as a CFC substitute in refrigeration
Executive Summary ES-19
-------
Figure ES-13
Substitution of Ozone
DoplDtlng Substances
HCFC-22 Production
Electrical Transmission
and Distribution
Magnesium Production
and Processing '
Aluminum Production j
Semiconductor
Manufacture
Portion of All
Emissions
5 10
MMTCE
15
applications. This trend is expected to continue for many
years, and will accelerate in the early part of the next
century as HCFCs, which are interim substitutes in many
applications, are themselves phased-out under the pro-
visions of the Copenhagen Amendments to the Montreal
Protocol.
Other Industrial Sources
HFCs, PFCs, and SF6 are also emitted from a num-
ber of other industrial processes. During the production
of primary aluminum, two PFCs—CF4 and C2F6—are
emitted as intermittent by-products of the smelting pro-
cess. Emissions from aluminum production were esti-
mated to have decreased by 48 percent between 1990
and 1998 due to voluntary emission reduction efforts by
the industry and falling domestic aluminum production.
HFC-23 is a by-product emitted during the pro-
duction of HCFC-22. Emissions from this source were
10.9 MMTCE in 1998, and have increased by 15 per-
cent since 1990. This increase is attributable to the 30
percent increase in HCFC-22 production that has oc-
curred since 1990; one third of this increase occurred
between 1997 and 1998. The intensity of HFC-23 emis-
sions (i.e., the amount of HFC-23 emitted per kilogram
of HCFC-22 manufactured), however, has declined sig-
nificantly since 1990.
The semiconductor industry uses combinations of
HFCs, PFCs, SF6 and other gases for plasma etching and
chemical vapor deposition processes. For 1998, it was
estimated that the U.S. semiconductor industry emitted
a total of 2.1 MMTCE. Emissions from this source cat-
egory have increased with the growth in the semicon-
ductor industry and the rising intricacy of chip designs.
The primary use of SF6 is as a dielectric in electri-
cal transmission and distribution systems. Fugitive emis-
sions of SF6 occur from leaks in and servicing of substa-
tions and circuit breakers, especially from older equip-
ment. Estimated emissions from this source increased by
25 percent from 1990, to 7.0 MMTCE in 1998.
Table ES-11: Emissions of HFCs, PFCs, and SF6 (MMTCE)
Source
Substitution of Ozone
Depleting Substances
Aluminum Production
HCFC-22 Production
Semiconductor Manufacture
Electrical Transmission and Distribution
Magnesium Production and Processing
Total
1990
0.3
5.4
9.5
0.8
5.6
1.7
23.3
1991
0.2
4.7
8.4
0.8
5.9
2.0
22.0
1992
0.4
4.4
9.5
0.8
6.2
2.2
23.5
1993
1.4
3.8
8.7
1.0
6.4
2.5
23.8
1994
2.7
3.2
8.6
1.1
6.7
2.7
25.1
1995
7.0
3.1
7.4
1.5
7.0
3.0
29.0
1996
9.9
3.2
8.5
1.9
7.0
3.0
33.5
1997
12.3
3.0
8.2
1.9
7.0
3.0
35.3
1998
14.5
2.8
10.9
2.1
7.0
3.0
40.3
Note: Totals may not sum due to independent rounding.
ES-20 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Box ES-4: Emissions of Ozone Depleting Substances
t Chlorofluorocarbons (CFCs) and other halogenated compounds were first emitted into the atmosphere this century. This family of
h man-made compounds includes CFCs, halons, methyl chloroform, carbon tetrachloride, methyl bromide, and hydrochlorofluorocarbons
j: (HCFCs). These substances have been used in a variety of industrial applications, including refrigeration, air conditioning, foam
jr blowing, solvent cleaning, sterilization, fire extinguishing, coatings, paints, and aerosols.
\ Because these compounds have been shown to deplete stratospheric ozone, they are typically referred to as ozone depleting
s-substances (ODSs). However, they are also potent greenhouse gases.
j-L Recognizing the harmful effects of these compounds on the ozone layer, in 1987 many governments signed the Montreal
^Protocol on Substances that Deplete the Ozone Layer to limit the production and importation of a number of CFCs and other
| halogenated compounds. The United States furthered its commitment to phase-out ODSs by signing and ratifying the Copenhagen
jlAmendments to the Montreal Protocol in 1992. Under these amendments, the United States committed to ending the production and
^importation of halons by 1994, and CFCs by 1996.
I The IPCC Guidelines do not include reporting instructions for estimating emissions of ODSs because their use is being phased-out
|under the Montreal Protocol. The United States "believes, however, that a greenhouse gas emissions inventory is incomplete without
£ these emissions; therefore, estimates for several Class I and Class II ODSs are provided in Table ES-12. Compounds are grouped by
plass according to their ozone depleting potential. Class I compounds are the primary ODSs; Class II compounds include partially
jUialogenated chlorine compounds (i.e., HCFCs), some of which were developed as interim replacements for CFCs. Because these
jiBCFC compounds are only partially halogenated, their hydrogen-carbon bonds are more vulnerable to oxidation in the troposphere
hnd, therefore, pose only one-tenth to one-hundredth the threat to stratospheric ozone compared to CFCs.
• It should be noted that the effects of these compounds on radiative forcing are not provided. Although many ODSs have relatively
- high direct GWPs, their indirect effects from ozone also a greenhouse gas destruction are believed to have negative radiative forcing
.effects, and therefore could significantly reduce the overall magnitude of their radiative forcing effects. Given the uncertainties
-surrounding the net effect of these gases, emissions are reported on an unweighted basis.
Table ES-12: Emissions of Ozone Depleting Substances (Gg)
Compound
f. Class 1
? CFC-11
'i" CFC-12
! CFC-11 3
P CFC-11 4
t CFC-115
:• Carbon Tetrachloride
r Methyl Chloroform
h Halon-1211
!: Halon-1301
Class II
f- HCFC-22
^ HCFC-123
I- HCFC-124
I; HCFC-141b
: HCFC-142b
i- HCFC-225ca/cb
^ Source: EPA
;•...+ Does not exceed 0.05 Gg
1990
53.5
112.6
26.4
4.7
4.2
32.3
158.3
1.0
1.8
79.8
+
+
+
+
+
1991
48.3
103.5
20.6
3.6
4.0
31.0
154.7
1.1
1.8
79.5
+
+
+
+
+
1992
45.1
80.5
17.1
3.0
3.8
21.7
108.3
1.0
1.7
79.5
0.3
0.4
+
0.7
+
1993
45.4
79.3
17.1
3.0
3.6
18.6
92.9
1.1
1.7
71.2
0.3
2.6
5.0
1.7
1994
36.6
57.6
8.6
1.6
3.3
15.5
77.4
1.0
1.7
71.4
0.5
4.8
12.4
4.6
1995
36.2
51.8
8.6
1.6
3.0
4.7
46.4
1.1
1.8
72.3
0.6
5.2
20.6
7.3
1996
26.6
35.5
0.3
3.2
+
1.1
1.9
73.2
0.7
5.6
25.4
8.3
1997
25.1
23.1
0.1
2.9
+
1.1
1.9
74.2
0.8
5.9
25.1
8.7
1998
24.9
21.0
0.1
2.7
1.1
1.9
75.1
0.9
6.1
26.7
9.0
Executive Summary ES-21
-------
Lastly, SF6 is also used as a protective covergas for
the casting of molten magnesium. Estimated emissions
from primary magnesium production and magnesium cast-
ing were 3.0 MMTCE in 1998, an increase of 76 percent
since 1990.
Criteria Pollutant Emissions
In the United States, carbon monoxide (CO), nitro-
gen oxides (NOX), nonmethane volatile organic com-
pounds (NMVOCs), and sulfur dioxide (SO2) are com-
monly referred to as "criteria pollutants," as termed in
the Clean Air Act. Criteria pollutants do not have a direct
global warming effect, but indirectly affect terrestrial ra-
diation absorption by influencing the formation and de-
struction of tropospheric and stratospheric ozone, or, in
the case of SO2, by affecting the absorptive characteris-
tics of the atmosphere. Carbon monoxide is produced
when carbon-containing fuels are combusted incom-
pletely. Nitrogen oxides (i.e., NO and NO2) are created
by lightning, fires, fossil fuel combustion, and in the
stratosphere from nitrous oxide (N2O). NMVOCs which
include such compounds as propane, butane, and ethane
are emitted primarily from transportation, industrial pro-
cesses, and non-industrial consumption of organic sol-
vents. In the United States, SO2 is primarily emitted from
the combustion of fossil fuels and by the metals industry.
Box ES-5: Sources and Effects of Sulfur Dioxide
:- Sulfur dioxide (S02) emitted into the atmosphere through natural and anthropogenic processes affects the Earth's radiative budget
'through its photochemical transformation into sulfate aerosols that can (1) scatter sunlight back to space, thereby reducing the
''; radiation reaching the Earth's surface; (2) affect cloud formation; and (3) affect atmospheric chemical composition (e.g., strato-
Ispheric ozone, by providing surfaces for heterogeneous chemical reactions). The overall effect of S02 derived aerosols on radiative
'forcing is believed to be negative (IPGC1996). However, because S02 is short-lived and unevenly distributed in the atmosphere, its
radiative forcing impacts are highly uncertain.
Sulfur dioxide is also a major contributor to the formation of urban smog, which can cause significant increases in acute and
f chronic respiratory diseases. Once S02 is emitted, it is chemically transformed in the atmosphere and returns to the Earth as the primary
^source of acid rain. Because of these harmful effects, the United States has regulated S02 emissions in the Clean Air Act.
'" Electric utilities are the largest source of S02 emissions in the United States, accounting for 62 percent in 1998. Coal combustion
;! contributes nearly all of those emissions (approximately 96 percent). Sulfur dioxide emissions have decreased in recent years, primarily
: as a result of electric utilities switching from high sulfur to low sulfur coal.
In part because of their contribution to the forma-
tion of urban smog—and acid rain in the case of SO2 and
NOX—criteria pollutants are regulated under the Clean
Air Act. These gases also indirectly affect the global cli-
mate by reacting with other chemical compounds in the
atmosphere to form compounds that are greenhouse gases.
Unlike other criteria pollutants, SO2 emitted into the at-
mosphere is believed to affect the Earth's radiative bud-
get negatively; therefore, it is discussed separately.
One of the most important indirect climate change
effects of criteria pollutants is their role as precursors
for tropospheric ozone formation. They can also alter
the atmospheric lifetimes of other greenhouse gases.
For example, CO interacts with the hydroxyl radical
the major atmospheric sink for methane emissions to
form CO2. Therefore, increased atmospheric concentra-
tions of CO limit the number of hydroxyl molecules
(OH) available to destroy methane.
Since 1970, the United States has published esti-
mates of annual emissions of criteria pollutants (EPA
1999).9 Table ES-13 shows that fuel combustion ac-
counts for the majority of emissions of these gases. In-
dustrial processes such as the manufacture of chemical
and allied products, metals processing, and industrial
uses of solvents are also significant sources of CO, NOX>
and NMVOCs.
9 NOX and CO emission estimates from agricultural residue burning were estimated separately, and therefore not taken from EPA
(1999).
ES-22 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Table ES-13: Emissions of NOX, CO, NMVOCs, and S02 (Gg)
: Gas/Activity
;; NOX
| -Stationary Combustion
^Mobile Combustion
j:- Oil and Gas Activities
; Industrial Processes
f ;. Solvent Use
|": Agricultural Residue Burning
'.:, Waste
tjCO
I Stationary Combustion
!,; Mobile Combustion
F : Oil and Gas Activities
f Industrial Processes
| Solvent Use
|r Agricultural Residue Burning
is Waste
jfNMVOCs
|1: Stationary Combustion
j-. Mobile Combustion
; Oil and Gas Activities
J: Industrial Processes
t; Solvent Use
? Agricultural Residue Burning
k Waste
;fS02
L Stationary Combustion
!i Mobile Combustion
; Oil and Gas Activities
•"" Industrial Processes
t- Solvent Use
fc Agricultural Residue Burning
P." Waste
1990
21,798
9,884
10,744
139
921
1
26
83
85,394
4,999
68,985
302
9,502
,4
623
979
18,795
912
8,037
555
3,179
5,217
NA
895
21,465
18,407
1,322
390
1,306
+
NA
38
1991
21,936
9,779
11,132
110
802
2
26
86
87,485
5,313
73,177
313
7,088
4
578
1,012
18,929
975
8,239
581
2,983
5,245
NA
907
20,903
17,959
1,373
343
1,187
+
NA
40
1992
22,176
9,914
11,224
134
785
2
29
87
84,589
5,583
71,543
337
5,401
5
688
1,032
18,527
1,011
7,862
574
2,811
5,353
NA
916
20,689
17,684
1,402
377
.1,186
_)_
NA
40
1993
22,398
10,080
11,294
111
774
2
23
112
84,716
5,068
72,210
337
5,421
4
544
1,133
18,708
901
7,919
588
2,893
5,458
NA
949
20,381
17,459
1,351
347
1,159
1
NA
65
1994
22,683
9,993
11,508
106
.939
2
32
103
88,911
5,007
74,057
307
7,708
5
717
1,111
19,290
898
8,223
587
3,043
5,590
NA
949
19,840
17,134
1,172
344
.1,135
1
NA
54
1995
22,177
9,822
11,294
100
842
3
27
89
80,093
5,383
67,433
316
5,291
5
590
1,075
18,613
973
7,621
582
2,859
5,609
NA
968
17,401
14,724
1,183
334
1,117
1
NA
43
1996
22,034
9,553
11,261
121
979
3
30
87
82,028
5,405
66,674
287
7,899
5
675
1,083
17,624
951
7,398
459
2,859
5,569
NA
388
18,695
15,981
1,208
300
1,167
NA
37
1997
22,153
9,728
11,289
121
890
q
O
32
89
79,284
4,455
65,301
292
7,432
5
704
1,095
17,469
770
7,169
461
3,002
5,672
NA
394
19,216
16,458
1,235
301
1,184
i
1
NA
37
1998
22,066
9,719 :
11,184 i
122
915 ;
0
£
34
90
78,082
4,491
63,780
296
7,669 '.
733
1,107
17,011
776
7,065 ;
464
*tU*t '
3,066 ;
5,239
NA :
400 ;
19,441
.16,635
1,261
QflQ
OUO J
1,204
-i
i
NA
38 :
^Source: (EPA 1999) except for estimates from agricultural residue burnina.
1 ~+ Does not exceed 0.5 Gg
plA (Not Available)
plote: Totals may not sum due to
independent rounding.
Executive Summary ES-23
-------
ES-24 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
1 . Introduction
is report presents estimates by the United States government of U.S. anthropogenic greenhouse gas emis
sions and sinks for the years 1990 through 1998. A summary of these estimates is provided in Table 1-4 and
Table 1-5 by gas and source category. The emission estimates in these tables are presented on both a full molecular
mass basis and on a Global Warming Potential (GWP) weighted basis in order to show the relative contribution of
each gas to global average radiative forcing.1'2 This report also discusses the methods and data used to calculate
these emission estimates.
In June of 1992, the United States signed the United Nations Framework Convention on Climate Change
(UNFCCC). The objective of the UNFCCC is "to achieve... stabilization of greenhouse gas concentrations in the
atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system." 3>4
Parties to the Convention, by signing, make commitments "to develop, periodically update, publish and make
available... national inventories of anthropogenic emissions by sources and removals by sinks of all greenhouse
gases not controlled by the Montreal Protocol, using comparable methodologies..."5 The United States views this
report as an opportunity to fulfill this commitment under UNFCCC.
In 1988, preceding the creation of the UNFCCC, the Intergovernmental Panel on Climate Change (IPCC) was
jointly established by the World Meteorological Organization (WMO) and the United Nations Environment Programme
(UNEP). The charter of the IPCC is to assess available scientific information on climate change, assess the environ-
mental and socio-economic impacts of climate change, and formulate response strategies (IPCC 1996). Under
Working Group 1 of the IPCC, nearly 140 scientists and national experts from more than thirty countries corroborated
in the creation of the Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories (BPCC/UNEP/OECD/
IEA 1997) to ensure that the emission inventories submitted to the UNFCCC are consistent and comparable between
nations. The Revised 1996 IPCC Guidelines were accepted by the IPCC at its Twelfth Session (Mexico City, 11-13
September 1996). The information provided in this inventory is presented in accordance with these guidelines.
1 See the section below entitled Global Warming Potentials for an explanation of GWP values.
2 See the section below entitled What is Climate Change? for an explanation of radiative forcing.
3 The term "anthropogenic", in this context, refers to greenhouse gas emissions and removals that are a direct result of human activities
or are the result of natural processes that have been affected by human activities (IPCC/UNEP/OECD/IEA 1997).
4 Article 2 of the Framework Convention on Climate Change published by the UNEP/WMO Information Unit on Climate Change. See
.
5 Article 4 of the Framework Convention on Climate Change published by the UNEP/WMO Information Unit on Climate Change (also
identified in Article 12). See .
Introduction 1-1
-------
Additionally, in order to fully comply with the Revised
1996IPCC Guidelines, the United States has provided
estimates of carbon dioxide emissions from fossil fuel
combustion using the IPCC Reference Approach in An-
nex O.
Overall, the purpose of an inventory of anthropo-
genic greenhouse gas emissions is (1) to provide a basis
for the ongoing development of methodologies for esti-
mating sources and sinks of greenhouse gases; (2) to
provide a common and consistent mechanism through
which Parties to the UNFCCC can estimate emissions
and compare the relative contribution of individual
sources, gases, and nations to climate change; and (3) as
a prerequisite for accounting for reductions and evaluat-
ing possible mitigation strategies.
What is Climate Change?
Climate change refers to long-term fluctuations in
temperature, precipitation, wind, and other elements of
the Earth's climate system.6 Natural processes such as
solar-irradiance variations, variations in the Earth's or-
bital parameters,7 and volcanic activity can produce
variations in climate. The climate system can also be
influenced by changes in the concentration of various
gases in the atmosphere, which affect the Earth's absorp-
tion of radiation.
The Earth naturally absorbs and reflects incoming
solar radiation and emits longer wavelength terrestrial
(thermal) radiation back into space. On average, the
absorbed solar radiation is balanced by the outgoing
terrestrial radiation emitted to space. A portion of this
terrestrial radiation, though, is itself absorbed by gases
in the atmosphere. The energy from this absorbed terres-
trial radiation warms the Earth's surface and atmosphere,
creating what is known as the "natural greenhouse ef-
fect." Without the natural heat-trapping properties of
these atmospheric gases, the average surface tempera-
ture of the Earth would be about 34°C lower (IPCC 1996).
Under the UNFCCC, the definition of climate
change is "a change of climate which is attributed di-
rectly or indirectly to human activity that alters the com-
position of the global atmosphere and which is in addi-
tion to natural climate variability observed over compa-
rable time periods."8 Given that definition, in its 1995
assessment of the science of climate change, the IPCC
concluded that:
Human activities are changing the atmospheric
concentrations and distributions of greenhouse
gases and aerosols. These changes can pro-
duce a radiative forcing by changing either
the reflection or absorption of solar radiation,
or the emission and absorption of terrestrial
radiation (IPCC 1996).
The IPCC went on to report in its assessment that
the "[g]lobal mean surface temperature [of the Earth] has
increased by between about 0.3 and 0.6 °C since the late
19th century..." (IPCC 1996) and finally concluded with
the following statement:
Our ability to quantify the human influence on
global climate is currently limited because the
expected signal is still emerging from the noise
of natural variability, and because there are
uncertainties in key factors. These include the
magnitude and patterns of long term natural
variability and the time-evolving pattern of forc-
ing by, and response to, changes in concentra-
tions of greenhouse gases and aerosols, and
land surface changes. Nevertheless, the bal-
ance of the evidence suggests that there is a
discernable human influence on global climate
(IPCC 1996).
Greenhouse Gases
Although the Earth's atmosphere consists mainly
of oxygen and nitrogen, neither plays a significant role
in enhancing the greenhouse effect because both are es-
sentially transparent to terrestrial radiation. The green-
house effect is primarily a function of the concentration
of water vapor, carbon dioxide, and other trace gases in
the atmosphere that absorb the terrestrial radiation leav-
6 The Earth's climate system comprises the atmosphere, oceans, biosphere, cryosphere, and geosphere.
7 For example, eccentricity, precession, and inclination.
8 Article 1 of the Framework Convention on Climate Change published by the UNEP/WMO Information Unit on Climate Change.
1 -2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
ing the surface of the Earth (IPCC 1996). Changes in the
atmospheric concentrations of these greenhouse gases
can alter the balance of energy transfers between the
atmosphere, space, land, and the oceans. A gauge of
these changes is called radiative forcing, which is a
simple measure of changes in the energy available to the
Earth-atmosphere system (IPCC 1996). Holding every-
thing else constant, increases in greenhouse gas concen-
trations in the atmosphere will produce positive radia-
tive forcing (i.e., a net increase in the absorption of en-
ergy by the Earth).
Climate change can be driven by changes in
the atmospheric concentrations of a number of
radiatively active gases and aerosols. We have
clear evidence that human activities have af-
fected concentrations, distributions and life
cycles of these gases (IPCC 1996).
Naturally occurring greenhouse gases include wa-
ter vapor, carbon dioxide (CO2), methane (CH4), nitrous
oxide (N2O), and ozone (O3). Several classes of haloge-
nated substances that contain fluorine, chlorine, or bro-
mine are also greenhouse gases, but they are, for the
most part, emitted solely by human activities. Chlorof-
luorocarbons (CFCs) and hydrochlorofluorocarbons
(HCFCs) are halocarbons that contain chlorine, while
halocarbons that contain bromine are referred to as
halons. Other fluorine containing halogenated sub-
stances include hydrofluorocarbons (MFCs),
perfluorocarbons (PFCs), and sulfur hexafluoride (SF6).
There are also several gases that, although they do not
have a direct radiative forcing effect, do influence the
formation and destruction of ozone, which does have
such a terrestrial radiation absorbing effect. These gases
referred to here as ozone precursors include carbon mon-
oxide (CO), oxides of nitrogen (NOX), and nonmethane
volatile organic compounds (NMVOCs).9 Aerosols ex-
tremely small particles or liquid droplets often produced
by emissions of sulfur dioxide (SO2) can also affect the
absorptive characteristics of the atmosphere.
Carbon dioxide, methane, and nitrous oxide are
continuously emitted to and removed from the atmo-
sphere by natural processes on Earth. Anthropogenic
activities, however, can cause additional quantities of
these and other greenhouse gases to be emitted or se-
questered, thereby changing their global average atmo-
spheric concentrations. Natural activities such as respi-
ration by plants or animals and seasonal cycles of plant
growth and decay are examples of processes that only
cycle carbon or nitrogen between the atmosphere and
organic biomass. Such processes except when directly
or indirectly perturbed out of equilibrium by anthropo-
genic activities generally do not alter average atmo-
spheric greenhouse gas concentrations over decadal
timeframes. Climatic changes resulting from anthropo-
genic activities, however, could have positive or nega-
tive feedback effects on these natural systems.
A brief description of each greenhouse gas, its
sources, and its role in the atmosphere is given below.
The following section then explains the concept of Glo-
bal Warming Potentials (GWPs), which are assigned to
individual gases as a measure of their relative average
global radiative forcing effect.
Water Vapor (H2O). Overall, the most abundant
and dominant greenhouse gas in the atmosphere is water
vapor. Water vapor is neither long-lived nor well mixed
in the atmosphere, varying spatially from 0 to 2 percent
(IPCC 1996). In addition, atmospheric water can exist
in several physical states including gaseous, liquid, and
solid. Human activities are not believed to directly af-
fect the average global concentration of water vapor;
however, the radiative forcing produced by the increased
concentrations of other greenhouse gases may indirectly
affect the hydrologic cycle. A warmer atmosphere has
an increased water holding capacity; yet, increased con-
centrations of water vapor affects the formation of clouds,
which can both absorb and reflect solar and terrestrial
radiation. Aircraft contrails, which consist of water va-
por and other aircraft emittants, are similar to clouds in
their radiative forcing effects (IPCC 1999).
Carbon Dioxide (CO2). In nature, carbon is cycled
between various atmospheric, oceanic, land biotic, ma-
rine biotic, and mineral reservoirs. The largest fluxes
occur between the atmosphere and terrestrial biota, and
' Also referred to in the U.S. Clean Air Act as "criteria pollutants."
Introduction 1-3
-------
between the atmosphere and surface water of the oceans.
In the atmosphere, carbon predominantly exists in its
oxidized form as CO2. Atmospheric carbon dioxide is
part of this global carbon cycle, and therefore its fate is a
complex function of geochemical and biological pro-
cesses. Carbon dioxide concentrations in the atmo-
sphere, as of 1994, increased from approximately 280
parts per million by volume (ppmv) in pre-industrial10
times to 358 ppmv, a 28 percent increase (IPCC 1996).n
The IPCC has stated that "[t]here is no doubt that this
increase is largely due to human activities, in particular
fossil fuel combustion..." (IPCC 1996). Forest clearing,
other biomass burning, and some non-energy produc-
tion processes (e.g., cement production) also emit no-
table quantities of carbon dioxide.
In its latest scientific assessment, the IPCC also
stated that "[t]he increased amount of carbon dioxide
[in the atmosphere] is leading to climate change and
will produce, on average, a global warming of the Earth's
surface because of its enhanced greenhouse effect al-
though the magnitude and significance of the effects are
not fully resolved" (IPCC 1996).
Methane (CH4). Methane is primarily produced
through anaerobic decomposition of organic matter in
biological systems. Agricultural processes such as wet-
land rice cultivation, enteric fermentation in animals,
and the decomposition of animal wastes emit CH4, as
does the decomposition of municipal solid wastes. Meth-
ane is also emitted during the production and distribu-
tion of natural gas and petroleum, and is released as a
by-product of coal mining and incomplete fossil fuel
combustion. The average global concentration of meth-
ane in the atmosphere was 1,720 parts per billion by
volume (ppbv) in 1994, a 145 percent increase from the
pre-industrial concentration of 700 ppbv (IPCC 1996).
It is estimated that 60 to 80 percent of current CH4 emis-
sions are the result of anthropogenic activities. Carbon
isotope measurements indicate that roughly 20 percent
of methane emissions are from fossil fuel consumption,
and an equal percentage is produced by natural wet-
lands, which will likely increase with rising tempera-
tures and rising microbial action (IPCC 1996).
Methane is removed from the atmosphere by re-
acting with the hydroxyl radical (OH) and is ultimately
converted to CO2. Increasing emissions of methane,
though, reduces the concentration of OH, and thereby
the rate of further methane removal (IPCC 1996).
Nitrous Oxide (N2O). Anthropogenic sources of
N2O emissions include agricultural soils, especially the
use of synthetic and manure fertilizers; fossil fuel com-
bustion, especially from mobile combustion; adipic (ny-
lon) and nitric acid production; wastewater treatment
and waste combustion; and biomass burning. The atmo-
spheric concentration of nitrous oxide (N2O) in 1994
was about 312 parts per billion by volume (ppbv), while
pre-industrial concentrations were roughly 275 ppbv.
The majority of this 13 percent increase has occurred
after the pre-industrial period and is most likely due to
anthropogenic activities (IPCC 1996). Nitrous oxide is
removed from the atmosphere primarily by the photolytic
action of sunlight in the stratosphere.
Ozone (O3). Ozone is present in both the upper
stratosphere,12 where it shields the Earth from harmful
levels of ultraviolet radiation, and at lower concentra-
tions in the troposphere,13 where it is the main compo-
nent of anthropogenic photochemical "smog." During
the last two decades, emissions of anthropogenic chlo-
rine and bromine-containing halocarbons, such as chlo-
rofluorocarbons (CFCs), have depleted stratospheric
ozone concentrations. This loss of ozone in the strato-
10 The pre-industrial period is considered as the time preceding the year 1750 (IPCC 1996).
11 Carbon dioxide concentrations during the last 1,000 years of the pre-industrial period (i.e., 750-1750), a time of relative climate
stability, fluctuated by about ±10 ppmv around 280 ppmv (IPCC 1996).
12 The stratosphere is the layer from the troposphere up to roughly 50 kilometers. In the lower regions the temperature is nearly
constant but in the upper layer the temperature increases rapidly because of sunlight absorption by the ozone layer. The ozone-layer
is the part of the stratosphere from 19 kilometers up to 48 kilometers where the concentration of ozone reaches up to 10 parts per
million.
13 The troposphere is the layer from the ground up to 11 kilometers near the poles and up to 16 kilometers in equatorial regions (i.e.,
the lowest layer of the atmosphere where people live). It contains roughly 80 percent of the mass of all gases in the atmosphere and
is the site for most weather processes, including most of the water vapor and clouds.
1 -4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
sphere has resulted in negative radiative forcing, repre-
senting an indirect effect of anthropogenic emissions of
•chlorine and bromine compounds (EPCC 1996).
Tropospheric ozone, which is also a greenhouse
gas, is produced from the oxidation of methane and
from reactions with precursor gases such as carbon mon-
oxide (CO), nitrogen oxides (NOX), and non-methane
volatile organic compounds (NMVOCs). This latter
group of ozone precursors is included in the category
referred to as "criteria pollutants" in the United States
under the Clean Air Act14 and its subsequent amend-
ments. The tropospheric concentrations of both ozone
and these precursor gases are short-lived and, there-
fore, spatially variable.
Halocarbons, Perfluorocarbons, and Sulfur
Hexafluoride (SF6). Halocarbons are, for the most part,
man-made chemicals that have both direct and indirect
radiative forcing effects. Halocarbons that contain chlo-
rine chlorofluorocarbons (CFCs), hydrochlorofluoro-
carbons (HCFCs), methyl chloroform, and carbon tetra-
chloride and bromine halons, methyl bromide, and
hydrobromofluorocarbons (HBFCs) result in strato-
spheric ozone depletion and are therefore controlled
under the Montreal Protocol on Substances that De-
plete the Ozone Layer. Although CFCs and HCFCs
include potent global warming gases, their net radia-
tive forcing effect on the atmosphere is reduced be-
cause they cause stratospheric ozone depletion, which
is itself an important greenhouse gas in addition to
shielding the Earth from harmful levels of ultraviolet
radiation. Under the Montreal Protocol, the United
States phased out the production and importation of
halons by 1994 and of CFCs by 1996. Under the
Copenhagen Amendments to the Protocol, a cap was
placed on the production and importation of HCFCs
by non-Article 515 countries beginning in 1996, and
then followed by a complete phase-out by the year 2030.
The ozone depleting gases covered under the Montreal
Protocol and its Amendments are not covered by the
UNFCCC; however, they are reported in this inventory
under Annex L.
Hydrofluorocarbons (HFCs), perfluorocarbons
(PFCs), and sulfur hexafluoride (SF6) are not ozone de-
pleting substances, and therefore are not covered un-
der the Montreal Protocol. They are, however, power-
ful greenhouse gases. HFCs primarily used as replace-
ments for ozone depleting substances but also emitted
as a by-product of the HCFC-22 manufacturing process
currently have a small aggregate radiative forcing im-
pact; however, it is anticipated that their contribution
to overall radiative forcing will increase (IPCC 1996).
PFCs and SF6 are predominantly emitted from various
industrial processes including aluminum smelting, semi-
conductor manufacturing, electric power transmission
and distribution, and magnesium casting. Currently,
the radiative forcing impact of PFCs, and SF6 is also
small; however, because they have extremely long at-
mospheric lifetimes, their concentrations tend to irre-
versibly accumulate in the atmosphere.
Carbon Monoxide (CO). Carbon monoxide has
an indirect radiative forcing effect by elevating con-
centrations'of CH4 and tropospheric ozone through
chemical reactions with other atmospheric constituents
(e.g., the hydroxyl radical) that would otherwise assist
in destroying CH4 and tropospheric ozone. Carbon
monoxide is created when carbon-containing fuels are
burned incompletely. Through natural processes in the
atmosphere, it is eventually oxidized to CO2. Carbon
monoxide concentrations are both short-lived in the
atmosphere and spatially variable.
Nitrogen Oxides (NOK). The primary climate
change effects of nitrogen oxides (i.e., NO and NO2) are
indirect and result from their role in promoting the for-
mation of ozone in the troposphere and, to a lesser de-
gree, lower stratosphere, where it has positive radiative
forcing effects. (NOX emissions injected higher in the
stratosphere16 can lead to stratospheric ozone deple-
14 [42 U.S.C §7408, CAA §108]
15 Article 5 of the Montreal Protocol covers several groups of countries, especially developing countries, with low consumption rates
ot ozone depleting substances. Developing countries with per capita consumption of less than 0.3 kg of certain ozone depleting
substances (weighted by their ozone depleting potential) receive financial assistance and a grace period of ten additional years in the
phase-out of ozone depleting substances.
16 Primarily from fuel combustion emissions from high altitude supersonic aircraft.
Introduction 1-5
-------
tion.) Additionally, NOX emissions from aircraft are ex-
pected to decrease methane concentrations, thus having
a negative radiative forcing effect (IPCC 1999). Nitro-
gen oxides are created from lightning, soil microbial
activity, biomass burning—both natural and anthropo-
genic fires—fuel combustion, and, in the stratosphere,
from nitrous oxide (N2O). Concentrations of NOX are
both relatively short-lived in the atmosphere and spa-
tially variable.
Nonmethane Volatile Organic Compounds
(NMVOCs). Nonmethane volatile organic compounds
include compounds such as propane, butane, and ethane.
These compounds participate, along with NOX, in the
formation of tropospheric ozone and other photochemi-
cal oxidants. NMVOCs are emitted primarily from trans-
portation and industrial processes, as well as biomass
burning and non-industrial consumption of organic sol-
vents. Concentrations of NMVOCs tend to be both short-
lived in the atmosphere and spatially variable.
Aerosols. Aerosols are extremely small particles
or liquid droplets found in the atmosphere. They can be
produced by natural events such as dust storms and vol-
canic activity or by anthropogenic processes such as
fuel combustion. Their effect upon radiative forcing is
to both absorb radiation and to alter cloud formation,
thereby affecting the reflectivity (i.e., albedo) of the
Earth. Aerosols are removed from the atmosphere prima-
rily by precipitation, and generally have short atmo-
spheric lifetimes. Like ozone precursors, aerosol con-
centrations and composition vary by region (IPCC1996).
Anthropogenic aerosols in the troposphere are pri-
marily the result of sulfur dioxide (SO^17 emissions from
fossil fuel and biomass burning. Overall, aerosols tend
to produce a negative radiative forcing effect (i.e., net
cooling effect on the climate), although because they
are short-lived in the atmosphere lasting days to weeks
their concentrations respond rapidly to changes in emis-
sions.18 Locally, the negative radiative forcing effects
of aerosols can offset the positive forcing of greenhouse
gases (IPCC 1996). "However, the aerosol effects do not
cancel the global-scale effects of the much longer-lived
greenhouse gases, and significant climate changes can
still result" (IPCC 1996). Emission estimates for sulfur
dioxide are provided in Annex M of this report.
Global Warming Potentials
A Global Warming Potential (GWP) is intended as
a quantified measure of the globally averaged relative
radiative forcing impacts of a particular greenhouse gas
(see Table 1-1). It is defined as the cumulative radiative
forcing both direct and indirect effects over a specified
tune horizon resulting from the emission of a unit mass
of gas relative to some reference gas (IPCC 1996). Di-
rect effects occur when the gas itself is a greenhouse gas.
Indirect radiative forcing occurs when chemical trans-
formations involving the original gas produces a gas or
gases that are greenhouse gases, or when a gas influ-
ences the atmospheric lifetimes of other gases. The ref-
erence gas used is CO2, and therefore GWP weighted
emissions are measured in million metric tons of carbon
equivalents (MMTCE). Carbon comprises 12/44ths of
carbon dioxide by weight. The relationship between
gigagrams (Gg) of a gas and MMTCE can be expressed
as follows:
where,
MMTCE = Million Metric Tons of Carbon
Equivalents
Gg = Gigagrams (equivalent to a thousand
metric tons)
GWP = Global Warming Potential
(ill _ Carbon to carbon dioxide molecular
44|
weight ratio.
MMT = Million Metric Tons
GWP values allow policy makers to compare the
impacts of emissions and reductions of different gases.
17 Sulfur dioxide is a primary anthropogenic contributor to the formation of "acid rain" and other forms of atmospheric acid
deposition.
18 Volcanic activity can inject significant quantities of aerosol producing sulfur dioxide and other sulfur compounds into the strato-
sphere, which can result in a longer negative forcing effect (i.e., a few years) (IPCC 1996).
1 -6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
According to the IPCC, GWPs typically have an uncer-
tainty of ±35 percent. The parties to the UNFCCC have
also agreed to use GWPs based upon a 100 year time
horizon although other time horizon values are available.
In addition to communicating emissions in units
of mass, Parties may choose also to use global
warming potentials (GWPs) to reflect their in-
ventories and projections in carbon dioxide-
equivalent terms, using information provided
by the Intergovernmental Panel on Climate
Change (IPCC) in its Second Assessment Re-
port. Any use of GWPs should be based on the
effects of the greenhouse gases over a 100-year
time horizon. In addition, Parties may also use
other time horizons.19
Greenhouse gases with long atmospheric lifetimes
(e.g., CO2, CH4, N2O, HFCs, PFCs, and SF6) tend to be
evenly distributed throughout the atmosphere, and con-
sequently global average concentrations can be deter-
mined. The short-lived gases such as water vapor, tropo-
spheric ozone, ozone precursors (e.g., NOX, CO, and
NMVOCs), and tropospheric aerosols (e.g., SO2 prod-
ucts), however, vary regionally, and consequently it is
difficult to quantify their global radiative forcing im-
pacts. No GWP values are attributed to these gases that
are short-lived and spatially inhomogeneous in the at-
mosphere. Other greenhouse gases not yet listed by the
Intergovernmental Panel on Climate Change (IPCC), but
are already or soon will be in commercial use include:
HFC-245fa, hydrofluoroethers (HFEs), and nitrogen
trifluoride (NF3).
Recent Trends in U.S. Greenhouse
Gas Emissions
Total U.S. greenhouse gas emissions rose in 1998
to l,834.6million metric tons of carbon equivalents
(MMTCE)20 (11.2 percent above 1990 baseline levels).
Table 1-1: Global Warming Potentials and
Atmospheric Lifetimes (Years)
IQas
Atmospheric Lifetime GWPa
tCarbon dioxide (C02)
JMethaneJCI-y13
aNitrous oxide (N20)
BEC-23
DiFC-125
|TiFC-T34a
StFC-143a
STHFC-152a
|HFC-227ea ,
SHFC-236fa
IHFC-43lOmee_ ".'
ITpc
IC^FB
it!4F10
BeF-H
f>~~ =— = • : — rr— — '• = '-•-.•
50-200
12±3
120
264
32.6
14.6
48.3
1.5
36.5
209
17.1
50,000
10,000
2,600
3,200
3,200
1
21
310
11,700
2,800
1,300
3,800
140
2,900
6,300
1,300
6,500
9,200
7,000
7,400
23,900
|§ource: (IPCC 1996)
||h 00 year time horizon
jjjhe methane GWP includes the direct effects and those
pndirect effects due to the production of tropospheric ozone and
fcsfratospheric water vapor. The indirect effect due to the
ftrbduction of C02 is not included.
K'. \-.i, ,/„,'_., .„„,„„ 1 J ., _,.' ; J_...-__^l-_.-J.
Figure 1-1
1750
1500-
1250
O 1000.
| 750
500
250
0 -
HFCs, PFCs, & SF6
Nitrous Oxide
1,6501.636
Methane
Carbon Dioxide
7AR 1,804 1,828 1,835
48 __ _
1990 1991 1992 1993 1994 1995 1996 1997 1998
19 Framework Convention on Climate Change; FCCC/CP/1996/15/Add.l; 29 October 1996; Report of the Conference of the Parties at
its second session; held at Geneva from 8 to 19 July 1996; Addendum; Part Two: Action taken by the Conference of the Parties at its
second session; Decision 9/CP.2; Communications from Parties included in Annex I to the Convention: guidelines, schedule and
process for consideration; Annex: Revised Guidelines for the Preparation of National Communications by Parties Included in Annex
I to the Convention; p. 18.
20 Estimates are presented in units of millions of metric tons of carbon equivalents (MMTCE), which weights each gas by its GWP value,
or Global Warming Potential (see previous section).
Introduction 1-7
-------
Figure 1-2
-1%
1991 1992 1993 1994 1995 1996 1997 1998
1991 1992 1993 1994 1995 1996 1997 1998
The single year increase in emissions from 1997 to 1998
was 0.4 percent (6.8 MMTCE), less than the 1.3 percent
average annual rate of increase for the 1990s. Figure 1-
1 through Figure 1-3 illustrate the overall trends in total
U.S. emissions by gas, annual changes, and absolute
changes since 1990.
As the largest source of U.S. greenhouse gas emis-
sions, CO2 from fossil fuel combustion, accounted for
80 percent of weighted emissions in 1998. Emissions
from this source grew by 11 percent (148.1 MMTCE)
from 1990 to 1998 and were also responsible for over 80
percent of the increase in national emissions during this
period. The annual increase in CO2 emissions from this
source was only 0.5 percent in 1998 lower than the
source's average annual rate of 1.3 percent during the
1990s despite a strong 3.9 percent increase in U.S. gross
domestic product.
In addition to economic growth, changes in CO2
emission from fossil fuel combustion are also correlated
with energy prices and seasonal temperatures. Excep-
tionally mild winter conditions in 1998 moderated
growth in CO2 emissions from fossil fuel combustion
below what would have been expected given the strength
of the economy and continued low fuel prices. Table 1-
2 shows annual changes in emissions during the last few
years of the 1990s for particular fuel types and sectors.
Carbon dioxide emissions from fossil fuel com-
bustion increased dramatically in 1996, due primarily
to two factors: 1) fuel switching by electric utilities
from natural gas to more carbon intensive coal as to
colder winter conditions and the associated rise in de-
mand for natural gas from residential, commercial and
industrial customers for heating caused gas prices to
rise sharply; and 2) higher consumption of petroleum
fuels for transportation. Milder weather conditions in
summer and winter moderated the growth in emissions
in 1997; however, the shut-down of several nuclear
power plants lead electric utilities to increase their con-
sumption of coal to offset the lost capacity. In 1998,
weather conditions were a dominant factor in slowing
the growth in emissions. Warm winter temperatures
resulted in a significant drop in residential, commer-
cial, and industrial natural gas consumption. This drop
in emissions from natural gas used for heating was pri-
marily offset by two factors: 1) electric utility emis-
sions, which increased in part due to a hot summer and
its associated air conditioning demand; and 2) in-
creased motor gasoline consumption for transportation.
Other significant trends in emissions from addi-
tional source categories over the nine year period from
1990 through 1998 included the following:
• Aggregate HFC and PFC emissions resulting from
the substitution of ozone depleting substances (e.g.,
CFCs) increased by 14.2 MMTCE. This increase
was partly offset, however, by reductions in PFC
emissions from aluminum production by 2.6
MMTCE (48 percent), which were the result of both
1 -8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Table 1-2: Annual Change in C02 Emissions from Fossil Fuel Combustion
for Selected Fuels and Sectors (MMTCE and Percent)
f-Sector
Electric Utility
t Electric Utility
•Jlectric Utility ,
j Transportation3
^Residential
iCommercial
i Industrial
: All Sectors"
Fuel Type
Coal
Petroleum
Natural Gas
Petroleum
Natural Gas
Natural Gas
Natural Gas
All Fuels"
;-a Excludes emissions from International Bunker
*£ Includes fuels and
sectors not shown in table.
1995-1996
24.5
1.4
(6.9)
13.8
5.8
1.9
4.7
49.4
Fuels.
5.7%
10.0%
(14.6%)
3.3%
8.1%
4.2%
3.4%
3.5%
1996-1997
14.3
2.2
3.3
1.1
(3.8)
0.9
(1.4)
19.4
3.1%
14.4%
8.1%
0.2%
(4.9%)
1.9%
(1.0%)
1.3%
1997-1998
5.5
7.3
4.2
7.2
(7.4)
(2.7)
(2.9)
7.5
1.2%
41 6%
9.8%
1 .7%
(10.0%)
(5.7%)
(2.0%) '•
0.5%
-----. - =
voluntary industry emission reduction efforts and
lower domestic aluminum production.
Combined N2O and CH4 emissions from mobile
combustion rose by 3.3 MMTCE (22 percent), pri-
marily due to increased rates of N2O generation in
highway vehicles.
Methane emissions from the manure management
activities have increased by 7.9 MMTCE (53 per-
cent) as the composition of the swine and dairy in-
dustries shift toward larger facilities. An increased
number of large facilities leads to an increased use
of liquid systems, which translates into increased
methane production.
Methane emissions from coal mining dropped by
6.2 MMTCE (26 percent) as a result of the mining of
less gassy coal from underground mines and the in-
creased use of methane from degasification systems.
Nitrous oxide emissions from agricultural soil man-
agement increased by 8.5 MMTCE (11 percent) as
fertilizer consumption and cultivation of nitrogen
fixing crops rose.
By 1998, all of the three major adipic acid produc-
ing plants had voluntarily implemented N2O abate-
ment technology; as a result, emissions fell by 3.0
MMTCE (60 percent). The majority of this decline
occurred from 1997 to 1998, despite increased pro-
duction.
Overall, from 1990 to 1998, total emissions of CO2,
CH4, and N2O increased by 153.7 (11 percent), 3.1 (2
percent), and 11.1 MMTCE (10 percent), respectively.
During the same period, weighted emissions of HFCs,
PFCs, and SF6 rose by 17.0 MMTCE (73 percent). De-
spite being emitted in smaller quantities relative to the
other principle greenhouse gases, emissions of HFCs,
PFCs, and SF6 are significant because of their extremely
high Global Warming Potentials and, in the cases of
PFCs and SF6, long atmospheric lifetimes. Conversely,
U.S. greenhouse gas emissions were partly offset by car-
bon sequestration in forests and in landfilled carbon,
which were estimated to be 12 percent of total emis-
sions in 1998.
As an alternative, emissions can be aggregated
across gases by the EPCC defined sectors, referred to
here as chapters. Over the nine year period of 1990 to
1998, total emissions in the Energy, Industrial Processes,
Agriculture, and Waste chapters climbed by 146.5 (10
percent), 18.5 (39 percent), 18.5 (14 percent), and 1.5
MMTCE (2 percent), respectively. Estimates of the quan-
tity of carbon sequestered in the Land-Use Change and
Forestry chapter, although based on projections, de-
clined by 105.5 MMTCE (33 percent).
Table 1-4 summarizes emissions and sinks from
all U.S. anthropogenic sources in weighted units of
MMTCE, while unweighted gas emissions and sinks in
gigagrams (Gg) are provided in Table 1-5. Alternatively,
emissions and sinks are aggregated by chapter in Table
Introduction 1-9
-------
Box 1 -1: Recent Trends in Various U.S. Greenhouse Gas Emissions-Related Data
There are several ways to assess a nation's greenhouse gas emitting intensity. These measures of intensity could be based on
aggregate energy consumption because energy-related activities21 are the largest sources of emissions, on fossil fuel consumption
only because almost all energy-related emissions involve the combustion of fossil fuels, on electricity consumption because electric
utilities were the largest sources of U.S. greenhouse gas emissions in 1998, on total gross domestic product as a measure of national
economic activity, or on a per capita basis. Depending upon which of these measures is used, the United States could appear to have
reduced orincreased its national greenhouse gas intensity. Table 1 -3 provides data on various statistics related to U.S. greenhouse gas
emissions normalized to 1990 as a baseline year. Greenhouse gas emissions in the U.S. have grown at an average annual rate of 1.3
percent since 1990. This rate is slightly slower than that for total energy or fossil fuel consumption thereby indicating an improved or
lower greenhouse gas emitting intensity and much slower than that for either electricity consumption or overall gross domestic
product. Emissions, however, are growing faster than national population, thereby indicating a worsening or higher greenhouse gas
emitting intensity on a per capita basis (see Figure 1-4). Overall, atmospheric C02 concentrations a function of many complex
anthropogenic and natural processes are increasing at 0.4 percent per year.
Table 1-3: Recent Trends in Various U.S. Data (Index 1990 = 100)
1991 1992 1993 1994 1995 1996 1997 1998 Growth
6HG Emissions8
Energy Consumption5
Fossil Fuel Consumption0
Electricity Consumption0
GDPd
Population8
Atmospheric C02 Concentration'
99
100
99
102
99
101
100
101
101
101
102
102
102
101
103
104
103
105
104
103
101
105
106
105
108
108
104
101
106
108
106
111
110
105
102
109
112
110
114
114
106
102
111
112
111
116
118
107
103
111
112
111
119
123
108
104
1.3%
1 .4%
1.4%
2.2%
2.6%
1.0%
0.4%
" 6WP weighted values
" Energy content weighted values. (DOE/EIA)
c {DOE/EIA)
a Gross Domestic Product in chained 1992 dollars (BEA 1999)
• (U.S. Census Bureau 1999)
1 Mauna Loa Observatory, Hawaii (Keeling and Whorf 1999)
» Average annual growth rate
Figure 1-4
106 n
102 -
98
90
1990 1991 1992 1993 1994 1995 1996 1997 1998
21 Energy-related activities are those that involve fossil fuel combustion (industrial, transportation, residential, and commercial end-use
sectors), and the production, transmission, storage, and distribution of fossil fuels.
1-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Table 1-4: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (MMTCE)
': Gas/Source
1990 1991 1992 1993 1994 1995 1996 1997 1998
C02
| Fossil Fuel Combustion
| Cement Manufacture
Natural Gas Flaring
! Lime Manufacture
: Waste Combustion
Limestone and Dolomite Use
> Soda Ash Manufacture and
Consumption
Carbon Dioxide Consumption
Land-Use Change and
Forestry (Sink)3
'. International Bunker Fuelsb
i CH4
Landfills
Enteric Fermentation
: Natural Gas Systems
Manure Management
Coal Mining
Petroleum Systems
Rice Cultivation
Stationary Sources
Mobile Sources
Wastewater Treatment
Petrochemical Production
Agricultural Residue Burning
Silicon Carbide Production
International Bunker Fuelsb
N20
Agricultural Soil Management
Mobile Sources
Nitric Acid
Stationary Sources
Manure Management
Human Sewage
Adipic Acid
Agricultural Residue Burning
Waste Combustion
International Bunker Fuelsb
HFCs, PFCs, and SF6
: Substitution of Ozone
Depleting Substances
HCFC-22 Production
Electrical Transmission and
Distribution
: Magnesium Production and
Processing
• Aluminum Production
Semiconductor Manufacture
Total Emissions
Net Emission (Sources and Sinks)
+ Does not exceed 0.05 MMTCE
1,340.3
1,320.1
9.1
2.5
3.0
2.8
1.4
1.1
0.2
(316.4)
32.2
177.9
58.2
32.7
33.0
15.0
24.0
7.4
2.4
2.3
1.5
0.9
0.3
0.2
+
+
108.2
75.3
13.8
4.9
3.8
3.4
2.0
5.0
0.1
0.1
0.3
23.3
0.3
9.5
5.6
1.7
5.4
0.8
1,649.7
1,333.3
1,326.1
1,305.8
8.9
2.8
3.0
3.0
1.3
1.1
0.2
(316.3)
32.7
177.7
58.1
32.8
33.4
15.5
22.8
7.5
2.3
2.4
1.5
0.9
0.3
0.2
+
+
110.5
76.3
14.6
4.9
3.8
3.6
2.0
5.2
0.1
0.1
0.3
22.0
0.2
8.4
5.9
2.0
4.7
0.8
1,636.2
1,320.0
1,350.4
1,330.1
8.9
2.8
3.1
3.0
1.2
1.1
0.2
(316.2)
30.0
179.4
59.1
33.2
33.9
16.0
22.0
7.2
2.6
2.4
1.5
0.9
0.3
0.2
+
+
113.3
78.2
15.7
5.0
3.9
3.5
2.0
4.8
0.1
0.1
0.3
23.5
0.4
9.5
6.2
2.2
4.4
0.8
1,666.6
1,350.5
1,383.3
1,361.5
9.4
3.7
3.1
3.1
1.1
1.1
0.2
(212.7)
27.2
178.7
59.6
33.7
34.6
17.1
19.2
6.9
2.4
2.4
1.5
0.9
0.4
0.1
+
+
113.8
77.3
16.5
5.1
3.9
3.7
2.0
5.2
0.1
0.1
0.2
23.8
1.4
8.7
6.4
2.5
3.8
1.0
1,699.7 1
1,487.0 1
1,404.8
1,382.0
9.8
3.8
3.2
3.1
1.5
1.1
0.2
(212.3)
26.7
181.6
59.9
34.5
34.3
18.8
19.4
6.7
2.7
2.4
1.5
0.9
0.4
0.2
+
+
121.5
83.5
17.1
5.3
4.0
3.8
2.1
5.5
0.1
0.1
0.2
25.1
2.7
8.6
6.7
2.7
3.2
1.1
,733.0
,520.7
1,416.5
1,392.0
10.0
4.7
3.4
3.0
1.9
1.2
0.3
(211.8)
27.5
184.1
60.5
34.9
34.0
19.7
20.3
6.7
2.6
2.5
1.4
0.9
0.4
0.2
+
118.8
80.4
17.4
5.4
4.0
3.7
2.1
5.5
0.1
0.1
0.2
29.0
7.0
7.4
7.0
3.0
3.1
1.5
1,748.5
1,536.6
1,466.2
1,441.3
10.1
4.5
3.6
3.1
2.0
1.2
0.3
(211.3)
27.9
183.1
60.2
34.5
34.6
20.4
18.9
6.5
2.4
2.6
1.4
0.9
0.4
0.2
121.5
82.4
17.5
5.6
4.2
3.8
2.1
5.7
0.1
0.1
0.2
33.5
9.9
8.5
7.0
3.0
3.2
1.9
1,804.4
1,593.1
1,486.4
1,460.7
10.5
4.2
3.7
3.4
2.3
1.2
0.4
(211.1)
29.9
183.8
60.2
34.2
34.1
22.1
18.8
6.5
2.6
2.3
1.4
0.9
0.4
0.2
122.4
84.2
17.3
5.8
4.2
3.9
2.1
4.7
0.1
0.1
0.3
35.3
12.3
8.2
7.0
3.0
3.0
1.9
1,827.9
1,616.8
1,494.0
1,468.2
10.7
3.9
3.7
3.5
2.4
1.2
0.4
(210.8)
31.3
180.9
58.8
33.7
33.6
22.9
17.8
6.3
2.7
2.3
1.3
0.9
0.4
0.2
119.4
83.9
17.2
5.8
4.3
4.0
2.2
2.0
0.1
0.1
0.3
40.3
14.5
10.9
7.0
3.0
2 8
2.1
1,834.6
1,623.8
; a Sinks are only included in net emissions total. Estimates of net carbon sequestration due to land-use change and forestry activities
exclude non-forest soils, and are based partially upon projections of forest carbon stocks.
; b Emissions from International Bunker Fuels are not
included in
totals.
Note: Totals may not sum due to independent rounding.
Introduction 1-11
-------
Table 1-5: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (Gg)
Gas/Source
1990
C02 4,914,351 4
Fossil Fuel Combustion 4,
Cement Manufacture
Natural Gas Flaring
Lime Manufacture
Waste Combustion
Limestone and Dolomite Use
Soda Ash Manufacture and
Consumption
Carbon Dioxide Consumption
840,483 4
33,278
9,097
11,092
10,345
5,113
4,144
800
Land-Use Change and Forestry (Sink)3 (1 ,1 59,994) (1 ,
International Bunker Fueisb 117,965
CH4
Landfills
Enteric Fermentation
Natural Gas Systems
Manure Management
Coal Mining
Petroleum Systems
Rice Cultivation
Stationary Sources
Mobile Sources
WastewaterTreatment
Petrochemical Production
Agricultural Residue Burning
Silicon Carbide Production
International Bunker Fuels"
H20
Agricultural Soil Management
Mobile Sources
Nitric Acid
Stationary Source
Manure Management
Human Sewage
Adipic Add
Agricultural Residue Burning
Waste Combustion
International Bunker Fuels'1
HFCs, PFCs, and SFS
Substitution of Ozone Depleting
Substances
Aluminum Production
HCFC-22 Production'
Semiconductor Manufacture
Electrical Transmission and
Distribution11
Magnesium Production and Processing11
NO,
CO
NMVOCs
31,054
10,171
5,712
5,770
2,613
4,184
1,294
414
404
257
150
56
30
1
2
1,280
891
163
58
45
40
23
59
1
1
3
M
M
M
3
M
1
+
21,798
85,394
18,795
1991
,862,349
,787,926
32,535
10,295
10,891
10,931
4,896
4,035
840
,159,646)
120,019
31,020
10,152
5,732
5,840
2,708
3,975
1,307
404
410
255
152
57
28
1
2
1,307
903
172
58
45
42
24
62
1
1
3
M
M
M
3
M
1
+
21,936
87,485
18,929
1992
4,951,561
4,876,887
32,792
10,169
11,245
10,993
4,502
4,091
882
(1,159,299)
109,965
31,329
10,321
5,804
5,923
2,801
3,835
1,262
453
425
257
154
60
33
1
2
1,340
925
185
59
46
42
24
57
1
1
3
M
M
M
3
M
1
+
22,176
84,589
18,527
1993
5,072,271
4,992,123
34,624
13,716
11,496
11,295
4,058
4,048
912
(779,935)
99,886
31,203
10,402
5,876
6,042
2,990
3,356
1,206
414
415
255
155
66
26
1
2
1,346
914
195
60
46
43
24
61
1
1
3
M
M
M
3
M
1
+
22,398
84,716
18,708
1994
1995
1996
5,150,787 5,193,841 5,376,081 5
5,067,248 5,103,838 5,284,901 5
36,087
13,800
11,895
11,308
5,541
4,012
898
(778,285)
98,017
31,711
10,452
6,016
5,987
3,283
3,390
1,175
476
416
253
157
70
34
1
2
1,437
988
202
63
47
44
25
65
1
1
3
M
M
M
3
M
1
+
22,683
88,911
19,290
36,847
17,164
12,624
11,104
6,987
4,309
968
(776,659)
101,014
32,147
10,566
6,094
5,931
3,447
3,550
1,168
445
437
251
158
72
28
1
2
1,406
951
206
64
48
44
25
66
1
1
3
M
M
M
2
M
1
+
22,177
80,093
18,613
37,079
16,506
13,179
11,504
7,499
4,273
1,140
1997
,449,974 5
,355,900 5
38,323
15,521
13,434
12,532
8,537
4,434
1,294
1998
,478,051
,383,502
39,227
14,214
13,627
12,889
8,854
4,325
1,413
(774,725) (774,083) (773,019)
102,197 109,788 114,700
31,972
10,508
6,032
6,041
3,567
3,301
1,143
420
446
246
160
75
32
1
2
1,437
975
207
67
49
45
25
67
1
1
3
M
M
M
3
M
•1
+
22,034
82,028
17,624
32,084
10,510
5,973
5,961
3,861
3,274
1,142
453
399
239
161
77
34
1
2
1,448
996
205
68
50
46
25
55
1
1
3
M
M
M
3
M
1
+
22,153
79,284
17,469
31,593
10,268
5,885
5,860
3,990
3,104
1,108
476
395
232
163
77
35
1
2
1,412
992
203
68
50
47
25
23
1
1
3
M
M
M
3
M
1
+
22,066
78,082
17,011
+ Does not exceed 0.5 Gg
M Mixture of multiple gases
• Sinks are not Included in C02 emissions total. Estimates of net carbon sequestration due to land-use change and forestry activities exclude non-forest soils, and are based
partially upon projections of forest carbon stocks.
B Emissions from International Bunker Fuels are not included in totals.
eHFC-23 emitted
*S?t emitted
Note; Totals may not sum due to independent rounding.
1-12 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
1-6 and Figure 1-5.
Methodology and Data Sources
Emissions of greenhouse gases from various
sources have been estimated using methodologies that
are consistent with the Revised 1996 IPCC Guidelines
for National Greenhouse Gas Inventories (IPCC/UNEP/
OECD/ffiA 1997). To the extent possible, the present
U.S. Inventory relies on published activity and emis-
sion factor data. Depending on the emission source
category, activity data can include fuel consumption or
deliveries, vehicle-miles traveled, raw material processed,
etc.; emission factors are factors that relate quantities of
emissions to an activity. For some sources, IPCC default
methodologies and emission factors have been employed.
However, for most emission sources, the IPCC default
Figure 1-5
methodologies were expanded and more comprehensive
methods were applied.
Inventory emission estimates from energy con-
sumption and production activities are based primarily
on the latest official fuel consumption data from the
Energy Information Administration (EIA) of the U.S. De-
partment of Energy. Emission estimates for NOX, CO,
and NMVOCs were taken directly, except where noted,
from the United States Environmental Protection
Agency's (EPA) report, National Air Pollutant Emission
Trends 1900 -1998 (EPA 1999), which is an annual EPA
publication that provides the latest estimates of regional
and national emissions of criteria pollutants. Emissions
of these pollutants are estimated by the EPA based on
statistical information about each source category, emis-
sion factors, and control efficiencies. While the EPA's
estimation methodologies for criteria pollutants are con-
ceptually similar to the IPCC recommended methodolo-
gies, the large number of sources EPA used in develop-
ing its criteria pollutant estimates makes it difficult to
reproduce the methodologies from EPA (1999) in this
inventory document. In these instances, the references
containing detailed documentation of the methods used
are identified for the interested reader. For agricultural
sources, the EPA criteria pollutant emission estimates
were supplemented using activity data from other agen-
cies. Complete documentation of the methodologies
and data sources used is provided in conjunction with
the discussion of each source and in the various annexes.
Emissions from fossil fuels combusted in ships and
aircraft engaged in the international transport of passen-
gers and cargo are not included in U.S. totals, but are
Table 1 -6: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks by Chapter/IPCC Sector (MMTCE)
Waste
Agriculture
Industrial
Processes
Land-Use
Change &
Forestry
(sink)
Chapter/IPCC Sector
1990 1991 1992 1993 1994 1995 1996 1997 1998
Energy
Industrial Processes
Agriculture
Land-Use Change and Forestry (Sink)*
Waste
Total Emissions
Net Emissions (Sources
and Sinks)
1,408.4
48.3
129.0
(316.4)
64.0
1,649.7
1,333.3
1,394.5
46.9
130.8
(316.3)
64.1
1,636.2
1,320.0
1 41 q
48
133
(316.
65
1,666
1,350
4
.3
.9
2)
.1
.6
.5
1,450.2
49.5
134.3
(212.7)
65.7
1,699.7
1,487.0
1,471.1
52.3
143.5
(212.3)
66.0
1,733.0
1,520.7
1,483.0
57.2
141.6
(211.8)
66.6
1,748.5
1,536.6
1,531.5
62.5
143.9
(211.3)
66.4
1,804.4
1,593.1
1,549.5
64.2
147.4
(211.1)
66.7
1,827.9
1,616.8
1,554.8
66.9
147.5
(210.8)
65.5
1,834.6
1,623.8
* Sinks are only included in net emissions total. Estimates of net carbon sequestration due to land-use change and forestry activities
exclude non-forest soils, and are based partially upon projections of forest carbon stocks.
Note: Totals may not sum due to independent rounding.
Introduction 1-13
-------
reported separately as international bunkers in accor-
dance with IPCC reporting guidelines (IPCC/UNEP/
OECD/D3A 1997). Carbon dioxide emissions from fuel
combusted within U.S. territories, however, are included
in U.S. totals.
Uncertainty in and
Limitations of Emission Estimates
While the current U.S. emissions inventory pro-
vides a solid foundation for the development of a more
detailed and comprehensive national inventory, it has
several strengths and weaknesses.
First, this inventory by itself does not provide a
complete picture of past or future emissions in the United
States; it only provides an inventory of U.S. emissions
for the years 1990 through 1998. However, the United
States believes that common and consistent inventories
taken over a period of time can and will contribute to
understanding future emission trends. The United States
produced its first comprehensive inventory of greenhouse
gas emissions and sinks in 1993, and intends to update
it annually, in conjunction with its commitments under
the UNFCCC. The methodologies used to estimate emis-
sions will also be updated periodically as methods and
information improve and as further guidance is received
from the IPCC and UNFCCC.
Secondly, there are uncertainties associated with
the emission estimates. Some of the current estimates,
such as those for CO2 emissions from energy-related ac-
tivities and cement processing, are considered to be fairly
accurate. For other categories of emissions, however, a
lack of data or an incomplete understanding of how emis-
sions are generated limits the scope or accuracy of the
estimates presented. Despite these uncertainties, the
Revised 1996 IPCC Guidelines for National Greenhouse
Box 1-2: Greenhouse Gas Emissions from Transportation Activities
Motor vehicle usage is increasing all over the world, including in the United States. Since the 1970s, the number of highway
vehicles registered in the United States has increased fasterthan the overall population, according to the Federal Highway Administra-
tion. Likewise, the number of miles driven up 21 percent from 1990 to 1998 and gallons of gasoline consumed each year in the United
States have increased relatively steadily since the 1980s, according to the Energy Information Administration. These increases in
motor vehicle usage are the result of a confluence of factors including population growth, economic growth, increasing urban sprawl,
and low fuel prices.
One of the unintended consequences of these changes is a slowing of progress toward cleaner air in both urban and rural parts
of the country. Passenger cars, trucks, motorcycles, and buses emit significant quantities of air pollutants with local, regional, and
global effects. Motor vehicles are major sources of carbon monoxide (CO), carbon dioxide (C02), methane (CH4), nonmethane
volatile organic compounds (NMVOCs), nitrogen oxides (NOX), nitrous oxide (N20), and hydrofluorocarbons (HFCs). Motor vehicles
are also important contributors to many serious air pollution problems, including ground-level ozone (i.e., smog), acid rain, fine
paniculate matter, and global warming. Within the United States and abroad, government agencies have taken strong actions to
reduce these emissions. Since the 1970s, the EPA has reduced lead in gasoline, developed strict emission standards for new
passenger cars and trucks, directed states to enact comprehensive motor vehicle emission control programs, required inspection and
maintenance programs, and more recently, introduced the use of reformulated gasoline to mitigate the air pollution impacts from motor
vehicles. New vehicles are now equipped with advanced emissions controls, which are designed to reduce emissions of nitrogen
oxides, hydrocarbons, and carbon monoxide.
Table 1-7 summarizes greenhouse gas emissions from all transportation-related activities. Overall, transportation activities exclud-
ing international bunker fuels accounted for an almost constant 26 percent of total U.S. greenhouse gas emissions from 1990 to
1998. These emissions were primarily C02 from fuel combustion, which increased by 11 percent from 1990 to 1998. However,
because of larger increases in N20 and HFC emissions during this period, overall emissions from transportation activities actually
increased by 12 percent.
1-14 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Table 1-7: Transportation-Related Greenhouse Gas Emissions (MMTCE)
Gas/Vehicle Type
1990
1991
1992 1993 1994 1995
1996 1997 1998
C02
: Passenger Gars
Light-Duty Trucks
Other Trucks
Buses
Aircraft3
, Boats and Vessels
Locomotives
• Other"
: International Bunker Fuels0
CH4
: Passenger Cars
Light-Duty Trucks
Other Trucks and Buses
Aircraft
Boats and Vessels
Locomotives
Other"
International Bunker Fuels0
N20
< Passenger Cars
; Light-Duty Trucks
Other Trucks and Buses
i Aircraft"
; Boats and Vessels
Locomotives
: Other"
International Bunker Fuels0
HFCs
Mobile Air Conditioners6
; Total0
399.6
169.1
77.4
56.3
2.7
48.2
15.1
7.3
23.6
32.2
1.5
0.7
0.5
0.1
+
+
+
0.1
+
13.8
8.1
4.2
0.6
0.5
0.1
0.1
0.2
0.3
+
+
414.8
391.5
167.6
77.1
54.2
2.8
46.1
14.4
6.8
22.3
32.7
1.5
0.7
0.5
0.1
+
+
+
0.1
+
14.6
8.0
5.1
. 0.7
0.5
0.1
0.1
0.2
0.3
+
+
407.5
401.1
171.7
77.1
55.9
2.9
45.5
18.5
7.3
22.3
30.0
1.5
0.6
0.6
0.1
+
+
+
0.1
+
15.7
8.4
5.8
0.7
0.5
0.1
0.1
0.2
0.3
0.2
0.2
418.5
409.1
173.3
80.4
59.1
3.0
45.8
17.3
6.7
23.6
27.2
1.5
0.6
0.6
0.2
+
+
+
0.1
+
16.5
8.6
6.4
0.7
0.5
0.1
0.1
0.2
0.2
0.7
0.7
427.8
422.3
172.2
87.1
62.1
3.3
48.0
17.0
7.9
24.8
26.7
1.5
0.6
0.6
0.2
+
+
+
0.1
+
17.1
8.8
6.6
0.8
0.5
0.1
0.1
0.2
0.2
1.8
1.8
442.7
427.7
175.0
88.9
63.6
3.5
46.8
17.0
8.1
24.8
27.5
1.4
0.6
0.6
0.2
+
+
0.1
+
17.4
8.9
6.8
0.8
0.5
0.1
0.1
0.2
0.2
2.6
2.6
449.2
441.7
178.5
91.1
67.7
3.0
49.1
18.1
8.7
25.4
27.9
1.4
0.6
0.5
0.2
+
+
0.1
17.5
8.9
6.8
0.9
0.5
0.1
0.1
0.2
0.2
3.7
3.7
464.3
443.4
180.0
92.1
70.1
3.2
48.8
13.7
9.0
26.5
29.9
1.4
0.5
0.5
0.2
+
,
0.1
17.3
8.7
6.8
0.9
0.5
0.1
0.1
0.2
0.3
4.7
4.7
466.8
450.2
185.1
94.6
70.3
3.2
49.4
12.5
9.0
26.3
31.3
1.3
0.5
0.5
0.2
+
0.1
17.2
8.6
6.8
1.0
0.5
0.1
0.1
0.2
0.3
4.7
4.7
473.5
+ Does not exceed 0.05 MMTCE
Note: Totals may not sum due to independent rounding.
a Aircraft emissions consist of emissions from all jet fuel (less bunker fuels) and aviation gas consumption.
b "Other" C02 emissions include motorcycles, construction equipment, agricultural machinery, pipelines, and lubricants.
f Emissions from International Bunker Fuels include emissions from both civilian and military activities, but are not included in totals.
d "Other" CH4 and N20 emissions include motorcycles, construction equipment, agricultural machinery, gasoline-powered recreational,
industrial, lawn and garden, light commercial, logging, airport service, other equipment; and diesel-powered recreational, industrial, lawn and
.garden, light construction, airport service.
8 Includes primarily HFC-134a
Box 1-3: Greenhouse Gas Emissions from Electric Utilities
*\ Like transportation, activities related to the generation, transmission, and distribution of electricity in the United States result in
.y significant greenhouse gas emissions. Table 1 -8 presents greenhouse gas emissions from electric utility-related activities. Aggregate
Remissions from electric utilities of all greenhouse gases increased by 15 percent from 1990 to 1998, and accounted for a relatively
^constant 29 percent of U.S. greenhouse emissions during the same period.22 The majority of these emissions resulted from the
kcombustion of coal in boilers to produce steam that is passed through a turbine to generate electricity. Overall, the generation of
^electricity results in a larger portion of total U.S. greenhouse gas emissions than any other activity.
22 Emissions from nonutility generators are not included in these estimates. Nonutilties were estimated to produce about 10 percent of
the electricity generated in the United States in 1998 (DOE and EPA 1999).
Introduction 1-15
-------
Table 1-8: Electric Utility-Related Greenhouse Gas Emissions (MMTCE)
Gas/Fuel Type or Source
»C02
Coal
," Natural Gas
Petroleum
Geothermal
CH4
Stationary Combustion (Utilities)
N20
Stationary Combustion (Utilities)
SF6
' Electrical Transmission and Distribution
Total
1990
476.6
409.0
41.2
26.4
0.1
0.1
0.1
2.0
2.0
5.6
5.6
484.3
19E
473
407
41
24
0
0
0
2
2
5
5
481
11
.2
.2
.1
.9
.1
.1
.1
.0
.0
.9
.9
.2
1992
472.7
411.8
40.7
20.2
0.1
0.1
0.1
2.0
2.0
6.2
6.2
481.0
1993
490.5
428.7
39.5
22.3
0.1
0.1
0.1
2.1
2.1
6.4
6.4
499.1
19£
493
429
44
20
0
0
2
2
b
6
502
14
.9
.5
.0
.5
+
.1
.1
.1
.1
./
.7
.9
1995
494.0
433.0
47.2
13.9
+
0.1
0.1
2.1
2.1
7.0
7.0
503.2
1996
513.0
457.5
40.3
15.3
+
0.1
0.1
2.2
2.2
7.0
7.0
522.4
1997
532.8
471.8
43.6
17.5
+
0.1
0.1
2.3
2.3
7.0
7.0
542.2
1998
549.9
477.3
47.8
24.8
+
0.1
0.1
2.3
2.3
7.0
7.0
559.3
+ Does not exceed 0.05 MMTCE
'Note: Totals may not sum due to independent rounding. Values do not include emissions from nonutility generators.
Gas Inventories (TPCC/UNEP/OECD/TEA1997) require
that countries provide single point estimates for each
gas and emission or removal source category. Within
the discussion of each emission source, specific factors
affecting the accuracy of the estimates are discussed.
Finally, while the TPCC methodologies provided
in the Revised 1996IPCC Guidelines represent baseline
methodologies for a variety of source categories, many
of these methodologies continue to be improved and
refined as new research and data becomes available. The
current U.S. inventory uses the IPCC methodologies
when applicable, and supplements them with other avail-
able methodologies and data where possible. The United
States realizes that additional efforts are still needed to
improve methodologies and data collection procedures.
Specific areas requiring further research include:
Incorporating excluded emission sources. Quan-
titative estimates of some of the sources and sinks of
greenhouse gas emissions are not available at this time.
In particular, emissions from some land-use activities
and industrial processes are not included in the inven-
tory either because data are incomplete or because meth-
odologies do not exist for estimating emissions from
these source categories. See Annex P for a discussion of
the sources of greenhouse gas emissions and sinks ex-
cluded from this report.
Improving the accuracy of emission factors. Fur-
ther research is needed in some cases to improve the
accuracy of emission factors used to calculate emissions
from a variety of sources. For example, the accuracy of
current emission factors applied to methane and nitrous
oxide emissions from stationary and mobile combustion
is highly uncertain.
Collecting detailed activity data. Although meth-
odologies exist for estimating emissions for some sources,
problems arise in obtaining activity data at a level of
detail in which aggregate emission factors can be ap-
plied. For example, the ability to estimate emissions of
methane and nitrous oxide from jet aircraft is limited
due to a lack of activity data by aircraft type and number
of landing and take-off cycles.
Applying Global Warming Potentials. GWP val-
ues have several limitations including that they are not
applicable to unevenly distributed gases and aerosols
such as tropospheric ozone and its precursors. They are
also intended to reflect global averages and, therefore,
do not account for regional effects. Overall, the main
uncertainties in developing GWP values are the estima-
tion of atmospheric lifetimes, assessing indirect effects,
choosing the appropriate integration time horizon, and
assessing instantaneous radiative forcing effects which
are dependent upon existing atmospheric concentra-
tions. According to the IPCC, GWPs typically have an
uncertainty of ±35 percent (IPCC 1996).
1-16 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Emissions calculated for the U.S. inventory reflect
current best estimates; in some cases, however, estimates
are based on approximate methodologies, assumptions,
and incomplete data. As new information becomes avail-
able in the future, the United States will continue to
improve and revise its emission estimates.
Organization of Report
In accordance with the IPCC guidelines for report-
ing contained in the Revised 1996 IPCC Guidelines for
National Greenhouse Gas Inventories (IPCC/UNEP/
OECD/IEA1997), this U.S. inventory of greenhouse gas
emissions and sinks is segregated into six sector-spe-
cific chapters, listed below in Table 1-9.
Within each chapter, emissions are identified by
the anthropogenic activity that is the source or sink of
the greenhouse gas emissions being estimated (e.g., coal
mining). Overall, the following organizational struc-
ture is consistently applied throughout this report:
Chapter/I PCC Sector: Overview of
emission trends for each IPCC defined sector
Source: Description of source pathway and emis-
sion trends from 1990 through 1998
— Methodology: Description of analytical
methods employed to produce emission esti-
mates
— Data Sources: Identification of data ref-
erences, primarily for activity data and emis-
sion factors
Table 1-9: IPCC Sector Descriptions
| Chapter/IPCC Sector Activities Included
— Uncertainty: Discussion of relevant is-
sues related to the uncertainty in the emission
estimates presented
Special attention is given to carbon dioxide from
fossil fuel combustion relative to other sources because
of its share of emissions relative to other sources and its
dominant influence on emission trends. For example,
each energy consuming end-use sector (i.e., residential,
commercial, industrial, and transportation), as well as
the electric utility sector, are treated individually. Ad-
ditional information for certain source categories and
other topics is also provided in several Annexes listed
in Table 1-10.
Changes in This Year's U.S.
Greenhouse Gas Inventory Report
Each year the EPA not only recalculates and re-
vises the emission estimates for all years that are pre-
sented in the Inventory of U.S. Greenhouse Gas Emis-
sions and Sinks but also attempts to improve the analy-
ses themselves through the use of better methods or data.
A summary of this year's changes is presented in the
following three sections and includes updates to histori-
cal data, changes in methodology, and other changes.
The magnitude of each change is also described.
Changes to historical data are generally due to
statistical data supplied by other agencies. Data sources
are provided for further reference.
For methodological changes, differences between
~ TEnergy
;" - Industrial Processes
t-_
^Solvent Use
i*-
Ir-...
' Agriculture
Emissions of all greenhouse gases resulting from stationary and mobile energy activities
including fuel combustion and fugitive fuel emissions.
By-product or fugitive emissions of greenhouse gases from industrial processes not directly
related to energy activities such as fossil fuel combustion.
Emissions, of primarily non-methane volatile organic compounds (NMVOCs), resulting from
the use of solvents.
Anthropogenic emissions from agricultural activities except fuel combustion and sewage
| emissions, which are addressed under Energy and Waste, respectively.
?; Land-Use Change and Forestry Emissions and removals from forest and land-use change activities, primarily carbon dioxide.
| Waste Emissions from waste management activities.
pource: (IPCC/UNEP/OECD/IEA 1997)
Introduction 1-17
-------
Table 1-10: List of Annexes
I ANNEX A Methodology for Estimating Emissions of C02 ANNEX H
I from Fossil Fuel Combustion
: ANNEX B Methodology for Estimating Emissions of CH4, ANNEX I
N20, and Criteria Pollutants from Stationary
= Combustion ANNEX J
= ANNEX C Methodology for Estimating Emissions of CH4,
" N20, and Criteria Pollutants from Mobile ANNEX K
' Combustion ANNEX L
• ANNEX D Methodology for Estimating Methane Emissions ANNEX M
: from Coal Mining ANNEX N
= ANNEX E Methodology for Estimating Methane Emissions ANNEX 0
" from Natural Gas Systems
> ANNEX F Methodology for Estimating Methane Emissions ANNEX P
i from Petroleum Systems ANNEX Q
ANNEX 6 Methodology for Estimating Emissions from ANNEX R
International Bunker Fuels Used by the U.S. ANNEX S
; Military ANNEX T
the previous Inventory and this Inventory are explained.
Many of the changes in methodology are due to a recent
series of IPCC good practice workshops held to assist in
the preparation of greenhouse gas inventories and in the
implementation of the Revised 1996 IPCC Guidelines
(IPCC/UNEP/OECD/IEA1997). Unless otherwise noted,
the methodological changes incorporated into this year's
Inventory reflect the recommendations of experts at these
IPCC good practice workshops. In general, when meth-
odological changes have been implemented, the entire
time series (1990 through 1998) has been recalculated
to reflect the change.
Changes in Historical Data
• In the CO2 Emissions from Fossil Fuel Combustion
section of the Energy chapter, most differences, as
compared to previous emission estimates, are due to
revised energy consumption data from the Energy
Information Administration (EIA 1999a, 1999c,
1999d) for selected years (see below for detail on an
additional small methodological change). In addi-
tion, a small error in estimates of CO2 emissions
from combustion of petroleum used for transporta-
tion has been corrected in this Inventory. Previ-
ously, the combustion efficiency had been inadvert-
ently applied to bunker fuel emissions prior to re-
moving them from the calculation of CO2 emissions
from petroleum used for transportation. In the cur-
rent Inventory, the combustion efficiency is correctly
Methodology for Estimating Methane Emissions
from Enteric Fermentation
Methodology for Estimating Methane Emissions
from Manure Management
Methodology for Estimating Methane Emissions
from Landfills _
Global Warming Potential Values
Ozone Depleting Substance Emissions
Sulfur Dioxide Emissions
Complete List of Sources
IPCC Reference Approach for Estimating C02
Emissions from Fossil Fuel Combustion
Sources of Greenhouse Gas Emissions Excluded
Constants, Units, and Conversions
Abbreviations
Chemical Symbols
Glossary r
applied once to all emissions after the subtraction
of bunker fuels. The combined data and method-
ological changes resulted in an average decrease
of 4.3 MMTCE (0.3 percent) in annual CO2 emis-
sions from fossil fuel combustion for 1990 through
1997.
In the Stationary Combustion (excluding CO2) sec-
tion of the Energy Chapter, differences from previ-
ous emission estimates are due to revised energy
consumption data from the EIA (1999a, 1999d) for
selected years. This revision resulted in an increase
of less than 0.1 MMTCE (0.6 percent) in annual
CH4 emissions and an average increase of less than
0.1 MMTCE (0.7 percent) in annual N2O emissions
from stationary combustion for 1990 through 1997.
In the Mobile Combustion (excluding CO2) sec-
tion of the Energy Chapter, differences with previ-
ous emission estimates for highway sources are due
to revised estimates of historical vehicle-miles-trav-
eled by the Federal Highway Administration (FHWA
1999). Extremely small differences exist in the
non-highway estimates due to revised historical
fuel consumption data from EIA (1999a, 1999c)
and FHWA (1999). These revisions caused an av-
erage increase of less than 0.1 MMTCE (3.0 per-
cent) in annual CH4 emissions and an increase of
0.3 MMTCE (1.9 percent) in annual N2O emissions
from mobile combustion for 1990 through 1997.
In the Natural Gas Systems section of the Energy
1-18 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Chapter, historical estimates of methane emissions
are revised based on the transmission pipeline mile-
age reported by the Office of Pipeline Safety (OPS).
Inventories in previous years relied on the American
Gas Association (AGA) for transmission pipeline
mileage data. AGA aggregates pipeline mileage data
as reported in FERC Form 2 "Annual Report of Natu-
ral Gas Pipeline Companies"; however, only inter-
state pipeline companies regulated by FERC sub-
mit Form 2. In contrast, OPS data is for all compa-
nies with more than one mile of transmission pipe-
line and includes both intra- and interstate pipe-
lines. Accordingly, OPS reports pipeline mileage
that is higher than that reported by AGA. Using the
new data, EPA recalculated historical emission esti-
mates, which resulted in increases for most years.
The historical emission estimates have increased an
average of 0.7 MMTCE (1.5 percent) in annual CH4
emissions for 1990 through 1997.
In the Natural Gas Flaring and Criteria Pollutant
Emissions in the Oil and Gas Activities section of
the Energy chapter, changes in emission estimates
for natural gas flaring are due to revisions in data
from the EIA (1999e). These revisions caused an
average increase of 0.2 MMTCE (5.8 percent) in
CO2 annual emissions for 1990 through 1997. The
EPA (1999b) has also revised estimates for criteria
pollutants from oil and gas activities for 1996 and
1997. These revisions resulted in average increases
of 3.5 percent in annual NOx emissions, and 3.9
percent in CO annual emissions from 1990 through
1997, and 1.0 percent in annual NMVOCs emissions
froml990 through 1997.
In the International Bunker Fuels section of the
Energy chapter, a small error in the 1990-1997 In-
ventory is corrected in this volume. Emissions from
combustion of distillate fuel in marine bunkers were
misreported by two years in that edition, presenting
1988 estimates for 1990, 1989 estimates for 1991,
and so forth. In addition, the activity data for for-
eign airlines at U.S. airports in 1997 have been ad-
justed slightly (BEA 1999). The combined data and
methodological changes resulted in an average de-
creases of 2.0 MMTCE (7.4 percent) in annual CO2
emissions, less than 0.1 MMTCE (10.9 percent) in
annual CH4 emissions and less than 0.1 MMTCE
(8.3 percent) in annual N2O emissions from interna-
tional bunker fuels for 1990 through 1997.
• In the Limestone and Dolomite Use section of the
Industrial Processes chapter, the 1997 value for lime-
stone and dolomite consumption was revised by the
United States Geological Survey (USGS 1999). This
data change resulted in an increase of 0.2 MMTCE,
or 9.3 percent, of CO2 emissions from limestone and
dolomite use in 1997.
• In the Carbon Dioxide Consumption section of the
Industrial Processes chapter, the 1997 value was re-
vised. The reference (Freedonia 1999) does not pro-
vide data for 1997, so it has been extrapolated using
annual growth rates from confkmed 1993 through
1996 values. Previously, the growth in CO2 produc-
tion was also applied to calculate CO2 used in EOR
applications. However, this year's data shows that
Freedonia holds EOR constant for 1996-1998. This
revision in data resulted in an average increase of
less than 0.1 MMTCE, or 5.3 percent, of CO2 emis-
sions from CO2 consumption for 1997.
• In the Petrochemical Production section of the In-
dustrial Processes chapter, the differences between
the 1990-1997 Inventory and this volume reflect
updated production data for ethylene, ethylene
dichloride, and methanol from the Chemical Manu-
facturers Association (CMA 1999). These updates
caused an average increase of less than 0.1 MMTCE
(1.5 percent) in annual CH4 emissions from petro-
chemical production for 1994 through 1997.
• In the Substitution of Ozone Depleting Substances
section of the Industrial Processes chapter, a review
of the current chemical substitution trends with in-
put from industry representatives resulted in updated
assumptions for the Vintaging Model, particularly
in the stationary refrigeration and foams sectors.
These updates resulted in an average decrease of 2.0
MMTCE (22.7 percent) in aggregate HFC, PFC, and
SF6 emissions from substitution of ozone depleting
substances for 1994 through 1997.
• In the Enteric Fermentation section of the Agricul-
ture chapter, the emission estimates for the 1990-
Introduction 1-19
-------
1998 Inventory have been recalculated using up-
dated animal population data. Specifically, animal
population data for 1994 through 1997 were up-
dated to reflect the recent publication of final live-
stock population estimates by USDA (1999a-h, n).
Also, horse population data for 1990 through 1998
were updated to reflect revised data from the Food
and Agriculture Organization (FAO 1999). The dairy
cow emission factors were also updated to reflect
revised milk production data. These data modifica-
tions caused an average increase of less than 0.1
MMTCE (less than 0.1 percent) in annual CH4 emis-
sions from enteric fermentation for 1990 through
1997.
Methodological Changes
Carbon Dioxide Emissions
from Fossil Fuel Combustion
The emission factor used to calculate emissions
from the combustion of residual fuel at electric utilities
was updated to 21.29 MMTCE/QBtu, based on new data
that EIA received from electric utilities (EIA 1999b).
The emission factor for residual fuel for all other sectors
remains at 21.49 MMTCE/QBtu.
Additionally, non-bunker jet fuel emissions from
military vehicles for 1990-1998, which are accounted
for under the transportation end-use sector, have been
estimated for the first time in this inventory. Data on jet
fuel expenditures by the U.S. military was supplied by
the Office of the Under Secretary of Defense (Environ-
mental Security), U.S. Department of Defense (DoD). Data
on fuel delivered to the military within the U.S. was pro-
vided from unpublished data by the Defense Energy
Support Center, under DoD's Defense Logistics Agency.
The quantity of fuel used was estimated using these data
sources. Jet fuel densities for each fuel type were ob-
tained from the Air Force (1998). The combined data
and methodological changes resulted in an average de-
crease of 4.3 MMTCE (0.3 percent) of CO2 annual emis-
sions from fossil fuel combustion for 1990 through 1997.
Petroleum Systems
EPA has restated the emissions of methane from
petroleum systems for 1998 and previous years, result-
ing in a substantial, 5.4 MMTCE, almost four-fold in-
crease in the estimate in CH4 from 1990 through 1997.
The new, higher estimate of methane emissions from pe-
troleum systems is based on work sponsored by EPA and
presented in Estimates of Methane Emissions from the
U.S. Oil Industry (EPA 1999a). Where the previous esti-
mates of methane emissions from the petroleum industry
used emission and activity factors based on top-down,
broad categories of activities, the revised approach is
based on a more detailed, bottom-up analysis of 70 dif-
ferent crude oil handling and processing activities from
the wellhead to refining.
The overall approach to these new petroleum sec-
tor estimates is now consistent with the detailed, bot-
tom-up analysis that has been used for several years to
estimate methane emissions from the natural gas indus-
try. As with natural gas, the new approach to estimating
methane emissions from petroleum systems is based on
a detailed characterization of the petroleum sector, which
describe the emissions producing sources within the
sector. Under this approach, EPA has developed emis-
sions factors for each emission producing activity that
describes the rate of annual emissions per activity. The
emissions factors derive largely from Radian Interna-
tional LLC (Radian 1996e). Other sources of emissions
factors include data from various reports and documents
of the American Petroleum Institute, EPA, Minerals Man-
agement Service (MMS) reports, Gas Research Institute
(GRI), Canadian Association of Petroleum Producers
(CAPP), and various industry peer review panels. Activ-
ity factors are used to generalize the emissions to the
entire industry and are multiplied by the emission fac-
tors to generate the total emissions estimates. The ma-
jor sources of activity factors include various reports
from the Energy Information Administration (EIA), API,
Radian, EPA, MMS, the Oil &Gas Journal, and peer
review panels.
International Bunker Fuels
1 -20 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
International bunker fuel emissions resulting from
military aviation and marine activities for 1990-1998
have been estimated for the first time in this inventory.
Data on jet fuel expenditures by the U.S. military was
supplied by the Office of the Under Secretary of Defense
(Environmental Security), U.S. Department of Defense
(DoD). Estimates of the percentage of each services'
total operations that are international operations were
developed by DoD. Military aviation bunkers include
international operations, operations conducted from na-
val vessels at sea, and operations conducted from U.S.
installations principally over international water in di-
rect support of military operations at sea. Data on fuel
delivered to the military within the U.S. was provided
from unpublished data by the Defense Energy Support
Center, under DoD's Defense Logistics Agency. Together,
the data allow the quantity of fuel used in military inter-
national operations to be estimated. Jet fuel densities for
each fuel type were obtained (Air Force 1998). The com-
bined data and methodological changes resulted in an
average decreases of 2.0 MMTCE (7.4 percent) in an-
nual CO2 emissions, less than 0.1 MMTCE (10.9 per-
cent) in annual CH4 emissions and less than 0.1 MMTCE
(8.3 percent) in annual N2O emissions from international
bunker fuels for 1990 through 1997.
Cement Manufacture
During clinker production some of the clinker pre-
cursor materials remain in the kiln as non-calcinated,
partially calcinated, or fully calcinated cement kiln dust.
The emissions attributable to the calcinated portion of
the cement kiln dust are not accounted for by the clinker
emission factor. These additional CO2 emissions were
estimated as 2 percent of the CO2 emissions calculated
from clinker production. The previous inventory did
not include cement kiln dust emissions estimates. These
additional emissions from cement kiln dust were com-
bined with the emissions from clinker production to cal-
culate total cement production emissions. This method-
ological change resulted in an average increase of 0.2
MMTCE (2.0 percent) in annual CO2 emissions from
cement manufacture for 1990 through 1997.
Lime Manufacture
During the calcination stage of lime manufacture,
CO2 is driven off as a gas and normally exits the system
with the stack gas. Carbon dioxide emissions were esti-
mated by applying a CO2 emission factor to the total
amount of lime produced. The emission factor used in
this analysis is the product of the mass of CO2 released
per unit of lime, and the average calcium plus magne-
sium oxide (CaO + MgO) content of lime. In previous
inventories the average calcium plus magnesium oxide
content of lime was not factored into the emissions fac-
tor. The inclusion of the CaO or CaO + MgO content of
lime in the current inventory, was recommended by the
National Lime Association (Males 1999). Lime industry
experts believe that approximately 93 percent is a repre-
sentative value for lime's average calcium plus magne-
sium oxide content (ASTM 1996; Schwarzkopf 1995).
The remainder is composed of silica, aluminum, and iron
oxides (3.83 percent) and CaCO3 (3.41 percent). These
other compounds are present because limestone feed is
not 100 percent pure, nor is the conversion process 100
percent efficient (Males 1999). This yields an emission
factor of 0.73 tons of CO2 per ton of lime produced. In
the previous Inventory, CaO was considered to be 100
percent of limestone, thus yielding an emission factor of
0.785 tons of CO2 per ton of lime produced. This meth-
odological change resulted in an average decrease of
0.2 MMTCE (6.8 percent) in annual CO2 emissions from
lime manufacture for 1990 through 1997.
Adipic Acid Production
The equation used to estimate N2O emissions from
adipic acid production was changed from the previous
Inventory to include both a destruction factor and an
abatement system utilization factor. The N2O destruc-
tion factor represents the amount of N2O expressed as a
percentage of N2O emissions that are destroyed by the
currently installed abatement technology. The abate-
ment system utilization factor represents the percent of
time that the abatement equipment operates. This meth-
odological change resulted hi an average increase of 0.3
MMTCE (7.5 percent) in annual N2O emissions from
Introduction 1-21
-------
adipic acid production for 1990 through 1997.
Nitric Acid Production
An estimated 20 percent of nitric acid plants in the
United States are equipped with Non-Selective Catalytic
Reduction (NSCR) technology (Choe, et al. 1993). In
the process of destroying NOx, NSCR systems also de-
stroy 80 to 90 percent of the N2O. Hence, the emission
factor is equal to (9.5 x 0.80) + (2 x 0.20) = 8 kg N2O per
metric ton HNO3. In previous Inventories the emission
factor was calculated without weighting the percent of
plants using NSCR and Selective Catalytic reduction
(SCR) technologies, thus the previous emission factor
was 5.5 kg N2O per metric ton HNO3. This methodologi-
cal change resulted in an average increase of 1.7 MMTCE
(46.2 percent) in annual N2O emissions from nitric acid
production for 1990 through 1997.
Aluminum Production
PFC emissions from aluminum production were
estimated by multiplying an emission factor by the an-
nual production. In the previous Inventory, PFC emis-
sions were estimated using a single per unit emission
factor for 1990, and emissions for 1991 through 1996
were estimated with emission factors that incorporated
data on reductions in anode effects provided by alumi-
num companies through the Voluntary Aluminum Indus-
try Partnership (VAIP). The current inventory combines
data on smelter operating parameters (anode effect fre-
quency and anode effect duration) with slope coefficients
that relate the operating parameters to emissions of CF4
and C2F6. The operating parameter data has been reported
by smelters and the slope coefficients are based upon
measurements taken at the individual smelters. In cases
where data reports or smelter specific coefficients are
unavailable, technology-specific defaults have been
used. These revisions in methodology resulted in an
average increase of 0.3 MMTCE (8.3 percent) in annual
PFC emissions from aluminum production for 1990
through 1997.
Semiconductor Manufacture
HFC, PFC, and SF6 emissions from semiconductor
manufacture were estimated using silicon chip manufac-
turing characteristics and data provided through the
Emission Reduction Partnership for the Semiconductor
Industry. For previous Inventories, emissions were esti-
mated based on gas sales data from 1994, emission fac-
tors for the most commonly used gases, and projections
regarding the growth of semiconductor sales and the ef-
fectiveness of emission reduction efforts. For the 1998
Inventory, emissions have been recalculated using an
improved estimation method that uses two sets of data.
For 1990 through 1994, emissions were estimated based
on the historical consumption of silicon (square centi-
meters), the estimated average number of interconnect-
ing layers in the chips produced, and an estimated per-
layer emission factor. The average number of layers per
chip was based on industry estimates of silicon consump-
tion by line-width and of the number of layers per line-
width. The per-layer emission factor was based on the
total annual emissions reported by the participants in
the EPA's Emission Reduction Partnership for the Semi-
conductor Industry. For the three years for which gas
sales data were available (1992 through 1994), the esti-
mates derived using the new method are within 10 per-
cent of the estimates derived using gas sales data and
average values for emission factors and GWP values.
For 1995 through 1998, emissions were estimated
based on the total annual emissions reported by the par-
ticipants in the EPA's Emission Reduction Partnership
for the Semiconductor Industry. Partners estimate their
emissions using a range of methods; the partners with
relatively high emissions typically multiply estimates
of their PFC consumption by process-specific emission
factors that they have either measured or obtained from
suppliers manufacturing equipment and based tools. To
estimate total U.S. emissions from semiconductor manu-
facturing based on reported partner emissions, aper-plant
emissions factor was estimated for the partners. This
per-plant emission factor was then applied to plants op-
erated by semiconductor manufacturers who were not
partners, considering the varying characteristics of the
plants operated by partners and non-partners (e.g., typi-
1 -22 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
cal plant size and type of device produced). The result-
ing estimate of non-partner emissions was added to the
emissions reported by the partners to obtain total U.S.
emissions. These revisions in methodology resulted in
an average increase of 0.4 MMTCE (72.4 percent) in
annual HFC, PFC, and SF6 emissions from semiconduc-
tor manufacture for 1990 through 1997.
Manure Management
The methodology for estimating N2O emissions
from manure management no longer assumes that 20
percent of the manure nitrogen volatilizes before N2O
production and therefore is not available for N2O pro-
duction. This assumption was used in previous Invento-'
ries to correct for what appeared to be an inconsistency
with the Agricultural Soil Management emission esti-
mate methodologies, which account for indirect N2O
emissions from nitrogen volatilized from managed ma-
nure systems, as well as from nitrogen in applied animal
manure. However, as a result of efforts carried out by the
IPCC in their work on inventory "good practice," the
determination was made that there is not an inconsis-
tency. Through this process, it became clear that the
total amount of manure nitrogen in managed systems is
available for both N2O production (accounted for in
the Manure Management calculations) and nitrogen
volatilization (accounted for in the Agricultural Soil
Management calculations). Therefore, this step has been
removed so that the methodology corresponds with the
guidance described in IPCC/UNEP/OECD/IEA (1997).
This results in a larger amount of nitrogen available for
N2O production.
Additionally, all animal population data, except
horses, for 1994 through 1997 were updated to reflect
the recent publication of final livestock population es-
timates for 1994 through 1997 from USD A (USDA
1999a-f, i-o). Horse population data for 1990 through
1998 were updated to reflect updated data from the Food
and Agriculture Organization (FAO 1999). These meth-
odological and data changes together cause an average
increase of 1.9 MMTCE (11.2 percent) in annual CH4
emissions and 0.8 MMTCE (28.3 percent) in annual
N2O emissions from manure management for 1990
through 1997.
Rice Cultivation
The climatic conditions of Arkansas (in 1998),
southwest Louisiana, Texas, and Florida allow for a sec-
ond, or ratoon, rice crop to be grown each year. This
second rice crop is produced from re-growth on the
stubble after the first crop has been harvested. For the
1990-1998 U.S. Inventory, the approach used to esti-
mate emissions from rice cultivation was modified to
account for emissions from ratooned and primary areas
separately. In this Inventory, data was collected on the
flooding season length, area cultivated, and emissions
rate range for both the primary and ratoon crops. In
previous Inventories, emissions from the primary and
ratoon seasons were not estimated separately. Instead,
flooding season lengths and a daily emission factor
range that are representative of the primary crop were
used to estimate emissions from both the primary and
ratooned areas. This approach was assumed to result in
a reasonable first approximation for the ratooned areas
because the higher daily average emissions from ra-
tooned areas are at least somewhat canceled out by a
shorter ratoon flooding season (compared to the pri-
mary crop). For the current Inventory, information on
ratoon flooding season lengths was collected from agri-
cultural extension agents in the states that practice ra-
tooning, and emission factors for both the primary sea-
son and the ratoon season were derived from published
results of field experiments in the United States. This
change caused an average decrease of 0.2 MMTCE (6.3
percent) in annual CH4 emissions from rice cultivation
for 1990 through 1997.
Agricultural Soil Management
The current Inventory includes two new sources of
nitrogen that were not accounted for in previous inven-
tories: land application of sewage sludge and produc-
tion of non-alfalfa forage legumes. The current Inven-
tory also includes several data and methodological
changes relative to the previous Inventory. Three changes
to the data have been made. First, an error was found in
a conversion factor used to calculate organic fertilizer
nitrogen consumption; correcting this factor has resulted
Introduction 1-23
-------
in higher organic fertilizer consumption statistics and
lower synthetic fertilizer consumption statistics. Sec-
ond, crop production statistics for some crops have
changed due to the use of updated statistics from the
U.S. Department of Agriculture (USDA 1994a, 1998).
Third, a more recent data source has been used to esti-
mate the annual areas under histosol cultivation, result-
ing in higher area estimates for the entire time series
(USDA1994b).
Two methodological changes have also been
made. First, the emission factor for histosol cultivation
has been revised upward as a result of new guidance
proposed by the IPCC in their work on inventory "good
practice." Second, in the indirect calculations for leach-
ing and runoff, the total amount of applied nitrogen has
been assumed to be subject to leaching and runoff, rather
than just the unvolatilized portion. This change was
also a result of work carried out under the "good prac-
tice" inventory effort. Through this process, it became
clear that the methodology assumes all of the volatil-
ized nitrogen redeposits. Therefore, in order to simplify
the methodology, rather than including a volatilization
and subsequent redeposition step, the leaching and run-
off fraction is just applied to all the applied nitrogen in
a single calculation. This change to the leaching and
runoff calculation has resulted in an increase in the emis-
sion estimates for this process. All the changes taken
together (i.e., the inclusion of the two additional sources
of applied nitrogen, combined with the data changes
and the methodological changes) resulted in an average
increase in the annual emissions from agricultural soil
management of 10.2 MMTCE (14.7 percent) relative to
the estimates in the previous Inventory.
Agriculture Residue Burning
This inventory includes three methodological
changes as compared to previous Inventories. Previous
calculations on rice production in Florida were based on
the assumption that the Sem-Chi Rice Co. accounted for
all of Florida's rice production. However, this Inventory
uses revised production data to include acreage from
additional producers. Average production per acre for
Florida for all years was assumed to be the same as 1998
productivity of Sem-Chi Rice. Total production in
Florida for 1990 through 1998 was estimated using this
average productivity and the revised annual acreage.
The methodology for estimating the percentage of
rice crop residue burned from rice was also revised. In
the previous Inventory, the percentage of rice burned
was assumed to be 3 percent in all states except Califor-
nia. To obtain a more accurate estimate for this Inven-
tory, estimates of the percentage of rice area burned per
year for 1990 through 1998 in each of the seven rice
burning states were obtained from agricultural exten-
sion agents. A weighted (by area) national average per-
cent area burned was calculated for each year.
Additionally, production numbers for corn were
changed to include only corn from grain. Corn for si-
lage was included in the previous Inventory, but is now
excluded because there is no resulting residue. Histori-
cal crop production data, which previously had been
taken from annual USDA summary reports, was revised
using two USDA reports of final crop estimates (USDA
1994a, 1998). These methodological changes cause an
average decrease of less than 0.1 MMTCE (17.1 percent)
in annual CH4 emissions and less than 0.1 MMTCE (12.9
percent) in annual N2O emissions from agriculture resi-
due burning for 1990 through 1997.
Landfills
The methodology used to estimate recovered land-
fill gas was altered from the previous Inventory. Previ-
ous landfill gas recovery estimates (1990-1997) were
based on 1990 and 1992 data obtained from Govern-
mental Advisory Associates (GAA 1994). The 1998 In-
ventory reflects estimates of landfill gas recovered per
year based on site-specific data collected from vendors
of flaring equipment, and a database on landfill gas-to-
energy (LFGTE) projects compiled by the EPA's Land-
fill Methane Outreach Program (LMOP). Based on the
information provided by vendors, the EPA estimated to-
tal methane recovered due to the use of 235 flares for
1990 through 1998. This estimate likely underestimates
emissions because the EPA believes that more than 700
flares are in use at landfills in the United States. The
1 -24 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
EPA is currently working with the Solid Waste Associa-
tion of North America (SWANA) to better characterize
the emissions reduced by flaring and expects to present
a revised estimate in the next Inventory. Additionally,
the LMOP database provided data on landfill gas flow
and energy generation for 237 out of approximately 260
operational LFGTE projects. From this data, the EPA
was able to estimate the methane emissions avoided due
to LFGTE projects.
The EPA assumes that emissions from industrial
landfills are equal to seven percent of the total methane
emissions from municipal landfills. The amount of meth-
ane oxidized is assumed to be ten percent of the meth-
ane generated (Liptay et al. 1998). To calculate net meth-
ane emissions, methane recovered and oxidized is sub-
tracted from methane generated at municipal and indus-
trial landfills. The 1990 through 1997 emission esti-
mates were updated for this Inventory according to the
revised recovery estimates. This change resulted in an
average decrease in the annual estimates of total CH4
emissions from landfills of 1.6 MMTCE (2.3 percent)
relative to the estimates in the previous Inventory.
Human Sewage
The assumptions used to estimate N2O emissions
from human sewage changed slightly from those used
for the previous Inventory. The total estimate of nitro-
gen in human sewage was decreased by the amount of
nitrogen added to soils via sewage sludge applications
which are accounted for under the Agricultural Soil Man-
agement source category.
Annually variable population and per capita pro-
tein consumption factors were obtained from the U.S.
Census Bureau and the United Nations Food and Agri-
culture Organization (FAO), respectively. Protein con-
sumption estimates are updated by the FAO annually.
However, data for protein intake was unavailable for 1998
and therefore, the value of per capita protein consump-
tion for the previous year was used. In addition, the
protein intake estimate for 1997 was unavailable for the
1997 Inventory. Thus, this Inventory reflects an updated
1997 protein intake estimate published in 1998. These
methodological changes for the 1990 through 1997 es-
timates resulted in an average annual decrease in N2O
emissions from Human Sewage of 0.2 MMTCE (6.8 per-
cent) relative to the estimates in the previous Inventory.
Other Changes
Two source categories have been added in the cur-
rent Inventory. First, CO2 emissions from the combus-
tion of plastics in municipal solid waste are now reported
in the Waste Combustion section. Previously, only N2O
emissions had been estimated. The second, an addition
in Land-Use Change and Forestry, addresses the storage
of carbon resulting from the disposal of yard trimmings
in landfills. Yard trimmings, a sizeable portion of mu-
nicipal solid waste, are a significant carbon sink when
landfilled.
The IPCC Reporting Tables, presented in Annex N
of the 1990-1997 Inventory, have been removed from
this Inventory. A new, more detailed, common reporting
format has been developed by the UNFCCC as a substi-
tute for those tables. The United States intends to sub-
mit information to the UNFCCC Secretariat using this
common reporting format in a separate report.
Introduction 1-25
-------
1 -26 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
2. Energy
Figure 2-1
1,468
Energy-related activities were the primary sources of U.S. anthropogenic greenhouse gas emissions, accounting
for 85 percent of total emissions on a carbon equivalent basis in 1998. This included 99, 34, and 18 percent of
the nation's carbon dioxide (CO^, methane (CH^, and nitrous oxide (N2O) emissions, respectively. Energy-related CO2
emissions alone constituted 80 percent of national emissions from all sources on a carbon equivalent basis, while the non-
CO2 emissions from energy represented a much smaller portion of total national emissions (4 percent collectively).
Emissions from fossil fuel combustion comprise the vast majority of energy-related emissions, with CO2 being the
primary gas emitted (see Figure 2-1). Due to the relative importance of fossil fuel combustion-related CO2 emissions, they
are considered separately from other emissions. Fossil fuel combustion also emits CH4 and N2O, as weU as criteria pollut-
ants such as nitrogen oxides (NOX), carbon monoxide (CO), and non-methane volatile organic compounds (NMVOCs).
Mobile fossil fuel combustion was the second largest source of N2O emissions in the United States, and overall energy-
related activities were collectively the largest source of crite-
ria pollutant emissions.
Energy-related activities other than fuel combustion,
such as the production, transmission, storage, and distribu-
tion of fossil fuels, also emit greenhouse gases. These emis-
sions consist primarily of CH4 from natural gas systems, pe-
troleum systems, and coal mining. Smaller quantities of CO2,
CO, NMVOCs, and NOX are also emitted.
The combustion of biomass and biomass-based fuels
also emits greenhouse gases. Carbon dioxide emissions from
these activities, however, are not included in national emis-
sions totals in the Energy chapter because biomass fuels are
of biogenic origin. It is assumed that the carbon released
when biomass is consumed is recycled as U.S. forests and
crops regenerate, causing no net addition of CO2 to the at-
mosphere. The net impacts of land-use and forestry activities
on the carbon cycle are accounted for in the Land-use change and Forestry chapter. Emissions of other greenhouse gases
from the combustion of biomass and biomass based fuels are included in national totals under stationary and mobile
combustion.
Overall, emissions from energy-related activities have increased from 1990 to 1998 due, in part, to the strong
performance of the U.S. economy. Over this period, the U.S. Gross Domestic Product (GDP) grew approximately 23
Fossil Fuel Combustion
Natural Gas Systems
Mobile Sources
Coal Mining
Stationary Sources I
Petroleum Systems H
Natural Gas Flaring |
Portion of All
Emissions
20
40
MMTCE
60
Energy 2-1
-------
Table 2-1: Emissions from Energy (MMTCE)
Gas/Source
C02
Fossil Fuel Combustion
Natural Gas Raring
Biomass-Wood*
International Bunker Fuels*
Biomass-Ethanol*
Fossil Fuel Carbon in Products*
CH4
Natural Gas Systems
Coal Mining
Petroleum Systems
Stationary Combustion
Mobile Combustion
International Bunker Fuels*
N20
Mobile Combustion
Stationary Combustion
International Bunker Fuels*
Total
1990
1,322.6
1,320.1
2.5
55.6
32.2
1.6
(69.4)
68.2
33.0
24.0
7.4
2.3
1.5
+
17.6
13.8
3.8
0.3
1,408.4
1991
1,308.6
1,305.8
2.8
56.2
32.7
1.2
(69.0)
67.5
33.4
22.8
7.5
2.4
1.5
+
18.3
14.6
3.8
0.3
1,394.5
1992
1,332.8
1,330.1
2.8
59.0
30.0
1.5
(70.7)
67.0
33.9
22.0
7.2
2.4
1.5
+
19.5
15.7
3.9
0.3
1,419.4
1993
1,365.2
1,361.5
3.7
58.8
27.2
1.7
(73.5)
64.6
34.6
19.2
6.9
2.4
1.5
+
20.4
16.5
3.9
0.2
1,450.2
1994
1,385.7
1,382.0
3.8
61.4
26.7
1.8
(78.4)
64.3
34.3
19.4
6.7
2.4
1.5
+
21.1
17.1
4.0
0.2
1,471.1
1995
1,396.6
1,392.0
4.7
64.2
27.5
2.0
(79.2)
64.9
34.0
20.3
6.7
2.5
1.4
+
21.4
17.4
4.0
0.2
1,483.0
1996
1,445.8
1,441.3
4.5
66.1
27.9
1.4
(80.7)
64.0
34.6
18.9
6.5
2.6
1.4
+
21.7
17.5
' 4.2
0.2
1,531.5
1997
1,464.9
1,460.7
4.2
62.9
29.9
1.8
(84.3)
63.1
34.1
18.8
6.5
2.3
1.4
+
21.5
17.3
4.2
0.3
1,549.5
.•••••:••„':-"?•.••*• ..«
1998 '
1,472.1
1,468.2 ,
3.9 ;
64.2 :
31.3 i
2.0
(85.6) '
61.3
33.6
17.8
6.3
2.3 :
1.3
+ . -.
21.5
17.2
4.3
OO
.3
1,554.8
+ Does not exceed 0.05 MMTCE .
* These values are presented for informational purposes only and are not included or are already accounted for in totals.
Note: Totals may not sum due to independent rounding.
Table 2-2: Emissions from Energy (Tg)
Gas/Source
tC°2
" Fossil Fuel Combustion
- Natural Gas Flaring
Biomass-Wood*
International Bunker Fuels*
Biomass-Ethanol*
Fossil Fuel Carbon in Products*
CH4
Natural Gas Systems
Coal Mining
Stationary Combustion
Petroleum Systems
Mobile Combustion
International Bunker Fuels*
N20
Mobile Combustion
Stationary Combustion
International Bunker Fuels*
1990
4,849.6
4,840.5
9.1
203.8
118.0
5.7
(254.5)
11.9
5.8
4.2
0.4
1.3
0.3
+
0.2
0.2
+
+_
1991
4,798.2
4,787.9
10.3
205.9
120.0
4.5
(253.2)
11.8
5.8
4.0
0.4
1.3
0.3
+
0.2
0.2
+
±_
1992
4,887.1
4,876.9
10.2
216.5
110.0
5.5
(259.1)
11.7
5.9
3.8
0.4
1.3
0.3
+
0.2
0.2
+
±_
1993
5,005.8
4,992.1
13.7
215.4
99.9
6.1
(269.4)
11.3
6.0
3.4
0.4
1.2
0.3
+
0.2
0.2
+
±_
1994
5,081.0
5,067.2
13.8
225.3
98.0
6.7
(287.4)
11.2
6.0
3.4
0.4
1.2
0.3
+
0.2
0.2
+
±_
1995
5,121.0
5,103.8
17.2
235.2
101.0
7.2
(290.6)
11.3
5.9
3.6
0.4
1.2
0.3
+
0.3
0.2
+
±_
1996
5,301.4
5,284.9
16.5
162.5
102.2
5.1
(295.9)
11.2
6.0
3.3
0.4
1.1
0.2
+
0.3
0.2
+
±_
1997
5,371.4
5,355.9
15.5
230.5
109.8
6.7
(309.0)
11.0
6.0
3.3
0.4
1.1
0.2
+
0.3
0.2
+
—
1998
5,397.7
5,383.5
14.2
235.6
114.7
7.3
(313.8)
10.7
5.9
3j
.1
0.4
1.1
On
.2
+
0.3
0.2
+
+
+ Does not exceed 0.05 Tg .
* These values are presented for informational purposes only and are not included or are already accounted for in totals.
Note: Totals may not sum due to independent rounding.
2-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
percent, or at an average annual rate of 3.1 percent. This
robust economic activity increased the demand for fossil
fuels, with an associated increase in greenhouse gas emis-
sions. Table 2-1 summarizes emissions for the Energy
chapter in units of million metric tons of carbon equiva-
lents (MMTCE), while unweighted gas emissions in
Teragrams (Tg) are provided in Table 2-2. Overall, emis-
sions due to energy-related activities were 1,554.8
MMTCE in 1998, an increase of 10 percent since 1990.
Carbon Dioxide Emissions from
Fossil Fuel Combustion
Carbon dioxide emissions from fossil fuel com-
bustion grew by 0.5 percent from 1997 to 1998. Excep-
tionally mild winter conditions in 1998 moderated
growth in CO2 emissions from fossil fuel combustion
below what would have been expected given the strength
of the economy and continued low fuel prices. Overall,
CO2 emissions from fossil fuel combustion have in-
creased by 11.2 percent since 1990.
Eighty-five percent of the energy consumed in the
United States was produced through the combustion of
fossil fuels such as coal, natural gas, and petroleum (see
Figure 2-2 and Figure 2-3). Of the remaining portion, 8
percent was supplied by nuclear electric power and 7
percent by renewable energy (EIA 1999a).
As fossil fuels are combusted, the carbon stored in
the fuels is emitted as CO2 and smaller amounts of other
gases, including methane (CH4), carbon monoxide (CO),
and non-methane volatile organic compounds
(NMVOCs). These other gases are emitted as a by-prod-
uct of incomplete fuel combustion.1
The amount of carbon in fuels varies significantly
by fuel type. For example, coal contains the highest
amount of carbon per unit of useful energy. Petroleum
has roughly 75 percent of the carbon per unit of energy
as coal, and natural gas has only about 55 percent.2 Pe-
troleum supplied the largest share of U.S. energy de-
mands, accounting for an average of 39 percent of total
energy consumption over the period of 1990 through
1998. Natural gas and coal followed in order of impor-
tance, accounting for an average of 24 and 22 percent of
total consumption, respectively. Most petroleum was
consumed in the transportation end-use sector, while the
vast majority of coal was used by electric utilities, with
natural gas consumed largely in the industrial and resi-
dential end-use sectors (see Figure 2-4) (EIA 1999a).
Emissions of CO2 from fossil fuel combustion in-
creased at an average annual rate of 1.3 percent from
Figure 2-2
1" A An 'II -aS»"" •^^'"^'&^yi?^'""-a^
7.5% Renewable
7.6% Nuclear
22.9% Coal
23.2% Natural Gas
38.8% Petroleum
Source: DOE/EIA-0384(99), Annual Energy Review 1998
Table 1.3, July 1999
Figure 2-3
1990 1991 1992 1993 1994 1995 1996 1997 1998
Source: DOE/EIA-0384(99), Annual Energy Review 1998,
Table 1.3, July 1999
1 See the sections entitled Stationary Combustion and Mobile Combustion for information on non-CO, gas emissions from fossil fuel
combustion.
2 Based on national aggregate carbon content of all coal, natural gas, and petroleum fuels combusted in the United States.
Energy 2-3
-------
1990 to 1998. The major factor behind this trend was a
robust domestic economy, combined with relatively low
energy prices (see Figure 2-5). For example, petroleum
prices reached historic lows in 1998, with prices in many
cases less than those seen in the 1970s before the oil
Figure 2-4
Noto: Utilities also include emissions of 0.04 MMTCE
from goothermal based eletricity generation
crisis. After 1990, when CO2 emissions from fossil fuel
combustion were 1,320.1 MMTCE (4,840.5 Tg), there
was a slight decline of emissions in 1991 due to a na-
tional economic downturn, followed by an increase to
1,468.2 MMTCE (5,383.5 Tg) in 1998 (see Figure 2-5,
Table 2-3, and Table 2-4).
Since 1990, overall fossil fuel consumption in-
creased significantly. Higher coal consumption during
the period accounted for about 36 percent of the change
hi total CO2 emissions from fossil fuel combustion, pe-
troleum accounted for 42 percent, and natural gas for 21
percent.
In regard to annual changes from 1997 to 1998,
absolute emissions from petroleum and coal increased by
11.5 and 5.1 MMTCE, respectively. Increased demand
for transportation fuels and by electric utilities were the
primary causes of the growth in emissions from petroleum
combustion, while record electricity demand drove most
of the increase in emissions from coal combustion. Emis-
sions from natural gas combustion, however, decreased
Table 2-3: CO, Emissions from Fossil Fuel Combustion by Fuel Type and Sector (MMTCE)
1990
1991 1992 1993 1994 1995 1996 1997 1998
Coal
Residential
Commercial
Industrial
Transportation
Electric Utilities
U.S. Territories
Natural Gas
Residential
Commercial
Industrial
Transportation
Electric Utilities
U.S. Territories
Petroleum
Residential
Commercial
Industrial
Transportation
Electric Utilities
U.S. Territories
Geothermal*
Total
480.9
1.6
2.4
67.7
409.0
0.2
272.8
65.1
38.8
117.9
9.8
41.2
_
566.4
23.9
18.0
100.0
389.1
26.4
9.0
0.1
1,320.1
475.2
1.4
2.2
64.1
407.2
0.2
277.7
67.5
40.4
119.7
8.9
41.1
-
552.9
24.4
17.1
94.2
381.9
24.9
10.5
0.1
1,305.8
477.5
1.5
2.2
61.8
411.8
0.2
286.0
69.4
41.5
125.6
8.8
40.7
-
566.5
24.8
16.1
104.2
391.6
20.2
9.6
0.1
1,330.1
493.9
1.5
2.2
61.4
428.7
0.2
296.4
73.4
43.1
131.1
9.3
39.5
-
571.1
26.2
14.9
98.0
399.2
22.3
10.5
0.1
1,361.5
495.2
1.4
2.1
61.9
429.5
0.2
301.4
71.7
42.9
132.6
10.2
44.0
-
585.4
25.3
14.9
102.0
411.5
20.5
11.2
+
1,382.0
498.1
1.4
2.1
61.4
433.0
0.3
313.6
71.7
44.8
139.5
10.4
47.2
-
580.3
25.7
15.0
97.5
416.7
13.9
11.5
+
1,392.0
520.5
1.4
2.1
59.2
457.5
0.3
319.2
77.5
46.7
144.2
10.6
40.3
-
601.7
27.2
14.6
103.3
430.5
15.3
10.8
+
1,441.3
534.5
1.5
2.2
58.7
471.8
0.3
318.8
73.7
47.6
142.8
11.2
43.6
~
607.3
27.0
13.8
105.8
431.6
17.5
11.7
+
1,460.7
539.6
1.5
2.2
58.4
477.3
0.3
309.7
66.3
44.9
139.9
10.8
47.8
"
618.9
27.0
13.8
101.8
438.8
24.8
12.8
+
1,468.2
- Not applicable
+ Does not exceed 0.05 MMTCE
* Although not technically a fossil fuel, geothermal energy-related C02 emissions are included for reporting purposes.
Note: Totals may not sum due to independent rounding.
2-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Figure 2-5
•••••^ •___T**'^ -^^ ""^"^
i—r "
1979
1991
1995
Source: DOE/EIA-0384(97), Annual Energy Review 1999,
Table 3.1, July 1999
by 9.1 MMTCE (2.9 percent), again due in large part to
the mild winter conditions and lower heating demands.
An analysis was performed by the EIA (1999c) to
examine the effects of weather conditions on U.S. fuel
consumption patterns. The analysis—using the EIA's
Short-Term Forecasting System—found that if normal
weather conditions had existed in 1998, overall CO2
emissions from fossil fuel combustion would have in-
creased by about 1.2 percent above weather-adjusted
emissions in 1997, instead of the actual 0.5 percent in-
crease.3 See also Box 2-1 and Table 2-11 for additional
discussion on overall emission trends and Figure 2-9 for
data on heating degree days.4
Table 2-4: C02 Emissions from Fossil Fuel Combustion by Fuel Type and Sector (Tg)
Ifuel/Sector
1990
1991
1992
1993 1994
1995
1996 1997
Coal
1; Residential
'«p. Commercial
i ,'-' Industrial
p: Transportation
t Electric Utilities
!:- U.S. Territories
f Natural Gas
^. Residential
|i Commercial
t Industrial
'fe. Transportation
|" Electric Utilities
I , U.S. Territories
tPetroleum
L" .. Residential
fc Commercial
|r Industrial
| Transportation
|T: Electric Utilities
Jir U.S. Territories
|Geothermal*
fTotal
1,763.3
5.8
8.7
248.4
+
1,499.7
0.6
1,000.3
238.5
142.4
432.2
36.0
151.1
-
2,076.7
87.7
66.1
366.5
1,426.5
96.8
33.1
0.2
4,840.5
1,742.3
5.3
8.0
235.0
+
1,493.2
0.7
1,018.1
247.3
148.2
439.0
32.8
150.6
_
2,027.4
89.4
62.6
345.5
1,400.2
91.2
38.6
0.2
4,787.9
1,750.9
5.4
8.1
226.6
+
1,510.0
0.8
1,048.6
254.5
152.3
460.4
32.1
149.3
_
2,077.2
90.9
59.1
382.1
1,436.0
73.9
35.2
0.2
4,876.9
1,811.1
5.3
8.1
225.1
4.
1,571.7
0.9
1,086.7
269.1
158.2
480.6
33.9
144.9
_
2,094.1
96.1
54.7
359.5
1,463.7
81.8
38.3
0.2
4,992.1
1,815.7
5.2
7.8
227.1
+
1,574.7
0.9
1,105.0
262.9
157.2
486.3
37.2
161.4
.
2,146.4
92.8
54.7
373.8
1,508.9
75.0
41.1
0.2
5,067.2
1,826.2
5.1
7.6
225.0
+
1,587.5
0.9
1,149.7
263.0
164.3
511.4
38.1
173.0
.
2,127.8
94.4
54.9
357.4
1,527.8
51.0
42.3
0.1
5,103.8
1,908.3
5.2
7.8
217.0
1,677.4
0.9
1,170.4
284.2
171.2
528.6
38.7
147.7
2,206.1
99.7
53.6
378.9
1,578.4
56.0
39.5
5,284.9
1,959.9
5.5
8.2
215.3
1,730.0
1.0
1,168.9
270.2
174.5
523.6
41.0
159.7
2,226.9
98.9
50.8
387.8
1,582.4
64.1
42.9
0.1
5,355.9
1,978.7
5.4
8.1
214.0
1,750.2
1.0
1,135.4
243.1
164.5
512.9
39.6
175.3
2,269.2
99.1
50.5
373.2
1,608.9
90.8
46.9
0.1
5,383.5
pH- Does not exceed 0.05 Tg
| * Although not technically a fossil fuel, geothermal energy-related C02 emissions are included for reporting purposes
| Note: Totals may not sum due to independent rounding.
3 The 1.2 percent growth rate in EIA's weather adjusted model is actually the average annual growth rate between 1990 and 1998. The
EIA goes on to state that given the high rate of economic growth in 1998, the increase in weather adjusted emissions between 1997 and
1998 would likely have been even greater.
4 Degree days are relative measurements of outdoor air temperature. Heating degree days are deviations of the mean daily temperature
below 65° F, while cooling degree days are deviations of the mean daily temperature above 65° F. Excludes Alaska and Hawaii. Normals
are based on data from 1961 through 1990.
Energy 2-5
-------
For the purpose of international reporting the DPCC
(IPCC/UNEP/OECD/IEA 1997) requires that particular
adjustments be made to national fuel consumption sta-
tistics. Certain fossil fuel-based products are used for
manufacturing plastics, asphalt, or lubricants. A portion
of the carbon consumed for these non-energy products
can be sequestered for long periods of time. To account
for the fact that the carbon in these fuels ends up in
products instead of being combusted (i.e., oxidized and
released into the atmosphere), the fraction of fossil fuel-
based carbon in manufactured products is subtracted from
emission estimates. The IPCC (1997) also requires that
CO2 emissions from the consumption of fossil fuels for
aviation and marine international transport activities
(i.e., international bunker fuels) be reported separately,
and not included in national emission totals. Both esti-
mates of carbon in products and international bunker
fuel emissions for the United States are provided in Table
2-5 and Table 2-6.
End-Use Sector Consumption
When analyzing CO2 emissions from fossil fuel
combustion, four end-use sectors were defined: indus-
trial, transportation, residential, and commercial. Elec-
tric utilities also emit CO2; however, these emissions
occur as they combust fossil fuels to provide electricity
to one of the four end-use sectors. For the discussion
below, electric utility emissions have been distributed
to each end-use sector based upon their share of national
electricity consumption. This method of distributing
emissions assumes that each sector consumes electricity
from an equally carbon-intensive electricity source. In
reality, sources of electricity vary widely in carbon in-
tensity. By giving equal carbon-intensity weight to each
sector's electricity consumption, for example, emissions
attributed to the industrial end-use sector may be over-
estimated, while emissions attributed to the residential
end-use sector may be underestimated. After the end-use
sectors are discussed, emissions from electric utilities
are addressed separately. Emissions from U.S. territories
Table 2-5: Fossil Fuel Carbon in Products and C02 Emissions from International Bunker Fuel Combustion (MMTCE)
Category/Sector
Fossil Fuel Carbon in Products
Industrial
Transportation
Territories
International Bunker Fuels*
Aviation*
Marine*
1990
69.4
67.5
1.8
0.2
32.2
12.7
19.4
1991
69.0
67.2
1.6
0.3
32.7
12.7
20.0
1992
70.7
68.9
1.6
0.1
30.0
12.9
17.1
1993
73.5
71.6
1.7
0.2
27.2
13.0
14.3
1994
78.4
76.5
1.7
0.2
26.7
13.2
13.6
1995
79.2
77.4
1.7
0.2
27.5
13.9
13.6
1996
80.7
78.7
1.6
0.3
27.9
14.2
13.7
1997
84.3
82.2
1.7
0.4
29.9
15.2
14.7
1998
85.6
83.3
1.8
0.4
31.3
15.5
15.8
* See International Bunker Fuels for additional detail.
Note: Totals may not sum due to independent rounding.
Table 2-6: Fossil Fuel Carbon in Products and C02 Emissions from International Bunker Fuel Combustion (Tg C02)
Category/Sector
1990
Fossil Fuel Carbon in Products 254.5
Industrial
Transportation
Territories
International Bunker Fuels*
Aviation*
Marine*
* See International Bunker Fuels
Note: Totals may not sum due to
247.3
6.5
0.7
118.0
46.7
71.2
for additional detail.
1991
253.2
246.4
5.8
0.9
120.0
46.7
73.3
1992
259.1
252.6
6.0
0.5
110.0
47.1
62.8
1993
269.4
262.6
6.1
0.7
99.9
47.6
52.3
1994
287.4
280.4
6.3
0.6
98.0
48.3
49.7
1995
290.6
283,
6,
0
101,
51
49
,6
,2
.7
.0
.1
.9
1996
295.9
288.6
6.0
1.2
102.2
52.1
50.1
1997
309.0
301.3
6.4
1.4
109.8
55.9
53.9
,-.-_. , .,„.,. ,3
1998
313.8 i
305,
6,
1
.0 ;
,7
.6
114.7
56
b/
.9
•8 :
independent rounding. ;
2-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Table 2-7: C02 Emissions from Fossil Fuel Combustion by End-Use Sector (MMTCE)*
' End-Use Sector
Industrial
Combustion
Electricity
"Transportation
: Combustion
Electricity
Residential
'',"- Combustion
:: Electricity
Commercial
'c Combustion
Electricity
; U.S. Territories
Total
1990
451.7
285.6
166.2
399.6
398.9
0.7
252.9
90.6
162.4
206.7
59.2
147.4
9.2
1,320.1
1991
440.3
278.1
162.2
391.5
390.8
0.7
257.0
93.3
163.7
206.3
59.7
146.7
10.7
1,305.8
1992
458.0
291.6
166.4
401.1
400.4
0.7
255.8
95.7
160.1
205.4
59.9
145.5
9.8
1,330.1
1993
458.0
290.5
167.5
409.1
408.5
0.7
271.6
101.0
170.5
212.0
60.2
151.8
10.7
1,361.5
1994
466.2
296.5
169.7
422.3
421.7
0.7
268.2
98.4
169.8
213.8
59.9
153.9
11.5
1,382.0
1995
464.4
298.3
166.0
427.7
427.1
07
269.8
98.8
170.9
218.3
61.9
156.4
11.8
1,392.0
1996
477.3
306.7
170.6
441.7
441.1
0 7
285.4
106.1
179.3
225.9
63.4
162.5
11.0
1,441.3
1997
482.5
307.3
175.3
443.4
442.7
0 7
284.7
102.2
182.6
238.0
637
174.4
12.0
1,460.7 1
1998
478.9
300.0
178.9
450.3
4496
0 7
286.8
94.8
192.0
239.3
609
178.4
13.0
468.2
j^Note: Totals may not sum due to independent rounding.
are allocated based on electricity consumption by each end-use sector.
Figure 2-6
are also calculated separately due to a lack of end-use-
specific consumption data. Table 2-7 and Figure 2-6 sum-
marize CO2 emissions from direct fossil fuel combustion
and pro-rated emissions from electricity consumption
by end-use sector.
The overall demand for energy in the United States
and other countries fluctuates in response to general eco-
nomic conditions, energy prices, and weather. For ex-
ample, a year with strong economic growth, low energy
prices, and severe summer and winter weather condi-
tions would be expected to have proportionally greater
emissions from fossil fuel combustion than a year with
poor economic performance, high energy prices, and mild
average temperatures. Except for 1991, economic growth
in the United States during the 1990s has fluctuated but
overall been robust, and energy prices have been low
and declining. Average U.S. temperatures, however have
fluctuated more significantly, with hotter summer tem-
peratures in 1998 stimulating electricity demand and
warmer winter temperatures reducing demand for heat-
ing fuels.
Longer-term changes in energy consumption pat-
terns are a function of variables that affect the scale of
consumption (e.g., population, number of cars, and size
of houses) and the efficiency with which energy is used
in equipment (e.g., cars, power plants, steel mills, and
light bulbs) and consumer behavior (e.g., bicycling or
tele-commuting to work instead of driving).
Carbon dioxide emissions, however, are also a func-
tion of the type fuel combusted and its carbon intensity.
Producing heat or electricity using natural gas or wind
energy instead of coal, for example, can reduce or even
eliminate the CO2 emissions associated with energy con-
sumption (see Box 2-1).
Industrial End-Use Sector
The industrial end-use sector accounted for ap-
proximately one-third of CO2 emissions from fossil fuel
combustion. On average, nearly 63 percent of these emis-
sions resulted from the direct consumption of fossil fu-
els in order to meet industrial demand for steam and
Energy 2-7
-------
process heat. The remaining 37 percent resulted from
their consumption of electricity for uses such as motors,
electric furnaces, ovens, and lighting.
The industrial end-use sector includes activities
such as manufacturing, construction, mining, and agri-
culture. The largest of these activities in terms of energy
consumption is manufacturing, which was estimated in
1994 to have accounted for 80 percent of industrial en-
ergy consumption (EIA 1997). Therefore, in general
emissions from the industrial end-use are fairly corre-
lated with economic growth, however, certain activities
within the sector, such as heating of industrial buildings
and agriculture, are also affected by weather conditions.
According to current EIA sectoral definitions, the
industrial sector also includes emissions from nonutility
generators (e.g., independent power producers) who pro-
duce electricity for their own use, to sell to large con-
sumers, or to sell on the wholesale electricity market.
The number and quantity of electricity generated by
nonutilities has increased significantly as many states
have begun deregulating their electricity markets. In
future inventories, these nonutility generators will be
removed from the industrial sector and incorporated into
a single sector with electric utilities.
Although the largest share of emissions was attrib-
uted to the industrial end-use sector, from 1990 to 1998,
its emissions grew the least in percentage terms (6 per-
cent). From 1997 to 1998, emissions actually declined
slightly (1 percent), likely due in part to lower output by
some energy intensive industries—such as primary met-
als—and weather-related changes in agricultural activi-
ties.
The industry was also the largest user of fossil fu-
els for non-energy applications. Fossil fuels can be used
for producing products such as fertilizers, plastics, as-
phalt, or lubricants that can sequester or store carbon for
long periods of time. Asphalt used in road construction,
for example, stores carbon essentially indefinitely. Simi-
larly, fossil fuels used in the manufacture of materials
like plastics can also store carbon, if the material is not
burned. The amount of carbon contained in industrial
products made from fossil fuels rose 24 percent between
1990 and 1998, to 85.6 MMTCE (313.8 Tg CO2).
Transportation End-Use Sector
Transportation was second to the industrial end-
use sector in terms of U.S. CO2 emissions from fossil fuel
combustion, accounting for slightly over 30 percent-
excluding international bunker fuels. Almost all of the
energy consumed in this end-use sector came from pe-
troleum-based products, with nearly two-thirds due to
gasoline consumption in automobiles and other high-
way vehicles. Other uses, including diesel fuel for the
trucking industry and jet fuel for aircraft, accounted for
the remainder.
Following the overall trend in U.S. energy con-
sumption, fossil fuel combustion for transportation grew
steadily after declining in 1991, resulting in an increase
in CO2 emissions of 33 percent from 1990 to 454.9
MMTCE (1,668.0 Tg) in 1998. This increase was prima-
rily the result of greater motor gasoline and jet fuel con-
sumption. It was slightly offset by decreases in the con-
sumption of residual fuel.
Overall, motor vehicle fuel efficiency stabilized
in the 1990s after increasing steadily since 1977 (EIA
1999a). This trend was due, in part, to a decline in gaso-
line prices and new motor vehicle sales being increas-
ingly dominated by less fuel-efficient light-duty trucks
and sport-utility vehicles (see Figure 2-7 and Figure 2-8).
Moreover, declining petroleum prices during the 1990s,
combined with a strong economy and a growing popula-
tion, were largely responsible for an overall increase in
vehicle miles traveled (EIA 1999a).
Figure 2-7
1972 1975 1978 1981 1984 1987 1990 1993 1996
Source for gasoline prices: DOE/EIA-0384(97), Annual Energy
Review 1997, July, 1998, Table 5.22
Source for motor vehicle fuel efficiency: DOT/FHWA, Highway
Statistics Summary to 1995, Highway Statistic 1996, 1997,1998.
2-8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Table 2-8 provides a detailed breakdown of CO2
emissions by fuel category and vehicle type for the
transportation end-use sector. Fifty-eight percent of
the emissions from this end-use sector were the result
of the combustion of motor gasoline in passenger cars
and light-duty trucks. Diesel highway vehicles and
jet aircraft were also significant contributors, each ac-
counting for 14 percent of CO2 emissions from the
transportation end-use sector.
Residential and Commercial End-Use Sectors
From 1990 to 1998, the residential and commer-
cial end-use sectors, on average, accounted for 20 and 16
percent, respectively, of CO2 emissions from fossil fuel
combustion. Both end-use sectors were heavily reliant
on electricity for meeting energy needs, with about two-
thirds of their emissions attributable to electricity con-
sumption for lighting, air conditioning, and operating
appliances. The remaining emissions were largely due to
the direct consumption of natural gas and petroleum prod-
ucts, primarily for heating and cooking needs. Coal con-
sumption was a minor component of energy use in both
the residential and commercial end-use sectors.
Unlike in other major end-use sectors, emissions
from residences and commercial buildings did not de-
cline during the economic downturn in 1991, but in-
stead decreased in 1994, then grew steadily through
1998. This difference in overall trends compared to other
Figure 2-8
All Motor Vehicles
IU
1972 19751978 1981198419871990 1993 1996
Source: DOT/FHWA, Highway Statistics Summary to 1995,
Highway Statistics 1996, Highway Statistics 1997,1998.
end-use sectors is because energy consumption in resi-
dences and commercial buildings is affected proportion-
ately more by the weather than by prevailing economic
conditions. Both end-use sectors are also affected by
population and regional migration trends.
In 1998, winter conditions in the United States
were extremely warm, with heating degree days 12 per-
cent below normal (see Figure 2-9). Due primarily to
these warm winter conditions, emissions from natural
gas consumption in residences and commercial estab-
lishments declined by an impressive 10 and 6 percent,
respectively.
In 1998, electricity consumption in the residential
and commercial end-use sectors increased by 4.5 and
1.7 percent, respectively. These increases were partly the
result of air conditioning related demand and the hotter
than normal summer in 1998, with cooling degree days
12 percent above normal (see Figure 2-10). U.S. tem-
peratures during June, July, and August of 1998 were on
average 10 percent higher than normal levels.6 In the
month of June, alone, residential customers increased
their consumption of electricity by 17 percent above
that for the same period the previous year (EIA 1999b).
Electric Utilities
The United States relied on electricity to meet a
significant portion of its energy requirements. Electric-
Figure 2-9
120
5 100
90
80 J
Normal
(4,576 Heating Degree Days)
1990 1991 1992 1993 1994 1995 1996 1997 1998
Note: Excludes Alaska and Hawaii
Source: DOE/EIA-0384(97), Annual Energy Review 1998, July,
1999, Table 1.7 and 1.8.
5 Degree days are relative measurements of outdoor air temperature. Heating degree days are deviations of the mean daily temperature
below 65° F. Excludes Alaska and Hawaii. Normals are based on data from 1961 through 1990.
6 Measured in terms of cooling degree days. Normals defined by the average between 1961 and 1990.
Energy 2-9
-------
Table 2-8: CO, Emissions from Fossil Fuel Combustion in Transportation End-Use Sector (MMTCE)
Fuel/Vehicle Type
Motor Gasoline
Passenger Cars
Light-Duty Trucks
Other Trucks
Motorcycles
Buses
Construction Equipment
Agricultural Machinery
Boats (Recreational)
Distillate Fuel Oil (Diesel)
Passenger Cars
Light-Duty Trucks
Other Trucks
Buses
Construction Equipment
Agricultural Machinery
Boats (Freight)
Locomotives
Marine Bunkers
Jet Fuel
General Aviation
Commercial Air Carriers
Military Vehicles
Aviation Bunkers
Other*
.Aviation Gasoline
I General Aviation
: Residual Fuel Oil
; Boats (Freight)
Marine Bunkers
Natural Gas
; Passenger Cars
•: Light-Duty Trucks
Buses
Pipeline
LPG
Light-Duty Trucks
Other Trucks
Buses
Electricity
Buses
Locomotives
Pipeline
Lubricants
Total (including bunkers)
1990
260.6
167.1
74.8
11.3
0.4
0.6
0.6
1.2
4.6
75.7
1.9
2.4
44.8
2.1
2.9
6.3
4.9
7.2
3.1
60.1
1.7
32.2
9.8
12.7
3.6
0.8
0.8
21.9
5.6
16.4
9.8
+
+
9.8
0.4
0.1
0.2
-j.
0.7
+
0.1
0.6
1.8
431.8
1991
259.2
165.8
74.6
11.2
0.4
0.6
0.6
1.2
4.7
72.6
1.9
2.4
42.9
2.2
2.8
6.2
4.8
6.6
2.9
58.1
1.5
29.8
9.7
12.7
4.3
0.8
0.8
22.0
4.9
17.1
8.9
+
+
8.9
0.3
0.1
0.2
+
0.7
+
0.1
0.5
1.6
424.2
1992
263.1
169.8
74.5
11.2
0.4
0.6
0.6
1.2
4.7
75.3
2.0
2.4
44.5
2.2
2.9
6.3
5.0
7.2
2.9
57.6
1.3
30.6
7.5
12.9
5.3
0.8
0.8
23.0
8.8
14.3
8.8
+
+
8.8
0.3
0.1
0.2
+
0.7
+
0.1
0.5
1.6
431.1
1993
269.0
171.2
77.7
11.7
0.5
0.7
0.6
2.0
4.6
77.3
2.0
2.6
47.2
2.3
2.9
6.3
4.6
6.5
2.9
58.1
1.1
31.3
7.2
13.0
5.5
0.7
0.7
19.4
8.1
11.4
9.3
+
+
9.2
0.3
0.1
0.2
+
0.7
+
0.1
0.5
1.6
436.4
1994
273.3
170.2
84.1
10.4
0.5
0.9
0.6
2.1
4.5
82.5
2.0
2.8
51.4
2.3
2.9
6.3
4.5
7.8
2.4
60.4
1.2
32.0
6.2
13.2
7.8
0.7
0.7
19.1
7.9
11.2
10.2
+
+
10.1
0.5
0.2
0.3
+
0.7
+
0.1
0.5
1.7
449.1
1995
278.9
172.9
85.7
10.9
0.5
0.8
0.7
2.2
5.3
83.8
2.0
3.0
52.4
2.7
2.8
6.2
4.3
7.9
2.5
60.0
1.4
33.1
5.8
13.9
5.6
0.7
0.7
18.5
7.4
11.1
10.4
+
+
10.4
0.5
0.3
0.3
+
0.7
+
0.1
0.5
1.7
455.3
1996
284.1
176.4
87.4
11.1
0.5
0.6
0.7
2.1
5.4
89.8
2.1
3.6
56.5
2.4
3.0
6.5
5.0
8.6
2.2
62.7
1.6
34.1
5.4
14.2
7.4
0.7
0.7
19.2
7.7
11.4
10.6
+
+
10.5
0.3
0.1
0.1
+
0.7
+
0.1
0.5
1.6
469.6
1997
286.5
177.9
88.1
11.0
0.5
0.6
0.7
2.2
5.5
93.5
2.2
3.9
58.9
2.5
3.0
6.7
5.0
8.8
2.5
63.3
1.7
35.3
4.8
15.2
6.3
0.7
0.7
15.5
3.2
12.2
11.2
+
+ .
11.1
0.2
0.1
0.1
+
0.7
+
0.2
0.5
1.7
473.4
1998
294.6
182.9
90.6
11.4
0.5
0.6 •:
0.7 ',,
2.3
5.6
94.0
2.1
3.9
58.8
2.5
3.0
6.7
5.0
8O
.8
3.1
64.2
1.7
35.8
5.0
15.5
6.2
0.7
0.7
14.5
.1.9
12.7
10.8
+
+ :
10.7
0.2
0.1
0.1
+
0.7
"*"
0.2
0.5
1.8
481.5
'= Note: Totals may not sum due to
International bunker fuels.
' Including but not limited to fuel
+ Does not exceed 0.05 MMTCE
independent rounding. Estimates include emissions from the combustion
blended with heating oils and fuel used for chartered aircraft flights.
of both aviation and marine
2-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Figure 2-10
120
110
o 100 -
80 J
Normal
(1,193 Cooling Degree Days)
1990 19911992 1993 1994 1995 1996 1997 1998
Note: Excludes Alaska and Hawaii
Source: DOE/EIA-0384(97), Annual Energy Review 1998
July, 1999, Table 1.7 and 1.8.
Figure 2-11
Commercial
400
1972 1975 1978 1981 1984 1987 1990 1993 1996
Note: The transportation end-use sector consumes minor
quanties of electricty.
ity was consumed primarily in the residential, commer-
cial, and industrial end-use sectors for uses such as light-
ing, heating, electric motors, appliances, electronics, and
air conditioning (see Figure 2-11).
In 1998, retail sales by electric utilities increased
in all end-use sectors due largely to robust economic
growth and the year's summer weather conditions. The
summer of 1998 for the United States was exceptionally
warm, with cooling degree days 14 percent above nor-
mal (see Figure 2-10).7 As a result, in part, of increased
demand for electricity, especially for air conditioning,
emissions from electric utilities rose by 3.2 percent from
1997 to 1998.
To generate the majority of this electricity, utili-
ties combusted fossil fuels, especially coal. In 1998, elec-
tric utilities were the largest producers of CO2 emissions
from fossil fuel combustion, accounting for 37 percent.
Electric utilities were responsible for such a large share
of emissions partly because they rely on more carbon
intensive coal for a majority of their primary energy.
Some of the electricity consumed in the United States
was generated using low or zero CO2 emitting technolo-
gies such as hydroelectric or nuclear energy. In 1998,
however, coal, natural gas, and petroleum were used to
produce the majority—52, 15, and 4 percent, respec-
tively—of the electricity generated by utilities in the
United States (EIA 1999b).
Electric utilities were the dominant consumer of
coal in the United States, accounting for 88 percent in
1998. Consequently, changes in electricity demand have
a significant impact on coal consumption and associ-
ated CO2 emissions. In fact, electric utilities consumed
record amounts of coal (18,717 TBtu) in 1998. Overall,
emissions from coal burned at electric utilities increased
by 17 percent from 1990 to 1998. This increase in coal-
related emissions from was alone responsible for 46 per-
cent of the overall rise in CO2 emissions from fossil fuel
combustion.
In addition to this rise in consumption of coal,
consumption of both natural gas and petroleum also rose
in 1998 by 10 and 42 percent, respectively (EIA 1999f).
This dramatic change in petroleum consumption was
due mainly to a drop in petroleum prices (26 percent or
the lowest price in 20 years) and the increased electric-
ity demand which required the use of idle or
underutilized petroleum units (EIA 1999b).
Demand for fossil fuels by electric utilities is also
affected by the supply of electricity from other energy
sources. In 1998, there was a significant decline in hy-
droelectric generation (8.5 percent) due mainly to re-
duced snowfall in the Northwest (EIA 1999b). This de-
7 Cooling degree days in 1998 were approximately 3 standard deviations above the normal value (i.e., average of 1961 to 1990).
Energy 2-11
-------
Box 2-1: Sectoral Carbon Intensity Trends Related to Fossil Fuel and Overall Energy Consumption ^
Fossil fuels areThTpred^minant source of energy in the United States, andrarbon dioxide (C02) is emitted as a product from their
complete combustion. Useful energy, however, can be generated from many other sources that do not emit C02 in the energy
conversion process." In the United States, useful energy is also produced from renewable (i.e., hydropower, biofuels, geothermal,
solar, and wind) and nuclear sources.
Energy-related COZ emissions can be reduced by not only lowering total energy consumption (e.g., through conservation
measures) but also by lowering the carbon intensity of the energy sources employed (e.g., fuel switching from coal to natural gas).
The amount of carbon emitted-in the form of C02-from the combustion of fossil fuels is dependent upon the carbon content of
the fuel and the fraction of that carbon that is oxidized. Fossil fuels vary in their average carbon content, ranging from about 13.7
MMTCE/EJ for natural gas to 26.4 MMTCE/EJ for coal and petroleum coke.9 In general, the carbon intensity of fossil fuels is the
highestfor coal products, followed by petroleum and then natural gas. Other sources of energy, however, may be directly or indirectly
carbon neutral (I e 0 MMTCE/EJ). Energy generated from nuclear and many renewable sources do not result in direct emissions of
CO, Biofuels such as wood and ethanol are also considered to be carbon neutral, as the C02 emitted during combustion is assumed
to be offset by the carbon sequestered in the growth of new biomass.10 The overall carbon intensity of the U.S. economy is thus
dependent upon the quantity and combination of fuels and other energy sources employed to meet demand.
Table 2-9 provides a time series of the carbon intensity for each sector of the U.S. economy. The time series incorporates only the
energy consumed from the direct combustion of fossil fuels in each sector. For example, the carbon intensity for the residential sector
does not include the energy from or emissions related to the consumption of electricity for lighting or wood for heat. Looking only at
this direct consumption of fossil fuels, the residential sector exhibited the lowest carbon intensity, which was related to the large
percentage of energy derived from natural gas for heating. The carbon intensity of the commercial sector was greater than the
residential sector for the period from 1990 to 1996, but then declined to an equivalent level as commercial businesses shifted away
from petroleum to natural gas. The industrial sector was more dependent on petroleum and coal than either the residential or
commercial sectors, and thus had higher carbon intensities over this period. The carbon intensity of the transportation sector was
closely related to the carbon content of petroleum products (e.g., motor gasoline and jet fuel), which were the primary sources of
energy. Lastly, the electric utility sector had the highest carbon intensity due to its heavy reliance on coal for generating electricity.
Table 2-9: Carbon Intensity from Direct Fossil Fuel Combustion by Sector (MMTCE/EJ)
Sector
Residential3
Commercial3
Industrial3
Transportation3
Electric Utilities6
All Sectors0
1990
14.7
15.2
16.9
18.3
22.4
18.7
1991
14.7
15.1
16.8
18.3
22.4
18.7
1992
14.6
15.0
16.7
18.3
22.4
18.6
1993
14.6
14.9
16.6
18.3
22.5
18.7
1994
14.6
14.9
16.6
18.3
22.4
18.6
1995
14.6
14.8
16.5
18.2
22.4
18.6
1996
14.6
14.8
16.4
18.2
22.6
18.6
1997
14.7
14.7
16.4
18.2
22.6
18.6
1998
14.7
14.8
16.4
18.2
22.5
18.7
» Does not Include electricity or renewable energy consumption.
b Does not include electricity produced using nuclear or renewable energy.
c Does not include nuclear or renewable energy consumption. no/iTo
Note: Excludes non-energy fuel use emissions and consumption. Exajoule (EJ) = 10l8joules = 0.9479
8 C02 emissions, however, may be generated from upstream activities (e.g., manufacture of the equipment).
9 One cxajoule (EJ) is equal to 1018 joules or 0.9478 QBtu.
'» This statement assumes that there is no net loss of biomass-based carbon associated with the land use practices used to produce these
biomass fuels.
2-12 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
In contrast to Table 2-9, Table 2-10 presents carbon intensity values that incorporate energy consumed from all sources (i.e., fossil
fuels, renewables, and nuclear). In addition, the emissions related to the generation of electricity have been attributed to both electric
utilities and the sector in which that electricity was eventually consumed.11 This table, therefore, provides a more complete picture of
Jhe actual carbon intensity of each sector per unit of energy consumed. The transportation sector in Table 2-10 emerges as the most
carbon intensive when all sources of energy are included, due to its almost complete reliance on petroleum products and relatively
|minor amount of biomass based fuels such as ethanol. The "other end-use sectors" (i.e. the residential, commercial, and industrial
^-sectors) use significant quantities of biofuels such as wood, thereby lowering the overall carbon intensity. The carbon intensity of
|:electric utilities differs greatly from the scenario in Table 2-9 where only the energy consumed from the direct combustion of fossil fuels
f was included. This difference is due almost entirely to the inclusion of electricity generation from nuclear and hydropower sources
4 which do not emit carbon dioxide.
Table 2-10: Carbon Intensity from Energy Consumption by Sector (MMTCE/EJ)
if
Sector
1990 1991 1992 1993 1994
|; Transportation3
I Other End-Use Sectors3'6
jt Electric Utilities0
tAll Sectors"
18.2
14.9
15.3
15.8
18.2
14.7
15.0
15.6
18.2
14.7
15.2
15.7
18.2
14.8
15.3
15.7
18.2
14.7
15.2
15.7
18.1
14.4
14.8
15.5
18.1
14.5
14.9
15.5
18.1
14.7
15.4
15.7
18.1
14.7
15.4
15.7
- . —. 1 — •- «"u.««") and direct renewable energy consumption
,._' Other End-Use Sectors include the residential, commercial, and industrial sectors.
I;.? Includes electricity generation from nuclear and renewable sources.
it- ^ Includes nuclear and renewable energy consumption.
,;Jote: Excludes non-energy fuel use emissions and consumption. Assumed that residential consumed all of the biofuel-based energy and 50
^percent of the solar energy in the combined EIA residential/commercial sector category. Exajoule (EJ) = 1018 joules = 0.9479 QBtu!
j By comparing the values in Table 2-9 and Table 2-10, a
| -couple of observations can be made. The usage of renewable
| and nuclear energy sources has resulted in a significantly lower
; carbon intensity of the U.S. economy. However, over the nine
[year period of 1990 through 1998, the carbon intensity of
f U.S. fossil fuel consumption has been fairly constant, as the
j proportion of renewable and nuclear energy technologies has
| not changed significantly.
I;._. Figure 2-12 and Table 2-11 present the detailed C02emis-
tsion trends underlying the carbon intensity differences and
| changes described in Table 2-9. In Figure 2-12, changes over
pime in both overall end-use sector-related emissions and elec-
|tricit,y-related emissions for each year since 1990 are high-
| lighted. In Table 2-11 changes in emissions since 1990 are
^presented by sector and fuel type to provide more detail on
|these changes.
Figure 2-12
Dark shaded columns relate to changes in emissions from
electricity consumption. Lightly shaded columns relate to
changes in emissions from direct fossil fuel combustion.
Residential Commercial Industrial Transportation
11 In other words, the emissions from the generation of electricity are intentionally double counted by attributing them both to utilities
and the sector m which electricity consumption occurred.
Energy 2-13
-------
Table 2-11: Change in C02 Emissions from Direct Fossil Fuel Combustion Since 1990 (MMTCE)
Sector/Fuel "type
Residential
Coal
Natural Gas
Petroleum
Commercial
Coal
Natural Gas
Petroleum
Industrial
Coal
Natural Gas
Petroleum
Transportation
Coal
Natural Gas
Petroleum
Electric Utility
Coal
Natural Gas
Petroleum
U.S. Territories
Coal
Natural Gas
Petroleum
All Sectors
+ Does not exceed
- Not applicable
1991
2.7
(0.1)
2.4
0.5
0.4
(0.2)
1.6
(0.9)
(7.5)
(3.6)
1.9
(5.7)
(8.1)
(0.9)
(7.2)
(3.4)
(1.8)
(0.1)
(1.5)
1.5
+
-
1.5
(14.3)
0.05 MMTCE
1992
5.1
(0.1)
4.4
0.9
0.6
(0.2)
2.7
(1-9)
6.0
(5.9)
7.7
4.2
1.5
(1.1)
2.6
(3.9)
2.8
(0.5)
(6-2)
0.6
+
-
0.6
9.9
1993
10.5
(0.1)
8.3
2.3
1.0
(0.2)
4.3
(3.1)
4.9
(6.3)
13.2
(1.9)
9.6
(0.6)
10.1
13.9
19.7
(1.7)
(4.1)
1.5
0.1
-
1.4
41.4
1994
7.9
(0.2)
6.6
1.4
0.7
(0.3)
4.0
(3.1)
10.9
(5.8)
14.7
2.0
22.8
0.3
22.5
17.3
20.5
2.8
(6.0)
2.3
0.1
-
2.2
61.8
1995
8.3
(0.2)
6.7
1.8
2.6
(0.3)
6.0
(3.0)
12.7
(6.4)
21.6
(2.5)
28.2
0.6
27.3
17.4
24.0
6.0
(12.5)
2.6
0.1
"
2.5
71.8
1996
15.5
(0.2)
12.4
3.3
4.2
(0.3)
7.9
(3.4)
21.1
(8.6)
26.3
3.4
42.1
0.7
41.4
36.4
48.5
(0.9)
(11.1)
1.8
0.1
"
1.8
121.2
1997
11.6
(0.1)
8.6
3.1
4.4
(0.1)
8.7
(4.2)
21.7
(9.0)
24.9
5.8
43.9
1.4
42.5
56.2
62.8
2.4
(8.9)
2.8
0.1
2.7
140.6
1998
4.2
(0.1)
1.2
.1 -
1.6
(0.2)
.u
(4.3)
14.5
(9.4)
22.0
1.8
50.7
1.0 :
49.7
73.3
68.3
6.6
(1.6)
.9
.1
.0
148.1
Note: Totals may not sum due to independent rounding. ;
cline, however, offset by a slightly larger increase in
electricity generation at nuclear power plants (7 percent)
after seven generating units, that had previously been
idle, were brought back into service (EIA 1999b).
It is important to note that the electric utility sec-
tor includes only regulated utilities. According to cur-
rent EIA sectoral definitions, nonutility generators of
electricity (e.g., independent power producers, qualify-
ing cogenerators, and other small power producers) are
included in the industrial sector. These nonutility gen-
erators produce electricity for their own use, to sell to
large consumers, or to sell on the wholesale electricity
market. The number and quantity of electricity gener-
ated by nonutilities has increased significantly as many
states have begun deregulating their electricity markets.
A recent report by the U.S. Department of Energy
and the EPA (DOE and EPA 1999) estimated emissions
from the entire electric power industry, including regu-
lated utilities and nonutilities. According to this report
CO2 emissions from nonutilities in 1998 were 56
MMTCE, bringing combined emissions from electricity
generation up to 41 percent (605.5 MMTCE) of total
U.S. CO2 emissions from fossil fuel combustion, versus
37 percent from utilities alone. In other words,
nonutilities were responsible for 10 percent of emissions
from electricity generation. The growth in nonutility
emissions from 1997 to 1998 was 9 percent. In future
inventories, these nonutility generators will be removed
from the industrial sector and incorporated into a single
sector with electric utilities.
Methodology
The methodology used by the United States for
estimating CO2 emissions from fossil fuel combustion is
conceptually similar to the approach recommended by
the IPCC for countries that intend to develop detailed,
2-14 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
sectoral-based emission estimates (IPCC/UNEP/OECD/
IEA 1997). A detailed description of the U.S. methodol-
ogy is presented in Annex A, and is characterized by the
following steps:
1. Determine fuel consumption by fuel type and
sector. By aggregating consumption data by sector (e.g.,
commercial, industrial, etc.), primary fuel type (e.g., coal,
petroleum, gas), and secondary fuel category (e.g., mo-
tor gasoline, distillate fuel oil, etc.), estimates of total
U.S. fossil fuel consumption for a particular year were
made. The United States does not include territories in
its national energy statistics; therefore, fuel consump-
tion data for territories was collected separately.12
2. Determine the total carbon content of fuels
consumed. Total carbon was estimated by multiplying
the amount of fuel consumed by the amount of carbon in
each fuel. This total carbon estimate defines the maxi-
mum amount of carbon that could potentially be released
to the atmosphere if all of the carbon in each fuel were
converted to CO2. The carbon content coefficients used
by the United States are presented in Annex A.
3. Subtract the amount of carbon stored in prod-
ucts. Non-energy uses of fossil fuels can result in storage
of some or all of the carbon contained in the fuel for
some period of time, depending on the end-use. For ex-
ample, asphalt made from petroleum can sequester up to
100 percent of the carbon for extended periods of time,
while other fossil fuel products, such as lubricants or
plastics, lose or emit some carbon when they are used
and/or burned as waste. Aggregate U.S. energy statistics
include consumption of fossil fuels for non-energy uses;
therefore, the portion of carbon that remains in products
after they are manufactured was subtracted from poten-
tial carbon emission estimates. The amount of carbon
remaining in products was based on the best available
data on the end-uses and fossil fuel products. These non-
energy uses occurred in the industrial and transporta-
tion sectors and U.S. territories.13
4. Adjust for carbon that does not oxidize during
combustion. Because combustion processes are not 100
percent efficient, some of the carbon contained in fuels
is not emitted to the atmosphere. Rather, it remains be-
hind as soot and ash. The estimated amount of carbon
not oxidized due to inefficiencies during the combus-
tion process was assumed to be 1 percent for petroleum
and coal and 0.5 percent for natural gas (see Annex A).
5. Subtract emissions from international bunker
fuels. According to the IPCC guidelines (IPCC/UNEP/
OECD/IEA 1997) emissions from international transport
activities, or bunker fuels, should not be included in
national totals. Because U.S. energy consumption sta-
tistics include these bunker fuels—distillate fuel oil, re-
sidual fuel oil, and jet fuel—as part of consumption by
the transportation sector, emissions from international
transport activities were calculated separately and sub-
tracted from emission estimates for the transportation
sector. The calculations for emissions from bunker fuels
follow the same procedures used for emissions from con-
sumption of all fossil fuels (i.e., estimation of consump-
tion, determination of carbon content, and adjustment
for the fraction of carbon not oxidized).
6. Allocate transportation emissions by vehicle
type. Because the transportation end-use sector was the
largest direct consumer of fossil fuels in the United
States,14 a more detailed accounting of carbon dioxide
emissions is provided. For fuel types other than jet fuel,
fuel consumption data by vehicle type and transporta-
tion mode were used to allocate emissions by fuel type
calculated for the transportation end-use sector. Specific
data by vehicle type were not available for 1998; there-
fore, the 1997 percentage allocations were applied to
1998 fuel consumption data in order to estimate emis-
sions in 1998. Military vehicle jet fuel consumption was
provided by the Defense Energy Support Center, under
Department of Defense's (DoD) Defense Logistics Agency
and the Office of the Undersecretary of Defense (Envi-
T H- '" Sam°a' °Uam' Puert° Rico' U'S- Vifgin Islands- Wake Isla"d> and other U.S.
cific Islands) is included in this report and contributed emissions of!3 MMTCE in 1998.
"See Waste Combustion section of Waste chapter for discussion of emissions from the combustion of plastics in the municipal solid
W3.SEC Slcftm. • "•
t C0nsidered a final end-use sector' because they consume energy solely to provide electricity to the other
Energy 2-15
-------
ronmental Security). The difference between total U.S.
jet fuel consumption (as reported by DOE/EIA) and ci-
vilian air carrier consumption for both domestic and in-
ternational flights (as reported by DOTVBTS and BEA)
plus military jet fuel consumption is reported as "other"
under the jet fuel category in Table 2-8, and includes
such fuel uses as blending with heating oils and fuel
used for chartered aircraft flights.
Data Sources
Data on fuel consumption for the United States
and its territories, carbon content of fuels, and percent of
carbon sequestered in non-energy uses were obtained
directly from the Energy Information Administration
(EIA) of the U.S. Department of Energy (DOE). Fuel con-
sumption data were obtained primarily from the Monthly
Energy Review (EIA 1999f) and various EIA databases.
Data on military jet fuel use was supplied by the Office
of the Under Secretary of Defense (Environmental Secu-
rity) and the Defense Energy Support Center (Defense
Logistics Agency) of the U.S. Department of Defense
(DoD). Estimates of international bunker fuel emissions
are discussed in the section entitled International Bun-
ker Fuels.
IPCC (IPCC/UNEP/OECD/IEA 1997) provided
combustion efficiency rates for petroleum and natural
gas. Bechtel (1993) provided the fraction oxidation val-
ues for coal. Vehicle type fuel consumption"oVata for the
allocation of transportation sector emissions were pri-
marily taken from the Transportation Energy Databook
prepared by the Center for Transportation Analysis at
Oak Ridge National Laboratory (DOE 1993,1994,1995,
1996, 1997, 1998). All jet fuel and aviation gasoline
was assumed to have been consumed in aircraft. Densi-
ties for each military jet fuel type were obtained from the
Air Force (1998).
Carbon intensity estimates were developed using
nuclear and renewable energy data from EIA (1998a)
and fossil fuel consumption data as discussed above and
presented in Annex A.
For consistency of reporting, the IPCC has recom-
mended that national inventories report energy data (and
emissions from energy) using the International Energy
Agency (EEA) reporting convention and/or BEA data. Data
in the IEA format are presented "top down"—that is,
energy consumption for fuel types and categories are
estimated from energy production data (accounting for
imports, exports, stock changes, and losses). The result-
ing quantities are referred to as "apparent consumption."
The data collected in the United States by EIA, and used
in this inventory, are, instead, "bottom up" in nature. In
other words, they are collected through surveys at the
point of delivery or use and aggregated to determine
national totals.
It is also important to note that EIA uses gross
calorific values (GCV) (i.e., higher heating values) as its
reporting standard for energy statistics. Fuel consump-
tion activity data presented here have not been adjusted
to correspond to international standard, which are to re-
port energy statistics in terms of net calorific values
(NCV) (i.e., lower heating values).
Uncertainty
For estimates of CO2 from fossil fuel combustion,
the amount of CO2 emitted, in principle is directly re-
lated to the amount of fuel consumed, the fraction of the
fuel that is oxidized, and the carbon content of the fuel.
Therefore, a careful accounting of fossil fuel consump-
tion by fuel type, average carbon contents of fossil fuels
consumed, and consumption of products with long-term
carbon storage should yield an accurate estimate of CO2
emissions.
There are uncertainties, however, concerning the
consumption data sources, carbon content of fuels and
products, and carbon oxidation efficiencies. For example,
given the same primary fuel type (e.g., petroleum), the
amount of carbon contained in the fuel per unit of useful
energy can vary. Non-energy uses of the fuel can also
create situations where the carbon is not emitted to the
atmosphere (e.g., plastics, asphalt, etc.) or is emitted at a
delayed rate. The proportions of fuels used in these non-
energy production processes that result in the sequestra-
tion of carbon have been assumed. Additionally, ineffi-
ciencies in the combustion process, which can result in
ash or soot remaining unoxidized for long periods, were
also assumed. These factors all contribute to the uncer-
tainty in the CO2 estimates.
2-16 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Other sources of uncertainty are fuel consumption
by U.S. territories and bunker fuels consumed by the
military. The United States does not collect as detailed
energy statistics for its territories as for the fifty states
and the District of Columbia. Therefore estimating both
emissions and bunker fuel consumption by these territo-
ries is difficult.
For Table 2-8, uncertainties also exist as to the
data used to allocate CO2 emissions from the transporta-
tion end-use sector to individual vehicle types and trans-
port modes. In many cases, bottom up estimates of fuel
consumption by vehicle type do not match top down
estimates from EIA. Further research is planned to better
allocate detailed transportation end-use sector emissions.
For the United States, however, these uncertainties
impact on overall CO2 emission estimates are believed
to be relatively small. For the United States, CO2 emis-
sion estimates from fossil fuel combustion are consid-
ered accurate within one or two percent. See, for example,
Marland and Pippin (1990).
Stationary Combustion
(excluding C02)
Stationary combustion encompasses all fuel com-
bustipn activities except those related to transportation
(i.e., mobile combustion). Other than carbon dioxide
(CO2), which was addressed in the previous section, gases
from stationary combustion include the greenhouse gases
methane (CH4) and nitrous oxide (N2O) and the criteria
pollutants nitrogen oxides (NOX), carbon monoxide (CO),
and non-methane volatile organic compounds
(NMVOCs).15 Emissions of these gases from stationary
sources depend upon fuel characteristics, technology
type, usage of pollution control equipment, and ambi-
ent environmental conditions. Emissions also vary with
the size and vintage of the combustion technology as
well as maintenance and operational practices.
Nitrous oxide and NOX emissions from stationary
combustion are closely related to air-fuel mixes and com-
bustion temperatures, as well as the characteristics of
any pollution control equipment that is employed. Car-
bon monoxide emissions from stationary combustion
are generally a function of the efficiency of combustion
and the use of emission controls; they are highest when
less oxygen is present in the air-fuel mixture than is nec-
essary for complete combustion. These conditions are
most likely to occur during start-up and shut-down and
during fuel switching (e.g., the switching of coal grades
at a coal-burning electric utility plant). Methane and
NMVOC emissions from stationary combustion are pri-
marily a function of the CH4 content of the fuel, com-
bustion efficiency, and post-combustion controls.
Emissions of CH4 increased slightly from 1990 to
1996, but fell to just below the 1990 level in 1998 to 2.3
MMTCE (395 Gg). This decrease in emissions was pri-
marily due to lower wood consumption in the residen-
tial sector. Nitrous oxide emissions rose 12 percent since
1990 to 4.3 MMTCE (50 Gg) in 1998. The largest source
of N2O emissions was coal combustion by electric utili-
ties, which alone accounted for 54 percent of total N2O
emissions from stationary combustion in 1998. Overall,
though, stationary combustion is a small source of CH4
and N2O in the United States.
In contrast, stationary combustion was a signifi-
cant source of NOX emissions, but a smaller source of CO
and NMVOCs. In 1998, emissions of NOX from station-
ary combustion represented 44 percent of national NOX
emissions, while CO and NMVOC emissions from sta-
tionary combustion contributed approximately 6 and 5
percent, respectively, to the national totals. From 1990
to 1998, emissions of NOX were fairly constant, while
emissions of CO and NMVOCs decreased by 10 and 15
percent, respectively.
The decrease in CO and NMVOC emissions from
1990 to 1998 can largely be attributed to decreased resi-
dential wood consumption, which is the most signifi-
cant source of these pollutants from stationary combus-
tion. Overall, NOX emissions from energy varied due to
fluctuations in emissions from electric utilities. Table
2-12 through Table 2-15 provide CH4 and N2O emission
Sulfur dioxide (SO2) emissions from stationary combustion are addressed in Annex M.
Energy 2-17
-------
Table 2-12: CH4 Emissions from Stationary Combustion (MMTCE)
1
J
I
I
L
?
:
Illl
„
-
§
Sector/Fuel Type
Electric Utilities
Coal
Fuel Oil
Natural Gas
Wood
Industrial
Coal
Fuel Oil
Natural Gas
Wood
Commercial/Institutional
Coal
Fuel Oil
Natural Gas
Wood*
Residential
Coal
Fuel Oil
Natural Gas
Wood
Total
1990
0.1
0.1
+
+
+
0.8
0.2
0.1
0.2
0.3
0.1
+
0.1
0.1
NA
1.3
0.1
0.1
0.1
0.9
2.3
1991
0.1
0.1
+
+
+
0.8
0.1
0.1
0.2
0.3
0.1
+
+
0.1
NA
1.3
0.1
0.1
0.1
1.0
2.4
1992
0.1
0.1
+
+
+
0.8
0.1
0.1
0.2
0.3
0.1
+
+
0.1
NA
1.4
0.1
0.1
0.1
1.1
2.4
1993
0.1
0.1
+
+
+
0.8
0.1
0.1
0.3
0.3
0.1
+
+
0.1
NA
1.3
0.1
0.1
0.1
1.0
2.4
1994
0.1
0.1
+
+
+
0.9
0.1
0.1
0.3
0.4
0.1
+
+
0.1
NA
1.3
0.1
0.1
0.1
0.9
2.4
1995
0.1
0.1
+
+
+
0.9
0.1
0.1
0.3
0.4
0.1
+
+
0.1
NA
1.4
0.1
0.1
0.1
1.0
2.5
1996
0.1
0.1
+
+
+
0.9
0.1
0.1
0.3
0.4
0.1
+
+
0.1
NA
1.4
0.1
0.1
0.1
1.1
2.6
1997
0.1
0.1
+
+
+
0.9
0.1
0.1
0.3
0.4
0.1
+
+
0.1
NA
1.1
0.1
0.1
0.1
0.8
2.3
1998
0.1
.1 :
: +
+ -. :
~^" ,;
0.9
0.1
0.1
0.3
0.4
0.1 ..:
+
0.1
NA
1.1
0.1
0.1
0.1
0.8
2.3
+ Does not exceed 0.05 MMTCE
i ' Commercial/institutional emissions from the combustion of wood are included under the residential sector.
1 Note: Totals may not sum due io independent rounding.
Table 2-13: N20 Emissions from Stationary Combustion (MMTCE)
Sector/Fuel Type
Electric Utilities
f Coal
Fuel Oil
: Natural Gas
Wood
Industrial
Coal
* Fuel Oil
" Natural Gas
Wood
Commercial/Institutional
Coal
Fuel Oil
Natural Gas
Wood*
Residential
Coal
Fuel Oil
Natural Gas
Wood
Total
+ Does not exceed 0.05 MMTCE
1990
2.0
1.9
0.1
+
_l_
1.4
0.3
0.4
0.1
0.6
0.1
+
+
NA
0.3
0.1
0.2
3.8
1991
2.0
1.9
0.1
+
+
1.4
0.3
0.4
0.1
0.6
0.1
+
+
NA
0.3
0.1
0.2
3.8
NA (Not Available)
* Commercial/institutional emissions from the combustion of
Note: Totals may not sum due to
1992
2.0
1.9
+
+
+
1.5
0.3
0.4
0.1
0.7
0.1
+
+
NA
0.3
0.1
0.2
3.9
1993 1994 1995
2.1
2.0
0.1
+
+
1.5
0.3
0.4
0.1
0.7
0.1
+
+
NA
0.3
0.1
0.2
3.9
wood are included under the
2.1
2.0
+
+
+
1.5
0.3
0.5
0.1
0.7
0.1
+
+
NA
0.3
0.1
0.2
4.0
residential
2.1
2.0
+
+
+
1.5
0.3
0.4
0.1
0.7
0.1
+
+
NA
0.3
0.1
0.2
4.0
sector.
1996
2.2
2.1
+
+
+
1.6
0.3
0.5
0.1
0.8
0.1
+
+
NA
0.3
0.1
0.2
4.2
1997
2.3
2.2
+
+
+
1.6
0.3
0.5
0.1
0.8
0.1
+
+
NA
0.3
0.1
0.2
4.2
-^.'--•-•.•i^l
1998
2.3
2.2 -:
.1 :
+
+
1.6
0.3
0.5
0.1 :
0.8 i
0.1
NA
0.3
0.1
0.2 ;
4.3
.
independent rounding. ;
2-18 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Table 2-14: CH4 Emissions from Stationary Combustion (Gg)
Kector/Fuel Type
f« Clectric Utilities
Coal
»,- Fuel Oil
ffi.
t Natural Gas
m_ Wood
fTndustrial
|1 Coal
jfe ._ Fuel Oil
£:: Natural Gas
JT Wood
Commercial/Institutional
\cl Coal
^ Fuel Oil
Jr Natural Gas
';.". Wood*
.Residential
|*^ Coal
E, Fuel Oil
k- Natural Gas
T Wood
!-- -• i
F Total
1990
23
16
4
3
1
140
27
17
40
55
23
1
9
13
NA
218
19
13
21
166
404
1991
23
16
4
3
1
138
26
16
41
55
23
1
9
13
NA
227
17
13
22
175
410
1992
22
16
3
3
1
143
25
17
43
58
23
1
8
14
NA
237
17
13
23
184
425
1993
23
17
3
3
1
145
25
17
45
59
23
1
8
14
NA
224
17
14
24
169
415
1994
23
17
3
3
1
151
25
18
45
63
23
1
8
14
NA
219
17
13
24
166
416
1995
23
17
2
3
155
24
17
48
65
23
1
8
15
NA
236
16
14
24
183
437
1996
23
18
2
•3
\j
159
24
18
49
68
24
1
7
15
NA
240
17
14
26
183
446
1997
24
19
p
C.
159
23
19
49
68
24
•t
i
7
/
16
NA
191
17
14
24
135
399
1998
26
19
H
I
160
23
18
48
70
23
7
/
15
NA
187
17
14
22
133
395
pIA (Not Available)
jr*: Commercial/institutional emissions from the combustion of
j-Jote: Totals may not sum due to independent rounding.
wood are included under the residential sector.
Table 2-15: N20 Emissions from Stationary Combustion (Gg)
pSector/Fuel Type
I Electric Utilities
£,- Coal
E: Fuel Oil
H- Natural Gas
SL, Wood
^Industrial
|; Coal
1 Fuel Oil
%- Natural Gas
5- Wood
| Commercial/Institutional
£••• Coal
1 ----- Fuel Oil
|: - Natural Gas
1;: Wood*
Residential
:-.. Coal
f~ Fuel Oil
f- Natural Gas
J- Wood
1 Total
f + Does not exceed 0.5 Gg
| NA (Not Available)
1990
24
23
1
_j_
+
17
4
5
1
7
1
_^_
1
NA
3
~h
1
+
2
45
1991
24
22
1
,
17
4
5
1
7
1
,
1
NA
4
_|_
1
+
2
45
£•*- Commercial/institutional emissions from the combustion
jt Note: Totals may not sum due to independent rounding.
F--;- -
1992
24
23
1
17
3
5
1
8
1
,
+
NA
4
4-
1
+
2
46
of wood
1993
25
24
1
17
3
5
1
8
1
^
+
NA
4
t
1
2
46
are included
1994
25
24
1
+
18
3
5
1
8
1
+
.
NA
4
,
1
2
47
1995
25
24
+
18
3
5
1
9
1
+
^
NA
4
.
1
2
48
iqqfi
?6
25
+
19
7
5
g
1
+
+
NA
4
(
.
1
2
/|q
1997
27
26
+
19
Q
K
-1
Q
+
+
NA
3
1
2
50
under the residential sector.
1998
27
26 ;
H
+
19
3:
c
9'
' '
+
+
NA ;
3
+ ;
p :
50
;
Energy 2-19
-------
Table 2-16: NOX, CO, and NMVOC Emissions
from Stationary Combustion in 1998 (Gg)
Sector/Fuel "type
NOX
CO NMVOC
Electric Utilities
Coal
Fuel Oil
Natural Gas
Wood
Internal Combustion
Industrial
Coal
Fuel Oil
Natural Gas
Wood
Other Fuels3
Internal Combustion
Commercial/Institutional
Coal
Fuel Oil
Natural Gas
Wood
Other Fuels3
Residential
Coatb
Fuel Oil"
Natural Gas"
Wood
Other Fuels0
Total
5,535
4,894
189
310
NA
141
2,997
613
209
1,141
NA
115
918
364
33
85
219
NA
27
823
NA
NA
NA
32
791
9,719
377
231
16
78
NA
53
1,090
91
52
314
NA
314
319
134
12
17
55
NA
50
2,891
NA
NA
NA
2,636
255
4,491
48
26
4
8
NA
9
166
5
6
46
NA
40
69
23
1
3
12
NA
8
539
NA
NA
NA
500
39
776
NA (Not Available)
Note: Totals may not sum due to independent rounding.
See Annex B for emissions in 1990 through 1998.
» "Other Fuels" include LP6, waste oil, coke oven gas,
coke, and non-residential wood (EPA 1999).
b Coal, fuel oil, and natural gas emissions are included
in the "Other Fuels" category (EPA 1999).
c "Other Fuels" include LPG, waste oil, coke oven gas,
and coke (EPA 1999).
estimates from stationary sources by sector and fuel type.
Estimates of NOX, CO, and NMVOC emissions in 1998
are given in Table 2-16.16
Methodology
Methane and nitrous oxide emissions were esti-
mated by multiplying emission factors (by sector and
fuel type) by fossil fuel and wood consumption data.
National coal, natural gas, fuel oil, and wood consump-
tion data were grouped into four sectors—industrial, com-
mercial/institutional, residential, and electric utilities.
For NOX, CO, and NMVOCs, the major source cat-
egories included in this section are those used in EPA
(1999): coal, fuel oil, natural gas, wood, other fuels (in-
cluding LPG, coke, coke oven gas, and others), and sta-
tionary internal combustion. The EPA estimates emis-
sions of NOX, CO, and NMVOCs by sector and fuel source
using a "bottom-up" estimating procedure. In other words,
emissions were calculated either for individual sources
(e.g., industrial boilers) or for multiple sources combined,
using basic activity data as indicators of emissions. De-
pending on the source category, these basic activity data
may include fuel consumption, fuel deliveries, tons of
refuse burned, raw material processed, etc.
The EPA derived the overall emission control effi-
ciency of a source category from published reports, the
1985 National Acid Precipitation and Assessment Pro-
gram (NAPAP) emissions inventory, and other EPA data-
bases. The U.S. approach for estimating emissions of NOX,
CO, and NMVOCs from stationary source combustion, as
described above, is consistent with the methodology rec-
ommended by the IPCC (ff CC/UNEP/OECD/IEA1997).
More detailed information on the methodology
for calculating emissions from stationary sources, includ-
ing emission factors and activity data, is provided in
Annex B.
Data Sources
Emissions estimates forNOx, CO, and NMVOCs in
this section were taken directly from the EPA's National
Air Pollutant Emissions Trends: 1900 -1998 (EPA 1999).
Fuel consumption data were provided by the U.S. En-
ergy Information Administration's Annual Energy Re-
view (EIA 1999a) and Monthly Energy Review (EIA
1999b). Emission factors were provided by the Revised
1996 IPCC Guidelines for National Greenhouse Gas
Inventories (IPCC/UNEP/OECD/ffiA 1997).
Uncertainty
Methane emission estimates from stationary
sources are highly uncertain, primarily due to difficul-
ties in calculating emissions from wood combustion (i.e.,
fireplaces and wood stoves). The estimates of CH4 and
* Sec Annex B for a complete time series of criteria pollutant emission estimates for 1990 through 1998.
2-20 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
N2O emissions presented are based on broad indicators
of emissions (i.e., fuel use multiplied by an aggregate
emission factor for different sectors), rather than specific
emission processes (i.e., by combustion technology and
type of emission control). The uncertainties associated
with the emission estimates of these gases are greater
than with estimates of CO2 from fossil fuel combustion,
which mainly rely on the carbon content of the fuel com-
busted. Uncertainties in both CH4 and N2O estimates are
due to the fact that emissions are estimated based on
emission factors representing only a limited subset of
combustion conditions. For the criteria pollutants, un-
certainties are partly due to assumptions concerning
combustion technology types, age of equipment, emis-
sion factors used, and activity data projections.
Mobile Combustion (excluding C02)
Mobile combustion emits greenhouse gases other
than CO2, including methane (CH4), nitrous oxide (N2O),
and the criteria pollutants carbon monoxide (CO), nitro-
gen oxides (NOX), and non-methane volatile organic
compounds (NMVOCs).
As with stationary combustion, N2O and NOX emis-
sions are closely related to fuel characteristics, air-fuel
mixes, combustion temperatures, as well as usage of pol- '
lution control equipment. Nitrous oxide, in particular,
can be formed by the catalytic processes used to control
NOX and CO emissions. Carbon monoxide emissions from
mobile source combustion are significantly affected by
combustion efficiency and presence of post-combustion
emission controls. Carbon monoxide emissions are high-
est when air-fuel mixtures have less oxygen than required
for complete combustion. This occurs especially in idle,
low speed and cold start conditions. Methane and
NMVOC emissions from motor vehicles are a function of
the CH4 content of the motor fuel, the amount of hydro-
carbons passing uncombusted through the engine, and
any post-combustion control of hydrocarbon emissions,
such as catalytic converters.
Emissions from mobile combustion were estimated
by transport mode (e.g., highway, air, rail, and water) and
fuel type—motor gasoline, diesel fuel, jet fuel, aviation
gas, natural gas, liquefied petroleum gas (LPG), and re-
sidual fuel oil—and vehicle type. Road transport ac-
counted for the majority of mobile source fuel consump-
tion, and hence, the majority of mobile combustion emis-
sions. Table 2-17 through Table 2-20 provide CH4 and
N2O emission estimates from mobile combustion by ve-
hicle type, fuel type, and transport mode. Estimates of
NOX, CO, and NMVOC emissions in 1998 are given in
Table 2-21.1T
Mobile combustion was responsible for a small
portion of national CH4 emissions but were the second
largest source of N2O in the United States. From 1990 to
1998, CH4 emissions declined by 10 percent, to 1.3
MMTCE (232 Gg). Nitrous oxide emissions, however,
rose 25 percent to 17.2 MMTCE (203 Gg) (see Figure
2-13). The reason for this conflicting trend was that the
control technologies employed on highway vehicles in
the United States lowered CO, NOX, NMVOC, and CH4
emissions, but resulted in higher average N2O emission
rates. Fortunately, since 1994 improvements in the emis-
sion control technologies installed on new vehicles have
reduced emission rates of both NOX and N2O per vehicle
mile traveled. Overall, CH4 and N2O emissions were domi-
Figure 2-13
1990 1991 1992 1993 1994 1995 1996 1997 1998
See Annex C for a complete time series of criteria pollutant emission estimates for 1990 through 1998.
Energy 2-21
-------
Table 2-17: CH4 Emissions from Mobile Combustion (MMTCE)
s Fuel Type/Vehicle Type
LGasoline Highway
! Passenger Cars
? Light-Duty Trucks
1 Heavy-Duty Vehicles
Motorcycles
* Diesel Highway
Passenger Cars
: Light-Duty Trucks
! Heavy-Duty Vehicles
"Non-Highway
? Ships and Boats
Locomotives
Farm Equipment
Construction Equipment
Aircraft
Other*
L Total
1990
1.3
0.7
0.5
0.1
0.1
+
0.1
0.1
+
+
+
+
+
+
1.5
1991
1.3
0.7
0.5
0.1
0.1
+
0.1
0.1
+
+
+
+
+
+
1.5
1992
1.3
0.6
0.6
0.1
0.1
+
0.1
0.1
+
+
+
+
+
+
1.5
1993
1.3
0.6
0.6
0.1
0.1
+
0.1
0.1
+
+
+
+
+
+
1.5
1994
1.3
0.6
0.6
0.1
0.1
+
0.1
0.1
+
+
+
+
+
+
1.5
1995
1.2
0.6
0.6
0.1
0.1
+
0.1
0.1
+
+
+
+
+
1.4
1996
1.2
0.6
0.5
0.1
0.1
0.1
0.1
+
1.4
1997
1.2
0.5
0.5
0.1
+"
0.1
4^.
+
0.1
0.1
- 4.
_(_
_|-
_i_
-)-
1.4
l"998
1.2
0.5
0.5 :
0.1
0.1
0.1
0.1
4-
+
+
+
:_
1.3
+ Does not exceed 0.05 MMTCE
%^33X£&i ySSS^SL utility equipment, heavy-duty gasoline powered uti.ity equipment, and heavy-duty
" diesel powered utility equipment.
Table 2-18: N20 Emissions from Mobile Combustion (MMTCE)
Fuel Type/Vehicle Type
Gasoline Highway
Passenger Cars
Light-Duty Trucks
Heavy-Duty Vehicles
Motorcycles
Diesel Highway
Passenger Cars
Light-Duty Trucks
Heavy-Duty Vehicles
Non-Highway
Ships and Boats
Locomotives
Farm Equipment
Construction Equipment
Aircraft
Other*
Total
1990 1
12.5
8.0
4.2
0.2
0.5
+
0.4
0.8
0.1
0.1
0.1
0.5
+
' 13.8
991 1
13.3
8.0
5.1
0.2
0.5
+
0.5
0.8
0.1
0.1
0.1
0.5
+
14.6
1 992
14.4
8.3
5.8
0.2
0.5
+
0.5
0.8
0.1
0.1
0.1
0.5
+
15.7
1993
15.2
8.6
6.4
0.3
0.5
+
0.5
0.8
0.1
0.1
0.1
0.5
+
16.5
1994
15.7
8.8
6.6
0.3
0.6
+
0.5
0.8
0.1
0.1
0.1
0.5
"*"
17.1
1995
16.0
8.9
6.8
0.3
0.6
+
0.5
0.8
0.1
0.1
0.1
0.5
"""
17.4
1996
16.0
8.9
6.8
0.3
0.6
0.6
0.8
0.1
0.1
0.1
0.5
17.5
1997
15.9
8.7
6.8
0.3
_[_
0.6
0.6
0.8
0.1
0.1
0.1
0.5
17.3
ligV
15.8
8.6
6.8 j
0.4
0.6
+ .
_j.
0.6
0.8
0.1
0.1 '
0.1 ;
+
0.5 •]
_L :
17.2
+ Does not exceed 0.05 MMTCE
utiHty equipment, heavy-duty gasoline powered utiiity equipment, and heavy-duty
diesel powered utility equipment.
2-22 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Table 2-19: CH4 Emissions from Mobile Combustion (Gg)
Fuel Type/Vehicle Type
Gasoline Highway
; Passenger Cars
; Light-Duty Trucks
Heavy-Duty Vehicles
Motorcycles
; Diesel Highway
; Passenger Cars
Light-Duty Trucks
Heavy-Duty Vehicles
Non-Highway
Ships and Boats
Locomotives
Farm Equipment
Construction Equipment
_ Aircraft
Other*
TotaT
1990
226
124
82
16
4
10
+
+
10
21
3
3
6
1
8
1
257
1991
224
114
91
15
4
10
+
+
10
21
4
2
5
1
7
1
255
1992
225
109
96
15
4
10
+
+
10
21
4
3
6
1
7
1
257
1993
223
104
99
16
4
11
+
+
10
21
4
2
5
1
7
1
255
1994
221
102
98
17
4
11
-l-
+
11
21
4
2
5
-)
7
1
253
1995
218
100
97
17
4
12
4-
~
_]_
~
11
22
4
Q
u
R
u
-1
I
7
1
251
1996
213
98
94
16
A
*\
12
11
22
A
T
q
o
1
I
7
1
246
1997
207
95
92
16
12
+
+
12
20
0
O
•\
1
7
1
239
1998
201
94
88
16
12
+
+
12 '.
19
3;
2'
7
1
232
Note: Totals may not sum due to independent rounding.
* "Other" includes snowmobiles, small gasoline powered utility equipment
'•• diesel powered utility equipment.
heavy-duty gasoline powered utility equipment, and heavy-duty
Table 2-20: N20 Emissions from Mobile Combustion (Gg)
Fuel Type/Vehicle Type
Gasoline Highway
: Passenger Cars
Light-Duty Trucks
Heavy-Duty Vehicles
Motorcycles
Diesel Highway
Passenger Cars
Light-Duty Trucks
Heavy-Duty Vehicles
Non-Highway
Ships and Boats
Locomotives
Farm Equipment
Construction Equipment
Aircraft
Other*
Total
1990
148
95
50
2
+
6
+
+
5
10
1
1
1
+
6
+
163
1991
157
95
. 60
2
6
+
5
9
1
1
1
6
+
172
1992
170
99
68
3
6
,
6
10
1
1
1
5
185
1993
179
101
75
3
6
6
9
1
1
1
5
195
1994
186
104
78
3
7
R
\J
10
.,
-j
6
202
1995
189
106
80
4
T
7
i
10
o
£-
1
1
I
I
5
206
1996
190
105
81
+
7
/
+
+
-7
10
-1
1
1
I
fi
u
207
1997
188
103
81
+
+
9
•\
\
-\
i
H
1
+
205
1998
187
102
80
+
+
7
9
-4
1
203
Note: Totals may not sum due to independent rounding
P°wered
^avy-duty
Energy 2-23
-------
Table 2-21: MOX, CO, and NMVOC Emissions from
Mobile Combustion in 1998 (Gg)
Fuel "type/Vehicle Type
Gasoline Highway
Passenger Cars
Light-Duly Trucks
Heavy-Duty Vehicles
L Motorcycles
.Diesel Highway
Passenger Cars
' Light-Duty Trucks
Heavy-Duty Vehicles
Non-Highway
: Ships and Boats
Locomotives
Farm Equipment
Construction Equipment
Aircraft'
Other"
Total
NOX
4,617
2,574
1,739
293
11
1,736
31
11
1,694
4,832
971
903
913
1,120
152
773
11,184
— — — — — — *
CO NMVOCs
44,300
24,357
16,988
2,783
173
1,410
27
10
1,374
18,069
2,085
110
573
1,166
865
13,271
63,780
4,630
2,534
1,828
233
35
201
11
5
186
2,234
742
A ~7
47
120
208
160
956
7,065
-* Aircraft estimates include only emissions related to LTD cycles,
and therefore do not include cruise altitude emissions.
* "Other' includes gasoline powered recreational, industrial, lawn
'and garden, light commercial, logging, airport service, other
equipment; and diesel powered recreational, industrial, lawn and
garden, light construction, airport service.
Note: Totals may not sum due to independent rounding. See
Annex C for emissions in 1990 through 1998.
nated by gasoline-fueled passenger cars and light-duty
gasoline trucks.
Emissions of criteria pollutants generally in-
creased from 1990 through 1994, after which there were
decreases of 3 (NOX) to 14 (CO) percent by 1998. A
drop in gasoline prices combined with a strengthening
U.S. economy caused the initial increase. These factors
pushed the vehicle miles traveled (VMT) by road sources
up, resulting in increased fuel consumption and higher
emissions. Some of this increased activity was later off-
set by an increasing portion of the U.S. vehicle fleet
meeting established emissions standards.
Fossil-fueled motor vehicles comprise the single
largest source of CO emissions in the United States and
are a significant contributor to NOX and NMVOC emis-
sions. In 1998, CO emissions from mobile combustion
contributed 74 percent of national CO emissions and
51 and 38 percent of NOX and NMVOC emissions, re-
spectively. Since 1990, emissions of CO and NMVOCs
from mobile combustion decreased by 8 and 12 per-
cent, respectively, while emissions of NOX increased by
4 percent.
Methodology
Estimates for CH4 and N2O emissions from mo-
bile combustion were calculated by multiplying emis-
sion factors by measures of activity for each category.
Depending upon the category, activity data included
such information as fuel consumption, fuel deliveries,
and vehicle miles traveled (VMT). Emission estimates
from highway vehicles were based on VMT and emis-
sion factors by vehicle type, fuel type, model year, and
control technology. Fuel consumption data was em-
ployed as a measure of activity for non-highway ve-
hicles and then fuel-specific emission factors were ap-
plied.18 A complete discussion of the methodology
used to estimate emissions from mobile combustion is
provided in Annex C.
The EPA (1999) provided emissions estimates of
NOX, CO, and NMVOCs for eight categories of highway
vehicles,19 aircraft, and seven categories of off-high-
way vehicles.20
Data Sources
Emission factors used in the calculations of CH4
and N2O emissions are presented in Annex C. The Re-
vised 1996 IPCC Guidelines (IPCC/UNEP/OECD/IEA
1997) provided emission factors for CH4, and were de-
veloped using MOBILESa, a model used by the Envi-
«• The consumption of international bunker fuels is not included in these activity data, but are esthnated separately under the
International Bunker Fuels source category.
^•SSSKJSWK^^
heavy-duty diesel trucks and buses, and motorcycles.
2-24 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
ronmental Protection Agency (EPA) to estimate exhaust
and running loss emissions from highway vehicles. The
MOBUJESa model uses information on ambient tempera-
ture, vehicle speeds, national vehicle registration distri-
butions, gasoline volatility, and other variables in order
to produce these factors (EPA 1997).
Emission factors for N2O from gasoline highway
vehicles came from EPA (1998). This report contains
emission factors for older passenger cars—roughly pre-
1992 in California and pre-1994 in the rest of the United
States—from published references, and for newer cars
from a recent testing program at EPA's National Vehicle
and Fuel Emissions Laboratory (NVFEL). These emis-
sion factors for gasoline highway vehicles are lower than
the U.S. default values in the Revised 1996IPCC Guide-
lines, but are higher than the European default values,
both of which were published before the more recent
tests and literature review conducted by the NVFEL. The
U.S. default values in the Revised 1996 IPCC Guide-
lines were based on three studies that tested a total of
five cars using European rather than U.S. test protocols.
More details may be found in EPA (1998).
Emission factors for gasoline vehicles other than
passenger cars were scaled from those for passenger cars
with the same control technology, based on their rela-
tive fuel economy. This scaling was supported by lim-
ited data showing that light-duty trucks emit more N2O
than passenger cars with equivalent control technology.
The use of fuel-consumption ratios to determine emis-
sion factors is considered a temporary measure only, to
be replaced as additional testing data are available. For
more details, see EPA (1998). Nitrous oxide emission
factors for diesel highway vehicles were taken from the
European default values found in the Revised 1996 IPCC
Guidelines (IPCC/UNEP/OECD/IEA 1997). There is little
data addressing N2O emissions from U.S. diesel-fueled
vehicles, and in general, European countries have had
more experience with diesel-fueled vehicles. U.S. de-
fault values in the Revised 1996 IPCC Guidelines were
used for non-highway vehicles.
Activity data were gathered from several U.S. gov-
ernment sources including EIA (1999a), EIA (1999b),
FHWA (1998), BEA (1999), DESC (1999), DOC (1999),
FAA (1999)., and DOT/BTS (1999). Control technology
data for highway vehicles were obtained from the EPA's
Office of Mobile Sources. Annual VMT data for 1990
through 1998 were obtained from the Federal Highway
Administration's (FHWA) Highway Performance Moni-
toring System database, as noted in EPA (1999).
Emissions estimates for NOX, CO, NMVOCs were
taken directly from the EPA's National Air Pollutant
Emissions Trends, 1900 -1998 (EPA 1999).
Uncertainty
Mobile source emission estimates can vary sig-
nificantly due to assumptions concerning fuel type and
composition, technology type, average speeds, type of
emission control equipment, equipment age, and oper-
ating and maintenance practices. Fortunately, detailed
activity data for mobile combustion were available, in-
cluding VMT by vehicle type for highway vehicles. The
allocation of this VMT to individual model years was
done using the profile of U.S. vehicle usage by vehicle
age in 1990 as specified in MOBILE 5a. Data to develop
a temporally variable profile of vehicle usage by model
year instead of age was not available.
Average emission factors were developed based
on numerous assumptions concerning the age and model
of vehicle; percent driving in cold start, warm start, and
cruise conditions; average driving speed; ambient tem-
perature; and maintenance practices. The factors for regu-
lated emissions from mobile combustion—CO, NOX, and
hydrocarbons—have been extensively researched, and
thus involve lower uncertainty than emissions of un-
regulated gases. Although methane has not been singled
out for regulation in the United States, overall hydrocar-
bon emissions from mobile combustion—a component
of which is methane—are regulated.
Compared to methane, CO, NOX, and NMVOCs,
there is relatively little data available to estimate emis-
sion factors for nitrous oxide. Nitrous oxide is not a
criteria pollutant, and measurements of it in automo-
bile exhaust have not been routinely collected. Re-
search data has shown that N2O emissions from vehicles
with catalytic converters are greater than those without
emission controls, and that vehicles with aged cata-
Energy 2-25
-------
lysts emit more than new ones. The emission factors
used were, therefore, derived from aged cars (EPA
I998b). The emission factors used for Tier 0 and older
cars were based on tests of 28 vehicles; those for newer
vehicles were based on tests of 22 vehicles. This sample
is small considering that it is being used to character-
ize the entire U.S. fleet, and the associated uncertainty
is therefore large. Currently, N2O gasoline highway
emission factors for vehicles other than passenger cars
are scaled based on those for passenger cars and their
relative fuel economy. Actual measurements should be
substituted for this procedure when they become avail-
able. Further testing is needed to reduce the uncertainty
in emission factors for all classes of vehicles, using
realistic driving regimes, environmental conditions, and
fuels.
Although aggregate jet fuel and aviation gaso-
line consumption data has been used to estimate emis-
sions from aircraft, the recommended method for esti-
mating emissions in the Revised 1996IPCC Guidelines
is to use data by specific aircraft type (IPCC/UNEP/
OECD/IEA 1997). The IPCC also recommends that
cruise altitude emissions be estimated separately using
fuel consumption data, while landing and take-off (LTO)
cycle data be used to estimate near-ground level emis-
sions. The EPA is attempting to develop revised esti-
mates based on this more detailed activity data, and
these estimates are to be presented in future inventories.
Overall, uncertainty for N2O emissions esti-
mates is considerably higher than for CH4, CO, NOX,
or NMVOC; however, all these gases involve far
more uncertainty than CO2 emissions from fossil fuel
combustion.
U.S. jet fuel and aviation gasoline consumption is
currently all attributed to the transportation sector by
EIA, and it is assumed here that it is all used to fuel
aircraft. However it is likely that some fuel purchased by
airlines is not necessarily be used in aircraft, but instead
used to power auxiliary power units, in ground equip-
ment, and to test engines. Some jet fuel may also be used
for other purposes such as blending with diesel fuel or
heating oil.
Lastly, in EPA (1999), U.S. aircraft emission esti-
mates for CO, NOx, and NMVOCs are based upon land-
ing and take-off (LTO) cycles and consequently only
capture near ground-level emissions, which are more rel-
evant for air quality evaluations. These estimates also
include both domestic and international flights. There-
fore, estimates presented here overestimate IPCC-defmed
domestic CO, NOX, and NMVOC emissions by including
LTO cycles by aircraft on international flights but un-
derestimate because they do not include emissions from
aircraft on domestic flight segments at cruising altitudes.
Coal Mining
All underground and surface coal mining liberates
(i.e., releases) methane as part of normal operations. The
amount of methane liberated during mining is primarily
dependent upon the amount of methane stored in the
coal and the surrounding strata. This in situ methane
content is a function of the quantity of methane gener-
ated during the coal formation process and its ability to
migrate through the surrounding strata over time. The
degree of coalification—defined by the rank or quality
of the coal formed—determines the amount of methane
generated; higher ranked coals generate more methane.
The amount of methane remaining in the coal and sur-
rounding strata depends upon geologic characteristics
such as pressure within a coal seam. Deeper coal depos-
its tend to retain more of the methane generated during
coalification. Accordingly, deep underground coal seams
generally have higher methane contents than shallow
coal seams or surface deposits.
Underground coal mines contribute the largest
share of methane emissions. All underground coal mines
employ ventilation systems to ensure that methane lev-
els remain within safe concentrations. These systems can
exhaust significant amounts of methane to the atmo-
sphere in low concentrations. Additionally, over twenty
gassy U.S. coal mines supplement ventilation systems
with degasification systems. Degasification systems are
wells drilled from the surface or boreholes drilled inside
the mine that remove large volumes of methane before,
during or after mining. In 1998,12 coal mines collected
methane from degasification systems and sold this gas
to a pipeline, thus reducing emissions to the atmosphere.
2-26 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Table 2-22: CH4 Emissions from Coal Mining (MMTCE)
1 Activity
Underground Mining
Liberated
Recovered & Used
Surface Mining
Post- Mining (Underground)
Post-Mining (Surface)
Total
- Note: Totals may not sum due to
1990
17.1
18.8
(1.6)
2.8
3.6
0.5
24.0
1991
16.4
18.1
(1.7).
2.6
3.4
0.4
22.8
1992
15.6
17.8
(2.1)
2.6
3.3
0.4
22.0
1993
13.3
16.0
(2.7)
2.5
3.0
0.4
19.2
1994
13.1
16.3
(3.2)
2.6
3.3
0.4
19.4
1995
14.2
17.7
(3.4)
2.4
3.3
0.4
20.3
1996
12.6
16.5
(3.8)
2.5
3.4
0.4
18.9
1997
12.3
16.8
(4.6)
2.6
3.5
0.4
18.8
1998
11.4
16.1
(4.8) ;
2.6
3.4
0.4
17.8
independent rounding. :
Table 2-23: CH4 Emissions from Coal Mining (Gg)
Activity
^Underground Mining
Liberated
Recovered & Used
-Surface Mining
_Post- Mining (Underground)
Post-Mining (Surface)
Total
Note: Totals may not sum due to
1990
2,991
3,279
(288)
488
626
79
4,184
1991
2,863
3,152
(289)
450
589
73
3,975
1992
2,731
3,102
(372)
449
582
73
3,835 .
1993
2,328
2,795
(468)
434
523
71
3,356
1994
2,289
2,848
(559)
455
572
74
3,390
1995
2,487
3,086
(599)
425
567
69
3,550
1996
2,204
2,875
(671)
436
590
71
3,301
1997
2,141
2,938
(797)
451
609
73
3,274
1998
1,983
2,814
(831) :
450 I
598
73
3,104
independent rounding.
Surface coal mines also release methane as the overbur-
den is removed and the coal is exposed; however, the
level of emissions is much lower than underground
mines. Additionally, after coal has been mined, small
amounts of methane retained in the coal are released
during processing, storage, and transport.
Total methane emissions in 1998 were estimated
to be 17.8 MMTCE (3,104.2 Gg), declining 26 percent
since 1990 (see Table 2-22 and Table 2-23). Of this
amount, underground mines accounted for 64 percent,
surface mines accounted for 15 percent, and post-min-
ing emissions accounted for 22 percent. With the excep-
tion of 1994 and 1995, total methane emissions declined
in each successive year during this period. In 1993, meth-
ane generated from underground mining dropped to a
low of 2,327.7 Gg, primarily due to labor strikes at many
large underground mines. In 1995, there was an increase
in methane emissions from underground mining due to
particularly increased emissions at the highest-emitting
coal mine in the country. The decline in methane emis-
sions from underground mines is the result of the mining
of less gassy coal, and an increase in gas recovery and
use. Surface mine emissions and post-mining emissions
remained relatively constant from 1990 to 1998.
In 1994, EPA's Coalbed Methane Outreach Program
(CMOP) began working with the coal industry and other
stakeholders to identify and remove obstacles to invest-
ments in coal mine methane recovery and use projects.
Reductions attributed to CMOP were estimated to be 0.7,
0.8, 1.0, 1.3, and 1.7 MMTCE in 1994 through 1998,
respectively, compared to business-as-usual emissions.
Methodology
The methodology for estimating methane emis-
sions from coal mining consists of two steps. The first
step involves estimating methane emissions from under-
ground mines. Because of the availability of ventilation
system measurements, underground mine emissions can
be estimated on a mine-by-mine basis and then summed
to determine total emissions. The second step involves
estimating emissions from surface mines and post-min-
ing activities by multiplying basin-specific coal pro-
Energy 2-27
-------
duction by basin-specific emissions factors.
Underground mines. Total methane emitted from
underground mines is estimated to be the quantity of meth-
ane liberated from ventilation systems, plus methane lib-
erated from degasification systems, minus methane re-
covered and used. The Mine Safety and Heath Adminis-
tration (MSHA) samples methane emissions from ventila-
tion systems for all mines with detectable21 methane con-
centrations. These mine-by-mine measurements are used
to estimate methane emissions from ventilation systems.
Some of the higher-emitting underground mines
also use degasification systems (e.g., wells or boreholes)
that remove methane before, during, or after mining.
This methane can then be collected for use or vented to
the atmosphere. Various approaches are employed to
estimate the quantity of methane collected by each of
the more than twenty mines using these systems, de-
pending on available data. For example, some mines
report to EPA the amounts of methane liberated from
their degasification systems. For mines that sell recov-
ered methane to a pipeline, pipeline sales data are used
to estimate degasification emissions. Finally, for those
mines for which no other data are available, default
recovery efficiency values are developed, depending
on the type of degasification system employed.
Finally, the amount of methane recovered by
degasification systems and then used (i.e., not vented)
is estimated. This calculation is complicated by the
fact that methane is rarely recovered and used during
the same year in which the particular coal seam is mined.
In 1998, 12 active coal mines sold recovered methane
to pipelines. Emissions avoided for these projects are
estimated using gas sales data reported by various state
agencies, and information supplied by coal mine op-
erators regarding the number of years in advance of
mining that gas recovery occurs. Additionally, some of
the state agencies provide individual well production
information, which is used to assign gas sales to a par-
ticular year.
Surface Mines and Post-Mining Emissions. Sur-
face mining and post-mining methane emissions are
estimated by multiplying basin-specific coal produc-
tion by basin-specific emissions factors. For surface
mining, emissions factors are developed by assuming
that surface mines emit from one to three times as much
methane as the average in situ methane content of the
coal. This accounts for methane released from the strata
surrounding the coal seam. For this analysis, it is as-
sumed that twice the average in-situ methane content
is emitted. For post-mining emissions, the emission fac-
tor is assumed to be from 25 to 40 percent of the aver-
age in situ methane content of coals mined in the ba-
sin. For this analysis, it is assumed that 32.5 percent of
the average in-situ methane content is emitted.
Data Sources
The Mine Safety and Health Administration pro-
vides mine-specific information on methane liberated
from ventilation systems at underground mines. EPA
develops estimates of methane liberated from
degasification systems at underground mines based on
available data for each of the mines employing these
systems. The primary sources of data for estimating
emissions avoided at underground mines are gas sales
data published by state petroleum and natural gas agen-
cies, information supplied by mine operators regarding
the number of years in advance of mining that gas re-
covery occurred, and reports of gas used on-site. An-
nual coal production data are taken from the Energy
Information Agency's Coal Industry Annual (see Table
2-24) (EIA 1991, 1992, 1993, 1994, 1995, 1996, 1997,
1998,1999). Data on in situ methane content and emis-
sions factors are taken from EPA (1993).
Uncertainty
The emission estimates from underground venti-
lation systems are based upon actual measurement data
for mines with detectable methane emissions. Accord-
ingly, the uncertainty associated with these measure-
ments is estimated to be low. Estimates of methane lib-
erated from degasification systems are less certain be-
cause EPA assigns default recovery efficiencies for a
subset of U.S. mines. Compared to underground mines,
21 MSHA records coal mine methane readings with concentrations of greater than 50 ppm (parts per million) methane. Readings below
this threshold are considered non-detectable.
2-28 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Table 2-24: Coal Production (Thousand Metric Tons)
Year
Underground
Surface
Total
"1990
1991
1992
1993
1994
1995
1996
• 1997
: 1998*
384,250
368,635
368,627
318,478
362,065
359,477
371,816
381,620
377,397
546,818
532,656
534,290
539,214
575,529
577,638
593,315
607,163
636,972
931,068
901,291
902,917
857,692
937,594
937,115
965,131
988,783
1,014,369
if-*Jotal production for 1998 provided by EIA. Underground and
^surface proportions are estimated based on 1997 EIA data.
there is considerably more uncertainty associated with
surface mining and post-mining emissions because of
the difficulty in developing accurate emissions factors
from field measurements. Because underground emis-
sions comprise the majority of total coal mining emis-
sions, the overall uncertainty is estimated to be only
±15 percent.22 Currently, the estimate does not include
emissions from abandoned coal mines because of lim-
ited data. The EPA is conducting research on the feasi-
bility of including an estimate in future years.
Natural Gas Systems
Methane emissions from natural gas systems are
generally process related, with normal operations, rou-
tine maintenance, and system upsets being the primary
contributors. Emissions from normal operations in-
clude: natural gas combusting engine and turbine ex-
haust, bleed and discharge emissions from pneumatic
devices, and fugitive emissions from system compo-
nents. Routine maintenance emissions originate from
pipelines, equipment, and wells during repair and main-
tenance activities. Pressure surge relief systems and
accidents can lead to system upset emissions.
The U.S. natural gas system encompasses hun-
dreds of thousands of wells, hundreds of processing
facilities, hundreds of thousands of miles of transmis-
sion pipeline, and over a million miles of distribution
pipeline. The system, though, can be divided into four
stages, each with different factors affecting methane
emissions, as follows:
Field Production. In this initial stage, wells are
used to withdraw raw gas from underground formations.
Emissions arise from the wells themselves, treatment
facilities, gathering pipelines, and process units such
as dehydrators and separators. Fugitive emissions and
emissions from pneumatic devices account for the ma-
jority of emissions. Emissions from field production
accounted for approximately 24 percent of methane
emissions from natural gas systems between 1990 and
1998. Emissions rose between 1990 and 1996 due to
an increased number of producing gas wells and re-
lated equipment, but returned to the 1990 level of 8.0
MMTCE in 1998 due to a decrease in domestic pro-
duction and improvements in technology coupled with
the normal .replacement of older equipment.
Processing. In this stage, processing plants re-
move various constituents from the raw gas before it is
injected into the transmission system. Fugitive emis-
sions from compressors, including compressor seals,
were the primary contributor from this stage. Process-
ing plants accounted for about 12 percent of methane
emissio'ns from natural gas systems during the period
of 1990 through 1998.
Transmission and Storage. Natural gas transmis-
sion involves high pressure, large diameter pipelines
that transport gas long distances from field produc-
tion areas to distribution centers or large volume cus-
tomers. Throughout the transmission system, compres-
sor stations pressurize the gas to move it through the
pipeline. Fugitive emissions from compressor stations
and metering and regulating stations accounted for
the majority of the emissions from transmission. Pneu-
matic devices and engine exhaust were smaller sources
of emissions from transmission facilities. A gradual
increase in transmission pipeline mileage has in-
creased methane emissions from natural gas transmis-
sion. Methane emissions from transmission and stor-
age accounted for approximately 40 percent of the
emissions from natural gas systems during the period
of 1990 through 1998.
Natural gas is also injected and stored in under-
: Preliminary estimate
Energy 2-29
-------
Table 2-25: CH4 Emissions from Natural Gas Systems (MMTCE)
Stage
Field Production
Processing
•Transmission and Storage
Distribution
Total
Note: Totals may not sum due
1990
8.0
4.0
12.7
8.3
33.0
to independent
1991
8.2
4.0
12.9
8.4
33.4
rounding.
1992
8.5
4.0
12.9
8.6
33.9
1993
8.7
4.0
13.1
8.8
34.6
1994
8.3
4.1
13.3
8.6
34.3
1995
8.4
4.1
13.0
8.5
34.0
1996
8.5
4.1
13.1
8.9
34.6
1997
8.2
4.1
13.2
8.6
34.1
1998
8.0
4.0
13.5
.1
33.6
:
Table 2-26: CH4 Emissions from Natural Gas Systems (Gg)
"stage
1990
1991 1992 1993 1994 1995 1996 1997 1998
Field Production
Processing
Transmission and Storage
Distribution
1,404 1,427
702 693
2,223 2,250
1,441 1,470
1,478
698
2,252
1,496
1,513
704
2,290
1,535
1,450
724
2,314
1,499
1,469
712
2,273
1,477
1,489
708
2,291
1,553
1,435 1,388
710 698
2,313 2,357
1,504 1,416
=TbtaI
5,770 5,840 5,923 6,042 5,987 5,931 6,041 5,961 5,860
* Note: Totals may not sum due to independent rounding.
ground formations during periods of low demand, and
withdrawn, processed, and distributed during peri-
ods of high demand. Compressors and dehydrators
were the primary contributors to emissions from these
storage facilities. Less than one percent of total emis-
sions from natural gas systems can be attributed to
storage facilities.
Distribution. The distribution of natural gas re-
quires the use of low-pressure pipelines to deliver gas
to customers. There were 955,000 miles of distribution
pipelines (i.e., main) in 1997 (the latest year for which
distribution pipeline mileage data is available), increas-
ing from a 1990 figure of just over 837,000 miles (AGA,
1998). Distribution system emissions, which account
for approximately 24 percent of emissions from natural
gas systems, resulted mainly from fugitive emissions
from gate stations and non-plastic piping. An increased
use of plastic piping, which has lower emissions than
other pipe materials, has reduced the growth in emis-
sions from this stage.
Overall, natural gas systems emitted 33.6
MMTCE (5,860 Gg) of methane in 1998, a slight in-
crease over 1990 emissions of 33.0 MMTCE (5,770) in
1990 (see Table 2-25 and Table 2-26). Even though
transmission and distribution pipeline mileage and
natural gas production have increased from 1990 to
1998, emissions over that period have remained rela-
tively constant. Improvements in management prac-
tices and technology, along with the normal replace-
ment of older equipment, helped to stabilize emissions.
In addition, EPA's Natural Gas STAR Program, initiated
in 1993, is working with the gas industry to promote
profitable practices that reduce methane emissions. The
program is estimated to have reduced emissions by 0.7,
1.2, 1.3, 1.8 and 2.2 MMTCE in 1994 through 1998,
respectively. In Table 2-25 and Table 2-26, Natural Gas
STAR reductions are included in the emission estimates
for each sector of the natural gas industry and are also
reflected in the total emission estimate.
Methodology
The foundation for the estimate of methane emis-
sions from the U.S. natural gas industry is a detailed
study by the Gas Research Institute and EPA (GRI/EPA
1996). The GRI/EPA study developed over 100 detailed
emission factors and activity levels through site visits to
selected gas facilities, and arrived at a national point
estimate for 1992. Since publication of this study, EPA
conducted additional analysis to update the activity data
for some of the components of the system, particularly
field production equipment. Summing emissions across
individual sources in the natural gas system provided a
2-30 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
1992 baseline emission estimate from which the emis-
sions for the period 1990 through 1998 were derived.
Apart from the year 1992, detailed statistics on
each of the over 100 activity levels were not available
for the time series 1990 through 1998. To estimate these
activity levels, aggregate annual statistics were ob-
tained on select driving variables, including: number
of producing wells, number of gas plants, miles of trans-
mission pipeline, miles of distribution pipeline, and
miles of distribution services. By assuming that the
relationships among these variables remained constant
(e.g., the number of heaters per well remained the same),
the statistics on these variables formed the basis for
estimating other activity levels.
For the period 1990 through 1995, the emission
factors were held constant. A gradual improvement in
technology and practices is expected to reduce the emis-
sion factors slightly over time. To reflect this trend, the
emission factors were reduced by about 0.2 percent per
year starting with 1996, a rate that, if continued, would
lower the emission factors by 5 percent in 2020. See
Annex E for more detailed information on the method-
ology and data used to calculate methane emissions
from natural gas systems.
Data Sources
Activity data were taken from the American Gas
Association (AGA1991,1992,1993,1994,1995,1996,
1997, 1998, 1999), Natural Gas Annual (EIA 1998),
and Natural Gas Monthly (EIA 1998), Independent Pe-
troleum Association of America (IPAA 1990, 1991,
1992, 1993, 1994, 1995, 1996, 1997, 1998), and the
Department of Transportation's Office of Pipeline Safety
(OPS 2000). The U.S. Department of Interior (DOI1997,
1998, 1999) supplied offshore platform data. All emis-
sion factors were taken from GRI/EPA (1996).
Uncertainty
The heterogeneous nature of the natural gas in-
dustry makes it difficult to sample facilities that are
completely representative of the entire industry. Be-
cause of this, scaling up from model facilities intro-
duces a degree of uncertainty. Additionally, highly vari-
able emission rates were measured among many system
components, making the calculated average emission
rates uncertain. Despite the difficulties associated with
estimating emissions from this source, the uncertainty
in the total estimated emissions are believed to be on
the order of ±40 percent.
Petroleum Systems
Methane emissions from petroleum systems are
primarily associated with crude oil production, trans-
portation, and refining operations. During each of these
activities, methane is released to the atmosphere as fu-
gitive emissions, vented emissions, operational upset
emissions, and emissions from the combustion of fuels.
These activities and associated methane emissions are
detailed below.
Production Field Operations. Production field
operations account for approximately 97 percent of to-
tal methane emissions from petroleum systems. The ma-
jor sources of methane from production operations are
venting from storage tanks and pneumatic devices, well-
head fugitives, combustion products, and process up-
sets. Vented methane from oil wells, storage tanks, and
related production field processing equipment was the
primary contributor to emissions from the oil industry,
accounting for, on average, 89 percent. Field storage
tanks and natural-gas-powered pneumatic devices used
to operate valves and small pumps were the dominant
contributors to venting emissions. Oil wells and off-
shore platforms accounted for most of the remaining
venting emissions.
Fugitive and combustion emissions from produc-
tion field operations accounted for three percent and
two percent, respectively, of total methane emissions
from the oil industry. Most fugitive methane emissions
in the field were from oil wellheads and the equipment
used to separate natural gas and water from the crude
oil. Nearly all of the combustion emissions in the field
were from engine exhaust. The EPA expects future emis-
sions from production fields to decline as the number
of oil wells declines and crude production slows.
Crude Oil Transportation. Crude transportation
activities accounted for less than one half percent of
total methane emissions from the oil industry. Venting
Energy 2-31
-------
Table 2-27: CH4 Emissions from Petroleum Systems (MMTCE)
s. - " "" ' '-"'-""'
Activity
: Production Field Operations
Tank venting
Pneumatic device venting
; Wellhead fugitives
• Combustion & process upsets
Misc. venting & fugitives
-Crude Oil Transportation
Refining
Total
+ Does not exceed 0.05 MMTCE
1990
7.2
3.2
3.2
0.1
0.3
0.4
0.1
7.4
1991
7.3
3.3
3.2
0.1
0.3
0.4
0.1
7.5
1992
7.1
3.1
3.1
0.1
0.3
0.4
0.1
7.2
1993
6.7
3.0
3.0
0.1
0.3
0.4
0.1
6.9
1994
6.6
2.9
2.9
0.1
0.3
0.4
0.1
6.7
1995
6.5
2.8
2.9
0.1
0.3
0.4
0.1
6.7
"i "" s^V"--'
1996
6.4
2.8
2.8
0.1
0.3
0.4
0.1
6.5
< ygjfljj^
6.4
2.8
2.8
0.1
0.3
0.4
0.2
6.5
1998 '
•• " ••• '- • 3
6.2
2.7
2.7
0.1
0.3 ;
0.4 . • ,
0.2 ;
6.3
Note: Totals may not sum due to independent rounding.
Table 2-28: CH4 Emissions from Petroleum Systems (Gg)
Activity
Production Field Operations
Tank venting
Pneumatic device venting
Wellhead fugitives
Combustion & process upsets
Misc. venting & fugitives
Crude Oil Transportation
Refining
Tnfal
1990
1,263
564
559
24
46
70
7
25
1,294
1991
1,276
570
564
26
46
70
6
24
1,307
1992
1,232
548
545
25
45
69
6
24
1,262
1993
1,175
519
521
24
45
67
6
25
1,206
1994
1,144
502
506
25
45
66
25
1,175
1995
1,136
493
507
25
45
66
6"
25
1,168
1996
1,111
485
491
25
45
65
26
1,143
1997
1,109
484
490
24
46
65
27
1,142
1998
1,075
466
475
24
45
64
6
27
1,108
, Note: Totals may not sum due to independent rounding. ........
from tanks and marine vessel loading operations ac-
counted for the majority of methane emissions from crude
oil transportation. Fugitive emissions, almost entirely
from floating roof tanks, accounted for the remainder.
Crude Oil Refining. Crude oil refining processes
and systems accounted for only two percent of total
methane emissions from the oil industry because most
of the methane in crude oil is removed or escapes be-
fore the crude oil is delivered to the refineries. Within
refineries, vented emissions accounted for 86 percent,
while fugitive and combustion emissions were seven
percent each. Refinery system blowdowns for mainte-
nance and the process of asphalt blowing—with air to
harden it—were the primary venting contributors. Most
of the fugitive emissions from refineries were from leaks
in the fuel gas system. Refinery combustion emissions
accumulate from small amounts of unburned methane
in process heater stack emissions and from unburned
methane in engine exhausts and flares.
The EPA estimates total methane emissions from
petroleum systems in 1998 were 6.3 MMTCE (1,108
Gg). Since 1990, emissions declined gradually prima-
rily due to a decline in domestic oil production. Emis-
sion' estimates are provided below in Table 2-27 and
Table 2-28.
Methodology
The EPA's methodology for estimating methane
2-32 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
emissions from petroleum systems is based on a compre-
hensive study of methane emissions from U.S. petroleum
systems,,Estimates of Methane Emissions from the U.S.
Oil Industry (Draft Report) (EPA 1999). The study esti-
mated emissions from 70 activities occurring in petro-
leum systems from the oil wellhead through crude oil
refining, including 39 activities for crude oil production
field operations, 11 for crude oil transportation activi-
ties, and 20 for refining operations. Annex F explains the
emission estimates for these 70 activities .in greater de-
tail. The estimate of methane emissions from petroleum
systems does not include emissions downstream from oil
refineries because these emissions are very small com-
pared to methane emissions upstream from oil refineries.
The methodology for estimating methane emis-
sions from the 70 oil industry activities employs emis-
sion factors and activity factors initially developed in
EPA (1999). The EPA estimates emissions for each activ-
ity by multiplying emission factors (e.g., emission rate
per equipment item or per activity) by their correspond-
ing activity factor (e.g., equipment count or frequency
of activity). The report (EPA 1999) provides emission
factors and activity factors for all activities except those
related to offshore oil production. For offshore oil pro-
duction, the EPA calculated an emission factor by divid-
ing an emission estimate from the Minerals Manage-
ment Service (MMS) by the number of platforms (the
activity factor).
The EPA collected activity factors for 1990 through
1998 from a wide variety of historical resources. For
1995, data on activity factors were available; however,
some activity factor data are not reported for other years.
When activity factor data were not available, the EPA
employed one of three options. Where appropriate, the
activity factor was assumed to be directly proportional
to annual oil production. (Proportionality constants were
calculated by dividing the activity factor for 1995 by
the annual oil production for 1995. The resulting pro-
portionality constants were then multiplied by the an-
nual oil production in years for which activity factors
must be estimated.) In other cases, the activity factor was
kept constant between 1990 and 1998. Lastly, 1997 data
was used when 1998 data were not yet available.
Emission factors were held constant for the period
1990 through 1998, with the exception of engine emis-
sions. Over time, more efficient engines are used to drive
pumps, compressors, and generators. The emission fac-
tor for these engines was adjusted accordingly.
Data Sources
Nearly all emission factors were taken from earlier
work performed by Radian International LLC (Radian
1996e). Other emission factors were taken from API pub-
lication 4638 (API 1996), EPA default values, MMS re-
ports (MMS 1995 and 1999), the Exploration and Pro-
duction (E&P) Tank model (API and GRI), reports by the
Canadian Association of Petroleum Producers (CAPP
1992 and 1993), and consensus of industry peer review
panels.
The EPA uses many references to obtain activity
factors. Among the more important references are En-
ergy Information Administration annual and monthly
reports (EIA 1995, 1996, 1997, 1998), the API Basic
Petroleum Data Book (API 1997 and 1999), the GRI/
EPA report (Radian 1996a-d), Methane Emissions from
the Natural Gas Industry, consensus of industry peer
review panels, MMS reports (MMS 1995 and 1999), and
the Oil & Gas Journal (OGJ 1990-1998a,b). Appendix F
liable2-29: Uncertainty in CH4 Emissions from Production Field Operations (Gg)
I Activity
Hfcnk venting (point estimate)
|r - Low
p»— High
t Pneumatic devices (point estimate)
fe Low
^i High
1990
564
423
705
559
372
698
1991
570
427
712
564
376
705
1992
548
411
685
545
363
681
1993
519
389
649
521
347
651
1994
502
377
628
506
338
633
1995
493
370
617
507
338
634
1996
485
364
606
491
328
614
1997
484
363
605
490
327
613
1998
466
349
582 :
475
317 :
594
Energy 2-33
-------
provides a complete list of references.
Uncertainty
The detailed, bottom-up analysis used to evaluate
U.S. petroleum systems for the current Inventory reduces
the uncertainty related to the methane emission estimates
compared to previous estimates. However, a number of
uncertainties remain. Because published activity factors
are not available every year for all 70 activities ana-
lyzed for petroleum systems, the EPAmust estimate some
of them. For example, there is uncertainty associated
with the estimate of annual venting emissions in pro-
duction field operations because a recent census of tanks
and other tank battery equipment, such as separators and
pneumatic devices, is not available. These uncertainties
are important because the production sector accounted
for 97 percent of total 1998 methane emissions from
petroleum systems. Uncertainties are also associated with
emission factors because highly variable emission rates
are summarized in one emission factor. The majority of
methane emissions occur during production field opera-
tions, where methane can first escape crude oil, so a bet-
ter understanding of tank battery equipment and tanks
would reduce the uncertainty associated with the esti-
mate of methane emissions from petroleum systems. Table
2-29 provides emission estimate ranges given the uncer-
tainty in the estimates of vented emissions from produc-
ing field tanks and pneumatic devices.
Natural Gas Flaring and Criteria
Pollutant Emissions from
Oil and Gas Activities
The flaring of natural gas from oil wells is a small
source of carbon dioxide (CO2). In addition, oil and gas
activities also release small amounts of nitrogen oxides
(NOX), carbon monoxide (CO), and nonmethane volatile
organic compounds (NMVOCs). This source accounts
for only a small proportion of overall emissions of each
of these gases. Emissions of CO2, NOX, and CO from
petroleum and natural gas production activities were all
less than 1 percent of national totals, while NMVOC
emissions were roughly 2 percent of national totals.
Carbon dioxide emissions from petroleum produc-
tion result from natural gas that is flared (i.e., combusted)
at the production site. Barns and Edmonds (1990) noted
that of total reported U.S. venting and flaring, approxi-
mately 20 percent may be vented, with the remaining 80
percent flared; however, it is now believed that flaring
accounts for an even greater proportion, although some
venting still occurs. Methane emissions from venting
are accounted for under Petroleum Systems. For 1998,
the CO2 emissions from the flaring were estimated to be
approximately 3.4 MMTCE (12,296 Gg), an increase of
148 percent since 1990 (see Table 2-30).
Criteria pollutant emissions from oil and gas pro-
duction, transportation, and storage, constituted a rela-
tively small and stable portion of the total emissions of
these gases from the 1990 to 1998 (see Table 2-31).
Methodology
The estimates for CO2 emissions were prepared
using an emission factor of 14.92 MMTCE/QBtu of flared
gas, and an assumed flaring efficiency of 100 percent.
The quantity of flared gas was estimated as the total
reported vented and flared gas minus a constant 12,031
million cubic feet, which was assumed to be vented.23
Criteria pollutant emission estimates for NOX, CO,
and NMVOCs were determined using industry-published
production data and applying average emission factors.
Data Sources
Activity data in terms of total natural gas vented
and flared for estimating CO2 emissions from natural gas
flaring were taken from EIA's Natural Gas Annual (EIA
1998). The emission and thermal conversion factors were
also provided by EIA (see Table 2-32).
EPA (1999) provided emission estimates for NOX,
CO, and NMVOCs from petroleum refining, petroleum
product storage and transfer, and petroleum marketing
operations. Included are gasoline, crude oil and distil-
late fuel oil storage and transfer operations, gasoline
bulk terminal and bulk plants operations, and retail gaso-
Sec the methodological discussion under Petroleum Systems for the basis of the portion of natural gas assumed vented.
2-34 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Table 2-30: C02 Emissions from Natural Gas Flaring
1-:'
_
pT.:
«,
£•• — L
1™ •"
r-
^ -
&*
Z=±-~ ':
Year
1990
1991
1992
1993
1994
1995
1996
1997
1998
MMTCE
2.5
2.8
2.8
3.7
3.8
4.7
,4.5 ../
4.2
3.9
Gg
9,097
10,295
,10,169
13,716
13,800
17,164
16,506
15,521
. 14,214
•'•/. .'•"• :.'v , .-•,-•,-.-
Table 2-31: NOX, NMVOCs, and CO Emissions
from Oil and Gas Activities (Gg)
g^_
ET:
S7^
^=-
=-'
t^ '
— '
sr
|j — ,
fc
Year
1990
1991
1992
1993
1994
1995
1996
1997
1998
NOX
139
110
134
111
106
100
121
121
122
CO
302
313
337
337
307
316
287
292
296
NMVOCs
555
581
574
588
587
582
459
461
464
.,
!
- J
line service stations operations.
Uncertainty
Uncertainties in CO2 emission estimates primarily
arise from assumptions concerning what proportion of
natural gas is flared and the flaring efficiency. The por-
tion assumed vented as methane in the methodology for
Petroleum Systems is currently held constant over the
period 1990 through 1998 due to the uncertainties in-
volved in the estimate. Uncertainties in criteria pollut-
ant emission estimates are partly due to the accuracy of
the emission factors used and projections of growth.
International Bunker Fuels
Table 2-32: Total Natural Gas Reported
Vented and Flared (Million Ft3) and
Thermal Conversion Factor (Btu/Ft3)
p~,
jr
f^ Year
r^^iggo
1991
1992
1993
1*^1994
t*4995
~1996
1997
1998
tr
..„-,,,; ;•..-.«.•••.}.:-
Vented
and Flared
150,415
169,909
167,519
226,743
.. „ 228,336
; 28*3,739
•:. 272,117
. 263,819
261,000
: Therrnal
Conversion
Factor
1,106
1,108
1,110
1,106
.. 1,105
1,106
1,109 ;
1,107
1,107
Emissions resulting from the combustion of fuels
used for international transport activities, termed inter-
national bunker fuels under the United Nations Frame-
work Convention on Climate Change (UNFCCC), are
currently not included in national emission totals, but
are reported separately based upon location of fuel sales.
The decision to report emissions from international bun-
ker fuels separately, instead of allocating them to a par-
ticular country, was made by the Intergovernmental Ne-
gotiating Committee in establishing the Framework
Convention on Climate Change.24 These decisions are
reflected in the Revised 1996IPCC Guidelines, in which
countries are requested to report emissions from ships or
aircraft that depart from their ports with fuel purchased
within national boundaries and are engaged in interna-
tional transport separately from national totals (IPCC/
UNEP/OECD/IEA 1997). The Parties to the UNFCCC
have yet to decide on a methodology for allocating these
emissions.25
Greenhouse gases emitted from the combustion of
international bunker fuels, like other fossil fuels, include
carbon dioxide (CO2), methane (CH4), nitrous oxide
(N2O), carbon monoxide (CO), oxides of nitrogen (NOX),
nonmethane volatile organic compounds (NMVOCs),
paniculate matter, and sulfur dioxide (SO2).26 Two trans-
port modes are addressed under the IPCC definition of
24 See report of the Intergovernmental Negotiating Committee for a Framework Convention on Climate Change on the work of its ninth
session, held at Geneva from 7 to 18 February 1994 (A/AC.237/55, annex I, para. Ic) (contact secretariat@unfccc.de).
25 See FCCC/SBSTA/1996/9/Add.l and Add.2 for a discussions of allocation options for international bunker fuels (see http-//
www.unfccc.de/fccc/docs/1996/sbsta/09a01.pdf and /09a02.pdf).
Energy 2-35
-------
international bunker fuels: aviation and marine. Emis-
sions from ground transport activities—by road vehicles
and trains—even when crossing international borders
are allocated to the country where the fuel was loaded
into the vehicle and, therefore, are not counted as bun-
ker fuel emissions.
The IPCC Guidelines distinguish between differ-
ent modes of air traffic. Civil aviation comprises aircraft
used for the commercial transport of passengers and
freight, military aviation comprises aircraft under the
control of national armed forces, and general aviation
applies to recreational and small corporate aircraft. The
IPCC Guidelines further define international bunker fuel
use from civil aviation as the fuel combusted for civil
(e.g., commercial) aviation purposes by aircraft arriving
or departing on international flight segments. However,
as mentioned above, and in keeping with the IPCC
Guidelines, only the fuel purchased in the United States
and used by aircraft taking-off (i.e., departing) from the
United States are reported here. The standard fuel used
for civil aviation is kerosene-type jet fuel, while the typi-
cal fuel used for general aviation is aviation gasoline.27
Emissions of CO2 from aircraft are a function of
fuel use, whereas emissions per flight or ton-mile in the
case of cargo, are a function of flight path, fuel effi-
ciency of the aircraft and its engines, occupancy, and
load factor. Methane, N2O, CO, NOX, and NMVOC emis-
sions depend upon engine characteristics, flight condi-
tions, and flight phase (i.e., take-off, climb, cruise, de-
cent, and landing). Methane, CO, and NMVOCs are the
product of incomplete combustion and occur mainly
during the landing and take-off phases. In jet engines,
N2O and NOX are primarily produced by the oxidation of
atmospheric nitrogen, and the majority of emissions oc-
cur during the cruise phase. The impact of NOX on atmo-
spheric chemistry depends on the altitude of the actual
emission. The cruising altitude of supersonic aircraft,
near or in the ozone layer, is higher than that of subsonic
aircraft. At this higher altitude, NOX emissions contrib-
ute to ozone depletion.28 At the cruising altitudes of
subsonic aircraft, however, NOX emissions contribute to
the formation of ozone. At these lower altitudes, the posi-
tive radiative forcing effect of ozone is most potent.29
The vast majority of aircraft NOX emissions occur at these
lower cruising altitudes of commercial subsonic aircraft
(NASA 1996).30
International marine bunkers comprise emissions
from fuels burned by ocean-going ships of all flags that
are engaged in international transport. Ocean-going ships
are generally classified as cargo and passenger carrying,
military (i.e., navy), fishing, and miscellaneous support
ships (e.g., tugboats). For the purpose of estimating green-
house gas emissions, international bunker fuels are solely
related to cargo and passenger carrying vessels, which is
the largest of the four categories, and military vessels.
Two main types of fuels are used on sea-going vessels:
distillate diesel fuel and residual fuel oil. Carbon diox-
ide is the primary greenhouse gas emitted from marine
shipping. In comparison to aviation, the atmospheric
impacts of NOX from shipping are relatively minor, as
the emissions occur at ground level.
Overall, aggregate greenhouse gas emissions in
1998 from the combustion of international bunker fuels
from both aviation and marine activities decreased by 3
percent since 1990, to 31.6 MMTCE (see Table 2-33).
Although emissions from international flights depart-
ing from the United States have increased significantly
(22 percent), emissions from international shipping voy-
ages departing the United States appear to have decreased
by 19 percent since 1990. Increased military activity
during the Persian Gulf War resulted in an increased level
of military marine emissions in 1990 and 1991; civilian
marine emissions during this period exhibited a similar
trend.31 Since 1994, marine emissions have steadily in-
» Sulfur dioxide emissions from jet aircraft and marine vessels, although not estimated here, are mainly determined by the sulfur
content of the fuel. In the U.S., jet fuel, distillate diesel fuel, and residual fuel oil average sulfur contents of 0.05, 0.3, and 2.3 percent,
respectively. These percentages are generally lower than global averages.
27 Naphtha-type jet fuel is used primarily by the military in turbojet and turboprop aircraft engines.
28 In 1996, there were only around a dozen civilian supersonic aircraft in service around the world which flew at these altitudes,
however.
29 However, at this lower altitude, ozone does little to shield the earth from ultraviolet radiation.
30 Cruise altitudes for civilian subsonic aircraft generally range from 8.2 to 12.5 km (27,000 to 41,000 feet).
2-36 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
creased. The majority of these emissions were in the form
of carbon dioxide; however, small amounts of CH4 and
N2O were also emitted. Of the criteria pollutants, emis-
sions of NOX by aircraft at cruising altitudes are of pri-
mary concern because of their effects on ozone forma-
tion (see Table 2-34).
Emissions from both aviation and marine interna-
tional transport activities are expected to grow in the
future as both air traffic and trade increase, although
emission rates should decrease over time due to techno-
logical changes.32
Methodology
Emissions of CO2 were estimated through the ap-
plication of carbon content and fraction .oxidized fac-
tors to fuel consumption activity data. This approach is
analogous to that described under CO2 from Fossil Fuel
Combustion. A complete description of the methodol-
ogy and a listing of the various factors employed can
be found in Annex A. See Annex G for a specific discus-
Table 2-33: Emissions from International Bunker Fuels (MMTCE)
uas/ivioae
C02
Aviation
Marine
CH4
Aviation
Marine
N20
Aviation
Marine
Total
+ Dnf>s nnt pyr-ppri n nc;
1990
32.2
12.7
19.4
+
+
0.3
0.1
0.1
32.5
I MMTfC
1991
32.7
12.7
20.0
+
+
0.3
0.1
0.2
33.0
1992
30.0
12.9
17.1
+
0.3
0.1
0.1
30.3
1993
27.2
13.0
14.3
0.2
0.1
0.1
27.5
1994
26.7
13.2
13.6
0.2
0.1
0.1
27.0
1995
27.5
13.9
13.6
"*"
0.2
0.1
0.1
27.8
1996
27.9
14.2
13.7
+
+
0.2
0.1
0.1
28.1
1997
29.9
15.2
14.7
+
+
0.3
0.1
0.1
30.2
1998
31.3
15.5
15.8
+
+
0.3
0.2
0.1
31.6
^ Note: Totals may not,sum due to indePendent rounding. Includes aircraft cruise altitude emissions.
Table 2-34: Emissions from International Bunker Fuels (Gg)
Gas/Mode
C02
Aviation
Marine
CM,
Aviation
I Marine
Nn
• Aviation
Marine
CO
Aviation
:• Marine
NOX
Aviation
Marine
NMVOC
Aviation
Marine
Note: Totals may not sum
1990
117,965
46,728
71,237
•\
1
3
-)
2
118
77
42
2,093
184
1,908
62
12
51
1991
120,019
46,682
73,337
2
-i
1
3
i
2
120
77
43
2,148
184
1,964
64
11
52
due to independent rounding.
1992
109,965
47,143
62,822
2
•i
1
3
1
2
114
77
37
1,870
186
1,683
56
12
45
1993
99,886
47,615
52,270
2
0
3
-)
109
78
31
1,591
188
1,403
49
12
37
Includes aircraft cruise
1994
98,017
48,327
49,690
2
1
0
3
•|
109
80
29
1,523
191
1,332
47
12
35
1995
101,014
51,093
49,921
2
1
0
3
2
-j
113
84
29
1,541
202
1,339
48
13
36
1996
102,197
52,135
50,062
2
1
0
3
2
1
115
86
29
1,548
207
1,341
49
13
36
altitude emissions.
1997
109,788
55,899
53,889
2
2
0
3
2
1
124
92
32
1,665
221
1,444
52
14
38
1998
114,700
56,917
57,783
2
2
1
3
2
1
128
94
34
1,776
225
1,550
55
14
41
31 See Uncertainty section for a discussion of data quality issues
Energy 2-37
-------
sion on the methodology used for estimating emissions
from international bunker fuel use by the U.S. military.
Emission estimates for CH4, N2O, CO, NOX, and
NMVOCs were calculated by multiplying emission fac-
tors by measures of fuel consumption by fuel type and
mode. Activity data for aviation included solely jet
fuel consumption statistics, while the marine mode in-
cluded both distillate diesel and residual fuel oil.
Data Sources
Carbon content and fraction oxidized factors for
kerosene-type and naptha-type jet fuel, distillate fuel
oil, and residual fuel oil were taken directly from the
Energy Information Administration (EIA) of the U.S.
Department of Energy and are presented in Annex A.
Heat content and density conversions were taken from
EIA (1998) and USAF (1998). Emission factors used in
the calculations of CH4, N2O, CO, NOX, and NMVOC
emissions were taken from the Revised 1996 IPCC
Guidelines (IPCC/UNEP/OECD/1EA1997). For aircraft
emissions, the following values, in units of grams of
pollutant per kilogram of fuel consumed (g/kg), were
employed: 0.09 for CH4, 0.1 for N2O, 5.2 for CO, 12.5
for NOX, and 0.78 for NMVOCs. For marine vessels con-
suming either distillate diesel or residual fuel oil the
following values, in the same units, except where noted,
were employed: 0.03 for CH4,0.08 for N20,1.9 for CO,
87 for NOX, and 0.052 g/MJ for NMVOCs.
Activity data on aircraft fuel consumption were
collected from three government agencies. Jet fuel con-
sumed by U.S. flag air carriers for international flight
segments was supplied by the Bureau of Transporta-
tion Statistics (DOT/BTS 1999). It was assumed that 50
percent of the fuel used by U.S. flagged carriers for
international flights—both departing and arriving in
the United States—was purchased domestically for
flights departing from the United States. In other words,
only one-half of the total annual fuel consumption es-
timate was used in the calculations. Data on jet fuel
expenditures by foreign flagged carriers departing U.S.
airports was taken from unpublished data collected by
the Bureau of Economic Analysis (BEA) under the U.S.
Department of Commerce (BEA 1999). Approximate
average fuel prices paid by air carriers for aircraft on
international flights was taken from DOT/BTS (1999)
and used to convert the BEA expenditure data to gal-
lons of fuel consumed. Data on jet fuel expenditures by
the U.S. military was supplied by the Office of the Un-
der Secretary of Defense (Environmental Security), U.S.
Department of Defense (DoD). Estimates of the percent-
age of each services' total operations that were interna-
tional operations were developed by DoD. Military
aviation bunkers included international operations,
operations conducted from naval vessels at sea, and
operations conducted from U.S. installations principally
over international water in direct support of military
operations at sea. Data on fuel delivered to the military
within the United States was provided from unpublished
data by the Defense Energy Support Center, under DoD's
Defense Logistics Agency (DESC 1999). Together, the
data allow the quantity of fuel used in military interna-
tional operations to be estimated. Jet fuel densities for
each fuel type were obtained from a report from the
U.S. Air Force (USAF 1998). Finaljetfuel consumption
estimates are presented in Table 2-35. See Annex G for
additional discussion of military data.
Activity data on distillate diesel and residual fuel
oil consumption by cargo or passenger carrying marine
vessels departing from U.S. ports were taken from un-
published data collected by the Foreign Trade Division
Table 2-35: Aviation Jet Fuel Consumption for International Transport (Million Gallons)
^Nationality
p:s. Carriers
• Foreign Carriers
|D.S. Military
irotal
i
ftNote: Totals may
I
1990
1,982
2,062
862
4,905
1991
1,970
2,075
855
4,900
1992
2,069
2,185
700
4,954
1993
2,078
2,252
677
5,007
1994
2,155
2,326
608
5,090
1995
2,256
2,549
581
5,385
1996
2,329
2,629
540
5,497
' 1997
2,482
2,918
496
5,895
not sum due to independent rounding.
1998 "*
2,363
3,138 '••<
502 ':
6,003 I
2-38 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
of the U.S. Department of Commerce's Bureau of the
Census (DOC 1998). Activity data on distillate diesel
consumption by military vessels departing from U.S.
ports were provided by the Defense Energy Support Cen-
ter (DESC). The total amount of fuel provided to naval
vessels was reduced by 13 percent to account for fuel
used while the vessels were not-underway (i.e., in port).
Data on the percentage of steaming hours underway ver-
sus not-underway were provided by the U.S. Navy. These
fuel consumption estimates are presented in Table 2-36.
Uncertainty
Emission estimates related to the consumption of
international bunker fuels are subject to the same uncer-
tainties as those from domestic aviation and marine mo-
bile combustion emissions; however, additional uncer-
tainties result from the difficulty in collecting accurate
fuel consumption activity data for international trans-
port activities separate from domestic transport activi-
ties.33 For example, smaller aircraft on shorter routes
often carry sufficient fuel to complete several flight seg-
ments without refueling in order to minimize time spent
at the airport gate or take advantage of lower fuel prices
at particular airports. This practice, called tankering,
when done on.international flights, complicates the use
of fuel sales data for estimating bunker fuel emissions.
Tankering is less common with the type of large, long-
range aircraft that make many international flights from
the United States, however. Similar practices occur in
the marine shipping industry where fuel costs represent
a significant portion of overall operating costs and fuel
prices vary from port to port, leading to some tankering
from ports with low fuel costs.
Particularly for aviation, the DOT/BTS (1998) in-
ternational flight segment fuel data used for U.S. flagged
carriers does not include smaller air carriers and unfortu-
nately defines flights departing to Canada and some
flights to Mexico as domestic instead of international.
As for the BEA (1998) data on foreign flagged carriers,
there is some uncertainty as to the average fuel price,
and to the completeness of the data. It was also not pos-
sible to determine what portion of fuel purchased by
foreign carriers at U.S. airports was actually used on do-
mestic flight segments; this error, however, is believed
to be small.34
Although aggregate fuel consumption data has
been used to estimate emissions from aviation, the rec-
ommended method for estimating emissions of gases
other than CO2 in the Revised 1996IPCC Guidelines is
to use data by specific aircraft type (IPCC/UNEP/OECD/
IEA 1997). The IPCC also recommends that cruise alti-
tude emissions be estimated separately using fuel con-
sumption data, while landing and take-off (LTO) cycle
data be used to estimate near-ground level emissions of
gases other than CO2.35 The EPA is developing revised
Table 2-36: Marine Fuel Consumption for International Transport (Million Gallons)
| Fuel Type
js Residual Fuel Oil
f Distillate Diesel Fuel & Other
f; U.S. Military Naval Fuels
|;TgtaJ
| Note: Totals may not sum due to
f-
1990
5,137
598
522
6,257
independent
1991
5,354
595
481
6,431
rounding.
1992
4,475
561
491
5,527
3
4
1993
,567
609
44R
KM
3
4
1994
,504
510
364
,378
1995
3,495
573
334
4,402
1996
3,583
456
362
4,402
1997
3,843
421
477
4,740
1998
3,974
627
506
5,107
33 See uncertainty discussions under CO2 from Fossil Fuel Combustion and Mobile Combustion
Energy 2-39
-------
estimates based on this more detailed activity data, and
these estimates are to be presented in future inventories.
There is also concern as to the reliability of the
existing DOC (1998) data on marine vessel fuel con-
sumption reported at U.S. customs stations due to the
significant degree of inter-annual variation. Of note is
that fuel consumption data were not available for the
year 1990; therefore, an average of 1989 and 1991 data
was employed.
Wood Biomass and Ethanol
Consumption
The combustion of biomass fuels—such as wood,
charcoal, and wood waste—and biomass-based fuels—
such as ethanol from corn and woody crops—generates
carbon dioxide (CO^. However, in the long run the car-
bon dioxide emitted from biomass consumption does
not increase atmospheric carbon dioxide concentrations,
assuming the biogenic carbon emitted is offset by the
uptake of CO2 resulting from the growth of new biom-
ass. As a result, CO2 emissions from biomass combustion
have been estimated separately from fossil fuel-based
emissions and are not included in the U.S. totals. Net
carbon fluxes from changes in biogenic carbon reser-
voirs in wooded or crop lands are accounted for in the
Land-Use Change and Forestry chapter.
In 1998, CO2 emissions due to burning of woody
biomass within the industrial and residential/commer-
cial sectors and by electric utilities were about 64.2
MMTCE (235,554 Gg) (see Table 2-37 and Table 2-38).
As the largest consumer of woody biomass, the indus-
trial sector in 1998 was responsible for 83 percent of the
CO2 emissions from this source. The combined residen-
tial/commercial36 sector was the second largest emitter,
making up 16 percent of total emissions from woody
biomass. The commercial end-use sector and electric
utilities accounted for the remainder.
Biomass-derived fuel consumption in the United
States consisted mainly of ethanol use in the transporta-
tion sector. Ethanol is primarily produced from corn
Table 2-37: CO, Emissions from Wood Consumption by End-Use Sector (MMTCE)
End-Use Sector
Electric Utility
Industrial
Residential/Commercial
Total
Note: Totals may not sum
1990
0.5
42.4
12.7
55.6
1991
0.5
42.3
13.4
56.2
1992
0.5
44.5
14.1
59.0
1993
0.4
45.4
12.9
58.8
1994
0.4
48.3
12.7
61.4
1995
0.4
49.8
14.0
64.2
1996
0.4
51.6
14.0
66.1
1997
0.4
52.1
10.4
62.9
1998
0.4
53.6
10.2
64.2
due to independent rounding.
Table 2-38: CO, Emissions from Wood Consumption by End-Use Sector (Gg)
End-Use Sector
Electric Utility
Industrial
Residential/Commercial
Tola!
1990
1,715
155,614
46,424
203,753
1991
1,698
155,232
48,981
205,910
1
1
163
51
216
iqq?
,725
,195
537
457
1993
1,636
166,480
47,303
215,419
1994
1,635
177,145
46,504
225,284
1995
1,356
182,658
51,218
235,232
1996
1,580
189,370
51,440
242,390
1997
1,542
190,968
37,959
230,470
1998
1,598
196,561
37,395
235,554
,3
• if
Note: Totals may not sum due to independent rounding.
« It should be noted that in the EPA's National Air Pollutant Emissions Trends, 1900 - 1998 (EPA 1999), U.S. aviation emission
estimates for CO?NOx and NMVOCs are based solely upon LTO cycles and consequently only capture near ground-level emissions,
more relevant for air quality evaluations. These estimates also include both domestic and international flights. Therefore,
g™cn under Mobile Source Fossil Fuel Combustion overestimate IPCC-defined domestic CO, NOX, and NMVOC emissions by
dinE Indingandtake-off (LTO) cycles by aircraft on international flights but underestimate because they do not include
emtston! Sm afrcraft on domestic flight segments at cruising altitudes. EPA (1998) is also likely to include emissions from ocean-
going vessels departing from U.S. ports on international voyages.
36 For this emissions source, data are not disaggregated into residential and commercial sectors.
2-40 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Table 2-39: C02 Emissions from
Ethanol Consumption
Year
MMTCE
1990
1991
1992
1993
1994
1995
1996
1997
1998
1.6
1.2
1.5
1.7
1.8
2.0 "
1.4
1.8
2.0
5,701
4,519
5,492
6,129
6,744
7,230
5,145
6,744
7,300
grown in the Midwest, and was used mostly in the Mid-
west and South. Pure ethanol can be combusted, or it can
be mixed with gasoline as a supplement or octane-en-
hancing agent. The most common mixture is a 90 per-
cent gasoline, 10 percent ethanol blend known as gaso-
hol. Ethanol and ethanol blends are often used to fuel
public transport vehicles such as buses, or centrally fu-
eled fleet vehicles. Ethanol and ethanol blends are be-
lieved to burn "cleaner" than gasoline (i.e., lower in NOX
and hydrocarbon emissions), and have been employed
in urban areas with poor air quality. However, because
ethanol is a hydrocarbon fuel, its combustion emits CO2.
In 1998, the United States consumed an estimated
105 trillion Btus of ethanol. Emissions of CO2 in 1998
due to ethanol fuel burning were estimated to be ap-
proximately 2.0 MMTCE (6,744 Gg) (see Table 2-39).
Ethanol production dropped sharply in the middle
of 1996 because of short corn supplies and high prices.
Plant output began to increase toward the end of the
growing season, reaching close to normal levels at the
end of the year. However, total 1996 ethanol production
fell far short of the 1995 level (EIA 1997). Production in
1998 returned to normal historic levels.
Methodology
Woody biomass emissions were estimated by con-
verting U.S. consumption data in energy units (17.2 mil-
lion Btu per short ton) to megagrams (Mg) of dry matter
using EIA assumptions. Once consumption data for each
sector were converted to megagrams of dry matter, the
carbon content of the dry fuel was estimated based on
default values of 45 to 50 percent carbon in dry biomass.
The amount of carbon released from combustion was
estimated using 87 percent for the fraction oxidized (i.e.,
combustion efficiency). Ethanol consumption data in
energy units were also multiplied by a carbon coeffi-
cient (18.96 mg C/Btu) to produce carbon emission esti-
mates.
Data Sources
Woody biomass consumption data were provided
Table 2-40: Woody Biomass Consumption
by Sector (Trillion Btu)
[-Year
1990
1991
11992
h993
•1:1994
?1995
,1996
1,1997
t1998
r =~°
^
Industrial
1,948
1,943
2,042
2,084
2,217
2,286
2,370
2,390
2,460
Residential/
Commercial
581
613
645
592
582
641
644
475
468
Electric
Utility
21
21
22 ;
20
20
17
20
19
20
. . .- .- -.. . - - ii •
laoie i-4i: tinanoi uonsumption
Year
1990
1991
1992
1993
1994
~~.-. 1995
1996
- 1997
1998
_ -- — • — .
Trillion Btu
82
65
79
88
97
104
74
97
105
Energy 2-41
-------
by EIA (1999) (see Table 2-40). The factor for convert-
ing energy units to mass was supplied by EIA (1994).
Carbon content and combustion efficiency values were
taken from the Revised 1996 1PCC Guidelines (IPCC/
UNEP/OECD/IEA1997).
Emissions from ethanol were estimated using con-
sumption data from EIA (1999) (see Table 2-41). The
carbon coefficient used was provided by OTA (1991).
Uncertainty
The combustion efficiency factor used is believed
to under estimate the efficiency of wood combustion
processes in the United States. The IPCC emission factor
has been used because better data are not yet available.
Increasing the combustion efficiency would increase
emission estimates. In addition, according to EIA (1994)
commercial wood energy use is typically not reported
because there are no accurate data sources to provide
reliable estimates. Emission estimates from ethanol pro-
duction are more certain than estimates from woody bio-
mass consumption due to better activity data collection
methods and uniform combustion techniques.
2-42 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
3. Industrial Processes
Greenhouse gas emissions are produced as a by-product of various non-energy-related industrial activities.
That is, these emissions are produced from an industrial process itself and are not directly a result of energy
consumed during, the process. For example, raw materials can be chemically transformed from one state to another.
This transformation can result in the release of greenhouse gases such as carbon dioxide (CO2), methane (CH4), or
nitrous oxide (N2O). The processes addressed in this chapter include cement production, lime manufacture, limestone
and dolomite use (e.g., flux stone, flue gas desulfurization, and glass manufacturing), soda ash production and use,
CO2 consumption, kon and steel production, ammonia manufacture, ferroalloy production, aluminum production,
petrochemical production, silicon carbide production, adipic acid production, and nitric acid production (see Figure
3-1).1
In addition to the three Figure 3-1
greenhouse gases listed above,
there are also industrial sources
of several classes of man-made
fluorinated compounds called
hydrofluorocarbons (MFCs),
perfluorocarbons (PFCs), and
sulfur hexafluoride (SF6). The
present contribution of these
gases to the radiative forcing ef-
fect of all anthropogenic green-
house gases is small; however, be-
cause of their extremely long life-
times, they will continue to ac-
cumulate in the atmosphere as
long as emissions continue. Sul-
fur hexafluoride, itself, is the
most potent greenhouse gas the
IPCC has ever evaluated. Usage
of these gases, especially MFCs,
Substitution of Ozone Depleting Substances
HCFC-22 Production
Cement Manufacture
Electrical Transmission and Distribution
Nitric Acid
Lime Manufacture
Magnesium Production and Processing
Aluminum Production
Limestone and Dolomite Use
Semiconductor Manufacture
Adipic Acid
Soda Ash Manufacture and Consumption
Petrochemical Production
Carbon Dioxide Consumption
Silicon Carbide Production
Portion of All
Emissions
<0.05
6 8 10
MMTCE
12 14
Carbon dioxide emissions from iron and steel production, ammonia manufacture, ferroalloy production, and aluminum production
are accounted for in the Energy chapter under Fossil Fuel Combustion of industrial coking coal, natural gas, and petroleum coke.
Industrial Processes 3-1
-------
Table 3-1: Emissions from Industrial Processes (MMTCE)
Gas/Source
C02
Cement Manufacture
Lime Manufacture
Limestone and Dolomite Use
Soda Ash Manufacture and
Consumption
Carbon Dioxide Consumption
Iron and Steel Production*
Ammonia Manufacture*
Ferroalloy Production*
Aluminum Production*
CH4
Petrochemical Production
Silicon Carbide Production
N20
Adipic Acid Production
Nitric Acid Production
HFCs, PFCs, and SF6
Substitution of Ozone
Depleting Substances
Aluminum Production
HCFC-22 Production
Semiconductor Manufacture
Electrical Transmission and
Distribution
Magnesium Production and
Processing
Total
1990
14.8
9.1
3.0
1.4
1.1
0.2
23.9
6.3
0.5
1.6
0.3
0.3
+
9.9
5.0
4.9
23.3
0.3
5.4
9.5
0.8
5.6
1.7
48.3
1991
14.5
8.9
3.0
1.3
1.1
0.2
19.2
6.4
0.4
1.7
0.3
0.3
+
10.1
5.2
4.9
22.0
0.2
4.7
8.4
0.8
5.9
2.0
46.9
1992
14.6
8.9
3.1
1.2
1.1
0.2
20.7
6.7
0.4
1.6
0.3
0.3
+
9.8
4.8
5.0
23.5
0.4
4.4
9.5
0.8
6.2
2.2
48.3
1993
15.0
9.4
3.1
1.1
1.1
0.2
21.0
6.4
0.4
1.5
0.4
0.4
+
10.2
5.2
5.1
23.8
1.4
3.8
8.7
1.0
6.4
2.5
49.5
1994
16.0
9.8
3.2
1.5
1.1
0.2
21.6
6.6
0.4
1.3
0.4
0.4
+
10.9
5.5
5.3
25.1
2.7
3.2
8.6
1.1
6.7
2.7
52.3
1995
16.8
10.0
3.4
1.9
1.2
0.3
22.2
6.5
0.4
1.4
0.4
0.4
+
11.0
5.5
5.4
29.0
7.0
3.1
7.4
1.5
7.0
3.0
57.2
1996
17.2
10.1
3.6
2.0
1.2
0.3
21.6
6.7
0.5
1.4
0.4
0.4
+
11.3
5.7
, 5.6
33.5
9.9
3.2
8.5
1.9
7.0
3.0
62.5
1997
18.0
10.5
3.7
2.3
1.2
0.4
21.6
6.6
0.5
1.4
0.4
0.4
+
10.5
4.7
5.8
35.3
12.3
3.0
8.2
1.9
7.0
3.0
64.2
1998
18.4
10.7
3.7
2.4
1.2
0.4
21.9
7.3
0.5
1.5
0.4
0.4
+
7.7
2.0
5.8
40.3
14.5
2.8
10.9
2.1
7.0
3.0
66.9
+ Does not exceed 0.05 MMTCE
* Emissions from these sources are accounted for in the Energy chapter and are not included in the Industrial Processes totals.
Note: Totals may not sum due to independent rounding.
is growing rapidly as they are the primary substitutes for
ozone depleting substances (ODSs), which are being
phased-out under the Montreal Protocol on Substances
that Deplete the Ozone Layer. In addition to ODS substi-
tutes, HFCs, PFCs, and other fluorinated compounds are
employed and emitted by a number of other industrial
sources in the United States. These industries include alu-
minum production, HCFC-22 production, semiconduc-
tor manufacture, electric power transmission and distribu-
tion, and magnesium metal production and processing.
In 1998, industrial processes generated emissions
of 67.0 MMTCE, or 3.7 percent of total U.S. greenhouse
gas emissions. Carbon dioxide emissions from all indus-
trial processes were 18.4 MMTCE (67,447 Gg) in the
same year. This amount accounted for only 1 percent of
national CO2 emissions. Methane emissions from pet-
rochemical and silicon carbide production resulted in
emissions of approximately 0.4 MMTCE (78 Gg) in 1998,
which was less than 1 percent of U.S. CH4 emissions.
Nitrous oxide emissions from adipic acid and nitric acid
production were 7.7 MMTCE (91 Gg) in 1998, or 6 per-
cent of total U.S. N2O emissions. In the same year, com-
bined emissions of HFCs, PFCs and SF6 totaled 40.5
MMTCE. Overall, emissions from industrial processes
increased by 39 percent from 1990 to 1998, due mainly
to growth in the use of HFCs.
Emission estimates are presented in this chapter
for several industrial processes that are actually ac-
2 See Annex P for a discussion of emission sources excluded.
3-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Table 3-2: Emissions from Industrial Processes (Gg)
I Gas/Source
,C02
- Cement Manufacture
f Lime Manufacture
Limestone and Dolomite Use
Soda Ash Manufacture and
:> Consumption
Carbon Dioxide Consumption
Iron and Steel Production3
Ammonia Manufacture3
Ferroalloy Production3
? Aluminum Production3
m i
CH4
Petrochemical Production
Silicon Carbide Production
N20
',-. Adipic Acid Production
, Nitric Acid Production
HFCs, PFCs, andSF6
" Substitution of Ozone
Depleting Substances
Aluminum Production
HCFC-22 Production"
Semiconductor Manufacture
Electrical Transmission and
Distribution0
Magnesium Production and
Processing0
54
33
11
5
4
87
23
1
5
1990
,427
,278
,092
,113
,144
800
600
138
809
951
57
56
1
117
59
58
M
M
M
3
M
1
+
1991
53,197
32,535
10,891
4,896
4,035
840
70,560
23,364
1,580
6,058
58
57
1
119
62
58
M
M
M
3
M
1
+
1992
53,512
32,792
11,245
4,502
4,091
882
75,840
24,391
1,579
5,942
61
60
1
116
57
59
M
M
M
3
M
1
+
1993
55,137
34,624
11,496
4,058
4,048
912
77,120
23,399
1,516
5,432
67
66
1
121
61
60
M
M
M
3
M
1
+
1994
58,432
36,087
11,895
5,541
4,012
898
79,040
24,316
1,607
4,850
71
70
1
129
65
63
M
M
M
3
M
1
+
1995
61,735
36,847
12,624
6,987
4,309
968
81,440
23,682
1,625
4,961
72
72
1
130
66
64
M
M
M
2
M
1
+
63
37
13
7
4
1
79
24
1
5
1996
,170
,079
,179
,499
,273
140
040
390
695
258
76
75
1
134
67
67
M
M
M
3
M
1
+
1997
66,021
38,323
13,434
8,537
4,434
1,294
79,360
24,346
1,789
5,296
77
77
1
124
55
68
M
M
M
3
M
-)
+
1998
67,447
39,227
13,627
8,854
4,325
1,413
80,160
26!880
1,790
5,458
78
77
1
91
23
68
M
•5
U
M
-)
+
I M (Mixture of gases)
; a Emissions from these sources are accounted for in the Energy chapter and are not included
i J3 HFC-23 emitted
h~SF6 emitted
j: Note: Totals may not sum due to independent rounding.
in the Industrial Processes .totals.
counted for within the Energy chapter. Although CO2
emissions from iron and steel production, ammonia
manufacture, ferroalloy production, and aluminum pro-
duction are not the result of the combustion of fossil
fuels for energy, their associated emissions are captured
in the fuel data for industrial coking coal, natural gas,
industrial coking coal, and petroleum coke, respectively.
Consequently, if all emissions were attributed to their
appropriate chapter, then emissions from energy would
decrease by roughly 31 MMTCE in 1998, and industrial
process emissions would increase by the same amount.
Greenhouse gases are also emitted from a number
of industrial processes not addressed in this chapter. For
example, caprolactam—a chemical feedstock for the
manufacture of nylon 6,6—and urea production are be-
lieved to be industrial sources of N2O emissions. How-
ever, emissions for these and other sources have not been
estimated at this time due to a lack of information on the
emission processes, manufacturing data, or both. As more
information becomes available, emission estimates for
these processes will be calculated and included in future
greenhouse gas emission inventories, although their con-
tribution is expected to be small.2
The general method employed to estimate emis-
sions for industrial processes, as recommended by the
Intergovernmental Panel on Climate Change (IPCC),
generally involved multiplying production data for each
process by an emission factor per unit of production.
The emission factors used were either derived using cal-
culations that assume precise and efficient chemical re-
actions or were based upon empirical data in published
references. As a result, uncertainties in the emission co-
Industrial Processes 3-3
-------
efficients can be attributed to, among other things, inef-
ficiencies in the chemical reactions associated with each
production process or to the use of empirically derived
emission factors that are biased and, therefore, may not
represent U.S. national averages. Additional sources of
uncertainty specific to an individual source category
are discussed in each section.
Table 3-1 summarizes emissions for the Industrial
Processes chapter in units of million metric tons of car-
bon equivalents (MMTCE), while unweighted gas emis-
sions in Gigagrams (Gg) are provided in Table 3-2.
Cement Manufacture
Cement manufacture is an energy and raw material
intensive process resulting in the generation of carbon
dioxide (CO2) from both the energy consumed in mak-
ing the cement and the chemical process itself.3 Cement
production accounts for about 2.4 percent of total glo-
bal industrial and energy-related CO2 emissions (IPCC
1996), and the United States is the world's third largest
cement producer. Cement is manufactured in almost ev-
ery state and is used in all of them. Carbon dioxide,
emitted from the chemical process of cement produc-
tion, represents one of the largest sources of industrial
CO2 emissions in the United States.
During the cement production process, calcium
carbonate (CaCO3) is heated in a cement kiln at a tem-
perature of about 1,300°C (2,400°F) to form lime (i.e.,
calcium oxide or CaO) and CO2. This process is known
as calcination or calcining. Next, the lime is combined
with silica-containing materials to produce clinker (an
intermediate product), with the earlier by-product CO2
being released to the atmosphere. The clinker is then
allowed to cool, mixed with a small amount of gypsum,
and used to make Portland cement. The production of
masonry cement from Portland cement requires additional
lime and, thus, results in additional CO2 emissions. How-
ever, this additional lime is already accounted for in the
Lime Manufacture source category in this chapter; there-
fore, the additional emissions from making masonry ce-
ment from clinker are not counted in this source's total.
They are presented here for informational purposes only.
In 1998, U.S. clinker production—including
Puerto Rico—totaled 75,859 thousand metric tons, and
U.S. masonry cement production reached 3,910 thou-
sand metric tons (USGS 1999). The resulting emissions
of CO2 from clinker production were estimated to be
10.7 MMTCE (39,227 Gg) (see Table 3-3). Emissions
from masonry production from clinker raw material were
estimated to be 0.02 MMTCE (88 Gg) in 1998, but again
are accounted for under Lime Manufacture.
After falling in 1991 by 2 percent from 1990 lev-
els, cement production emissions have grown every year
since. Overall, from 1990 to 1998, emissions increased
by 18 percent. In 1998, output by cement plants increased
2 percent over 1997, to 75,859 thousand metric tons.
Cement is a critical component of the construction in-
dustry; therefore, the availability of public construction
funding, as well as overall economic growth, will have
considerable influence on cement production in the fu-
ture. In the near term, a strong domestic economy is a
key factor in maintaining high demand for construction
materials and, hence, growth in the cement industry and
associated CO2 emissions.
Table 3-3: CO, Emissions from Cement Production*
f*- "•"
L
t.
j
P '
r"
1-
Year
-, . - 1990
1991
1992
1993
1994
" 1995
' •-•••••-< 1996
1997
1998
'• Totals exclude CO
rom clinker, which
MMTCE
9.1
8.9
8.9
9.4
9.8
'To.O'
10.1
10.5
10.7
2 emissions from
are accounted for
Gg
33,278
32,535
32,792
34,624
36,087
36,847'
37,079
38,323
39,227
making masonry cement
under Lime Manufacture.
3 The CO2 emissions related to the consumption of energy for cement manufacture are accounted for under CO2 from Fossil Fuel
Combustion in the Energy chapter.
3-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Methodology
Carbon dioxide emissions from cement manufac-
ture are created by the chemical reaction of carbon-con-
taining minerals (i.e., calcining limestone). While in the
kiln, limestone is broken down into CO2 and lime with
the CO2 released to the atmosphere. The quantity of the
CO2 emitted during cement production is directly pro-
portional to the lime content of the clinker. During cal-
cination, each mole of CaCO3 (i.e., limestone) heated in
the clinker kiln forms one mole of lime (CaO) and one
mole of CO2:
CaCO3 + heat -> CaO + CO2
Carbon dioxide emissions were estimated by ap-
plying an emission factor, in tons of CO2 released per
ton of clinker produced, to the total amount of clinker
produced. The emission factor used in this analysis is
the product of the average lime fraction for clinker of
64.6 percent (EPCC/UNEP/OECD/IEA 1997) and a con-
stant reflecting the mass of CO2 released per unit of lime.
This yields an emission factor of 0.507 tons of CO2 per
ton of clinker produced. The emission factor was calcu-
lated as follows:
[44.01 g/moleCOj
2 =
56.08 g/mole CaoJ
0.507 tons CO2/ton clinker
During clinker production, some of the clinker pre-
cursor materials remain in the kiln as non-calcinated,
partially calcinated, or fully calcinated cement kiln dust
(CKD). The emissions attributable to the calcinated por-
tion of the CKD are not accounted for by the clinker
emission factor. The IPCC recommends that these addi-
tional CKD CO2 emissions should be estimated as 2 per-
cent of the CO2 emissions calculated from clinker pro-
duction. Total cement production emissions were calcu-
lated by adding the emissions from clinker production
to the emissions assigned to CKD (IPCC/OECD/IEA
1999).
Masonry cement requires additional lime over and
above the lime used in clinker production. In particular,
non-plasticizer additives such as lime, slag, and shale
are added to the cement, increasing its weight by ap-
proximately 5 percent. Lime accounts for approximately
60 percent of this added weight. Thus, the additional
lime is equivalent to roughly 2.86 percent of the starting
amount of the product, since:
0.6 x 0.05/(1 + 0.05) = 2.86%
An emission factor for this added lime can then be
calculated by multiplying this percentage (2.86 percent)
by the molecular weight ratio of CO2 to CaO (0.785) to
yield 0.0224 metric tons of additional CO2 emitted for
every metric ton of masonry cement produced.
As previously mentioned, the CO2 emissions from
the additional lime added .during masonry cement pro-
duction are accounted for in the section on CO2 emis-
sions from Lime Manufacture. Thus, these emissions were
estimated in this chapter for informational purposes only,
and are not included in the cement emission totals.
Data Sources
The activity data for clinker and masonry cement
production (see Table 3-4) were obtained from U.S. Geo-
logical Survey (USGS 1992, 1995, 1996, 1997, 1998,
1999). The data were compiled by USGS through ques-
tionnaires sent to domestic clinker and cement manu-
facturing plants. The 1998 value for masonry cement
production was furnished by Hendrick van Oss, USGS.
Uncertainty
The uncertainties contained in these estimates are
primarily due to uncertainties in the lime content of clin-
ker, in the amount of lime added to masonry cement, and
in the percentage of CKD recycled inside the clinker
kiln. The lime content of clinker varies from 64 to 66
percent. CKD loss can range from 1.5 to 8 percent de-
pending upon plant specifications. Additionally, some
Table 3-4: Cement Production
(Thousand Metric Tons)
Year
Clinker
Masonry
t~- '-"
r
pr -
F-..
fc_
ip'r---
jjJt-.-r- -• -
8r---..
jfe " '
&£"-
r-
ST."
JF^: -
KLr
1990
1991
1992
1993
1994
1995
_: 1996
1997
1998
64,355
62,918
63,415
66,957
69,786
71,257
71,706
74,112
75,859
3,209
2,856
3,093
2,975
3,283
3,603
3,469
3,634
3,910
Industrial Processes 3-5
-------
amount of CO2 is reabsorbed when the cement is used
for construction. As cement reacts with water, alkaline
substances such as calcium hydroxide are formed. Dur-
ing this curing process, these compounds may react with
CO2 in the atmosphere to create calcium carbonate. This
reaction only occurs in roughly the outer 0.2 inches of
surface area. Because the amount of CO2 reabsorbed is
thought to be minimal, it was not estimated.
Lime Manufacture
Lime, or calcium oxide (CaO),4 is an important
manufactured product with many industrial, chemical,
and environmental applications. Its major uses are in
steel making, flue gas desulfurization (FGD) at coal-fired
electric power plants, construction, pulp and paper manu-
facturing, and water purification. Lime has historically
ranked fifth in total production of all chemicals in the
United States.
Lime production involves three main processes:
stone preparation, calcination, and hydration. Carbon
dioxide is generated during the calculation stage, when
limestone—mostly calcium carbonate (CaCO3)—is
roasted at high temperatures in a kiln to produce CaO
and CO2. The CO2 is driven off as a gas and is normally
emitted to the atmosphere. Some of the CO2 generated
during the production process, however, is recovered at
Table 3-5: Net C02 Emissions
from Lime Manufacture
Year
1990
1991
1992
1993
1994
1995
1996
1997
1998
MMTCE
3.0
3.0
3.1
3.1 :
3.2 ;
3.4
3.6 :
3.7
3.7
some facilities for use in sugar refining and precipitated
calcium carbonate (PCC)5 production. It is also impor-
tant to note that for certain applications, lime reabsorbs
CO2 during use (see Uncertainty, below).
Lime production in the United States—including
Puerto Rico—was reported to be 20,100 thousand met-
ric tons in 1998 (USGS 1999). This resulted in CO2 emis-
sions of 3.7 MMTCE (13,627 Gg) (see Table 3-5 and
Table 3-6).
At the turn of the century, over 80 percent of lime
consumed in the United States went for construction uses.
However, the contemporary lime market is distributed
across its four end-use categories as follows: metallurgi-
cal uses, 39 percent; environmental uses, 26 percent;
chemical and industrial uses, 24 percent, and construc-
tion uses, 9 percent. Domestic lime manufacture has in-
creased every year since 1991, when it declined by 1
percent from 1990 levels. Production in 1998 increased
2 percent over the previous year to about 20,100 thou-
sand metric tons. Overall, from 1990 to 1998, lime pro-
duction, and hence process CO2 emissions, increased by
23 percent. The increase in production is attributed in
part to growth in demand for environmental applications,
especially flue gas desulfurization technologies. In 1993,
the U.S. Environmental Protection Agency (EPA) com-
pleted regulations under the Clean Air Act capping sul-
Table 3-6: C02 Emissions
from Lime Manufacture (Gg)
F"
i
t'
i|
k"
1
f
l:
!•
r
p
Year
1990
1991
1992
1993
1994
1995
1996
1997
1998
Potential
11,574
11,454
11,843
12,261
12,699
13,502
14,013
14,378
14,670
Recovered*
(483)
(563)
(598)
(765)
. (804)
(878)
(834)
(944)
(1,043)
;; • .'•-,'---'-'--•• = • -,'!>
Net
Emissions
11,092
10,891
11,245
11,496
11,895
12,624
13,179
13,434
13,627
For sugar refining and precipitated calcium carbonate
production
l_Note: Totals may not sum due to independent rounding.
4 Lime also exists in a dolomitic form (CaO-MgO).
5 Precipitated calcium carbonate is a specialty filler used in premium-quality coated and uncoated papers.
3-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
fur dioxide (SO2) emissions from electric utilities. Lime
scrubbers' high efficiencies and increasing affordability
have allowed the FGD end-use to expand from 12 per-
cent of total lime consumption in 1994 to 15 percent in
1998 (USGS 1999).
Methodology
During the calcination stage of lime manufacture,
CO2 is driven off as a gas and normally exits the system
with the stack gas. Carbon dioxide emissions were esti-
mated by applying a CO2 emission factor to the total
amount of lime produced. The emission factor used in
this analysis is the product of a constant reflecting the
mass of CO2 released per unit of lime and the average
calcium plus magnesium oxide (CaO + MgO) content
for lime. This yields an emission factor of 0.73 tons of
CO2 per ton of lime produced. The emission factor was
calculated as follows:
[(44.01 g/mole CO2) -H (56.08 g/mole CaO)]
x (0.93 CaO/lime) = 0.73 g CO2/g lime
Lime production in the United States was 20,100
thousand metric tons in 1998 (USGS 1999), resulting in
potential CO2 emissions of 14,670 Gg. Some of the CO2
generated during the production process, however, was
recovered for use in sugar refining and precipitated cal-
cium carbonate (PCC) production. Combined lime manu-
facture by these producers was 1,785 thousand metric
tons in 1998, generating 1.0 Gg of CO2. It was assumed
Table 3-7: Lime Production and Lime Use
for Sugar Refining and PCC (Thousand Metric Tons)
E Year
£• 1"0
r- 1991
r- 1992
fc 1993
$•-- 1994
!- 1995
I-, 1996
IT 1997 '
IS— 1998
I7" ' '
Production
15,859
15,694
16,227
16,800
17,400
18,500
19,200
19,700
20,100
Use
826
964
1,023
1,310
1,377
1,504
1,428
1,616
1,785
that approximately 80 percent of the CO2 involved in
sugar refining and PCC was recovered.
Data Sources
The activity data for lime manufacture and lime
consumption by sugar refining and precipitated calcium
carbonate (PCC) for 1990 through 1992 (see Table 3-7)
were obtained from USGS (1991,1992); for 1993 through
1994 from Michael Miller (1995); for 1995 through 1998
from USGS (1997, 1998, 1999). The CaO purity of lime
was obtained from ASTM (1996) and Schwarzkopf (1995).
Uncertainty
The term "lime" is actually a general term that in-
cludes various chemical and physical forms of this com-
modity. Uncertainties in the emission estimate can be at-
tributed to slight differences in the chemical composition
of these products. For example, although much care is
taken to avoid contamination during the production pro-
cess, lime typically contains trace amounts of impurities
such as iron oxide, alumina and silica. Due to differences
in the limestone used as a raw material, a rigid specifica-
tion of lime material is impossible. As a result, few plants
manufacture lime with exactly the same properties.
In addition, a portion of the CO2 emitted during
lime manufacture will actually be reabsorbed when the
lime is consumed. As noted above, lime has many differ-
ent chemical, industrial, environmental, and construction
applications. In many processes, CO2 reacts with the lime
to create calcium carbonate (e.g., water softening). Car-
bon dioxide reabsorption rates vary, however, depending
on the application. For example, 100 percent of the lime
used to produce precipitated calcium carbonate (PCC)
reacts with CO2; whereas most of the lime used in steel-
making reacts with impurities such as silica, sulfur, and
aluminum compounds. A detailed accounting of lime use
in the United States and further research into the associ-
ated processes are required to quantify the amount of CO2
that is reabsorbed.6 As more information becomes avail-
able, this emission estimate will be adjusted accordingly.
6 Representatives of the National Lime Association estimate that CO2 reabsorption that occurs from the use of lime offsets as much as
a third of the CO2 emissions from calcination.
Industrial Processes 3-7
-------
In some cases, lime is generated from calcium car-
bonate by-products at paper mills and water treatment
plants.7 The lime generated by these processes is not in-
cluded in the USGS data for commercial lime consump-
tion. In the paper industry, mills that employ the sulfate
process (i.e., Kraft) consume lime in order to causticize a
waste sodium carbonate solution (i.e., black liquor). Most
sulfate mills recover the waste calcium carbonate after
the causticizing operation and calcine it back into lime—
thereby generating CO2—for reuse in the pulping pro-
cess. However, some of these mills capture the CO2 re-
leased in this process to be used as precipitated calcium
carbonate (PCC). Further research is necessary to deter-
mine to what extent CO2 is released to the atmosphere
through generation of lime by paper mills.
In the case of water treatment plants, lime is used
in the softening process. Some large water treatment
plants may recover their waste calcium carbonate and
calcine it into quicklime for reuse in the softening pro-
cess. Further research is necessary to determine the de-
gree to which lime recycling is practiced by water treat-
ment plants in the United States.
Limestone and Dolomite Use
Limestone (CaCO3) and dolomite
(CaCO3MgCO3)8 are basic raw materials used by a wide
variety of industries, including construction, agricul-
ture, chemical, metallurgy, glass manufacture, and envi-
ronmental pollution control. Limestone is widely dis-
tributed throughout the world in deposits of varying sizes
and degrees of purity. Large deposits of limestone occur
in nearly every state in the United States, and significant
quantities are extracted for industrial applications. For
some of these applications, limestone is sufficiently
Table 3-8: C02 Emissions from Limestone & Dolomite Use (MMTCE)
Activity
Flux Stone
Glass Making
FGD
Total
1990
0.8
0.5
1.4
1991
0.7
0.6
1.3
1992
0.6
0.1
0.5
1.2
1993
0.5
0.1
0.5
1.1
1994
0.8
0.1
0.6
1.5
1995
1.1
0.1
0.7
1.9
1996
1.2
0.2
0.7
2.0
1997
1.4
0.2
0.8
2.3
1998
1.5
0.2
0.8
2.4
Does not exceed 0.05 MMTCE
Note: Totals may not sum due to independent rounding.
Table 3-9: CO? Emissions from Limestone & Dolomite Use (Gg)
Activity
Flux Stone
Limestone
Dolomite
Glass Making
Limestone
Dolomite
FGD
Tola!
NA (Not Available)
Note: Totals may not sum
1990
3,002
2,550
451
189
189
NA
1,922
5,113
1991
2,699
2,294
406
170
170
NA
2,027
4,896
1992
2,314
1,957
357
218
218
NA
1,971
4,502
1993
1,903
1,597
306
274
274
NA
1,880
4,058
1994
2,950
2,108
842
356
356
NA
2,235
5,541
1995
3,903
2,523
1,380
526
421
105
2,558
6,987
1996
4,249
3,330
919
555
445
110
2,695
7,499
1997
5,042
3,970
1,072
593
475
118
2,902
8,537
1998
5,327
4,194
1,132
626
502
124
2,902
8,854
due to independent rounding.
7 Some carbide producers may also regenerate lime from their calcium hydroxide by-products, which does not result in emissions of
CO,. In making calcium carbide, quicklime is mixed with coke and heated in electric furnaces. The regeneration of lime in this process
is done using a waste calcium hydroxide (hydrated lime) [CaC2 + 2H2O -> C2H2 + Ca(OH)J, not calcium carbonate [CaCO3]. Thus, the
calcium hydroxide is heated in the kiln to simply expel the water [Ca(OH)2 + heat -s- CaO + H2O] and no CO2 is released to the
atmosphere.
8 Limestone and dolomite are collectively referred to as limestone by the industry, and intermediate varieties are seldom distinguished.
3-8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
heated during the process to generate CO2 as a by-prod-
uct. Examples of such applications include limestone
used as a flux or purifier in metallurgical furnaces, as a
sorbent in flue gas desulfurization (FGD) systems for
utility and industrial plants, or as a raw material in glass
manufacturing.
In 1998, approximately 17,268 thousand metric
tons of limestone and 2,597 thousand metric' tons of do-
lomite were used for these applications. Overall, both
limestone and dolomite usage resulted in aggregate CO2
emissions of 2.4 MMTCE (8,854 Gg) (see Table 3-8 and
Table 3-9).
Emissions in 1998 increased 4 percent from the
previous year. Although they decreased slightly in 1991,
1992, and 1993, CO2 emissions from this source have
since increased 73 percent from the 1990 baseline. In
the future, increases in demand for crashed stone are
anticipated. Demand for crashed stone from the trans-
portation sector continues to drive growth in limestone
and dolomite use. The Transportation Equity Act for the
21st Century, which commits over $200 billion dollars
to highway work through 2003, promises to maintain
the upward trend in consumption.
Methodology
Carbon dioxide emissions were calculated by mul-
tiplying the amount of limestone consumed by an aver-
age carbon content for limestone, approximately 12.0
percent for limestone and 13.2 percent for dolomite
(based on stoichiometry). Assuming that aU of the car-
bon was released into the atmosphere, the appropriate
emission factor was multiplied by the annual level of
consumption for flux stone, glass manufacturing, and
FGD systems to determine emissions.
Data Sources
Consumption data for 1990 through 1998 of lime-
stone and dolomite used as flux stone and in glass manu-
facturing (see Table 3-10) were obtained from the USGS
(1991, 1993, 1996, 1997, 1998, 1999). Consumption
data for limestone used in FGD were taken from unpub-
lished survey data in the Energy Information
Administration's Form EIA-767, "Steam Electric Plant
Operation and Design Report," (EIA 1997, 1998). For
1990 and 1994, the USGS did not provide a breakdown
of limestone and dolomite production by end-use and
for 1998 the end-use breakdowns had not yet been final-
ized at the time of publication. Consumption figures for
these years were estimated by assuming that limestone
and dolomite accounted for the same percentage of total
crashed stone consumption for a given year as the aver-
age of the percentages for the years before and after (ex-
ception: 1990 and 1998 consumption were estimated
using the percentages for only 1991 and 1997, respec-
tively). Furthermore, starting in 1996, USGS discontin-
ued reporting glass manufacture separately. From 1996
onward, limestone used in glass manufacture is estimated
based on its percent of total crashed stone for 1995.
Table 3-10: Limestone & Dolomite Consumption (Thousand Metric Tons).
Activity
1990
1991
1992
1993
1994
1995
1996
1997
Flux Stone
Limestone
Dolomite
Glass Making
Limestone
Dolomite
FGD
NA
(N<
3t Available)
5,797
932
430
NA
4,369
5,213
838
386
NA
4,606
4,447
738
495
NA
4,479
3,631
632
622
NA
4,274
4,792
1,739
809
NA
5,080
5,734
2,852
958
216
5,815
7,569
1,899
1,011
228
6,125
9,024
2,215
1,079
243
6,595
1998
9,533
2,340
1,140
257
6,595
Industrial Processes 3-9
-------
It should be noted that there is a large quantity of
crushed stone reported to the USGS under the category
"unspecified uses". A portion of this consumption is
believed to be limestone or dolomite used as flux stone
and for glass manufacture. The quantity listed for "un-
specified uses" was, therefore, allocated to each reported
end-use according to each end-uses fraction of total con-
sumption in that year.9
Uncertainty
Uncertainties in this estimate are due in part, to
variations in the chemical composition of limestone. In
addition to calcite, limestone may contain smaller
amounts of magnesia, silica, and sulfur. The exact speci-
fications for limestone or dolomite used as flux stone
vary with the pyrometallurgical process, the kind of ore
processed, and the final use of the slag. Similarly, the
quality of the limestone used for glass manufacturing
will depend on the type of glass being manufactured.
Uncertainties also exist in the activity data. Much of the
limestone consumed in the United States is reported as
"other unspecified uses;" therefore, it is difficult to ac-
curately allocate this unspecified quantity to the correct
end-uses. Furthermore, some of the limestone reported
as "limestone" is believed to actually be dolomite, which
has a higher carbon content than limestone.
Soda Ash
Manufacture and Consumption
Soda ash (sodium carbonate, Na2CO3) is a white
crystalline solid that is readily soluble in water and
strongly alkaline. Commercial soda ash is used as a raw
material in a variety of industrial processes and in many
familiar consumer products such as glass, soap and de-
tergents, paper, textiles, and food. It is used primarily as
an alkali, either in glass manufacturing or simply as a
material that reacts with and neutralizes acids or acidic
Table 3-11: C02 Emissions
from Soda Ash Manufacture and Consumption
Year
r :,.' ;•:,: -1990
1991
, 1992
1993
, - 1994
. ...' .,:,. :• ,1995 ;" .; ..""...' .;.'
1996
1997
1998
MMTCE
1.1
1.1
1.1 "
1.1
1.1
• 1.2
1.2
1.2
1.2
Table 3-12: C02 Emissions from
Soda Ash Manufacture and Consumption (Gg)
r Year
fc 1990
r 1991
L 1992
f 1993
T 1994
r : 1995
is • 1996
L 1997
^ : 1998
> Note: Totals
Si"
Manufacture
Ii435
1,429
1,451 "
1,412
1,422
1,607
1,587
1,666
J ' 1,607"""
may not sum due to
Consumption
2,709
2,605
2,639
2,635
2,590
2,702
2,685
2,768
2,718
Total
4,144
4,035
4,091
4,048
4,012
4,309
4,273
4,434
4,325
independent rounding.
substances. Internationally, two types of soda ash are
produced—natural and synthetic. The United States pro-
duces only natural soda ash and is the largest soda ash-
producing country in the world. Trona is the principal
ore from which natural soda ash is made.
Only two states produce natural soda ash: Wyo-
ming and California. Of these two states, only Wyoming
has net emissions of CO2. This difference is a result of
the production processes employed in each state.10 Dur-
ing the production process used in Wyoming, natural
sources of sodium carbonate are heated and transformed
into a crude soda ash that requires further refining. Car-
bon dioxide (CO2) is generated as a by-product of this
* This approach was recommended by USGS.
» In California soda ash is manufactured using sodium carbonate-bearing brines instead of trona ore. To extract the sodium carbonate
the complexTr neTare first treated with CO2 in carbonation towers to convert the sodium carbonate into sodium bicarbonate which
hen rcc pitates from the brine solution. The precipitated sodium bicarbonate is then calcined back into sodium carbonate. Although
CO" rgenerated as . by-product, the CO2 is recovered and recycled for use in the carbonation stage and is never actually released.
3-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
reaction, and is eventually emitted into the atmosphere.
In addition, CO2 may also be released when soda ash is
consumed.
In 1998, CO2 emissions from the manufacture of
soda ash from trona were approximately 0.4 MMTCE
(1,600 Gg). Soda ash consumption in the United States
also generated 0.7 MMTCE (2,700 Gg) of CO2 in 1998.
Total emissions from this source in 1998 were then 1.2
MMTCE (4,325 Gg) (see Table 3-11 and Table 3-12).
Emissions have fluctuated since 1990. These fluctua-
tions were strongly related to the behavior of the export
market and the U.S. economy. Emissions in 1998 de-
creased by 2 percent from the previous year, but have
increased 4 percent since 1990.
The United States has the world's largest deposits
of trona and represents about one-third of total world
soda ash output. The distribution of soda ash by end-use
in 1998 was glass making, 49 percent; chemical produc-
tion, 27 percent; soap and detergent manufacturing, 11
percent; distributors, 5 percent; flue gas desulfurization,
3 percent; pulp and paper production, 2 percent; and
water treatment and miscellaneous combined for the re-
maining 3 percent (USGS 1999).
Soda ash production and consumption decreased
by 3.5 and 1.8 percent from 1997 values, respectively.
Exports are a driving force behind U.S. soda ash produc-
tion and the Asian economic crisis beginning in late
1997 has been cited as a major cause for the drop in
world soda ash demand. Moderate growth (between 1.5
and 2 percent) is expected for 1999 as the Asian economy
recovers and as demand in South America continues to
grow (USGS 1999).
Construction is currently underway on a major soda
ash plant that will use a new feedstock—nahcolite, a
natural sodium bicarbonate found in deposits in
Colorado's Piceance Creek Basin. By 2001, the plant is
expected to be mining more than 1.4 million tons of
nahcolite per year and converting it into 1 million tons
of soda ash (C&EN, 1999). Part of this process involves
the stripping of CO2. At this point, it is unknown whether
any CO2 will be released to the atmosphere or captured
and used for conversion back to sodium bicarbonate.
Methodology
During the production process, trona ore is calcined
in a rotary kiln and chemically transformed into a crude
soda ash that requires further processing. Carbon dioxide
and water are generated as by-products of the calcination
process. Carbon dioxide emissions from the calcination
of trona can be estimated based on the following chemi-
cal reaction:
2(Na3H(C03)2 x2H20) -^ 3Na2CO3 + 5H2O + CO2
[trona] [soda ash]
Based on this formula, approximately 10.27 metric
tons of trona are required to generate one metric ton of
CO2. Thus, the 16.5 million metric tons of trona mined in
1998 for soda ash production (USGS 1999) resulted in
CO2 emissions of approximately 0.4 MMTCE (1,600 Gg).
Once manufactured, most soda ash is consumed in
glass and chemical production, with minor amounts in
soap and detergents, pulp and paper, flue gas desulfur-
ization and water treatment. As soda ash is consumed for
these purposes, additional CO2 is usually emitted. In
these applications, it is assumed that one mole of carbon
is released for every mole of soda ash used. Thus, ap-
proximately 0.113 metric tons of carbon (or 0.415 met-
ric tons of CO2) are released for every metric ton of soda
ash consumed.
Data Sources
The activity data for trona production and soda
ash consumption (see Table 3-13) were taken from USGS
Table 3-13: Soda Ash Manufacture
and Consumption (Thousand Metric Tons)
it? •:
fe^r
££:...:
|t: • - • -
m«— -
)5B--;--- •'
m- •- - •:-
r-
ft;" ~
l^-Soda
& '
Year
-1990
1991
1992
:•; .'1.993:.
.1994
1995
1996
1997
-1998
ash manufactured
Manufacture*
14,734
14,674
14,900
14,500
....14,600
16,500
16,300
17,100
16,500
from trona ore only.
Consumption
6,527 ":
6,278
6,360
6,350
6,240
6,510
6,470 :
6,670
6,550 •
tf
Industrial Processes 3-11
-------
(1993, 1994,1995,1998, and 1999). Soda ash manufac-
ture and consumption data were collected by the USGS
from voluntary surveys of the U.S. soda ash industry. All
six of the soda ash manufacturing operations in the United
States completed surveys to provide data to the USGS.
Uncertainty
Emissions from soda ash manufacture are consid-
ered to be relatively certain. Both the emissions factor
and activity data are reliable. However, emissions from
soda ash consumption are dependent upon the type of
processing employed by each end-use. Specific informa-
tion characterizing the emissions from each end-use is
limited. Therefore, uncertainty exists as to the accuracy
of the emission factors.
Carbon Dioxide Consumption
Carbon dioxide (CO2) is used for a variety of ap-
plications, including food processing, chemical produc-
tion, carbonated beverages, and enhanced oil recovery
(EOR). Carbon dioxide used for EOR is injected into the
ground to increase reservoir pressure, and is therefore
considered sequestered.11 For the most part, however, CO2
used in non-EOR applications will eventually enter the
atmosphere.
Carbon dioxide is produced from a small number
of natural wells, as a by-product from the production of
Table 3-14: C02 Emissions
from Carbon Dioxide Consumption
Year
1990
1991
1992
1993
1994
1995
1996
1997
1998
MMTCE
0.2
0.2
0.2
0.2
0.2
0.3
0.3
0.4
0.4
Gg
800
840
882
912
898
968
1,140
1,294
1,413
chemicals (e.g., ammonia), or separated from crude oil
and natural gas. Depending on the raw materials that are
used, the by-product CO2 generated during these pro-
duction processes may already be accounted for in the
CO2 emission estimates from fossil fuel consumption
(either during combustion or from non-fuel uses). For
example, ammonia is primarily manufactured using natu-
ral gas as a feedstock. Carbon dioxide emissions from
this process are accounted for in the Energy chapter un-
der Fossil Fuel Combustion and, therefore, are not in-
cluded here.
In 1998, CO2 emissions from this source not ac-
counted for elsewhere were 0.4 MMTCE (1,413 Gg) (see
Table 3-14). This amount represents an increase of 9 per-
cent from the previous year and is 77 percent higher
than emissions in 1990.
Methodology
Carbon dioxide emission estimates were based on
CO2 consumption with the assumption that the end-use
applications, except enhanced oil recovery, eventually
release 100 percent of the CO2 into the atmosphere. Car-
bon dioxide consumption for uses other than enhanced
oil recovery was about 7,067 thousand metric tons in
1998. The Freedonia Group estimates that, in the United
States, there is an 80 to 20 percent split between CO2
produced as a by-product and CO2 produced from natu-
ral wells. Thus, emissions are equal to 20 percent of CO2
consumption. The remaining 80 percent was assumed to
Table 3-15: Carbon Dioxide Consumption
Year
Thousand Wletric Tons
1990
1991
1992
1993
1994
1995
1996
1997
1998
4,000
4,200
4,410
4,559
4,488
4,842
5,702
6,468
7,067
"It is unclear to what extent the CO2 used for EOR will be re-released. For example, the CO2 used for EOR may show up at the wellhead
±^r^^^^
assumed that all of the CO2 remains sequestered.
3-12 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
already be accounted for in the CO2 emission estimates
from other categories (the most important being Fossil
Fuel Combustion).
Data Sources
Carbon dioxide consumption data (see Table 3-15)
were obtained from Industrial Gases to 2003, published
by the Freedonia Group Inc. (1994, 1996, 1999). The
1999 report contains actual data for 1998 only. Data for
1996 were obtained by personal communication with
Paul Ita of the Freedonia Group Inc. (1997). Data for
1997 production was calculated from annualized growth
rates for 1994 through 1996 while the 1997 value for
enhanced oil recovery was set equal to the 1998 value.
The percent of carbon dioxide produced from natural
wells was obtained from Freedonia Group Inc. (1991).
Uncertainty
Uncertainty exists in the assumed allocation of
carbon dioxide produced from fossil fuel by-products
(80 percent) and carbon dioxide produced from wells
(20 percent). In addition, it is possible that CO2 recovery
exists in particular end-use sectors. Contact with several
organizations did not provide any information regard-
ing recovery. More research is required to determine the
quantity, if any, that may be recovered.
Iron and Steel Production
In addition to being an energy intensive process,
the production of kon and steel also generates process-
related emissions of CO2. Iron is produced by first reduc-
ing iron oxide (ore) with metallurgical coke in a blast
furnace to produce pig iron (impure iron of about 4 to
4.5 percent carbon by weight). Carbon dioxide is pro-
duced as the coke used in this process is oxidized. Steel
(less than 2 percent carbon by weight) is produced from
pig iron in a variety of specialized steel furnaces. The
majority of CO2 emissions come from the production of
iron, with smaller amounts evolving from the removal of
carbon from pig iron to produce steel.
Emissions of CO2 from iron and steel production
in 1998 were 21.9 MMTCE (80,200 Gg). Emissions
fluctuated significantly from 1990 to 1998 due to
changes in domestic economic conditions and changes
in imports and exports. Forecasts for iron and steel pro-
duction remain mixed. Despite a 5 percent increase in
capital expenditures during 1998, plant capacity utili-
zation sank below 80 percent and steel imports contin-
ued to climb.
CO2 emissions from iron and steel production are
not included in totals for the Industrial Processes chap-
ter because they are accounted for with Fossil Fuel Com-
bustion emissions from industrial coking coal in the
Energy chapter.12 Emissions estimates are presented here
for informational purposes only (see Table 3-16). Addi-
tional CO2 emissions also occur from the use of lime-
stone or dolomite flux during production; however,
these emissions are accounted for under Limestone and
Dolomite Use.
Methodology
Carbon dioxide emissions were calculated by mul-
tiplying annual estimates of pig kon production by the
ratio of CO2 emitted per unit of iron produced (1.6 met-
ric ton CO2/metric ton iron). The emission factor em-
ployed was applied to both pig iron production and in-
tegrated pig iron plus steel production; therefore, emis-
sions were estimated using total U.S. pig kon produc-
tion for all uses including making steel.
Table 3-16: C02 Emissions from
Iron and Steel Production
~ Year
.--. ,1990
"'--•- 1991
1992
1993
-; 1994
? 1995
---. 1996
-'V 1997
r': 1998
•,-- — i -
MMTCE
23.9
19.2
20.7
21.0
21.6
22.2
21.6
21,6
21.9
Gg
87,600
70,560
75,840
77,120
79,040
81,440
79,040
79,360
80,160
12 Although the C02 emissions from the use of industrial coking coal as a reducing agent should be included in the Industrial Processes
chapter, information to distinguish individual non-energy uses of fossil fuels is unfortunately not available in DOE/EIA fuel statistics.
Industrial Processes 3-13
-------
Table 3-17: Pig Iron Production
Year
1990
1991
1992
1993
1994
1995
1996
1997
1998
Thousand Metric Tons i
54,750 ;
44,100 :
47,400
48,200 :
49,400 ;
50,900
49,400
49,600 :
50,100 I
'<
Data Sources
The emission factor was taken from the Revised
1996IPCC Guidelines (IPCC/UNEP/OECD/ffiA 1997).
Production data for 1990 through 1997 (see Table 3-17)
were obtained from the U.S. Geological Survey's (USGS)
Minerals Yearbook: Volume I-Metals and Minerals
(USGS 1996, 1997, 1998); data for 1998 were obtained
from USGS's Mineral Commodity Summaries (1999).
Uncertainty
The emission factor employed was assumed to be
applicable to both pig iron production and integrated
pig iron plus steel production. This assumption was made
because the uncertainty in the factor is greater than the
additional emissions generated when steel is produced
from pig iron. Using plant-specific emission factors could
yield a more accurate estimate, but these factors were
not available. The most accurate alternative would be to
calculate emissions based on the amount of reducing
agent used, rather than on the amount of iron or steel
produced; however, these data were also not available.
Ammonia Manufacture
Emissions of CO2 occur during the production of
ammonia. In the United States, roughly 98 percent of
synthetic ammonia is produced by catalytic steam re-
forming of natural gas, and the remainder is produced
using naphtha (a petroleum fraction) or the electrolysis
of brine at chlorine plants (EPA 1997). The former two
fossil fuel-based reactions produce carbon monoxide and
hydrogen gas; however, the latter reaction does not lead
to CO2 emissions. Carbon monoxide (CO) in the first
two processes is transformed into CO2 in the presence of
a catalyst (usually a metallic oxide). The hydrogen gas
is diverted and combined with nitrogen gas to produce
ammonia. The CO2, included in a gas stream with other
process impurities, is absorbed by a scrubber solution.
In regenerating the scrubber solution, CO2 is released.
(catalyst)
CH4+H20->4H2+C02
3H2+N2 ->2NH3
Emissions of CO2 from ammonia production in
1998 were 7.3 MMTCE (26,900 Gg). Carbon dioxide
emissions from this source are not included in totals for
the Industrial Processes chapter because these emissions
are accounted for with non-energy use of natural gas
under Fossil Fuel Combustion in the Energy chapter.13
Emissions estimates are presented here for informational
purposes only (see Table 3-18).
Table 3-18: C02 Emissions
from Ammonia Manufacture
£.:".
I"
F-"
|i" ...:„
£
if '
i
1s
S"
§: •
t
Year
1991
,-J9§2
1993
1994
1995
1996
1997
1998
----- •- •- •- • •••-
MMTCE
6.4
6.7
6.4
6.6
6.5
6.7
6.6
7.3
Gg
23',364
24,391
23,399
24,316
23,682
24,390
24,346
26,880
13 Although the CO, emissions from the use of natural gas as a feedstock should be included in the Industrial Processes chapter,
information to distinguish individual non-energy uses of fossil fuels is unfortunately not available in DOE/EIA fuel statistics.
3-14 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Methodology
Emissions of CO2 were calculated by multiply-
, ing annual estimates of ammonia production by an
emission factor (1.5 ton CO2/ton ammonia). It was as-
sumed that all ammonia was produced using catalytic
steam reformation, although small amounts may have
been produced using chlorine brines. The actual amount
produced using this latter method is not known, but
assumed to be small.
Data Sources
The emission factor was taken from the Revised
1996IPCC Guidelines (IPCC/UNEP/OECD/ffiA 1997).
Ammonia production data (see Table 3-19) were ob-
tained from the Census Bureau of the U.S. Department
of Commerce (Census Bureau 1998, 1999) as reported
in Chemical and Engineering News, "Facts & Figures
for the Chemical Industry."
Uncertainty
It is uncertain how accurately the emission factor
used represents an average across all ammonia plants.
By using natural gas consumption data for each ammo-
nia plant, more accurate estimates could be calculated.
However, these consumption data are often considered
confidential and are difficult to acquire. All ammonia
Table 3-19: Ammonia Manufacture
isF
l~
F
F.
f-
E:" -
E::
j~_
|i-
r
L.
Year
1990
1991
1992
1993
1994
1995
1996
1997
1998
Thousand Metric Tons
- 15,425
15,576
16,261
15,599
16,211
15,788
16,260
16,231
17,920
1
production in this analysis was assumed to be from the
same process; however, actual emissions could differ
because processes other than catalytic steam reforma-
tion may have been used.
Ferroalloy Production
Carbon dioxide is emitted from the production of
several ferroalloys. Ferroalloys are composites of iron
and other elements often including silicon, manganese,
and chromium. When incorporated in alloy steels,
ferroalloys are used to alter the material properties of the
steel. Estimates from two types of ferrosilicon (50 and
75 percent silicon) and silicon metal (about 98 percent
silicon) have been calculated. Emissions from the pro-
duction of ferrochromium and ferromanganese are not
included here because of the small number of manufac-
turers of these materials. As a result, government infor-
mation disclosure rules prevent the publication of pro-
duction data for them. Similar to emissions from the pro-
duction of iron and steel, CO2is emitted when metallur-
gical coke is oxidized during a high-temperature reac-
tion with iron and the selected alloying element. Due to
the strong reducing environment, CO is initially pro-
duced. The CO is eventually oxidized, becoming CO2. A
representative reaction equation for the production of
50 percent ferrosilicon is given below:
Fe2O3 +2SiO2 +7C -> 2FeSi + 7CO
Emissions of CO2 from ferroalloy production in
1998 were 0.5 MMTCE (1,800 Gg). Carbon dioxide
emissions from this source are not included in the totals
for the Industrial Processes chapter because these emis-
sions are accounted for in the calculations for industrial
coking coal under Fossil Fuel Combustion in the En-
ergy chapter.14 Emission estimates are presented here for
informational purposes only (see Table 3-20).
14 Although the CO2 emissions from the use of industrial coking coal as a reducing agent should be included in the Industrial Processes
chapter, information to distinguish individual non-energy uses of fossil fuels is unfortunately not available in DOE/EIA fuel statistics.
Industrial Processes 3-15
-------
Table 3-20: C02 Emissions
from Ferroalloy Production
Year
1990
1991
1992
1993
1994
1995
1996
1997
1998
MMTCE
0.5
0.4
0.4
0.4
0.4
0.4
0.5
0.5
0.5
Gg
1,809
1,580 ;
1,579
1,516
1,607
1,625 ;
1,695
1,789
1,790
Methodology
Emissions of CO2 were calculated by multiplying
annual estimates of ferroalloy production by material-
specific emission factors. Emission factors were applied
to production data for ferrosilicon 50 and 75 percent
(2.35 and 3.9 metric ton CO2/metric ton, respectively)
and silicon metal (4.3 metric tqn CO2/metric ton). It was
assumed that all ferroalloy production was produced us-
ing coking coal, although some ferroalloys may have
been produced with wood, other biomass, or graphite
carbon inputs.
Data Sources
Emission factors were taken from the Revised 1996
1PCC Guidelines (IPCC/UNEP/OECD/IEA 1997).
Ferroalloy production data for 1990 through 1997 (see
Table 3-21) were obtained from the U.S. Geological
Survey's (USGS) Minerals Yearbook: Volume I—Metals
and Minerals (USGS, 1991, 1992, 1993, 1994, 1995,
1996, 1997, 1998); data for 1998 were obtained from
USGS (1999) Mineral Industry Surveys: Silicon in De-
cember 1998.
Uncertainty
Although some ferroalloys may be produced us-
ing wood or other biomass as a carbon source, informa-
tion and data regarding these practices were not avail-
able. Emissions from ferroalloys produced with wood
would not be counted under this source because wood-
based carbon is of biogenic origin.15 Emissions from
ferroalloys produced with graphite inputs would be
counted in national totals, but may generate differing
amounts of CO2 per unit of ferroalloy produced com-
pared to the use of coking coal. As with emissions from
iron and steel production, the most accurate method for
these estimates would be basing calculations on the
amount of reducing agent used in the process, rather
than on the amount of ferroalloys produced. These data
were not available, however.
Petrochemical Production
Small amounts of methane (CH4) are released dur-
ing the production of some petrochemicals. Petrochemi-
cals are chemicals isolated or derived from petroleum or
natural gas. Emissions are presented here from the pro-
duction of five chemicals: carbon black, ethylene, eth-
ylene dichloride, styrene, and methanol.
Carbon black is an intensely black powder made
by the incomplete combustion of an aromatic petroleum
feedstock. Almost all output is added to rubber to impart
strength and abrasion resistance, and the tire industry is
by far the largest consumer. Ethylene is consumed in the
production processes of the plastics industry including
polymers such as high, low, and linear low density poly-
ethylene (HOPE, LDPE, LLDPE), polyvinyl chloride
(PVC), ethylene dichloride, ethylene oxide, and
ethylbenzene. Ethylene dichloride is one of the first
Table 3-21: Production of Ferroalloys (Metric Tons)
fe.-,
PYear
1 1990
M991
! 1992
1993
£1994
E1995
£1996
11997
r1998
-
Ferrosilicon
50%
321,385
230,019
238,562
199,275
198,000
181,000
182,000
"" 175,000
166,000
Ferrosilicon
75%
109,566
101,549
79,976
94,437
112,000
128,000
132,000
147,000
144,000
Silicon
Metal
145,744
149,570
164,326
158,000
164,000
163,000
175,000
187,000
195,000
_
15 Emissions and sinks of biogenic carbon are accounted for in the Land-Use Change and Forestry chapter.
3-16 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
manufactured chlorinated hydrocarbons with reported
production as early as 1795. In addition to being an
important intermediate in the synthesis of chlorinated
hydrocarbons, ethylene dichloride is used as an indus-
trial solvent and as a fuel additive. Styrene is a common
precursor for many plastics, rubber, and resins. It can be
found in many construction products, such as foam in-
sulation, vinyl flooring, and epoxy adhesives. Metha-
nol is an alternative transportation fuel as well as a prin-
ciple ingredient in windshield wiper fluid, paints, sol-
vents, refrigerants, and disinfectants. In addition, metha-
nol-based acetic acid is used in making PET plastics and
polyester fibers. The United States produces close to one
quarter of the world's supply of methanol.
Aggregate emissions of CH4 from petrochemical
production in 1998 were 0.4 MMTCE (77 Gg) (see Table
3-22). Production levels of all five chemicals increased
from 1990 to 1998. Petrochemicals are currently in over-
supply and production for 1999 and 2000 is expected to
decrease.
Table 3-22: CH4 Emissions
from Petrochemical Production
Year
1990
1991
1992
1993
1994
1995
1996
1997
1998
MMTCE
0.3
0.3
0.3
0.4
0.4
0.4
0.4
0.4
0.4
Gg
56
57
60
66
70
72
75
77
77
Methodology
Emissions of CH4 were calculated by multiplying
annual estimates of chemical production by an emission
factor. The following factors were used: 11 kg CH4/met-
ric ton carbon black, 1 kg CH4/metric ton ethylene, 0.4
kg CH4/metric ton ethylene dichloride,16 4 kg CH4/met-
ric ton styrene, and 2 kg CH4/metric ton methanol. These
emission factors were based upon measured material bal-
ances. Although the production of other chemicals may
also result in methane emissions, there were not suffi-
cient data to estimate their emissions.
Data Sources
Emission factors were taken from the Revised 1996
IPCC Guidelines (IPCC/UNEP/OECD/ffiA 1997). An-
nual production data (see Table 3-23) were obtained from
the Chemical Manufacturers Association Statistical
Handbook (CMA 1999).
Uncertainty
The emission factors used here were based on a
limited number of studies. Using plant-specific factors
instead of average factors could increase the accuracy of
the emissions estimates, however, such data were not
available. There may also be other significant sources of
methane arising from petrochemical production activi-
ties that have not been included in these estimates.
Silicon Carbide Production
Methane is emitted from the production of silicon
carbide, a material used as an industrial abrasive. To make
silicon carbide (SiC), quartz (SiO2) is reacted with car-
Table 3-23: Production of Selected Petrochemicals (Thousand Metric Tons)
Chemical
i Carbon Black
1 Ethylene
jTEthylene Dichloride
IrStyrene
pflethanol
, _ — ,_^_i_»^,U4ii;t
1990
1,306
16,542
6,282
3,637
3,785
-i^jui-AU^-i-y -
1991
1,225
18,124
6,221
3,681
3,948
. - : .- :,,.-. ..
1992
1,365
18,563
6,872
4,082
3,666
fa~.- = - ----- -. T" --
1993
1,452
18,709
8,141
4,565
4,782
1994
1,492
20,201
8,482
.5,112
4,904
' .,- --- ----- —
1995
1,524
21,199
7,829
5,167
4,888
1996
1,560
22,197
8,596
5,387
5,330
.- ^--.-'n- --- ';•-£,•-
1997
1,588
23,088
9,152
5,171
5,806
j". •:.•-.- --- :.
1998
1,610
23,474
8,868
5,183
5,693
- . •.1-,.,' ,. -=._.
Industrial Processes 3-17
-------
bon in the form of petroleum coke. Methane is produced
during this reaction from volatile compounds in the pe-
troleum coke. Although CO2 is also emitted from this
production process, the requisite data were unavailable
for these calculations. Regardless, they are already ac-
counted for under CO2 from Fossil Fuel Combustion in
the Energy chapter. Emissions of CH4 from silicon car-
bide production in 1998 (see Table 3-24) were 1 Gg (less
than 0.05 MMTCE).
Methodology
Emissions of CH4 were calculated by multiplying
annual estimates of silicon carbide production by an
emission factor (11.6 kg CH4/metric ton silicon carbide).
This emission factor was derived empirically from mea-
surements taken at Norwegian silicon carbide plants
(IPCC/UNEP/OECD/ffiA 1997).
Data Sources
The emission factor was taken from the Revised
1996IPCC Guidelines (IPCC/UNEP/OECD/ffiA 1997).
Production data for 1990 through 1998 (see Table 3-25)
were obtained from the Minerals Yearbook: Volume I-
Metals and Minerals, Manufactured Abrasives (USGS,
1991, 1992,1993,1994,1995,1996,1997,1998,1999).
Uncertainty
The emission factor used here was based on one
study of Norwegian plants. The applicability of this fac-
tor to average U.S. practices at silicon carbide plants is
uncertain. A better alternative would be to calculate
Table 3-25: Production of Silicon Carbide
Table 3-24: CH4 Emissions
from Silicon Carbide Production
Year
1990
1991
1992
1993
1994
1995
1996
1997
1998
Metric Tons
105,000
78,900
84,300
74,900
84,700
75,400
73,600
68,200
69,800
Year MMTCE
;-•-• 1990 + ' '•'•"•"
.,-.- •., 1991 +
::'•' • 1992" ~ +
.7,7, = 1993 +
'''"' 1996 -•"-••"'^••-"V
:: . : """1997 "" +
1998 +
+ Does not exceed 0.05 MMTCE
eg
..... .j
1
- 1
1 ' '
1
"' ^'-"1 "•
1 •
1
3
, -s
emissions based on the quantity of petroleum coke used
during the production process rather than on the amount
of silicon carbide produced. These data were not avail-
able, however.
Adipic Add Production
Adipic acid production has been identified as an
anthropogenic source of nitrous oxide (N2O) emissions.
Worldwide, there are few adipic acid plants. The United
States is the major producer with three companies in
four locations accounting for approximately one-half of
world production. Adipic acid is a white crystalline solid
used in the manufacture of synthetic fibers, coatings,
plastics, urethane foams, elastomers, and synthetic lu-
bricants. Commercially, it is the most important of the
aliphatic dicarboxylic acids, which are used to manu-
facture polyesters. Ninety percent of all adipic acid pro-
duced in the United States is used in the production of
nylon 6,6. It is also used to provide some foods with a
"tangy" flavor.
Adipic acid is produced through a two-stage pro-
cess during which N2O is generated in the second stage.
The first stage of manufacturing usually involves the
oxidation of cyclohexane to form a cyclohexanone /
cyclohexanol mixture. The second stage involves oxi-
dizing this mixture with nitric acid to produce adipic
acid. Nitrous oxide is generated as a by-product of the
nitric acid oxidation stage and is emitted, in the waste
gas stream. Process emissions from the production of
adipic acid will vary with the types of technologies and
3-18 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
level of emissions controls employed by a facility. In
1990, two of the three major adipic acid producing plants
implemented N2O abatement technologies and as of
1998, all of the major adipic acid production facilities
had control systems in place.17 Only one small plant
does not control for N2O, representing approximately 3
percent of production.
Adipic acid production for 1998 was 866 thou-
sand metric tons. Nitrous oxide emissions from this source
were estimated to be 2.0 MMTCE (23 Gg) in 1998 (see
Table 3-26).
In 1998, adipic acid production reached its high-
est level in fourteen years. This increase is chiefly due to
rising demand for engineering plastics. Though produc-
tion continues to increase, emissions have been signifi-
cantly reduced due to the widespread installation of
pollution control measures. By 1998, all of the three
major producing plants had voluntarily implemented
N2O abatement technology, which resulted in an overall
reduction of emissions by approximately 60 percent.
Methodology
Nitrous oxide emissions were calculated by multi-
plying adipic acid production by the ratio of N2O emit-
ted per unit of adipic acid produced and adjusting for
the actual percentage of N2O released as a result of plant-
specific emission controls. Because emissions of N2O in
the United States are not regulated, emissions have not
been well characterized. However, on the basis of ex-
periments (Thiemens and Trogler 1991), the overall re-
Table 3-26: N20 Emissions
from Adipic Acid Production
1 Year
t 1990
|;_ 1991
|- 1992
fe 1993
1 1994
Y 1995
1996
r 1997
!: 1998
L_ _____
MMTCE
5.0.
5.2
4.8
5.2
5.5
5.5
5.7
4.7
2.0
.. J,_^_^^_^.1,:J.__._.
Gg
59
62
57
61
65
66
67
55
23
action stoichiometry for N2O production in the prepara-
tion of adipic acid was estimated at approximately 0.3
kg of N2O per kilogram of product. Emissions are deter-
mined using the following equation:
N2O emissions = [production of adipic acid]
x [0.3 kg N2O / kg adipic acid]
x [1 - (N2O destruction factor
x abatement system utility factor) ]
The "N2O destruction factor" represents the
amount of N2O expressed as a percentage of N2O emis-
sions that are destroyed by the currently installed abate-
ment technology. The "abatement system utility factor"
represents the percent of time that the abatement equip-
ment operates. Overall in the U.S., 63 percent of produc-
tion employs catalytic destruction, 34 percent uses ther-
mal destruction, and 3 percent of production has no N2O
abatement measures. The N2O abatement system destruc-
tion factor is assumed to be 95 percent for catalytic abate-
ment and 98 percent for thermal abatement (Reimer
1999a, 1999b). The abatement system utility factor is
assumed to be 95 percent for catalytic abatement and 98
percent for thermal abatement (Reimer 1999a, 1999b).
Data Sources
Adipic acid production data for 1990 through 1995
(see Table 3-27) were obtained from Chemical and Engi-
neering News, "Facts and Figures" and "Production of
Top 50 Chemicals" (C&EN 1992, 1993, 1994, 1995,
1996). For 1996 and 1997 data were projected from the
Table 3-27: Adipic Acid Production
r
CZ/
f."
g_^r
t
!'•: '
tT"
E—
*»—
tfc- V,
Year
1990
: 1991
1992
1993
1994
'. 1995
1996
1997
;, 1998
Thousand
Metric Tons
735
771
708
765
815
816
. .. .. 835
860
.,., „ ,. ,,866
C--. '--'- '.-• /• --•.--•.. - - • -; •' .' ,
"During 1997, the N2O emission controls installed by the third plant operated for approximately a quarter of the year.
Industrial Processes 3-19
-------
J995 manufactured total based upon suggestions from
industry contacts. For 1998, production data were ob-
tained from Chemical Week, Product focus: adipic acid/
adiponitrile (CW 1999). The emission factor was taken
from Thiemens, M.H. and W.C. Trogler (1991). Adipic
acid plant capacities for 1998 were updated using Chemi-
cal Week, Product focus: adipic acid/adiponitrile (CW
1999). Plant capacities for previous years were obtained
from Chemical Market Reporter (1998).
Uncertainty
Because N2O emissions are controlled in some adi-
pic acid production facilities, the amount of N2O that is
actually released will depend on the level of controls in
place at a specific production plant. Thus, in order to
calculate accurate emission estimates, it is necessary to
have production data on a plant-specific basis. In most
cases, however, these data are confidential. As a result,
plant-specific production figures were estimated by al-
locating total adipic acid production using existing plant
capacities. This creates a degree of uncertainty in the
adipic acid production data used to derive the emission
estimates as it is necessary to assume that all plants oper-
ate at equivalent utilization levels.
The emission factor was based on experiments
(Thiemens and Trogler 1991) that attempt to replicate
the industrial process and, thereby, measure the reaction
stoichiometry for N2O production in the preparation of
adipic acid. However, the extent to which the lab results
are representative of actual industrial emission rates is
not known.
Nitric Acid Production
Nitric acid (HNO3) is an inorganic compound used
primarily to make synthetic commercial fertilizers. It is
also a major component in the production of adipic
acid—a feedstock for nylon—and explosives. Virtually
all of the nitric acid produced in the United States is
manufactured by the catalytic oxidation of ammonia
(EPA 1997). During this reaction, N2O is formed as a by-
product and is released from reactor vents into the atmo-
sphere.
Table 3-28: N20 Emissions
from Nitric Acid Production
Year
1990
1991
1992
1993
1994
1995
1996
1997
1998
MMTCE
4.9
4.9
5.0
5.1
5.3
5.4
5.6
5.8
5.8
Gg
58
58
59
60
63
64
67
68
68
Currently, the nitric acid industry controls for NO
and NO2 i.e., NOX. As such the industry uses a combina-
tion of non-selective catalytic reduction (NSCR) and
selective catalytic reduction (SCR) technologies. In the
process of destroying NOx, NSCR systems are also very
affective at destroying N2O. However, NSCR units are
generally not preferred in modern plants because of high
energy costs and associated high gas temperatures.
NSCRs were widely installed in nitric plants built be-
tween 1971 and 1977. Currently, it is estimated that ap-
proximately 20 percent of nitric acid plants use NSCR
(Choe, et al. 1993). The remaining 80 percent use SCR
or extended absorption, neither of which is known to
reduce N2O.
Nitric acid production was 8,504 thousand metric
tons in 1998 (C&EN 1999). Nitrous oxide emissions from
this source were estimated at 5.8 MMTCE (68 Gg) (see
Table 3-28). Nitric acid production for 1998 decreased 1
percent from the previous year, but has increased 18 per-
cent since 1990.
Methodology
Nitrous oxide emissions were calculated by multi-
plying nitric acid production by the amount of N2O emit-
ted per unit of nitric acid produced. An emissions factor
of 8 kg N2O / tonne HNO3 was used and represents a
combined factor comprising of 2 kg for plants using non-
selective catalytic reduction (NSCR) systems and 9.5 kg
for plants not equipped with NSCR (Reimer & Slaten
1992). An estimated 20 percent of HNO3 plants in the
3-20 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
U.S. were equipped with NSCR (Choe, et al. 1993). In the
process of destroying NOX, NSCR systems also destroy
80 to 90 percent of the N2O. Hence, the emission factor is
equal to (9.5 x 0.80) + (2 x 0.20) = 8 kg N2O / mt HNO3.
Data Sources
Nitric acid production data for 1990 through 1998
(see Table 3-29) were obtained from Chemical and Engi-
neering News, "Facts and Figures" (C&EN 1999). The
emission factor range was taken from Reimer, R.A., Parrett,
R.A., and Slaten, C.S. (1992).
Uncertainty
In general, the nitric acid industry is not well cat-
egorized. A significant degree of uncertainty exists in
Table 3-29: Nitric Acid Production
R
r Year
£"""v 1990
! 1991
: 1992
; 1993
- 1994
1995
fc 1996
1997
----- • 1998
Thousand Metric Tons
7,196 :
7,191
7,381
7,488
7,905
8,020
8,351
8,557
8,504
nitric acid production figures because nitric acid plants
are often part of larger production facilities, such as fer-
tilizer or explosive manufacturing. As a result, only a
small volume of nitric acid is sold on the market making
production figures difficult to track. Emission factors
are also difficult to determine because of the large num-
ber of plants using many different technologies. Based
on expert judgment, it is estimated that the N2O destruc-
tion factor for NSCR nitric acid facilities is associated
with an uncertainty of approximately ±10 percent.
Substitution of Ozone Depleting
Substances
Hydrofluorocarbons (HFCs) and perfluorocarbons
(PFCs) are used primarily as alternatives to several classes
of ozone-depleting substances (ODSs) that are being
phased out under the terms of the Montreal Protocol and
the Clean Air Act Amendments of 1990.18 Ozone deplet-
ing substances—chlorofluorocarbons (CFCs), halons,
carbon tetrachloride, methyl chloroform, and
hydrochlorofluorocarbons (HCFCs)—are used in a vari-
ety of industrial applications including refrigeration and
air conditioning equipment, solvent cleaning, foam pro-
duction, sterilization, fire extinguishing, and aerosols.
Although HFCs and PFCs, unlike ODSs, are not harmful
Table 3-30: Emissions of HFCs and PFCs from ODS Substitution (MMTCE)
iGas
ff=**~
pHFC-23
IHEC-125
tHFC-134a
IHFC-143a
|HFC-236fa
&4F10
|C6F14
^Olhers*
potal
l£r= -
1990
+
+
0.2
+
+
+
+
0.1
0.3
1991 1992
+ +
+ 0.2
0.2 0.2
• + • +
' + ' - -.+...
+ +
. ..+ +
+ +
0.2 0.4
1993
+
0.4
.1.0
+
+
+
. + -
1 +';
1.4
1994
+
0.2
2.3
0.1
+
+
0.2
2.7
1995
"+;
0.4
5.2
0.1
^_
+
1.3
7.0
1996
+
0.5
6.9
0.2
,
+
2.3
9.9
1997
0.6
8.5
0.4
,
+
2.7
12.3
1998
' - -;
0.8
9.8
05
0.3
+
3.1
14.5
FfDoes not exceed 0.05 MMTCE
f* Others include HFC-152a, HFC-227ea, HFC-4310mee and PFC/PFPEs, which are a proxy for a diverse collection of PFCs and
Ijerfluoropoiyethers (PFPEs) employed for solvent applications. For estimating purposes, the GWP value used for PFC/PFPEs was based
pponC6F14.
pote: Totals may not sum due to independent rounding.
18 [42 U.S.C § 7671, CAA § 601]
Industrial Processes 3-21
-------
Table 3-31: Emissions of HFCs and PFCs from ODS Substitution (Mg)
Gas
HFC-23
HFC-125
HFC-1343
HFC-1433
HFC-236fa
G^Fio
Others*
1990 1991
+ +
+ +
564 564
+ +
+ +
+ +
M M
1992
+
236
626
+
+
+
M
1993
+
481
2,885
12
+
+
M
1994
+
295
6,408
63
+
+
M
1995
2
459
14,596
132
+
^
M
1996
5
637
19,350
234
+
^
M
1997
9
828
24,065
358
18
^
M
1998
15
1,027
27,693
506
148
+
M
M (Mixture of Gases)
+ Does not exceed 0.5 Mg .
* Others include HFC-152a, HFC-227ea, HFC-4310mee and PFC/PFPEs, which are a proxy for a diverse collection of PFCs and
perfluoropolyethers (PFPEs) employed for solvent applications.
house gases. Emission estimates for HFCs and PFCs used
as substitutes for ODSs are provided in Table 3-30 and
Table 3-31.
In 1990 and 1991, the only significant emissions
of HFCs and PFCs as substitutes to ODSs were relatively
small amounts of HFC-152a—a component of the re-
frigerant blend R-500 used in chillers—and HFC-134a
in refrigeration end-uses. Beginning in 1992, HFC-134a
was used in growing amounts as a refrigerant in motor
vehicle air conditioners and in refrigerant blends such
as R-404.19 In 1993, use of HFCs in foams and aerosols
began, and in 1994 these compounds also found appli-
cations as solvents and sterilants. In 1995, ODS substi-
tutes for halons entered widespread use in the United
States as halon production was phased-out.
The use and subsequent emissions of HFCs and
PFCs as ODS substitutes increased dramatically, from
small amounts in 1990, to 14.5 MMTCE in 1998. This
increase was the result of efforts to phase-out CFCs and
other ODSs in the United States. This trend is expected
to continue for many years, and will accelerate in the
early part of the next century as HCFCs, which are in-
terim substitutes in many applications, are themselves
phased out under the provisions of the Copenhagen
Amendments to the Montreal Protocol.
Methodology and Data Sources
The EPA used a detailed vintaging model of ODS-
containing equipment and products to estimate the ac-
tual—versus potential—emissions of various ODS sub-
stitutes, including HFCs and PFCs. The name of the model
refers to the fact that the model tracks the use and emis-
sions of various compounds for the annual "vintages" of
new equipment that enter service in each end-use. This
vintaging model predicts ODS and ODS substitute use
in the United States based on modeled estimates of the
quantity of equipment or products sold each year con-
taining these chemicals and the amount of the chemical
required to manufacture and/or maintain equipment and
products over time. Emissions for each end-use were es-
timated by applying annual leak rates and release pro-
files, which account for the lag in emissions from equip-
ment as they leak over time. By aggregating the data for
more than 40 different end-uses, the model produces es-
timates of annual use and emissions of each compound.
The major end-use categories defined in the
vintaging model to characterize ODS use in the United
States were: refrigeration and air conditioning, aerosols,
solvent cleaning, fire extinguishing equipment, steril-
ization, and foams.
The vintaging model estimates HFC and PFC use
and emissions resulting from their use as replacements
for ODSs by undertaking the following steps:
" R-404 contains HFC-125, HFC-143a, and HFC-134a.
3-22 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Step 1: Estimate ODS Use in the United States Prior
to Phase-out Regulations
The model begins by estimating CFC, halon, me-
thyl chloroform, and carbon tetrachloride use prior to
the restrictions on the production of these compounds
in the United States. For modeling purposes, total ODS
use was divided into more than 40 separate end-uses.
The methodology used to estimate baseline ODS use
varied depending on the end-use under consideration.
The next section describes the methodology used for
estimating baseline ODS use in the refrigeration, air con-
ditioning, and fire extinguishing (halon) end-uses. The
subsequent section details the methodology used for all
other end-uses.
Step 1.1: Estimate Baseline ODS
Use for Refrigeration, Air Conditioning,
and Fire Extinguishing
For each equipment type, the model estimates the
total stock of ODS-containing equipment during the
period 1985 to 1997. The key data required to develop
stock estimates for each end-use were as follows:
• Total stock of ODS-containing equipment in use in the
United States in 1985
• The annual rate of growth in equipment consumption
in each end-use
• The retirement function for equipment in each end-use
Historical production and consumption data were
collected for each equipment type to develop estimates
of total equipment stock in 1985. For some end-uses, the
only data.available were estimates of ODS usage. In these
cases, the total 1985 stock was estimated by dividing
total ODS use by the average charge of ODS in a typical
piece of equipment.
Stocks of ODS-containing equipment change over
time. In the vintaging model, the growth in equipment
stocks in each end-use was simulated after 1985 using
growth rates that define the total number of pieces of
new equipment added to the stock each year. The model
also uses a retirement function to calculate the length of
time each piece of equipment is expected to remain in
service. These retirement functions are a critical part of
the vintaging model because they determine the speed
at which the stock of equipment turns over and is re-
placed by new equipment. In this analysis, point esti-
mates of the average lifetime of equipment in each end-
use were used to develop retirement functions. These
retirement functions assume 100 percent survival of
equipment up to this average age and zero percent sur-
vival thereafter.
Given these data, the total equipment stock in ser-
vice in a given year / was estimated as the equipment
stock in the year (f-1), plus new equipment added to the
stock in year t, minus retirements in year t.
Annual ODS use was then estimated for each equip-
ment type during the period 1985 through 1998. Be-
cause control technologies can reduce particular kinds
of ODS use, use estimates were broken down by type of
use (e.g., use in new equipment at manufacture and use
required to maintain existing equipment). Baseline esti-
mates of ODS use were based on the following data col-
lected for each equipment type:
ODS charge size (the number of kilograms of ODS
installed in new equipment during manufacture)
ODS required to maintain existing equipment (In
many end-uses, chemical must be regularly added to
equipment to replace chemical emitted from the equip-
ment. Such emissions result from normal leakage and
from loss during servicing of the equipment.)
With these data, ODS usage for each refrigeration,
air conditioning, and fire extinguishing end-use was
calculated using the following equation:
(Total stock of existing equipment in use) x (ODS
required to maintain each unit of existing equipment) +
(New equipment additions) x (ODS charge size)
Step 1.2: Estimate Baseline ODS Use in Foams,
Solvents, Sterilization, and Aerosol End-Uses
For end-uses other than refrigeration, air condi-
tioning, and fire extinguishing, a simpler approach was
used because these end-uses do not require partial re-
filling of existing equipment each year. Instead, such
equipment either does not require any ODS after initial
Industrial Processes 3-23
-------
production (e.g., foams and aerosols), or requires com-
plete re-filling or re-manufacturing of the equipment
each year (e.g., solvents and sterilants). ODS use does
not need to be differentiated between new and existing
equipment for these end-uses. Thus, it is not necessary
to track the stocks of new and existing equipment sepa-
rately over time.
The approach used for these end-uses was to esti-
mate total ODS use in 1985 based on available industry
data. Future ODS use was estimated using growth rates
that predict ODS consumption growth in these end-uses
over time, based upon input from industry.
Step 2: Specification and
Implementation of Control Technologies
Having established a baseline for ODS equipment
in 1985, the vintaging model next defines controls that
may be undertaken for purposes of reducing ODS use
and emissions within each end-use. The following con-
trols were implemented in the model:
• Replacement of ODS used in the manufacturing of
new equipment or in the operation of existing
equipment (i.e., retrofits) with alternative chemi-
cals, such as HFCs and PFCs
• Replacement of ODS-based processes or products
with alternative processes or products (e.g., the use
of aqueous cleaning to replace solvent cleaning
withCFC-113)
• Modification of the operation and servicing of
equipment to reduce use and emission rates
through the application of engineering and recy-
cling controls
Assumptions addressing these types of controls in
each end-use were used to develop "substitution sce-
narios" that simulate the phase-out of ODSs in the United
States by end-use. These scenarios represent the EPA's
best estimates of the use of control technologies towards
the phase-out ODS in the United States, and are periodi-
cally reviewed by industry experts.
In addition to the chemical substitution scenarios,
the model also assumes that a portion of ODS substitutes
are recycled during servicing and retirement of the equip-
ment. Recycling is assumed to occur in the refrigeration
and air conditioning and fire extinguishing end-uses.
The substitution scenarios defined for each equip-
ment type were applied to the relevant equipment stocks.
The equipment life-cycle was then simulated after the
imposition of controls. Substitute chemical use and emis-
sions—including HFCs and PFCs—were calculated for
each scenario using the methods described below.
Step 3: Estimate ODS Substitute
Use and Emissions (HFCs and PFCs)
ODS substitute use (i.e., HFC and PFC use) was
calculated using the same routine described above for
refrigeration, air conditioning, and fire extinguishing
equipment. In terms of chemical usage, a key question
was whether implementation of a given ODS substitute
in an end-use changed the quantity of chemical required
to manufacture new equipment or service existing equip-
ment. In this analysis, it was assumed that the use of ODS
alternatives in new equipment—including HFCs and
PFCs—did not change the total charge of initial chemi-
cal used in the equipment in each end-use. For certain
refrigeration and air conditioning end-uses, however, it
was assumed that new equipment manufactured with
HFCs and PFCs would have lower leak rates than older
equipment. Existing ODS-containing equipment that was
retrofitted with HFCs or PFCs was assumed to have a
higher leak rate than new HFC/PFC equipment.
The use of HFCs and PFCs in all other end-uses
was calculated by simply replacing ODS use with the
chemical alternatives defined in the substitution sce-
narios. The use of HFCs and PFCs was not assumed to
change the quantity of chemical used in new or existing
equipment for these end-uses.
The vintaging model estimates HFC and PFC emis-
sions over the lifetime of equipment in each end-use.
Emissions may occur at the following points in the life-
time of the equipment:
• Emissions upon manufacture of equipment
• Annual emissions from equipment (due to normal
leakage, and if applicable, servicing of equipment)
• Emissions upon retirement of equipment
The emissions that occur upon manufacture of re-
frigeration and air conditioning equipment were assumed
to be less than 0.1 percent. Annual emissions of HFCs
and PFCs from equipment—due to normal leakage and
3-24 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
servicing—were assumed to be constant each year over
the life of the equipment. The quantity of emissions at
disposal is a function of the prevalence of recycling at
disposal.
Emissions for open cell foam were assumed to be
100 percent in the year of manufacture. Closed cell foams
were assumed to emit a portion of total HFC/PFC use
upon manufacture, a portion at a constant rate over the
lifetime of the foam, and the rest at disposal. There were
no foam recycling technologies in use in the United
States; therefore, HFCs and PFCs remaining in closed
cell foam were assumed to be emitted by the end of the
product lifetime.
Emissions were assumed to occur at manufacture,
during normal operation, and upon retirement of fire
extinguishing systems. Emissions at manufacture were
assumed to be negligible and emissions upon disposal
were assumed to be minimal because of the use of recov-
ery technologies.
For solvent applications, 15 percent of the chemi-
cal used in equipment was assumed to be emitted in that
year. The remainder of the-used solvent was assumed to
be reused or disposed without being released to the at-
mosphere.
For sterilization applications, all chemicals that
were used in the equipment were assumed to be emitted
in that year.
All HFCs and PFCs used in aerosols were assumed
to be emitted in the same year. No technologies were
known to exist that recycle or recover aerosols.
Uncertainty
Given that emissions of ODS substitutes occur from
thousands of different kinds of equipment and from mil-
lions of point and mobile sources throughout the United
States, emission estimates must be made using analyti-
cal tools such as the EPA vintaging model or the meth-
ods outlined in IPCC/UNEP/OECD/DEA (1997). Though
the EPA!s model is more comprehensive than the IPCC
methodology, significant uncertainties still exist with
regard to the levels of equipment sales, equipment char-
acteristics, and end-use emissions profiles that-were used
to estimate annual emissions for the various compounds.
Aluminum Production
Aluminum is a light-weight, malleable, and corro-
sion resistant metal that is used in many manufactured
products including aircraft, automobiles, bicycles, and
kitchen utensils. The United States was the largest pro-
ducer of primary aluminum, with 17 percent of the world
total in 1998 (USGS 1999). The United States was also a -
major importer of primary aluminum. The production of
primary aluminum—in addition to consuming large quan-
tities of electricity—results in emissions of several green-
house gases including carbon dioxide (CO2) and two
perfluorocarbons (PFCs): perfluoromethane (CF4) and
perfluoroethane (C2F6).
Occasionally, sulfur hexafluoride (SF6) is also used
by the aluminum industry as a fluxing and degassing
agent in experimental and specialized casting operations.
In these cases it is normally mixed with argon, nitrogen,
and/or chlorine and blown through molten aluminum;
however, this practice is not used by primary aluminum
production firms in the United States and is not believed
to be extensively used by secondary casting firms. Where
it does occur, the concentration of SF6 in the mixture is
small and a portion of the SF6 is decomposed in the
process (Waite and Bernard 1990, Corns 1990). It has
been estimated that 230 Mg of SF6 were used by the
aluminum industry in the United States and Canada
(Maiss and Brenninkmeijer 1998); however, this esti-
mate is highly uncertain. Emissions of SF6 have not been
Industrial Processes 3-25
-------
Table 3-32: C02 Emissions
from Aluminum Production
Year
1990
1991
1992
1993
1994
1995
1996
1997
1998
MMTCE
1.6
1.7
1.6
1.5
1.3
1.4
1.4
1.4
1.5
Gg
5,951
6,058
5,942
5,432
4,850
4,961
5,258
5,296
5,458
Table 3-33: PFC Emissions
from Aluminum Production (MMTCE)
Year
1990
1991
1992
1993
. 1994
1995
1996
1997
1998
Note: Totals
CF4
4.7
4.1
3.9
3.3
2.8
2.8
2.8
2.6
2.5
may not sum due
C2F6
0.7
0.6
0.6
0.4
0.4
0.4
0.4
0.3
0.3
to independent
Total
5.4
4.7
4.4
3.8
3.2
3.1
3.2
3.0
2.8
rounding.
estimated for this source.
Carbon dioxide is emitted during the aluminum
smelting process when alumina (aluminum oxide, A12O3)
is reduced to aluminum using the Hall-Heroult reduc-
tion process. The reduction of the alumina occurs through
electrolysis in a molten bath of natural or synthetic cryo-
lite (Na3AlF6). The reduction cells contain a carbon lin-
ing that serves as the cathode. Carbon is also contained
in the anode, which can be a carbon mass of paste, coke
briquettes, or prebaked carbon blocks from petroleum
coke. During reduction, some of this carbon is oxidized
and released to the atmosphere as CO2.
Process emissions of CO2 from aluminum produc-
tion were estimated at 1.5 MMTCE (5,500 Gg) in 1998
(see Table 3-32). The CO2 emissions from this source,
however, are accounted for under the non-energy use
portion of CO2 from Fossil Fuel Combustion of petro-
leum coke and tar pitch in the Energy chapter. Thus, to
avoid double counting, CO2 emissions from aluminum
production are not included in totals for the Industrial
Processes chapter. They are provided here for informa-
tional purposes only.
In addition to CO2 emissions, the aluminum pro-
duction industry was also the largest source of PFC emis-
sions in the United States. During the smelting process,
when the alumina ore content of the electrolytic bath
falls below critical levels requked for electrolysis, rapid
voltage increases occur, termed "anode effects." These
anode effects cause carbon from the anode and fluorine
from the dissociated molten cryolite bath to combine,
thereby producing fugitive emissions of CF4 and C2F6.
In general, the magnitude of emissions for a given level
of production depends on the frequency and duration of
these anode effects. The more frequent and long-lasting
the anode effects, the greater the emissions.
Primary aluminum production-related emissions
of PFCs are estimated to have declined 48 percent since
1990 to 2.5 MMTCE of CF4 (1.42 Gg) and 0.3 MMTCE
of C2F6 (0.12 Gg) in 1998, as shown in Table 3-33 and
Table 3-34. This decline was both due to reductions in
domestic aluminum production and actions taken by
aluminum smelting companies to reduce the frequency
and duration of anode effects. The EPA supports alumi-
num smelters with these efforts through the Voluntary
Aluminum Industrial Partnership (VAIP).
U.S. primary aluminum production for 1998—to-
taling 3,713 thousand metric tons—increased slightly
from 1997. This increase can be attributed to the reintro-
Table 3-34: PFC Emissions
from Aluminum Production (Gg)
Ijr
1
^.
SL -- "•
r=ri • • • -
¥-' -~ "
§;
f- - '
t.. , •.
*-
IT.: ' '•'-
jr -
P
Year
1990
1991
1992
1993
1994
1995
1996
1997
1998
CF4
2.67
2.32
2.18
1.88
1.57
1.57
1.60
1.49
1.42'
C2F6
0.28
0.24
0.23
0.18
""•"" ™-o:tS ':'""
0.14
0.14
0.13
0.12
' *"'*, .
3-26 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
duction of previously idled production capacity (USGS
1999). In general, U.S. primary aluminum production is
very responsive to imports, mainly from Russia and other
republics of the Former Soviet Union. For example, in
1994 these countries exported 60 percent more ingots
(metal cast for easy transformation) to the United States
than in 1993, leading to a significant decline in domes-
tic production. However, 1998 imports from Russia were
10 percent below their peak level in 1994 (USGS 1999).
The transportation industry remained the largest
domestic consumer of aluminum, accounting for about
29 percent (USGS 1998). Leading automakers have an-
nounced new automotive designs that will expand the
use of aluminum materials in the near future. The U.S.
Geological Survey believes that demand for and pro-
duction of aluminum will continue to increase.
Methodology
Carbon dioxide is generated during alumina reduc-
tion to aluminum metal following the reaction below:
2A12O3 + 3C -> 4A1 + 3CO2
The CO2 emission factor employed was estimated
from the production of primary aluminum metal and the
carbon consumed by the process. During alumina reduc-
tion, approximately 1.5 to 2.2 metric tons of CO2 are emit-
ted for each metric ton of aluminum produced
(Abrahamson 1992). Based upon the mass balance for a
"typical" aluminum smelter (Drexel University Project
Team 1996), the emission factor was set at 1.5 metric tons
CO2 per metric ton of aluminum smelted. This value is at
the low end of the Abrahamson (1992) range.
The CO2 emissions from this source are already
accounted for under CO2 Emissions from Fossil Fuel
Combustion in the Energy chapter.20 Thus, to avoid
double counting, CO2 emissions from aluminum pro-
duction are not included in totals for the Industrial Pro-
cesses chapter.
PFC emissions from aluminum production were
estimated using a per unit production emission factor
that is expressed as a function of operating parameters
(anode effect frequency and duration), as follows:
PFC (CF4 or C2F6) kg/ton Al = S x Anode Effect
Minutes/Cell-Day
where:
S = Slope coefficient
Anode Effect Minutes/Cell-Day = Anode Effect
Frequency x Anode Effect Duration
The slope coefficient was established for each
smelter based on actual field measurements, where avail-
able, or default coefficients by technology-type based
on field measurements. Once established, the slope co-
efficient was used along with smelter anode effect data,
collected by aluminum companies and reported to the
VAIP, to estimate emissions factors over time. Emissions
factors were multiplied by annual production to esti-
mate annual emissions at the smelter level. Emissions
were then aggregated across smelters to estimate national
emissions. The methodology used to estimate emissions
is consistent with the methodologies recommended by
the IPCC (IPCC/UNEP/OECD/EEA1997).
Data Sources
Primary aluminum production data for 1990
through 1997 (see Table 3-35) were obtained from USGS,
Mineral Industry Surveys: Aluminum Annual Report
(USGS 1995, 1998). The data for 1998 were taken from
Table 3-35: Production of Primary Aluminum
jfeY •-::,"- -.-"---- • '- --.-
&";- - -" "" "," , -- '- ,; -- -'"
i_^_ r Year
i^: ,,1990
L 1991
~ -'-' 1992
ir: : r 1993
fc;:~.:- 1994
»-r ---.•:. •--,,; .M 995
:: •;-... . : ...;• 1.996.
e^_i- _., 1 997
R ""•/'- 1998
Thousand
Metric Tons
4,048
.. .... - 4,121
4,042
3,695
3,299
3,375
3,577
3,603
3,713
is unfortunately not available in DOE/EIA fuel statistics.
Industrial Processes 3-27
-------
Mineral Industry Surveys: Aluminum in January 1999
(USGS 1999). The USGS requested data from the 13 do-
mestic producers, all of whom responded. The CO2 emis-
sion factor range was taken from Abrahamson (1992).
The mass balance for a "typical" aluminum smelter was
taken from Drexel University Project Team (1996).
PFC emission estimates were provided by the
EPA's Climate Protection Division in cooperation with
participants in the Voluntary Aluminum Industrial Part-
nership (VAIP) program.
Uncertainty
Uncertainty exists as to the most accurate CO2
emission factor for aluminum production. Emissions vary
depending on the specific technology used by each
plant. However, evidence suggests that there is little varia-
tion in CO2 emissions from plants utilizing similar tech-
nologies (IPCCAJNEP/OECD/ffiA 1997). A less uncer-
tain method would be to calculate emissions based upon
the amount of carbon—in the form of petroleum coke or
tar pitch—consumed by the process; however, this type
of information was not available.
For PFC emission estimates, the uncertainty in the
aluminum production data is relatively low (± 1 to 2
percent) compared to the uncertainty in the emissions
factors (± 10 to 50 percent). Uncertainty in the emissions
factors arises from the lack of comprehensive data for
both the slope coefficients and anode effect data. Cur-
rently, insufficient measurement data exist to quantify a
relationship between PFC emissions and anode effect
minutes for all smelters. Future inventories will incorpo-
rate additional data reported by aluminum companies
and ongoing research into PFC emissions from alumi-
num production.
Emissions of SF5 from aluminum fluxing and de-
gassing have not been estimated. Uncertainties exist
as to the quantity of SF6 used by the aluminum indus-
try and its rate of destruction as it is blown' through
molten aluminum.
HCFC-22 Production
Trifluoromethane (HFC-23- or CHF3) is generated
as a by-product during the manufacture of
chlorodifluoromethane (HCFC-22), which is primarily
employed in refrigeration and air conditioning systems
and as a chemical feedstock for manufacturing synthetic
polymers. Since 1990, production and use of HCFC-22
has increased significantly as it has replaced chlorofluo-
rocarbons (CFCs) in many applications. Because HCFC-
22 depletes stratospheric ozone, HCFC-22 production
for non-feedstock uses is scheduled to be phased out by
, 2020 under the U.S. Clean Air Act.21 Feedstock produc-
tion, in contrast, is permitted to continue indefinitely.
HCFC-22 is produced by the reaction of chloro-
form (CHC13) and hydrogen fluoride (HF) in the pres-
ence of a catalyst, SbCl5. The reaction of the catalyst
and HF produces SbClxFy, (where x + y = 5), which reacts
with chlorinated hydrocarbons to replace chlorine at-
oms with fluorine. The HF and chloroform are introduced
by submerged piping into a continuous-flow reactor that
contains the catalyst in a hydrocarbon mixture of chlo-
roform and partially fluorinated intermediates. The va-
pors leaving the reactor contain HCFC-21 (CHC12F),
HCFC-22 (CHC1F2), HFC-23 (CHF3), HC1, chloroform,
and HF. The under-fluorinated intermediates (HCFC-21) ~
and chloroform are then condensed and returned to the
reactor, along with residual catalyst, to undergo further
fluorination. The final vapors leaving the condenser are
primarily HCFC-22, HFC-23, HC1 and residual HF. HC1
is recovered as a useful byproduct, and the HF is re-
moved. Once separated from HCFC-22, the HFC-23 is
generally vented to the atmosphere as an unwanted by-
product, or may be captured for use in a limited number
of applications.
Emissions of HFC-23 in 1998 were estimated to be
10.9 MMTCE (3.4 Gg), which represents a 15 percent
increase in emissions since 1990 (see Table 3-36). This
increase is attributable to the 30 percent increase in HCFC-
22 production that occurred since 1990; one third of this
* As construed, interpreted, and applied in the terms and conditions of the Montreal Protocol on Substances that Deplete the Ozone
Layer. [42 U.S.C. §7671m(b), CAA §614]
3-28 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Table 3-36: HFC-23 Emissions
from HCFC-22 Production
f" -
W:_ •
^—
— - "
&*;• '
f^~ '
g^ -, ,
! ,'
s=.
M.-:.^-
S,^-,'-.,
Year
1990 .':'.
1991
1992
1993
:1994
:1995
1996
1997
1998
1 ..'... ,.- . . ..• ~.-~
MMTCE
9.5
.8.4
9.5
, ,8,7
8.6
-7.4. . ,
8.5
8.2
10.9
.' ..;.:.- •-. -..•'•;.'..
fig
3.0
2.6
3.0
2.7 ...
2.7
2.3
2.7
2.6
3.4
•'.".Vr :
-. .1
' "1
"•
-• - - <(
•-.«
increase occurred between 1997 and 1998. Separately,
the intensity of HFC-23 emissions (the amount of HFC-23
emitted per kilogram of HCFC-22 manufactured) has de-
clined significantly since 1990.
In the future, production of HCFC-22 in the United
States is expected to decline as non-feedstock HCFCs pro-
duction is phased-out. In contrast, feedstock production is
anticipated to continue growing steadily, mainly for manu-
facturing Teflon® and other chemical products. All U.S.
producers of HCFC-22 are participating in a voluntary
program with the EPA to reduce HFC-23 emissions.
Methodology
The EPA studied the conditions of HFC-23 gen-
eration, methods for measuring emissions, and technolo-
gies for emissions control. This effort was undertaken in
cooperation with the manufacturers of HCFC-22.
The methodology employed for estimating emis-
sions was based upon measurements of critical feed com-
ponents at individual HCFC-22 production plants. Indi-
vidual producers also measured HFC-23 concentrations
in their output stream by gas chromatography. Using mea-
surements of feed components and HFC-23 concentra-
tions in output streams, the amount of HFC-23 generated
was estimated. HFC-23 concentrations were determined
at the point the gas leaves the chemical reactor; therefore,
estimates also include fugitive emissions.
Data Sources
Emission estimates were provided by the EPA's Cli-
mate Protection Division in cooperation with the U.S.
manufacturers of HCFC-22.
Uncertainty
A high level of confidence has been attributed to
the HFC-23 concentration data employed because mea-
surements were conducted frequently and accounted for
day-to-day and process variability. It is estimated that the
emissions reported are within 20 percent of the true value.
This methodology accounted for the declining intensity
of HFC-23 emissions over time. The use of a constant
emission factor would not have allowed for such account-
ing. Earlier emission estimates assumed that HFC-23 emis-
sions were between 2 and 4 percent of HCFC-22 produc-
tion on a mass ratio basis. By 1996, the rate of HFC-23
generated as a percent of HCFC-22 produced dropped, on
average, below 2 percent in the United States.
Semiconductor Manufacture
The semiconductor industry uses multiple long-
lived fluorinated gases in plasma etching and chemical
vapor deposition (CVD) processes. The gases most com-
monly employed are trifluoromethane (HFC-23),
perfluoromethane (CF4), perfluoroethane (C2F6), nitro-
gen trifluoride (NF3), and sulfur hexafluoride (SF6), al-
though other compounds such as perfluoropropane (C3F8)
and perfluorocyclobutane (c-C4F8) are also used. The
exact combination of compounds is specific to the pro-
cess employed.
Plasma etching is performed to provide pathways
for the electrical conducting material to connect indi-
vidual circuit components in the silicon, using HFCs,
PFCs, SF6 and other gases in plasma. The etching pro-
cess creates fluorine atoms that react at the semiconduc-
tor surface according to prescribed patterns to selectively
remove substrate material. A single semiconductor wa-
fer may require as many as 100 distinct process steps
that utilize these gases. Chemical vapor deposition cham-
bers, used for depositing materials that will act as insula-
tors and wires, are cleaned periodically using PFCs and
other gases. During the cleaning cycle the gas is con-
verted to fluorine atoms in plasma, which etches away
residual material from chamber walls, electrodes, and
chamber hardware. However, due to the low destruction
efficiency (high dissociation energy) of PFCs, a portion
of the gas flowing into the chamber flows unreacted
Industrial Processes 3-29
-------
through the chamber and, unless emission abatement
technologies are used, this portion is emitted into the
atmosphere.
In addition to being directly used in the manufac-
turing processes, these gases can also be transformed
during the process into a different HFC or PFC com-
pound, which is then exhausted into the atmosphere.
For example, when either CHF3 or C2F6 is used in clean-
ing or etching, CF4 is often generated and emitted as a
process by-product.
For 1998, it was estimated that total weighted emis-
sions of all fluorinated greenhouse gases by the U.S.
semiconductor industry were 2.1 MMTCE. Combined
emissions of all fluorinated greenhouse gases are pre-
sented in Table 3-37 below. The rapid growth of this
industry and the increasing complexity of semiconduc-
tor products could increase emissions in the future.
Methodology
Emissions were estimated using two sets of data.
For 1990 through 1994, emissions were estimated based
on the historical consumption of silicon (square centi-
meters), the estimated average number of interconnect-
ing layers in the chips produced, and an estimated per-
layer emission factor. (The number of layers per chip,
and hence the PFC emissions per square centimeter of
silicon, increases as the line-width of the chip decreases.)
The average number of layers per chip was based on
industry estimates of silicon consumption by line-width
Table 3-37: Emissions of Fluorinated Greenhouse
Gases from Semiconductor Manufacture
Year
1990
1991
1992
1993
1994
1995
1996
1997
1998
* Combined radiative forcing effect of a
MMTCE* 1
0.8
0.8
0.8 :
1.0 :
1.1
1.5
1.9
1.9
2.1
II gases. .•
and of the number of layers per line-width. The per-layer
emission factor was based on the total annual emissions
reported by the participants in the EPA's PFC Emission
Reduction Partnership for the Semiconductor Industry.
For the three years for which gas sales data are available
(1992 through 1994), the estimates derived using his-
torical silicon consumption are within 10 percent of the
estimates derived using gas sales data and average val-
ues for emission factors and GWPs.
For 1995 through 1998, emissions were estimated
based on total annual emissions reported by participants
in the EPA's PFC Emission Reduction Partnership for
the Semiconductor Industry. As part of the program, part-
ners estimated their emissions using a range of methods;
the partners with relatively high emissions typically mul-
tiplied estimates of their PFC consumption by process-
specific emission factors that they have either measured
or obtained from suppliers of PFC-based manufacturing
equipment. To estimate total U.S. emissions from semi-
conductor manufacturing based on reported partner emis-
sions, a per-plant emissions factor was estimated for the
partners. This per-plant emission factor was then applied
to PFC-using plants operated by semiconductor manu-
facturers who were not partners, considering the varying
characteristics of the plants operated by partners and
non-partners (e.g., typical plant size and type of device
produced). The resulting estimate of non-partner emis-
sions was added to the emissions reported by the part-
ners to obtain total U.S. emissions.
Data Sources
Aggregate emissions estimates for the semicon-
ductor manufacturers participating in the PFC Emission
Reduction Partnership were provided by manufacturers
(partners). Estimates of the numbers of plants operated
by partners and non-partners, and information on the
characteristics of those plants, were derived from the In-
ternational Fabs on Disk database. Estimates of silicon
consumed by line-width from 1990 through 1994 were
derived from information from VLSI Research, and the
number of layers per line-width was obtained from the
Semiconductor Industry Association's 1997 National
Technology Roadmap.
3-30 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Uncertainty
Emission estimates for this source are improving,
but are still relatively uncertain. Emissions vary depend-
ing upon the total amount of gas used and the tool and
process in which the gas is used, but not all semiconduc-
tor manufacturers track this information. In addition, the
relationship between the emissions from semiconductor
manufacturers participating in the PFC Emission Reduc-
tion Partnership and total U.S. emissions from semicon-
ductor manufacturing is uncertain.
Electrical Transmission
and Distribution
The largest use for sulfur hexafluoride (SF6), both
domestically and internationally, is as an electrical in-
sulator in equipment that transmits and distributes elec-
tricity. It has been estimated that 30 percent of the world-
wide use of SF6 is leaked from electrical transmission
and distribution equipment (Maiss and Brenninkmeijer
1998). The gas has been employed by the electric power
industry in the United States since the 1950s because of
its dielectric strength and arc-quenching characteristics.
It is used in gas-insulated substations, circuit breakers, '
and other switchgear. Sulfur hexafluoride has replaced
flammable insulating oils in many applications and al-
lows for more compact substations in dense urban areas.
Fugitive emissions of SF6 can escape from gas-
insulated substations and switchgear through seals, es-
Table 3-38: SF6 Emissions
from Electrical Transmission and Distribution
£ Year
!•" 1990
i- 1991
ir~ 1992
&-. •,-- 1993
S-- 1994
S~ 1995
r 1996
"• ,1997
rr •;.: 1998,,
sS~~
MMTCE
5.6
5.9
_ 6.2
6.4
6.7
7.0
7.0
7.0
7.0
Gg
0.86
0.90
0.95
.. 0.99
1.03
1.07
1.07
1.07
1.07
pecially from older equipment. It can also be released
during equipment installation and when equipment is
opened for servicing, which typically occurs every few
years. In the past, some utilities vented SF6 to the atmo-
sphere during servicing; however, it is believed that in-
creased awareness and the relatively high cost of the gas
have reduced this practice.
Emissions of SF6 from electrical transmission and
distribution systems were estimated to be 7.0 MMTCE
(1.07 Gg) in 1998. This quantity amounts to a 25 per-
cent increase over the estimate for 1990 (see Table 3-38).
Methodology
The EPA developed its methodology for estimat-
ing SF6 emissions from electrical transmission and dis-
tribution systems in 1994. The method estimates actual
emissions of SF6 using a top-down, or production-based
approach. Specifically, emissions were calculated based
upon the following factors: 1) the estimated U.S. pro-
duction capacity for SF6, 2) the estimated use of this
production capacity, 3) the fraction of U.S. SF6 produc-
tion estimated to be sold annually to fill or refill electri-
cal equipment, and 4) the fraction of these sales esti-
mated to replace emitted gas.
Based on information gathered from chemical
manufacturers, it was estimated that in 1994 U.S. pro-
duction capacity for SF6 was approximately 3,000 met-
ric tons. It was assumed that plants were operating at 90
percent capacity, which was consistent with industry
averages and implied that 2,700 metric tons of SF6 were
produced in 1994. It was further assumed that 75 percent
of U.S. SF6 sales were made to electric utilities and elec-
trical transmission and distribution equipment manu-
facturers. This assumption is consistent with the esti-
mate given in Ko, et al. (1993) that worldwide, 80 per-
cent of SF6 sales is for electrical transmission and distri-
bution systems. Seventy-five percent of annual U.S. pro-
duction in 1994 was 2,000 metric tons.
Finally, it was assumed that approximately 50 per-
cent of this production, or 1.0 thousand metric tons, re-
placed gas emitted into the atmosphere in 1994. This
amount is equivalent to 6.7 MMTCE (when rounding is
performed at the end of the calculation). EPA's estimate
was based on information that emissions rates from this
Industrial Processes 3-31
-------
equipment were significant and atmospheric measure-
ments that indicated that most of the SF6 produced inter-
nationally since the 1950s had been released. Emissions
from electrical equipment were known to occur from the
service and disposal of the equipment and leaks during
operation. Leaks from older equipment were reported to
release up to 50 percent of the equipment's charge per
year, although leaks from newer equipment were reported
to release considerably less (e.g., less than 1 percent of
the charge per year).
It was assumed that emissions have remained con-
stant at 7 MMTCE since 1995.
Data Sources
Emission estimates were provided by EPA's Cli-
mate Protection Division in cooperation with U.S. elec-
tric utilities and chemical producers.
Uncertainty
There is currently little verifiable data for estimat-
ing SF6 emissions from electrical transmission and dis-
tribution systems. Neither U.S. gas consumption nor
emission monitoring data were available when these es-
timates were developed. The EPA has recently launched
a voluntary program with electrical power systems to
reduce emissions of SF6 from equipment used to trans-
mit and distribute electricity such as high voltage cir-
cuit breakers, substations, transformers, and transmis-
sion lines. The EPA anticipates that better information
on SF6 emissions from electrical equipment will be pro-
vided through its voluntary agreements with electrical
utilities that use SF6 in equipment.
Magnesium
Production and Processing
The magnesium metal production and casting in-
dustry uses sulfur hexafluoride (SF6) as a covergas to
prevent the violent oxidation of molten magnesium in
the presence of air. Small concentrations of SF6 in com-
bination with carbon dioxide and/or air are blown over
molten magnesium metal to induce and stabilize the for-
mation of a protective crust. A minute portion of the SF6
applied reacts with the magnesium to form a thin mo-
lecular film of mostly magnesium oxide and some mag-
nesium fluoride. Little conversion or destruction of SF6
occurs in the magnesium production or casting processes,
and it is currently assumed that all SF6 is emitted to the
atmosphere. SF6 has been used in this application around
the world for the last twenty years. It has largely replaced
salt fluxes, sulfur dioxide (SO2), and boron trifluoride
(BF3), which are toxic and more corrosive at higher con-
centrations.
For 1998, a total of 3.0 MMTCE (0.5 Gg) of SF6
was estimated to have been emitted by the magnesium
industry, 76 percent more than was estimated for 1990
(see Table 3-39). There are no significant plans for ex-
pansion of primary magnesium production in the United
States, but demand for magnesium metal for die casting
is growing as auto manufacturers design more magne-
sium parts into vehicle models. The increased demand
for primary magnesium is expected to be met by magne-
sium producers located outside the United States
Methodology
Emissions were estimated from gas usage informa-
tion supplied to the EPA by primary magnesium produc-
ers. Consumption was assumed to equal emissions in the
Table 3-39: SF6 Emissions from
Magnesium Production and Processing
p
i
tr
F
.
h •
it-- --
>-•
s"~
K"Tl " „ '"
i~- '."..
yt i
Year
1990
1991
1992
1993
1994
1995
1996
1997
1998
MMTCE
1.7
2.0
2.2
2.5
2.7
3.0
3.0
" 3.0
3.0
Gg
0.3
0.3
0.3
•0.4
0.4
0.5 " .'•••
0.5
" •"• '"0.5 : •"
0.5
, ,„„
3-32 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Box 3-1: Potential Emission Estimates of HFCs, PFCs, and SF6
^ Emissions of HFCs, PFCs and SF6 from industrial processes can be estimated in two ways, either as potential emissions or as actual
^missions. Emission estimates in this chapter are "actual emissions," which are defined by the Revised 1996IPCC Guidelines for
jational Greenhouse Gas Inventories (IPCC1997) as estimates that take into account the time lag between consumption and
pssions. In contrast, "potential emissions" are defined to be equal to the amount of a chemical consumed in a country, minus the
amount of a chemical recovered for destruction or export in the year of consideration. Potential emissions will generally be'greater for
pjven year than actual emissions, since some amount of chemical consumed will be stored in products or equipment and will not
to emitted to the atmosphere until a later date, if ever. Because all chemicals consumed will eventually be emitted into the atmosphere
In the long term the cumulative emission estimates using the two approaches should be equivalent unless the chemical is captured
rndJestroyed. Although actual emissions are considered to be the more accurate estimation approach for a single year, estimates of
potential emissions are provided for informational purposes.
|: Separate estimates of potential emissions were not made for industrial processes that fall into the following categories:
By-product emissions. Some emissions do not result from the consumption or use of a chemical, but are the unintended by-
products of another process. For such emissions, which include emissions of CF4 and C2F6 from aluminum production and of
IHFC-23 from HCFC-22 production, the distinction between potential and actual emissions is not relevant.
I* P°te/7f/a/em/ss/o/7sf/?a?^
|— that there is no delay between consumption and emission and that no destruction of the chemical takes place. It this case actual
jp- emissions equal potential emissions.
^r Emissions that are not easily defined. In some processes, such as semiconductor manufacture, the gases used in the process
jg may be destroyed or transformed into other compounds, which may also be greenhouse gases. It is therefore not logical to
|E estimate potential emissions based on consumption of the original chemical.
|: Table 3-40 presents potential emission estimates for HFCs and PFCs from the substitution of ozone depleting substances and SF6
^missions from electrical transmission and distribution and other miscellaneous sources such as tennis shoes and sound insulating
p/indows.2* Potential emissions associated with the substitution for ozone depleting substances were calculated through a combina-
||on of the EPA's Vintaging Model and information provided by U.S. chemical manufacturers. For other SF6 sources, estimates were
||as_ed_on an assumed U.S. SF6 production capacity and plant utilization to estimate total sales. The portion of this amount used for
pagnesium processing and assumed to be used for semiconductor manufacture were subtracted.
"40:1998 Potential and! flGtual Emissions of HFCs, PFCs, and SF6 from Selected Sources (MMTCE)
iSource
Potential
Actual
^Substitution of Ozone Depleting Substances
tAluminum Production
I HCFC-22 Production
f-rSemiconductor Manufacture
^Magnesium Production and Processing
Bother SF6 Sources*
45.7
3.0
15.0
40.3
2.8
10.9
2.1
3.0
7.0
j.-- Not applicable.
|*lncludes Electrical Transmission and Distribution and, in the case of potential emissions, other miscellaneous sources.
See Annex P for a discussion of sources of SF6 emissions excluded from the actual
emissions estimates in this report.
Industrial Processes 3-33
-------
same year. Although not directly employed, the Norwe-
gian Institute for Air Research (NIAR1993) has reported
a range of emission factors for primary magnesium pro-
duction as being from 1 to 5 kg of SF6 per metric ton of
magnesium. A survey of magnesium die casters has also
reported an average emission factor of 4.1 kg of SF6 per
metric ton of magnesium parts die cast (Gjestland and
Magers 1996).
Data Sources
Emission estimates were provided by the EPA's
Climate Protection Division in cooperation with the U.S.
primary magnesium metal producers and casting firms.
Uncertainty
There are a number of uncertainties in these esti-
mates, including the assumption that SF6 does not react
nor decompose during use. It is possible that the melt
surface reactions and high temperatures associated with
molten magnesium would cause some gas degradation.
As is the case for other sources of SF6 emissions, verifi-
able SF6 consumption data for magnesium production
and processing in United States were not available. The
EPA has recently launched a voluntary partnership with
magnesium producers and casters to reduce emissions of
SF6 from magnesium production and processing. The
EPA anticipates that data provided by magnesium firms
will improve future SF6 emission estimates.
Sulfur hexafluoride may also be used as a covergas
for the casting of molten aluminum with a high magne-
sium content; however, it is uncertain to what extent this
practice actually occurs.
Industrial Sources
of Criteria Pollutants
In addition to the main greenhouse gases addressed
above, many industrial processes generate emissions of
criteria air pollutants. Total emissions of nitrogen ox-
ides (NOX), carbon monoxide (CO), and nonmethane
volatile organic compounds (NMVOCs) from non-en-
ergy industrial processes from 1990 to 1998 are reported
in Table 3-41.
Methodology and Data Sources
The emission estimates for this source were taken
directly from the EPA's National Air Pollutant Emis-
sions Trends, 1900-1998 (EPA 1999). Emissions were
calculated either for individual categories or for many
categories combined, using basic activity data (e.g., the
amount of raw material processed) as an indicator of
emissions. National activity data were collected for in-
dividual categories from various agencies. Depending
on the category, these basic activity data may include
data on production, fuel deliveries, raw material pro-
cessed, etc.
Activity data were used in conjunction with emis-
sion factors, which together relate the quantity of emis-
sions to the activity. Emission factors are generally avail-
able from the EPA's Compilation of Air Pollutant Emis-
sion Factors, AP-42 (EPA 1997). The EPA currently de-
rives the overall emission control efficiency of a source
category from a variety of information sources, includ-
ing published reports, the 1985 National Acid Precipita-
tion and Assessment Program emissions inventory, and
other EPA databases.
Uncertainty
Uncertainties in these estimates are partly due to
the accuracy of the emission factors used and accurate
estimates of activity data.
3-34 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Table 3-41: NOX, CO, and NMVOC Emissions from Industrial Processes (Gg)
Gas/Source
i NOX
Chemical & Allied
Product Manufacturing
i" 'Metals Processing
f Storage and Transport
; Other Industrial Processes
' Miscellaneous*
CO
; Chemical & Allied
r Product Manufacturing
Metals Processing
! Storage and Transport
: Other Industrial Processes
Miscellaneous*
NMVOCs
Chemical & Allied
Product Manufacturing
Metals Processing
Storage and Transport
Other Industrial Processes
Miscellaneous*
* Miscellaneous includes the following
1990
921
152
88
3
343
335
9,502
1,074
2,395
69
487
5,479
3,179
575
111
1,356
364
774
categories:
Storage, and Disposal Facilities under the Resource
agricultural fires or slash/prescribed burning, which
1991
802
149
69
5
319
259
7,088
1,022
2,333
25
497
3,210
2,983
644
112
1,390
355
482
1992
5
1
2
1
785
148
74
4
328
231
,401
,009
,264
15
494
,619
2,811
1
649
113
,436
376
238
catastrophic/accidental
Conservation
1993
774
141
75
4
336
219
5,421
992
2,301
46
538
1,544
2,893
636
112
1,451
401
292
1994
939
145
82
5
353
354
7,708
1,063
2,245
22
544
3,833
3,043
627
114
1,478
397
428
1995
5
1
2
1
2
1
842
144
89
5
362
242
,291
,109
,159
22
566
,435
,859
599
113
,499
409
240
release, other combustion, health
and Recovery Act),
cooling towers,
1996
979
130
69
5
343
433
7,899
668
1,383
72
533
5,242
2,859
332
409
1,193
398
525
services.
and fugitive dust.
are accounted for under the Agricultural Residue Burning
Note: Totals may not sum due to independent rounding.
source.
1997
890
131
70
5
348
334
7,432
676
1,416
73
546
4,721
3,002
332
422
1,211
400
637
1998
915
133
72
5
354
351
7,669
684
1,449
74
559
4,903
3,066
336
435
1,225
409
662
TSDFs (Transnnrt
It does not
include
Industrial Processes 3-35
-------
3-36 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
4. Solvent Use
T:
•he use of solvents and other chemical prod
ucts can result in emissions of various ozone
precursors (i.e., criteria pollutants).1 Nonmethane vola-
tile organic compounds (NMVOCs), commonly referred
to as "hydrocarbons," are the primary gases emitted from
most processes employing organic or petroleum based
solvents, along with small amounts of carbon monoxide
(CO) and oxides of nitrogen (NOX) whose emissions are
associated with control devices used to reduce NMVOC
emissions. Surface coatings accounted for just under a
majority of NMVOC emissions from solvent use—44
percent in 1998—while "non-industrial"2 uses accounted
for about 37 percent and degreasing applications for 8
percent. Overall, solvent use accounted for approxi-
mately 30 percent of total U.S. emissions of NMVOCs-in
1998, and increased less than 1 percent since 1990.
Although NMVOCs are not considered direct
greenhouse gases, their role as precursors to the forma-
tion of ozone—which is a greenhouse gas—results in
their inclusion in a greenhouse gas inventory. Emissions
from solvent use have been reported separately by the
United States to be consistent with the inventory report-
ing guidelines recommended by the IPCC. These guide-
lines identify solvent use as one of the major source
categories for which countries should report emissions.
In the United States, emissions from solvents are prima-
rily the result of solvent evaporation, whereby the lighter
hydrocarbon molecules in the solvents escape into the
atmosphere. The evaporation process varies depending
on different solvent uses and solvent types. The major
categories of solvents uses include: degreasing, graphic
arts, surface coating, other industrial uses of solvents
(i.e., electronics, etc.), dry cleaning, and non-industrial
uses (i.e., uses of paint thinner, etc.). Because many of
these industrial applications also employ thermal incin-
eration as a control technology, CO and NOX combus-
tion by-products are also reported with this source cat-
egory.
Total emissions of nitrogen oxides (NOX),
nonmethane volatile organic compounds (NMVOCs), and
carbon monoxide (CO) from 1990 to 1998 are reported
in Table 4-1.
Methodology
Emissions were calculated by aggregating solvent
use data based on information relating to solvent uses
from different applications such as degreasing, graphic
arts, etc. Emission factors for each consumption category
were then applied to the data to estimate emissions. For
example, emissions from surface coatings were mostly
due to solvent evaporation as the coatings solidify. By
applying the appropriate solvent emission factors to the
type of solvents used for surface coatings, an estimate of
emissions was obtained. Emissions of CO and NOX result
primarily from thermal and catalytic incineration of sol-
1 Solvent usage in the United States also results in the emission of small amounts of hydrofluorocarbons (HFCs) and hydrofluoroethers
(HFEs), which are included under Substitution of Ozone Depleting Substances in the Industrial Processes chapter.
2 "Non-industrial" uses include cutback asphalt, pesticide application adhesives, consumer solvents, and other miscellaneous
applications.
Solvent Use 4-1
-------
Table 4-1: Emissions of NOX, CO, and NMVOC from Solvent Use (Gg)
Hi
-
f
1
(1
!1
,
'_
1
;;
»
-
I
f-
i
i
1,
f"
.
•
Activity
NOX
Degreasing
Graphic Arts
Dry Cleaning
Surface Coating
Other Industrial Processes3
Non-Industrial Processes"
CO
Degreasing
Graphic Arts
Dry Cleaning
Surface Coating
Other Industrial Processes8
Non-Industrial Processes'3
NMVOCs
Degreasing
Graphic Arts
Dry Cleaning
Surface Coating
Other Industrial Processes3
Non-Industrial Processes'"
1990
1
+
+
+
1
+
+
4
+
+
+
+
4
+
5,217
675
249
195
2,289
85
1,724
1991
2
+
+
+
1
+
+
4
+
+
+
1
3
+
5,245
651
273
198
2,287
89
1,746
1992
2
+
1
+
2
+
+
5
+
+
+
1
3
+
5,353
669
280
203
2,338
93
1,771
1 Includes rubber and plastics manufacturing, and other miscellaneous
" Includes cutback asphalt, pesticide application
Note: Totals may not sum due to
4 Does not exceed 0.5 Gg
adhesives,
consumer
1993
2
+
1
+
2
+
+
4
+
+
+
1
3
+
5,458
683
292
204
2,388
93
1,798
applications.
solvents, and
1994
2
+
1
+
2
+
+
5
+
+
1
1
3
+
5,590
703
302
207
2,464
90
1,825
1995
3
+
1
+
2
+
+
5
+
+
1
1
3
+
5,609
716
307
209
2,432
87
1,858
other miscellaneous
1996
3
+
1
+
2
+
+
5
+
+
1
1
3
+
5,569
604
300
171
2,501
87
1,906
1997
3
+
1
+
2
+
+
5
+
+
1
1
3
+
5,672
623
302
173
2,558
88
1,928
1998 ;
2
+ ;
: + ~. •
' + ,
1 .'- •
+ - :
''-+'" "'
5
+ .. j
' + :
1 '..'•'
1 :
3 !
+
5,239
429 ;
304 !
175
2,291
90 .•
1,950 '
-
applications. , - . ;
independent rounding. :
vent laden gas streams from painting booths, printing
operations, and oven exhaust.
Data Sources
The emission estimates for this source were taken
directly from the EPA's National Air Pollutant Emis-
sions Trends, 2900-1998 (EPA 1999). Emissions were
calculated either for individual categories or for many
categories combined, using basic activity data (e.g., the
amount of solvent purchased) as an indicator of emis-
sions. National activity data were collected for individual
applications from various agencies.
Activity data were used in conjunction with emis-
sion factors, which together relate the quantity of emis-
sions to the activity. Emission factors are generally avail-
able from the EPA's Compilation of Air Pollutant Emis-
sion Factors, AP-42 (EPA 1997). The EPA currently de-
rives the overall emission control efficiency of a source
category from a variety of information sources, includ-
ing published reports, the 1985 National Acid Precipita-
tion and Assessment Program emissions inventory, and
other EPA data bases.
Uncertainty
Uncertainties in these estimates are partly due to
the accuracy of the emission factors used and the reli-
ability of correlations between activity data and actual
emissions.
4-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
5. Agriculture
Figure 5-1
i gricultural activities contribute directly to emissions of greenhouse gases through a variety of processes.
iThis chapter includes the following sources: enteric fermentation in domestic livestock, livestock manure
management, rice cultivation, agricultural soil management, and agricultural residue burning (see Figure 5-1). Agri-
culture-related land-use activities, such as conversion of
grassland to cultivated land, are discussed in the Land-
Use Change and Forestry chapter.
In 1998, agricultural activities were responsible
for emissions of 148.4 MMTCE, or 8 percent of total
U.S. greenhouse gas emissions. Methane (CH4) and ni-
trous oxide (N2O) were the primary greenhouse gases
emitted by agricultural activities. Methane emissions
from enteric fermentation and manure management rep-
resent about 19 and 13 percent of total CH4 emissions
from anthropogenic activities, respectively. Of all do-
mestic animal types, beef and dairy cattle were by far the
largest emitters of methane. Rice cultivation and agri-
cultural crop residue burning were minor sources of meth-
ane. Agricultural soil management activities such as fer-
tilizer application and other cropping practices were the
largest source of U.S. N2O emissions, accounting for 71
percent. Manure management and agricultural residue
burning were also smaller sources of N2O emissions.
Table 5-1 and Table 5-2 present emission estimates for the Agriculture chapter. Between 1990 and 1998, CH4
emissions from agricultural activities increased by 19 percent while N2O emissions increased by 12 percent. In
addition to CH4 and N2O, agricultural residue burning was also a minor source of the criteria pollutants carbon
monoxide (CO) and nitrogen oxides (NOX).
Agricultural Soil
Management
Agricultural
Residue Burning
Agriculture 5-1
-------
defined for each category of animal, reflecting the feeds
and forages consumed by cattle type and region. Using
this model, emission factors were derived for each com-
bination of animal type and representative diet. Based
upon the level of use of each diet in the five regions,
average regional emission factors for each of the nine
cattle types were derived.2 These emission factors were
then multiplied by the applicable animal populations
from each region.
For dairy and beef cows and replacements, emis-
sion estimates were developed using regional emission
factors. Dairy cow emission factors were modified to re-
flect changing—primarily increasing—milk production
per cow over time in each region. All other emission
factors were held constant over time. Emissions from
other cattle types were estimated using national average
emission factors.
Emissions estimates for other animal types were
based upon average emission factors representative of
entire populations of each animal type. Methane emis-
sions from these animals accounted for a minor portion
of total methane emissions from livestock in the United
States. Also, the variability in emission factors for each
of these other animal types (e.g., variability by age, pro-
duction system, and feeding practice within each ani-
mal type) is less than that for cattle.
See Annex H for more detailed information on the
methodology and data used to calculate methane emis-
sions from enteric fermentation.
Data Sources
The emission estimates for all domestic livestock
were determined using a mechanistic model of rumen
digestion and emission factors developed hi EPA (1993).
For dairy and beef cows and replacements, regional emis-
sion factors were used from EPA (1993). Emissions from
other cattle types were estimated using national average
emission factors from EPA (1993). Methane emissions
from sheep, goats, pigs, and horses were estimated by
using emission factors utilized in Crutzen et al. (1986)
and annual population data from U.S. Department of
Agriculture statistical reports (USDA 1994a-b, 1995a-d,
1996, 1997, 1998a-c, 1999a-i). These emission factors
are representative of typical animal sizes, feed intakes,
and feed characteristics in developed countries. The
methodology employed in EPA (1993) is the same as
those recommended in IPCC (1997). All livestock popu-
lation data were taken from USDA statistical reports. See
the following section on manure management for a com-
plete listing of reports cited. Table 5-5 provides a sum-
mary of cattle population and milk production data.
Uncertainty
The diets analyzed using the rumen digestion
model include broad representations of the types of feed
consumed within each region. Therefore, the full diver-
sity of feeding strategies employed in the United States
is not represented and the emission factors used may be
biased. The rumen digestion model, however, has been
validated by experimental data. Animal population and
production statistics, particularly for beef cows and other
grazing cattle, are also uncertain. Overall, the uncertainty
in the emission estimate is estimated to be roughly "20
percent (EPA 1993).
Manure Management
The management of livestock manure can produce
anthropogenic methane (CH4) and nitrous oxide (N2O)
emissions. Methane is produced by the anaerobic de-
composition of manure. Nitrous oxide is produced as
part of the nitrogen cycle through the nitrification and
Table 5-5: Cow Populations (Thousands)
and Milk Production (Million Kilograms)
BE":-
•*
p1.
l"l'"
^r
f
s*
~; .
_V^ ;
te»-
Year
1990
: 1991
•1992
1993
-1994
1995
^1996
1997
1998
= Dairy
Population
10,007
""9,883""
9,714
9,679
9,504
9,4'9l
9,410
9,309
9,200
Beef
Cow
Population
: 32,677
"" 32^96CI "
33,453"
34,132
35,101
'35,645
35,509
34,629
34,143
1
Milk
Production
'" "~ 'l:'67,b=06 :
": ~m&5 •"
"6M4T
68,328
69,673
70,440
69,857
70,802
71,415,
: •. ' ; ,n_. ,----",-
2 Feed intake of bulls does not vary significantly by region, so only a national emission factor was derived for this cattle type.
5-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
denitrification of the organic nitrogen in livestock ma-
nure and urine.
When livestock and poultry manure is stored or
treated in systems that promote anaerobic conditions
(e.g., as a liquid in lagoons, ponds, tanks, or pits), the
decomposition of materials in manure tends to produce
methane. When manure is handled as a solid (e.g., in
stacks or pits) or deposited on pastures and range lands,
it tends to decompose aerobically and produce little or
no methane. A number of other factors related to how the
manure is handled also affect the amount of methane
produced: 1) air temperature and moisture affect the
amount of methane produced because they influence
the growth of the bacteria responsible for methane for-
mation; 2) methane production generally increases with
rising temperature and residency time; and 3) for non-
liquid based manure systems, moist conditions (which
are a function of rainfall and humidity) favor methane
production. Although the majority of manure is handled
as a solid, producing little methane, the general trend in
manure management, particularly for dairy and swine
producers, is one of increasing usage of liquid systems.
The composition of the manure also affects the
amount of methane produced. Manure composition var-
ies by animal type and diet. The greater the energy con-
tent and digestibility of the feed, the greater the poten-
tial for methane emissions. For example, feedlot cattle
fed a high energy grain diet generate manure with a high
methane-producing capacity. Range cattle feeding on a
low energy diet of forage material produce manure with
roughly half the methane-producing potential of feed-
lot cattle manure.
The amount of N2O produced depends on the ma-
nure and urine composition, the type of bacteria involved
in the process and the amount of oxygen and liquid in
the manure system. Nitrous oxide emissions result from
livestock manure and urine that is managed using liquid
and slurry systems, as well as manure and urine that is
collected and stored as a solid. Nitrous oxide emissions
from unmanaged livestock manure and urine on pas-
tures, ranges, and paddocks, as well as from manure and
urine that is spread onto fields either directly as "daily
spread," or after it is removed from manure management
systems (e.g., lagoon, pit, etc.) is accounted for and dis-
cussed under Agricultural Soil Management.
Table 5-6, Table 5-7, and Table 5-8 provide esti-
mates of methane and N2O emissions from manure man-
agement by animal category. Estimates for methane emis-
sions in 1998 were 22.9 MMTCE (3,990 Gg), 53 percent
higher than in 1990. The majority of the increase in
methane emissions was from swine and dairy cow ma-
nure and are attributed to shifts by the swine and dairy
industries towards larger facilities. Larger swine and dairy
farms tend to use flush or scrape liquid systems. Thus the
shift towards larger facilities is translated into an in-
creasing use of liquid systems. This shift was accounted
for by incorporating weighted methane conversion fac-
tor (MCF) values calculated from the 1997 farm-size dis-
tribution reported in the 7997 Census of Agriculture
(USDA 1999m). An increase in feed consumption by dairy
cows to maximize milk production is also accounted for
in the estimates. A detailed description of the methodol-
ogy is provided in Annex I.
Total N2O emissions from managed manure sys-
tems in 1998 were estimated to be 4.0 MMTCE (47 Gg).
The 19 percent increase in N2O emissions from 1990 to
1998 can be partially attributed to an increase in the
population of poultry and swine. The population of beef
cattle in feedlots, which tend to use managed manure
systems, also increased. As stated previously, N2O emis-
sions from unmanaged livestock manure is accounted
for under Agricultural Soil Management. Methane emis-
sions were mostly unaffected by this increase in the beef
cattle population because feedlot cattle use solid stor-
age systems, which produce little methane.
Methodology
The methodologies presented in EPA (1993) form
the basis of the methane emissions estimates for each
animal type. The calculation of emissions requires the
following information:
• Amount of manure produced (amount per head times
number of head)
• Portion of the manure that is volatile solids (by ani-
mal type)
• Methane producing potential of the volatile solids
(by animal type)
Agriculture 5-5
-------
Table 5-6: CH4 and N,0 Emissions from Manure Management (MMTCE)
Animal Type
CH.
Dairy Cattle
Beef Cattle
Swine
Sheep
Goats
Poultry
Horses
N20
Dairy Cattle
Beef Cattle
Swine
Sheep
Goats
Poultry
Horses
Total
+ Does not exceed 0.05 MMTCE
Note: Totals may not sum due to
1990 1991
15.0 15.5
4.3 4.3
1.1 1.2
7.9 8.3
+ +
1.5 1.5
0.2 0.2
3.4 3.6
0.1 0.1
1.4 1.6
0.1 0.2
0.0 0.0
0.0 0.0
1.6 1.7
0.1 0.1
18.3 19.1
independent rounding.
1992
16.0
4.4
1.2
8.7
+
1.6
0.2
3.5
0.1
1.5
0.2
0.0
0.0
1.7
0.1
19.6
1993
17.1
4.5
1.2
9.6
+
1.6
0.2
3.7
0.1
1.5
0.2
0.0
0.0
1.8
0.1
20.8
1994
18.8
4.8
1.3
10.8
+
1.7
0.2
3.8
0.1
1.6
0.2
0.0
0.0
1.8
0.1
22.6
1995
19.7
4.9
1.3'
11.6
+
1.7
0.2
3.7
0.1
1.5
0.2
0.0
0.0
1.9
0.1
23.5
1996
20.4
5.1
1.3
12.1
+
1.7
0.2
3.8
0.1
1.5
0.2
0.0
0.0
1.9
0.1
24.3
1997
22.1
5.4
1.3
13.5
+
1.8
0.2
3.9
0.1
1.5
0.2
0.0
0.0
2.0
0.1
26.0
1998
22.9
5.3
1.3
14.2
*
1.8
0.2
4.0
0.1
1.6
0.2
0.0
0.0
2.0
0.1
26.9
Table 5-7: CH4 Emissions from Manure Management (Gg)
Animal Type
Dairy Cattle
Beef Cattle
Swine
Sheep
Poultry
Horses
Total
Note: Totals may not sum due to
1990 1991
747 751
200 205
1,371 1,451
4 4
1 1
261 268
29 29
2,613 2,708
independent rounding.
1992
762
206
1,523
4
1
275
30
2,801
1993
791
212
1,668
3
1
284
30
2,990
1994
843
219
1,894
3
1
292
31
3,283
1995
864
221
2,031
3
1
297
31
3,447
1996
896
229
2,106
3
1
301
31
3,567
1997
941
229
2,349
3
308
31
3,861
1998
933
233
2,475
3
314
31
3,990
Table 5-8: N20 Emissions from Manure Management (Gg)
Animal Type
Dairy Cattle
Beef Cattle
Swine
Sheep
Goats
;<. Poultry
Horses
Total
1990 1991
1.0 1.0
16.7 18.4
1.7 1.8
0.5 0.5
0.1 0.1
19.1 19.8
0.7 0.7
39.8 42.1
1992
1.0
17.2
1.9
0.4
0.1
20.4
0.7
41.7
1993
1.0
18.1
1.9
0.4
0.1
21.0
0.7
43.3
1994
1.1
18.5
2.0
0.4
0.1
21.7
0.7
44.4
1995
1.1
17.6
2.0
0.4
0.1
22.3
0.7
44.2
1996
1.2
18.0
1.9
0.3
0.1
23.0
0.7
45.3
1997
1.2
18.3
2.0
0.3
0.1
23.5
0.8
46.3
1998
1.2
18.9
2.1
0.3
0.1
23.9
0.8
47.3
; Note: Totals may not sum due to independent rounding.
5-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
• Extent to which the methane producing potential is
realized for each type of manure management sys-
tem (by state and manure management system)
• Portion of manure managed in each manure man-
agement system (by state and animal type)
For swine and dairy cattle —the two largest emit-
ters of methane—estimates were developed using state-
level animal population data and average weighted
MCFs for each state. These weighted MCFs were deter-
mined for each farm size category based on the general
relationship between farm sizes and manure system us-
age, where larger facilities will tend to use liquid sys-
tems. These values were further adjusted to harmonize
with emissions reported in EPA (1993). For other animal
types, 1990 state-level emission estimates from the de-
tailed analysis presented in EPA (1993) were scaled by
the change in the state population.
Nitrous oxide emissions were estimated by first
determining manure management system usage. Manure
system usage for swine and dairy cows were based on
assumptions of system usage for the respective popula-
tions' farm size distribution. Total Kjeldahl nitrogen3
production was calculated for all livestock using live-
stock population data and nitrogen excretion rates. Ni-
trous oxide emission factors specific to the type of ma-
nure management system were then applied to total ni-
trogen production to estimate N2O emissions.
See Annex I for more detailed information on the
methodology and data used to calculate methane emis-
sions from manure management. The same activity data
were also used to calculate N2O emissions.
Data Sources
Annual livestock population data for all livestock
types except horses were obtained from the U.S. Depart-
ment of Agriculture's National Agricultural Statistics
Service (USDA 1994a, 1995 a-e, 1996a-b, 1997a-b,
1998a-d, 1999a-k). Horse population data were obtained
from the FAOSTAT database (FAO 1999). Data on farm
size distribution for dairy cows and swine were taken
from the U.S. Department of Commerce (DOC 1995,
1987). Manure management system usage data for other
livestock were taken from EPA (1992). Nitrogen excre-
tion rate data were developed by the American Society
of Agricultural Engineers (ASAE 1999). Nitrous oxide
emission factors were taken from IPCC/UNEP/OECD/
DBA (1997). Manure management systems characterized
as "Other" generally refers to deep pit and litter systems.
The IPCC N2O emission factor for "other" systems (0.005
kg N2O/kg N excreted), was determined to be inconsis-
tent with the characteristics of these management sys-
tems. Therefore, in its place the solid storage/drylot emis-
sion factor was used.
Uncertainty
The primary factors contributing to the uncertainty
in emission estimates are a lack of information on the
usage of various manure management systems in each
state and the exact methane generating characteristics
of each type of manure management system. Because of
significant shifts in the swine and dairy sectors toward
larger farms, it is believed that increasing amounts of
manure are being managed in liquid manure manage-
ment systems. The existing estimates reflect these shifts
in the weighted MCFs based on the 1997 farm-size data.
However, the assumption of a direct relationship between
farm-size and liquid system usage may not apply in all
cases. In addition, the methane generating characteris-
tics of each manure management system type are based
on relatively few laboratory and field measurements, and
may not match the diversity of conditions under which
manure is managed nationally.
The N2O emission factors published in IPCC/
UNEP/OECD/IEA (1997) were also derived using lim-
ited information. The IPCC factors are global averages;
U.S.-specific emission factors may be significantly dif-
ferent. Manure and urine in anaerobic lagoons and liq-
uid/slurry management systems produce methane at dif-
ferent rates, and would in all likelihood produce N2O at
different rates, although a single emission factor was used
for both system types.
' Total Kjeldahl nitrogen is a measure of organically bound nitrogen and ammonia nitrogen.
Agriculture 5-7
-------
Rice Cultivation
Most of the world's rice, and all rice in the United
States, is grown on flooded fields. When fields are
flooded, aerobic decomposition of organic material
gradually depletes the oxygen present in the soil and
floodwater, causing anaerobic conditions in the soil to
develop. Once the environment becomes anaerobic, meth-
ane is produced through anaerobic decomposition of
soil organic matter by methanogenic bacteria. As much
as 60 to 90 percent of the methane produced, however, is
oxidized by aerobic methanotrophic bacteria in the soil
(Holzapfel-Pschorn et al. 1985, Sass et al. 1990). Some
of the methane is also leached away as dissolved meth-
ane in floodwater that percolates from the field. The re-
maining un-oxidized methane is transported from the
submerged soil to the atmosphere primarily by diffusive
transport through the rice plants. Some methane also
escapes from the soil via diffusion and bubbling through
floodwaters.
The water management system under which rice is
grown is one of the most important factors affecting
methane emissions. Upland rice fields are not flooded,
and therefore are not believed to produce methane. In
deepwater rice fields (i.e., fields with flooding depths
greater than one meter), the lower stems and roots of the
rice plants are dead so the primary methane transport
pathway to the atmosphere is blocked. The quantities of
methane released from deepwater fields, therefore, are
believed to be significantly less than the quantities re-
leased from areas with more shallow flooding depths.
Some flooded fields are drained periodically during the
growing season, either intentionally or accidentally. If
water is drained and soils are allowed to dry sufficiently,
methane emissions decrease or stop entirely. This is due
to soil aeration, which not only causes existing soil meth-
ane to oxidize but also inhibits further methane produc-
tion in soils. All rice in the United States is grown under
continuously flooded conditions; none is grown under
deepwater conditions.
Other factors that influence methane emissions
from flooded rice fields include fertilization practices
(especially the use of organic fertilizers,) soil tempera-
ture, soil type, cultivar selection, and cultivation prac-
tices (e.g., tillage, and seeding and weeding practices).
The factors that determine the amount of organic mate-
rial that is available to decompose, i.e., organic fertilizer
use, soil type, cultivar type4, and cultivation practices,
are the most important variables influencing methane
emissions over an entire growing season because the
total amount of methane released depends primarily on
the amount of organic substrate available. Soil tempera-
ture is known to be an important factor regulating the
activity of methanogenic bacteria, and therefore the rate
of methane production. However, although temperature
controls the amount of time it takes to convert a given
amount of organic material to methane, that time is short
relative to a growing season, so the dependence of emis-
sions over an entire growing season on soil temperature
is weak. The application of synthetic fertilizers has also
been found to influence methane emissions; in particu-
lar, both nitrate and sulfate fertilizers (e.g., ammonium
nitrate, and ammonium sulfate) appear to inhibit meth-
ane formation. In the United States, soil types, soil tem-
peratures, cultivar types, and cultivation practices for
rice vary from region to region, and even from farm to
farm. However, most rice farmers utilize organic fertiliz-
ers in the form of rice residue from the previous crop,
which is left standing, disked, or rolled into the fields.
Most farmers also apply synthetic fertilizer to their fields,
usually urea. Nitrate and sulfate fertilizers are not com-
monly used in rice cultivation in the United States. In
addition, the climatic conditions of Arkansas, southwest
Louisiana, Texas, and Florida allow for a second, or ra-
toon, rice crop. This second rice crop is produced on the
stubble after the first crop has been harvested. Because
the first crop's stubble is left behind in ratooned fields,
the amount of organic material that is available for de-
composition is considerably higher than with the first
(i.e., primary) crop. Methane emissions from ratoon crops
have been found to be considerably higher than those
from the primary crop.
Rice cultivation is a small source of methane emis-
sions in the United States (2 percent). Rice is cultivated
The roots of rice plants shed organic material. The amount of root exudates produced varies among cultivar types.
5-8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
in seven states: Arkansas, California, Florida, Louisiana,
Mississippi, Missouri, and Texas. Estimates of total an-
nual CH4 emissions from rice cultivation range from 2.3
to 2.7 MMTCE (404 to 476 Gg CH4) for the years 1990
to 1998 (Table 5-9 and Table 5-10). There was no appar-
ent trend over the nine year period, although total emis-
sions increased by 15 percent between 1990 and 1998
due to an increase in harvested area.
The factors that affect the rice area harvested vary
from state to state.5 In Florida, the state having the small-
est harvested rice area, rice acreage is largely a function
of sugarcane acreage. Sugarcane fields are flooded each
year to control pests, and on this flooded land a rice crop
is grown along with a ratoon crop of sugarcane
(Schueneman 1997). In Missouri, rice acreage is affected
by weather (e.g., rain during the planting season may
prevent the planting of rice), the price differential be-
tween soybeans and rice (e.g., if soybean prices are higher,
then soybeans may be planted on some of the land which
would otherwise have been planted in rice), and govern-
ment support programs (Stevens 1997). The price differ-
ential between soybeans and rice also affects rice acre-
age in Mississippi. Rice in Mississippi is usually rotated
with soybeans, but if soybean prices increase relative to
rice prices, then some of the acreage that would have
been planted in rice, is instead planted in soybeans (Street1
1997). In Texas, rice production, and thus, harvested area,
are affected by both government programs and the cost
of production (Klosterboer 1997). California rice area is
influenced by water availability as well as government
programs and commodity prices. In Louisiana, rice area
is influenced by government programs, weather condi-
tions (e.g., rainfall during the planting season), as well
as the price differential between rice and corn and other
crops (Saichuk 1997). Arkansas rice area has been influ-
enced in the past by government programs. However,
Table 5-9: CH4 Emissions from Rice Cultivation (MMTCE)
i; State
r '
: Arkansas
i California
i . Florida
Louisiana
I Mississippi
i; Missouri
f Texas
^ Total
1990 1991
0.7 0.7
0.4 0.4
• + +
0.7 0.7
0.1 0.1
0.1 0.1
0.3 0.3
2.4 2.3
1992
0.8
0.4
+
0.8
0.1
0.1
0.3
2.6
1993
0.7
0.5
+
0.7
0.1
0.1
0.3
2.4
1994
0.8
0.5
+
0.8
0.1
0.1
0.3
2.7
1995
0.8
0.5
+
0.8
0.1
0.1
0.3
2.6
1996
0.7
0.5
+
0.7
0.1
0.1
0.3
2.4
1997
0.8
0.5
+
0.8
0.1
0.1
0.2
2.6
1998
0.9
0.5
+
0.8
0.1
0.1
0.3
2.7
r+ Does not exceed 0.05 MMTCE
| Note: Totals may
Table 5-1 0:CH
; State
1 Arkansas
1 California
I Florida -
^Louisiana
^Mississippi
^Missouri
(Texas
iTotal
ifNote: Totals may
not sum due to independent rounding.
4 Emissions from Rice Cultivation (Gg)
1990 1991
121 127
72 65
3 4
127 119
26 23
10 12
. 55 54
414 404
not sum due to independent rounding.
1992
139
72
5
145
23
14
55
453
1993
124
80
5
124
23
12
47,
414
1994
143
89
5
145
23
16
55
476
1995
135
85
5
133
23
14
50
445
1996
118
91
5
125
23
12
47
420
1997
140
94
4
136
23
15
40
453
1998
154
87
4
145
23
18
44
476
4
The statistic "area harvested" accounts for double cropping, i.e., if one hectare is cultivated twice in one year, then that hectare is
counted as two hectares harvested.
Agriculture 5-9
-------
due to the phase-out of these programs nationally, which
began in 1996, spring commodity prices have had a
greater effect on the amount of land planted in rice in
recent years (Mayhew 1997).
Methodology
The Revised 1996IPCC Guidelines (IPCC/UNEP/
OECD/IEA1997) recommend applying a seasonal emis-
sion factor to the annual harvested rice area to estimate
annual CH4 emissions. This methodology assumes that
a seasonal emission factor is available for all growing
conditions. Because season lengths are quite variable
both within and among states in the United States, and
because flux measurements have not been taken under
all growing conditions in the United States, an earlier
IPCC methodology (IPCC/UNEP/OECD/IEA 1995) has
been applied here, using season lengths that vary slightly
from the recommended approach. The 1995 IPCC Guide-
lines recommend multiplying a daily average emission
factor by growing season length and annual harvested
area. The IPCC Guidelines suggest that the "growing"
season be used to calculate emissions based on the as-
sumption that emission factors are derived from mea-
surements over the whole growing season rather than
just the flooding season. Applying this assumption to
the United States, however, would result in an overesti-
mate of emissions because the emission factors devel-
oped for the United States are based on measurements
over the flooding, rather than the growing, season. There-
fore, the method used here is based on the number of
days of flooding during the growing season, and a daily
average emission factor, which is multiplied by the har-
vested area. Agricultural extension agents in each of the
seven states in the United States that produce rice were
contacted to determine water management practices and
flooding season lengths in each state. Although all con-
tacts reported that rice growing areas were continuously
flooded, flooding season lengths varied considerably
among states; therefore, emissions were calculated sepa-
rately for each state.
Emissions from ratooned and primary areas are es-
timated separately. Information on ratoon flooding sea-
son lengths was collected from agricultural extension
agents in the states that practice ratooning, and emis-
sion factors for both die primary season and the ratoon
season were derived from published results of field ex-
periments in the United States.
Data Sources
The harvested rice areas for the primary and ratoon
crops in each state are presented in Table 5-11. Data for
all states except Florida for 1990 through 1995 were
taken from U.S. Department of Agriculture's National
Agriculture Statistics Data—Historical Data (USDA
1999b). The data for 1996 through 1998 were obtained
from the Crop Production 1998 Summary (USDA 1999a).
Harvested rice areas in Florida from 1990 to 1998 were
obtained from Tom Schueneman (1999b, 1999c), a
Florida Agricultural Extension Agent. Acreages for the
ratoon crops were derived from conversations with the
agricultural extension agents in each state. In Arkansas,
ratooning occurred only in 1998, when the ratooned area
was less than 1 percent of the primary area (Slaton 1999a).
In the other three states in which ratooning is practiced
(i.e., Florida, Louisiana, and Texas), the percentage of
the primary area that was ratooned was constant over the
entire 1990 to 1998 period. In Florida, the ratooned area
was 50 percent of the primary area (Schueneman 1999a),
in Louisiana it was 30 percent (Linscombe 1999a), and
in Texas it was 40 percent (Klosterboer 1999a),
Information about flooding season lengths was
obtained from agricultural extension agents in each state
(Beck 1999, Guethle 1999, Klosterboer 1999b,
Linscombe 1999b, Scardaci 1999a and 1999b,
Schueneman 1999b, Slaton 1999b, Street 1999a and
1999b). These data are presented in Table 5-12.
To determine what daily methane emission factors
should be used for the primary and ratoon crops, meth-
ane flux information from all the rice field measurements
made in the United States was collected. Experiments in
which nitrate and sulfate fertilizers, or other substances
known to suppress methane formation, were applied, as
well as experiments in which measurements were not
made over an entire flooding season or in which flood-
waters were drained mid-season, were excluded from the
analysis. This left ten field experiments from California
(Cicerone et al. 1992), Texas (Sass et al. 1990, 1991a,
1991b, 1992), and Louisiana (Lindau et al. 1991, Lindau
5-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
and Bollich 1993, Lindau et al. 1993, Lindau et al. 1995, tilizer did not vary among experiments. In contrast, all
Lindau et al. 1998).6 These experimental results were the ratooned fields that received synthetic fertilizer had
then sorted by season and type of fertilizer amendment emission rates that were higher than the one ratoon ex-
(i.e., no fertilizer added, organic fertilizer added, and periment in which no synthetic fertilizer was applied.
synthetic and organic fertilizer added). The results for Given these results, the highest and lowest emission rates
the primary crop showed no consistent correlation be- measured in primary fields that received synthetic fertil-
tween emission rate and type or magnitude of fertilizer izer only — which bounded the results from fields that
application. Although individual experiments have
shown a significant increase in emissions when organic Tab|e 5.-| 2: Rjce Flooding Season Lengths (Days)
fertilizers are added, when the results were combined,
emissions from fields that receive organic fertilizers were
not found to be, on average, higher that those from fields
that receive synthetic fertilizer only. In addition, there
appeared to be no correlation between fertilizer applica-
tion rate and emission rate, either for synthetic or or-
-
ganic fertilizers. These somewhat surprising results are
probably due to other variables that have not been taken
into account, such as timing and mode of fertilizer ap-
plication, soil type, cultivar type, and other cultivation
practices. There were limited results from ratooned fields. 1
Of those that received synthetic fertilizers, there was no
consistent correlation between emission rate and amount j
of fertilizer applied, however, the type of synthetic fer- i
Table 5-1 1 : Rice Areas Harvested (Hectares)
f-State/Crop 1990 1991 1992 1993
EMansas
t Primary 485,633 509,915 558,478 497,774
L Ratoon* NA NA NA NA
il California 159,854 144,071 159,450 176,851
ffloricla
jT Primary 4,978 8,580 9,308 9,308
;;, Ratoon 2,489 4,290 4,654 4,654
§ Louisiana
;: Primary 220,558 206,394 250,911 214,488
•%_ Ratoon 66,168 61,918 75,273 64,346
I Mississippi 101,174 89,033 111,291 99,150
^Missouri 32,376 37,232 45,326 37,637
I: Texas
t Primary 142,857 138,810 142,048 120,599
|: Ratoon 57,143 55,524 56,819 48,240
t Total 1,273,229 1,255,767 1,413,557 1,273,047 1
JLNote: Totals may not sum due to independent rounding.
|* Arkansas ratooning only occurred in 1998.
r
«- ~ -----
£. Stale/Crop
£_ ' -
± Arkansas
L- Primary
Ratoon
t. California
ifci Florida
E3— ----. •.-•.. '-•-..-;;--
- Pnmary
iS i Ratoon
~- Louisiana
IT;- Primary. ...
pa .- Ratoon
fv= Mississippi
Jt~,; Missouri
~^_L Jexas
s,,, Primary
r;- Ratoon
K'-'::-- -,'• . ;• - .:'.'•" ' , -
1994 1995
574,666 542,291
NA NA
196,277 188,183
9,713 9,713
4,856 4,856
250,911 230,676
75,273 69,203
126,669 116,552
50,182 45,326
143,262 128,693
57,305 51,477
,489,114 1,386,969
Low
60
30
100
90
"40
90
70
68
.. 80
60
40
1996
473,493
NA
202,347
8,903
4,452
215,702
64,711
84,176
38,446
120,599
48,240
1,261,068
High
80
40
145
110
60
120
75
82
. 100
80
60
1997 1998
562,525 617,159
NA 202
208,822 193,444
7,689 8,094
3,845 4,047
235,937 250,911
70,781 75,273
96,317 108,458
47,349 57,871
104,816 114,529
41,926 45,811
1,380,008 1,475,799
-3
i
If
— .=»
11
-
*
•rf
;
4
s
""S
,,:
\
-1.
J
6 In some of these remaining experiments, measurements from individual plots were excluded from the analysis because of the reasons
just mentioned. In addition, one measurement from the ratooned fields (i.e., the flux of 2.041 g/m2/day in Lindau and Bollich 1993)
was excluded since this emission rate is unusually high compared to other flux measurements in the United States as well as in Europe
and Asia (IPCC/UNEP/OECD/IEA 1997).
Agriculture 5-11
-------
received both synthetic and organic fertilizers—was used
as the emission factor range for the primary crop, and the
lowest and highest emission rates measured in all the
ratooned fields was used as the emission factor range for
the ratoon crop. These ranges are 0.020 to 0.609 g/m2-
day for the primary crop, and 0.301 to 0.933 g/m2-day
for the ratoon crop.
Uncertainty
The largest uncertainty in the calculation of CH4
emissions from rice cultivation is associated with the emis-
sion factors applied. Daily average emissions, derived
from field measurements in the United States, vary by
more than one order of magnitude (IPCC/UNEP/OECD/
IEA 1997). This variability is due to differences in culti-
vation practices, particularly the type, amount, and mode
of fertilizer application; differences in cultivar type; and
differences in soil and climatic conditions. By separat-
ing primary from ratooned areas, this Inventory has ac-
counted for more of this variability than previous inven-
tories. However, a range for both the primary (0.315 g/
m2day ±93 percent) and ratoon crop (0.617 g/m2day ±51
percent) has been used in these calculations to reflect the
remaining uncertainty. Based on this range, total meth-
ane emissions from rice cultivation in 1998 were esti-
mated to have been approximately 0.43 to 5.0 MMTCE
(75 to 876 Gg 0*4), or 2.7 MMTCE ±84 percent.
Another source of uncertainty is in the flooding
season lengths used for each state. Hooding seasons in
each state may fluctuate from year to year, and thus a
range has been used to reflect this uncertainty. Even
within a state, flooding seasons can vary by county and
cultivar type (Linscombe 1999a).
The last source of uncertainty is in the practice of
flooding outside of the normal rice season. According to
the agriculture extension agents, all of the rice-growing
states practice this on some part of their rice acreage,
ranging from 5 to 33 percent of the rice acreage. Fields
are flooded for a variety of reasons: to provide habitat
for waterfowl, to provide ponds for crawfish production,
and to aid in rice straw decomposition. To date, methane
1 Nitrification is the aerobic microbial oxidation of ammonium to nitrate, and denitrification is the anaerobic microbial reduction of
nitrate to dinitrogen gas (IPCC/UNEP/OECD/IEA 1997). Nitrous oxide is a gaseous intermediate product in the reaction sequences of
both processes, which leaks from microbial cells into the soil atmosphere.
flux measurements have not been undertaken in these
flooded areas.
As scientific understanding improves, these emis-
sion estimates will be adjusted to better reflect these
variables.
Agricultural Soil Management
Nitrous oxide (N2O) is produced naturally in soils
through the microbial processes of nitrification and deni-
trification.7 A number of agricultural activities add ni-
trogen to soils, thereby increasing the amount of nitro-
gen available for nitrification and denitrification, and
ultimately the amount of N2O emitted. These activities
may add nitrogen to soils either directly or indirectly.
Direct additions occur through various soil management
practices (i.e., application of synthetic and organic fer-
tilizers, application of sewage sludge, application of
animal wastes, production of nitrogen-fixing crops, ap-
plication of crop residues, and cultivation of high or-
ganic content soils, which are also called histosols), and
through animal grazing (i.e., direct deposition of animal
wastes on pastures, range, and paddocks by grazing ani-
mals). Indirect additions occur through two mechanisms:
1) volatilization of applied nitrogen (i.e., fertilizer, sew-
age sludge and animal waste) as ammonia (NH3) and
oxides of nitrogen (NOX) and subsequent atmospheric
deposition of that nitrogen in the form of ammonium
(NH4) and oxides of nitrogen (NOX); and 2) surface run-
off and leaching of applied nitrogen into aquatic sys-
tems. Figure 5-2 illustrates these sources and pathways
of nitrogen additions to soils in the United States. Other
agricultural soil management practices, such as irriga-
tion, drainage, tillage practices, and fallowing of land,
can affect fluxes of N2O, as well as other greenhouse
gases, to and from soils. However, because there are sig-
nificant uncertainties associated with these other fluxes,
they have not been estimated.
Estimates of annual N2O emissions from agricul-
tural soil management range from 75.3 to 83.9 MMTCE
5-12 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Figure 5-2
Sources of N20 Emissions from Agricultural Soils
N Flows:
N Inputs
Direct N20 Emissions
Geophysical Processes
Indirect NaO Emissions
(891 to 992 Gg) for the years 1990 to 1998 (Table 5-13
and'Table 5-14).8 Emission levels fluctuated moderately
during the 1990 to 1993 period, increased sharply in
1994, and fluctuated again through 1998. These fluc-
tuations are largely a reflection of annual variations in
synthetic nitrogen fertilizer consumption and crop pro-
duction. Synthetic nitrogen fertilizer consumption, and
production of corn and most beans and pulses, increased
in 1994 due to the 1993 flooding of the North Central
region and the intensive cultivation that followed. From
1997 to 1998, N2O emission estimates decreased by 0.4
percent. Over the nine-year period, total emissions of
N2O increased by approximately 11 percent.
This N2O source category is divided into
three components: (1) direct emissions from
managed soils due to N applications and culti-
vation of histosols; (2) direct emissions from
managed soils due to grazing animals; and (3)
emissions from soils indirectly induced by ap-
plications of nitrogen. Except where specifi-
cally noted, the emission estimates for all three
components follow the methodologies in the
Revised 1996IPCC Guidelines (IPCC/UNEP/
OECD/ffiA 1997).
Direct N20 Emissions from Agricultural Soils
Estimates of N2O emissions from this
component are based on the total amount of
nitrogen that is applied to, or made available
to—in the case of histosol cultivation—soils
through various practices. The practices are:
(1) the application of synthetic and organic
fertilizers, (2) the application of sewage sludge,
(3) the application of livestock and poultry
waste through both daily spread and eventual
application of wastes that had been managed
in waste management systems (e.g., lagoons),
(4) the production of nitrogen-fixing crops, (5)
the application of crop residues, and (6) the
cultivation of histosols.
Annual synthetic and organic fertilizer consump-
tion data for the United States were taken from annual
publications on commercial fertilizer statistics (AAPFCO
1995, 1996, 1997, 1998; TVA 1990, 1992a,b, 1994).
Organic fertilizers included in these publications are
manure, compost, dried blood, sewage sludge, tankage9,
and "other". The manure portion of the organic fertiliz-
ers was subtracted from the total organic fertilizer con-
sumption data to avoid double counting10. Fertilizer
consumption data are recorded in "fertilizer year" totals
(i.e., July to June), which were converted to calendar
year totals by assuming that approximately 35 percent
Note that these emission estimates include applications of N to all soils, but the phrase "Agricultural Soil Management" is kept for
consistency with the reporting structure of the Revised 1996 IPCC Guidelines.
9 Tankage is dried animal residue, usually freed from fat and gelatin.
10 The manure used in commercial fertilizer is accounted for when estimating the total amount of animal waste nitrogen applied to soils.
Agriculture 5-13
-------
Table 5-13: N,0 Emissions from Agricultural Soil Methodology and Data Sources Management (MMTCE)
1990 1991 1992 1993 1994 1995 1996 1997 1998
Direct
Agricultural Soils
Grazing Animals
Indirect
Total
42.7
10.3
22.4
75.3
43.3
10.3
22.7
76.3
44.7
10.6
23.0
78.2
43.0
10.7
23.6
77.3
48.3
10.9
24.3
83.5
45.3
11.1
24.0
80.4
47.1
11.0
24.3
82.4
49.3
10.7
24.3
84.2
49.2
10.5
24.2
83.9
Note: Totals may not sum due to independent rounding.
Table 5-14: N20 Emissions from Agricultural Soil Management (Gg)
1990
1991 1992 1993 1994 1995 1996 1997 1998
Direct
Agricultural Soils
Grazing Animals
Indirect
Total
505
121
265
891
512
122
269
903
528
125
272
925
509
126
279
914
571
129
287
988
536
131
284
951
557
130
288
975
583
126
287
996
581
124
287
992
Note: Totals may not sum due to independent rounding.
of fertilizer usage occurred from July to December (TVA
1992b). July to December values were not available for
calendar year 1998, so a "least squares line" statistical
test using the past eight data points was used to arrive at
an approximate total. Data on the nitrogen content of
synthetic fertilizers were available in the published fer-
tilizer reports; however, these reports did not include
nitrogen content information for organic fertilizers. It
was assumed that 4.1 percent of non-manure organic fer-
tilizers on a mass basis was nitrogen (Terry 1997). An-
nual consumption of commercial fertilizers—synthetic
and non-manure organic—in units of nitrogen are pre-
sented in Table 5-15. The total amount of nitrogen con-
sumed from synthetic and non-manure organic fertiliz-
ers was reduced by 10 percent and 20 percent, respec-
tively, to account for the portion that volatilizes to NH3
and NOX (TPCC/UNEP/OECD/IEA1997).
Data collected by the U.S. Environmental Protec-
tion Agency (EPA) were used to derive annual estimates
of nitrogen additions from land application of sewage
sludge. Sewage sludge is generated from the treatment
of raw sewage in public or private wastewater treatment
works. Based on a 1988 questionnaire returned from 600
publicly owned treatment works (POTWs), the EPA esti-
mated that 5.4 million metric tons of dry sewage sludge
were generated in the United States in that year (EPA
1993). Of this total, 36 percent was applied to land—
including agricultural applications, compost manufac-
ture, forest land application, and the reclamation of min-
ing areas—34.0 percent was disposed in landfills, 10.3
percent was surface-disposed (in open dumps), 16.1 per-
cent was incinerated, and 6.3 percent was dumped into
the oceans (EPA 1993). In 1997, the EPA conducted a
nationwide state-by-state study that estimated that ap-
proximately 7 million metric tons of dry sewage sludge
were generated by 12,000 POTWs (Bastian 1999). The
same study concluded that 54 percent of sewage sludge
generated that year was applied to land. Sewage sludge
production increased between 1988 and 1997 due to
increases in the number of treatment plants and the mag-
nitude of industrial wastewater treated, as well as changes
in sewage treatment techniques. The proportion of sew-
age sludge applied to land increased due to the passage
of legislation in 1989 that banned all ocean dumping of
sewage, as well as stricter laws regulating the use of land-
fills for sewage disposal (Bastian 1999). To estimate sew-
age sludge production for the 1990 to 1998 period, the
values for 1988 and 1997 were linearly interpolated. To
5-14 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
estimate the proportion of sewage sludge that was ap-
plied to land, the values for 1988 and 1992 were linearly
interpolated; the 1992 value was estimated by assuming
all sewage sludge dumped in the ocean before 1992 was
land applied that year (i.e., 1991 was the last year ocean
dumping of sludge occurred). A second interpolation
was then calculated for the period 1992 to 1997 using
the 1997 value and the 1992 estimate. The rate of sew-
age sludge production destined for land application is
currently leveling off (Bastian 1999); in the absence of
more precise data for 1998, the 1997 estimate was used
for 1998. Anywhere between 1 to 6 percent of dry weight
sewage sludge is nitrogen, both in organic and inorganic
form (National Research Council 1996); 4 percent was
used as a conservative average estimate of the nitrogen
content in sewage sludge. Annual land application of
sewage sludge in units of nitrogen is presented in Table
5-15. As with non-manure organic fertilizer applications
to managed soils, it was assumed that 20 percent of the
sewage sludge nitrogen volatilizes. A portion of sewage
sludge is used as commercial fertilizer; application of
this nitrogen and associated N2O emissions are accounted
for under the organic fertilizer application category.
To estimate the amount of livestock and poultry
waste nitrogen applied to soils, it was assumed that all of
it will eventually be applied to soils with two excep-
tions. These exceptions are (1) the nitrogen in the poul-
try waste that is used as feed for ruminants (i.e., approxi-
mately 10 percent of the poultry waste), and (2) the ni-
trogen in the waste that is directly deposited onto fields
by grazing animals.11 Annual animal population data
for all livestock types, except horses, were obtained from
the USDANational Agricultural Statistics Service (USDA
1994b,c, 1995a,b, 1996a,b, 1997a,b, 1998a,b; 1999a-
g,i-m). Horse population data were obtained from the
FAOSTAT database (FAO 1999). Population data by ani-
mal type were multiplied by an average animal mass
constant (ASAE 1999) to derive total animal mass for
each animal type. Total Kjeldahl nitrogen12 excreted
per year (i.e., manure and urine) was then calculated us-
ing daily rates of nitrogen excretion per unit of animal
mass (ASAE 1999) (Table 5-16). The amount of animal
waste nitrogen directly deposited by grazing animals—
derived using manure management system usage data
and farm size (Safely et al. 1992, DOC 1995) as described
in the "Direct N2O Emissions from Grazing Animals"
section—was then subtracted from the total nitrogen.
Ten percent of the poultry waste nitrogen produced in
managed systems and used as feed for ruminants was
then subtracted. Finally, the total amount of nitrogen
from livestock and poultry waste applied to soils was
then reduced by 20 percent to account for the portion
that volatilizes to NH3 and NOX (IPCC/UNEP/OECD/
IEA 1997).
Table 5-15: Commercial Fertilizer Consumption &
Land Application of Sewage Sludge (Thousand Metric Tons of Nitrogen)
F ~ "-—
^Fertilizer Type
tSynthetic
pipn-Manure Organics
pSewage Sludge
| Note: The sewage sludge figures do
%
1990 1991 1992 1993 1994 1995
10,104 10,261 10,324 10,718 11,161 10,799
8 12 13 .11 11 14
94 103 112 120 127 135
not include sewage sludge used as commercial fertilizer.
1996
11,158
15
143
1997
11,172
15
151
1998
11,156
16
151
Table 5-16: Animal Excretion from Livestock and Poultry (Thousand Metric Tons of Nitrogen)
EotetMty
tr-:
EApplied to Soils
fPasture, Range, & Paddock '
f-:
1990 1991 1992 1993 1994 1995
3,695 3,804 3,812 3,864 3,933 3,913
4,830 4,850 4,972 5,021 5,132 5,221
1996
3,890
5,170
^^j^:^
1997
3,972
5,029
— ^---— -
1998
3,890
4,923
_= _^__.
11 An additional exception is the nitrogen in the waste that will runoff from waste management systems due to inadequate management.
There is insufficient information with which to estimate this fraction of waste nitrogen.
Total Kjeldahl nitrogen is a measure of organically bound nitrogen and ammonia nitrogen.
Agriculture 5-15
-------
Annual production statistics for some of the nitro-
gen-fixing crops (i.e., beans, pulses, and alfalfa) were
taken from U.S. Department of Agriculture reports (USDA
1994a, 1997c, 1998c, 1999h). These statistics are pre-
sented in Table 5-17. Crop product values for beans and
pulses were expanded to total crop dry biomass, in mass
units of dry matter, by applying residue to crop ratios
and dry matter fractions for residue from Strehler and
Stiitzle (1987). Crop production for the alfalfa were con-
verted to dry matter mass units by applying a dry matter
fraction value estimated at 80 percent (Mosier 1998). To
convert to units of nitrogen, it was assumed that 3 per-
cent of the total crop dry mass for all crops was nitrogen
(IPCC/UNEP/OECD/EEA1997).
There are no published annual production statis-
tics for non-alfalfa legumes used as forage in the United
States (i.e., red clover, white clover, birdsfoot trefoil,
arrowleaf clover, crimson clover, hairy vetch). Estimates
of average annual crop coverage density and crop area
were obtained through personal communications with
agricultural extension agents or faculty at agronomy and
soil science departments of universities. The estimates
of dry matter crop coverage density were obtained
through on-site experiment and measurement results
(Smith 1999, Peterson 1999, Mosjidis 1999). Estimates
of average annual crop areas at the national level are
reported in Taylor and Smith (1995). Estimates of an-
nual crop production were derived by multiplying the
crop coverage densities by the crop areas. Total nitrogen
content was estimated in the same manner as for alfalfa.
Annual production estimates for non-alfalfa forage le-
gumes are presented in Table 5-17.
To estimate the amount of nitrogen applied to soils
as crop residue, it was assumed that all residues from
corn, wheat, bean, and pulse production, except the frac-
tions that are burned in the field after harvest, were ei-
ther plowed under or left on the field.13 Annual produc-
tion statistics were taken from U.S. Department of Agri-
culture (USDA 1994a, 1997c, 1998c, 1999h). These sta-
tistics are presented in Table 5-17 and Table 5-18. Crop
residue biomass, in dry matter mass units, was calcu-
lated from the production statistics by applying residue
Table 5-17: Nitrogen Fixing Crop Production (Thousand Metric Tons of Product)
Product Type
Soybeans
Peanuts
Dry Edible Beans
Dry Edible Peas
Austrian Winter Peas
Lentils
Wrinkled Seed Peas
Alfalfa
Red Clover
White Glover
; Birdsfoot Trefoil
ii Arrowleaf Clover
'- Crimson Clover
Hairy Vetch
1990
52,416
1,635
1,469
108
6
66
42
75,671
62,438
40,700
12,375
2,044
818
500
1991
54,065
2,235
1,532
169
6
104
42
75,585
62,438
40,700
12,375
2,044
818
500
1992
59,612
1,943
1,026
115
4
71
24
71,795
62,438
40,700
12,375
2,044
818
500
1993
50,885
1,539
994
149
7
91
39
72,851
62,438
40,700
12,375
2,044
818
500
1994
68,444
1,927
1,324
102
2
84
34
73,787
62,438
40,700
12,375
2,044
818
500
1995
59,174
1,570
1,398
209
97
48
76,671
62,438
40,700
12,375
2,044
818
500
1996
64,780
1,661
1,268
121
60
25
72,137
62,438
40,700
12,375
2,044
818
500
1997
73,176
1,605
1,332
264
108
31
71,887
62,438
40,700
12,375
2,044
818
500
1998
75,028
1,783
1,398
269
88
31
74,398
62,438
40,700
12,375
2,044
818
500
Table 5-18: Corn and Wheat Production (Thousand Metric Tons of Product)
i Product Type 1990 1991 1992
1993
1994 1995
1996 1997
1998
Corn for Grain
Wheat
201534 189868 240,719 160,986 255,295187,970 234,518 233,864 247,943
74292 53,891 67,135 65,220 63,167 59,404 61,980 67,534 69,410
13 Although residue application mode would probably affect the magnitude of emissions, a methodology for estimating N2O emissions
for these two practices separately has not been developed yet.
5-16 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
to crop mass ratios and dry matter fractions for residue
from Strehler and Stiitzle (1987). For wheat and corn,
nitrogen contents were taken from Barnard and
Kristoferson (1985). For beans and pulses, it was assumed
that 3 percent of the total crop residue was nitrogen (IPCC/
UNEP/OECD/ffiA 1997). The crops whose residues were
burned in the field are corn, wheat, soybeans, and pea-
nuts. For these crop types, the total residue nitrogen was
reduced by 3 percent to subtract the fractions burned in
the field (see the Agricultural Residue Burning section).
Total crop nitrogen in the residues returned to soils
was then added to the unvolatilized applied nitrogen
from commercial fertilizers, sewage sludge, and animal
wastes, and the nitrogen fixation from bean, pulse, al-
falfa and non-alfalfa forage legume cultivation. The sum
was multiplied by the IPCC default emission factor
(0.0125 kg N2O-N/kg N applied) to estimate annual N2O
emissions from nitrogen applied to soils.
Statistics on the area of histosols cultivated each
year were not available; however, estimates for the years
1982 and 1992 were available from National Resources
Inventory (USDA 1994d). The area statistics for 1982
and 1992 were linearly interpolated to obtain area esti-
mates for 1990 and 1991, and linearly extrapolated to
obtain area estimates for 1993 to 1998 (Table 5-19). To
estimate annual N2O emissions from histosol cultiva-
tion, the histosol areas were multiplied by the default
emission factor (8 kg N2O-N/ha cultivated) recommended
in the draft IPCC paper on "good practice" in imple-
Table 5-19: Histosol Area Cultivated
(Thousand Hectares)
•--- Year
i- •.,.,,.
: - 1990
! 1991
~ 1992
1993
1994
r 1995
1996
: 1997
7 1998 ,
Area
1,013
1,005
998
. . - ' . . 991 .
984
976
969
_962
955
menting the Revised 1996 IPCC Guidelines (IPCC
1999a). This recommended emission factor is based on
the results of recent measurements that indicate that ni-
trous oxide emissions from cultivated organic soils in
mid-latitudes are higher than previously estimated.
Annual N2O emissions from nitrogen applied to soils
were then added to annual N2O emissions from histosol
cultivation to estimate total annual direct N2O emissions
from agricultural cropping practices (Table 5-20).
Direct N20 Emissions from Grazing Animals
Estimates of N2O emissions from this component
were based on animal wastes that are not used as animal
feed, or applied to soils, or managed in manure manage-
ment systems, but instead are deposited directly on soils
by animals in pastures, range, and paddocks.14 It was
assumed that all unmanaged wastes fall into this cat-
Table 5-20: Direct N20 Emissions from Agricultural Cropping Practices (MMTCE)
I Activity
fComm. Fertilizers & Sew.
l&DimaLWaste
p Fixation
| Crop Residue
j- (distosol Cultivation
Ifotal
1990
Sludge 15.2
4.9
15.1
6.4
1.1
42.7
1991
15.5
5.1
15.3
6.3
1.1
43.3
1992
15.6
5.1
15.8
7.1
1.1
44.7
1993
16.2
5.1
14.7
6.0
1.1
43.0
1994
16.9
5.2
17.1
8.0
1.0
48.3
1995
16.3
5.2
16.0
6.8
1.0
45.4
1996
16.9
5.2
16.5
7.5
1.0
47.2
1997
16.9
5.3
17.6
8.4
1.0
49.3
f. Note: Totals may not sum due to independent rounding.
1998
16.9
5.2 :
18.0 :
8.1
1.0 !
49.2 ;
The Revised 1996 IPCC Guidelines (IPCC/UNEP/OECD/IEA 1997) indicate that emissions from animal wastes managed in solid
storage and drylot should also be included in the emissions from soils (see footnote "c" in Table 4-22 in the Reference Manual)-
however, this instruction appeared to be an error (and footnote "b" should have been listed next to "Solid storage and drylot" in Table
4-22). Therefore, N2O emissions from livestock wastes managed in solid storage and drylot are reported under Manure Management
rather than here. (See Annex H for a discussion of the activity data used to calculate emissions from the manure management source
category.)
Agriculture 5-17
-------
egory (Safely et al. 1992), except for unmanaged dairy
cow wastes. Although it is known that there is a small
portion of dairy cattle that graze, there are no available
statistics for this category, and therefore the simplifying
assumption is made that all unmanaged dairy cow wastes
fall into the daily spread category. Estimates of nitrogen
excretion by the remaining animals were derived from
animal population and weight statistics, information on
manure management system usage hi the United States,
and nitrogen excretion values fo^each animal type.
Annual animal population data for all the remain-
ing livestock types, except horses, were obtained from
the USDA National Agricultural Statistics Service (USDA
1994b,c; 1995a,b; 1996a,b; 1997a,b; 1998a,b;,1999a-
g,i-m). Horse population data were obtained from the
FAOSTAT database (FAO 1999). Manure management
system utilization data for all livestock types except for
diary cattle and swine was taken from Safely et al (1992).
In the last few years, there has been a significant shift in
the dairy and swine industries toward larger, consoli-
dated facilities, which use manure management systems.
Based on the assumption that larger facilities have a
higher chance of using manure management systems,
farm-size distribution data reported in the 1992 and 1997
Census of Agriculture (DOC 1995, USDA 1999n) were
used to assess system utilization in the dairy and swine
industries. Populations in the larger farm categories were
assumed to utilize manure collection and storage sys-
tems; all the wastes from smaller farms were assumed to
be managed as pasture, range, and paddock. As stated
earlier, waste from manure collection and storage sys-
tems is covered under the manure management section.
Waste from pasture, range, and paddock is considered
direct depositing of waste, and is covered in this section.
For each animal type, the population of animals
within pasture, range, and paddock systems was multi-
plied by an average animal mass constant (ASAE 1999)
to derive total animal mass for each animal type. Total
Kjeldahl nitrogen excreted per year was then calculated
for each animal type using daily rates of nitrogen excre-
tion per unit of animal mass (ASAE 1999). Annual nitro-
gen excretion was then summed over all animal types
(see Table 5-21), and reduced by 20 percent to account
for the portion that volatilizes to NH3 and NOX. The re-
mainder was multiplied by the IPCC default emission
factor (0.02 kg N2O-N/kg N excreted) to estimate N2O
emissions (see Table 5-21).
Indirect N20 Emissions from
Nitrogen Applied to Managed Soils
This component accounts for N2O that is emitted
indirectly from nitrogen applied as commercial fertil-
izer, sewage sludge, and animal waste. Through volatil-
ization, some of this nitrogen enters the atmosphere as
NH3 and NOX, and subsequently returns to soils through
atmospheric deposition, thereby enhancing N2O produc-
tion. Additional nitrogen is lost from soils through leach-
ing and runoff, and enters groundwater and surface wa-
ter systems, from which a portion is emitted as N2O. These
two indirect emission pathways are treated separately,
although the activity data used are identical.
Estimates of total nitrogen applied as commercial
fertilizer, sewage sludge, and animal waste were derived
using the same approach as was employed to estimate the
direct soil emissions. Annual application rates for syn-
thetic and non-manure organic fertilizer nitrogen were
derived from commercial fertilizer statistics as described
above (AAPFCO 1995, 1996, 1997, 1998; TVA 1990,
1992a and b, 1994). Annual application rates for sewage
sludge were also derived as described above. Annual to-
tal nitrogen excretion data for livestock and poultry by
animal type were derived from EPA data, also as described
above, using population statistics (USDA 1994b,c;
1995a,b; 1996a,b; 1997a,b; 1998a,b; 1999a-g,i-m; DOC
1987; and FAO 1999), average animal mass constants
(ASAE 1999), and daily rates of nitrogen excretion per
unit of animal mass (ASAE 1999). Annual nitrogen ex-
cretion was then summed over all animal types.
To estimate N2O emissions from volatilization and
subsequent atmospheric deposition, the methodology
described in the Revised 1996IPCC Guidelines (IPCC/
UNEP/OECD/ESA1997) was followed, where it is assumed
that 10 percent of the synthetic fertilizer nitrogen and 20
percent of animal waste (i.e., livestock and poultry) nitro-
gen applied as fertilizer are volatilized to NH3 and NOX. It
was then assumed that 1 percent of the total deposited
nitrogen is emitted as N2O. The same NH3 and NOX vola-
tilization and N2O emission rates as those used for animal
5-18 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
waste fertilizer were used for nitrogen applied to land as
non-manure organic fertilizer and as sewage sludge. These
emission estimates are presented in Table 5-22.
To estimate N2O emissions from leaching and run-
off, it was assumed that 30 percent of the total nitrogen
applied to managed soils was lost to leaching and sur-
face runoff, and 2.5 percent of the lost nitrogen was emit-
ted as N2O (TPCC/UNEP/OECD/IEA1997). These emis-
sion estimates are also presented in Table 5-22.
Uncertainty
A number of conditions can affect nitrification and
denitrification rates in soils. These conditions vary
greatly by soil type, climate, cropping system, and soil
management regime, and their combined effect on the
processes leading to N2O emissions are not fully under-
stood. Moreover, the amount of added nitrogen from each
source that is not absorbed by crops or wild vegetation,
but remains in the soil and is available for production of
N2O, is uncertain. Therefore, it is not yet possible to
develop statistically valid estimates of emission factors
for all possible combinations of soil, climate, and man-
agement conditions. The emission factors used were mid-
point estimates based on measurements described in the
scientific literature, and as such, are representative of
current scientific understanding. Nevertheless, estimated
ranges around each midpoint estimate are wide; most
are an order of magnitude or larger (IPCC/UNEP/OECD/
IEA 1997; IPCC 1999a,b).
Uncertainties also exist in the activity data used to
derive emission estimates. In particular, the fertilizer sta-
tistics include only those organic fertilizers that enter
the commercial market, so some non-commercial fertil-
izer uses have not been captured. Statistics on sewage
sludge applied to soils were not available on an annual
basis; annual production and application estimates were
based on two data points that were calculated from sur-
veys that yielded uncertainty levels as high as 14 per-
cent (Bastian 1999). Also, the nitrogen content of or-
ganic fertilizers varies by type, as well as within indi-
vidual types; however, average values were used to esti-
mate total organic fertilizer nitrogen consumed. Similar
uncertainty levels are associated with the nitrogen con-
tent of sewage sludge. Conversion factors for the bean,
Table 5-21: Direct N20 Emissions from Pasture, Range, and Paddock Animals (MMTCE)
I"
pnimal Type
I Beef Cattle
ISwine
F-Sheep
|-Goats
poultry
jLHorses
| Total
1990
9.0
0.4
0.2
0.1
+
0.5
10.3
1991
9.1
0.4
0.2
0.1
+
0.5
10.3
1992
9.3
0.4
0.2
0.1
+
0.6
10.6
1993
9.5
0.4
0.2
0.1
+•
0.6
10.7
1994
9.8
0.3
0.2
0.1
+
0.6
10.9
1995
10.0
0.3
0.2
0.1
+
0.6
11.1
1996
10.0
0.2
0.2
0.1
+
0.6
11.0
1997
9.7
0.2
0.2
+
+
0.6
10.7
1998
9.5
0.2
0.2
+
+
0.6
10.5
' + Does not exceed 0.05 MMTCE
[Note: Totals may not sum due to independent rounding.
Table 5-22: Indirect N20 Emissions (MMTCE)
Activity 1990 1991
Volatilization & Aim. Deposition 3.7 3.7
i=._ Comm. Fertilizers & Sew. Sludge 1.4 1.4
rr- Animal Waste 2.3 2.3
-Surface Run-off & Leaching 18.7 19.0
L Comm. Fertilizers Sew. Sludge 10.2 10.3
f Animal Waste 8.6 8.7
pTotal 22.4 22J
"Mote: Totals may not sum due to independent rounding.
1992
3.8
1.4
2.4
19.2
10.4
8.8
23.0
1993
3.8
1.5
2.4
19.7
10.8
8.9
23.6
1994
3.9
1.5
2.4
20.4
11.3
9.1
24.3
1995
3.9
1.5
2.4
20.1
10.9
9.2
24.0
1996
4.0
1.5
2.4
20.4
11.3
9.1
24.3
1997
3.9
1.5
2.4
20.3
11.3
9.0
24.3
1998
3.9
1.5
2.4
20.3
11.3
9.0
24.2
*,,
1
dS
Agriculture 5-19
-------
pulse, alfalfa, and non-alfalfa legume production statis-
tics were based on a limited number of studies, and may
not be representative of all conditions in the United
States. It was assumed that the entire crop residue for
corn, wheat, beans, and pulses was returned to the soils,
with the exception of the fraction burned. A portion of
this residue may be disposed of through other practices,
such as composting or landfilling; however, data on these
practices are not available. The point estimates of yearly
production yields for non-alfalfa forage legumes carry a
high degree of uncertainty; many of the estimated aver-
age coverage densities and cover areas are based on a
combination of on-field experimentation and expert
judgment. Also, the amount of nitrogen that is added to
soils from non-alfalfa forage will depend at least in part
on grazing intensity, which has not been taken into ac-
count. Lastly, the livestock excretion values, while based
on detailed population and weight statistics, were de-
rived using simplifying assumptions concerning the
types of management systems employed; for example,
emissions due to grazing dairy cattle are probably un-
derestimated, while emissions due to soil application of
dairy cattle waste are overestimated.
Agricultural Residue Burning
Large quantities of agricultural crop residues are
produced by farming activities. There are a variety of
ways to dispose of these residues. For example, agricul-
tural residues can be plowed back into the field,
composted and then applied to soils, landfilled, or
burned in the field. Alternatively, they can be collected
and used as a fuel or sold in supplemental feed markets.
Field burning of crop residues is not considered a net
source of carbon dioxide (CO2) because the carbon re-
leased to the atmosphere as CO2 during burning is as-
sumed to be reabsorbed during the next growing season.
Crop residue burning is, however, a net source of meth-
ane (CH4), nitrous oxide (N2O), carbon monoxide (CO),
and nitrogen oxides (NOX), which are released during
combustion.
Field burning is not a common method of agricul-
tural residue disposal in the United States; therefore,
emissions from this source are minor. The primary crop
types whose residues are typically burned in the United
States are wheat, rice, sugarcane, corn, barley, soybeans,
and peanuts, and of these residues, less than 5 percent is
burned each year, except for rice.15 Annual emissions
Table 5-23: Emissions from Agricultural Residue Burning (MMTCE)
! Gas/Crop Type 1990 1991 1992
= CH4 0.2 0.2 0.2
i Wheat + + +
i Rice + + +
Sugarcane + + +
it Com 0.1 0.1 0.1
: Barley + + +
* Soybeans + + +
! Peanuts + + +
N20 0.1 0.1 0.1
is Wheat + + +
Rice + + +
Sugarcane + + +
Corn + + +
-, Barley + + +
Soybeans 0.1 0.1 0.1
: Peanuts + + +
Total 0.3 0.2 0.3
i+ Does not exceed 0.05 MMTCE
" Note: Totals may not sum due to independent rounding.
1993 1994 1995 1996 1997
0.1 0.2 0.2 0.2 0.2
+ + + + +
+ + + + +
+ + + + +
0.1 0.1 0.1 0.1 0.1
+ + + + +
+ 0.1 + + 0.1
+ + + + +
0.1 0.1 0.1 0.1 0.1
+ + + + +
+ + + + . +
+ + + + +.
+ + + + -t-
+ + + + +
+ 0.1 0.1 0.1 0.1
+ + + - + +
0.2 0.3 0.3 0.3 0.3
1998
0.2
: . + "
+
+
0.1
+
0.1
+
0.1
.; +
+
. '.. +
. +
+
0.1
+
0.3
15 The fraction of rice straw burned each year is significantly higher than that for other crops (see "Data Sources" discussion below).
5-20 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
from this source over the period 1990 through 1998 av-
eraged approximately 0.2 MMTCE (31 Gg) of CH4, 0.1
MMTCE (1 Gg) of N20,650 Gg of CO, and 29 Gg of NOX
(see Table 5-23 and Table 5-24).
Methodology
The methodology for estimating greenhouse gas
emissions from field burning of agricultural residues is
consistent with the Revised 1996IPCC Guidelines (IPCC/
UNEP/OECD/IEA 1997). In order to estimate the
amounts of carbon and nitrogen released during burn-
ing, the following equations were used:
Carbon Released = (Annual Crop Production) x
(Residue/Crop Product Ratio) x (Fraction of Residues
Burned in situ) x (Dry Matter content of the Residue) x
(Burning Efficiency) x (Carbon Content of the Residue)
x (Combustion Efficiency)16
Nitrogen Released = (Annual Crop Production) x
(Residue/Crop Product Ratio) x (Fraction of Residues
Table 5-24: Emissions from Agricultural Residue Burning (Gg)
Gas/Crop Type
1990 1991 1992 1993 1994 1995 1996 1997
CH4
Wheat
Rice
'-.; Sugarcane
Corn
Barley
Soybeans
Peanuts
N20
Wheat
Rice
Sugarcane
Corn
Barley
Soybeans
Peanuts
CO
Wheat
Rice
Sugarcane
Corn
Barley
Soybeans
Peanuts
NOX
Wheat
Rice
Sugarcane
Corn
Barley
Soybeans
Peanuts
30
7
2
1
12
1
7
4
1
+
+
+
+
4
1
+
623
137
48
18
254
15
148
2
26
1
1
4-
8
4
14
+
28
5
2
1
11
1
7
+
1
4
4.
4.
4.
4
1
4.
578
99
47
20
240
16
153
3
26
1
1
4
8
4
14
+
33
6
3
1
14
1
8
4,
1
4.
+
4.
4.
4.
1
4
688
124
54
20
304
16
168
3
29
1
2
+
10
4.
16
.. + . .
26
6
2
1
10
1
7
-t-
1
+
_l_
4.
+
1
544
120
40
20
203
14
144
2
23
1
1
6
14
- +
34
6
2
1
15
1
9
1
+
_l_
4.
1
717
116
49
20
322
13
193
3
32
1
1
10
18
+
28
5
2
1
11
1
8
1
+
•f-
+
H-
1
590
109
41
20
237
13
167
2
27
1
1
8
16
32
5
2
1
14
1
9
1
+
+
+
+
1
675
114
47
19
296
14
183
2
30
1
1
9
17 ,
34
6
2
1
14
10
1
4.
+
+
1
704
124
42
21
295
13
207
2
32
1
9
20
35
6
1
15
1
10
•j
4
•|
733
128
44
22
313
12
212
2
34
1 «.
10
20
Note: Totals may not sum due to independent rounding.
Burning Efficiency is defined as the fraction of dry biomass exposed to burning that actually burns. Combustion Efficiency is
defined as the fraction of carbon in the fire that is oxidized completely to CO2. In the methodology recommended by the IPCC the
burning efficiency" is assumed to be contained in the "fraction of residues burned" factor. However, the number used here to estimate
the "fraction of residues burned" does not account for the fraction of exposed residue that does not burn. Therefore a "burning
efficiency factor" was added to the calculations.
Agriculture 5-21
-------
Burned in sitii) x (Dry Matter Content of the Residue) •
(Burning Efficiency) x (Nitrogen Content of the Resi-
due) x (Combustion Efficiency)
Emissions of CH4 and CO were calculated by mul-
tiplying the amount of carbon released by the appropri-
ate IPCC default emission ratio (i.e., CH4-C/C or CO-C/
C). Similarly, N2O and NOX emissions were calculated
by multiplying the amount of nitrogen released by the
appropriate IPCC default emission ratio (i.e., N2O-N/N
orNCy-N/N).
Data Sources
The crop residues that are burned in the United
States were determined from various state level green-
house gas emission inventories (ILENR 1993, Oregon
Department of Energy 1995, Wisconsin Department of
Natural Resources 1993) and publications on agricul-
tural burning in the United States (Jenkins et al. 1992,
Turn et al. 1997, EPA 1992).
Crop production data were taken from the USDA's
Field Crops, Final Estimates 1987-1992, 1992-1997
(USDA 1994, 1998) and Crop Production 1998 Sum-
mary (USDA 1999), except data on the production of
rice in Florida, which USDA does not estimate. To esti-
mate Florida rice production, an average 1998 value for
ice productivity (i.e., metric tons rice/acre) was obtained
from Sem-Chi Rice, which produces the majority of rice
in Florida (Smith 1999), and multiplied by total Florida
rice acreage each year (Schueneman 1999c). The pro-
duction data for the crop types whose residues are burned
are presented in Table 5-25.
The percentage of crop residue burned was assumed
to be 3 percent for all crops in all years, except rice,
based on state inventory data (ILENR 1993, Oregon De-
partment of Energy 1995, Noller 1996, Wisconsin De-
partment of Natural Resources 1993, and Cibrowski
1996). Estimates of the percentage of rice acreage on
which residue burning took place were obtained on a
state-by-state basis from agricultural extension agents
in each of the seven rice-producing states (Guethle 1999,
Fife 1999, Klosterboer 1999a and 1999b, Slaton 1999a
and 1999b, Linscombe 1999a and 1999b, Schueneman
1999a and 1999b, Street 1999a and 1999b) (see Table
5-26 and Table 5-27). The estimates provided for each
state remained the same from year to year for all states,
with the exception of California. For California, it was
assumed that the annual percents of rice acreage burned
in Sacramento Valley are representative of burning in
the entire state, because the Valley accounts for over 95
percent of the rice acreage in California (Fife 1999). The
annual percents of rice acreage burned in Sacramento
Table 5-26: Percentage of Rice Area Burned By
JHate
l-State
i " ' '" •
^Arkansas
I- California
Plorida5 . ,. ,,...,_.. v,..,
f, Louisiana
|,, Mississippi
LMissouri
p'Texas "
hyalues provided in Table 5-27.
Percent Burned
10
variable3
. .... T .. , .
'""6 ;
10
3.5
•" 2
LiivPlnrirta —
•• _- . i ,qj
"* ;::;i^;,::::,-.M":^
3S
-;
Table 5-25: Agricultural Crop Production (Thousand Metric Tons of Product)
Crop
Wheat
Rice
Sugarcane
Com"
Bartey
Soybeans
Peanuts
Total
"Com for grain
1990
74,292
7,105
25,525
201,534
9,192
52,416
1,635
371,698
(I.e., excludes corn
1991
53,891
7,271
27,444
189,868
10,110
54,065
2,235
344,883
for silage).
1992
67,135
8,196
27,545
240,719
9,908
59,612
1,943
415,058
1993
65,220
7,127
28,188
160,986
8,666
50,885
1,539
322,612
1994
63,167
9,019
28,057
255,295
8,162
68,444
1,927
434,069
1995
59,404
7,935
27,922
187,970
7,824
59,174
1,570
351,799
1996
61,980
7,828
26,729
234,518
8,544
64,780
1,661
406,041
-rrrr-1 -..ap- -~, !
1997
67,534
8,339
28,766
233,864
7,835.
73,176
1,605
421,120
1998
69,410 •
8,570
30,588 ;
247,943 .:
7,674 '.
75,028 :
1 ,783 •
440,995
-
5-22 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Valley were obtained from Fife (1999). These values de-
clined over the 1990-1998 period because of a legis-
lated reduction in agricultural burning (see Table 5-27).
Because the percentage of rice acreage burned varied
from state to state, and from year to year within Califor-
nia, a weighted average national "percent burned" fac-
tor was derived for rice for each year (Table 5-27). The
weighting was based on rice area in each state.
Residue/crop product mass ratios, residue dry mat-
ter contents, residue carbon contents, and residue nitro-
gen contents for all crops except sugarcane, peanuts,
and soybeans were taken from Strehler and Stutzle
(1987). These data for sugarcane were taken from Uni-
versity of California (1977) and Turn et al. (1997). Resi-
due/crop product mass ratios and residue dry matter con-
tents for peanuts and soybeans were taken from Strehler
and Stutzle (1987); residue carbon contents for these
crops were set at 0.45 and residue nitrogen contents were
taken from Barnard and Kristoferson (1985). The value
Table 5-27: Percentage of Rice Area Burned
f Year
|-.; 1990
£1991
£1992
1-1993
C-T994
p995
f 1996
£1997
f;i998
K~-
California
43
43
43
26
24
20
27
16
19
.„.. t__, — _._„_. ,__. .__, — _ _
U.S. (weighted average)
12
12
12
10
10
9
11
9
9
for peanuts was set equal to the soybean value. These
assumptions are listed in Table 5-28. The burning effi-
ciency was assumed to be 93 percent, and the combus-
tion efficiency was assumed to be 88 percent for all crop
types (EPA 1994). Emission ratios for all gases (see Table
5-29) were taken from the Revised 1996 IPCC Guide-
lines (IPCC/UNEP/OECD/IEA 1997).
Uncertainty
The largest source of uncertainty in the calcula-
tion of non-CO2 emissions from field burning of agricul-
tural residues is in the estimates of the fraction of residue
of each crop type burned each year. Data on the fraction
burned, as well as the gross amount of residue burned
each year, are not collected at either the national or state
level. In addition, burning practices are highly variable
among crops, as well as among states. The fractions of
residue burned used in these calculations were based
upon information collected by state agencies and in pub-
Table 5-29: Greenhouse Gas Emission Ratios
fe"-;'1-
ii— ...
ji "
&-=•-•
fc-. '
Sv, -
Gas
CH4a
G0a
N20"
NO,"
Emission Ratio
_,,.__-,__- : ,. ; . .
0.005
0.060
0.007
0.121
/lass of carbon compound released (units of C) relative to
tmass of total carbon released from burning (units of C)
y Mass of nitrogen compound released (units of N) relative to
pffiass of total nitrogen released from burning (units of N)
St^'-- -'; -'-' :^'-r^..- ;v „. „.;: ,. ,,,,,,,.^- L;,, ._>.*. ,,__-^:l _
Table 5-28: Key Assumptions for Estimating Emissions from Agricultural Residue Burning3
ICrop
jjpWlieat
f.Rice
tSugarcane
rcorn
|-Bariey
fSoybeans
LPeanuts
pJhe burning efficiency
p-See Table 5-27.
Residue/
Crop Ratio
1.3
1.4
0.8
1.0
1.2
2.1
1.0
and combustion efficiency
Fraction of
Residue Burned
0.03
variable11
0.03
0.03
0.03
0.03
0.03
for all crops were assumed
Dry Matter
Fraction
0.85
0.85
0.62
0.78
0.85
0.87
0.90
to be 0.93 and 0.88,
Carbon
Fraction
0.4853
0.4144
0.4235
0.4709
0.4567
0.4500
0.4500
respectively.
Nitrogen
Fraction
0.0028
0.0067 :
0.0040
0.0081
0.0043 ;
0.0230
0.0230
' ..... : J ' ™J
,.;.--.'.- ,'- -•-, ,-'', '. ."-. -I..,,,',-*! v -„ ; y.,\ - -3
Agriculture 5-23
-------
lished literature. It is likely that these emission estimates
will continue to change as more information becomes
available in the future.
Other sources of uncertainty include the residue/
crop product mass ratios, residue dry matter contents,
burning and combustion efficiencies, and emission ra-
tios. A residue/crop product ratio for a specific crop can
vary among cultivars, and for all crops except sugar-
cane, generic residue/crop product ratios, rather than ra-
tios specific to the United States, have been used. Resi-
due dry matter contents, burning and combustion effi-
ciencies, and emission ratios, all can vary due to weather
and other combustion conditions, such as fuel geom-
etry. Values for these variables were taken from literature
on agricultural biomass burning.
5-24 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
6. Land-Use Change
and Forestry
This chapter provides an assessment of the net carbon dioxide (CO2) flux caused by (1) changes in forest
• carbon stocks, (2) changes in non-forest soil carbon stocks, and (3) changes in non-forest carbon stocks in
landfills. Six components of forest carbon stocks are analyzed: trees, understory, forest floor, forest soil, wood
products, and landfilled wood. The estimated CO2 flux from each of these forest components is based on carbon stock
estimates developed by the U.S. Forest Service, using methodologies that are consistent with the Revised 1996IPCC
Guidelines (IPCC/UNEP/OECD/EEA 1997). Changes in non-forest soil carbon stocks include mineral and organic
soil carbon stock changes due to agricultural land use and land management, and emissions of CO2 due to the
application of crushed limestone and dolomite to agricultural soils. The methods in the Revised 1996 IPCC Guide-
lines were used to estimate all three components of changes in non-forest soil carbon stocks. Changes in non-forest
carbon stocks in landfills are estimated for yard trimmings disposed in landfills using EPA's method of analyzing life
cycle GHG emissions and sinks associated with solid waste management (EPA 1998).
Unlike the assessments in other chapters, which are based on annual activity data, the flux estimates in this
chapter, with the exception of emissions from liming and carbon storage associated with yard trimmings disposed in
landfills, are based on periodic activity data in the form of forest and soil surveys. Carbon dioxide fluxes from forest
carbon stocks and from non-forest mineral and organic soils are calculated on an average annual basis over five- or
ten-year periods. The resulting annual averages are applied to years between surveys. As a result of this data
structure, estimated CO2 fluxes are constant over multi-year intervals. In addition, because the most recent national
forest survey was completed for the year 1992, the estimates of the CO2 flux from forest carbon stocks are based in part
on modeled projections of stock estimates for the year 2000.1
The previous U.S. Inventory included only a preliminary assessment of the net CO2 flux from two non-forest
soil components: use and management of organic soils and liming of agricultural soils. In the current Inventory,
revised estimates of flux from organic soils—based on revised activity data—updated flux estimates for liming of
agricultural soils—based on updated activity data—and flux estimates for non-forest mineral soils are included.
However, due to the lack of a national soil survey more recent than 1992, carbon flux estimates for non-forest mineral
and organic soils were not calculated for the 1993 through 1998 period. Therefore, the non-forest soil carbon flux
estimates are not included in the total fluxes reported for this chapter.
'The national forest survey for 1997 is expected to be completed this year. This survey will be used to develop revised forest carbon
flux estimates, which will be presented in the 1990-1999 version of the U.S. Inventory.
Land-Use Change and Forestry 6-1
-------
Estimates of total annual net CO2 flux from land-
use change and forestry decline from 316 to 211
MMTCE (1,160,000 to 773,000 Gg) net sequestration
between 1990 and 1998 (Table 6-1 and Table 6-2). The
decrease in annual net CO2 sequestration is due to a
maturation and slowed expansion of the U.S. forest cover
and a gradual decrease in the rate of yard trimmings dis-
posed in landfills; the abrupt shift between 1992 and
1993 is a result of the use of methodologies that incor-
porate periodic activity data and decadal, rather than
annual, stock estimates.
Changes in Forest Carbon Stocks
Globally, the most important human activity that
affects forest carbon fluxes is deforestation, particularly
the clearing of tropical forests for agricultural use. Tropi-
cal deforestation is estimated to have released nearly 6
billion metric tons of CO2 per year during the 1980s, or
about 23 percent of global CO2 emissions from anthro-
pogenic activities. Conversely, during this period about
7 percent of global CO2 emissions were offset by CO2
uptake due to forest regrowth in the Northern Hemisphere
(Houghton et al. 1995).
Table 6-1: Met CO, Flux from Land-Use Change and Forestry (MMTCE)*
Description
1990
1991
1992
1993 1994 1995
^TsT^TaTTIOT"
4.0 (74.0) (74.0)
~:sj—^nsy—IP;
lfcW-™.(i 4i ^,^,,.^-S*
y.oj
1996
1997 1998
Forests
". Trees
Understory
Forest Floor
Soil
Harvested Wood
Wood Products
LandfilledWood
Landfilled Yard Trimmings
(274.2)
(95.6)
(2.4)
(20.8)
(155.2)_
C[37-3)
lit7-
(274.2)
(95.6)
(2.4)
(20.8)
(155.2)
w^w^
(4.9)
(4.8)
(274.2)
(95.6)
(2.4)
(20.8)
(155.2)
(74.0)~
...; -
"
(17.9) (17.9) (17.9)
(171
-(74.0):
•(1.3),
(9.8)
(86.3)
(37.3)
(17.9)
(74.0)
"-(9.8)
(86.3)
(37.3)
(17.9)
- _
"(3.7) ~ (3.3) (2.7)
J19.4) (19.4) ^
(2.6) (2.3)
Total Net Flux
(3i6?o ......... (316:2)
Note- Parentheses indicate sequestration. Totals may not sum due to independent rounding. Shaded areas indicate values based on a
combination of historical data and projections. All other values are based on historical data only.
*The total net flux excludes flux estimates for non-forest soils due to incomplete flux estimates for organic and mineral soils for the 1990
through 1998 period.
Table 6-2: Net C02 Flux from Land-Use Change and Forestry (Gg)
Description 1990 1991 1992 1993
1994
1995
1996
1997
1998
Forests (1,005,400)
Trees (350,500)
Understory (8,800)
Forest Floor (76,300)
Soil (569JOO)
Harvested Wood
Wood Products 11(65,500)
LandfilledWood E(71,200)
LandfilledYard --1-
Trimmings (17,800)
(1,005,400)
(350,500)
(8,800)
(76,300)
i569JMje=ss^,|||gi||^
(65,500) (65,500) (65,500) (65,500)
(71,200) (71,200) (71,200) (71,200)
(1,005,400)
(350,500)
(8,800)
(76,300)
(569,100)
•627,900) (627,900)
TOOO) (271,300) (271,300) (271,300) (271,300)
:{OOQ) (4,6l)Op'=KSOP):''n:4'600) (4",600)
iS6l W100] (3t,800rl35,800) (35,800)
^'^^^•rajg^jjjj-j^^fjyxsi 6,300)
TspTjOTimSuO)(136^00)
(65,500) (65,500)
(71,200) (71,200)
(65,500)
(71,200)
(271,300) ;
(4,600):*
(35,800): '
@1Q,3QQ)~1
(136,800) *
(65,500) ;
(71,200):;;
(17,500) (17,100) (15.300) (13,600) (12,000) (10,000) (9.400) (8,300)
' _. .. ,,.,.,_'_..„_ , _________ '- ,_m— _— l-JH-
_
Total Net Flux tfJTllo^OO) ' (i ,151.700)
Note- Parentheses indicate sequestration. Totals may not sum due to independent rounding. Shaded areas indicate values based on a
combination of historical data and projections. All other values are based on historical data only.
*The total net flux excludes flux estimates for non-forest soils due to incomplete flux estimates for organic and mineral soils for the 1990
through 1998 period.
6-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
In the United States, the amount of forest land has
remained fairly constant during the last several decades.
The United States covers roughly 2,263 million acres, of
which 33 percent (737 million acres) is forest land
(Powell et al. 1993). The amount of forest land declined
by approximately 5.2 million acres between 1977 and
1987 (USFS 1990, Waddell et al. 1989), and increased
by about 5.3 million acres between 1987 and 1992
(Powell et al. 1993). These changes represent average
fluctuations of only about 0.1 percent per year. Other
major land-use categories in the United States include
range and pasture lands (29 percent), cropland (17 per-
cent), urban areas (3 percent), and other lands (18 per-
cent) (Daugherty 1995).
Given the low rate of change in U.S. forest land
area, the major influences on the current net carbon flux
from forest land are management activities and ongoing
impacts of previous land-use changes. These activities
affect the net flux of carbon by altering the amount of
carbon stored in forest ecosystems. For example, inten-
sified management of forests can increase both the rate
of growth and the eventual biomass density of the forest,
thereby increasing the uptake of carbon. The reversion
of cropland to forest land through natural regeneration
also will, over decades, result in increased carbon stor-
age in biomass and soils.
Forests are complex ecosystems with several inter-
related components, each of which acts as a carbon stor-
age pool, including:
• Trees (i.e., living trees, standing dead trees, roots,
stems, branches, and foliage)
• Understory vegetation (i.e., shrubs and bushes)
• The forest floor (i.e., woody debris, tree litter, and
humus)
• Soil
As a result of biological processes in forests (e.g.,
growth and mortality) and anthropogenic activities (e.g.,
harvesting, thinning, and replanting), carbon is continu-
ously cycled through these ecosystem components, as
well as between the forest ecosystem and the atmosphere.
For example, the growth of trees results in the uptake of
carbon from the atmosphere and storage of carbon in
2 For this reason, the term "apparent flux" is used in this chapter.
living biomass. As trees age, they continue to accumu-
late carbon until they reach maturity, at which point they
are relatively constant carbon stores. As trees die and
otherwise deposit litter and debris on the forest floor,
decay processes release carbon to the atmosphere and
also increase soil carbon. The net change in forest car-
bon is the sum of the net changes in the total amount of
carbon stored in each of the forest carbon pools over
time.
The net change in forest carbon, however, may not
be equivalent to the net flux between forests and the
atmosphere because timber harvests may not always re-
sult in an immediate flux of carbon to the atmosphere.2
Harvesting in effect transfers carbon from one of the "for-
est pools" to a "product pool." Once in a product pool,
the carbon is emitted over time as CO2 if the wood prod-
uct combusts or decays. The rate of emission varies con-
siderably among different product pools. For example,
if timber is harvested for energy use, combustion results
in an immediate release of carbon. Conversely, if timber
is harvested and subsequently used as lumber in a house,
it may be many decades or even centuries before the
lumber is allowed to decay and carbon is released to the
atmosphere. If wood products are disposed of in land-
fills, the carbon contained in the wood may be released
years or decades later, or may even be stored permanently
in the landfill.
In the United States, improved forest management
practices, the regeneration of previously cleared forest
areas, and timber harvesting and use have resulted in an
annual net uptake (i.e., sequestration) of carbon. Also,
due to improvements in U.S. agricultural productivity,
the rate of forest land clearing for crop cultivation and
pasture slowed in the late 19th century, and by 1920 this
practice had all but ceased. As farming expanded in the
Midwest and West, large areas of previously cultivated
land in the East were brought out of crop production,
primarily between 1920 and 1950, and were allowed to
revert to forest land or were actively reforested. The
impacts of these land-use changes are still affecting car-
bon fluxes from forests in the East. In addition to land-
use changes in the early part of this century, in recent
Land-Use Change and Forestry 6-3
-------
decades carbon fluxes from Eastern forests were affected
by a trend toward managed growth on private land, re-
sulting in a near doubling of the biomass density in east-
ern forests since the early 1950s. More recently, the 1970s
and 1980s saw a resurgence of federally sponsored tree-
planting programs (e.g., the Forestry Incentive Program)
and soil conservation programs (e.g., the Conservation
Reserve Program), which have focused on reforesting
previously harvested lands, improving timber manage-
ment activities, combating soil erosion, and converting
marginal cropland to forests. In addition to forest regen-
eration and management, forest harvests have also af-
fected net carbon fluxes. Because most of the timber that
is harvested from U.S. forests is used in wood products
and much of the discarded wood products are disposed
of by landfilling—rather than incineration—significant
quantities of this harvested carbon are transferred to long-
term storage pools rather than being released to the at-
mosphere. The size of these long-term carbon storage
pools has also increased over the last century.
As shown in Table 6-3 and Table 6-4, U.S. forest
components, wood product pools, and landfilled wood
were estimated to account for an average annual net se-
questration of 311.5 MMTCE (1,142,200 Gg CO2) from
1990 through 1992, and 208.6 MMTCE (764,700 Gg
CO2) from 1993 through 1998. The net carbon seques-
tration reported for 1998 represents an offset of about 14
percent of the 1998 CO2 emissions from fossil fuel corn-
Table 6-3: Net C02 Flux from U.S. Forests (MMTCE)
Description
Apparent Forest Flux
Trees
Understory
Forest Floor
Forest Soils
Apparent Harvested Wood Flux
Apparent Wood Product Flux
Apparent Landfilled Wood Flux
Ibtal Net Flux
1990
(274.2)
(95.6)
(2.4)
(20.8)
(155.2)
|(37.3)
C!(19.4)
'(311.5)
1991
(274.2)
(95.6)
(2.4)
(20.8)
(155.2)
(37.3)
(17.9)
(19.4)
(311.5)
1992
(274.2)
(95.6)
(2.4)
(20.8)
(155.2)
(37.3)
(17.9)
(19.4)
(311.5)
1993
(171.3)
Insl
J|863)
(37.3)
(17.9)
(19.4)
(208.6)
1994
(171.3)
(740)
(98)"
(863)
(37.3)
(17.9)
(19.4)
(208.6)
1995
(171.3)
' J7J.O)_r
_. ^.-
' (863)
"137.3T
_'..!&.'
(208.6)
1996
(171.3)
(74.0)
~ (isr
(9.8) '
""""(863)"'
(17.9) '
" (19.4)
(208.6)
1997
(171.3)
(74.0)
"PT
:.. HI
~(3T.3f
' (17.9)
(19.4)
(208.6)
1998
tntsT
. (74.Q)
Hi, .JWMlr'n.y^WJ^'j.li.Sf
*,**ffi
'""(37.3)"
(17.9)
(19.4)
(208.6)
«,„«&
, ''.£
s
- .:
Note: Parentheses indicate net carbon "sequestration" (i.e., sequestration or accumulation into the carbon pool minus emissions or harvest
from the carbon pool). The word "apparent" is used to indicate that an estimated flux is a measure of net change in carbon stocks, rather
than an actual flux to or from the atmosphere. The sum of the apparent fluxes in this table (i.e., total flux) is an estimate of the actual flux.
Shaded areas indicate values based on a combination of historical data and projections. All other values are based on historical data only.
Totals may not sum due to independent rounding.
Table 6-4: Net CO, Flux from U.S. Forests (Gg)
Description
Apparent Forest Flux
Trees
Understory
Forest Floor
Soil
Apparent Harvested
Wood Flux
Wood Products
Landfilled Wood
Total Net Flux
(1
1990
,005,400)
(350,500)
(8,800)
(76,300)
(569,100)
ir — — '~ — ' — " —
f; (136,800)
£^"(65,500)
I! .,(71,200)
^
,142,200)
1991
(1,005,400)
(350,500)
(8,800)
(76,300)
(569,100)
(136,800)
(65,500)
(71,200)
(1,142,200)
1992 1993
(1,005,400)
(350,500)
(8,800)
(76,300)
(569,100)
627,900)
:(271,300)
llipsi
I(35j8p0)
(136,800) (136,800)
(65,500) (65,500)
(71,200) (71,200)
(1,142,200) (764,700)
1994
1995 1996
1997
(627,900) (627,900) (627,900) (627,900)
(271,300) "(271, 300) [271,300) (271,300)'
"(4,600) (47600) _jpoo1_" f4$5o7
(35,800) (35,800 "ra&.lSb)
X«, ,A- .»,,!.r- .hi.,{k^^^<«aMA^.nniM.I!^MJ!lW)^
(136,800)
(65,500)
(71,200)
(764,700)
(136,800) (136,800)
(65,500) (65,500)
" (71;200) (71,200)
(764,700) (764,700)
__ (35,800)
(136,800)
(65,500)
,(71,200)
(764,700)
1998
(627^05)
"j27f,3R))
-(liCs'ffoT
(136,800)
(65,500)
'(71,200)
(764,700)
•™r^
•^
^
" i
_J
Note: Parentheses indicate net carbon "sequestration" (i.e., sequestration or accumulation into the carbon pool minus emissions or harvest
from the carbon pool). The word "apparent" is used to indicate that an estimated flux is a measure of net change in carbon stocks, rather
than an actual flux to or from the atmosphere. The sum of the apparent fluxes in this table (i.e., total flux) is an estimate of the actual flux.
Shaded areas Indicate values based on a combination of historical data and projections. All other values are based on historical data only.
Totals may not sum due to independent rounding.
6-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
bustion. The average annual net carbon sequestration
reported for 1993 through 1998 represents a 33 percent
decrease relative to the average annual net carbon se-
questration reported for 1990 through 1992. This over-
all decrease in annual net sequestration is due to changes
in the aggregate age structure of U.S. forests caused by
the maturation of existing forests and the slowed ex-
pansion of Eastern forest cover. The abrupt shift in an-
nual net sequestration from 1992 to 1993 is the result of
calculating average annual fluxes using periodic activ-
ity data as well as models that estimate and project
decadal rather than annual stock estimates.
Methodology
The methodology for estimating annual forest car-
bon flux in the United States differs from the method-
ologies employed for other activities because the forest
carbon flux estimates were derived from periodic sur-
veys rather than annual activity data. In addition, be-
cause the most recent survey was completed for 1992, a
combination of survey data and projected data, rather
than complete historical data, was used to derive some
of the annual flux estimates.
Timber stock data from national forest surveys were
used to derive estimates of carbon contained in the four
forest ecosystem components (i.e., trees, understory, for-
est floor, and soil) for the survey years. The apparent
annual forest carbon flux for a specific year was estimated
as the average annual change in the total forest carbon
stocks between the preceding and succeeding forest sur-
vey years. The most recent national forest surveys were
conducted for the years 1987 and 1992. Therefore, the
apparent annual forest carbon flux estimate for the years
1990 through 1992 was calculated from forest carbon
stocks derived from the 1987 and 1992 surveys. To esti-
mate the apparent annual forest carbon flux estimate for
the years 1993 through 1998, the 1992 forest carbon
stocks and forest carbon stocks for 2000, which were de-
rived from a projection of timber stocks, were used.3
Carbon stocks contained in the wood product and
landfilled wood pools were estimated for 1990 using
historical forest harvest data, and were estimated for 2000
using projections of forest harvest. Therefore, apparent
annual wood product and landfilled wood fluxes for the
years 1990 through 1998 were calculated from a 1990
historical estimate and a 2000 projection.4
The total annual net carbon flux from forests was
obtained by summing the apparent carbon fluxes associ-
ated with changes in forest stocks, wood product pools,
and landfilled wood pools.
The inventory methodology described above is
consistent with the Revised 1996IPCC Guidelines (IPCC/
UNEP/OECD/DEA 1997). The IPCC identifies two ap-
proaches to developing estimates of net carbon flux from
Land-Use Change and Forestry: (1) using average an-
nual statistics on land-use change and forest manage-
ment activities, and applying carbon density and flux
rate data to these activity estimates to derive total flux
values; or (2) using carbon stock estimates derived from
periodic inventories of forest stocks, and measuring net
changes in carbon stocks over time. The latter approach
was employed because the United States conducts peri-
odic surveys of national forest stocks. In addition, the
IPCC identifies two approaches to accounting for car-
bon emissions from harvested wood: (1) assuming that
all of the harvested wood replaces wood products that
decay in the inventory year so that the amount of carbon
in annual harvests equals annual emissions from har-
vests; or (2) accounting for the variable rate of decay of
harvested wood according to its disposition (e.g., prod-
uct pool, landfill, combustion). The latter approach was
applied for this inventory using estimates of carbon
stored in wood products and landfilled wood.5 Although
there are large uncertainties associated with the data used
to develop the flux estimates presented here, the use of
direct measurements from forest surveys and associated
estimates of product and landfilled wood pools is likely
3 Once the 1997 national forest survey is released, new annual estimates of forest carbon flux will be developed These new estimates
will be reported in the 1990-1999 U.S. Inventory.
4 These values will also be revised once the 1997 national forest survey is released.
5 This calculation does not account for carbon stored in imported wood products. It does include carbon stored in exports even if the
logs are processed in other countries (Heath et al. 1996).
Land-Use Change and Forestry 6-5
-------
to result in more accurate flux estimates than the alterna-
tive EPCC methodology.
Data Sources
The estimates of forest, product, and landfill car-
bon stocks used in this inventory to derive forest carbon
fluxes were obtained from Birdsey and Heath (1995),
Heath et al. (1996), and Heath (1997). The amount of
carbon in trees, understory vegetation, the forest floor,
and forest soil in 1987 and 1992 was estimated using
timber volume data collected by the U.S. Forest Service
(USFS) for those years (Waddell et al. 1989; PoweU et al.
1993). The timber volume data include timber stocks
on forest land classified as timberland, reserved forest
land, or other forest land6 in the contiguous United States,
but do not include stocks on forest land in Alaska, Ha-
waii, U.S. territories, or trees on non-forest land (e.g.,
urban trees).7 The timber volume data include estimates
by tree species, size class, and other categories.
The amount of carbon in trees, understory vegeta-
tion, the forest floor, and forest soil in 2000 was esti-
mated by Birdsey and Heath (1995) using the FORCARB
forest carbon model (Plantinga and Birdsey 1993) linked
to the TAMM/ATLAS forest sector model (Adams and
Haynes 1980; Alig 1985; Haynes and Adams 1985; Mills
andKincaid 1992). The forest stock projections for 2000,
therefore, are based on multiple variables, including pro-
jections of prices, consumption, and production of tim-
ber and wood products; and projections of forest area,
forest inventory volume, growth, and removals.
The amount of carbon in aboveground and below
ground tree biomass in forests was calculated by multi-
plying timber volumes by conversion factors derived
from studies in the United States (Cost et al. 1990, Koch
1989). Carbon stocks in the forest floor and understory
vegetation were estimated based on simple models (Vogt
et al. 1986) and review of numerous intensive ecosys-
tem studies (Birdsey 1992). Soil carbon stocks were
calculated using a model similar to Burke et al. (1989)
based on data from Post et al. (1982).
Carbon stocks in wood products in use and in wood
stored in landfills were estimated by applying the
HARVCARB model (Row and Phelps 1991) to histori-
cal harvest data from the USFS (Powell et al. 1993) and
harvest projections for 2000 (Adams and Haynes 1980;
Mills and Kincaid 1992). The HARVCARB model allo-
cates harvested carbon to disposition categories (i.e.,
products, landfills, energy use, and emissions), and tracks
the accumulation of carbon in different disposition cat-
egories over time.
Table 6-5 presents the carbon stock estimates for
forests—including trees, understory, forest floor, and for-
est soil—wood products, and landfilled wood used in
this inventory. The increase in all of these stocks over
time indicates that, during the examined periods, for-
ests, forest product pools, and landfilled wood all accu-
mulated carbon (i.e., carbon sequestration by forests was
greater than carbon removed in wood harvests and re-
leased through decay; and carbon accumulation in prod-
uct pools and landfills was greater than carbon emis-
sions from these pools by decay and burning).
Uncertainty
There are considerable uncertainties associated
with the estimates of the net carbon flux from U.S. forests.
The first source of uncertainty stems from the underlying
forest survey data. These surveys are based on a statisti-
cal sample designed to represent the wide variety of
growth conditions present over large territories. There-
fore, the actual timber volumes contained in forests are
represented by average values that are subject to sam-
pling and estimation errors. In addition, the forest survey
data that are currently available exclude timber stocks
on forest land in Alaska, Hawaii, U.S. territories, and trees
6 Forest land in the United States includes all land that is at least 10 percent stocked with trees of any size. Timberland is the most
produclive type of forest land, growing at a rate of 20 cubic feet per acre per year or more. In 1992, there were about 490 million acres
Of Timberlands, which represented 66 percent of all forest lands (Powell et al. 1993). Forest land classified as Timberland is unreserved
forest land that is producing or is capable of producing crops of industrial wood. The remaining 34 percent of forest land is classified
as Productive Reserved Forest Land, which is withdrawn from timber use by statute or regulation, or Other Forest Land, which includes
unreserved and reserved unproductive forest land.
7 Although forest carbon stocks in Alaska and Hawaii are large compared to the U.S. total, net carbon fluxes from forest stocks in Alaska
and Hawaii are believed to be minor. Net carbon fluxes from urban tree growth are also believed to be minor.
6-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Table 6-5: U.S. Forest Carbon Stock Estimates (Gg)
: Description
i987
1990
LForests
|:: Trees
Understory
Forest Floor
Forest Soil
Harvested Wood
Wood Products
Landfilled Wood
36,353,000
13,009,000
558,000
2,778,000
20,009,000
NA
IMA
NA
NA
NA
NA
NA
NA
3,739,000
2,061,000
1,678,000
1992
37,724,000
13,487,000
570,000
2,882,000
20,785,000
NA
NA
NA
2000
NA (Not Available)
Note: Forest carbon stocks do not include forest stocks in Alaska, Hawaii, U.S. territories, or trees on non-forest
land (e.g., urban trees); wood product stocks include exports, even if the logs are processed in other countries,
and exclude imports. Shaded areas indicate values based on projections. All other values are based on historical
data. Totals may not sum due to independent rounding.
on non-forest land (e.g., urban trees); however, net car-
bon fluxes from these stocks are believed to be minor.
The second source of uncertainty results from de-
riving carbon storage estimates for the forest floor, un-
derstory vegetation, and soil from models that are based
on data from forest ecosystem studies. In order to ex-
trapolate results of these studies to all forest lands, it was
assumed that they adequately describe regional or na-
tional averages. This assumption can potentially intro-
duce the following errors: (1) bias from applying data
from studies that inadequately represent average forest
conditions, (2) modeling errors (e.g., erroneous assump-
tions), and (3) errors in converting estimates from one
reporting unit to another (Birdsey and Heath 1995). In
particular, the impacts of forest management activities,
including harvest, on soil carbon are not well under-
stood. Moore et al. (1981) found that harvest may lead
to a 20 percent loss of soil carbon, while little or no net
change in soil carbon following harvest was reported in
another study (Johnson 1992). Since forest soils con-
tain over 50 percent of the total stored forest carbon in
the United States, this difference can have a large impact
on flux estimates.
The third source of uncertainty results from the
use of projections of forest carbon stocks for the year
2000 (Birdsey and Heath 1995) to estimate annual net
carbon sequestration from 1993 to 1998. These projec-
tions are the product of two linked models (i.e.,
FORCARB and TAMM/ATLAS) that integrate multiple
uncertain variables related to future forest growth and
economic forecasts. Because these models project
decadal rather than annual carbon fluxes, estimates of
annual net carbon sequestration from 1993 to 1998 are
calculated as average annual estimates based on pro-
jected long-term changes in U.S. forest stocks.
The fourth source of uncertainty results from in-
complete accounting of wood products. Because the
wood product stocks were estimated using U.S. harvest
statistics, these stocks include exports, even if the logs
were processed in other countries, and exclude imports.
Haynes (1990) estimates that imported timber accounts
for about 12 percent of the timber consumed in the United
States, and that exports of roundwood and primary prod-
ucts account for about 5 percent of harvested timber.
Changes in Non-Forest
Soil Carbon Stocks
The amount of organic carbon contained in soils
depends on the balance between inputs of photosyn-
thetically fixed carbon (i.e., organic matter such as de-
cayed detritus and roots) and loss of carbon through de-
composition. The quantity and quality of organic mat-
ter inputs, and the rate of decomposition, are determined
by the combined interaction of climate, soil properties,
and land-use. Agricultural practices and other land-use
activities, such as clearing, drainage, tillage, planting,
crop residue management, fertilization, and flooding,
can modify both organic matter inputs and decomposi-
tion, and thereby result in a net flux of carbon to or from
Land-Use Change and Forestry 6-7
-------
soils. In addition, the application of carbonate minerals
to soils through liming operations results in emissions
of CO2. The IPCC methodology for changes in non-
forest soil carbon stocks (IPCC/UNEP/OECD/ffiA 1997)
is divided into three categories of land-use/land-man-
agement activities: (1) agricultural land-use and land
management activities on mineral soils, especially land-
use change activities; (2) agricultural land-use and land
management activities on organic soils, especially cul-
tivation and conversion to pasture and forest; and (3)
liming of soils. Organic soils and mineral soils are treated
separately because each responds differently to land-
use practices.
Organic soils contain extremely deep and rich lay-
ers of organic matter. When these soils are cultivated,
tilling or mixing of the soil aerates the soil, thereby ac-
celerating the rate of decomposition and CO2 genera-
tion. Because of the depth and richness of the organic
layers, carbon loss from cultivated organic soils can con-
tinue over long periods of time. Conversion of organic
soils to agricultural uses typically involves drainage as
well, which also causes soil carbon oxidation. When
organic soils are disturbed, through cultivation and/or
drainage, the rate at which organic matter decomposes,
and therefore the rate at which CO2 emissions are gener-
ated, is determined primarily by climate, the composi-
tion (decomposability) of the organic matter, and the
specific land-use practices undertaken. The use of or-
ganic soils for upland crops results in greater carbon loss
than conversion to pasture or forests, due to deeper drain-
age and/or more intensive management practices
(Armentano and Verhoeven 1990, as cited in IPCC/UNEP/
OECD/IEA 1997).
Mineral soils contain considerably less organic
carbon than organic soils. Furthermore, much of the
organic carbon is concentrated near the soil surface. When
mineral soils undergo conversion from their native state
to agricultural use, as much as half of the soil organic
carbon can be lost to the atmosphere. The rate and ulti-
mate magnitude of carbon loss will depend on native
vegetation, conversion method and subsequent manage-
ment practices, climate, and soil type. In the tropics, 40-
60 percent of the carbon loss occurs within the first 10
years following conversion; after that, carbon stocks
continue to drop but at a much slower rate. In temperate
regions, carbon loss can continue for several decades.
Eventually, the soil will reach a new equilibrium that
reflects a balance between carbon accumulation from
plant biomass and carbon loss through oxidation. Any
changes in land-use or management practices that result
in increased biomass production or decreased oxidation
(e.g., crop rotations, cover crops, application of organic
amendments and manure, and reduction or elimination
of tillage) will result in a net accumulation of soil or-
ganic carbon until a new equilibrium is achieved.
Lime in the form of crushed limestone (CaCO3)
and dolomite (CaMg(CO3)2) is commonly added to agri-
cultural soils to ameliorate acidification. When these
compounds come in contact with acid soils, they de-
grade, thereby generating CO2. The rate of degradation
is determined by soil conditions and the type of mineral
applied; it can take several years for agriculturally-ap-
plied lime to degrade completely.
Of the three activities, use and management of
mineral soils was by far the most important in terms of
contribution to total flux during the 1990 through 1992
period (see Table 6-6 and Table 6-7). Because the most
recent national survey of land-use and management is
from 1992, carbon flux estimates for the years 1993
through 1998 for non-forest organic and mineral soils
are not included. Annual carbon sequestration on min-
eral soils for 1990 through 1992 was estimated at 18.2
MMTCE (66,600 Gg CO2), while annual emissions from
organic soils were estimated at 7.4 MMTCE (27,100 Gg
CO2). Between 1990 and 1998, liming accounted for
net annual emissions that ranged from 2.1 to 3.0 MMTCE
(7,700 to 11,000 Gg CO2). Total net annual CO2 flux
from all three activities on non-forest soils (use and man-
agement of mineral and organic soils, and liming of soils)
was negative over the 1990 to 1992 period (i.e., the com-
bined activities resulted in net carbon sequestration each
year). While organic soils and liming both accounted
for net CO2 emissions, the sum of emissions from both
activities was more than offset by carbon sequestration
in mineral soils.
The emission estimates and analysis for this source
are restricted to CO2 fluxes associated with the use and
6-8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Table 6-6: Net C02 Flux From Non-Forest Soils (MMTCE)
fc -•-_
^Description
f-
|_ Mineral Soils
|0rganic Soils
|liming of Soils
I'Note: Numbers in
"- NA: Not available.
£~
1990 1991
(18.2) (18.2)
7.4 7.4
2.2 2.8
1992
(18.2)
7.4
2.1
1993
NA
NA
2.1
1994
NA
NA
2.3
1995
NA
NA
2.5
1996
NA
NA
2.4
1997
NA
NA
2.4
parentheses indicate net carbon sequestration.
1998
NA ':
NA ;
3.0
• - T-— , ,£
Table 6-7: Net C02 Flux From Non-Forest Soils (Gg)
([Description 1990 1991 1992 1993 1994 1995 1996 1997
1998
t Mineral Soils
I Organic Soils
I Liming of Soils
(66,600) (66,600) (66,600) NA NA NA NA NA NA
27,100 27,100 27,100 NA NA NA NA NA NA
8,088 10,224 7,687 7,722 8,455 9,191 8,882 8,702 10,943
plote: Numbers in parentheses indicate net carbon sequestration. Totals might not add up due to independent rounding.
£ NA: Not available.
management of non-forest mineral and organic soils and
liming of soils. However, it is important to note that
land-use and land-use change activities may also result
in fluxes of non-CO2 greenhouse gases, such as methane
(CH4), nitrous oxide (N2O), and carbon monoxide (CO),
to and from soils. For example, when lands are flooded
with freshwater, such as during hydroelectric dam con-
struction, CH4 is produced and emitted to the atmosphere
due to anaerobic decomposition of organic material in
the soil and water column. Conversely, when flooded
lands, such as lakes and wetlands, are drained, anaerobic
decomposition and associated CH4 emissions will be re-
duced. Dry soils are a sink of CH4, so eventually, drain-
age may result in soils that were once a source of CH4
becoming a sink of CH4. However, once the soils be-
come aerobic, oxidation of soil carbon and other organic
material will result in elevated emissions of CO2. More-
over, flooding and drainage may also affect net soil fluxes
of N2O and CO, although these fluxes are highly uncer-
tain. The fluxes of CH4, and other gases, due to flooding
and drainage are not assessed in this inventory due to a
lack of activity data on the extent of these practices in
the United States as well as scientific uncertainties about
the variables that control fluxes.8
Methodology and Data Sources
The methodologies used to calculate CO2 emis-
sions from use and management of mineral and organic
soils and from liming follow the Revised 1996 IPCC
Guidelines (TPCC/UNEP/OECD/IEA1997), except where
noted below.
The estimates of annual net CO2 flux from mineral
soils are based on work by Eve et al. (2000). Eve at al.
developed total mineral soil carbon stock estimates for
1982 and 1992 by applying the default IPCC carbon
stock and carbon adjustment factors to area estimates
derived from U.S. databases on climate (Daly et al. 1994,
1998), soil types and land use and management (USDA
1994), and tillage practices (CTIC 1998). These data-
bases were linked to obtain total area for each combined
climate/soil/land-use/tillage category in 1982 and 1992.
To derive carbon stock estimates for each year, the areas
for each combined category were multiplied by the de-
fault IPCC values for soil carbon under native vegeta-
tion, and base, tillage, and input factors. The base, till-
age, and input factors were adjusted to account for use of
a ten-year accounting period, rather than the 20-year
period used in the IPCC Guidelines. The changes in
carbon stocks between 1982 and 1992 for all categories
However, methane emissions due to flooding of rice fields are included, as are nitrous oxide emissions from agricultural soils. These
are addressed under the Rice Cultivation and Agricultural Soil Management sections, respectively, of the Agriculture chapter.
Land-Use Change and Forestry 6-9
-------
were then summed, and divided by ten, to obtain an
estimate of total average annual change in carbon C
stocks (i.e., net flux) for that period. The 1997 National
Resources Inventory, which will be a 1997 update of
USDA (1994), had not been completed at the time this
version of the U.S. Inventory was compiled. Publication
of the 7997 National Resources Inventory will enable
mineral soil carbon stock estimates for 1997 to be devel-
oped, which will allow for estimation of annual average
mineral soil carbon flux for 1993 through 1998.
The estimates of annual CO2 emissions from or-
ganic soils are also based on Eve et al. (2000). The
procedure used is similar to that for mineral soils, except
that organic soils under native vegetation were excluded
from the database under the assumption mat they are not
significantly affected by human activity. Following the
IPCC methodology, only organic soils under intense
management were included, and the default IPCC rates
of carbon loss were applied to the total 1982 and 1992
areas for the climate/land-use categories defined in the
IPCC Guidelines. The area estimates were derived from
the same climatic, soil, and land-use/land management
databases that were used in the mineral soil calculations
(Daly et al. 1994, 1998; USDA 1994). As with mineral
soils, producing estimates for 1993 through 1998 will
be possible once the 1997 National Resources Inven-
tory is published.
Carbon dioxide emissions from degradation of
limestone and dolomite applied to agricultural soils were
calculated by multiplying the annual amounts of lime-
stone and dolomite applied (see Table 6-8), by CO2 emis-
sion factors (0.120 metric ton C/metric ton limestone,
0.130 metric ton C/metric ton dolomite).9 These emis-
sion factors are based on the assumption that all of the
carbon in these materials evolves as CO2. The annual
application rates of limestone and dolomite were de-
rived from estimates and industry statistics provided in
the U.S. Geological Survey's Mineral Resources Program
Crushed Stone Reports and Mineral Industry Surveys
(USGS 1993; 1995; 1996; 1997a,b; 1998a,b; 1999a,b).
To develop these data, the Mineral Resources Program
obtained production and use information by surveying
crushed stone manufacturers. Because some manufac-
turers were reluctant to provide information, the esti-
mates of total crushed limestone and dolomite produc-
tion and use are divided into three components: (1)
production by end-use, as reported by manufacturers (i.e.,
"specified" production); (2) production reported by
manufacturers without end-uses specified (i.e., "unspeci-
fied" production); and (3) estimated additional produc-
tion by manufacturers who did not respond to the survey
(i.e., "estimated" production). To estimate the total
amounts of crushed limestone and dolomite applied to
agricultural soils, it was assumed that the fractions of
"unspecified" and "estimated" production that were ap-
plied to agricultural soils were equal to the fraction of
"specified" production that was applied to agricultural
soils. In addition, data were not available in 1990,1992,
and 1998 on the fractions of total crushed stone produc-
tion that were limestone and dolomite, and on the frac-
tions of limestone and dolomite production that were
applied to soils. To estimate these data, average annual
fractions were derived from data in the other years (i.e.,
1991, 1993, and 1994 through 1997) and were applied
to the total crushed stone production statistics in 1990,
1992, and 1998.
Table 6-8: Quantities of Applied Minerals (Thousand Metric Tons)
Description
Limestone
Dolomite
1
15
2
[990
,807
,417
1991
19,820
3,154
1992
15,024
2,297
1993
15,340
2,040
1994
16,730
2,294
1995
17,913
2,747
1996
17,479
2,499
1997
16,539
2,989
1
21,
3,
998
,337
,262
* Note: the default emission factor for dolomite provided in the Workbook volume of the Revised 1996 IPCC Guidelines (IPCC/UNEP/
OECD/IEA 1997) is incorrect. The value provided is 0.122 metric ton carbon/metric ton of dolomite; the correct value is 0.130 metric
ion carbon/metric ton of dolomite.
6-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Uncertainty
Uncertainties in the flux estimates for mineral and
organic soils result from both the activity data and the
carbon stock and adjustment factors. Each of the datasets
used in deriving the area estimates has a level of uncer-
tainty that is passed on through the analysis, and the
aggregation of data over large areas necessitates a cer-
tain degree of generalization. The default EPCC values
used for estimates of mineral soil carbon stocks under
native vegetation, as well as for the base, tillage and
input factors, carry with them high degrees of uncer-
tainty, as these values represent broad regional averages
based on expert judgment. Moreover, measured carbon
loss rates from cultivated organic soils vary by as much
as an order of magnitude. In addition, this methodology
does not take into account changes in carbon stocks and
land-use trends that occurred over longer time periods.
Uncertainties in the estimates of emissions from
liming stem primarily from the methodology, rather than
the underlying activity data. It can take several years for
agriculturally-applied lime to degrade completely. The
BPCC method assumes that the amount of mineral ap-
plied in any year is equal to the amount that degrades in
that year, so annual application rates can be used to de-
rive annual emissions. Further research is required to
determine applied limestone degradation rates. More-
over, soil and climatic conditions are not taken into ac-
count in the calculations.
Table 6-9: Net C02 Flux from Non-Forest Carbon
Stocks in Landfills
^
r
i
E
r
t-
|
IK-
f-
T"
£-
Year
1990
1991
1992
1993
1994
1995
1996
1997
1998
MMTCE
(4.9)
(4.8)
(4.7)
(4.2)
(3.7)
(3-3)
(2.7)
(2.6)
(2.3)
Gg
(17,800)
(17,500)
(17,100)
(15,300)
(13,600)
(12,000)
(10,000)
(9,400)
(8,300)
|,ftote: Parentheses indicate sequestration.
Changes in Non-Forest Carbon
Stocks in Landfills
As is the case with landfilled forest products, car-
bon contained in landfilled yard trimmings can be stored
indefinitely. In the United States, yard trimmings (i.e.,
grass clippings, leaves, branches) comprise a significant
portion of the municipal waste stream. In 1990, the EPA
estimated discards of yard trimmings to landfills at over
21 million metric tons. Since then, programs banning or
discouraging disposal, coupled with a dramatic rise in
the number of composting facilities, have decreased the
disposal rate for yard trimmings; the 1998 landfill dis-
posal was about 10 million metric tons. The decrease in
the yard trimmings landfill disposal rate has resulted in
a decrease in the rate of landfill carbon storage from
about 4.9 MMTCE in 1990 to 2.3 MMTCE in 1998 (see
Table 6-9).
Yard trimmings comprise grass, leaves, and
branches and have long been a significant component of
the U.S. waste stream. In 1990, discards (i.e., landfilling
plus combustion) of yard trimmings were about 27.9
million metric tons, representing 17.9 percent of U.S.
disposal of municipal solid waste (EPA 1999). Unlike
most of the rest of the waste stream, yard trimmings dis-
posal has declined consistently in the 1990s—genera-
tion has declined at 3.3 percent per year, and recovery
(e.g., composting) has increased at an average annual
rate of 15 percent. Laws regulating disposal of yard
trimmings now affect over 50 percent of the U.S. popula-
tion, up from 28 percent in 1992 (EPA 1999). By 1997,
discards were about 15 million metric tons, representing
10 percent of U.S. municipal waste disposal.
Methodology
The methodology for estimating carbon storage is
based on a life cycle analysis of greenhouse gas emis-
sions and sinks associated with solid waste management
(EPA 1998). According to this methodology, carbon stor-
age is the product of the mass of yard trimmings dis-
posed, on a wet weight basis and a storage factor. The
storage factor is based on a series of experiments de-
signed to evaluate methane generation and residual or-
ganic material in landfills under average conditions
Land-Use Change and Forestry 6-11
-------
(Barlaz 1997). These experiments analyzed grass, leaves,
branches, and other materials, and were designed to pro-
mote biodegradation by providing ample moisture and
nutrients.
For purposes of this analysis, the composition of
yard trimmings was assumed to consist of 50 percent
grass clippings, 25 percent leaves, and 25 percent
branches. A different storage factor was used for each
component. The weighted average carbon storage fac-
tor is 0.19 Gg carbon per Gg of yard trimmings, as shown
in Table 6-10. Results, in terms of carbon storage, are
also shown.
Data Sources
The yard trimmings discard rate was taken from
the EPA report Characterization of Municipal Solid
Waste in the U.S.: 1998 Update (EPA 1999), which pro-
vides estimates for 1990 through 1997 and forecasts for
2000 and 2005. Yard trimmings discards for 1998 were
projected using the EPA (1999) forecast of generation
and recovery rates (decrease of 6 percent per year, in-
crease of 8 percent per year, respectively) for 1997
through 2000. This report does not subdivide discards
of individual materials into volumes landfilled and com-
busted, although it does provide an estimate of the over-
all distribution of solid waste between these two man-
Table 6-10: Composition of Yard Trimmings (%)
in MSW and Carbon Storage Factor
(Gg Carbon/Gg Yard Trimmings)
Component
Grass
Leaves
Branches
Total/
Weighted Average
Percent
50
25
25
100
Storage
Factor
0.11
0.36
0.19
0.19
agement methods (76 percent and 24 percent, respec-
tively) for the waste stream as a whole.10 Thus, yard
trimmings disposal to landfills is the product of the quan-
tity discarded and the proportion of discards managed
in landfills (see Table 6-11). The carbon storage factors
were obtained from EPA (1998).
Uncertainty
The principal source of uncertainty for the landfill
carbon storage estimates stem from an incomplete un-
derstanding of the long-term fate of carbon in landfill
environments. Although there is ample field evidence
that many landfilled organic materials remain virtually
intact for long periods, the quantitative basis for pre-
dicting long-term storage is based on limited laboratory
results under experimental conditions. In reality, there
is likely to be considerable heterogeneity in storage rates,
based on (1) actual composition of yard trimmings (e.g.,
oak leaves decompose more slowly than grass clippings)
and (2) landfill characteristics (e.g., availability of mois-
ture, nitrogen, phosphorus, etc.). Other sources of uncer-
tainty include the estimates of yard trimmings disposal
rates—which are based on extrapolations of waste com-
position surveys, and the extrapolation of a value for
1998 disposal from estimates for the period from 1990
through 1997.
Table 6-11: Yard Trimmings Disposal to Landfills
Year
1990
1991
1992
1993
1994
1995
1996
1997
1998
Metric Tons
21,236,000
20,822,000
20,408,000
18,168,000
16,203,000
14,265,200
11,962,300
11,197,000
9,929,500
.
10 Note that this calculation uses a different proportion for combustion than an earlier calculation in the waste combustion section
of Chapter 6. The difference arises from different sources of information with different definitions of what is included in the solid
waste stream.
6-12 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
7. Waste
lAfaste management and treatment activities are sources of greenhouse gas emissions (See Figure 7-1).
• If Landfills are the nation's largest source of anthropogenic methane emissions, accounting for 33 percent
of the U.S. total.1 Waste combustion is the second largest source in this sector, emitting carbon dioxide (CO2) and
nitrous oxide (N2O). Smaller amounts of methane are emitted from wastewater systems by bacteria used in various
treatment processes. Wastewater treatment systems are also a potentially significant source of N2O emissions;
however, methodologies are not currently available to develop a complete estimate. Nitrous oxide emissions from
the treatment of the human sewage component of wastewater were estimated, however, using a simplified methodol-
ogy. Nitrogen oxide (NOX), carbon monoxide (CO), and non-methane volatile organic compounds (NMVOCs) are
emitted by each of these sources, and are addressed separately at the end of this chapter. Asummary of greenhouse gas
emissions from the Waste chapter is presented in Table 7-1 and Table 7-2.
Overall, in 1998, waste activities generated emissions of 65.4 MMTCE, or 3.6 percent of total U.S. greenhouse
gas emissions.
Figure 7-1
Portion of All
Emissions
Human Sewage
Wastewater
Treatment
' ii i 1 1 1 1
0 10 20 30 40 50 60 70
MMTCE
Landfills
Landfills are the largest anthropogenic source of
methane (CH4) emissions in the United States. In 1998,
landfill emissions were approximately 58.8 MMTCE
(10,268 Gg). Emissions from municipal solid waste
(MSW) landfills, which received about 61 percent of the
total solid waste generated in the United States, ac-
counted for about 93 percent of total landfill emissions,
while industrial landfills accounted for the remainder.
Landfills also emit non-methane volatile organic com-
pounds (NMVOCs). There are over 2,300 landfills in the
United States (BioCycle 1999), with the largest landfills
receiving most of the waste and generating the majority
of the methane.
Waste 7-1
-------
Table 7-1: Emissions from Waste (MMTCE)
« ""
-Gas/Source
•
CH,
«»4
Landfills
i WastewaterTreatment
NoO
: Human Sewage
: Waste Combustion
i CO,
Waste Combustion
Total
Note: Totals may not sum due to
1990
59.1
58.2
0.9
2.1
2.0
0.1
2.8
2.8
64.0
1991
59.0
58.1
0.9
2.1
2.0
0.1
3.0
3.0
64.1
1992
60.0
59.1
0.9
2.1
2.0
0.1
3.0
3.0
65.1
1993
60.5
59.6
0.9
2.1
2.0
0.1
3.1
3.1
65.7
1994
60.8
59.9
0.9
2.2
2.1
0.1
3.1
3.1
66.1
1995
61.4
60.5
0.9
2.2
2.1
0.1
3.0
3.0
66.6
1996
61.1
60.2
0.9
2.2
2.1
0.1
3.1
3.1
66.4
1997
61.1
60.2
0.9
2.2
2.1
0.1
3.4
3.4
66.7
"1998 '
59.7
58.8
0.9
2.2
2.2
0.1
3.5
3.5 .
65.4
independent rounding.
Table 7-2: Emissions from Waste (Gg)
Gas/Source
CH4
Landfills
WastewaterTreatment
Nn
Human Sewage
Waste Combustion
CO,
Waste Combustion
Note: Totals may not sum due to
1990
10,320
10,170
150
24
23
1
10,344
10,344
independent
1991
10,303
10,151
151
24
24
1
10,931
10,931
rounding.
1992
10,475
10,321
153
25
24
1
10,992
10,992
1993
10,557
10,401
155
25
24
1
11,295
11,295
1994
10,608
10,451
156
26
25
1
11,307
11,307
1995
10,724
10,566
158
25
25
1
11,104
11,104
1996
10,667
10,507
159
26
25
1
11,504
11,504
1997
10,671
10,509
161
26
25
12,531
12,531
1998
10,430
10,267
162
26
25
-|
12,889
12,889
Methane emissions result from the decomposition
of organic landfill materials such as paper, food scraps,
and yard trimmings. This decomposition process is a
natural mechanism through which microorganisms de-
rive energy. After being placed in a landfill, organic
waste is initially digested by aerobic (i.e., in the pres-
ence of oxygen) bacteria. After the oxygen supply has
been depleted, the remaining waste is attacked by anaero-
bic bacteria, which break down organic matter into sub-
stances such as cellulose, amino acids, and sugars. These
substances are further broken down through fermenta-
tion into gases and short-chain organic compounds that
form the substrates for the growth of methanogenic bac-
teria. Methane-producing anaerobic bacteria convert
these fermentation products into stabilized organic ma-
terials and biogas consisting of approximately 50 per-
cent carbon dioxide (CO^ and 50 percent methane, by
volume.2 Methane production typically begins one or
two years after waste disposal in a landfill and may last
from 10 to 60 years.
Between 1990 and 1998, methane emissions from
landfills were relatively constant (see Table 7-3 and Table
7-4). The roughly constant emissions estimates are a re-
sult of two offsetting trends: (1) the amount of MSW in
landfills contributing to methane emissions increased,
thereby increasing the potential for emissions; and (2) the
amount of landfill gas collected and combusted by land-
fill operators also increased, thereby reducing emissions.
Methane emissions from landfills are a function of
several factors, including: (1) the total amount of MSW
in landfills, which is related to total MSW landfilled
annually for the last 30 years; (2) composition of the
waste-in-place; (3) the amount of methane that is recov-
ered and either flared or used for energy purposes; and
(4) the amount of methane oxidized in landfills instead
2 The percentage of CO2in biogas released from a landfill may be smaller because some CO2 dissolves in landfill water (Bingemer and
Crutzen 1987).
7-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Box 7-1: Biogenic Emissions and Sinks of Carbon
p- For many countries, C02 emissions from the combustion or degradation of biogenic materials is important because of the
^'significant amount of energy they derive from biomass (e.g., burning fuelwood). The fate of biogenic materials is also important when
i- evaluating waste management emissions (e.g., the decomposition of grass clippings or combustion of paper). The carbon contained
;: in paper and grass trimmings was originally removed from the atmosphere by photosynthesis, and under natural conditions, it would
eventually degrade and cycle back to the atmosphere as C02. The quantity of carbon that these degredation processes cycle through
the Earth's atmosphere, waters, soils, and biota is much greater than the quantity added by anthropogenic greenhouse gas sources.
But the focus of the United Nations Framework Convention on Climate Change is on anthropogenic emissions—emissions resulting
from human activities and subject to human control—because it is these emissions that have the potential to alter the climate by
! disrupting the natural balances in carbon's biogeochemical cycle, and enhancing the atmosphere's natural greenhouse effect.
'. Thus, if C02 emissions from biogenic materials (e.g., paper, wood products, and yard trimmings) result from materials grown on
: a sustainable basis, then those emissions are considered to mimic the closed loop of the natural carbon cycle —that is, they return
, to the atmosphere C02 that was originally removed by photosynthesis. Conversely, C02 emissions from burning fossil fuels or
i products such as plastics derived from fossil sources would not enterthe cycle were it not for human activity (i.e., they were removed
i from permanent fossil deposits). Likewise, CH4 emissions from landfilled waste would not be emitted were it not for the man-made
anaerobic conditions conducive to CH4 formation that exist in landfills.
' However, the removal of carbon from this cycling of carbon between the atmosphere and biogenic materials—which occurs when
; wastes of sustainable, biogenic origin (e.g., yard trimmings) are deposited in landfills—sequesters carbon. When wastes of sustain-
; able, biogenic origin are landfilled, and do not completely decompose, the carbon that remains is effectively removed from the global
carbon cycle. Landfilling of forest products and yard trimmings results in long-term storage of about 19 MMTCE and 2 to 5 MMTCE
; per year, respectively. Carbon storage that results from forest products and yard trimmings disposed in landfills is accounted for in
: Chapter 6 to comport with IPCC inventory reporting guidance regarding the tracking of carbon flows.
Box 7-2: Recycling and Greenhouse Gas Emissions and Sinks
jr • U.S. waste management patterns changed dramatically in the 1990s in response to changes in economic and regulatory factors.
| Perhaps the most significant change from a greenhouse gas perspective was the increase in the national average recycling rate, which
h climbed from 16 percent in 1990 to 28 percent in 1997 (EPA 1999).
I This change had an important effect on emissions in several areas, primarily in regard to emissions from waste and energy
"activities, as well as forestry sinks. The impact of increased recycling on greenhouse gas emissions can be best understood when
K emissions are considered from a life cycle perspective (EPA 1998). When a material is recycled, it is used in place of virgin inputs in
the manufacturing process, rather than being disposed and managed as waste. The substitution of recycled inputs for virgin inputs
reduces three types of emissions throughout the product life cycle. First, manufacturing processes involving recycled inputs generally
require less energy than those using virgin inputs. Second, the use of recycled inputs leads to reductions in process non-energy
emissions. Third, recycling reduces disposal and waste management emissions, including methane from landfills and nitrous oxide
and non-biogenic carbon dioxide emissions from combustion. In addition to greenhouse gas emission reductions from manufactur-
ing and disposal, recycling of paper products—which are the largest component of the U.S. wastestream—results in increased forest
carbon sequestration. When paper is recycled, fewer trees are needed as inputs in the manufacturing process; reduced harvest levels
result in older average forest ages, with correspondingly more carbon stored.
Waste 7-3
-------
of being released into the atmosphere. The estimated
total quantity of waste-in-place contributing to emis-
sions increased from about 4,926 Gg in 1990 to 5,907
Gg in 1998, an increase of 20 percent (see Annex J).
During this period, the estimated methane recovered and
flared from landfills increased as well. In 1990, for ex-
ample, approximately 1,110 Gg of methane was recov-
ered and combusted (i.e., used for energy or flared) from
landfills. In 1998, the estimated quantity of methane
recovered and combusted increased to 3,590 Gg.
Over the next several years, the total amount of
MSW generated is expected to increase slightly. The
percentage of waste landfilled, however, may decline
due to increased recycling and composting practices. In
addition, the quantity of methane that is recovered and
either flared or used for energy purposes is expected to
increase, partially as a result of a recently promulgated
regulation that requires large landfills to collect and
combust landfill gas (Federal Register 1996).
Methodology
Based on available information, methane emis-
sions from landfills were estimated to equal the methane
produced from municipal landfills, minus the methane
recovered and combusted, minus the methane oxidized
before being released into the atmosphere, plus the meth-
ane produced by industrial landfills.
The methodology for estimating CH4 emissions
from municipal landfills is based on a model that up-
dates the population of U.S. landfills each year. This
model is based on the pattern of actual waste disposal by
each individual landfill surveyed by the EPA's Office of
Solid Waste in 1987. A second model was employed to
estimate emissions from the landfill population (EPA
1993). For each landfill in the data set, the amount of
waste-in-place contributing to methane generation was
estimated using its year of opening, its waste acceptance
rate, year of closure, and design capacity. Data on na-
tional waste disposed in landfills each year was appor-
tioned by landfill. Emissions from municipal landfills
Table 7-3: CH4 Emissions from Landfills (MMTCE)
• Activity
MSW Landfills
Industrial Landfills
Recovered
Gas-to-Energy
Flared
Net Emissions
Note: Totals may not sum
1990
60.4
4.2
(4.6)
(1.7)
58.2
due to independent
1991
61.8
4.3
(4.9)
(3.0)
58.1
rounding.
1992
63.6
4.4
(5.2)
(3.6)
59.1
1993
65.5
4.5
(6.0)
(4.4)
59.6
1994
67.5
4.6
(6.8)
(5.5)
59.9
1995
69.5
4.8
(7.1)
(6.7)
60.5
1996
71.1
4.9
(8.0)
(7.8)
60.2
1997
72.8
5.0
(9.2)
(8.3)
60.2
Table 7-4: CH4 Emissions from Landfills (Gg)
Activity
MSW Landfills
Industrial Landfills
Recovered
Gas-to-Energy
Flared
Net Emissions
Note: Totals may not sum
1990
10,550
731
(811)
(299)
10,171
due to independent
1991
10,791
746
(861)
(524)
10,152
rounding.
1992
11,107
767
(915)
(637)
10,321
1993
11,431
787
(1,053)
(764)
10,402
1994
11,777
809
(1,183)
(952)
10,452
1995
12,138
833
(1,233)
(1,171)
10,566
1996
12,419
850
(1,397)
(1,363)
10,508
1997
12,705
868
(1,608)
(1,454)
10,510
1998 ;
74.3
5.1
(11.6
(9.0)
58.8
--'-r :
1998
12,974
883
(2,025)
(1,564)
10,268
7-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
were then estimated by multiplying the quantity of waste
contributing to emissions by emission factors (EPA 1993).
For further information see Annex J.
The estimated landfill gas recovered per year was
based on updated data collected from vendors of flaring
equipment, and a database compiled by the EPA's Land-
fill Methane Outreach Program (LMOP). Based on the
information provided by vendors, the methane com-
busted by the 235 flares in operation from 1990 to 1998
were estimated. This estimate likely underestimates
emissions. The EPA believes that more than 700 flares
exist in the United States, and so the EPA is working
with the Solid Waste Association of North America
(SWANA) to better characterize flaring activities. Addi-
tionally, the LMOP database provided data on landfill
gas flow and energy generation for 237 of the approxi-
mately 260 operational landfill gas-to-energy projects.
Emissions from industrial landfills were assumed
to be equal to 7 percent of the total methane emissions
from municipal landfills. The amount of methane oxi-
dized was assumed to be 10 percent of the methane gen-
erated (Liptay et al. 1998). To calculate net methane
emissions, methane recovered and oxidized was sub-
tracted from methane generated at municipal and indus-
trial landfills.
Data Sources
The landfill population model, including actual
waste disposal data from individual landfills, was devel-
oped from a survey performed by the EPA's Office of
Solid Waste (EPA 1988). National landfill waste dis-
posal data for 1991 through 1998 were obtained from
BioCycle (1999). Documentation on the landfill meth-
ane emissions methodology employed is available in
the EPA's Anthropogenic Methane Emissions in the
United States, Estimates for 1990: Report to Congress
(EPA 1993). Information on flares was obtained from
vendors, and information on landfill gas-to-energy
projects was obtained from the LMOP database.
Uncertainty
Several types of uncertainties are associated with
the estimates of methane emissions from landfills. The
primary uncertainty concerns the characterization of
landfills. Information is lacking on the area landfilled
and total waste-in-place—the fundamental factors that
affect methane production. In addition, the statistical
model used to estimate emissions is based upon meth-
ane generation at landfills that currently have devel-
oped energy recovery projects, and may not precisely
capture the relationship between emissions and various
physical characteristics of individual landfills. Overall,
uncertainty in the landfill methane emission rate is esti-
mated to be roughly ±30 percent.
Waste Combustion
Waste combustion involves the burning of garbage
and non-hazardous solids, referred to as municipal solid
waste (MSW). In 1996, there were approximately 137
municipal waste combustion plants in operation within
the United States (EPA 1999). Most of the organic materi-
als in MSW are of biogenic origin. Net CO2 emissions
resulting from combustion of biogenic materials are ac-
counted for under Land-Use Change and Forestry (see Box
7-1). However, one component—plastics—is of fossil ori-
gin, and is included as a source of CO2 emissions. Plastics
in the U.S. wastestream are primarily in the form of con-
tainers, packaging, and durable goods. Some other mate-
rials in the waste stream (e.g., some textiles and rubber) are
of fossil origin, but are not included in this estimate.
In addition, MSW combustion has been identified
as a source of nitrous oxide (N2O) emissions. N2O emis-
sions are dependent on the types of waste burned and
combustion temperatures (De Soete 1993).
Carbon dioxide emissions have risen 25 percent
since 1990, to about 3.5 MMTCE (12,900 Gg) in 1998, as
the volume of plastics hi MSW has increased (see Table
7-5 and Table 7-6). Nitrous oxide emissions from MSW
combustion were estimated to be 0.1 MMTCE (1 Gg) in
1998, and have not changed significantly since 1990.
Waste 7-5
-------
Table 7-5: C02 and N20 Emissions
from Waste Combustion (MMTCE)
Year
1990
1991
1992
1993
1994
1995
1996
1997
1998
C02
2.8
3.0
3.0
3.1
3.1
3.0
3.1
3.4
3.5
N20
0.1
0.1
0.1
0.1 ;
0.1
0.1 ;
0.1
0.1 ;
0.1
Table 7-6: C02 and N20 Emissions
from Waste Combustion (Gg)
Year
1990
1991
1992
1993
1994
1995
1996
1997
1998
C02
10,000
10,900
11,000
11,300
11,300
11,100
11,500
12,600
12,900
N20
1 ' :
1
1
1
1 :
1
1 ;
1 :
1
Methodology
In the report, Characterization of Municipal Solid
Waste in the United States (EPA 1999), the flows of plas-
tics in the U.S. wastestream are reported for seven resin
categories. The 1997 quantity generated, recovered, and
discarded for each resin is shown in Table 7-7. The re-
port does not provide estimates for individual materials
landfilled and combusted, although it does provide such
an estimate for the waste stream as a whole. To estimate
the quantity of plastics landfilled and combusted, total
discards were apportioned based on the proportions of
landfilling and combustion for the entire U.S. wastestream
(76 percent and 24 percent, respectively).
Fossil CO2 emissions for 1997 were estimated as
the product of plastic combusted, carbon content, and
combustion efficiency (see Table 7-8). The carbon con-
tent of each of the six types of plastics is listed, with the
value for "other plastics" assumed equal the weighted
average of the six categories. A combustion efficiency
of 98 percent was assumed.
Emissions for 1990 through 1996 were calculated
using the same approach. Estimates of the portion of
Table 7-7: 1997 Plastics in the Municipal Solid Waste Stream by Resin (Thousand Metric Tons)
Waste Pathway
Generation
Recovery
Discard
= Landfill
Combustion
Recovery*
Discard*
: Landfill*
Combustion*
PET
1,724
327
1,397
1,061
336
19%
81%
62%
19%
HOPE
4,200
381
3,819
2,903
916
9%
91%
69%
22%
PVC
1,198
0
1,198
910
288
0%
100%
76%
24%
LDPE/
LLDPE
4,881
91
4,790
3,641
1,149
2%
98%
75%
24%
PP
2,531
109
2,422
1,841
582
4%
96%
73%
23%
PS
1,905
9
1,896
1,441
455
0.5%
99.5%
76%
24%
Other
3,030
91
2,939
2,234
706
3%
97%
74%
23%
Total
19,469
1,007
18,462
14,030
4,432
5%
95%
72%
23%
" * As a percent of waste generation.
Note: Totals may not sum due to independent rounding. PET (polyethylene terephthalate), HOPE (high density polyethylene), PVC (polyvmyl
-chloride), LDPE/LLDPE ((linear) low density polyethylene), PP (polypropylene), PS (polystyrene).
7-6 inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Table 7-8: 1997 Plastics Combusted (Thousand Metric Tons), Carbon Content (%), and
Carbon Equivalent Combusted (Thousand Metric Tons)
5.1"
IJFactor
1 Quantity Combusted
fcparbon Content of Resin
jfCarbon Equivalent Combusted
iEmlssions (MMTCE)"
PET
336
62.5
210
0.2
HOPE
916
85.7
785
0.8
PVC
288
38.1
110
0.1
LDPE/
LLDPE
1,149
85.7
985
1.0
PP
582
85.7
498
0.5
PS
455
92.3
420
0.4
Other
706
67.9a
479
0.5
Total
4,432
3,487
3.4
IM Weighted average of other plastics.
j-P^Assumes 98 percent combustion efficiency.
St.
plastics in the wastestream in 1998 were not available;
therefore, they were projected by assuming 3 percent
annual growth rate in generation and a 5.4 percent growth
rate for recovery, based on reported trends (EPA 1999).
Estimates of N2O emissions from MSW combus-
tion in the United States are based on the methodology
outlined in the EPA's Compilation of Air Pollutant Emis-
sion Factors (EPA 1997). According to this methodol-
ogy, emissions of N2O from MSW combustion is the prod-
uct of the mass of MSW combusted, an emission factor
of N2O emitted per unit mass of waste combusted, and an
N2O emissions control removal efficiency. For MSW
combustion in the United States, an emission factor of
30 g N2O/metric ton MSW, and an estimated emissions
control removal efficiency of zero percent were used.
Data Sources
The estimates of CO2 emissions andN2O emissions
are based on different data sources. The fossil CO2 emis-
sions are a function of a specific material—plastics—as
reported by EPA (1999) in its characterization of the
municipal wastestream. The N2O emissions are a func-
tion of total waste combusted, as reported in the April
1999 issue of BioCycle (Glenn 1999). Table 7-9 pro-
vides MSW generation and percentage combustion data
for the total wastestream. The emission factor of N2O
emissions per quantity of MSW combusted was taken
from Olivier (1993).
Table 7-9: Municipal Solid Waste Generation
(Metric Tons) and Percent Combusted
|:
P"
IK:
£.-,-
ifc:
pa,-
ir
fe::'
|r;
5" .
Year
1990
'1991
1992
1993
1994
1995
1996
1997
1998
Waste Generation
266,541,881
254,796,765
264,843,388
278,572,955
293,109,556
296,586,430
297,268,188
309,075,035
340,090,022
Combusted (%)
11.5
10.0
11.0
10.0
10.0
10.0
10.0
9.0
7.5
As noted above, CO2 emissions from plastics are
based on (1) the carbon content of the various plastic
resins, and (2) an assumption of 98 percent combustion
efficiency, as reported in the EPA's life cycle analysis of
greenhouse gas emissions and sinks from management
of solid waste (EPA 1998).
Uncertainty
A source of uncertainty affecting both fossil CO2
and N2O emissions is the estimate of the MSW combus-
tion rate. The EPA (1999) estimates of plastics genera-
tion, discards, and combustion are subject to consider-
able error. Similarly, the BioCycle (Glenn 1999) esti-
mate of total waste combustion—used for the N2O esti-
mate—is based on a survey of state officials, who use
Waste 7-7
-------
differing definitions of solid waste and who draw from a variety
of sources of varying reliability and accuracy. Despite the differ-
ences in methodology and data sources, the two references—
EPA's Office of Solid Waste (EPA 1999) and BioCycle (Glenn
1999)—provide estimates of total solid waste combusted that
are relatively consistent (see Table 7-10).
The other principal source of uncertainty for the carbon
dioxide estimate is combustion efficiency. Municipal waste
combustors vary considerably in their efficiency as a function
of waste type, moisture content, combustion conditions, and
otherfactors. Thevalueof98percentassumedheremaynotbe
representative of typical conditions.
As with other combustion-related sources of N2O,
emissions are affected by combustion conditions (De
Soete 1993). In part, because insufficient data exists to
provide detailed estimates of N2O emissions for indi-
vidual combustion facilities, the estimates presented are
highly uncertain. The emission factor for N2O from MS W
combustion facilities used in the analysis is a default
used to estimate N2O emissions from facilities worldwide
(Olivier 1993). As such, it has a range of uncertainty of
an order of magnitude (between 25 and 293 g N2O/metric
ton MSW combusted) (Watanabe, et al. 1992). Due to a
lack of relevant information on the control of N2O emis-
sions from MSW combustion facilities in the United
States, the estimate of zero percent for N2O emissions
control removal efficiency is also uncertain.
Wastewater Treatment
The breakdown of organic material in wastewater
treatment systems produces methane when it occurs un-
der anaerobic conditions. The amount of methane pro-
duced is driven by the extent to which the organic ma-
terial is broken down under anaerobic versus aerobic
conditions. During collection and treatment, wastewa-
ter may be incidentally or deliberately managed under
anaerobic conditions. The methane produced during
deliberate anaerobic treatment is typically collected and
flared or combusted for energy. However, whenever
anaerobic conditions develop, some of the methane gen-
erated is incidentally released to the atmosphere. Un-
treated wastewater may also produce methane if con-
tained under anaerobic conditions.
The organic content, expressed in terms of bio-
chemical oxygen demand (BOD), determines the meth-
ane producing potential of wastewater. BOD represents
the amount of oxygen that would be required to com-
pletely consume the organic matter contained in the
wastewater through aerobic decomposition processes.
Under anaerobic conditions, wastewater with higher
BOD concentrations will produce more methane than
wastewater with lower BOD.
In 1998, methane emissions from municipal waste-
water were 0.9 MMTCE (163 Gg). Emissions have in-
creased since 1990 reflecting the increase in the U.S.
human population. Table 7-11 provides emission esti-
mates from domestic wastewater treatment.
Table 7-10: U.S. Municipal Solid Waste Combusted
by Data Source (Metric Tons)
Table 7-11: CH4 Emissions from
Domestic Wastewater Treatment
Year
1990
1991
1992
1993
1994
1995
1996
1997
1998
NA (Not Available)
EPA
28,939,680
30,236,976
29,656,638
29,865,024
29,474,928
32,241,888
32,740,848
32,294,240
NA
BioCycle
30,652,316
25,479,677
29,132,773
27,857,295
29,310,956
29,658,643
29,726,819
27,816,753
25,506,752
f~ — r-B vsrr
; I*
' I-
' fl "
*
: a-
i k_
.; t.
£
h
i
! f"
: 'fir
Year
1990
1991
1992
1993
1994
1995
1996
1997
1998
-Hi-,- - • ,",-
MMTCE
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
Gg
150
152
154
155
157
158
160
161
163
7-8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
At this time, data are not sufficient to estimate
methane emissions from industrial wastewater sources.
Further research is ongoing to quantify emissions from
this source.
Methodology
Wastewater methane emissions are estimated us-
ing the default IPCC methodology (IPCC/UNEP/OECD/
IEA 1997). The total population for each year was mul-
tiplied by a per capita wastewater BOD production rate
to determine total wastewater BOD produced. It was
assumed that, per capita, 0.05 kilograms of wastewater
BOD53 is produced per day and that 15 percent of waste-
water BODS is anaerobically digested. This proportion
of BOD was then multiplied by an emission factor of
0.22 GgCH4/Gg BODS.
Data Sources
National population data for 1990 to 1998 were
supplied by the U.S. Census Bureau (1999). The emis-
sion factor employed was taken from Metcalf and Eddy
(1972). Table 7-12 provides U.S. population and waste-
water BOD data.
Uncertainty
Domestic wastewater emissions estimates are
highly uncertain due to the lack of data on the occur-
rence of anaerobic conditions in treatment systems, es-
pecially incidental occurrences. It is also believed that
industrial wastewater is responsible for significantly more
methane emissions than domestic wastewater treatment.
Human Sewage
Sewage is disposed on land or discharged into
aquatic environments such as rivers and estuaries. Prior
to being disposed on land or in water, it may be depos-
ited in septic systems or treated in wastewater treatment
facilities. Nitrous oxide (N2O) may be generated during
each of these stages through nitrification and denitrifi-
cation of the nitrogen that is present in sewage. Nitrifi-
cation occurs aerobically and converts ammonium into
nitrate, while denitrification occurs anaerobically, and
converts nitrate into dinitrogen gas. Nitrous oxide is a
gaseous intermediate product in the reaction sequences
of both processes. In general, temperature, pH, biochemi-
cal oxygen demand (BOD), and nitrogen concentration
affect N2O generation from human sewage. BOD is the
amount of dissolved oxygen used by aerobic microor-
ganisms to completely consume the available organic
matter (Metcalf and Eddy 1972). The amount of protein
consumed by humans determines the quantity of nitro-
gen contained in sewage.
Nitrous oxide emission from human sewage were
estimated using the IPCC default methodology (IPCC/
UNEP/OECD/IEA 1997) with one exception. The IPCC
methodology assumes that N2O emissions associated
with land disposal and sewage treatment are negligible
and all sewage nitrogen is discharged directly into
aquatic environments. In the United States, however, a
certain amount of sewage nitrogen is applied to soils via
sewage sludge applications and therefore, not all sew-
age nitrogen enters aquatic environments.4 The N2O
estimates presented here account for the amount of ni-
trogen in sewage sludge applied to soils.
Table 7-12: U.S. Population (Millions)
and Wastewater BOD Produced (Gg)
Year
Population
BODS
1990
1991
1992
1993
1994
1995
1996
1997
1998
249.3
252.0
254.9
257.7
260.2
262.7
265.1
267.7
270.2
4,554
4,602
4,655
4,706
4,752
4,797
4,842
4,888
4,935
3 The 5-day biochemical oxygen demand (BOD) measurement (Metcalf and Eddy 1972).
4 The IPCC methodology is based on the total amount of nitrogen in sewage, which is in turn based on human protein consumption and
the fraction of nitrogen in protein (i.e., FracNPR). A portion of the total nitrogen in sewage in the United States is applied to soils in the
form of sewage sludge each year. This amount is estimated as part of agricultural soil management (see Chapter 6) and is subtracted
here from total nitrogen in human sewage to estimate sewage N2O emissions.
Waste 7-9
-------
Table 7-13: N20 Emissions from Human Sewage
Year
1990
1991
1992
1993
1994
1995
1996
1997
1998
MMTCE
2.0
2.0
2.0
2.0
2.1
2.1
2.1
2.1
2.2
Gg
23
24
24
24
25
25
25
25
25
Emissions of N2O from sewage nitrogen discharged
into aquatic environments were estimated to be 2.2
MMTCE (25 Gg N2O) in 1998. An increase in the U.S.
population and the per capita protein intake resulted in
an overall increase of 10 percent in N2O emissions from
human sewage between 1990 and 1998 (see Table 7-13).
Methodology
With the exception described above, N2O emis-
sions from human sewage were estimated using the IPCC
default methodology (EPCC/UNEP/OECD/IEA 1997).
This is illustrated below:
N2O(s) = [(Protein) x (Frac,^) x (NR People)] x
[l-FracN.SOIL]x(EF)x(44/28)
Where:
N2O(s) = N2O emissions from human sewage
Protein = Annual, per capita protein consumption
FracNPR = Fraction of nitrogen in protein
NR People = U.S. population
FracN^OIL = Fraction of sewage sludge N applied
to soils
EF = Emission factor (kg N20-N/kg sewage-N
produced)
(44/28) = The molecular weight ratio of N2O to N2
Data Sources
U.S. population data were taken from the U.S. Cen-
sus Bureau (1999). Data on the annual per capita pro-
tein consumption were provided by the United Nations
Food and Agriculture Organization (FAO1999) (see Table
7-14). Because data on protein intake were unavailable
for 1998, the value of per capita protein consumption
for the previous year was used. An emission factor has
not been specifically estimated for the United States, so
the default IPCC value (0.01 kg N2O-N/kg sewage-N
produced) was applied. Similarly, the fraction of nitro-
gen in protein (0.16 kg N/kg protein) was also obtained
from IPCC/UNEP/OECD/IEA (1997).
Uncertainty
The U.S. population (NR people), per capita pro-
tein intake data (Protein), and fraction of nitrogen in
protein (Frac^) are believed to be fairly accurate. There
is significant uncertainty, however, in the emission fac-
tor (EF) employed due to regional differences that would
likely affect N2O emissions but are not accounted for in
the default IPCC factor. Moreover, the underlying meth-
odological assumption that negligible N2O emissions
result from sewage treatment may be incorrect. In addi-
tion N2O emissions from industrial wastewater, which
have not been addressed in the IPCC Guidelines, have
not been estimated.
Table 7-14: U.S. Population (Millions) and Average
Protein Intake (kg/Person/Year)
Year
1990
1991
1992
1993
1994
1995
1996
1997
1998
Population
249.3
252.0
"254.9
257.7
260.2
262.7
^265.1
267.7
270.2
Protein
39.2
39.8
40.1
40.1
41.0
40.4
40.8
41.0
41.0
•••"'-• ' -'••'• "1
7-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Waste Sources of Criteria Pollutants
In addition to the main greenhouse gases addressed
above, waste generating and handling processes are also
sources of criteria air pollutant emissions. Total emis-
sions of nitrogen oxides (NOX), carbon monoxide (CO),
and nonmethane volatile organic compounds (NMVOCs)
from waste sources for the years 1990 through 1998 are
provided in Table 7-15.
Methodology and Data Sources
These emission estimates were taken directly from
theEPA's National Air Pollutant Emissions Trends, 1900-
1998 (EPA 1999). This EPA report provides emission
estimates of these gases by sector, using a "top down"
estimating procedure—emissions were calculated either
for individual sources or for many sources combined,
using basic activity data (e.g., the amount of raw mate-
rial processed) as an indicator of emissions. National
activity data were collected for individual source cat-
egories from various agencies. Depending on the source
category, these basic activity data may include data on
production, fuel deliveries, raw material processed, etc.
Activity data were used in conjunction with emis-
sion factors, which relate the quantity of emissions to
the activity. Emission factors are generally available
from the EPA's Compilation of Air Pollutant Emission
Factors, AP-42 (EPA 1997). The EPA currently derives
the overall emission control efficiency of a source cat-
egory from a variety of information sources, including
published reports, the 1985 National Acid Precipitation
and Assessment Program emissions inventory, and other
EPA data bases.
Uncertainty
Uncertainties in these estimates are primarily due
to the accuracy of the emission factors used and accurate
estimates of activity data.
Table 7-15: Emissions of NOX, CO, and NMVOC from Waste (Gg)
Gas/Source
NOX
Landfills
Wastewater Treatment
Waste Combustion3
Miscellaneous11
CO
Landfills
Wastewater Treatment
Waste Combustion3
Miscellaneous11
NMVOCs
Landfills
Wastewater Treatment
Waste Combustion3
Miscellaneous11
1990
83
+
+
82
+
979
1
+
978
+
895
58
57
222
558
1991
86
+
+
85
1
1,012
1
+
1,011
+
907
60
58
227
562
1992
87
+
+
86
1
1,032
2
+
1,030
+
916
63
61
230
563
1993
112
1
+
107
4
1,133
2
+
1,130
1
949
67
63
256
563
1994
103
1
+
99
3
1,111
2
+
1,108
1
949
73
64
248
564
1995
89
1
+
88
1
1,075
2
+
1,073
1
968
68
61
237
602
1996
87
1
+
86
1
1,083
4
+
1,079
1
388
18
57
237
76
1997
89
1
+
87
1
1,095
4
+
1,091
1
394
19
58
240
77
1998
90
1
88
1,107
4
1,103
1
400
19
59
243
79
+ Does not exceed 0.5 Gg
a Includes waste incineration and open burning (EPA 1999)
11 Miscellaneous includes TSDFs (Treatment, Storage, and Disposal Facilities under the Resource Conservation and Recovery Act [42 U S C
§ 6924, SWDA § 3004]) and other waste categories.
Note: Totals may not sum due to independent rounding.
Waste 7-11
-------
7-12 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
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8-22 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
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Annexes
The following seventeen annexes provide additional information to the material presented in the main body of this
report. Annexes A through J discuss methodologies for individual source categories in greater detail than was presented
in the main body of the report and include explicit activity data and emission factor tables. Annex K presents a table
of the Global Warming Potential values to weight greenhouse emission estimates.. Annexes L and M summarize U.S.
emissions of ozone depleting substances (e.g., CFCs and HCFCs) and sulfur dioxide (SO2), respectively. Annex N
provides a complete list of emission sources assessed in this report. Annex O presents the IPCC reference approach
for estimating CO2 emissions from fossil fuel combustion. Annex P addresses the criteria for the inclusion of an
emission source category and some of the sources that meet the criteria but are nonetheless excluded from U.S.
estimates. Annex Q provides some useful constants, unit definitions, and conversions. Annexes R and S provide a
listing of abbreviations and chemical symbols used. Finally, Annex T contains a glossary of terms related to greenhouse
gas emissions and inventories.
List of Annexes
Annex A Methodology for Estimating Emissions of CO2 from Fossil Fuel Combustion
Annex B Methodology for Estimating Emissions of CH4, N2O, and Criteria Pollutants from
Stationary Combustion
Annex C Methodology for Estimating Emissions of CH4, N2O, and Criteria Pollutants from Mobile
Combustion
Annex D Methodology for Estimating Methane Emissions from Coal Mining
Annex E Methodology for Estimating Methane Emissions from Natural Gas Systems
Annex F Methodology for Estimating Methane Emissions from Petroleum Systems
Annex G Methodology for Estimating Emissions from International Bunker Fuels Used by the
U.S. Military
Annex H Methodology for Estimating Methane Emissions from Enteric Fermentation
Annex I Methodology for Estimating Methane Emissions from Manure Management
Annex J Methodology for Estimating Methane Emissions from Landfills
Annex K Global Warming Potential Values
Annex L Ozone Depleting Substance Emissions
Annex M Sulfur Dioxide Emissions
Annex N Complete List of Sources
Annex O IPCC Reference Approach for Estimating CO2 Emissions from Fossil Fuel Combustion
Annex P Sources of Greenhouse Gas Emissions Excluded
Annex Q Constants, Units, and Conversions
Annex R Abbreviations
Annex S Chemical Symbols
Annex T Glossary
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Annex A
Methodology for Estimating Emissions of C02 from Fossil Fuel Combustion
Carbon dioxide (CO2) emissions from fossil fuel combustion were estimated using a "bottom-up" methodology
characterized by six steps. These steps are described below.
Step 1: Determine Energy Consumption by Fuel Type and Sector
The bottom-up methodology used by the United States for estimating CO2 emissions from fossil fuel combustion
is conceptually similar to the approach recommended by the Intergovernmental Panel on Climate Change (IPCC) for
countries that intend to develop detailed, sectoral-based emission estimates (IPCC/UNEP/OECD/IEA 1997). Basic
consumption data are presented in Columns 2-8 of Table A-l through Table A-9, with totals by fuel type in Column 8
and totals by end-use sector in the last rows. Fuel consumption data for the bottom-up approach were obtained directly
from the Energy Information Administration (EIA) of the U.S. Department of Energy. These data were first gathered
in physical units, and must be converted to their energy equivalents (see "Converting Physical Units to Energy Units"
in Annex Q). The EIA data were collected through surveys at the point of delivery or use; therefore, they reflect the
reported consumption of fuel by end-use sector and fuel type. Individual data elements were supplied by a variety of
sources within EIA. Most information was taken from published reports, although some data were drawn from
unpublished energy studies and databases maintained by EIA.
Energy consumption data were aggregated by end-use sector (i.e., residential, commercial, industrial,
transportation, electric utilities, and U.S. territories), primary fuel type (e.g., coal, natural gas, and petroleum), and
secondary fuel type (e.g., motor gasoline, distillate fuel, etc.). The 1998 total energy consumption across all sectors,
including territories, and energy types was 80,632 trillion British thermal units (TBtu), as indicated in the last entry of
Column 8 in Table A-l. This total includes fuel used for non-energy purposes and fuel consumed as international
bunkers, both of which are deducted in later steps.
There are two modifications made in this report that may cause consumption information herein to differ from
figures given in the cited literature. These are the consideration of synthetic natural gas production and ethanol added
to motor gasoline.
First, a portion of industrial coal accounted for in EIA combustion figures is actually used to make "synthetic
natural gas" via coal gasification. The energy in this gas enters the natural gas stream, and is accounted for in natural
gas consumption statistics. Because this energy is already accounted for as natural gas, it is deducted from industrial
coal consumption to avoid double counting. This makes the figure for other industrial coal consumption in this report
slightly lower than most EIA sources.
Second, ethanol has been added to the motor gasoline stream for several years, but prior to 1993 this addition was
not captured in EIA motor gasoline statistics. Starting in 1993, ethanol was included in gasoline statistics. However,
because ethanol is a biofuel, which is assumed to result in no net CO2 emissions, the amount of ethanol added is
subtracted from total gasoline consumption. Thus, motor gasoline consumption statistics given in this report may be
slightly lower than in EIA sources.
There are also three basic differences between the consumption figures presented in Table A-1 through Table A-9
and those recommended in the IPCC emission inventory methodology.
First, consumption data in the U.S. inventory are presented using higher heating values (HHV)1 rather than the
lower heating values (LHV)2 reflected in the IPCC emission inventory methodology. This convention is followed
because data obtained from EIA are based on HHV.
' Also referred to as Gross Calorific Values (GCV).
Also referred to as Net Calorific Values (NCV).
A-1
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Second, while EIA's energy use data for the United States includes only the 50 U.S. states and the District of
Columbia, the data reported to the Framework Convention on Climate Change are to include energy consumption with in
territories.' Therefore, consumption estimates for U.S. territories were added to domestic consumption of fossil fuels.
Energy consumption data from U.S. territories are presented in Column 7 of Table A-l. It is reported separately from
domestic sectoral consumption, because it is collected separately by EIA with no sectoral disaggregation.
Third, the domestic sectoral consumption data in Table A-l include bunker fuels used for international transport
activities and non-energy uses of fossil fuels. The IPCC requires countries to estimate emissions from international
bunker fuels separately and exclude these emissions from national totals, so international bunker fuel emissions have
been estimated in Table A-l 0 and deducted from national estimates (see Step 4). Similarly, fossil fuels used to produce
non-energy products that store carbon rather than release it to the atmosphere are provided in Table A-11 and deducted
from national emission estimates (see Step 3).
Step 2: Determine the Carbon Content of All Fuels
The carbon content of combusted fossil fuels was estimated by multiplying energy consumption (Columns 2
through 8 of Table A-l) by fuel-specific carbon content coefficients (see Table A-12 and Table A-13) that reflect the
amount of carbon per unit of energy that was inherent in each fuel. The resulting carbon contents are sometimes referred
to as potential emissions, or the maximum amount of carbon that could potentially be released to the atmosphere if all
carbon in the fuels were converted to CO2. The carbon content coefficients used in the U.S. inventory were derived by
EIA from detailed fuel information and are similar to the carbon content coefficients contained in the IPCC's default
methodology (IPCC/UNEP/OECD/IEA1997), with modifications reflecting fuel qualities specific to the United States.
Step 3: Adjust for the amount of Carbon in Products
Depending on the end-use, non-energy uses of fossil fuels can result in long term storage of some or all of the
carbon contained in the fuel. For example, asphalt made from petroleum can sequester up to 100 percent of the carbon
contained in the petroleum feedstock for extended periods of time. Other non-energy fossil fuel products, such as
lubricants or plastics also store carbon, but can lose or emit some of this carbon when they are used and/or burned as
waste.3
The amount of carbon in non-energy fossil fuel products was based upon data that addressed the fraction of carbon
that remains in products after they are manufactured, with all non-energy use attributed to the industrial, transportation,
and territories end-use sectors. This non-energy consumption is presented in Table A-l 1. This data was then multiplied
by fuel-specific carbon content coefficients (Table A-12 and Table A-13) to obtain the carbon content of the fuel, or
the maximum amount of carbon that could remain in non-energy products (Columns 5 and 6 of Table A-ll). This
carbon content was then multiplied by the fraction of carbon assumed to actually have remained in products (Column
7 of Table A-ll), resulting in the final estimates by sector and fuel type, which are presented in Columns 8 through 10
of Table A-ll. The fractions of carbon remaining in products were based on EIA data.
Step 4: Subtract Carbon from International Bunker Fuels
Emissions from international transport activities, or international bunker fuel consumption, were not included in
national totals as required by the IPCC (IPCC/UNEP/OECD/IEA 1997). There is currently disagreement internationally
as to how these emissions should be allocated, and until this issue is resolved, countries are asked to report them
separately. EIA energy statistics, however, include bunker fuels—jet fuel for aircraft, and distillate fuel oil and residual
fuel oil for marine shipping—as part of fuel consumption by the transportation sector. To compensate for this inclusion,
international bunker fuel emissions" were calculated separately (see Table A-10) and the carbon content of these fuels
was subtracted from the transportation sector. International bunker fuel emissions from military activities were
' Sec Waste Combustion section of the Waste chapter for a discussion of emissions from the combustion of plastics in the minicipal solid waste
sue am.
4 Refer to the International Bunker Fuels section of the Energy chapter for a description of the methodology for distinguishing between bunker
and non-bunker fuel consumption.
A-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
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developed using data provided by the Department of Defense as described in the International Bunker Fuels section of
the Energy chapter and in Annex G. The calculations of international bunker fuel emissions followed the same
procedures used for other fuel emissions (i.e., estimation of consumption, determination of carbon content, and
adjustment for the fraction of carbon not oxidized).
Step 5: Account for Carbon that Does Not Oxidize During Combustion
Because combustion processes are not 100 percent efficient, some of the carbon contained in fuels is not emitted
to the atmosphere. Rather, it remains behind as soot, particulate matter and ash. The estimated fraction of carbon not
oxidized in U.S. energy conversion processes due to inefficiencies during combustion ranges from 0.5 percent for
natural gas to 1 percent for petroleum and coal. Except for coal these assumptions are consistent with the default values
recommended by the IPCC (IPCC/UNEP/OECD/IEA 1997). In the U.S. unoxidized carbon from coal combustion was
estimated to be no more than one percent (Bechtel 1993). Table A-12 presents fractions oxidized by fuel type, which
are multiplied by the net carbon content of the combusted energy to give final emissions estimates.
Step 6: Summarize Emission Estimates
Actual CO2 emissions in the United States were summarized by major fuel (i.e., coal, petroleum, natural gas,
geothermal) and consuming sector (i.e., residential, commercial, industrial, transportation, electric utilities, and
territories). Adjustments for international bunker fuels and carbon in non-energy products were made. Emission
estimates are expressed in terms of million metric tons of carbon equivalents (MMTCE).
To determine total emissions by final end-use sector, emissions from electric utilities were distributed to each end-
use sector according to its share of aggregate electricity consumption (see Table A-14). This pro-rated approach to
allocating emissions from electric utilities may overestimate or underestimate emissions for particular sectors due to
differences in the average carbon content of utility fuel mixes.
A-3
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Table A-10: 1998 Emissions From International Bunker Fuel Consumption
Bunker Fuel Carbon Content Carbon Content Fraction Emissions
Consumption Coefficient (MMTCE) Oxidized (MMTCE)
Fuel Type
Distillate Fuel Oil
Jet Fuel
Residual Fuel Oil
Total
(TBtu) (MMTCE/QBlu)5
87
811
595
1,493
Table A-11: 1998 Carbon In Non-Energy
1
Fuel Type
Industrial Coking Coal
Natural Gas
Asphalt & Road Oil
LPG
Lubricants
Pentanes Plus
Petrochemical Feedstocks
Naphtha (<401 deg. F)
Other Oil (>401 deg. F)
Still Gas
Petroleum Coke
Special Naphtha
Other Wax & Misc.
Distillate Fuel Oil
Residual Fuel Oil
Waxes
Miscellaneous
Total
2 3
Non-energy Use
(TBtu)
Ind. Trans.
25
377
1,263
1,582
191 180
264
584
819
0
306
107
7
50
42
119
6,290 180
19.95
19.33
21.49
Products
4
Carbon Content
Coefficient
(MMTCE/QBtu)
25.55
14.47
20.62
16.86
20.24
18.24
18.14
19.95
17.51
27.85
19.86
19.95
21.49
19.81
20.23
3.2 0.99
15.8 0.99
12.9 0.99
31.9
5 6
Carbon Content
(MMTCE)
Ind. Trans.
0.64
5.46
26.03
26.67
3.86 3.65
4.82
10.59
16.33
0.00
8.53
2.13
0.14
1.08
0.84
2.41
114.94 3.65
3.1
15.7
12.8
31.6
7
Fraction
Sequestered
0.75
1.00
1.00
0.80
0.50
0.80
0.75
0.50
0.80
0.50
0.00
0.50
0.50
1.00
1.00
8
9 10
Carbon Stored (MMTCE)
Ind.
0.48
5.46
26.03
21.34
1.93
3.85
7.95
8.17
0.00
4.26
0.00
0.07
0.54
0.84
2.41
83.33
Trans. Total
0.48
5.46
26.03
21.34
1.82 3.75
3.85
7.95
8.17
0.00
426
T.C.U
0.00
0.07
0.54
0.84
2.41
1.82 85.15
One QBtu is one quadrillion Btu, or 10" Btu. This unit is commonly referred to as a "Quad."
A-13
-------
Table A-12: Key Assumptions for Estimating Carbon Dioxide Emissions
Carbon Content Coefficient
Fuel Type (MMTCE/QBtu)
Coal
Residential Coal
Commercial Coal
Industrial Coking Coal
Industrial Other Coal
Coke Imports
Transportation Coal
Utility Coal
U.S. Territory Coal (bit)
Natural Gas
Petroleum
Asphalt & Road Oil
Aviation Gasoline
Distillate Fuel Oil
Jet Fuel
Kerosene
LPG
Lubricants
Motor Gasoline
Residual Fuel
Other Petroleum
AvGas Blend Components
Crude Oil
MoGas Blend Components
Misc. Products
Naphtha (<401 deg. F)
Other Oil (>401deg.F)
Pentanes Plus
Petrochemical Feedstocks
Petroleum Coke
Still Gas
Special Naphtha
Unfinished Oils
Waxes
Other Wax & Misc.
Geothermal
[a]
[a]
[a]
[a]
27.85
NC
[a]
25.14
14.47
20.62
18.87
19.95
[a]
19.72
[a]
20.24
[a]
21.49
20.23
18.87
[a]
19.39
20.23
18.14
19.95
18.24
19.37
27.85
17.51
19.86
20.23
19.81
19.81
2.05
Fraction
Oxidized
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.995
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
IPCC (IPCC/UNEP/OECD/1EA1997, vol. 2).
-Not applicable
NC (Not Calculated)
[a] These coefficients vaiy annually due to fluctuations in fuel quality (see Table A-13).
A-14 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Table A-13: Annually Variable Carbon Content Coefficients by Year (MMTCE/QBtu)
Fuel Type
Residential Coal
Commercial Coal
Industrial Coking Coal
Industrial Other Coal
Utility Coal
LP6
Motor Gasoline
Jet Fuel
Crude Oil
Source: EIA(1999c)
1990
25.92
25.92
25.51
25.58
25.68
16.99
19.41
19.40
20.16
1991
26.00
26.00
25.51
25.60
25.69
16.98
19.41
19.40
20.18
1992
26.13
26.13
25.51
25.62
25.69
16.99
19.42
19.39
20.22
1993
25.97
25.97
25.51
25.61
25.71
16.97
19.43
19.37
20.22
1994
25.95
25.95
25.52
25.63
25.72
17.01
19.45
19.35
20.21
1995
26.00
26.00
25.53
25.63
25.74
17.00
19.38
19.34
20.23
1996
25:92
25.92
25.55
25.61
25.74
16.99
19.36
19.33
20.25
1997
26.00
26.00
25.56
25.63
25.76
16.99
19.35
19.33
20.24
1998
26.00
26.00
25.56
25.63
25.76
16.99
19.33
19.33
20.24
Table A-14: Electricity Consumption by End-Use Sector (Billion Kilowatt-Hours)
End-Use Sector
Residential
Commercial
Industrial
Transportation
U.S. Territories*
Total
1990
924
839
946
4
-
2,713
1991
955
856
947
4
-
2,762
1992
936
851
973
4
.-
2,763
1993
995
886
977
4
-
2,861
1994
1,008
914
1,008
4
-
2,935
1995
1,043
954
1,013
4
3,013
1996
1,082
981
1,030
4
3,098
1997
1,076
1,027
1,033
4
3,140
1998
1,124
1,045
1 047
4
3,220
- Not applicable
Source: EIA(1999a)
A-15
-------
-------
Annex
Methodology for Estimating Emissions of CH4, N20, and Criteria Pollutants from
Stationary Combustion
Estimates of CH4 and N20 Emissions
Methane (CH4) and nitrous oxide (N2O) emissions from stationary combustion were estimated using IPCC
emission factors and methods. Estimates were obtained by multiplying emission factors—by sector and fuel type—by
fossil fuel and wood consumption data. This "top-down" methodology is characterized by two basic steps, described
below. Data are presented in Table B-l through Table B-5.
Step 1: Determine Energy Consumption by Sector and Fuel Type
Greenhouse gas emissions from stationary combustion activities were grouped into four sectors: industrial,
commercial/institutional, residential, and electric utilities. For CH4 and N2O, estimates were based upon consumption
of coal, gas, oil, and wood. Energy consumption data were obtained from EIA's Monthly Energy Review (1999b), and
adjusted to lower heating values assuming a 10 percent reduction for natural gas and a 5 percent reduction for coal and
petroleum fuels. Table B-l provides annual energy consumption data for the years 1990 through 1998.
Step 2: Determine the Amount of CH4 and N20 Emitted
Activity data for each sector and fuel type were then multiplied by emission factors to obtain emissions estimates.
Emission factors were taken from the Revised 1996 IPCC Guidelines (IPCC/UNEP/OECD/IEA 1997). Table B-2
provides emission factors used for each sector and fuel type.
Estimates of NOX, CO, and NMVOC Emissions
For criteria pollutants, the major source categories included were those identified in EPA (1999): coal, fuel oil,
natural gas, wood, other fuels (i.e., bagasse, liquefied petroleum gases, coke, coke oven gas, and others), and stationary
internal combustion, which includes emissions from internal combustion engines not used in transportation. The EPA
(1999) periodically estimates emissions of NOX, CO, and NMVOCs by sector and fuel type using a "bottom-up"
estimating procedure. In other words, the emissions were calculated either for individual sources (e.g., industrial
boilers) or for many sources combined, using basic activity data (e.g., fuel consumption or deliveries, etc.) as indicators
of emissions. The EPA (1999) projected emissions for years subsequent to their bottom-up estimates. The national
activity data used to calculate the individual categories were obtained from various sources. Depending upon the
category, these activity data may include fuel consumption or deliveries of fuel, tons of refuse burned, raw material
processed, etc. Activity data were used in conjunction with emission factors that relate the quantity of emissions to the
activity. Table B-3 through Table B-5 present criteria pollutant emission estimates for 1990 through 1998.
The basic calculation procedure for most source categories presented in EPA (1999) is represented by the
following equation:
Ep>s = Asx EfptS x (1
where,
E = emissions
p = pollutant
B-1
-------
s = source category
A — activity level
EF = emission factor
C = percent control efficiency
The EPA currently derives the overall emission control efficiency of a category from a variety of sources,
including published reports, the 1985 National Acid Precipitation and Assessment Program (NAPAP) emissions
inventory, and other EPA databases. The U.S. approach for estimating emissions of NOX, CO, and NMVOCs from
stationary combustion as described above is similar to the methodology recommended by the IPGC
(IPCC/UNEP/OECD/IEA 1997).
Table B-1: Fuel Consumption by Stationary Combustion for Calculating CH4 and N20 Emissions (Tbtu)
Fuel/End-Use Sector
Coal
Residential
Commercial/Institutional
Industry
Utilities
Petroleum
Residential
Commercial/Institutional
Industry
Utilities
Natural Gas
Residential
Commercial/Institutional
Industry
Utilities
Wood
Residential & Commercial
Industrial
Utilities
1990
18,935
62
93
2,693
16,088
11,741
1,266
907
8,318
1,250
18,579
4,519
2,698
8,500
2,861
2,550
581
1,948
21
1991
18,699
56
84
2,545
16,012
11,390
1,293
861
8,058
1,178
18,964
4,685
2,808
8,618
2,854
2,577
613
1,943
21
1992
18,802
57
86
2,468
16,192
11,714
1,312
813
8,638
951
19,514
4,821
2,884
8,980
2,829
2,709
645
2,042
22
1993
19,428
57
86
2,445
16.841
11,642
1,387
753
8,450
1,052
20,230
5,097
2,996
9,393
2,744
2,696
592
2,084
20
1994
19,468
55
83
2,464
16.867
11,929
1,340
753
8,867
968
20,580
4,980
2,978
9,565
3,057
2,819
582
2,217
20
1995
19,567
54
81
2,442
16,990
11,466
1,363
757
8,689
658
21,416
4,981
3,113
10,045
3,276
2,944
641
2,286
17
1996
20,448
55
83
2,357
17,953
11,980
1,441
741
9,073
725
21,800
5,383
3,244
10,376
2,798
3,034
644
2,370
20
1997
20,981
58
87
2,336
18,500
12,315
1.432
705
9,356
822
21,749
5,118
3,306
10,300
3,025
2,884
475
2,390
19
1998
21,175
57
86
2,315
18,717
12,469
1,432
701
9,170
1,166
21,135
4,605
3,117
10,093
3,320
2,948
468
2,460
20
B-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Table B-2: CH4 and N20 Emission Factors by Fuel Type and Sector (g/GJ)1
Fuel/End-Use Sector
Coal
Residential
Commercial/Institutional
Industry
Utilities
Petroleum
Residential
Commercial/Institutional
Industry
Utilities
Natural Gas
Residential
Commercial/Institutional
Industry
Utilities
Wood
Residential
Commercial/Institutional
Industrial
Utilities
CH.
300
10
10
1
10
10
2
3
5
5
5
1
300
300
30
30
N,0
1.4
1.4
1.4
1.4
0.6
0.6
0.6
0.6
0.1
0.1
0.1
0.1
4.0
4.0
4.0
4.0
1G3 (Gigajoule) = 10' joules. One joule = 9.486x10"' Btu
B-3
-------
Table B-3: NO. Emissions from Stationary Combustion (Gg)
Sector/Fuel Type
Electric Utilities
Coal
Fuel Oil
Natural gas
Wood
Internal Combustion
Industrial
Coal
Fuel Oil
Natural gas
Wood
Other Fuels"
Internal Combustion
Commercial/Institutional
Coal
Fuel Oil
Natural gas
Wood
Other Fuels'
Residential
Coal"
Fuel Oil"
Natural Gas"
Wood
Other Fuels"
Total
1990
6,045
5,119
200
513
NA
213
2,754
530
240
1,072
NA
119
792
336
36
88
181
NA
31
749
NA
NA
NA
42
708
9,884
1991
5,914
5,043
192
526
NA
152
2,703
517
215
1,134
NA
117
720
333
33
80
191
NA
29
829
NA
NA
NA
45
784
9,779
1992
5,901
5,062
154
526
NA
,. 159
2,786
521
222
1,180
NA
115
748
348
35
84
204
NA
25
879
NA
NA
NA
48
831
9,914
1993
6,034
5,211
163
500
NA
160
2,859
534
222
1,207
NA
113
783
360
37
84
211
NA
28
827
NA
NA
NA
40
787
10,080
1994
5,956
5,113
148
536
NA
159
2,855
546
219
1,210
NA
113
767
365
36
86
215
, NA
28
817
NA
NA
NA
40
777
9,993
1995
5,792
5,061
87
510
NA
134
2,852
541
224
1,202
NA
111
774
365
35
94
210
NA
27
813
NA
NA
NA
44
769
9,822
1996
5,496
5,027
94
239
NA
136
2,907
594
206
1,106
NA
112
890
346
31
81
208
NA
25
804
NA
NA
NA
44
760
9,553
1997
5,614
5,089
117
269
NA
138
2,952
604
208
1,124
NA
113
904
355
32
83
214
NA
26
807
NA
NA
NA
32
775
9,728
1998
5,535
4,894
189
310
NA
141
2,997
613
209
1,141
NA
115
918
364
33
85
219
NA
27
823
NA
NA
NA
32
791
9,719
NA (Not Available)
' "Other Fuels" include LPG, waste oil, coke oven gas, coke,
" Coal, fuel oil, and natural gas emissions are included in the
Note: Totals may not sum due to independent rounding.
and non-residential wood (EPA 1999).
'Other Fuels" category (EPA 1999).
B-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Table B-4: CO Emissions from Stationary Combustion (Gg)
Sector/Fuel Type
Electric Utilities
Coal
Fuel Oil
Natural gas
Wood
Internal Combustion
Industrial
Coal
Fuel Oil
Natural gas
Wood
Other Fuels'
Internal Combustion
Commercial/Institutional
Coal
Fuel Oil
Natural gas
Wood
Other Fuels"
Residential
Coal"
Fuel Oil"
Natural Gas"
Wood
Other Fuels'
Total
1990
329
213
18
46
NA
52
798
95
67
205
NA
253
177
205
13
16
40
NA
136
3,668
NA
NA
NA
3,430
238
4,999
1991
317
212
17
46
NA
41
835
92
54
257
NA
242
189
196
13
16
40
NA
128
3,965
NA
NA
NA
3,711
255
5,313
1992
318
214
14
47
NA
43
867
92
58
272
NA
239
205
204
13
16
46
NA
128
4,195
NA
NA
NA
3,930
265
5,583
1993
329
224
15
45
NA
46
946
92
60
292
NA
259
243
207
14
16
48
NA
129
3,586
NA
NA
NA
3,337
249
5,068
1994
335
224
13
48
NA
50
944
91
60
306
NA
260
228
212
13
16
49
NA
134
3,515
NA
NA
NA
3,272
243
5,007
1995
338
227
9
49
NA
52
958
88
64
313
NA
270
222
211
14
17
49
NA
132
3,876
NA
NA
NA
3,628
248
5,383
1996
354
225
10
69
NA
50
1,058
88
51
305
NA
305
309
126
11
16
52
NA
47
3,867
NA
NA
NA
3,622
244
5,405
1997
366
230
11
73
NA
52
1,074
89
52
309
NA
309
314
130
12
16
54
NA
48
2,885
NA
NA
NA
2,636
249
4,455
1998
377
231
16
78
NA
53
1,090
91
52
314
NA
314
319
134
12
17
55
NA
50
2,891
NA
NA
NA
2,636
255
4,491
1 "Other Fuels" include LPG, waste oil, coke oven gas, coke,
" Coal, fuel oil, and natural gas emissions are included in the
Note: Totals may not sum due to independent rounding.
and non-residential wood (EPA 1999).
'Other Fuels" category (EPA 1999).
B-5
-------
Table B-5: NMVOC Emissions from Stationary Combustion (Gg)
Sector/Fuel Type
Electric Utilities
Coal
Fuel Oil
Natural gas
Wood
Internal Combustion
Industrial
Coal
Fuel Oil
Natural gas
Wood
Other Fuels"
Internal Combustion
Commercial/Institutional
Coal
Fuel Oil
Natural gas
Wood
Other Fuels*
Residential
Coal"
Fuel Oil"
Natural Gas"
Wood
Other Fuels"
Total
1990
43
25
5
2
NA
11
165
7
11
52
NA
46
49
18
1
3
7
NA
8
686
NA
NA
NA
651
35
912
1991
40
25
5
2
NA
9
177
5
10
54
NA
• 47
61
18
1
2
8
NA
7
739
NA
NA
NA
704
35
975
1992
40
25
4
2
NA
9
169
7
11
47
NA
45
60
20
1
3
9
NA
7
782
NA
NA
NA
746
36
1,011
1993
41
26
4
2
NA
9
169
5
11
46
NA
46
60
22
1
3
10
NA
8
670
NA
NA
NA
633
36
901
1994
41
26
4
2
NA
9
178
7
11
57
NA
45
58
21
1
3
10
NA
8
657
NA
NA
NA
621
36
898
1995
40
26
2
2
NA
9
187
5
11
66
NA
45
59
21
1
3
10
NA
8
726
NA
NA
NA
689
37
973
1996
44
25
3
7
NA
9
161
5
6
45
NA
39
67
22
1
3
11
NA
7
724
NA
NA
NA
687
37
951
1997
46
26
3
7
NA
9
163
5
6
45
NA
39
68
22
1
3
11
NA
8
538
NA
NA
NA
500
38
770
1998
48
26
4
8
NA
9
166
5
6
46
NA
40
69
23
1
3
12
NA
8
539
NA
NA
NA
500
39
776
NA (Not Available)
• "Other Fuels" include LPG, waste oil, coke oven gas, coke, and non-residential wood (EPA 1999).
" Coal, fuel oil, and natural gas emissions are included in the "Other Fuels" category (EPA 1999).
Note: Totals may not sum due to independent rounding.
B-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Annex C
Methodology for Estimating Emissions of CH4, N20, and Criteria Pollutants from
Mobile Combustion
Estimates of CH4 and N20 Emissions
Greenhouse gas emissions from mobile combustion are reported by transport mode (e.g., road, rail, air, and water),
vehicle type, and fuel type. The EPA does not systematically track emissions of CH4 and N2O as in EPA (1999);
therefore, estimates of these gases were developed using a methodology similar to that outlined in the Revised 1996
IPCC Guidelines (IPCC/UNEP/OECD/IEA 1997).
Step 1: Determine Vehicle Miles Traveled or Fuel Consumption by Vehicle Type, Fuel
Type, and Model Year
Activity data were obtained from a number of U.S. government agency publications. Depending on the category,
these basic activity data included such information as fuel consumption, fuel deliveries, and vehicle miles traveled
(VMT). The activity data for highway vehicles included estimates of VMT by vehicle type and model year from EPA
(1999) and the MOBILESa emissions model (EPA 1997).
National VMT data for gasoline and diesel highway vehicles are presented in Table C-l and Table C-2,
respectively. Total VMT for eachhighway category (i.e., gasoline passenger cars, light-duty gasoline tracks, heavy-duty
gasoline vehicles, diesel passenger cars, light-duty diesel trucks, heavy-duty diesel vehicles, and motorcycles) were
distributed across 25 model years based on the temporally fixed age distribution of VMT by the U.S. vehicle fleet in
1990 (see Table C-3) as specified in MOBILESa. Activity data for gasoline passenger cars and light-duty trucks in
California were developed separately due to the different emission control technologies deployed in that state relative
to the rest of the country. Unlike the rest of the United States, beginning in model year 1994, a fraction of the computed
California VMT for gasoline passenger cars and light-duty trucks was attributed to low emission vehicles (LEVs).
LEVs have not yet been widely deployed in other states. Based upon U.S. Department of Transportation statistics for
1994, it was assumed that 8.7 percent of national VMT occurred in California.
Activity data for non-highway vehicles were based on annual fuel consumption statistics by transportation mode
and fuel type. Consumption data for distillate and residual fuel oil by ships and boats (i.e., vessel bunkering),
construction equipment, farm equipment, and locomotives were obtained from EIA (1999b). In the case of ships and
boats, the EIA (1999b) vessel bunkering data was reduced by the amount of fuel used for marine international bunkers.'
Data on the consumption of jet fuel in aircraft were obtained directly from DOT/BTS, as described under CO2 from
Fossil Fuel Combustion, and were reduced by the amount allocated to international bunker fuels. Data on aviation
gasoline consumed in aircraft were taken from FAA (1999). Data on the consumption of motor gasoline by ships and
boats, construction equipment, farm equipment, and locomotives data were drawn from FHWA (1998). For these
vehicles, 1997 fuel consumption data were used as a proxy because 1998 data were unavailable. The activity data used
for non-highway vehicles are included in Table C-4.
Step 2: Allocate VMT Data to Control Technology Type for Highway Vehicles
For highway sources, VMT by vehicle type for each model year were distributed across various control
technologies as shown in Table C-5, Table C-6, Table C-7, Table C-8, and Table C-9. Again, California gasoline-fueled
passenger cars and light-duty trucks were treated separately due to that state's distinct vehicle emission
standards—including the introduction of Low Emission Vehicles (LEVs) in 1994—compared with the rest of the United
States. The categories "Tier 0" and "Tier 1" were substituted for the early three-way catalyst and advanced three-way
catalyst categories, respectively, as defined in the Revised 1996IPCC Guidelines, Tier 0, Tier 1, and LEV are actually
1 See International Bunker Fuels.
C-1
-------
U.S. emission regulations, rather than control technologies; however, each does correspond to particular combinations
of control technologies and engine design. Tier 1 and its predecessor Tier 0 both apply to vehicles equipped with three-
way catalysts. The introduction of "early three-way catalysts," and "advance three-way catalysts" as described in the
Revised 1996IPCC Guidelines, roughly correspond to the introduction of Tier 0 and Tier 1 regulations (EPA 1998).
Step 3: Determine the Amount of CH4 and N20 Emitted by Vehicle, Fuel, and Control Technology Type
Emissions of CH4 and N2O from non-highway vehicles were calculated by multiplying emission factors in
IPCC/UNEP/OECD/IEA (1997) by activity data for each vehicle type as described in Step 1 (see Table C-10 and Table
C-l 1). The CH4 emission factors for highway sources were derived from the EPA's MOBILESa mobile source
emissions model (EPA 1997). The MOBILESa model uses information on ambient temperature, diurnal temperature
range, altitude, vehicle speeds, national vehicle registration distributions, gasoline volatility, emission control
technologies, fuel composition, and the presence or absence of vehicle inspection/maintenance programs in order to
produce these factors.
Emissions of N2O—in contrast to CH4, CO, NOX, and NMVOCs—have not been extensively studied and are
currently not well characterized. The limited number of studies that have been performed on highway vehicle emissions
of N2O have shown that emissions are generally greater from vehicles with catalytic converter systems than those
without such controls, and greater from aged than from new catalysts. These systems control tailpipe emissions of NOX
(i.e., NO and NOj) by catalytically reducing NOX to N2. Suboptimal catalyst performance, caused by as yet poorly
understood factors, results in incomplete reduction and the conversion of some NOX to N2O rather than to N2.
Fortunately, newer vehicles with catalyst and engine designs meeting the more recent Tier 1 and LEV standards have
shown reduced emission rates of both NOX andN2O.
In order to better characterize the process by which N2O is formed by catalytic controls and to develop a more
accurate national emission estimate, the EPA's Office of Mobile Sources—at its National Vehicle and Fuel Emissions
Laboratory (N VFEL)—conducted a series of tests in order to measure emission rates of N2O from used Tier 1 and LEV
gasoline-fueled passenger cars and light-duty trucks equipped with catalytic converters. These tests and a review of the
literature were used to develop the emission factors for N2O (EPA 1998). The following references were used in
developing the N2O emission factors for gasoline-fueled highway passenger cars presented in Table C-10:
LEVs. Tests performed at NVFEL (EPA 1998)2
Tier 1. Tests performed at NVFEL (EPA 1998)
Tier 0. Smith and Carey (1982), Barton and Simpson (1994), and one car tested at NVFEL (EPA
1998)
Oxidation Catalyst. Smith and Carey (1982), Urban and Garbe (1979)
Non-Catalyst. Prigent and de Soete (1989), Dasch (1992), and Urban and Garbe (1979)
Nitrous oxide emission factors for other types of gasoline-fueled vehicles—light-duty trucks, heavy-duty vehicles,
and motorcycles—were estimated by adjusting the factors for gasoline passenger cars, as described above, by their
relative fuel economies. This adjustment was performed using the carbon dioxide emission rates in the Revised 1996
IPCC Guidelines (IPCC/UNEP/OECD/IEA 1997) as a proxy for fuel economy (see Table C-10). Data from the
literature and tests performed at NVFEL support the conclusion that light-duty trucks have higher emission rates than
passenger cars. However, the use of fuel-consumption ratios to determine emission factors is considered a temporary
measure only, to be replaced as soon as real data-are available.
1 It was assumed that LEVs would be operated using low-sulfur fuel (i.e., Indolene at 24 ppm sulfur). All other NVFEL tests were performed
using a standard commercial fuel (CAAB at 285 ppm sulfur). Emission tests by NVFEL have consistently exhibited higher N2O emission rates
from higher sulfur fuels on Tier 1 and LEV vehicles.
C-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
The resulting N2O emission factors employed for gasoline highway vehicles are lower than the U S default values
presented in the Revised 1996IPCC Guidelines, but are higher than the European default values, both of which were
published before the more recent tests and literature review conducted by the NVFEL. The U S defaults in the
Guidelines were based on three studies that tested a total of five cars using European rather than U.S. test procedures.
Nitrous oxide emission factors for diesel highway vehicles were taken from the European default values found
in the Revised 1996IPCC Guidelines (IPCC/UNEP/OECD/IEA 1997). There is little data addressing N2O emissions
from U.S. diesel-fueled vehicles, and in general, European countries have had more experience with diesel-fueled
vehicles. U.S. default values in the Revised 1996 IPCC Guidelines were used for non-highway vehicles.
Compared to regulated tailpipe emissions, there is relatively little data available to estimate emission factors for
N20. Nitrous oxide is not a criteria pollutant, and measurements of it in automobile exhaust have not been routinely
collected. Further testing is needed to reduce the uncertainty in nitrous oxide emission factors for all classes of vehicles
using realistic driving regimes, environmental conditions, and fuels.
Estimates of NOX, CO, and NMVOC Emissions
The emission estimates of NOX, CO, and NMVOCs for mobile combustion were taken directly from the EPA's
National Air Pollutant Emissions Trends, 1900 - 1998 (EPA 1999). This EPA report provides emission estimates for
these gases by sector and fuel type using a "top down" estimating procedure whereby emissions were calculated using
basic activity data, such as amount of fuel delivered or miles traveled, as indicators of emissions. Table O12 through
Table C-14 provide complete emissions estimates for 1990 through 1998.
Table C-1: Vehicle Miles Traveled for Gasoline Highway Vehicles (10s Miles)
Year
1990
1991
1992
1993
1994
1995
1996
1997
1998
Passenger
Cars'
1,268.19
1,223.05
1,235.38
1,238.52
1,266.89
1,295.30
1,322.82
1,336.47
1,366.67
Light-Duty Heavy-Duty Passenger Cars Light-Duty
Trucks' Vehicles Motorcycles tCKt Trunk* /rai"
520.28
588.03
640.07
675.29
692.39
715.38
738.84
761.00
778.20
42.08
42.88
43.66
46.01
49.65
50.79
51.84
53.66
54.87
9.64
9.30
9.37
9.37
9.59
9.80
9.91
9.96
10.18
120.85 4Q RR
116.54
117.72
118.02
120.72
123.43
126.05
127.35
130.23
56.03
60.99
64.35
65.98
68.17
70.40
72.52
74.15
b California WIT for passenger cars and light-duty trucks was treated separately and estimated as 8.7 percent of national total
Source: VMT data are the same as those used in EPA (1999).
-------
Table C-2: Vehicle Miles Traveled for Diesel Highway Vehicles (10s Miles)
Year
1990
1991
1992
1993
1994
1995
1996
1997
1998
Passenger Light-Duty
Cars Trucks
19.2
18.5
18.7
18.7
19.1
19.6
20.0
20.2
20.6
4.7
5.3
5.8
6.1
6.3
6.5
6.7
6.8
6.9
Heavy-Duty
Vehicles
109.9
112.4
115.5
120.0
127.0
133.8
137.5
143.0
146.3
Table C-3: VMT Profile by Vehicle Age (Years) and Vehicle/Fuel Type for Highway Vehicles (Percent of VMT)
Vehicle Age
1
2
3
4
5
6
7
8
g
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
LDGV
4.9%
7.9%
8.3%
8.2%
8.4%
8.1%
7.7%
5.6%
5.0%
5.1%
5.0%
5.4%
4.7%
3.7%
2.4%
1.9%
1.4%
1.5%
1.1%
0.8%
0.6%
0.5%
0.4%
0.3%
1.0%
LDGT
6.3%
8.4%
8.4%
8.4%
8.4%
6.9%
5.9%
4.4%
3.6%
3.1%
3.0%
5.3%
4.7%
4.6%
3.6%
2.8%
1.7%
2.2%
1.7%
1.4%
0.9%
0.8%
0.8%
0.5%
2.5%
HDGV
2.3%
4.7%
4.7%
4.7%
4.7%
3.8%
3.3%
2.1%
2.6%
2.9%
3.4%
6.4%
5.4%
5.8%
5.1%
3.8%
4.3%
4.1%
3.5%
2.9%
2.1%
2.2%
2.2%
1.4%
11.7%
LDDV
4.9%
7.9%
8.3%
8.2%
8.4%
8.1%
7.7%
5.6%
5.0%
5.1%
5.0%
5.4%
4.7%
3.7%
2.4%
1.9%
1.4%
1.5%
1.1%
0.8%
0.6%
0.5%
0.4%
0.3%
1.0%
LDDT
6.3%
8.4%
8.4%
8.4%
8.4%
6.9%
5.9%
4.4%
3.6%
3.1%
3.0%
5.3%
4.7%
4.6%
3.6%
2.8%
1.7%
2.2%
1.7%
1.4%
0.9%
0.8%
0.8%
0.5%
2.5%
HDDV
3.4%
6.7%
6.7%
6.7%
6.7%
7.3%
6.1%
4.0%
4.1%
5.1%
5.3%
6.6%
5.5%
5.7%
4.5%
1.9%
2.3%
2.8%
2.4%
1.6%
1.1%
0.9%
0.7%
0.5%
1.6%
MC
14.4%
16.8%
13.5%
10.9%
8.8%
7.0%
5.6%
4.5%
3.6%
2.9%
2.3%
9.7%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
LDGT (light-duty gas trucks)
HOGV (heavy-duty gas vehicles)
LDDV (diesel passenger cars, also referred to as light-duty diesel vehicles)
LDDT (light-duty diesel trucks)
HDDV (heavy-duty diesel vehicles)
MC (motorcycles)
C-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 19SO-1998
-------
Table C-4: Fuel Consumption for Non-Highway Vehicles by Fuel Type {U.S. gallons)
Vehicle Type/Year
Aircraft*
1990
•i Ann
1991
1992
4 rino
1993
j f\f\t
1994
J f\f\C
1995
4 nnc
1996
•inft?
1997
H nno
1998
Ships and Boats'1
1990
1991
1992
1993
1994
1995
1996
1997
1998
Construction Equipment1
1990
1991
1992
1993
1994
1995
1996
1997
*t CirtO
1998
Farm Equipment"
1990
1991
1992
1993
1994
1995
1996
1997
1998
Locomotives
1990
1991
1992
1993
1994
1995
1996
1997
1998
Residual
-
-
-
-
-
-
-
-
1,165,580,227
1,486,167,178
2,347,064,583
2,758,924,466
2,499,868,472
2,994,692,916
2,280,373,162
1,005,997,126
666,587,222
-
.
-
-
-
-
-
-
"
-
-
-,
-
-
-
.
~
25,422
6,845
8,343
4,065
5,956
6,498
9,309
3,431
2,587
Diesel
-
-
'
-
-
-
-
-
1,829,927,570
1,806,653,451
1,820,275,621
1,661,285,902
1,746,597,258
1,636,189,216
1,952,357,254
1,917,777,070
1,498,285,988
2,508,300,000
2,447,400,000
2,287,642,000
2,323,183,000
2,437,142,000
2,273,162,000
2,386,973,000
2,385,236,000
2,432,182,000
3,164,200,000
3,144,200,000
3,274,811,000
3,077,122,000
3,062,436,000
3,093,224,000
3,225,029,000
3,206,359,000
2,965,006,000
3,210,111,000
3,026,292,000
3,217,231,000
2,906,998,000
3,063,441.000
3,191.023,000
3,266,861,000
3,067,400,000
2,833,276,000
Jet Fuel
17,055,286,001
16,022,943,658
16,444,526,173
16,686,897,872
17,150,828,119
17,882,934,898
18,453,097,849
19,265,762,116
19,271,920,783
-
-
_
-
_
-
..
.
.
.
.
.
.
.
-
_
.
.
.
.
- .
.
.
-
.
_
_
_
_
_
_
_
-
Other
355,100,000
355,600,000
300,000,000
273,000,000
268,200,000
289,300,000
290,500,000
294,200,000
297,800,000
1,300,400,000
1,709,700,000
1,316,170,000
873,687,000
896,700,000
1,060,394,000
993,671,000
993,671,000
993,671,000
1,523,600,000
1,384,900,000
1,492,200,000
1,270,386,667
1,312,161,667
1,351,642,667
1,365,550,667
1,365,550,667
1,365,550,667
812,800,000
776,200,000
805,500,000
845,320,000
911,996,000
926,732,000 .
918,085,000
918,085,000
918,085,000
.
.
.
b Other fuel motor gasoline.
' Construction Equipment includes snowmobiles. Other fuel is motor gasoline.
"Other fuel is motor gasoline. .
C-5
-------
Table C-5: Control Technology Assignments for Gasoline Passenger Cars (Percent of VMT)*
Model Years
1973-1974
1975
1976-1977
1978-1979
1980
1981
1982
1983
1984-1993
1994
1995
1996-1998
Non-catalyst
100%
20%
15%
10%
5%
Oxidation
80%
85%
90%
88%
15%
14%
12%
TierO
7%
85%
86%
88%
100%
60%
20%
TieM
40%
80%
100%
•Excluding California VMT
Table C-6: Control Technology Assignments for Gasoline Light-Duty Trucks (Percent of VMT)*
Model Years
1973-1974
1975
1976
1977-1978
1979-1980
1981
1982
1983
1984
1985
1986
1987-1993
1994
1995
1996-1998
Non-catalyst
100%
30%
20%
25%
20%
Oxidation
70%
80%
75%
80%
95%
90%
80%
70%
60%
50%
5%
TierO
5%
10%
20%
30%
40%
50%
95%
60%
20%
TIerl
40%
80%
100%
' Excluding California VMT
Table C-7: Control Technology Assignments for California Gasoline Passenger Cars and
Light-Duty Trucks (Percent of VMT)
Model Years Non-catalyst Oxidation TierO
TieM
LEV
1973-1974
1975-1979
1980-1981
1982
1983
1984-1991
1992
1993
1994
1995
1996-1998
100%
100%
15%
14%
12%
85%
86%
88%
100%
60%
20%
40%
80%
90%
85%
80%
10%
15%
20%
C-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Table C-8: Control Technology Assignments for Gasoline Heavy-Duty Vehicles (Percent of VMT)
Model Years
s 1981
1982-1984
1985-1986
1987
1988-1989
1990-1998
Uncontrolled
100%
95%
Non-catalyst
95%
70%
60%
45%
Oxidation
5%
5%
15%
25%
30%
TierO
15%
15%
25%
Table C-9: Control Technology Assignments for Diesel Highway VMT
Vehicle Type/Control Technology Model Years
Diesel Passenger Cars and Light-Duty Trucks
Uncontrolled
Moderate control
Advanced control
Heavy-Duty Diesel Vehicles
Uncontrolled
Moderate control
Advanced control
Motorcycles
Uncontrolled
Non-catalyst controls
1966-1982
1983-1995
1996-1998
1966-1972
1983-1995
1996-1998
1966-1995
1996-1998
C-7
-------
Table C-10: Emission Factors (g/km) for CH4 and N20 and "Fuel Economy" (g COj/km)"
for Highway Mobile Combustion
Vehicle Type/Control Technology
N,0
CH4 g COj/km
Gasoline Passenger Cars
Low Emission Vehicles'
Tierl
TierO
Oxidation Catalyst
Non-Catalyst
Uncontrolled
Gasoline Light-Duty Trucks
Low Emission Vehicles"
Tierl
TierO
Oxidation Catalyst
Non-Catalyst
Uncontrolled
Gasoline Heavy-Duty Vehicles
TierO
Oxidation Catalyst"
Non-Catalyst Control
Uncontrolled
Diesel Passenger Cars
Advanced
Moderate
Uncontrolled
Diesel Light Trucks
Advanced
Moderate
Uncontrolled
Diesel Heavy-Duty Vehicles
Advanced
Moderate
Uncontrolled
Motorcycles
Non-Catalyst Control
Uncontrolled
0.0176
0.0288
0.0507
0.0322
0.0103
0.0103
0.0249
0.0400
0.0846
0.0418
0.0117
0.0118
0.1729
0.0870
0.0256
0.0269
0.0100
0.0100
0.0100
0.0200
0.0200
0.0200
0.0300
0.0300
0.0300
0.0042
0.0054
0.025
0.030
0.040
0.070
0.120
0.135
0.030
0.035
0.070
0.090
0.140
0.135
0.075
0.090
0.125
0.270
0.01
0.01
0.01
0.01
0.01
0.01
0.04
0.05
0.06
0.13
0.26
280
285
298
383
531
506
396
396
498
498
601
579
1,017
1,036
1,320
1,320
237
248
319
330
331
415
987
1,011
1,097
219
266
1 Applied to California VMT only.
* Methane emission factor assumed based on light-duty trucks oxidation catalyst value.
e The carbon emission factor (g COj/km) was used as a proxy for fuel economy because of the greater number of significant figures compared to the
km/L values presented in (IPCC/UNEP/OECD/IEA1997).
C-8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Table C-11: Emission Factors for CH4 and N20 Emissions from Non-Highway Mobile Combustion (g/kg Fuel)
Vehicle Type/Fuel Type
N,0
CH4
Ships and Boats
Residual
Distillate
Gasoline
Locomotives
Residual
Diesel
Coal
Farm Equipment
Gas/Tractor
Other Gas
Diesel/Tractor
Other Diesel
Construction
Gas Construction
Diesel Construction
Other Non-Highway
Gas Snowmobile
Gas Small Utility
Gas HD Utility
Diesel HD Utility
Aircraft
Jet Fuel
Aviation Gasoline
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.1
0.04
0.23
0.23
0.23
0.25
0.25
0.25
0.45
0.45
0.45
0.45
0.18
0.18
0.18
0.18
0.18
0.18
0.087
2.64
Table C-12: NOX Emissions from Mobile Combustion, 1990-1998 (Gg)
Fuel Type/Vehicle Type
Gasoline Highway
Passenger Cars
Light-Duty Trucks
Heavy-Duty Vehicles
Motorcycles
Diesel Highway
Passenger Cars
Light-Duty Trucks
Heavy-Duty Vehicles
Non-Highway
Ships and Boats
Locomotives
Farm Equipment
Construction Equipment
Aircraft11
Other*
Total
1990
4,356
2,910
1,140
296
11
2,031
35
6
1,989
4,357
906
843
819
1,003
143
642
10,744
1991
4,654
3,133
1,215
296
10
2,035
34
7
1,995
4,443
953
842
837
1,020
141
650
11,132
1992
4,788
3,268
1,230
280
11
1,962
35
7
1,920
4,474
924
858
854
1,036
142
661
11,224
1993
4,913
3,327
1,289
286
11
1,900
36
7
1,857
4,482
884
857
870
1,052
142
676
11,294
1994
5,063
3,230
1,503
318
11
1,897
35
9
1,854
4,548
896
859
886
1,069
146
692
11,508
1995
4,804
3,112
1,378
301
12
1,839
35
9
1,795
4,651
903
898
901
1,090
150
709
11,294
1996
4,770
2,691
1,769
298
11
1,803
31
11
1,760
4,688
951
836
913
1,109
151
727
11,261
1997
4,733
2,647
1,774
301
11
1,787
31
11
1,745
4,770
962
870
915
1,119
151
754
11,289
1998
4,617
2,574
1,739
293
11
1,736
?1
11
1,694
4,832
971
903
Q10
1,120
152
773
11,184
,,,j V.,,,VVIH,,V ,wlul,v,u w t_, v wjrwi&o, cuiu uiciciuic uu nui iiii/iuuc uiuioc afUuluc emlSSIOnS
"Other" includes gasoline powered recreational, industrial, lawn and garden, light commercial, logging, airport service,' other equipment- and diesel
powered recreatonal, industrial, lawn and garden, light construction, airport service. HP.
Note: Totals may not sum due to independent rounding.
C-9
-------
Table C-13: CO Emissions from Mobile Combustion, 1990-1998 (Gg) c
Fuel Type/Vehicle Type
Gasoline Highway
Passenger Cars
Light-Duty Trucks
Heavy-Duty Vehicles
Motorcycles
Diesel Highway
Passenger Cars
Light-Duty Trucks
Heavy-Duty Vehicles
Non-Highway
Ships and Boats
Locomotives
Farm Equipment
Construction Equipment
Aircraft1
Other"
Total
1990
51,332
33,746
12,534
• 4,863
190
1,147
28
5
1,115
16,506
2,040
110
527
1,148
820
11,860
68,985
1991
55,104
36,369
13,621
4,953
161
1,210
27
5
1,177
16,863
2,053
109
537
1,171
806
12,187
73,177
1992
53,077
35,554
13,215
4,145
163
1,227
28
6
1,193
17,239
2,054
113
547
1,194
818
12,514
71,543
1993
53,375
35,357
13,786
4,061
172
1,240
30
6
1,205
17,595
2,053
108
557
1,216
821
12,840
72,210
1994
54,778
33,850
15,739
5,013
177
1,316
29
7
1,280
17,962
2,059
104
566
1,238
830
13,165
74,057
• Aircraft estimates include only emissions related to LTD cycles, and therefore do not include crui
* "Other" includes gasoline powered recreational, industrial, lawn and garden, light commercial, lo
powered recreational, industrial, lawn and garden, light construction, airport service.
Note: Totals may not sum due to independent rounding.
1995
47,767
30,391
13,453
3,741
182
1,318
30
7
1,281
18,347
2,064
103
575
1,258
855
13,492
67,433
1996
46,965
25,894
17,483
3,416
171
1,354
27
10
1,318
18,354
2,069
102
582
1,249
861
13,492
66,674
1997
45,477
24,998
17,186
3,123
170
1,394
27
10
1,358
18,430
2,082
106
581
1,220
859
13,582
65,301
1998
44,300
24,357
16,988
2,783
173
1,410
27
10
1,374
18,069
2,085
110
573
1,166
865
13,271
63,780
se altitude emissions.
gging, airport service, other equipment; and diesel
Table C-14: NMVOCs Emissions from Mobile Combustion, 1990-1998 (Gg)
Fuel Type/Vehicle Type
Gasoline Highway
Passenger Cars
Light-Duty Trucks
Heavy-Duty Vehicles
Motorcycles
Diesel Highway
Passenger Cars
Light-Duty Trucks
Heavy-Duty Vehicles
Non-Highway
Ships and Boats
Locomotives
Farm Equipment
Construction Equipment
Aircraft1
Other*
Total
1990
5,444
3,524
1,471
392
56
283
11
2
269
2,310
743
48
133
204
163
1,019
8,037
1991
5,607
3,658
1,531
384
33
290
11
3
276
2,342
747
47
133
208
161
1,046
8,239
1992
5,220
3,447
1,440
303
30
288
12
3
274
2,354
729
49
132
212
162
1,070
7,862
1993
5,248
3,427
1,494
296
31
288
12
3
273
2,382
731
47
132
216
160
1,096
7,919
1994
5,507
3,367
1,731
375
33
300
12
4
284
2,416
738
45
131
220
159
1,123
8,223
1995
4,883
3,071
1,478
297
37
290
12
274
2,449
738
45
130
225
161
1,150
7,621
1996
4,743
2,576
1,869
266
33
238
11
223
2,417
738
44
129
223
161
1,122
7,398
1997
4,614
2,504
1,830
247
32
221
11
206
2,334
742
46
124
216
160
1,046
7,169
1998
4,630
2,534
1,828
233
35
201
11
186
2,234
742
47
120
208
160
956
7,065
rUlUldll CaUIIICHCO HiMiuuo wuijr **nii»*uiwii*/ i WIUIWM »** •- • ** •*/ •»•—, _..__. — ^
6 "Other Includes gasoline powered recreational, industrial, lawn and garden, light commercial, logging, airport service,
powered recreational, industrial, lawn and garden, light construction, airport service.
other equipment; and diesel
Note: Totals may not sum due to independent rounding.
C-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Annex D
Methodology for Estimating Methane Emissions from Coal Mining
The methodology for estimating methane emissions from coal mining consists of two distinct steps. The first step
addresses emissions from underground mines. For these mines, emissions are estimated on a mine-by-mine basis and
then are summed to determine total emissions. The second step of the analysis involves estimating methane emissions
for surface mines and post-mining activities. In contrast to the methodology for underground mines, which uses mine-
specific data, the surface mine and post-mining activities analysis consists of multiplying basin-specific coal production
by basin-specific emission factors.
Step 1: Estimate Methane Liberated and Methane Emitted from Underground Mines
Underground mines generate methane from ventilation systems and from degasification systems. Some mines
recover and use methane generated from degasification systems, thereby reducing emissions to the atmosphere. Total
methane emitted from underground mines equals the methane liberated from ventilation systems, plus the methane
liberated from degasification systems, minus methane recovered and used.
Step 1.1 Estimate Methane Liberated Irom Ventilation Systems
All coal mines with detectable methane emissions' use ventilation systems to ensure that methane levels remain
within safe concentrations. Many coal mines do not have detectable levels of methane, while others emit several million
cubic feet per day (MMCFD) from their ventilation systems. On a quarterly basis, the U.S. Mine Safety and Health
Administration (MSHA) measures methane emissions levels at underground mines. MSHA maintains a database of
measurement data from all underground mines with detectable levels of methane in their ventilation air. Based on the
four quarterly measurements, MSHA estimates average daily methane liberated at each of the underground mines with
detectable emissions.
For the years 1990 through 1996, MSHA emissions data were obtained for a large but incomplete subset of all
mines with detectable emissions. This subset includes mines emitting at least 0.1 MMCFD for some years and at least
0.5 MMCFD for other years, as shown in Table D-l. Well over 90 percent of all ventilation emissions were
concentrated in these subsets. For 1997, the complete MSHA database for all 586 mines with detectable methane
emissions was obtained. These mines were assumed to account for 100 percent of methane liberated from underground
mines. The 1998 emissions dataset from MSHA includes mines emitting at least 0.1 MMCFD.
Using the complete database from 1997, the proportion of total emissions accounted for by mines emitting more
and less than 0.1 MMCFD or 0.5 MMCFD was estimated (see Table D-l). These proportions were then applied to the
years 1990 through 1998 to account for the less than 10 percent of ventilation emissions coming from mines without
MSHA data.
Average daily methane emissions were multiplied by 365 to determine the annual emissions for each mine. Total
ventilation emissions for a particular year were estimated by summing emissions from individual mines.
1 MSHA records coal mine methane readings with concentrations of greater than 50 ppm (parts per million) methane. Readings below this
threshold are considered non-detectable.
D-1
-------
Table D-1: Mine-Specific Data Used to Estimate Ventilation Emissions
Year Individual Mine Data Used
1990 All Mines Emitting at Least 0.1 MMCFD (Assumed to Account for 97.8% of Total)*
1991 1990 Emissions Factors Used Instead of Mine-Specific Data
1992 1990 Emissions Factors Used Instead of Mine-Specific Data
1993 All Mines Emitting at Least 0.1 MMCFD (Assumed to Account for 97.8% of Total)*
1994 All Mines Emitting at Least 0.1 MMCFD (Assumed to Account for 97.8% of Total)*
1995 All Mines Emitting at Least 0.5 MMCFD (Assumed to Account for 94.1 % of Total)*
1996 All Mines Emitting at Least 0.5 MMCFD (Assumed to Account for 94.1% of Total)*
1997 All Mines with Detectable Emissions (Assumed to Account for 100% of Total)
1998 All Mines Emitting at Least 0.1 MMCFD (Assumed to Account for 97.8% of Total)* :
* Factor derived from a complete set of individual mine data collected for 1997.
Step 1.2 Estimate Methane Liberated from Degasification Systems
Coal mines use several different types of degasification systems to remove methane, including vertical wells and
horizontal boreholes to recover methane prior to mining of the coal seam. Gob wells and cross-measure boreholes
recover methane from the overburden (i.e., GOB area) after mining of the seam (primarily in longwall mines).
MSHA collects information about the presence and type of degasification systems in some mines, but does not
collect quantitative data on the amount of methane liberated. Thus, the methodology estimated degasification emissions
on a mine-by-mine basis based on other sources of available data. Many of the coal mines employing degasification
systems have provided EPA with information regarding methane liberated from their degasification systems. For these
mines, this reported information was used as the estimate. In other cases in which mines sell methane recovered from
degasification systems to a pipeline, gas sales were used to estimate methane liberated from degasification systems (see
Step 1.3). Finally, for those mines that do not sell methane to a pipeline and have not provided information to EPA,
methane liberated from degasification systems was estimated based on the type of system employed. For example, for
coal mines employing gob wells and horizontal boreholes, the methodology assumes that degasification emissions
account for 40 percent of total methane liberated from the mine.
Step 1.3: Estimate Methane Recovered from Degasification Systems and Used (Emissions Avoided)
In 1998, twelve active coal mines had developed methane recovery and use projects and sold the recovered
methane to a pipeline. One coal mine also used some recovered methane in a thermal dryer in addition to selling gas
to a pipeline. Where available, state agency gas sales data were used to estimate emissions avoided for these projects.
Emissions avoided were attributed to the year in which the coal seam was mined. For example, if a coal mine recovered
and sold methane using a vertical well drilled five years in advance of mining, the emissions avoided associated with
those gas sales were attributed to the year during which the well was mined-through (e.g., five years after the gas was
sold). In order to estimate emissions avoided for those coal mines using degasification methods that recover methane
in advance of mining, information was needed regarding the amount of gas recovered and the number of years in
ad vance of mining that wells were drilled. In most cases, coal mine operators provided this information, which was then
used to estimate emissions avoided for a particular year. Additionally, several state agencies provided production data
for individual wells. For some mines, these individual well data were used to assign gas sales from individual wells to
the appropriate emissions avoided year.
Step 2: Estimate Methane Emitted from Surface Mines and Post-Mining Activities
Mine-specific data were not available for estimating methane emissions from surface coal mines or for post-mining
activities. For surface mines and post-mining activities, basin-specific coal production was multiplied by a basin-
specific emission factor to determine methane emissions.
D-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Step 2.1: Define the Geographic Resolution of the Analysis and Collect Coal Production Data
The first step in estimating methane emissions from surface mining and post-mining activities was to define the
geographic resolution of the analysis and to collect coal production data at mat level of resolution. The analysis was
conducted by coal basin as defined in Table D-2, which presents coal basin definitions by basin and by state.
The Energy Information Agency's (EIA) Coal Industry Annual reports state- and county-specific underground
and surface coal production by year. To calculate production by basin, the state level data were grouped into coal basins
using the basin definitions listed in Table D-2. For two states—West Virginia and Kentucky—county-level production
data was used for the basin assignments because coal production occurred from geologically distinct coal basins within
these states. Table D-3 presents the coal production data aggregated by basin.
Step 2.2: Estimate Emissions Factors for Each Emissions Type
Emission factors for surface mined coal were developed from the in situ methane content of the surface coal in
each basin. Based on an analysis presented in EPA (1993), surface mining emission factors were estimated to be from
1 to 3 times the average in situ methane content in the basin. For this analysis, the surface mining emission factor was
determined to be twice the in situ methane content in the basin. Furthermore, the post-mining emission factors used
were estimated to be 25 to 40 percent of the average in situ methane content in the basin. For this analysis, the post-
mining emission factor was determined to be 32.5 percent of the in situ methane content in the basin. Table D-4
presents the average in situ content for each basin, along with the resulting emission factor estimates.
Step 2.3: Estimate Methane Emitted
The total amount of methane emitted was calculated by multiplying the coal production in each basin by the
appropriate emission factors.
Total annual methane emissions is equal to the sum of underground mine emissions plus surface mine emissions
plus post-mining emissions. Table D- 5 and Table D-6 present estimates of methane liberated, used, and emitted for
1990 through 1998. Table D-7 provides emissions by state.
D-3
-------
Table D-2: Coal Basin Definitions by Basin and by State
Basin
States
Northern Appalachian Basin
Central Appalachian Basin
Warrior Basin
Illinois Basin
South West and Rockies Basin
North Great Plains Basin
West Interior Basin
Northwest Basin
Maryland, Ohio, Pennsylvania, West VA North
Kentucky East, Tennessee, Virginia, WestVA South
Alabama
Illinois, Indiana, Kentucky West
Arizona, California, Colorado, New Mexico, Utah
Montana, North Dakota, Wyoming
Arkansas, Iowa, Kansas, Louisiana, Missouri, Oklahoma, Texas
Alaska, Washington
State
Basin
Alabama
Alaska
Arizona
Arkansas
California
Colorado
Illinois
Indiana
Iowa
Kansas
Kentucky East
Kentucky West
Louisiana
Maryland
Missouri
Montana
New Mexico
North Dakota
Ohio
Oklahoma
Pennsylvania.
Tennessee
Texas
Utah
Virginia
Washington
West Virginia South
West Virginia North
Wyoming
Warrior Basin
Northwest Basin
South West And Rockies Basin
West Interior Basin
South West And Rockies Basin
South West And Rockies Basin
Illinois Basin
Illinois Basin
West Interior Basin
West Interior Basin
Central Appalachian Basin
Illinois Basin
West Interior Basin
Northern Appalachian Basin
West Interior Basin
North Great Plains Basin
South West And Rockies Basin
North Great Plains Basin
Northern Appalachian Basin
West Interior Basin
Northern Appalachian Basin
Central Appalachian Basin
West Interior Basin
South West And Rockies Basin
Central Appalachian Basin
Northwest Basin
Central Appalachian Basin
Northern Appalachian Basin
North Great Plains Basin
D-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Table D-3: Annual Coal Production (Thousand Short Tons)
Underground Coal Production
Basin
Northern Appalachia
Central Appalachia
Warrior
Illinois
S. WesVRockies
N. Great Plains
West Interior
Northwest
Total
Surface Coal Production
Basin
Northern Appalachia
Central Appalachia
Warrior
Illinois
S. West/Rockies
N. Great Plains
West Interior
Northwest
Total
1990
103,865
198,412
17,531
69,167
32,754
1,722
105
0
423,556
1990
60,761
94,343
11,413
72,000
43,863
249,356
64,310
6,707
602,753
1991
103,450
181,873
17,062
69,947
31,568
2,418
26
0
406,344
1991
51,124
91,785
10,104
63,483
42,985
259,194
61,889
6,579
587,143
1992
105,220
177,777
15,944
73,154
31,670
2,511
59
0
406,335
1992
50,512
95,163
9,775
58,814
46,052
258,281
63,562
6,785
588,944
1993
77,032
164,845
15,557
55,967
35,409
2,146
100
0
351,056
1993
48,641
94,433
9,211
50,535
48,765
275,873
60,574
6,340
594,372
1994
100,122
170,893
14,471
69,050
41,681
2,738
147
0
399,102
1994
44,960
106,129
8,795
51,868
49,119
308,279
58,791
6,460
634,401
1995
98,103
166,495
17,605
69,009
42,994
2,018
25
0
396,249
1995
39,372
106,250
7,036
40,376
46,643
331,367
59,116
6,566
636,726
1996
106,729
171,845
18,217
67,046
43,088
2,788
137
0
409,850
1996
39,788
108,869
6,420
44,754
43,814
343,404
60,912
6,046
654,007
1997
112,135
177,720
18,505
64,728
44,503
2,854
212
0
420,657
1997
40,179
113,275
5,963
46,862
48,374
349,612
59,061
5,945
669,271
1998
116,460
170,750
17,405
62,674
45,314
3,183
217
0
416,002
1998
41,283
108,874
5,608
47,502
50,304
384,596
57,980
5,982
702,130
Total Coal Production
Basin
Northern Appalachia
Central Appalachia
Warrior
Illinois
S. Wesl/Rockies
N. Great Plains
West Interior
Northwest
Total
1990
164,626
292,755
28,944
141,167
76,617
251,078
64,415
6,707
1,026,309
1991
154,574
273,658
27,166
133,430
74,553
261,612
61,915
6,579
993,487
1992
155,732
272,940
25,719
131,968
77,722
260,792
63,621
6,785
995,279
1993
125,673
259,278
24,768
106,502
84,174
278,019
60,674
6,340
945,428
1994
145,082
277,022
23,266
120,918
90,800
311,017
58,938
6,460
1995
137,475
272,745
24,641
109,385
89,637
333,385
59,141
6,566
1,033,503 1,032,975
1996
146,517
280,714
24,637
111,800
86,902
346,192
61,049
6,046
1,063,857 1
1997
152,314
290,995
24,468
111,590
92,877
352,466
59,273
5,945
,0829,928
1998
157,743
279,624
23,013
110,176
95,618
387,779
58,197
5,982
1,118,132
Source: EIA (1990-99), Coal Industry Annual. U.S. Department of Energy, Washington, DC, Table 3.
Note: Totals may not sum due to independent rounding.
D-5
-------
Table D-4: Coal Surface and Post-Mining Methane Emission Factors (ft3 Per Short Ton)
Surface Average Underground Average Surface Mine Post-Mining Post Mining
Basin in situ Content In situ Content Factors Surface Factors Underground
Northern Appalachia
Central Appalachia
Warrior
Illinois
S. West/Rockies
N. Great Plains
West Interior
Northwest
49.3
49.3
49.3
39.0
15.3
3.2
3.2
3.2
171.7
330.7
318.0
57.20
225.8
41.67
41.67
41.67
98.6
98.6
98.6
78.0
30.6
6.4
6.4
6.4
16.0
16.0
16.0
12.7
5.0
1.0
1.0
1.0
16.0
16.0
16.0
12.7
5.0
1.0
1.0
1.0
Source: EPA (1993), Anthropogenic Methane Emissions in the United States: Estimates for 1990, Report to Congress, U.S. Environmental Protection
Agency, Air and Radiation, April.
Table D- 5: Underground Coal Mining Methane Emissions (Billion Cubic Feet)
Activity
Ventilation Output
Adjustment Factor for Mine Data"
Adjusted Ventilation Output
DegasificatJon System Liberated
Total Underground Liberated
Recovered & Used
Total
1990
112
97.8%
114
57
171
(15)
156
1991
NA
NA
NA
NA
164
(15)
149
1992
NA
NA
NA
NA
162
(19)
142
1993
95
97.8%
97
49
146
(24)
121
1994
96
97.8%
98
50
149
(29)
119
1995
102
91.4%
111
50
161
(31)
130
1996
90
91.4%
99
51
150
<3b)
115
1997
96
100.0%
96
57
153
(42)
112
1998
91
97.8%
93
54
147
(43)
103
'Referto Table D-1:
Note: Totals may not sum due to independent rounding.
Table D-6: Total Coal Mining Methane Emissions (Billion Cubic Feet)
Activity
Underground Mining
Surface Mining
Post-Mining (Underground)
Post-Mining (Surface)
Total
1990
156
25
33
4
218
1991
149
23
31
4
207
1992
142
23
30
4
200
1993
121
23
27
4
175
1994
119
24
30
4
177
1995
130
22
30
4
185
1996
115
23
31
4
172
1997
112
24
31
4
171
1998
103
23
31
4
162
Note: Totals may not sum due to independent rounding.
D-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Table D-7: Total Coal Mining Methane Emissions by State (Million Cubic Feet)
State
Alabama
Alaska
Arizona
Arkansas
California
Colorado
Illinois
Indiana
Iowa
Kansas
Kentucky
Louisiana
Maryland
Missouri
Montana
New Mexico
North Dakota
Ohio
Oklahoma
Pennsylvania
Tennessee
Texas
Utah
Virginia
Washington
West Virginia
Wyoming
Total
1990
33,650
13
402
+
2
10,117
10,643
3,149
3
5
21,229
24
510
20
280
905
217
4,710
13
22,573
800
415
4,562
45,883
37
56,636
1,382
218,180
1993
27,000
12
433
+
0
7,038
8,737
2,623
1
3
19,823
23
245
5
267
1,186
238
4,110
14
26,437
350
406
4,512
30,454
35
39,477
1,578
175,007
1994
30,713
12
464
+
0
9,029
10,624
2,791
+
2
21,037
26
256
6
310
1,223
240
4,377
52
24,026
338
389
3,696
26,782
36
38,565
1,782
176,781
1995
39,945
13
425
4-
0
8,541
11,106
2,106
0
2
19,103
28
259
4
294
980
224
3,900
14
27,086
366
392
3,541
19,898
36
44,894
1,977
185,134
1996
30,808
11
371
+
0
5,795
10,890
2,480
0
2
18,292
24
287
5
283
856
222
3,992
14
26,567
418
410
4,061
19,857
34
44,380
2,090
172,149
1997
26,722
11
417
+
0
9,057
8,571
3,088
0
3
20,089
26
296
3
305
961
220
4,313
132
30,339
390
397
4,807
16,990
33
41,454
2,122
170,746
1998
26,910
10
403
+'
0
6,016
7,974
3,223
0
3
18,801
24
281
3
319
1,017
227
4,150
137
31,313
309
391
5,059
9,514
35
43,402
2,361
161,883
Note: The emission estimates provided above are inclusive of emissions from underground mines, surface mines and post-mining activities. The
following states have neither underground nor surface mining and thus report no emissions as a result of coal mining: Connecticut, Delaware, Florida,
Georgia, Hawaii, Idaho, Maine, Massachusetts, Michigan, Minnesota, Mississippi, Nebraska, Nevada, New Hampshire, New Jersey, New York, North
Carolina, Oregon, Rhode Island, South Carolina, South Dakota, Vermont, and Wisconsin. Emission estimates are not given for 1991 and 1992 because
underground mine data was not available for those years.
D-7
-------
-------
Annex E
Methodology for Estimating Methane Emissions from Natural Gas Systems
The following steps were used to estimate methane emissions from natural gas systems.
Step 1: Calculate Emission Estimates for Base Year 1992 Using GRI/EPA Study
The first step in estimating methane emissions from natural gas systems was to develop a detailed base year
estimate of emissions. The study by GRI/EPA (1996) divides the industry into four stages to construct a detailed
emission inventory for the year 1992. These stages include: field production, processing, transmission and storage (i.e.,
both underground and liquefied gas storage), and distribution. This study produced emission factors and activity data
for over 100 different emission sources within the natural gas system. Emissions for 1992 were estimated by
multiplying activity levels by emission factors for each system component and then summing by stage. Since
publication, the EPA has updated activity data for some of the components in the system. Table E-l displays the 1992
GRI/EPA activity levels and emission factors for venting and flaring from the field production stage, and the current
EPA activity levels and emission factors. The data in Table E-l is a representative sample of data used to calculate
emissions from all stages.
Step 2: Collect Aggregate Statistics on Main Driver Variables
As detailed data on each of the over 100 sources were not available for the period 1990 through 1998, activity
levels were estimated using aggregate statistics on key drivers, including: number of producing wells (IPAA1990,1991,
1992, 1993,1994, 1995, 1996, 1997, 1998), number of gas plants (AGA 1990, 1991, 1992,1993, 1994, 1995, 1996,
1997,1998,1999), miles of transmission pipeline (OPS 2000), miles of distribution pipeline (AGA 1990,1991,1992,
1993,1994,1995,1996,1997,1998), miles of distribution services (AGA 1990,1991,1992,1993,1994,1995,1996,
1997, 1998), and energy consumption (EIA 1999). Data on the distribution of gas mains by material type was not
available for certain years from AGA. For those years, the average distribution by type was held constant. Table E-2
provides the activity levels of some of the key drivers in the natural gas analysis.
Step 3: Estimate Emission Factor Changes Over Time
For the period 1990 through 1995, the emission factors were held constant, based on 1992 values. An assumed
improvement in technology and practices was estimated to reduce emission factors by 5 percent by the year 2020. This
assumption, annualized, amounts to a 0.2 percent decline in the 1996 emission factor, a 0.4 percent decline in the 1997
emission factor, and a 0.6 percent decline in the 1998 emission factor.
Step 4: Estimate Emissions for Each Source
Emissions from each sector of the natural gas industry were estimated by multiplying the activity factors by
emission factors and then subtracting the Natural Gas STAR emission reductions as reported by industry STAR
Partners. Total emissions were estimated by adding the emission estimates from each sector. Table E-3 provides
emission estimates for venting and flaring emissions from the field production stage.
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-------
Table E-3: CH4 Emission Estimates for Venting and Flaring from the Field Production Stage (Mg)
Activity
1990 1991 1992 1993 1994 1995 1996 1997 1998
Drilling and Well Completion
Completion Flaring
Normal Operations
Pneumatic Device Vents
Chemical Injection Pumps
Kimray Pumps
Dertydrator Vents
Compressor Exhaust Vented
Gas Engines
Routine Maintenance
Well Workovers
Gas Wells
Well Clean Ups(LP Gas Wells)
5.4
5.5
5.6
5.7
5.8
5.9
6.1
6.0
6.0
567778 578,313 602,291 618,531 635,276 655,386 692,033 687,785 686,403
36,449 37,323 39,053 40,277 41,668 43,111 45,666 45,256 45,166
134247 136,380 140,566 143,211 144,040 147,191 151,572 149,506 149,206
41,436 42,095 43,387 44,203 44,459 45,432 46,784 46,146 46,054
119,284 121,498 126,535 129,947 133,465 137,690 145,389 144,497 144,206
531 540 556 567 570 582 600 591 590
101,118 102,725 105.878 107,870 108,494 110,868 114,168 112,612 112,386
UIUIIUU1*II*7
Vessel BD
Pipeline BD
Compressor BD
Compressor Starts
Upsets
Pressure Relief Valves
ESD
Mishaps
256
1,710
1,548
3,462
326
6,764
925
261
1,729
1.573
3,518
332
6.827
936
271
1.772
1,627
3,640
346
6,767
959
278
1,799
1,662
3,718
355
6,646
974
284
1,818
1,687
3,773
365
6,773
984
292
1,852
1,730
3.871
376
6,882
1,003
306
1,908
1,802
4,031
397
6,834
1,033
303
1,894
1,786
3,995
395
6,816
1,025
302
1,890
1,782
3,987
394
7,006
1,023
E-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Annex F
Methodology for Estimating Methane Emissions from Petroleum Systems
The methodology for estimating methane emissions from petroleum systems is based on the 1999 EPA draft
report, Estimates of Methane Emissions from.the U.S. Oil Industry (EPA 1999). Seventy activities that emit methane
from petroleum systems were examined for this report. Most of the activities analyzed involve crude oil production field
operations, which accounted for 97 percent of total oil industry emissions. Crude transportation and refining accounted
for the remaining emissions at about one and two percent each, respectively.
The following steps were taken to estimate methane emissions from petroleum systems.
Step 1: Calculate a Detailed Emission Estimate for 1995 Based on the 1999 EPA Report
The emission factors used for the 1995 estimate of methane emissions did not require any changes from those used
in the 1999 EPA draft report. An industry peer review process identified improvements to activity data for oil wells
and tank venting. These recommendations were incorporated into the estimate provided in this inventory. In addition,
the EPA reviewed data on the number of oil well completions each year for two years after the initial estimates to ensure
that late reports were incorporated. Enhanced data sources were found for the number of offshore oil production
platforms and the number of crude oil loadings into marine vessels. The activity factors for all years are updated to
include data from these sources. This format is used as a basis for estimating emissions for the other years in the time
series: 1990-94 and 1996-98 by including the appropriate activity factors for each year.
Step 2: Collect Oil Industry Activity Data
Several approaches were used to develop annual activity data for 1990 through 1994 and 1996 through 1998.
Most activity data were updated annually at the same level of detail as the 1995 estimate, using reports from the U.S.
Department of Energy (DOE) and the oil industry. For cases in which annual data were not available but the activity
factors were known to correlate well with changes in oil production rates, the activity factors were scaled from a base
year in proportion to annual oil production rate changes. For a small number of sources, 1998 data were not yet
available. In these cases, the 1997 activity factors were used. In the few cases where no data was located, activity data
based on oil industry expert judgment were used.
Step 3: Selection Emission Factors
The 1995 emission factors were used for all years -1990 through 1998. Many of the emission factors are based
on field tests performed several years ago while others were taken from more recent work. The more recently developed
emission factors use tank emission models developed by the American Petroleum Institute for estimating emissions from
fixed roof and floating roof tanks.
Step 4: Estimate Emissions for Each Activity
Emissions from each of the 70 petroleum system activities analyzed were estimated by multiplying the activity
data for each year by the corresponding emission factor. Table F-l, Table F-2, and Table F-3 provide the 1998 activity
factors, emission factors, and emission estimates. Table F- 4 provides a summary of emission estimates for the years
1990 through 1998.
F-1
-------
Table F-1: CH4 Emissions from Petroleum Production Field Operations
Activity/Equipment
Vented Emissions:
Oil Tanks
Pneumatic Devices, High Bleed
Pneumatic Devices, Low Bleed
Chemical Injection Pumps
Vessel Slowdowns
Compressor Slowdowns
Compressor Starts
Stripper wells
Well Completion Venting
Well Workovers
Pipeline Pigging
Offshore Platforms, Gulf of Mexico
Offshore Platforms, Other U.S. Areas
Total Vented Emissions
Fugitive Emissions:
Offshore Platforms, Gulf of Mexico
Offshore Platforms, Other U.S. Areas
Oil Wellheads (heavy crude)
Oil Wellheads (light crude)
Separators (heavy crude)
Separators (light crude)
Heater/Treaters (light crude)
Headers (heavy crude)
Headers (light crude)
Floating Roof Tanks
Compressors
Large Compressors
Sates Areas
Pipelines
Well Drilling
Battery Pumps
Total Fugitive Emissions
Combustion Emissions:
Gas Engines
Heaters
Well Drilling
Flares
Offshore Platforms, Gulf of Mexico
Offshore Platforms, Other U.S. Areas
Total Emissions from Combustion
Process Upset Emissions:
Platform Emergency Shutdowns
Pressure Relief Valves
Well Blowouts Offshore
Well Blowouts Onshore
Total Emissions from Upsets
Total (excluding stripper wells)
Emission
Factor Units
18scf of CH^/bbl crude
345 scfd CHt/device
35 scfd CH/device
248 scfd CH
-------
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-------
Table F- 3: CH4 Emissions from Petroleum Refining
Activity/Equipment
Vented Emissions:
Tanks
System Slowdowns
Asphalt Blowing
Total Vented Emissions
Fugitive Emissions:
Fuel Gas System
Floating Roof Tanks
Wastewater Treating
Cooling Towers
Total Fugitive Emissions
Combustion Emissions:
Atmospheric Distillation
Vacuum Distillation
Thermal Operations
Catalytic Cracking
Catalytic Reforming
Catalytic Hydrocracking
Hydrorefining
Hydrotreat'ng
Alkylation/Polymerization
Aromatics/lsomeration
Lube Oil Processing
Engines
Flares
Total Combustion Emissions
Tolal
Table F- 4: Summary of CH4
Activity
Production Field Operations
Tank venting
Pneumatic device venting
Wellhead fugitives
Combustion & process upsets
Misc. venting & fugitives
Crude Oil Transportation
Refining
Total
Emission
Factor Units
20.6 scfCIVMbbl
137 scfCIVMbbl
2,555 scfCH
-------
Annex G
Methodology for Estimating Emissions from International Bunker Fuels used by the
U.S. Military
Bunker fuel emissions estimates for the Department of Defense (DoD) were developed using data generated by
the Defense Energy Support Center for aviation and naval fuels. The Defense Energy Support Center (DESC) of the
Defense Logistics Agency (DLA) prepared a special report based on data in the Defense Fuels Automated Management
System (DFAMS). DFAMS contained data for 1995 through 1998, but the data set was not complete for years prior
to 1995. Fuel quantities for 1990 to 1994 were estimated based on a back-calculation of the 1995 DFAMS values using
DLA aviation and marine fuel procurement data.
Step 1: Omit Extra-Territorial Fuel Deliveries
Beginning with the complete DFAMS data set for each year, the first step in the development of DoD related
emissions from international bunker fuels was to identify data that would be representative of international bunker fuel
consumption as that term is defined by decisions of the UNFCCC (i.e., fuel sold to a vessel, aircraft, or installation
within the United States or its territories and used in international maritime or aviation transport). Therefore, fuel data
was categorized by the location of fuel delivery in order to identify and omit all extra-territorial fuel
transactions/deliveries (i.e., sales abroad). Table G-l displays the fuels that remain at the completion of step 1,
summarized by fuel type.
Step 2: Omit Fuel Transactions Received by Military Services that are not Considered to be
International Bunker Fuels
Next, the records were sorted by Military Service. The following assumptions were made regarding bunker fuel
use by Service, leaving only the Navy and Air Force as users of military international bunker fuels.
• Only fuel delivered to a ship, aircraft, or installation in the United States can be a potential international
bunker fuel. Fuel consumed in international aviation or marine transport should be included in the bunker
fuel estimate of the country where the ship or aircraft was fueled. Fuel consumed entirely within a
country's borders is not bunker fuel.
• Based on discussions with the Army staff, only an extremely small percentage of Army aviation emissions,
and none of its watercraft emissions, qualified as bunker fuel emissions. The magnitude of these emissions
was judged to be insignificant when compared to Air Force and Navy emissions. Based on this, Army
bunker fuel emissions are assumed to be zero.
• Marine Corps aircraft operating while embarked consume fuel reported as delivered to the Navy. Bunker
fuel emissions from embarked Marine Corps aircraft are reported in the Navy bunker fuel estimates.
Bunker fuel emissions from other Marine Corps operations and training are assumed to be zero.
Bunker fuel emissions from other DoD and non-DoD activities (i.e., other federal agencies) that purchase
fuel from DESC are assumed to be zero.
Step 3: Omit Land-Based Fuels
Navy and Air Force land-based fuels (i.e., fuel not used by ships or aircraft) were also omitted. The remaining
fuels, listed below, were potential DoD international bunker fuels.
Marine: naval distillate fuel (F76) and marine gas oil (MGO).
• Aviation: aviation gasoline (113 and 130) and jet fuels (JP8, JP5, JP4, JAA, JA1, and JAB).
G-1
-------
Step 4: Determine Bunker Fuel Percentages
Next it was necessary to determine what percent of the marine and aviation fuels were used as international bunker
fuels. Military aviation bunkers include international operations (i.e., sorties that originate in the United States and end
in a foreign country), operations conducted from naval vessels at sea, and operations conducted from U.S. installations
principally over international water in direct support of military operations at sea (e.g., anti-submarine warfare). For
the Air Force, a bunker fuel weighted average was calculated based on flying hours by major command. International
flights were weighted by an adjustment factor to reflect the fact that they typically last longer than domestic flights. In
addition, a fuel use correction factor was used to account for the fact that transport aircraft burn more fuel per hour than
most tactical aircraft. The Air Force bunker fuel percentage was determined to be 13.2 percent. This percentage was
multiplied by total annual Air Force aviation fuel delivered for U.S. activities, producing an estimate for bunker fuel
consumed by the Air Force. The Naval Aviation bunker fuel percentage of total fuel was calculated using flying hour
data from Chief ofNaval Operations Flying Hour Projection System Budget Analysis Report for FY1998, and estimates
of bunker fuel percent of flights provided by the fleet. The Navy Aviation bunker fuel percentage, determined to be 40.4
percent, was multiplied by total annual Navy aviation fuel delivered for U.S. activities, yielding total Navy aviation
bunker fuel consumed.
For marine bunkers, fuels consumed while ships were underway were assumed to be bunker fuels. The Navy
reported that 87 percent of vessel operations were underway, while the remaining 13 percent of operations occurred in
port (i.e., pierside). Therefore, the Navy maritime bunker fuel percentage was determined to be 87 percent. Table G-2
and Table G-3 display DoD bunker fuel totals for the Navy and Air Force.
Step 5: Calculate Emissions from International Bunker Fuels
Bunker fuel totals were multiplied by appropriate emission factors to determine GHG emissions.
The rows labeled 'U.S. Military' and 'U.S. Military Naval Fuels' within Tables 2-35 and 2-36 of the inventory
were based on the international bunker fuel totals provided in Table G-2 and Table G-3, below. Carbon dioxide
emissions from Aviation Bunkers and distillate Marine Bunkers presented in Table 2-8 are the total of military plus civil
aviation and civil marine bunker fuels, respectively. The military component of each total is based on fuels tallied in
Table G-2 and Table G-3. Carbon dioxide emissions from Military Vehicles (e.g., ships, aircraft, and land-based
vehicles) presented in Table 2-8 of the Inventory were calculated by subtracting Total Aviation Bunker Fuel in Table
G-2 from the Aviation Subtotal in Table G-l. Motor gasoline totals presented in Table G-l were estimated using data
provided by the military services.
6-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
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Table 6-2: Total U.S. DoD Aviation Bunker Fuel (Million Gallons)
Fuel Type/Service
Jet Fuels
JP8
Navy
Air Force
JP5
Navy
Air Force
JP4
Navy
Air Force
JAA
Navy
Air Force
JA1
Navy
Air Force
JAB
Navy
Air Force
AVGAS (113, 130)
Navy
Air Force
Navy Subtotal
Air Force Subtotal
Total
1990
861.75
445.62
56.74
388.88
370.53
365.29
5.25
31.90
0.02
31.88
13.70
8.45
5.25
-t-
+
+
+
+
+
+
+
+
430.50
431.25
861.76
1991
855.08
442.17
56.30
385.87
367.66
362.46
5.21
31.65
0.02
31.63
13.60
8.39
5.21
+
+
+
+
+
+
+
+
+
427.17
427.92
855.08
1992
699.85
361.90
46.08
315.82
300.92
296.66
4.26
25.90
0.02
25.89
11.13
6.86
4.27
+
+
+
+
+
+
+
+
+
349.62
350.23
699.85
1993
676.68
349.92
44.56
305.36
290.95
286.83
4.12
25.05
0.02
25.03
10.76
6.64
4.12
+
+
+
+
+
+
+
+
+
338.04
338.64
676.68
1994
608.35
314.58
40.06
274.53
261.57
257.87
3.70
22.52
0.01
22.50
9.67
5.97
3.71
+
+
+
+
+
+
+
+
+
303.91
304.44
608.35
1995
580.93
300.40
38.25
262.15
249.78
246.25
3.54
21.50
0.01
21.49
9.24
5.70
3.54
+
+
+
+
+
+
+
+
+
290.21
290.72
580.93
1996
539.53
308.81
39.84
268.97
219.40
216.09
3.31
1.05
0.00
1.05
10.27
6.58
3.69
+
+
+
+
+
+
+
+
+
262.51
277.02
539.53
1997
495.65
292.01
46.92
245.09
194.16
191.15
3.01
0.05
0.00
0.05
9.42
5.88
3.54
+
+
+
+
+
+
+
+
+
243.95
251.70
495.65
1998
501.66
306.39
53.81
252.59
184.38
181.36
3.02
0.03
0.00
0.03
10.84
6.63
4.21
+
+
0.01
0.01
+
+
0.01
0.01
+
241.81
259.86
501.67
+ Does not exceed 0.005 million gallons.
Table G-3: Total U.S. DoD Maritime Bunker Fuel (Million Gallons)
Marine Distillates
1990
1991
1992 1993
1994
1995 1996
1997
1998
Navy-MGO
Navy-F76
522.37 481.15
+
491.47
448.27 364.01 333.82
30.34
331.88
35.57
441.65
31.88
474.23
Total
522.37 481.15 491.47 448.27 364.01 333.82 362.22 477.22 506.11
+ Does not exceed 0.005 million gallons.
6-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Annex H
Methodology for Estimating Methane Emissions from Enteric Fermentation
The following steps were used to estimate methane emissions from enteric fermentation in livestock.
Step 1: Collect Livestock Population Data
All livestock population data, except for horses, were taken from U.S. Department of Agriculture (USDA)
statistical reports. For each animal category, the USDA publishes monthly, annual, and multi-year livestock population
and production estimates. Multi-year reports include revision to earlier published data. Recent reports were obtained
from the USDA Economics and Statistics System website, at , while historical
data were downloaded from the USDA-National Agricultural Statistics Service (NASS) website at
.
The Food and Agriculture Organization (FAO) publish horse population data. These data were accessed from
the FAOSTAT database at . Table H-1 summarizes the published population data by animal type.
Step 2: Estimate Emission Factors for Dairy Cows
Regional dairy cow emission factors from the 1993 Report to Congress (EPA 1993) were used as the starting point
for the analysis. These emission factors were used to calibrate a model of methane emissions from dairy cows. The
model applies revised regional emission factors that reflect changes in milk production per cow over time. Increases
in milk production per cow, in theory, require increases in feed intake, which lead to higher methane emissions per cow.
Table H-2 presents the emission factors per head by region used for dairy cows and milk production. The regional
definitions are from EPA (1993).
Step 3: Estimate Methane Emissions from Dairy Cattle
Dairy cow emissions for each state were estimated by multiplying the published state populations by the regional
emission factors, as calculated in Step 2. Dairy replacement emissions were estimated by multiplying national
replacement populations by a national emission factor. The USDA reported the number of replacements 12 to 24
months old as "milk heifers." It is assumed that the number of dairy cow replacements 0 to 12 months old was
equivalent to the number 12 to 24 months old replacements.
Step 4: Estimate Methane Emissions from Beef Cattle
Beef cattle methane emissions were estimated by multiplying published cattle populations by emission factors.
Emissions from beef cows and replacements were estimated using state population data and regional emission developed
by EPA (1993), as shown in Table H-3. Emissions from slaughter cattle and bulls were estimated using national data
and emission factors. The emission factors for slaughter animals represent their entire life, from birth to slaughter.
Consequently, the emission factors were multiplied by the national data on total steer and heifer slaughters rather than
live populations of calves, heifers, and steers grown for slaughter. Slaughter population numbers were,taken from
USDA datasets. The Weanling and Yearling mix was unchanged from earlier estimates derived from discussions with
industry representatives.
Step 5: Estimate Methane Emissions from Other Livestock
Methane emissions from sheep, goats, swine, and horses were estimated by multiplying published national
population estimates by the national emission factor for each year.
A summary of emissions is provided in Table H-4. Emission factors, national average or regional, are shown by
animal type in Table H-5.
H-1
-------
Table H-1: Livestock Population (Thousand Head)
Animal Type
Dairy
Cows
Replacements 0-1 2
Replacements 12-24
Beef
Cows
Replacements 0-1 2
Replacements 12-24
Slaughter-Weanlings
Slaughter-Yearlings
Bulls
Other
Sheep
Goats
Horses
Hogs
1990 1991 1992 1993
10,007 9,883 9,714 9,679
4,135 4,097 4,116 4,088
4,135 4,097 4,116 4,088
32,677 32,960 33,453 34,132
5,141 5,321 5,621 5,896
5,141 5,321 5,621 5,896
5,199 5,160 5,150 5,198
20,794 20,639 20,600 20,794
2,180 2,198 2,220 2,239
11,358 11,174 10,797 10,201
2,545 2,475 2,645 2,605
5,650 5,650 5,850 5,900
53,941 56,478 58,532 58,016
Table H-2: Dairy Cow CH4 Emission Factors and Milk Production
Region
Dairy Cow Emission Factors
North Atlantic
South Atlantic
North Central
South Central
West
Milk Production (kg/year)
North Atlantic
South Atlantic
North Central
South Central
West
1990 1991 1992
(kg/head)
116.2 118.5 121.3
127.7 128.7 132.3
105.0 105.7 107.8
116.2 116.1 117.9
130.4 129.6 132.7
6,574 6,811 7,090
6,214 6,300 6,622
6,334 6,413 6,640
5,696 5,687 5,849
8,339 8,255 8,573
1994 1995 1996 1997
9,504 9,491 9,410 9,309
4,062 4,011 3,895 3,829
4,062 4,011 3,895 3,829
35,101 35,645 35,509 34,629
6,132 6,076 5,844 5,671
6,132 6,076 5,844 5,671
5,408 5,612 5,580 5,692
21,632 22,450 22,322 22,770
2,306 2,392 2,392 2,325
9,825 8,982 8,458 8,015
2,605 2,495 2,545 2,295
6,000 6,000 6,050 6,150
59,951 58,899 56,220 58,728
Per Cow
1993 1994 1995 1996 1997
121.0 121.7 124.3 124.8 125.8
132.2 134.2 132.9 133.6 136.5
107.6 109.2 110.9 110.5 112.2
119.0 121.2 121.7 121.4 121.5
132.6 137.9 135.7 135.1 136.9
7,055 7,134 7,391 7,439 7,546
6,608 6,783 6,667 6,730 6,986
6,623 6,791 6,980 6,936 7,123
5,951 6,160 6,206 6,180 6,182
8,559 9,107 8,884 8,822 8,999
1998
9,200
3,793
3,793
34,143
5,282
5,282
5,666
22,663
2,235
7,817
2,045
6,150
62,043
1998
125.8
136.5
111.8
120.5
139.4
7,693
6,847
7,441
6,204
8,991
Table H-3: CH4 Emission Factors Beef Cows and Replacements (kg/Head/Year)
Region Replacements (0-12) Replacements (12-24)
North Atlantic
South Atlantic
North Central
South Central
West
19.2 63.8
22.7 67.5
20.4 60.8
23.6 67.7
22.7 64.8
Mature Cows
61.5
70.0
59.5
70.9
69.1
H-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Table H-4: Methane Emissions from Livestock Enteric Fermentation (Gg)
Animal Type
Dairy
Cows
Replacements 0-12
Replacements 12-24
Beef
Cows
Replacements 0-12
Replacements 12-24
Slaughter-Weanlings
Slaughter-Yearlings
Bulls
Other
Sheep
Goats
Horses
Hogs
Total
1990
1,474
1,150
81
243
3,951
2,181
115
334
120
984
218
286
91
13
102
81
5,712
1991
1,465
1,144
80
241
3,979
2,199
119
346
119
976
220
288
89
12
102
85
5,732
1992
1,473
1,150
81
242
4,039
2,233
125
365
119
974
222
293
86
13
105
88
5,804
1993
1,468
1,148
80
240
4,120
2,278
132
383
120
984
224
288
82
13
106
87
5,876
1994
1,471
1,152
80
239
4,256
2,342
137
398
125
1,023
231
290
79
13
108
90
6,016
1995
1,473
1,159
79
236
4,340
2,380
136
395
130
1,062
239
281
72
12
108
88
6,094
1996
1,454
1,149
76
229
4,305
2,371
130
380
129
1,056
239
274
68
13
109
84
6,032
1997
1,453
1,153
75
225
4,246
2,310
126
368
131
1,077
233
274
64
11
111
88
5,973
1998
1,443
1,146
74
223
4,165
2,279
118
342
131
1,072
223
277
63
10
111
93
5,885
Table H-5: Enteric Fermentation CH4 Emission Factors
Animal Type kg/head/year
Dairy
Cows
Replacements 0-12
Replacements 12-24
Beef
Cows
Replacements 0-12
Replacements 12-24
Slaughter-Weanlings
Slaughter-Yearlings
Bulls
Other
Sheep
Goats
Horses
Hogs
regional
19.6
58.8
regional
regional
regional
23.1
47.3
100.0
8.0
5.0
18.0
1.5
H-3
-------
-------
Annex I
Methodology for Estimating Methane Emissions from Manure Management
The following steps were used to estimate methane emissions from, the management of livestock manure.
Step 1: Collect Livestock Population Data ;
All livestock population data, except for horses, were taken from U.S. Department of Agriculture (USDA)
statistical reports. For each animal category, the USDA publishes monthly, annual, and multi-year livestock population
and production estimates. Multi-year reports include revisions to earlier published data. Recent reports were obtained
from the USDA Economics and Statistics System website, at , while historical
data were downloaded from the USDA National Agricultural 'Statistics Service (NASS) website at
.
Dairy cow and swine population data by farm size for each state, useft in the weighted methane conversion factor
(MCF) calculation described in Step 2, were found in the 1992 Census of Agriculture published by the U.S. Department
of Commerce (DOC 1995), and the 1997 Census of Agriculture published by the USDA (1999n). This census is
conducted every five years. Data from the census is available at the USDA NASS website at
. ;
The Food and Agriculture Organization (FAO) publishes horse population data. These data were accessed from
the FAOSTAT database at . Table 1-1 summarizes the published population data by animal type.
Step 2: Estimate State Methane Conversion Factors for Dairy Cows and Swine
EPA (1992) provides an assessment of dairy and swine manure management practices used to estimate emissions
for 1990. Based on this assessment and the relationship between farm sizes and manure management systems, an
average weighted MCF was assigned to each dairy and swine farm size category. These weighted MCFs indicate the
portion of the methane producing potential realized for each category. MCFs applied to larger farms were higher than
those applied to smaller farm sizes because larger farms tend to use liquid manure management systems, which produce
more methane. ;
Using the dairy cow and swine populations by farm size in the 1992 and 1997 Census of Agriculture for each state,
weighted average dairy and swine MCFs were calculated for each state for each of these years. This weighted MCF
value represents the mix of manure management practices in each state. The overall increase in average state MCFs
between 1992 and 1997 is caused by a shift in dairy cow and swine populations towards larger facilities, which reflects
the increasing use of liquid systems. '
Calculated weighted MCFs for 1992 were used for 1990 and 1991. The calculated weighted MCFs for 1997 were
used for 1998. MCF values for the years in between (i.e., 1993 through 1996) were calculated by interpolating between
the two sets of calculated weighted MCFs. Table 1-2 and Table 1-3 present the weighted dairy and swine MCF values
for each year. i
Step 3: Estimate Methane Emissions from Swine
For each state, the total swine population was multiplied by volatile solids (VS) production rates to determine total
VS production. Estimated state level emissions were calculated as the product of total VS production in each year
multiplied by the maximum methane production potential for swine manurte (B0), and the weighted average state MCF
for the corresponding year. Total U.S. emissions are the sum of the state level emissions. The VS production rate and
maximum methane production potential are shown in Table 1-4.
-------
Step 4: Estimate Methane Emissions from Dairy Cattle
Methane emissions from dairy cow manure were estimated using the same method as emissions from swine
(Step 3), but with an added analysis to estimate changes in manure production associated with changes in feed intake,
or dry matter intake (DMi). It is assumed that manure and VS production will change linearly with changes in dry matter
intake (DMi).
Changes in DMi were calculated reflecting changes in feed intake associated with changes in milk production per
cow per year. To estimate the changes in feed intake, a simplified emission factor model was used for dairy cow enteric
fermentation emissions (see Annex H). This model estimates the change in DMi overtime relative to 1990, which was
used to calculate VS production by dairy cows by state, as summarized in the following equation: (Dairy cow
population)' (VS produced per cow) ' (DMi scaling factor). Methane emissions were then calculated as follows: (VS
produced) ' (Maximum methane production potential for dairy cow manure) ' (State-specific MCF). Total emissions
were finally calculated as the sum of the state level emissions. The 1990 VS production rate and maximum methane
production potential are shown in Table 1-4.
Step 5: Estimate Methane Emissions for Other Animals
The 1990 methane emissions for the other animal types were estimated using the detailed method described in
EPA (1993). This approach is based on animal-specific manure characteristics and management system data. This
process was not repeated for subsequent years for these other animal types. Instead, national populations of each of the
animal types were used to scale the 1990 emissions estimates to the period 1991 through 1999.
Emission estimates are summarized in Table 1-5.
1-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Table 1-1: Livestock Population (1,000 Head)
Animal Type
Dairy Cattle
Dairy Cows
Dairy Heifers
Swine
Market
Breeding
Beef Cattle
Feedlot Steers
Feedlot Heifers
Feedlot Cow/Other
NOF Bulls
NOF Calves
NOF Heifers
NOF Steers
NOF Cows
Sheep
Ewes >1yr
Rams/Weth > 1yr
Ewes < 1yr
Rams/Weth < 1yr
Sheep on Feed
Goats
Poultry
Hens > 1yr
Pullets laying
Pullets > 3mo
Pullets < 3mo
Chickens
Broilers
Other (Lost)
Other (Sold)
Turkeys
Horses
1990
14,143
10,007
4,135
53,941
47,042
6,898
86,064
7,252
3,749
89
2,180
23,909
8,744
7,552
32,588
11,356
7,961
369
1,491
381
1.154
2,545
1,703,036
119,551
153,916
34,222
38,945
6,545
1,172,830
6,971
41,672
128,384
5,650
1991
13,980
9,883
4,097
56,478
49,247
7,230
87,265
7,927
4,142
100
2,198
23,854
8,831
7,355
32,860
11,174
7,799
361
1,464
373
1,177
2,475
1,767,513
117,178
162,943
34,272
42,344
6,857
1,227,430
7,278
39,707
129,505
5,650
1992
13,830
9,714
4,116
58,532
51,276
7,255
88,545
7,404
3,882
94
2,220
24,118
9,263
8,206
33,358
10,797
7,556
350
1,432
366
1,093
2,645
1,832,308
121,103
163,397
34,710
45.160
7,113
1,280,498
7,025
41,538
131,764
5,850
1993
13,767
9,679
4,088
58,016
50,859
7,156
90,319
7,841
4,091
99
2,239
24,209
9,730
8,079
34,032
10,201
7,140
331
1,349
348
1,032
2,605
1,895,851
131,688
158,938
33,833
47,941
7,240
1,338,862
6,992
39,606
130,750
5,900
1994
13,566
9,504
4,062
59,951
52,669
7,282
92,570
8,034
4,111
101
2,306
24,586
10,323
8,109
35,000
9,825
6,839
315
1,287
332
1,052
2,605
1,966,050
135,094
163,433
33,159
46,694
7,369
1,403,508
7,124
39,402
130,266
6,000
1995
13,502
9,491
4,011
58,899
51,973
6,926
94,390
7,625
3,921
99
2,392
25,170
10,805
8,831
35,545
8,982
6,256
286
1,179
298
964
2,495
2,034,213
133,841
165,230
34,004
47,365
7,637
1,465,134
12,212
35,901
132,889
6,000
1996
13,305
9,410
3,895
56,220
49,581
6,639
94,269
7,806
4,049
99
2,392
25,042
10,819
8,651
35,411
8,458
5,898
270
1,110
282
898
2,545
2,096,618
138,048
165,874
33,518
48,054
7,243
1,519,352
12,072
34,860
137,597
6,050
1997
13,138
9,309
3,829
58,728
51,887
6,839
92,290
7,943
4,108
99
2,325
24,363
10,768
8,153
34,530
8,015
5,595
253
1,053
261
853
2,295
2,147,851
140,966
171,171
35,578
54,766
7.549
1,552,840
9,851
38,197
136,932
6,150
1998
12,992
9,200
3,793
62,043
55,192
6,850
90,730
8,199
4,238
98
2,235
24,001
10,219
7,696
34,044
7,817
5,481
249
1,038
261
788
2,045
2,189,254
150,778
169,916
39,664
56,054
7,682
1,586,856
10,686
38,754
128,865
6,150
1-3
-------
Table 1-2: Dairy Cow Weighted MCF Values
Stale
Alabama
Alaska
Arizona
Arkansas
California
Colorado
Connecticut
Delaware
Florida
Georgia
Hawaii
Idaho
Illinois
Indiana
Iowa
Kansas
Kentucky
Louisiana
Maine
Maryland
Massachusetts
Michigan
Minnesota
Mississippi
Missouri
Montana
Nebraska
Nevada
New Hampshire
New Jersey
New Mexico
New York
North Carolina
North Dakota
Ohio
Oklahoma
Oregon
Pennsylvania
Rhode Island
South Carolina
South Dakota
Tennessee
Texas
Utah
Vermont
Virginia
. Washington
West Virginia
Wisconsin
Wyoming
1990
0.23
0.35
0.45
0.09
0.44
0.31
0.19
0.21
0.41
0.27
0.40
0.23
0.07
0.06
0.04
0.09
0.06
0.19
0.10
0.15
0.13
0.12
0.04
0.17
0.07
0.16
0.08
0.36
0.12
0.13
0.42
0.11
0.20
0.05
0.07
0.13
0.25
0.06
0.07
0.29
0.06
0.14
0.31
0.21
0.11
0.17
0.29
0.11
0.05
0.12
1991
0.23
0.35
0.45
0.09
0.44
0.31
0.19
0.21
0.41
0.27
0.40
0.23
0.07
0.06
0.04
0.09
0.06
0.19
0.10
0.15
0.13
0.12
0.04
0.17
0.07
0.16
0.08
0.36
0.12
0.13
0.42
0.11
0.20
0.05
0.07
0.13
0.25
0.06
0.07
0.29
0.06
0.14
0.31
0.21
0.11
0.17
0.29
0.11
0.05
0.12
1992
0.23
0.35
0.45
0.09
0.44
0.31
0.19
0.21
0.41
0.27
0.40
0.23
0.07
0.06
0.04
0.09
0.06
0.19
0.10
0.15
0.13
0.12
0.04
0.17
0.07
0.16
0.08
0.36
0.12
0.13
0.42
0.11
0.20
0.05
0.07
0.13
0.25
0.06
0.07
0.29
0.06
0.14
0.31
0.21
0.11
0.17
0.29
0.11
0.05
0.12
1993
0.22
0.35
0.45
0.09
0.44
0.33
0.20
0.20
0.42
0.28
0.40
0.26
0.07
0.07
0.05
0.09
0.07
0.19
0.11
0.15
0.13
0.14
0.05
0.18
0.07
0.17
0.08
0.38
0.13
0.14
0.43
0.12
0.21
0.05
0.08
0.13
0.27
0.07
0.06
0.29
0.07
0.14
0.32
0.23
0.13
0.17
0.31
0.11
0.06
0.12
1994
0.20
0.35
0.45
0.09
0.44
0.35
0.21
0.19
0.42
0.29
0.40
0.29
0.08
0.08
0.06
0.09
0.07
0.19
0.11
0.16
0.14
0.15
0.06
0.19
0.08
0.18
0.09
0.39
0.14
0.14
0.43
0.14
0.21
0.05
0.09
0.13
0.29
0.08
0.04
0.29
0.08
0.15
0.33
0.25
0.14
0.18
0.33
0.11
0.07
0.13
1995
0.19
0.35
0.45
0.09
0.44
0.37
0.22
0.19
0.43
0.30
0.40
0.32
0.09
0.08
0.07
0.09
0.07
0.19
0.12
0.16
0.15
0.17
0.07
0.19
0.08
0.19
0.09
0.40
0.15
0.15
0.44
0.16
0.22
0.05
0.09
0.13
0.31
0.08
0.03
0.29
0.08
0.16
0.34
0.27
0.16
0.18
0.35
0.12
0.08
0.13
1996
0.18
0.35
0.45
0.09
0.44
0.38
0.22
0.18
0.43
0.31
0.40
0.36
0.10
0.09
0.08
0.09
0.08
0.19
0.13
0.17
0.15
0.18
0.08
0.20
0.08
0.20
0.10
0.42
0.16
0.16
0.44 ~
0.18
0.23
0.05
0.10
0.13
0.33
0.09
0.02
0.29
0.09
0.16
0.35
0.29
0.17
0.18
0.38
0.12
0.09
0.13
1997
0.16
0.35
0.45
0.09
0.44
0.40
0.23
0.18
0.44
0.32
0.40
0.39
0.11
0.10
0.09
0.09
0.08
0.19
0.13
0.17
0.16
0.20
0.09
0.21
0.09
0.21
0.10
0.43
0.17
0.16
0.45
0.19
0.24
0.05
0.11
0.14
0.35
0.10
0.01
0.29
0.09
0.17
0.37
0.30
0.18
0.19
0.40
0.13
0.10
0.14
1998
0.16
0.35
0.45
0.09
0.44
0.40
0.23
0.18
0.44
0.32
0.40
0.39
0.11
0.10
0.09
0.09
0.08
0.19
0.13
0.17
0.16
0.20
0.09
0.21
0.09
0.21
0.10
0.43
0.17
0.16
0.45
0.19
0.24
0.05
0.11
0.14
0.35
0.10
0.01
0.29
0.09
0.17
0.37
0.30
0.18
0.19
0.40
0.13
0.10
0.14
1-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Table 1-3: Swine Weighted MCF Values
State
Alabama
Alaska
An'zona
Arkansas
California
Colorado
Connecticut
Delaware
Florida
Georgia
Hawaii
Idaho
Illinois
Indiana
Iowa
Kansas
Kentucky
Louisiana
Maine
Maryland
Massachusetts
Michigan
Minnesota
Mississippi
Missouri
Montana
Nebraska
Nevada
New Hampshire
New Jersey
New Mexico
New York
North Carolina
North Dakota
Ohio
Oklahoma
Oregon
Pennsylvania
Rhode Island
South Carolina
South Dakota
Tennessee
Texas
Utah
Vermont
Virginia
Washington
West Virginia
Wisconsin
Wyoming
1990
0.15
0.35
0.44
0.37
0.28
0.28
0.01
0.19
0.13
0.19
0.40
0.16
0.23
0.25
0.20
0.18
0.17
0.30
0.01
0.25
0.26
0.24
0.20
0.21
0.17
0.23
0.18
0.32
0.24
0.14
0.30
0.13
0.42
0.10
0.15
0.18
0.20
0.19
0.38
0.24
0.12
0.15
0.18
0.19
0.01
0.19
0.19
0.06
0.14
0.10
1991
0.15
0.35
0.44
0.37
0.28
0.28
0.01
0.19
0.13
0.19
0.40
0.16
0.23
0.25
0.20
0.18
0.17
0.30
0.01
0.25
0.26
0.24
0.20
0.21
0.17
0.23
0.18
0.32
0.24
0.14
0.30
0.13
0.42
0.10
0.15
0.18
0.20
0.19
0.38
. 0.24
0.12
0.15
0.18
0.19
0.01
0.19
0.19
0.06
0.14
0.10
1992
0.15
0.35
0.44
0.37
0.28
0.28
0.01
0.19
0.13
0.19
0.40
0.16
0.23
0.25
0.20
0.18
0.17
0.30
0.01
0.25
0.26
0.24
0.20
0.21
0.17
0.23
0.18
0.32
0.24
0.14
0.30
0.13
0.42
0.10
0.15
0.18
0.20
0.19
0.38
0.24
0.12
0.15
0.18
0.19
0.01
0.19
0.19
0.06
0.14
0.10
1993
0.19
0.35
0.35
0.38
0.30
0.29
0.01
0.16
0.11
0.20
0.40
0.13
0.25
0.26
0.23
0.20
0.20
0.30
0.01
0.24
0.21
0.26
0.23
0.25
0.20
0.26
0.20
0.26
0.19
0.12
0.25
0.16
0.42
0.14
0.17
0.24
0.19
0.22
0.31
0.27
0.15
0.18
0.15
0.24
0.01
0.23
0.19
0.05
0.15
0.08
1994
0.22
0.35
0.27
0.39
0.33
0.30
0.01
0.12
0.09
0.22
0.40
0.10
0.27
0.28
0.25
0.22
0.23
0.30
0.01
0.23
0.16
0.28
0.26
0.29
0.22
0.30
0.22
0.20
0.15
0.09
0.19
0.20
0.43
0.17
0.19
0.29
0.18
0.25
0.23
0.30
0.18
0.20
0.11
0.29
0.01
0.27
0.20
0.04
0.17
0.06
1995
0.25
0.35
0.18
0.40
0.35
0.31
0.01
0.08
0.06
0.23
0.40
0.07
0.28
0.29
0.28
0.24
0.27
0.30
0.01
0.23
0.11
0.30
0.28
0.33
0.25
0.33
0.24
0.14
0.10
0.06
0.13
0.24
0.43
0.21
0.21
0.34
0.18
0.28
0.16
0.33
0.21
0.23
0.08
0.34
0.01
0.31
0.21
0.03
0.19
0.05
1996
0.29
0.35
0.10
0.41
0.38
0.32
0.01
0.05
0.04
0.24
0.40
0.04
0.30
0.31
0.30
0.27
0.30
0.30
0.01
0.22
'0.06
0.32
0.31
0.37
0.27
0.36
0.26
0.07
0.06
0.04
0.07
0.28
0.44
0.24
0.23
0.39
0.17
0.31
0.08
0.36
0.24
0.26
0.05
0.39
0.01
0.36
0.22
0.02
0.20
0.03
1997
0.32
0.35
0.01
0.42
0.40
0.33
0.01
0.01
0.01
0.25
0.40
0.01
0.32
0.32
0.32
0.29
0.33
0.30
0.01
0.21
0.01
0.34
0.34
0.42
0.30
0.40
0.28
0.01
0.01
0.01
0.01
0.32
0.45
0.28
0.25
0.44
0.16
0.34
0.01
0.39
0.27
0.28
0.01
0.44
0.01
0.40
0.22
0.01
0.22
0.01
1998
0.32
0.35
0.01
0.42
0.40
0.33
0.01
0.01
0.01
0.25
0.40
0.01
0.32
0.32
0.32
0.29
0.33
0.30
0.01
0.21
0.01
0.34
0.34
0.42
0.30
0.40
0.28
0.01
0.01
0.01
0.01
0.32
0.45
0.28
0.25
0.44
0.16
0.34
0.01
0.39
0.27
0.28
0.01
0.44
0.01
0.40
0.22
0.01
0.22
0.01
1-5
-------
Table 1-4: Dairy Cow and Swine Constants
Description
Dairy Cow
Typical Animal Mass (kg)
kg VS/day per 1000 kg mass
Maximum Methane Generation
Potential (BJ m3 methane/kg VS
640
10
0.24
Market Swine Breeding
Swine
116
8.5
0.47
Source
181 ASAE (1999)
8.5 ASAE (1999)
0.47 EPA (1992)
Table 1-5: CH4 Emissions from Livestock Manure Management (Gg)
Animal Type
Dairy Cattle
Dairy Cows
Dairy Heifers
Swine
Market
Breeding
Beef Cattle
Feedlot Steers
Feediot Heifers
Feedlot Cow/Other
NOF Bulls
NOF Calves
NOF Heifers
NOF Steers
NOF Cows
Sheep
Ewes > 1 yr
Rams/Weth > 1 yr
Ewes < 1 yr
Rams/Weth < 1 yr
Sheep on Feed
Goals
Poultry
Hens > 1 yr
Pullets laying
Pullets > 3 mo
Pullets < 3 mo
Chickens
Broilers
Other (Lost)
Other (Sold)
Turkeys
Horses
1990
746.8
580.9
165.9
1,371.1
1,116.6
254.5
200.5
28.2
16.0
0.4
6.5
19.3
15.7
14.2
100.2
3.8
3.1
0.1
0.2
0.1
0.2
0.9
261.3
53.7
56.2
8.4
6.1
2.3
97.7
0.8
9.3
26.7
28.8
1991
750.8
586.5
164.3
1,450.9
1,180.8
270.1
205.0
31.0
17.7
0.5
6.6
19.0
15.7
13.5
100.9
3.7
3.0
0.1
0.2
0.1
0.2
0.9
268.3
51.3
60.0
8.4
6.9
2.4
102.3
0.8
9.3
26.9
28.8
1992
762.2
597.1
165.1
1,522.9
1,248.1
274.8
206.3
29.1
16.7
0.4
6.7
19.3
16.4
15.2
102.5
3.6
2.9
0.1
0.2
0.1
0.2
0.9
275.1
55.9
57.9
8.4
6.8
2.5
106.7
0.8
9.2
27.0
29.8
1993
791.3
627.3
164.0
1,667.6
1,367.4
300.1
212.1
31.0
17.6
0.4
6.7
19.4
17.3
15.1
104.6
3.4
2.7
0.1
0.2
0.1
0.2
0.9
284.2
59.1
59.4
7.6
7.4
2.7
111.6
0.8
8.9
26.8
30.0
1994
843.4
680.5
162.9
1,893.9
1,558.0
336.0
218.9
31.5
18.7
0.6
6.9
19.7
18.5
15.0
108.0
3.3
2.6
0.1
0.2
0.1
0.2
0.9
291.7
59.8
60.9
7.9
7.0
2.7
117.0
0.8
8.7
26.9
30.5
1995
863.5
702.7
160.9
2,031.3
1,682.5
348.8
221.1
30.1
18.0
0.6
7.2
20.1
19.6
16.2
109.4
3.0
2.4
0.1
0.2
0.1
0.2
0.9
296.6
57.5
61.9
7.8
7.4
2.8
122.1
1.4
8.1
27.5
30.5
1996
895.8
739.6
156.2
2,106.2
1,744.7
361.5
228.8
34.6
21.5
0.7
7.3
20.0
19.7
15.2
109.9
2.8
2.3
0.1
0.2
0.1
0.2
0.9
301.3
58.0
61.4
7.7
7.3
2.6
126.6
1.3
7.8
28.5
30.8
1997
940.5
786.9
153.6
2,348.5
1,950.3
398.2
228.5
33.1
21.0
0.7
7.1
19.7
20.7
17.3
108.9
2.6
2.1
0.1
0.2
0.0
0.2
0.8
308.2
58.9
62.4
8.7
8.1
2.8
129.4
1.0
8.4
28.4
31.3
1998
933.4
781.2
152.1
2,475.0
2,073.6
401.4
232.7
36.7
22.3
0.7
7.0
19.6
20.7
16.3
109.4
2.6
2.1
0.1
0.2
0.0
0.2
0.7
314.5
61.1
63.8
9.7
8.2
2.8
132.2
1.1
8.4
27.1
31.3
1-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Annex J
Methodology for Estimating Methane Emissions from Landfills
Landfill methane is produced from a complex process of waste decomposition and subsequent fermentation under
anaerobic conditions. The amount and rate of methane production depends upon the characteristics of the landfilled
material and the surrounding environment. To estimate the amount of methane produced in a landfill in a given year,
the following information is needed: the quantity of waste in the landfill, the waste characteristics, the residence time
of the waste in the landfill, and the landfill capacity.
The amount of methane emitted from a landfill is less than the amount produced in a landfill. If no measures are
taken to extract the methane, a portion of it will oxidize as it travels through the top layer of the landfill cover. The
portion of the methane that oxidizes turns primarily to carbon dioxide (CO2). If the methane is extracted and combusted
(e.g., flared or used for energy), then that portion of the methane produced in the landfill will not be emitted as methane,
but again, would be converted to CO2. In general, CO2 emissions are of biogenic origin and primarily result from the
decomposition, either aerobic or anaerobic, of organic matter such as food or yard wastes.
Methane emissions are driven by the quantity of waste in landfills overtime. From an analysis of the population
of municipal solid waste (MSW) landfills, landfill-specific data are extracted and used in an emissions model to estimate
the amount of methane produced by municipal solid waste. Although not explicitly modeled, methane emissions from
industrial landfills are assumed to be seven percent of the total methane generated from MSW at landfills. Total
methane emissions are estimated by adding the methane from MSW landfills, subtracting the amount recovered or used
for energy or flared, subtracting the amount oxidized in the soil, and adding emissions from industrial landfills. The
steps taken to estimate emissions from U.S. landfills for the years 1990 through 1998 are discussed in greater detail
below.
Step 1: Estimate Municipal Solid Waste-in-Place Contributing to Methane Emissions
First, landfills were characterized as of 1990 based on EPA's landfill survey (EPA 1988). Each landfill was
characterized in terms of its year of opening, waste acceptance during operation, year of closure, and design capacity.
Following characterization of the landfill population, waste was simulated to be placed in these landfills. For 1991
through 1998, waste disposal estimates were based on annual BioCycle (1999) data. Landfills were simulated to open
and close based on waste disposal rates. If landfills reached their design capacity, they were simulated to close. New
landfills were simulated to open when a significant shortfall in disposal capacity was predicted. Simulated new landfills
were assumed to be larger, on average, reflecting the trend toward fewer and more centralized facilities. The analysis
updated the landfill characteristics each year, calculating the total waste-in-place and the profile of waste disposal over
time. Table J- 1 shows the amount of waste landfilled each year and the total estimated waste-in-place contributing to
methane emissions.
Step 2: Estimate Landfill Methane Production
Emissions for each landfill were estimated by applying the emissions model (EPA 1993) to the landfill waste-in-
place contributing to methane production. Total emissions were then calculated as the sum of emissions from all
landfills.
J-1
-------
Step 3: Estimate Industrial Landfill Methane Production
Industrial landfills receive waste from factories, processing plants, and other manufacturing activities. Because
no data were available on methane generation at industrial landfills, emissions from industrial landfills were assumed
to equal seven percent of the total methane emitted from MSW landfills (EPA 1993). These emissions are shown in
Table J-2.
Step 4: Estimate Methane Emissions Avoided
The quantity of methane flared - without a landfill gas-to-energy (LFGTE) system— was based on data collected
from flaring equipment vendors. These data included information on the quantity of flares, landfill gas flow rates, and
year of flare installation. Total methane recovered was estimated by multiplying the number of flares by a landfill gas
flow rate provided by a flaring equipment vendor.
The quantity of methane avoided due to LFGTE systems was estimated based on the data included in a database
compiled by EPA's Landfill Methane Outreach Program (LMOP). Using data on landfill gas flow and energy
generation, the total direct methane emissions avoided were estimated.
Step 5: Estimate Methane Oxidation
As discussed above, a portion of the methane escaping from a landfill through its cover oxidizes in the top layer
of the soil. The amount of oxidation that occurs is uncertain and depends upon the characteristics of the soil and the
environment. For purposes of this analysis, it was assumed that ten percent of the methane produced, minus the amount
of gas recovered for flaring or LFGTE projects, was oxidized in the soil (Liptay et al. 1998).
Step 6: Estimate Total Methane Emissions
Total methane emissions were calculated by adding emissions from MSW and industrial waste, and subtracting
methane recovered and oxidized, as shown in Table J-2.
Table J-1: Municipal Solid Waste (MSW) Contributing to Methane Emissions (Tg)
Description
Total MSW Generated'
Percent of MSW Landfilled'
Total MSW Landfilled
MSW Contributing to Emissions"
1990
267
77%
206
4,926
1991
255
76%
194
5,027
1992
265
72%
191
5,162
1993
279
71%
198
5,292
1994
293
67%
196
5,428
1995
297
63%
187
5,560
1996
297
62%
184
5,677
1997
309
61%
189
5,791
1998
340
61%
207
5,907
• Source- BloCycle (1999). The data, originally reported in short tons, are converted to metric tons.
11 The EPA emissions model (EPA 1993) defines all waste that has been in place for less than 30 years as contributing to methane emissions.
J-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Table J-2: Methane Emissions from Landfills (Gg)
Activity
MSW Generation
Large Landfills
Medium Landfills
Small Landfills
Industrial Generation
Potential Emissions
Emissions Avoided
Landfill Gas-to-Energy
Flare
Oxidation
Net Emissions
1990
11,599
4,534
5,791
1,273
731
12,330
(811)
(299)
(1,049)
10,171
1991
11,837
4,625
5,912
1,300
746
12,582
(861)
(524)
(1,045)
10,152
1992
12,168
4,767
6,070
1,332
767
12,935
(915)
(637)
(1,062)
10,321
1993
12,499
4,918
6,222
1,359
787
13,287
(1,053)
(764)
(1,068)
10,402
1994
12,848
5,115
6,348
1,385
809
13,658
(1,183)
(952)
(1,071)
10,452
1995
13,220
5,298
6,514
1,407
833
14,052
(1,233)
(1,171)
(1,081)
10,566
1996
13,492
5,464
6,605
1,423
850
14,342
(1,397)
(1,363)
(1,073)
10,508
1997
13,776
5,641
6,697
1,438
868
14,644
(1,608)
(1,454)
(1,071)
10.510
Note: I otais may not sum due to independent rounding.
1998
14,017
5,811
6,752
1,453
883
14,900
(2,025)
(1,564)
(1,043)
10,268
Table J-3: Municipal Solid Waste Landfill Size Definitions (Gg)
Description
Waste-in-Place
Small Landfills
Medium Landfills
Large Landfills
<400
400-2,000
> 2,000
J-3
-------
-------
Annex K
Global Warming Potential Values
Table K-1: Global Warming Potentials and Atmospheric Lifetimes (Years)
Gas
Carbon dioxide (C02)
Methane (CH4)b
Nitrous oxide (N20)
HFC-23
HFC-125
HFC-134a
HFC-1433
HFC-152a
HFC-227ea
HFC-236fa
HFC-4310mee
CF4
C2F6
C4F10
C6F,4
SFB
Atmospheric Lifetime
50-200
12±3
120
264
32.6
14.6
48.3
1.5
36.5
209
17.1
50,000
10,000
2,600
3,200
3,200
GWP1
1
21
310
11,700
2,800
1,300
3,800
140
2,900
6,300
1,300
6,500
9,200
7,000
7,400
23,900
Source: (IPCC 1996)
1100 year time horizon
" The methane GWP includes the direct effects and those indirect effects due to the production of tropospheric ozone and stratospheric water vapor. The
indirect effect due to the production of C02 is not included.
K-1
-------
-------
Annex L
Ozone Depleting Substance Emissions
Ozone is present in both the stratosphere1, where it shields the Earth from harmful levels of ultraviolet radiation,
and at lower concentrations in the troposphere2, where it is the main component of anthropogenic photochemical
"smog". Chlorofluorocarbons (CFCs) and other compounds that contain chlorine or bromine have been found to destroy
ozone in the stratosphere, and are commonly referred to as ozone-depleting substances (ODSs). If left unchecked, ozone
depletion could result in a dangerous increase of ultraviolet radiation reaching the earth's surface. In 1987, nations
around the world signed the Montreal Protocol on Substances that Deplete the Ozone Layer. This landmark agreement
created an international framework for limiting, and ultimately eliminating, the use and emission of most ozone depleting
substances, which are used in a variety of industrial applications, including refrigeration and air conditioning, foam
blowing, fire extinguishing, aerosol propellants, sterilization, and solvent cleaning.
In the United States, the Clean Air Act Amendments of 1990 provide the legal instrument for implementation of
the Montreal Protocol controls. The Clean Air Act classifies ozone depleting substances as either Class I or Class II,
depending upon the ozone depletion potential (OOP) of the compound.3 The production of CFCs, halons, carbon
tetrachloride, and methyl chloroform-all Class I substances-has already ended in the United States. However, because
stocks of these chemicals remain available and in use, they will continue to be emitted for many years from applications
such as refrigeration and air conditioning equipment, fire extinguishing systems, and metered dose inhalers. As a result,
emissions of Class I compounds will continue, in ever decreasing amounts, into the early part of the next century. Class
II substances, which are comprised of hydrochlorofluorocarbons (HCFCs), are being phased-out at a later date because
of their lower ozone depletion potentials. These compounds are serving as interim replacements for Class I compounds
in many industrial applications. The use and emissions of HCFCs in the United States is anticipated to increase over
the next several years. Under current controls, the production of all HCFCs in the United States will end by the year
2030.
In addition to contributing to ozone depletion, CFCs, halons, carbon tetrachloride, methyl chloroform, and HCFCs
are also significant greenhouse gases. The total impact of ozone depleting substances on global warming is not clear,
however, because ozone is also a greenhouse gas. The depletion of ozone in the stratosphere by ODSs has an indirect
negative radiative forcing, while most ODSs have a positive direct radiative forcing effect. The IPCC has prepared both
direct GWPs and net (i.e., combined direct and indirect effects) GWP ranges for some of the most common ozone
depleting substances (IPCC 1996). Direct GWPs account for the direct global warming impact of the emitted gas. Net
GWP ranges account for both the direct impact of the emitted gas and the indirect effects resulting from the destruction
of ozone.
Although the IPCC emission inventory guidelines do not include reporting emissions of ozone depleting
substances, the United States believes that no inventory is complete without the inclusion of these emissions. Emission
estimates for several ozone depleting substances are provided in Table L-l.
The stratosphere is the layer from the top of the troposphere up to about 50 kilometers. Approximately 90 percent of atmospheric ozone lies within
the stratosphere. The greatest concentration of ozone occurs in the middle of the stratosphere, in a region commonly called the ozone layer.
2 The troposphere is the layer from the ground up to about 11 kilometers near the poles and 16 kilometers in equatorial regions (i.e., the lowest layer
of the atmosphere, where humans live). It contains roughly 80 percent of the mass of all gases in the atmosphere and is the site for weather processes
including most of the water vapor and clouds.
Substances with an ozone depletion potential of 0.2 or greater are classified as Class I. All other substances that may deplete stratospheric ozone
but which do not have an ODP of 0.2 or greater, are classified as Class II.
L-1
-------
Table L-1: Emissions of Ozone Depleting Substances (Gg)
Compound
Class 1
CFC-11
CFC-12
CFC-113
CFC-114
CFC-115
Carbon Tetrachloride
Methyl Chloroform
Halon-1211
Halon-1301
Class II
HCFC-22
HCFC-123
HCFC-124
HCFC-141b
HCFC-142b
HCFC-225ca/cb
Source: EPA
•t- Does not exceed 0.05 Gg
1990
53.5
112.6
26.4
4.7
4.2
32.3
158.3
1.0
1.8
79.8
+
+
+
+
+
1991
48.3
103.5
20.6
3.6
4.0
31.0
154.7
1.1
1.8
79.5
+
+
+
+
+
1992
45.1
80.5
17.1
3.0
3.8
21.7
108.3
1.0
1.7
79.5
0.3
0.4
+
0.7
H-
1993
45.4
79.3
17.1
3.0
3.6
18.6
92.9
1.1
1.7
71.2
0.3
2.6
5.0
1.7
+
1994
36.6
57.6
8.6
1.6
3.3
15.5
77.4
1.0
1.7
71.4
0.5
4.8
12.4
4.6
+
1995
36.2
51.8
8.6
1.6
3.0
4.7
46.4
1.1
1.8
72.3
0.6
5.2
20.6
7.3
+
1996
26.6
35.5
+
0.3
3.2
+
+
1.1
1.9
73.2
0.7
5.6
25.4
8.3
+
1997
25.1
23.1
+
0.1
2.9
+
• +
1.1
1.9
74.2
0.8
5.9
25.1
8.7
+
1998
24.9
21.0
+
0.1
2.7
+
+
1.1
1.9
75.1
0.9
6.1
26.7
9.0
+
Methodology and Data Sources
Emissions of ozone depleting substances were estimated using two simulation models: the Atmospheric and
Health Effects Framework (AHEF) and the EPA's Vintaging Model.
AHEF contains estimates of U.S. domestic use of each of the ozone depleting substances. These estimates were
based upon data that industry reports to the EPA and other published material. The annual consumption of each
compound was divided into various end-uses based upon historical trends and research into specific industrial
applications. These end-uses include refrigerants, foam blowing agents, solvents, aerosol propellants, sterilants, and
fire extinguishing agents.
With the exception of aerosols, solvents, and certain foam blowing agents, emissions of ozone depleting
substances are not instantaneous, but instead occur gradually over time (i.e., emissions in a given year are the result
of both ODS use in that year and use in previous years). Each end-use has a certain release profile, which gives the
percentage of the compound that is released to the atmosphere each year until all releases have occurred. In
refrigeration equipment, for example, the initial charge is released or leaked slowly over the lifetime of the equipment,
which could be 20 or more years. In addition, not all of the refrigerant is ultimately emitted—some will be recovered
when the equipment is retired from operation.
The AHEF model was used to estimate emissions of ODSs that were in use prior to the controls implemented
under the Montreal Protocol. This included CFCs, halons, carbon tetrachloride, methyl chloroform, and HCFC-22.
Certain HCFCs, such as HCFC-123, HCFC-124, HCFC-141b, HCFC-142b, HCFC-225ca and HCFC-225cb, have
also entered the market as interim substitutes for ODSs. Emissions estimates for these compounds were taken from
the EPA's Vintaging Model.
The Vintaging Model was used to estimate the use and emissions of various ODS substitutes, including HCFCs.
The name refers to the fact that the model tracks the use and emissions of various compounds by the annual "vintages"
of new equipment that enter service in each end-use. The Vintaging Model is a "bottom-up" model. Information was
collected regarding the sales of equipment that use ODS substitutes and the amount of the chemical required by each
unit of equipment. Emissions for each end-use were estimated by applying annual leak rates and release profiles, as
in the AHEF. By aggregating the data for more than 40 different end-uses, the model produces estimates of annual
use and emissions of each compound.
L-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Uncertainties
Uncertainties exist with regard to the levels of chemical production, equipment sales, equipment characteristics,
and end-use emissions profiles that are used by these models.
L-3
-------
-------
Annex M
Sulfur Dioxide Emissions
Sulfiir dioxide (SO2) emitted into the atmosphere through natural and anthropogenic processes affects the
Earth's radiative budget through photochemical transformation into sulfate aerosols that can (1) scatter sunlight back
to space, thereby reducing the radiation reaching the Earth's surface; (2) affect cloud formation; and (3) affect
atmospheric chemical composition (e.g., stratospheric ozone, by providing surfaces for heterogeneous chemical
reactions). The overall effect of SO2 derived aerosols on radiative forcing is believed to be negative (IPCC 1996).
However, because SO2 is short-lived and unevenly distributed through the atmosphere, its radiative forcing impacts
are highly uncertain. Sulfur dioxide emissions have been provided below in Table M-l.
The major source of SO2 emissions in the United States was the burning of sulfur containing fuels, mainly coal.
Metal smelting and other industrial processes also released significant quantities of SO2. As a result, the largest
contributors to overall U.S. emissions of SO2 were electric utilities, accounting for 58 percent in 1998 (see Table M-
2). Coal combustion accounted for approximately 94 percent of SO2 emissions from electric utilities in the same year.
The second largest source was industrial fuel combustion, which produced 20 percent of 1998 SO2 emissions.
Overall, SO2 emissions in the United States decreased by 9 percent from 1990 to 1998. The majority of this decline
came from reductions from electric utilities, primarily due to increased consumption of low sulfur coal from surface
mines in western states.
Sulfur dioxide is important for reasons other than its effect on radiative forcing. It is a major contributor to the
formation of urban smog and acid rain. As a contributor to urban smog, high concentrations of SO2 can cause
significant increases in acute and chronic respiratory diseases. In addition, once SO2 is emitted, it is chemically
transformed in the atmosphere and returns to earth as the primary contributor to acid deposition, or acid rain. Acid
rain has been found to accelerate the decay of building materials and paints, and to cause the acidification of lakes
and streams and damage trees. As a result of these harmful effects, the United States has regulated the emissions of
SO2 under the Glean Air Act. The EPA has also developed a strategy to control these emissions via four programs:
(1) the National Ambient Air Quality Standards program,1 (2) New Source Performance Standards,2 (3) the New
Source Review/Prevention of Significant Deterioration Program,3 and (4) the sulfur dioxide allowance program.
4
References
EPA (1999) National Air Pollutant Emissions Trends Report, 1900-1998, U.S. Environmental Protection
Agency, Office of Air Quality Planning and Standards, Research Triangle Park, NC.
'[42 U.S.C § 7409, CAA § 109]
2[42U.S.C§74H,CAA§111]
3[42 U.S.C § 7473, CAA § 163]
4[42 U.S.C § 7651, CAA § 401]
-------
Table M-1: S02 Emissions (Gg)
Sector/Source
Energy
Stationary Combustion
Mobile Combustion
Oil and Gas Activities
Industrial Processes
Chemical Manufacturing
Metals Processing
Storage and Transport
Other Industrial Processes
Miscellaneous*
Solvent Use
Degreasing
Graphic Arts
Dry Cleaning
Surface Coating
Other Industrial
Non-industrial
Agriculture
Agricultural Burning
Waste
Waste Combustion
Landfills
Wastewater Treatment
Miscellaneous Waste
Total
1990
20,120
18,407
1,322
390
1,306
269
658
6
362
11
+
+
+
NA
+
+
NA
NA
NA
38
38
+
+
+
21,465
1991
19,675
17,959
1,373
343
1,187
254
555
9
360
10
+
+
+
NA
+
+
NA
NA
NA
40
39
+
-i-
1
20,903
1992
19,463
17,684
1,402
377
1,186
252
558
8
360
9
+
+
+
+
+
+
NA
NA
NA
40
39
+
+
1
20,689
1993
19,157
17,459
1,351
347
1,159
244
547
4
355
9
1
+
+
NA
+
+
NA
NA
NA
65
56
+
+
8
20,331
1994
18,650
17,134
1,172
344
1,135
249
510
1
361
14
1
+
+
+
+
+
NA
NA
NA
54
48
+
+
5
19,840
1995
16,241
14.724
1,183
334
1,117
260
481
2
365
9
1
+
+
+
+
+
NA
NA
NA
43
42
+
1
+
17,401
1996
17,490
15,981
1,208
300
1,167
445
387
2
316
17
+
+
+
+
+
+
NA
NA
NA
37
36
+
+
+
18,695
1997
17,994
16,458
1,235
301
1,184
451
395
2
321
14
1
+
+
+
+
+
NA
NA
NA
37
36
+
+
+
19,216
1998
18,199
16,635
1,261
303
1,204
457
402
3
327
15
1
+
+
+
+
+
NA
NA
NA
38
37
+
+
+
19,441
Source: (EPA 1999)
* Miscellaneous Includes other combustion and fugitive dust categories.
+ Does not exceed 0.5 Gg
NA (Not Available)
Note: Totals may not sum due to independent rounding.
Table M-2: SO, Emissions from Electric Utilities (Gg)
Fuel Type
1990 1991 1992 1993 1994 1995 1996 1997 1998
Coal
Petroleum
Natural Gas
Misc. Internal Combustion
13,807 13,687 13,448 13,179 12,985 10,526 11,010 11,378 11,272
580 591 495 555 474 375 395 443 662
111118211
45 41 42 45 48 50 51 53 54
Total 14.432 14.320 13.986 13,779 13.507 10.959 11.459 11.875 11,990
Source: (EPA 1999)
Note: Totals may not sum due to independent rounding.
M-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Annex N
Complete List of Sources
Chapter/Source
Gas(es)
Energy
Carbon Dioxide Emissions from Fossil Fuel Combustion
Stationary Combustion (excluding C02)
Mobile Combustion (excluding C02)
Coal Mining
Natural Gas Systems
Petroleum Systems
Natural Gas Flaring and Criteria Pollutant Emissions from Oil and Gas Activities
International Bunker Fuels
Wood Biomass and Ethanol Consumption
C02
CH4, N20, CO, NOX, NMVOC
CH4, N20, CO, NOX, NMVOC
CH4
CH4
CH4
C02, CO, NOX, NMVOC
C02, CH4, NjA CO, NOX, NMVOC
CO,
Industrial Processes
Cement Manufacture
Lime Manufacture
Limestone and Dolomite Use
Soda Ash Manufacture and Consumption
Carbon Dioxide Consumption
Iron and Steel Production
Ammonia Manufacture
Ferroalloy Production
Petrochemical Production
Silicon Carbide Production
Adipic Acid Production
Nitric Acid Production
Substitution of Ozone Depleting Substances
Aluminum Production
HCFC-22 Production
Semiconductor Manufacture
Electrical Transmission and Distribution
Magnesium Production and Processing
Industrial Sources of Criteria Pollutants
C02
C02
C02
C02
C02
C02
C02
C02
CH4
CH4
N20
N20
MFCs, PFCs"
C02, CF4, C2F6
HFC-23
HFCs, PFCs, SFe"
SF6
SF6
CO, NO,, NMVOC
Solvent Use
CO, NO,, NMVOC
Agriculture
Enteric Fermentation
Manure Management
Rice Cultivation
Agricultural Soil Management
Agricultural Residue Burning
CH4
CH4, N20
CH4
N20
CH4. N20. CO. NOx
Land-Use Change and Forestry
Changes in Forest Carbon Stocks
Changes in Non-Forest Soil Carbon Stocks
Changes in Non-Forest Carbon Stocks in Landfills
C02 (sink)
C02 (sink)
CO, (sink)
Waste
Landfills
Wastewater Treatment
Human Sewage
Waste Combustion
Waste Sources of Criteria Pollutants
CH4
CH4
N20
C02l N20
CO, NO,, NMVOC
" In 1998, included HFC-23, HFC-125, HFC-134a, HFC-143a, HFC-152a, HFC-227ea, HFC-236fa, HFC-4310mee, C4F10, C6FM, PFC/PFPEs
" Included such gases as HFC-23, CF4, C2F6, SF6
N-1
-------
-------
Annex O
IPCC Reference Approach for Estimating C02 Emissions from Fossil Fuel
Combustion
It is possible to estimate carbon emissions from fossil fuel consumption using alternative methodologies and
different data sources than those described in Annex A. For example, the IPCC requires countries hi addition to their
"bottom-up" sectoral methodology to complete a "top-down" Reference Approach for estimating carbon dioxide
emissions from fossil fuel combustion. Section 1.3 of the Revised 1996 IPCC Guidelines for National Greenhouse Gas
Inventories: Reporting Instructions states, "If a detailed, Sectoral Approach for energy has been used for the estimation
of CO2 from fuel combustion you are still asked to complete...the Reference Approach...for verification purposes"
(IPCC/UNEP/OECD/IEA 1997). This reference method estimates fossil fuel Consumption by adjusting national
aggregate fuel production data for imports, exports, and stock changes rather than relying on end-user consumption
surveys. The basic principle is that once carbon-based fuels are brought into a national economy, they are either saved
in some way (e.g., stored in products, kept in fuel stocks, or left unoxidized in ash) or combusted, and therefore the
carbon in them is oxidized and released into the atmosphere. Accounting for actual consumption of fuels at the sectoral
or sub-national level is not required. The following discussion provides the detailed calculations for estimating CO2
emissions from fossil fuel combustion from the United States using the IPCC-recommended Reference Approach,
Step 1: Collect and Assemble Data in Proper Format
To ensure the comparability of national inventories, the IPCC has recommended that countries report energy data
using the International Energy Agency (IEA) reporting convention. National energy statistics were collected in physical
units from several DOE/EIA documents in order to obtain the necessary data on production, imports, exports, and stock
changes.1 These data are presented in Table O-1.
The carbon content of fuel varies with the fuel's heat content. Therefore, for an accurate estimation of CO2
emissions, fuel statistics should be provided on an energy content basis (e.g., BTU's or joules). Because detailed fuel
production statistics are typically provided in physical units (as in) Table O-1, they were converted to units of energy
before carbon emissions were calculated. Fuel statistics were converted to their energy equivalents by using conversion
factors provided by DOE/EIA. These factors and their data sources are displayed in Table O-1. The resulting fuel data
are provided in Table O-2.
Step 2: Estimate Apparent Fuel Consumption
The next step of the IPCC Reference Approach is to estimate "apparent consumption" of fuels within the country.
This requires a balance of primary fuels produced, plus imports, minus exports, and adjusting for stock changes. In this
way, carbon enters an economy through energy production and imports (and decreases in fuel stocks) and is transferred
out of the country through exports (and increases in fuel stocks). Thus, apparent consumption of primary fuels
(including crude oil, natural gas liquids, anthracite, bituminous, subbituminous and lignite coal, and natural gas) can
be calculated as follows:
Production + Imports - Exports - Stock Change
Flows of secondary fuels (e.g., gasoline, residual fuel, coke) should be added to primary apparent consumption.
The production of secondary fuels, however, should be ignored in the calculations of apparent consumption since the
carbon contained in these fuels is already accounted for in the supply of primary fuels from which they were derived
For the United States, national aggregate energy statistics typically exclude data on the U.S. territories. As a result, national statistics were
adjusted to include U.S. territories data. The territories include Puerto Rico, U.S. Virgin Islands, Guam, American Samoa, Wake Island, and
U.S. Pacific Islands. Consumption data were used for the territories because they are thought to be more reliable than production, import,
export, and stock change data.
0-1
-------
(e.g., the estimate for apparent consumption of crude oil already contains the carbon from which gasoline would be
refined). Flows of secondary fuels should therefore be calculated as follows:
Imports - Exports - Stock Change
Note that this calculation can result in negative numbers for apparent consumption. This is a perfectly acceptable
result since it merely indicates a net export or stock increase in the country of that fuel when domestic production is not
considered.
The IPCC Reference Approach calls for estimating apparent fuel consumption before converting to a common
energy unit. However, certain primary fuels in the United States (e.g., natural gas and steam coal) have separate
conversion factors for production, imports, exports, and stock changes. In these cases, it is not appropriate to multiply
apparent consumption by a single conversion factor since each of its components have different heat contents.
Therefore, United States fuel statistics were converted to their heat equivalents before estimating apparent consumption.
The energy value of bunker fuels used for international transport activities was subtracted before computing energy
totals.2 Results are provided in Table O-2.
Step 3: Estimate Carbon Emissions
Once apparent consumption is estimated, the remaining calculations are virtually identical to those for the
"bottom-up" Sectoral Approach (see Annex A). That is:
• Potential carbon emissions were estimated using fuel-specific carbon coefficients (see Table O-3).3
• The carbon sequestered in non-energy uses of fossil fuels (e.g., plastics or asphalt) was then
estimated and subtracted from the total amount of carbon (see Table O-4).
• Finally, to obtain actual carbon emissions, net carbon emissions were adjusted for any carbon that
remained unoxidized as a result of incomplete combustion (e.g., carbon contained in ash or soot).4
Step 4: Convert to C02 Emissions
Because the IPCC reporting guidelines recommend that countries report greenhouse gas emissions on a full
molecular weight basis, the final step in estimating CO2 emissions from fossil fuel consumption was converting from
units of carbon to units of CO2. Actual carbon emissions were multiplied by the molecular-to-atomic weight ratio of
CO2 to carbon (44/12) to obtain total carbon dioxide emitted from fossil fuel combustion in teragrams (Tg). The results
are contained in Table O-5.
Comparison Between Sectoral and Reference Approaches
These two alternative approaches can both produce reliable estimates that are comparable within a few percent.
The major difference between methodologies employed by each approach lies hi the energy data used to derive carbon
emissions (i.e., the actual reported consumption for the Sectoral Approach versus apparent consumption derived for the
Reference Approach). In theory, both approaches should yield identical results. In practice, however, slight
discrepancies occur. For the United States, these differences are discussed below.
1 Bunker fuels refer to quantities of fuels used for international transportation. The IPCC methodology accounts for these fuels as part of the
energy balance of the country in which they were delivered to end-users. Carbon dioxide emissions from the combustion of these fuels were
estimated separately and were not included in U.S. national totals. This is done to ensure that all fuel is accounted for in the methodology and so
that the IPCC is able to prepare global emission estimates.
5 Carbon coefficients from EIA were used wherever possible. Because EIA did not provide coefficients for coal, the IPCC-recommended
emission factors were used in the top-down calculations for these fuels. See notes in Table N-4, Table O-3, Table O-4 for more specific source
information.
4 For the portion of carbon that is unoxidized during coal combustion, the IPCC suggests a global average value of 2 percent. However, because
combustion technologies in the United States are more efficient, the United States inventory uses 1 percent in its calculations for petroleum and
coal and 0.5 percent for natural gas.
0-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
Differences in Total Amount of Energy Consumed
Table O-7 and Table O-85 summarize the differences between the Reference and Sectoral approaches in estimating
total energy consumption in the United States. Although theoretically the two methods should arrive at the same
estimate for U.S. energy consumption, the Reference Approach provides an energy total that is 2.0 percent lower than
the Sectoral Approach for 1998. The greatest difference lies in the higher estimate of petroleum consumption with the
Sectoral Approach (4.0 percent).
There are several potential sources for the discrepancies in consumption estimates:
• Product Definitions. The fuel categories in the Reference Approach are different from those used
in the Sectoral Approach, particularly for petroleum. For example, the Reference Approach
estimates apparent consumption for crude oil. Crude oil is not typically consumed directly, but
refined into other products. As a result, the United States does not focus on estimating the energy
content of crude oil, but rather estimating the energy content of the various products resulting
from crude oil refining. The United States does not believe that estimating apparent consumption
for crude oil, and the resulting energy content of the crude oil, is the most reliable method for the
United States to estimate its energy consumption. Other differences in product definitions include
using sector-specific coal statistics in the Sectoral Approach (i.e., residential, commercial,
industrial coking, industrial other, and transportation coal), while the Reference Approach
characterizes coal by rank (i.e. anthracite, bituminous, etc.). Also, the liquefied petroleum gas
(LPG) statistics used in the bottom-up calculations are actually a composite category composed of
natural gas liquids (NGL) and LPG.
Heat Equivalents. It can be difficult to obtain heat equivalents for certain fuel types, particularly
for categories such as "crude oil" where the key statistics are derived from thousands of producers
in the United States and abroad. For heat equivalents by coal rank, it was necessary to refer back
to EIA's State Energy Data Report 1992 (1994) because this information is no longer published.
Possible inconsistencies in U.S. Energy Data. The United States has not focused its energy data
collection efforts on obtaining the type of aggregated information used in the Reference
Approach. Rather, the United States believes that its emphasis on collection of detailed energy
consumption data is a more accurate methodology for the United States to obtain reliable energy
data. Therefore, top-down statistics used in the Reference Approach may not be as accurately
collected as bottom-up statistics applied to the Sectoral Approach.
Balancing Item. The Reference Approach uses apparent consumption estimates while the
Sectoral Approach uses reported consumption estimates. While these numbers should be equal,
there always seems to be a slight difference that is often accounted for in energy statistics as a
"balancing item."
Differences in Estimated C02 Emissions
Given these differences in energy consumption data, the next step for each methodology involved estimating
emissions of CO2. Table O-8 summarizes the differences between the two methods in estimated carbon emissions for
1998. Although complete data and calculations are not presented, comparison tables are also presented for 1995 through
1997 emissions in Table O-9 through Table O-14.
As shown previously, the Sectoral Approach resulted in a 2.0 percent higher estimate of energy consumption in
the United States than the Reference Approach, but the resulting emissions estimate for the Reference Approach is 0.8
percent higher. While both methods' estimates of natural gas emissions are almost exactly the same, coal and petroleum
emission estimates from the Reference Approach are higher than the Sectoral Approach. Potential reasons for these
patterns may include:
* Although complete energy consumption data and calculations are not presented, comparison tables are also presented for 1995-1998
0-3
-------
• Product Definitions. Coal data is aggregated differently in each methodology, as noted above,
with United States coal data typically collected in the format used for the Sectoral Approach. This
results in more accurate estimates than in the Reference Approach. Also, the Reference Approach
relies on a "crude oil" category for determining petroleum-related emissions. Given the many
sources of crude oil in the United States, it is not an easy matter to track potential differences in
carbon content between different sources of crude, particularly since information on the carbon
content of crude oil is not regularly collected.
• Carbon Coefficients. The Reference Approach relies on several default carbon coefficients
provided by IPCC (IPCC/UNEP/OECD/IEA 1997), while the Sectoral Approach uses category-
specific coefficients that are likely to be more accurate. Also, as noted above, the carbon
coefficient for.crude oil is not an easy value to obtain given the many sources and grades of crude
oil consumed in the United States.
Although the two approaches produce similar results, the United States believes that the "bottom-up" Sectoral
Approach provides a more accurate assessment of CO2 emissions at the fuel level. This improvement in accuracy is
largely a result of the data collection techniques used in the United States, where there has been more emphasis on
obtaining the detailed products-based information used in the Sectoral Approach than obtaining the aggregated energy
flow data used in the Reference Approach. The United States believes that it is valuable to understand both methods.
References
EIA (1998a) Annual Energy Review 1998, DOE/EIA- 0384(98)-annual, Energy Information Administration, U.S.
Department of Energy, Washington, DC.
EIA (1998b) Coal Industry Annual - 1998, DOE/EIA 0584(98)-annual, Energy Information Administration, U.S.
Department of Energy, Washington, DC.
EIA (1998c) Emissions of Greenhouse Gases in the United States 1998, DOE/EIA 0573(98)-annual, Energy
Information Administration, U.S. Department of Energy, Washington, DC.
EIA (1998d) Monthly Energy Review, DOE/EIA 0035(98)-monthly, Energy Information Administration, U.S.
Department of Energy, Washington, DC. November.
EIA (1998e) Petroleum Supply Annual -1998, DOE/EIA 0340(98)-annual, Energy Information Administration, U.S.
Department of Energy, Washington, DC, Volume I.
EIA (1994) State Energy Data Report 1992, DOE/EIA 0214(92)-annual, Energy Information Administration, U.S.
Department of Energy, Washington, DC.
IPCC/UNEP/OECD/IEA (1997) Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories, Paris:
Intergovernmental Panel on Climate Change, United Nations Environment Programme, Organization for
Economic Co-Operation and Development, International Energy Agency.
0-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
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Table 0-5: 1998 Non-Energy Carbon Stored in Products
Fuel Type
Consumption for Non-
Energy Use (TBtuL
Carbon Coefficients Carbon Content
(MMTCE/QBlu) (MMTCE)
Carbon Sequestered
Fraction (MMTCE)
Coal
Natural Gas
Asphalt & Road Oil
LPG
Lubricants
Pentanes Plus
Petrochemical Feedstocks
Petroleum Coke
Special Naptha
Waxes/Misc.
Misc. U.S. Territories Petroleum
Total
25.1
377.2
1,262.6
1,582.0
371.0
264.0
[1]
306.3
107.3
[1]
[1]
25.6
14.5
20.6
16.9
20.2
18.2
HI
27.9
19.9
[1]
[1]
0.6
5.5
26.0
26.7
7.5
4.8
[1]
8.5
2.1
[1]
[1]
0.8
1.0
1.0
0.8
0.5
0.8
[1]
0.5
0.0
[1]
[1]
0.48
5.46
26.03
21.34
3.75
3.85
16.11
4.26
0.00
3.86
0.43
85.58
[1] Values for Misc. U.S. Territories Petroleum, Petrochemical Feedstocks and Waxes/Misc. are not shown because these categories are aggregates of
numerous smaller components.
Note: Totals may not sum due to independent rounding.
Table 0-6: Reference Approach C02 Emissions from Fossil Fuel Consumption (MMTCE unless otherwise noted)
Fuel Category
Coal
Petroleum
Natural Gas
Total
Potential Carbon
Emissions
552.4
710.5
315.9
1,578.8
Carbon
Sequestered
0.5
79.6
5.5
85.6
Net Carbon
Emissions
552.0
630.8
310.5
1,493.3
Fraction Oxidized
(percent)
99.0%
99.0%
99.5%
-
C02 Emissions
546.4
624.5
308.9
1,479.9
C02 Emissions
(Tg)
2,003.6
2,289.9
1,132.7
5,426.2
Note: Totals may not sum due to independent rounding.
Table 0-7: 1998 Energy Consumption in the United States: Sectoral vs. Reference Approaches (Tbtu)
Approach
Sectoral3
Reference (Apparent)3
Difference
Coal
21,185.6
?1 175 1
0.0%
Natural Gas
21,885.1
21,833.5
-0.2%
Petroleum
37,358.5
35,783.1
-4.2%
Total
80,429.2
78,791.7
-2.0%
1 Includes U.S. territories
Note: Totals may not sum due to independent rounding.
Table 0-8: 1998 C02 Emissions from Fossil Fuel Combustion by Estimating Approach (MMTCE)
Approach
Sectoral3
Reference3
Difference
Coal
539.6
5464
1.3%
Natural Gas
309.7
308.9
-0.2%
Petroleum
618.9
624.5
0.9%
Total
1,468.2
1,479.9
0.8%
' Includes U.S. territories
Note: Totals may not sum due to independent rounding.
0-9
-------
Table 0-9: 1997 Energy Consumption in the United States: Sectoral vs. Reference Approaches (TBtu)
Approach
Sectoral1
Reference (Apparent)"
Difference
Coal
20,991.8
21,042.0
0.2%
Natural Gas
22,525.5
22,536.9
0.1%
Petroleum
36,700.1
35,237.6
-4.0%
Total
80,217.4
78,816.5
-1.7%
1 Includes U.S. territories
Note: Totals may not sum due to independent rounding.
Table 0-10: 1997 CO, Emissions from Fossil Fuel Combustion by Estimating Approach (MMTCE)
Approach
Coal
Natural Gas
Petroleum
Total
Sectoral'
Reference"
534.5
542.8
318.8
318.8
607.3
614.7
1,460.7
1,476.3
Difference
1.5%
0.0%
1.2%
1.1%
• Includes U.S. territories
Note: Totals may not sum due to independent rounding.
Table 0-11: 1996 Energy Consumption in the United States: Sectoral vs. Reference Approaches (Tbtu)
Approach
Sectoral*
Reference (Apparent)"
Difference
Coal
20,458.6
20,355
-0.5%
Natural Gas
22,533.7
22,547
0.1%
Petroleum
36,142.7
34,523
-4.5%
Total
79,135
77,426
-2.2%
1 Includes U.S. territories
Note: Totals may not sum due to independent rounding.
Table 0-12: 1996 C02 Emissions from Fossil Fuel Combustion by Estimating Approach (MMTCE)
Approach Coal Natural Gas Petroleum Total
Sectoral"
Reference"
520.5
525.2
319.2
319.3
601.7
603.1
1,441.3
1,447.6
Difference
0.9%
0.0%
0.2%
0.4%
1 Includes U.S. territories
Note: Totals may not sum due to independent rounding.
0-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
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Table 0-13: 1995 Energy Consumption in the United States: Sectoral vs. Reference Approaches (Tbtu)
Approach
Sectoral3
Reference (Apparent)8
Difference
Coal
19,577.2
19,434
-0.7%
Natural Gas
22,138.0
22,152
0.1%
Petroleum
34,991.8
33,174
-5.2%
Total
76,707
74,759
-2.5%
1 Includes U.S. territories
Note: Totals may not sum due to independent rounding.
Table 0-14: 1995 C02 Emissions from Fossil Fuel Combustion by Estimating Approach (MMTCE)
Approach
Coal
Natural Gas
Petroleum
Total
Sectoral'
Reference"
498.1
501.6
313.6
313.6
580.3
576.9
1,391.9
1,392.1
Difference
0.7%
0.0%
-0.6%
0.0%
• Includes U.S. territories
Note: Totals may not sum due to independent rounding.
0-11
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-------
Annex P
Sources of Greenhouse Gas Emissions Excluded
Although this report is intended to be a comprehensive assessment of anthropogenic1 sources and sinks of
greenhouse gas emissions for the United States, certain sources have been identified yet excluded from the estimates
presented for various reasons. Before discussing these sources, however, it is important to note that processes or
activities that are not anthropogenic in origin or do not result in a net source or sink of greenhouse gas emissions are
intentionally excluded from a national inventory of anthropogenic greenhouse gas emissions. In general, processes or
activities that are not anthropogenic are considered natural (i.e., not directly influenced by human activity) in origin and,
as an example, would include the following:
• Volcanic eruptions
CO2 exchange (i.e., uptake or release) by oceans
Natural forest firesz
• CH4 emissions from wetlands not affected by human induced land-use changes
Some processes or activities may be anthropogenic in origin but do not result in net emissions of greenhouse gases,
such as the respiration of CO2 by people or domesticated animals. Given a source category that is both anthropogenic
and results in net greenhouse gas emissions, reasons for excluding a source related to an anthropogenic activity include
one or more of the following:
• There is insufficient scientific understanding to develop a reliable method for estimating
emissions at a national level.
Although an estimating method has been developed, data were not adequately available to
calculate emissions.
Emissions were implicitly accounted for within another source category (e.g., CO2 from Fossil
Fuel Combustion).
It is also important to note that the United States believes the exclusion of the sources discussed below introduces
only a minor bias in its overall estimate of U.S. greenhouse gas emissions.
Separate Cruise and LTO Emissions from the Combustion of Jet Fuel
The combustion of jet fuel by aircraft results in emissions of CH4, N2O, CO, NOX, and NMVOCs. The emissions
per mass of fuel combusted during landing/take-off (LTO) operations differ from those during aircraft cruising.
Accurate estimation of these emissions requires a detailed accounting of LTO cycles and fuel consumption during
cruising by aircraft model (e.g., Boeing 747-400). Sufficient data for separately calculating near ground-level emissions
during landing and take-off and cruise altitude emissions by aircraft model were not available for this report, (see
Revised 1996IPCC Guidelines for National Greenhouse Gas Inventories: Reference Manual, pp. 1.93 - 1.96)
1 The term "anthropogenic", in this context, refers to greenhouse gas emissions and removals that are a direct result of human activities or are the
result of natural processes that have been affected by human activities (IPCC/UNEP/OECD/IEA 1997).
2 In some cases forest fires that are started either intentionally or unintentionally are viewed as mimicking natural burning processes which have
been suppressed by other human forest management activities. The United States does not consider forest fires within its national boundaries to
be a net source of greenhouse emissions.
P-1
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C02 from Burning in Coal Deposits and Waste Piles
Coal is periodically burned in deposits and waste piles. It has been estimated that the burning of coal in deposits
and waste piles would represent less than 1.3 percent of total U.S. coal consumption, averaged over ten-years. Because
there is currently no known source of data on the quantity of coal burned in waste piles and there is uncertainty as to
the fraction of coal oxidized during such burnings, these CO2 emissions are not currently estimated. Further research
would be required to develop accurate emission factors and activity data for these emissions to be estimated (see Revised
1996IPCC Guidelines for National Greenhouse Gas Inventories: Reference Manual, p. 1.112 -1.113).
Fossil C02 from Petroleum and Natural Gas Wells, C02 Separated from Natural Gas, and
C02 from Enhanced Oil Recovery (EOR)
Petroleum and natural gas well drilling, petroleum and natural gas production, and natural gas
processing—including removal of CO2—may result in emissions of CO2 that was at one time stored in underground
formations.
Carbon dioxide and other gases are naturally present in raw natural gas, in proportions that vary depending on the
geochemical circumstances that caused the formation of the gas. After the heavier gases are removed during processing,
small amounts of carbon dioxide may be allowed to remain in the natural gas. If the amount of CO2 sufficiently lowers
the heating value of the natural gas, it is typically extracted by amine scrubbing and, in most cases, released into the
atmosphere. These emissions can be estimated by calculating the difference between the average carbon dioxide content
of raw natural gas and the carbon dioxide content of pipeline gas. The Energy Information Administration (El A)
estimates that annual CO2 emissions from scrubbing are about 4 million metric tons of carbon. Because of imprecision
in the reporting of U.S. natural gas production and processing, emissions estimates from energy production sources may
be double-counted or under-reported, and thus are uncertain.
Carbon dioxide is also injected into underground deposits to increase crude oil reservoir pressure in a field
technique known as enhanced oil recovery (EOR). It is thought that much of the injected CO2 may be effectively and
permanently sequestered, but the fraction of injected CO2 that is re-released remains uncertain. The fraction re-released
varies from well to well depending upon the field geology and the gas capture/re-injection technology employed at the
wellhead. Over time, carbon dioxide may also seep into the producing well and mix with the oil and natural gas present
there. If the gas portion of this mixture has a sufficiently high energy content, it may be collected and sent to a natural
gas plant; if not, it may be vented or flared. The EIA estimates that the amount of CO2 used for EOR is on the order
of 12 million metric tons, of which emissions would be some fraction yet to be defined. This figure is based on the
difference between U.S. Department of Commerce sales figures for industrial CO2 (17 million metric tons) minus the
5 million metric tons reported by the Freedonia Group that is used for purposes other than EOR. Further research into
EOR is required before the resulting CO2 emissions can be adequately quantified. (See Carbon Dioxide Consumption
in the Industrial Processes chapter).
Carbon Sequestration in Underground Injection Wells
Organic hazardous wastes are injected into underground wells. Depending on the source of these organic
substances (e.g., derived from fossil fuels) the carbon in them may or may not be included in U.S. CO2 emission
estimates. Sequestration of carbon containing substances in underground injection wells may be an unidentified sink.
Further research is required if this potential sink is to be quantified.
CH4 from Abandoned Coal Mines
Abandoned coal mines are a source of CH4 emissions. In general, many of the same factors that affect emissions
from operating coai mines will affect emissions from abandoned mines such as the permeability and gassiness of the
coal, the mine's depth, geologic characteristics, and whether it has been flooded. A few gas developers have recovered
P-2 Inventory of U.S. greenhouse Gas Emissions and Sinks: 1990-1998
-------
methane from abandoned mine workings; therefore, emissions from this source may be significant. Further research
and methodological development is needed if these emissions are to be estimated.
C02 from "Unaccounted for" Natural Gas
There is a discrepancy between the amount of natural gas sold by producers and that reported as purchased by
consumers. This discrepancy, known as "unaccounted for" or unmetered natural gas, was assumed to be the sum of
leakage, measurement errors, data collection problems, undetected non-reporting, undetected over reporting, and
undetected under reporting. Historically, the amount of gas sold by producers has always exceeded that reportedly
purchased by consumers; therefore, some portion of unaccounted for natural gas was assumed to be a source of CO2
emissions. In other words, it was assumed that consumers were underreporting their usage of natural gas. In
DOE/EIA's energy statistics for 1996, however, reported consumption of natural gas exceeded the amount sold by
producers. Therefore, the historical explanation given for this discrepancy has lost credibility and unaccounted for
natural gas is no longer used to calculate CO2 emissions.
C02 from Shale Oil Production
Oil shale is shale saturated with kerogen.3 It can be thought of as the geological predecessor to crude oil. Carbon
dioxide is released as a by-product of the process of producing petroleum products from shale oil. As of now, it is not
cost-effective to mine and process shale oil into usable petroleum products. The only identified large-scale oil shale
processing facility in the United States was operated by Unocal during the years 1985 to 1990. There have been no
known emissions from shale oil processing in the United States since 1990 when the Unocal facility closed.
CH4 from the Production of Carbides other than Silicon Carbide
Methane (CH4) may be emitted from the production of carbides because the petroleum coke used in the process
contains volatile organic compounds, which form CH4 during thermal decomposition. Methane emissions from the
production of silicon carbide were estimated and accounted for, but emissions from the production of calcium carbide
and other carbides were not. Further research is needed to estimate CH4 emissions from the production of calcium
carbide and other carbides other than silicon carbide. (See Revised 1996IPCC Guidelines for National Greenhouse
Gas Inventories: Reference Manual, pp. 2.20 - 2.21)
C02 from Calcium Carbide and Silicon Carbide Production
Carbon dioxide is formed by the oxidation of petroleum coke in the production of both calcium carbide and silicon
carbide. These CO2 emissions are implicitly accounted for with emissions from the combustion of petroleum coke in
the Energy chapter. There is currently not sufficient data on coke consumption to estimate emissions from these sources
explicitly. (See Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories: Reference Manual pp 220
- 2.21)
C02 from Graphite Consumption in Ferroalloy and Steel Production
The CO2 emissions from the three reducing agents used in ferroalloy and steel production—coke, wood (or
biomass), and graphite—are accounted for as follows:
Emissions resulting from the use of coke are accounted for in the Energy chapter under Fossil
Fuel Combustion.
3 Kerogen is fossilized insoluble organic material found in sedimentary rocks, usually shales, which can be converted to petroleum uroducts bv
distillation. }
P-3
-------
Estimating emissions from the use of wood or other biomass materials is unnecessary because
these emissions should be accounted for in Land-Use Change and Forestry chapter if the biomass
is harvested on an unsustainable basis.
The CO2 emissions from the use of graphite, which is produced from petroleum by-products, may
be accounted for in the Energy chapter, although further analysis is required to determine if these
emissions are being properly estimated. The CO2 emissions from the use of natural graphite,
however, have not been accounted for in the estimate.
Emissions from graphite electrode consumption—versus its use as a reducing agent—in ferroalloy and steel
production may at present only be accounted for in part under fossil fuel combustion if the graphite used was derived
from a fossil fuel substrate, versus natural graphite ore. Further research into the source and total consumption of
graphite for these purposes is required to explicitly estimate emissions. (See Iron and Steel Production and Ferroalloy
Production in the Industrial Processes chapter)
N20 from Caprolactam Production
Caprolactam is a widely used chemical intermediate, primarily to produce nylon-6. All processes for producing
caprolactam involve the catalytic oxidation of ammonia, with N2O being produced as a by-product. Caprolactam
production could be a significant source of N2O—it has been identified as such in the Netherlands. More research is
required to determine this source's significance because there is currently insufficient information available on
caprolactam production to estimate emissions in the United States. (See Revised 1996IPCC Guidelines for National
Greenhouse Gas Inventories: Reference Manual, pp. 2.22 - 2.23)
N20 from Cracking of Certain Oil Fractions
In order to improve the gasoline yield in crude oil refining, certain oil fractions are processed in a catcracker.
Because crude oil contains some nitrogen, N2O emissions may result from this cracking process. There is currently
insufficient data to develop a methodology for estimating these emissions. (See Revised 1996 IPCC Guidelines for
National Greenhouse Gas Inventories: Reference Manual, p. 2.23)
CH4 from Coke Production
Coke production may result in CH4 emissions. Detailed coke production statistics were not available for the
purposes of estimating CH4 emissions from this minor source. (See Petrochemical Production in the Industrial Processes
chapter and the Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories: Reference Manual, p. 2.23)
C02 from Metal Production
Coke is used as a reducing agent in the production of some metals from their ores, including magnesium,
chromium, lead, nickel, silicon, tin, titanium, and zinc. Carbon dioxide may be emitted during the metal's production
from the oxidization of this coke and, in some cases, from the carbonate ores themselves (e.g., some magnesium ores
contain carbonate). The CO2 emissions from coke oxidation are accounted for in the Energy chapter under Fossil Fuel
Combustion. The CO2 emissions from the carbonate ores are not presently accounted for, but their quantities are thought
to be minor. (See Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories: Reference Manual, p. 2.37
-2.38)
N20 from Acrylonitrile Production
Nitrous oxide may be emitted during acrylonitrile production. No methodology was available for estimating these
emissions, and therefore further research is needed if these emissions are to be included, (see Revised 1996 IPCC
Guidelines for National Greenhouse Gas Inventories: Reference Manual, p. 2.22)
P-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
-------
SF6 from Aluminum Fluxing and Degassing
Occasionally, sulfur hexafluoride (SF6) is used by the aluminum industry as a fluxing and degassing agent in
experimental and specialized casting operations. In these cases it is normally mixed with argon, nitrogen, and/or
chlorine and blown through molten aluminum; however, this practice is not used by primary aluminum production firms
in the United States and is not believed to be extensively used by secondary casting firms. Where it does occur, the
concentration of SF6 in the mixture is small and a portion of the SF6 is decomposed in the process (Waite and Bernard
1990, Corns 1990). It has been estimated that 230 Mg of SF6 were used by the aluminum industry in the United States
and Canada (Maiss and Brenninkmeijer 1998); however, this estimate is highly uncertain.
Miscellaneous SF6 Uses
Sulfur hexafluoride may be used in gas-filled athletic shoes, in foam insulation, for dry etching, in laser systems,
as an atmospheric tracer gas, for indoor air quality testing, for laboratory hood testing, for chromatography, in tandem
accelerators, in sound-insulating windows, in tennis balls, in loudspeakers, in shock absorbers, and for certain
biomedical applications. Data need to be gathered and methodologies developed if these emissions are to be estimated.
A preliminary global assessment of aggregate emissions from these applications can be found in Maiss, M. and C.A.M.
Brenninkmeijer (1998).
C02 from Solvent Incineration
CO2 may be released during the incineration of solvents. Although emissions from this source are believed to be
minor, data need to be gathered and methodologies developed if these emissions are to be estimated.
C02 from Non-Forest Soils
Non-forest soils emit CO2 from decaying organic matter and carbonate minerals—the latter may be naturally
present or mined and later applied to soils as a means to adjust their acidity. Soil conditions, climate, and land-use
practices interact to affect the CO2 emission rates from non-forest soils. The U.S. Forest Service has developed a model
to estimate CO2 emissions from forest soils; a similar model is currently being developed for non-forest soils. A new
methodology has been implemented in order to estimate CO2 fluxes from non-forest soils in the 1990 to 1992 period,
but more recent data that would permit the inclusion of 1993 to 1998 estimates in this Inventory had not yet been
released (see Changes in Non-Forest Carbon Stocks in the Land-Use Change and Forestry chapter).
N20 from Domestic House Animal Waste Deposited on Soils
A substantial amount of liquid and solid waste is produced by domestic animals that are kept as pets. A
preliminary methodology was developed to estimate nitrous oxide (N2O) emissions from the deposition of domestic
house animal (i.e., dogs and cats) waste on lawns, fields and parks. Estimates calculated with this methodology suggest
that, in 1990, approximately 330 thousand metric tons of nitrogen originating as domestic house animal waste were
deposited on soils resulting in approximately 0.8 MMTCE of N2O emissions from soils. To estimate the amount of
nitrogen deposited by domestic house animals, only those excretions that remained on land surfaces -as opposed to
wastes that were collected by owners and are managed as municipal solid waste - were included. Annual dog and cat
population numbers were obtained from the Pet Food Institute.4 Annual nitrogen excretion rates were estimated from
protein intake. The recommended protein intake for an average size adult of each animal type5 was multiplied by the
average amount of nitrogen per unit of protein (0.16 kg N/kg protein, from the Revised 1996 JPCC Guidelines) to
4 Pet Food Institute (1999) Pet Incidence Trend Report. Pet Food Institute, Washington DC.
5 Bright, S. (1999) Personal communication between Marco Alcaraz of ICF Consulting and Susan Bright of the Dupont Animal Clinic
Washington, D.C., August 1999.
P-5
-------
estimate nitrogen consumption. It was then assumed that 95 percent of this nitrogen was excreted, either in solid or
liquid form (i.e., it was assumed that 5 percent was retained for fur and milk production). Of the total nitrogen
excretion, 90 percent was assumed to occur through liquid waste, with the balance from solid waste.6 Both cat and dog
populations were divided into urban and rural fractions, using the metropolitan and non-metropolitan human population
categories, respectively, of the U.S. Census Bureau.7 Both liquid and solid wastes from the urban cat population, and
solid waste from the urban dog population were assumed to be collected (i.e., not deposited on soils). Nitrous oxide
emission estimates from domestic house animal excretion were calculated in the same manner as performed for
estimating emissions from livestock excretion. Producing these estimates involved making a number of simplifying
assumptions regarding average animal size and protein consumption, as well as the proportions of animal populations
residing in urban and rural areas and the proportions of wastes that are deposited on land. Further methodological
development and data collection is required in order to reduce the uncertainty involved in the domestic house animal
excretion estimates.
C02 from Food Scraps Disposed in Landfills
A certain amount of food scraps generated from food processing or as leftovers join the waste stream and are
landfilled. Nationally, an estimated 0.1 MMTCE per year are stored in the form of organic carbon contained in food
scraps in landfills, acting as a carbon sink. A portion of the landfilled food scraps becomes a source of methane
emissions, which offset the sink estimates to an extent. Further data collection on the amount and composition of food
scraps generated and landfilled is required in order to reduce the uncertainty associated with this estimate.
CH4 from Land-Use Changes Including Wetlands Creation or Destruction
Wetlands are a known source of methane (CH4) emissions. When wetlands are destroyed, CH4 emissions may
be reduced. Conversely, when wetlands are created (e.g., during the construction of hydroelectric plants), CH4
emissions may increase. Grasslands and forest lands may also be weak sinks for CH4 due to the presence of
methanotrophic bacteria that use CH4 as an energy source (i.e., they oxidize CH4 to CO2). Currently, an adequate
scientific basis for estimating these emissions and sinks does not exist, and therefore further research and
methodological development is required.
CH4 from Septic Tanks and Drainfields
Methane (CH4) is produced during the biodegradation of organics in septic tanks if other suitable electron-
acceptors (i.e., oxygen, nitrate, or sulfate) besides CO2 are unavailable. Such conditions are called methanogenic. There
were insufficient data and methodological developments available to estimate emissions from this source.
N20 from Wastewater Treatment
As a result of nitrification and denitrification processes, nitrous oxide (N2O) may be produced and emitted from
both domestic and industrial wastewater treatment plants. Nitrogen-containing compounds are found in wastewater due
to the presence of both human excrement and other nitrogen-containing constituents (e.g. garbage, industrial wastes,
dead animals, etc.). The portion of emitted N2O which originates from human excrement is currently estimated under
the Human Sewage source category—based upon average dietary assumptions. The portion of emitted N2O which
originates from other nitrogen-containing constituents is not currently estimated. Further research and methodological
development is needed if these emissions are to be accurately estimated.
* Swcnson, M J. and W.G. Reece, eds. (1993) Duke's Physiology of Domestic Animals. Cornell University Press. 11th Edition.
' U.S. Census Bureau (1999)
P-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
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CH4 from Industrial Wastewater Treatment
Methane (CH4) may be produced during the biodegradation of organics in wastewater if other suitable electron-
acceptors (i.e. oxygen, nitrate, or sulfate) besides CO2 are unavailable. Such conditions are called methanogenic.
Methane produced from domestic wastewater treatment plants is accounted for in the Waste chapter. These emissions
are estimated by assuming an average 5-day biological oxygen demand (BOD5) per capita contribution in conjunction
with the approximation that 15 percent of wastewater's BOD5 is removed under methanogenic conditions. This method
itself needs refinement. It is not clear if industrial wastewater sent to domestic wastewater treatment plants, which may
contain biodegradable material, are accounted for in the average BOD5 per capita method when this wastewater is sent
to domestic wastewater treatment plants. Additionally, CH4 emissions from methanogenic processes at industrial
wastewater treatment plants are not currently estimated. Further research and methodological development is needed
if these emissions are to be accurately estimated. (See Wastewater Treatment in the Waste chapter)
P-7
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Annex Q
Constants, Units, and Conversions
Metric Prefixes
Although most activity data for the U.S. is gathered in customary U.S. units, these units are converted into metric
units per international reporting guidelines. The following table provides a guide for determining the magnitude of
metric units.
Table Q-1: Guide to Metric Unit Prefixes
Prefix/Symbol
atto (a)
femto (f)
pico (p)
nano (n)
micro {jj )
mill! (m)
centi (c)
deci (d)
deca (da)
hecto (h)
kilo (k)
mega (M)
giga (G)
tera(T)
peta (P)
exa (E)
Factor
io-18
10'15
•io-12
1C'9
10'6
10'3
10'2
10'1
10
102
103
106
109
1012
1015
1018
Unit Conversions
1 kilogram =
1 pound =
1 short ton =
1 metric ton =
1 cubic meter =
1 cubic foot
1 U.S. gallon
1 barrel (bbl) =
1 barrel (bbl) =
1 liter
2.205 pounds
0.454 kilograms
2,000 pounds
1,000 kilograms
= 0.9072 metric tons
= 1.1023 short tons
35.315 cubic feet
0.02832 cubic meters
3.785412 liters
0.159 cubic meters
42 U.S. gallons
0.1 cubic meters
Q-1
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1foot
1 meter
1m!le
1 kilometer
0.3048 meters
3.28 feet
1.609 kilometers
0.622 miles
1 acre = 43,560 square feet = 0.4047 hectares = 4,047 square meters
1 square mile = 2.589988 square kilometers
To convert degrees Fahrenheit to degrees Celsius, subtract 32 and multiply by 5/9
To convert degrees Celsius to Kelvin, add 273.15 to the number of Celsius degrees
Density Conversions1
Methane 1 cubic
Carbon dioxide 1 cubic
Natural gas liquids
Unfinished oils
Alcohol
Liquefied petroleum gas
Aviation gasoline
Naphtha jet fuel
Kerosene jet fuel
Motor gasoline
Kerosene
Naphtha
Distillate
Residual oil
Lubricants
Bitumen
Waxes
Petroleum coke
Petrochemical feedstocks
Special naphtha
Miscellaneous products
meter = 0
meter = 1
1 metric ton
1 metric ton
1 metric ton
1 metric ton
1 metric ton
1 metric ton
1 metric ton
1 metric ton
1 metric ton
1 metric ton
1 metric ton
1 metric ton
1 metric ton
1 metric ton
1 metric ton
1 metric ton
1 metric ton
1 metric ton
1 metric ton
= 0.67606 kilograms
= 1.85387 kilograms
= 11.6 barrels
= 7.46 barrels
= 7.94 barrels
= 11.6 barrels
= 8.9 barrels
= 8.27 barrels
= 7.93 barrels
= 8.53 barrels
= 7.73 barrels
= 8.22 barrels
= 7.46 barrels
= 6.66 barrels
= 7.06 barrels
= 6.06 barrels
= 7.87 barrels
= 5.51 barrels
= 7.46 barrels
= 8.53 barrels
= 8.00 barrels
1,844.2 liters
1,186.04 liters
1,262.36 liters
1,844.2 liters
1,415.0 liters
1,314.82 liters
1,260.72 liters
1,356.16 liters
1,228.97 liters
1,306.87 liters
1,186.04 liters
1,058.85 liters
1,122.45 liters
963.46 liters
1,251.23 liters
876.02 liters
1,186.04 liters
1,356.16 liters
1,271.90 liters
Energy Conversions
Converting Various Energy Units to Joules
The common energy unit used in international reports of greenhouse gas emissions is the joule. A joule is the
energy required to push with a force of one Newton for one meter. A terajoule (TJ) is one trillion (1012) joules. A
British thermal unit (Btu, the customary U.S. energy unit) is the quantity of heat required to raise the temperature of one
pound of water one degree Fahrenheit at or near 39.2 Fahrenheit.
1TJ= 2.388x10" calories
23.88 metric tons of crude oil equivalent
947.8 million Btus
277,800 kilowatt-hours
'Reference: EIA(1998a)
Q-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
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Converting Various Physical Units to Energy Units
Data on the production and consumption of fuels are first gathered in physical units. These units must be
converted to their energy equivalents. The values in the following table of conversion factors can be used as default
factors, if local data are not available. See Appendix A of EIA's Annual Energy Review 1997 (EIA 1998) for more
detailed information on the energy content of various fuels.
Table Q-2: Conversion Factors to Energy Units (Heat Equivalents)
Fuel Type (Units)
Factor
Solid Fuels (Million Btu/Short ton)
Anthracite coal 22.573
Bituminous coal 23.89
Sub-bituminous coal 17.14
Lignite 12.866
Coke 24.8
Natural Gas (Btu/Cubic foot) 1,027
Liquid Fuels (Million Btu/Barrel)
Crude oil 5.800
Natural gas liquids and LRGs 3.777
Other liquids 5.825
Motor gasoline 5.253
Aviation gasoline 5.048
Kerosene 5.670
Jet fuel, kerosene-type 5.670
Distillate fuel 5.825
Residual oil 6.287
Naphtha for petrochemicals 5.248
Petroleum coke 6.024
Other oil for petrochemicals 5.825
Special naphthas 5.248
Lubricants 6.065
Waxes 5.537
Asphalt 6.636
Still gas 6.000
Misc. products 5.796
Note: For petroleum and natural gas, Annual Energy Review 1997 (EIA 1998b). For coal ranks, Sfafe Energy Data Report 1992 (EIA 1993). All values
are given In higher heating values (gross calorific values).
References
EIA (1998a) Emissions of Greenhouse Gases in the United States, DOE/EIA-0573(97), Energy Information
Administration, U.S. Department of Energy. Washington, DC. October.
EIA (1998b) Annual Energy Review, DOE/EIA-0384(97), Energy Information Administration, U.S. Department of
Energy. Washington, DC. July.
EIA (1993) State Energy Data Report 1992, DOE/EIA-0214(93), Energy Information Administration, U.S.
Department of Energy. Washington, DC. December.
Q-3
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Annex R
Abbreviations
AFEAS Alternative Fluorocarbon Environmental Acceptability Study
AAPFCO American Association of Plant Food Control Officials
ASAE American Society of Agricultural Engineers
BEA Bureau of Economic Analysis, U.S. Department of Commerce
BOD5 Biochemical oxygen demand over a 5-day period
BTS Bureau of Transportation Statistics, U.S. Department of Transportation
Btu British thermal unit
CAAA Cltfan Air Act Amendments of 1990
C&EN Chemical and Engineering News
CFC Chlorofluorocarbon
CMA Chemical Manufacturers Association
CMOP Coalbed Mefliane Outreach Program
CVD Chemical vapor deposition
DIC Dissolved inorganic carbon
DOC U.S. Department of Commerce
DOD U.S. Department of Defense
DOE U.S. Department of Energy
DOT U.S. Department of Transportation
EIA Energy information Administration, U.S. Department of Energy
EPA U.S. Environmental Protection Agency
FAA Federal Aviation Administration
FAO Food and Agricultural Organization
FCCC Framework Convention on Climate Change
FHWA Federal Highway Administration
6AA Governmental Advisory Associates
GHG Greenhouse gas
GRI Gas Research Institute
GSAM Gas Systems Analysis Model
GWP Global warming potential
HBFC Hydrobromofluorocarbon
HCFC Hydrochlorofluorocarbon
HDGV Heavy duty gas vehicle
HDDV Heavy duty diesel vehicle
HFC Hydrofluorocarbon
HFE Hydrofluoroethers
ICAO International Civil Aviation Organization
IEA International Energy Association
ILENR Illinois Department of Energy and Natural Resources
IMO International Maritime Organization
IPAA Independent Petroleum Association of America
IPCC Intergovernmental Panel on Climate Change
LDDT Light duty diesel truck
LDDV Light duty diesel vehicle
LDGV Light duty gas vehicle
LDGT Light duty gas truck
LFG Landfill gas
LPG Liquefied petroleum gas(es)
MC Motorcycle
MMTCE Million metric tons of carbon equivalent
MSW Municipal solid waste
NIAR Norwegian Institute for Air Research
NMVOCs Nonmethane volatile organic compounds
NOX Nitrogen Oxides
R-1
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NWEL National Vehicle Fuel Emissions Laboratory
OAQPS EPA Office of Air Quality Planning and Standards
DOS Ozone depleting substances
OECD Organization of Economic Co-operation and Development
OMS EPA Office of Mobile Sources
ORNL Oak Ridge National Laboratory
OSHA Occupational Safety and Health Administration
OTA Office of Technology Assessment
PFC Perfluorocarbon
PFPE Pertluoropolyether
ppmv Parts per million(106) by volume
ppbv Parts per billion (109) by volume
pptv Parts per trillion (1012) by volume
SAE Society of Automotive Engineers
SNG Synthetic natural gas
TBtu Trillion Btu
TJ Terajoule
TSDF Hazardous waste treatment, storage, and disposal facility
TVA Tennessee Valley Authority
U.S. United States
USDA United States Department of Agriculture
USFS United States Forest Service
USGS United States Geological Survey
UNEP United Nations Environmental Programme
UNFCCC United Nations Framework Convention on Climate Change
VAIP EPA's Voluntary Aluminum Industrial Partnership
VMT Vehicle miles traveled
WMO World Meteorological Organization
R-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
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Annex S
Chemical Symbols
Table S-1: Guide to Chemical Symbols
Symbol
Name
Al
AI203
Br
C
CH4
C2H6
C3H8
CF4
C2F6
C-C4F8
CF3I
CFCI3
CF2CI2
CF3CI
C2F3CI3
CCI3CF3
C2F4CI2
C2F5CI
CHF2CI
C2F4HCI
C2FH3CI2
C2H3F2CI
C3F5HCI2
CCI4
CHCICCI2
CCI2CCI2
CH3Ci
CH3CCI3
CH2CI2
CHCI3
CHF3
CH3F?
C2HF5
C2H2F4
OH2FCF3
C2H3F3
C2H3F3
C2H4F2
C3HF7
C3H2F6
C3H3F5
CH2Br2°
CH2BrCI
Aluminum
Aluminum Oxide
Bromine
Carbon
Methane
Ethane.
Propane
Perfluoromethane
Perfluoroethane, hexafluoroethane
Perfluoropropane
Perfluorocyclobutane
Perfluoropentane
Perfluorohexane
Trifluoroiodomethane
Trichlorofluoromethane (CFC-11)
Dichlorodifluoromethane (CFC-12)
Chlorotrifluoromethane (CFC-13)
Trichlorotrifluoroethane (CFC-113)*
CFC-113a*
Dichlorotetrafluoroethane (CFC-114)
Chloropentafluoroethane (CFC-115)
Chlorodifluoromethane (HCFC-22)
HCFC-123
HCFC-124
HCFC-141b
HCFC-142b
HCFC-225ca/cb
Carbon tetrachloride
Trichloroethylene
Perchloroethylene, tetrachloroethene
Methylchloride
Methylchloroform
Methylenechloride
Chloroform, trichloromethane
HFC-23
HFC-32
HFC-41
HFC-125
HFC-134
HFC-134a
HFC-143*
HFC-1433*
HFC-152a
HFC-227ea
HFC-236fa
HFC-245ca
HFC-43-10mee
Dibromomethane
Dibromochloromethane
S-1
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CHBr3
CH3Br
CFjBrCI
CF3Br(CBrF3)
CO
C02
CaC03
CaMg(C03)2
CaO
Cl
F
Fe
Fe203
FeSi
H,H2
H20
H202
OH
N,N2
NH3
NH4+
HN03
NF3
N20
NO
N02
N03
Na
Na2C03
Na3AIF6
0,02
03
S
H2S04
SO*
Si
SiC
Si02
Tribromomethane
Methylbromlde
Bromodichloromethane (Halon 1211)
Bromotrifluoromethane (Halon 1301)
Carbon monoxide
Carbon dioxide
Calcium carbonate, Limestone
Dolomite
Calcium oxide, Lime
atomic Chlorine
Fluorine
Iron
Ferric oxide
Ferrosilicon
Atomic Hydrogen, molecular Hydrogen
Water
Hydrogen peroxide
Hydroxyl
atomic Nitrogen, molecular Nitrogen
Ammonia
Ammonium ion
Nitric Acid
Nitrogen trifluoride
Nitrous oxide
Nitric oxide
Nitrogen dioxide
Nitrate radical
Sodium
Sodium carbonate, soda ash
Synthetic cryolite
atomic Oxygen, molecular Oxygen
Ozone
atomic Sulfur
Sulfuric acid
Sulfur hexafluoride
Sulfur dioxide
Silicon
Silicon carbide
Quartz
Distinct isomers.
S-2 ' Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
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Annex T
Glossary
Abiotic.7 Nonliving. Compare biotic.
Absorption of radiation.1 The uptake of radiation by a solid body, liquid or gas. The absorbed energy may be
transferred or re-emitted.
Acid deposition.6 A complex chemical and atmospheric process whereby recombined emissions of sulfur and nitrogen
compounds are redeposited on earth in wet or dry form. See acid rain.
Acid rain.6 Rainwater that has an acidify content greater than the postulated natural pH of about 5.6. It is formed when
sulfur dioxides and nitrogen oxides, as gases or fine particles in the atmosphere, combine with water vapor and
precipitate as sulfuric acid or nitric acid in rain, snow, or fog. The dry forms are acidic gases or particulates.
See acid deposition.
Acid solution.7 Any water solution that has more hydrogen ions (H+) than hydroxide ions (OH-); any water solution
with a pH less than 7. See basic solution, neutral solution.
Acidic.7 See acid solution.
Adiabatic process.9 A thermodynamic change of state of a system such that no heat or mass is transferred across the
boundaries of the system. In an adiabatic process, expansion always results in cooling, and compression in
warming.
Aerosol.1&9 Particulate matter, solid or liquid, larger than a molecule but small enough to remain suspended in the
atmosphere. Natural sources include salt particles from sea spray, dust and clay particles as a result of
weathering of rocks, both of which are carried upward by the wind. Aerosols can also originate as a result of
human activities and are often considered pollutants. Aerosols are important in the atmosphere as nuclei for
the condensation of water droplets and ice crystals, as participants in various chemical cycles, and as absorbers
and scatters of solar radiation, thereby influencing the radiation budget of the Earth's climate system. See
climate, paniculate matter.
Afforestation.2 Planting of new forests on lands that have not been recently forested.
Air carrier8 An operator (e.g., airline) in the commercial system of air transportation consisting of aircraft that hold
certificates of, Public Convenience and Necessity, issued by the Department of Transportation, to conduct
scheduled or non-scheduled flights within the country or abroad.
Air pollutant. See air pollution.
Air pollution.7 One or more chemicals or substances in high enough concentrations in the air to harm humans, other
animals, vegetation, or materials. Such chemicals or physical conditions (such as excess heat or noise) are
called air pollutants.
Albedo.9 The fraction of the total solar radiation incident on a body that is reflected by it.
Alkalinity.6 Having the properties of a base with a pH of more than 7. A common alkaline is baking soda.
Alternative energy.6 Energy derived from nontraditional sources (e.g., compressed natural gas, solar, hydroelectric,
wind).
Anaerobic.6 A life or process that occurs in, or is not destroyed by, the absence of oxygen.
Anaerobic decomposition.2 The breakdown of molecules into simpler molecules or atoms by microorganisms that can
survive in the partial or complete absence of oxygen.
Anaerobic lagoon.2 A liquid-based manure management system, characterized by waste residing in water to a depth
of at least six feet for a period ranging between 30 and 200 days. Bacteria produce methane in the absence of
oxygen while breaking down waste.
¥-1
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Anaerobic organism.7 Organism that does not need oxygen to stay alive. See aerobic organism.
Antarctic "Ozone Hole."6 Refers to the seasonal depletion of stratospheric ozone in a large area over Antarctica. See
ozone layer.
Anthracite.2 A hard, black, lustrous coal containing a high percentage of fixed carbon and a low percentage of volatile
matter. Often referred to as hard coal.
Anthropogenic.2 Human made. In the context of greenhouse gases, emissions that are produced as the result of human
activities.
Arable land.7 Land that can be cultivated to grow crops.
Aromatic.6 Applied to a group of hydrocarbons and their derivatives characterized by the presence of the benzene ring.
Ash.6 The mineral content of a product remaining after complete combustion.
Asphalt.2 A dark-brown-to-black cement-like material containing bitumen as the predominant constituent. It is
obtained by petroleum processing. The definition includes crude asphalt as well as the following finished
products: cements, fluxes, the asphalt content of emulsions (exclusive of water), and petroleum distillates
blended with asphalt to make cutback asphalt.
Atmosphere.' The mixture of gases surrounding the Earth. The Earth's atmosphere consists of about 79.1 percent
nitrogen (by volume), 20.9 percent oxygen, 0.036 percent carbon dioxide and trace amounts of other gases.
The atmosphere can be divided into a number of layers according to its mixing or chemical characteristics,
generally determined by its thermal properties (temperature). The layer nearest the Earth is the troposphere,
which reaches up to an altitude of about 8 kilometers (about 5 miles) in the polar regions and up to 17
kilometers (nearly 11 miles) above the equator. The stratosphere, which reaches to an altitude of about 50
kilometers (31 miles) lies atop the troposphere. The mesosphere, which extends from 80 to 90 kilometers atop
the stratosphere, and finally, the thermosphere, or ionosphere, gradually diminishes and forms a fuzzy border
with outer space. There is relatively little mixing of gases between layers.
Atmospheric lifetime. See lifetime.
Atomic weight.6 The average weight (or mass) of all the isotopes of an element, as determined from the proportions
in which they are present in a given element, compared with the mass of the 12 is.otope of carbon (taken as
precisely 12.000), that is the official international standard; measured in daltons.
Atoms.7 Minute particles that are the basic building blocks of all chemical elements and thus all matter.
Aviation Gasoline.8 All special grades of gasoline for use in aviation reciprocating engines, as given in the American
Society for Testing and Materials (ASTM) specification D 910. Includes all refinery products within the
gasoline range that are to be marketed straight or in blends as aviation gasoline without further processing (any
refinery operation except mechanical blending). Also included are finished components in the gasoline range,
which will be used for blending or compounding into aviation gasoline.
Bacteria.7 One-celled organisms. Many act as decomposers that break down dead organic matter into substances that
dissolve in water and are used as nutrients by plants.
Barrel (bbl).6 A liquid-volume measure equal to 42 United States gallons at 60 degrees Fahrenheit; used in expressing
quantities of petroleum-based products.
Basic solution.7 Water solution with more hydroxide ions (OH-) than hydrogen ions (H+); water solutions with pH
greater than 7. See acid solution, alkalinity, acid.
Biodegradable.7 Material that can be broken down into simpler substances (elements and compounds) by bacteria or
other decomposers. Paper and most organic wastes such as animal manure are biodegradable. See
nonbiodegradable.
Biofuel.3&7 Gas or liquid fuel made from plant material (biomass). Includes wood, wood waste, wood liquors, peat,
railroad ties, wood sludge, spent sulfite liquors, agricultural waste, straw, tires, fish oils, tall oil, sludge waste,
waste alcohol, municipal solid waste, landfill gases, other waste, and ethanol blended into motor gasoline.
T-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
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Biogeochemical cycle.7 Natural processes that recycle nutrients in various chemical forms from the environment, to
organisms, and then back to the environment. Examples are the carbon, oxygen, nitrogen, phosphorus, and
hydrologic cycles.
Biological oxygen demand (BOD).7 Amount of dissolved oxygen needed by aerobic decomposers to break down the
organic materials in a given volume of water at a certain temperature over a specified time period. See BODS.
Biomass.7 Total dry weight of all living organisms that can be supported at each tropic level in a food chain. Also,
materials that are biological in origin, including organic material (both living and dead) from above and below
ground, for example, trees, crops, grasses, tree litter, roots, and animals and animal waste.
Biomass energy.' Energy produced by combusting biomass materials such as wood. The carbon dioxide emitted from
burning biomass will not increase total atmospheric carbon dioxide if this consumption is done on a sustainable
basis (i.e., if in a given period of time, regrowth of biomass takes up as much carbon dioxide as is released
from biomass combustion). Biomass energy is often suggested as a replacement for fossil fiiel combustion.
Biosphere.2&7 The living and dead organisms found near the earth's surface in parts of the lithosphere, atmosphere,
and hydrosphere. The part of the global carbon cycle that includes living organisms and biogenic organic
matter.
Biotic.7 Living. Living organisms make up the biotic parts of ecosystems. See abiotic.
Bitumen.7 Gooey, black, high-sulfur, heavy oil extracted from tar sand and then upgraded to synthetic fuel oil. See
tar sand.
Bituminous coal.2 A dense, black, soft coal, often with well-defined bands of bright and dull material. The most
common coal, with moisture content usually less than 20 percent. Used for generating electricity, making coke,
and space heating.
BODS.2 The biochemical oxygen demand of wastewater during decomposition occurring over a 5-day period. A
measure of the organic content of wastewater. See biological oxygen demand.
British thermal unit (Btu).3 The quantity of heat required to raise the temperature of one pound of water one degree
of Fahrenheit at or near 39.2 degrees Fahrenheit.
Bunker fuel.2 Fuel supplied to ships and aircraft for international transportation, irrespective of the flag of the carrier,
consisting primarily of residual and distillate fuel oil for ships and jet fuel for aircraft.
Bus.6&s A rubber-tired, self-propelled, manually steered vehicle that is generally designed to transport 30 individuals
or more. Bus types include intercity, school and transit.
Carbon black.2 An amorphous form of carbon, produced commercially by thermal or oxidative decomposition of
hydrocarbons and used principally in rubber goods, pigments, and printer's ink.
Carbon cycle.2 All carbon reservoirs and exchanges of carbon from reservoir to reservoir by various chemical,
physical, geological, and biological processes. Usually thought of as a series of the four main reservoirs of
carbon interconnected by pathways of exchange. The four reservoirs, regions of the Earth in which carbon
behaves in a systematic manner, are the atmosphere, terrestrial biosphere (usually includes freshwater systems),
oceans, and sediments (includes fossil fuels). Each of these global reservoirs may be subdivided into smaller
pools, ranging in size from individual communities or ecosystems to the total of all living organisms (biota).
Carbon dioxide.2 A colorless, odorless, non-poisonous gas that is a normal part of the ambient air. Carbon dioxide
is a product of fossil fuel combustion. Although carbon dioxide does not directly impair human health, it is
a greenhouse gas that traps terrestrial (i.e., infrared) radiation and contributes to the potential for global
warming. See global warming.
Carbon equivalent (CE).' A metric measure used to compare the emissions of the different greenhouse gases based
upon their global warming potential (GWP). Greenhouse gas emissions in the United States are most
commonly expressed as "million metric tons of carbon equivalents" (MMTCE). Global warming potentials
are used to convert greenhouse gases to carbon dioxide equivalents. See global \varmingpotential, greenhouse
gas.
T-3
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Carbon flux.9 The rate of exchange of carbon between pools (i.e., reservoirs).
Carbon intensity. The relative amount of carbon emitted per unit of energy or fuels consumed.
Carbon pool. * The reservoir containing carbon as a principal element in the geochemical cycle.
Carbon sequestration.' The uptake and storage of carbon. Trees and plants, for example, absorb carbon dioxide,
release the oxygen and store the carbon. Fossil fuels were at one time biomass and continue to store the carbon
until burned. See carbon sinks.
Carbon sinks.' Carbon reservoirs and conditions that take-in and store more carbon (i.e., carbon sequestration) than
they release. Carbon sinks can serve to partially offset greenhouse gas emissions. Forests and oceans are large
carbon sinks. See carbon sequestration.
Carbon tetrachloride (CCI<)." A compound consisting of one carbon atom and four chlorine atoms. It is an ozone
depleting substance. Carbon tetrachloride was widely used as a raw material in many industrial applications,
including the production of chlorofluorocarbons, and as a solvent. Solvent use was ended in the United States
when it was discovered to be carcinogenic. See ozone depleting substance.
Chemical reaction. 7 Interaction between chemicals in which there is a change in the chemical composition of the
elements or compounds involved.
Chlorofluorocarbons (CFCs).7 Organic compounds made up of atoms of carbon, chlorine, and fluorine. An example
is CFC-12 (CCyy, used as a refrigerant in refrigerators and air conditioners and as a foam blowing agent.
Gaseous CFCs can deplete the ozone layer when they slowly rise into the stratosphere, are broken down by
strong ultraviolet radiation, release chlorine atoms, and then react with ozone molecules. See Ozone Depleting
Substance.
Climate.I&9 The average weather, usually taken over a 30 year time period, for a particular region and time period.
Climate is not the same as weather, but rather, it is the average pattern of weather for a particular region.
Weather describes the short-term state of the atmosphere. Climatic elements include precipitation, temperature,
humidity, sunshine, wind velocity, phenomena such as fog, frost, and hail-storms, and other measures of the
weather. See weather.
Climate change.' The term "climate change" is sometimes used to refer to all forms of climatic inconsistency, but
because the Earth's climate is never static, the term is more properly used to imply a significant change from
one climatic condition to another. In some cases, "climate change" has been used synonymously with the term,
"global warming"; scientists however, tend to use the term in the wider sense to also include natural changes
in climate. See global warming, greenhouse effect, enhanced greenhouse effect, radiative forcing.
Climate feedback.' An atmospheric, oceanic, terrestrial, or other process that is activated by direct climate change
induced by changes in radiative forcing. Climate feedbacks may increase (positive feedback) or diminish
(negative feedback) the magnitude of the direct climate change.
Climate lag.' The delay that occurs in climate change as a result of some factor that changes very slowly. For
example, the effects of releasing more carbon dioxide into the atmosphere may not be known for some time
because a large fraction is dissolved in the ocean and only released to the atmosphere many years later.
Climate sensitivity.' The equilibrium response of the climate to a change in radiative forcing; for example, a doubling
of the carbon dioxide concentration. See radiative forcing.
Climate system (or Earth system).' The atmosphere, the oceans, the biosphere, the cryosphere, and the geosphere,
together make up the climate system.
Coal.2 A black or brownish black solid, combustible substance formed by the partial decomposition of vegetable matter
without access to air. The rank of coal, which includes anthracite, bituminous coal, subbituminous coal, and
lignite, is based on fixed carbon, volatile matter, and heating value. Coal rank indicates the progressive
alteration, or coalification, from lignite to anthracite. See anthracite, bituminous coal, subbituminous coal,
lignite.
Coal coke.2 A hard, porous product made from baking bituminous coal in ovens at temperatures as high as 2,000
degrees Fahrenheit. It is used both as a fuel and as a reducing agent in smelting iron ore in a blast furnace.
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Coal gasification.7 Conversion of solid coal to synthetic natural gas (SNG) or a gaseous mixture that can be burned
as a fuel.
Coal liquefaction.7 Conversion of solid coal to a liquid fuel such as synthetic crude oil or methanol.
Coalbed methane.2 Methane that is produced from coalbeds in the same manner as natural gas produced from other
strata. Methane is the principal component of natural gas.
Co-control benefit.10 It is the additional benefit derived from an environmental policy that is designed to control one
type of pollution, while reducing the emissions of other pollutants as well. For example, a policy to reduce
carbon dioxide emissions might reduce the combustion of coal, but when coal combustion is reduced, so too
are the emissions of particulates and sulfur dioxide. The benefits associated with reductions in emissions of
particulates and sulfur dioxide are the co-control benefits of reductions in carbon dioxide.
Cogeneration.7 Production of two useful forms of energy such as high-temperature heat and electricity from the same
process.
Combustion.2 Chemical oxidation accompanied by the generation of light and heat.
Commercial sector.8 An area consisting of non-housing units such as non-manufacturing business establishments (e.g.,
wholesale and retail businesses), health and educational institutions, and government offices.
Compost.7 Partially decomposed organic plant and animal matter that can be used as a soil conditioner or fertilizer.
Composting.7 Partial breakdown of organic plant and animal matter by aerobic bacteria to produce a material that can
be used as a soil conditioner or fertilizer. See compost.
Compound.7 Combination of two or more different chemical elements held together by chemical bonds. See element.
See inorganic compound, organic compound.
Concentration.7 Amount of a chemical in a particular volume or weight of air, water, soil, or other medium. See parts
per billion, parts per million.
Conference Of Parties (COP).10 The supreme body of the United Nations Framework Convention on Climate Change
(UNFCCC). It comprises more than 170 nations that have ratified the Convention. Its first session was held
in Berlin, Germany, in 1995 and is expected to continue meeting on a yearly basis. The COP's role is to
promote and review the implementation of the Convention. It will periodically review existing commitments
in light of the Convention's objective, new scientific findings, and the effectiveness of national climate change
programs. See United Nations Framework Convention on Climate Change.
Conifer.7 See coniferous trees.
Coniferous trees.7 Cone-bearing trees, mostly evergreens, that have needle-shaped or scale-like leaves. They produce
wood known commercially as softwood. See deciduous trees.
Criteria pollutant.2 A pollutant determined to be hazardous to human health and regulated under EPA's National
Ambient Air Quality Standards. The 1970 amendments to the Clean Air Act require EPA to describe the health
and welfare impacts of a pollutant as the "criteria" for inclusion in the regulatory regime. In this report,
emissions of the criteria pollutants CO, NOX, NMVOCs, and SO2 are reported because they are thought to be
precursors to greenhouse gas formation.
Crop residue.2 Organic residue remaining after the harvesting and processing of a crop.
Crop rotation.7 Planting the same field or areas of fields with different crops from year to year to reduce depletion
of soil nutrients. A plant such as corn, tobacco, or cotton, which remove large amounts of nitrogen from the
soil, is planted one year. The next year a legume such as soybeans, which add nitrogen to the soil, is planted.
Crude oil.2 A mixture of hydrocarbons that exist in liquid phase in underground reservoirs and remain liquid at
atmospheric pressure after passing through surface separating facilities. See petroleum.
Deciduous trees.7 Trees such as oaks and maples that lose their leaves during part of the year. See coniferous trees.
Decomposition.9 The breakdown of matter by bacteria and fungi. It changes the chemical composition and physical
appearance of the materials.
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Deforestation.' Those practices or processes that result in the conversion of forested lands for non-forest uses. This
is often cited as one of the major causes of the enhanced greenhouse effect for two reasons: 1) the burning or
decomposition of the wood releases carbon dioxide; and 2) trees that once removed carbon dioxide from the
atmosphere in the process of photosynthesis are no longer present.
Degradable.7 See biodegradable.
Desertification.' The progressive destruction or degradation of existing vegetative cover to form a desert. This can
occur due to overgrazing, deforestation, drought, and the burning of extensive areas. Once formed, deserts can
only support a sparse range of vegetation. Climatic effects associated with this phenomenon include increased
reflectivity of solar radiation, reduced atmospheric humidity, and greater atmospheric dust (aerosol) loading.
Distillate fuel oil.2 A general classification for the petroleum fractions produced in conventional distillation operations.
Included are products known as No. 1, No. 2, and No. 4 fuel oils and No. 1, No. 2, and No. 4 diesel fuels.
Used primarily for space heating, on and off-highway diesel engine fuel (including railroad engine fuel and
fuel for agricultural machinery), and electric power generation.
Economy.7 System of production, distribution, and consumption of economic goods.
Ecosystem.10 The complex system of plant, animal, fungal, and microorganism communities and their associated non-
living environment interacting as an ecological unit. Ecosystems have no fixed boundaries; instead their
parameters are set to the scientific, management, or policy question being examined. Depending upon the
purpose of analysis, a single lake, a watershed, or an entire region could be considered an ecosystem.
Electrons.7 Tiny particle moving around outside the nucleus of an atom. Each electron has one unit of negative charge
(-) and almost no mass.
Element.7 Chemicals such as hydrogen (H), iron (Fe), sodium (Na), carbon (C), nitrogen (N), or oxygen (O), whose
distinctly different atoms serve as the basic building blocks of all matter. There are 92 naturally occurring
elements. Another 15 have been made in laboratories. Two or more elements combine to form compounds
that make up most of the world's matter. See compound.
Emission inventory. A list of air pollutants emitted into a community's, state's, nation's, or the Earth's atmosphere
in amounts per some unit time (e.g. day or year) by type of source. An emission inventory has both political
and scientific applications.
Emissions coefficient/factor.2 A unique value for scaling emissions to activity data in terms of a standard rate of
emissions per unit of activity (e.g., grams of carbon dioxide emitted per barrel of fossil fuel consumed).
Emissions. 2 Releases of gases to the atmosphere (e.g., the release of carbon dioxide during fuel combustion).
Emissions can be either intended or unintended releases. See fugitive emissions.
Energy conservation.7 Reduction or elimination of unnecessary energy use and waste. See energy-efficiency.
Energy intensity.5 Ratio between the consumption of energy to a given quantity of output; usually refers to the amount
of primary or final energy consumed per unit of gross domestic product.
Energy quality.7 Ability of a form of energy to do useful work. High-temperature heat and the chemical energy in
fossil fuels and nuclear fuels are concentrated high quality energy. Low-quality energy such as low-
temperature heat is dispersed or diluted and cannot do much useful work.
Energy.3 The capacity for doing work as measured by the capability of doing work (potential energy) or the conversion
of this capability to motion (kinetic energy). Energy has several forms, some of which are easily convertible
and can be changed to another form useful for work. Most of the world's convertible energy comes from fossil
fuels that are burned to produce heat that is then used as a transfer medium to mechanical or other means in
order to accomplish tasks. In the United States, electrical energy is often measured in kilowatt-hours (kWh),
while heat energy is often measured in British thermal units (Btu).
Energy-efficiency. *** The ratio of the useful output of services from an article of industrial equipment to the energy
use by such an article; for example, vehicle miles traveled per gallon of fuel (mpg).
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Enhanced greenhouse effect.' The concept that the natural greenhouse effect has been enhanced by anthropogenic
emissions of greenhouse gases. Increased concentrations of carbon dioxide, methane, and nitrous oxide, CFCs,
HFCs, PFCs, SF6, NF3, and other photochemically important gases caused by human activities such as fossil
fuel consumption, trap more infra-red radiation, thereby exerting a warming influence on the climate. See
greenhouse gas, anthropogenic, greenhouse effect, climate.
Enhanced oil recovery.7 Removal of some of the heavy oil left in an oil well after primary and secondary recovery.
See primary oil recovery, secondary oil recovery.
Enteric fermentation.2 A digestive process by which carbohydrates are broken down by microorganisms into simple
molecules for absorption into the bloodstream of an animal.
Environment.7 All external conditions that affect an organism or other specified system during its lifetime.
Ethanol (C2H5OH). 8 Otherwise known as ethyl alcohol, alcohol, or grain spirit. A clear, colorless, flammable
oxygenated hydrocarbon with a boiling point of 78.5 degrees Celsius in the anhydrous state. In transportation,
ethanol is used as a vehicle fuel by itself (E100), blended with gasoline (E85), or as a gasoline octane enhancer
and oxygenate (10 percent concentration).
Evapotranspiration.10 The loss of water from the soil by evaporation and by transpiration from the plants growing
in the soil, which rises with air temperature.
Exponential growth.7 Growth in which some quantity, such as population size, increases by a constant percentage
of the whole during each year or other time period; when the increase in quantity over time is plotted, this type
of growth yields a curve shaped like the letter J.
Feedlot.7 Confined outdoor or indoor space used to raise hundreds to thousands of domesticated livestock. See
rangeland.
Fertilization, carbon dioxide.' An expression (sometimes reduced to 'fertilization') used to denote increased plant
growth due to a higher carbon dioxide concentration.
Fertilizer.7 Substance that adds inorganic or organic plant nutrients to soil and improves its ability to grow crops, trees,
or other vegetation. See organic fertilizer.
Flaring.9 The burning of waste gases through a flare stack or other device before releasing them to the air.
Fluidized bed combustion (FBC).7 Process for burning coal more efficiently, cleanly, and cheaply. A stream of hot
air is used to suspend a mixture of powdered coal and limestone during combustion. About 90 to 98 percent
of the sulfur dioxide produced during combustion is removed by reaction with limestone to produce solid
calcium sulfate.
Fluorocarbons.' Carbon-fluorine compounds that often contain other elements such as hydrogen, chlorine, or bromine.
Common fluorocarbons include chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs),
hydrofluorocarbons (HFCs), and perfluorocarbons (PFCs). See chlorofluorocarbons,
hydrochlorofluorocarbons, hydrofluorocarbons, perfluorocarbons.
Forcing mechanism.' A process that alters the energy balance of the climate system (i.e., changes the relative balance
between incoming solar radiation and outgoing infrared radiation from Earth). Such mechanisms include
changes in solar irradiance, volcanic eruptions, and enhancement of the natural greenhouse effect by emission
of carbon dioxide.
Forest.7 Terrestrial ecosystem (biome) with enough average annual precipitation (at least 76 centimeters or 30 inches)
to support growth of various species of trees and smaller forms of vegetation.
Fossil fuel. A general term for buried combustible geologic deposits of organic materials, formed from decayed plants
and animals that have been converted to crude oil, coal, natural gas, or heavy oils by exposure to heat and
pressure in the earth's crust over hundreds of millions of years. See coal, petroleum, crude oil, natural gas.
Fossil fuel combustion.' Burning of coal, oil (including gasoline), or natural gas. The burning needed to generate
energy release carbon dioxide by-products that can include unbumed hydrocarbons, methane, and carbon
monoxide. Carbon monoxide, methane, and many of the unbumed hydrocarbons slowly oxidize into carbon
dioxide in the atmosphere. Common sources of fossil fuel combustion include cars and electric utilities.
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Freon. See chlorofluorocarbon.
Fugitive emissions.2 Unintended gas leaks from the processing, transmission, and/or transportation of fossil fuels,
CFCs from refrigeration leaks, SF6 from electrical power distributor, etc.
Gasohol.7 Vehicle fuel consisting of a mixture of gasoline and ethyl or methyl alcohol; typically 10 to 23 percent
ethanol by volume.
General Aviation.8 That portion of civil aviation, which encompasses all facets of aviation except air carriers. It
includes any air taxis, commuter air carriers, and air travel clubs, which do not hold Certificates of Public
Convenience and Necessity. See air carriers.
General circulation model (GCM).' A global, three-dimensional computer model of the climate system which can
be used to simulate human-induced climate change. GCMs are highly complex and they represent the effects
of such factors as reflective and absorptive properties of atmospheric water vapor, greenhouse gas
concentrations, clouds, annual and daily solar heating, ocean temperatures and ice boundaries. The most recent
GCMs include global representations of the atmosphere, oceans, and land surface.
Geosphere.' The soils, sediments, and rock layers of the Earth's crust, both continental and beneath the ocean floors.
Geothermal energy.7 Heat transferred from the earth's molten core to under-ground deposits of dry steam (steam with
no water droplets), wet steam (a mixture of steam and water droplets), hot water, or rocks lying fairly close to
the earth's surface.
Global Warming Potential (GWP).' The index used to translate the level of emissions of various gases into a
common measure in order to compare the relative radiative forcing of different gases without directly
calculating the changes in atmospheric concentrations. GWPs are calculated as the ratio of the radiative
forcing that would result from the emissions of one kilogram of a greenhouse gas to that from the emission of
one kilogram of carbon dioxide over a period of time (usually 100 years). Gases involved in complex
atmospheric chemical processes have not been assigned GWPs. See lifetime.
Global warming. 10 The progressive gradual rise of the earth's surface temperature thought to be caused by the
greenhouse effect and responsible for changes in global climate patterns. See enhanced greenhouse effect,
greenhouse effect, climate change.
Grassland.7 Terrestrial ecosystem (biome) found in regions where moderate annual average precipitation (25 to 76
centimeters or 10 to 30 inches) is enough to support the growth of grass and small plants but not enough to
support large stands of trees.
Greenhouse effect.7 Trapping and build-up of heat in the atmosphere (troposphere) near the earth's surface. Some
of the heat flowing back toward space from the earth's surface is absorbed by water vapor, carbon dioxide,
ozone, and several other gases in the atmosphere and then reradiated back toward the earth's surface. If the
atmospheric concentrations of these greenhouse gases rise, the average temperature of the lower atmosphere
will gradually increase. See enhanced greenhouse effect, climate change, global warming.
Greenhouse gas (GHG).' Any gas that absorbs infrared radiation in the atmosphere. Greenhouse gases include, but
are not limited to, water vapor, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O),
hydrochlorofluorocarbons (HCFCs), ozone (O3), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and
sulfur hexafluoride (SF6). See carbon dioxide, methane, nitrous oxide, hydrochloroftuorocarbon, ozone,
hydrojluorocarbon, perfluorocarbon, sulfur hexafluoride.
Halocarbons.' Chemicals consisting of carbon, sometimes hydrogen, and either chlorine, fluorine, bromine or iodine.
Halons.' Compounds, also known as bromofluorocarbons, that contain bromine, fluorine, and carbon. They are
generally used as fire extinguishing agents and cause ozone depletion. Bromine is many times more effective
at destroying stratospheric ozone than chlorine. See ozone depleting substance.
Heat content.5 The amount of heat per unit mass released upon complete combustion.
Heat.7 Form of kinetic energy that flows from one body to another when there is a temperature difference between the
two bodies. Heat always flows spontaneously from a hot sample of matter to a colder sample of matter. This
is one way to state the second law of thermodynamics. See temperature.
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Higher heating value.5 Quantity of heat liberated by the complete combustion of a unit volume or weight of a fuel
assuming that the produced water vapor is completely condensed and the heat is recovered; also known as
gross calorific value. See lower heating value.
Histosol.9 Wet organic soils, such as peats and mucks.
Hydrocarbons.' Substances containing only hydrogen and carbon. Fossil fuels are made up of hydrocarbons. Some
hydrocarbon compounds are major air pollutants.
Hydrochlorofluorocarbons (HCFCs).' Compounds containing hydrogen, fluorine, chlorine, and carbon atoms.
Although ozone depleting substances, they are less potent at destroying stratospheric ozone than
chlorofluorocarbons (CFCs). They have been introduced as temporary replacements for CFCs and are also
greenhouse gases. See ozone depleting substance.
Hydroelectric power plant.7 Structure in which the energy of fading or flowing water spins a turbine generator to
produce electricity.
Hydrofluorocarbons (HFCs).' Compounds containing only hydrogen, fluorine, and carbon atoms. They were
introduced as alternatives to ozone depleting substances in serving many industrial, commercial, and personal
needs. HFCs are emitted as by-products of industrial processes and are also used in manufacturing. They do
not significantly deplete the stratospheric ozone layer, but they are powerful greenhouse gases with global
wanning potentials ranging from 140 (HFC-152a) to 11,700 (HFC-23).
Hydrologic cycle. The process of evaporation, vertical and horizontal transport of vapor, condensation, precipitation,
and the flow of water from continents to oceans. It is a major factor in determining climate through its
influence on surface vegetation, the clouds, snow and ice, and soil moisture. The hydrologic cycle is
responsible for 25 to 30 percent of the mid-latitudes' heat transport from the equatorial to polar regions.
Hydropower.7 Electrical energy produced by falling or flowing water. See hydroelectric power plant.
Hydrosphere.7 All the earth's liquid water (oceans, smaller bodies of fresh water, and underground aquifers), frozen
water (polar ice caps, floating ice, and frozen upper layer of soil known as permafrost), and small amounts of
water vapor in the atmosphere.
Industrial sector.8 Construction, manufacturing, agricultural and mining establishments.
Infrared radiation.' The heat energy that is emitted from all solids, liquids, and gases. In the context of the
greenhouse issue, the term refers to the heat energy emitted by the Earth's surface and its atmosphere.
Greenhouse gases strongly absorb this radiation in the Earth's atmosphere, and re-radiate some of it back
towards the surface, creating the greenhouse effect.
Inorganic compound.7 Combination of two or more elements other than those used to form organic compounds. See
organic compound.
Inorganic fertilizer.7 See synthetic fertilizer.
Intergovernmental Panel on Climate Change (IPCC).' The IPCC was established jointly by the United Nations
Environment Programme and the World Meteorological Organization in 1988. The purpose of the IPCC is
to assess information in the scientific and technical literature related to all significant components of the issue
of climate change. The IPCC draws upon hundreds of the world's expert scientists as authors and thousands
as expert reviewers. Leading experts on climate change and environmental, social, and economic sciences from
some 60 nations have helped the IPCC to prepare periodic assessments of the scientific underpinnings for
understanding global climate change and its consequences. With its capacity for reporting on climate change,
its consequences, and the viability of adaptation and mitigation measures, the IPCC is also looked to as the
official advisory body to the world's governments on the state of the science of the climate change issue. For
example, the IPCC organized the development of internationally accepted methods for conducting national
greenhouse gas emission inventories.
Irreversibilities.10 Changes that, once set in motion, cannot be reversed, at least on human time scales.
Jet fuel8 Includes both naphtha-type and kerosene-type fuels meeting standards for use in aircraft turbine engines.
Although most jet fuel is used in aircraft, some is used for other purposes such as generating electricity.
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Joule.' The energy required to push with a force of one Newton for one meter.
Kerogen.7 Solid, waxy mixture of hydrocarbons found in oil shale, with a fine grained sedimentary rock. When the
rock is heated to high temperatures, the kerogen is vaporized. The vapor is condensed and then sent to a
refinery to produce gasoline, heating oil, and other products. See oil shale, shale oil.
Kerosene.2 A petroleum distillate that has a maximum distillation temperature of 401 degrees Fahrenheit at the 10
percent recovery point, a final boiling point of 572 degrees Fahrenheit, and a minimum flash point of 100
degrees Fahrenheit. Used in space heaters, cookstoves, and water heaters, and suitable for use as an illuminant
when burned in wick lamps.
Kyoto Protocol.I0 This is an international agreement struck by 159 nations attending the Third Conference of Parties
(COP) to the United Nations Framework Convention on Climate Change (held in December of 1997 in Kyoto
Japan) to reduce worldwide emissions of greenhouse gases. If ratified and put into force, individual countries
have committed to reduce their greenhouse gas emissions by a specified amount. See Framework Convention
on Climate Change, Conference of Parties.
Landfill.7 Land waste disposal site in which waste is generally spread in thin layers, compacted, and covered with a
fresh layer of soil each day.
Lifetime (atmospheric).' The lifetime of a greenhouse gas refers to the approximate amount of time it would take for
the anthropogenic increment to an atmospheric pollutant concentration to return to its natural level (assuming
emissions cease) as a result of either being converted to another chemical compound or being taken out of the
atmosphere via a sink. This time depends on the pollutant's sources and sinks as well as its reactivity. The
lifetime of a pollutant is often considered in conjunction with the mixing of pollutants in the atmosphere; a long
lifetime will allow the pollutant to mix throughout the atmosphere. Average lifetimes can vary from about a
week (e.g., sulfate aerosols) to more than a century (e.g., CFCs, carbon dioxide). See residence time.
Light-duty vehicles.8 Automobiles and light trucks combined.
Lignite.2 A brownish-black coal of low rank with high inherent moisture and volatile matter content, used almost
exclusively for electric power generation. Also referred to as brown coal.
Liquefied natural gas (LNG).7 Natural gas converted to liquid form by cooling to a very low temperature.
Liquefied petroleum gas (LPG).2 Ethane, ethylene, propane, propylene, normal butane, butylene, and isobutane
produced at refineries or natural gas processing plants, including plants that fractionate new natural gas plant
liquids.
Litter.' Undecomposed plant residues on the soil surface. See decomposition.
Longwave radiation.9 The radiation emitted in the spectral wavelength greater than 4 micrometers corresponding to
the radiation emitted from the Earth and atmosphere. It is sometimes referred to as terrestrial radiation or
infrared radiation, although somewhat imprecisely. See infrared radiation.
Low Emission Vehicle (LEV).8 A vehicle meeting the low-emission vehicle standards.
Lower heating value.s Quantity of heat liberated by the complete combustion of a unit volume or weight of a fuel
assuming that the produced water remains as a vapor and the heat of the vapor is not recovered; also known
as net calorific value. See higher heating value.
Lubricant2 A substance used to reduce friction between bearing surfaces or as a process material, either incorporated
into other materials used as aids in manufacturing processes or as carriers of other materials. Petroleum
lubricants may be produced either from distillates or residues. Other substances may be added to impart or
improve useful properties. Does not include by-products of lubricating oil from solvent extraction or tars
derived from de-asphalting. Lubricants include all grades of lubricating oils from spindle oil to cylinder oil
and those used in greases. Lubricant categories are paraffinic and naphthenic.
Manure.7 Dung and urine of animals that can be used as a form of organic fertilizer.
Mass balance.' The application of the principle of the conservation of matter.
Mauna Loa.' An intermittently active volcano 13,680 feet (4,170 meters) high in Hawaii.
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Methane (CH4).' A hydrocarbon that is a greenhouse gas with a global warming potential most recently estimated at
21. Methane is produced through anaerobic (without oxygen) decomposition of waste in landfills, animal
digestion, decomposition of animal wastes, production and distribution of natural gas and petroleum, coal
production, and incomplete fossil fuel combustion. The atmospheric concentration of methane as been shown
to be increasing at a rate of about 0.6 percent per year and the concentration of about 1.7 per million by volume
(ppmv) is more than twice its pre-industrial value. However, the rate of increase of methane in the atmosphere
may be stabilizing.
Methanol (CH3OH).8 A colorless poisonous liquid with essentially no odor and little taste. It is the simplest alcohol
with a boiling point of 64.7 degrees Celsius. In transportation, methanol is used as a vehicle fuel by itself
(M100), or blended with gasoline (M85).
Methanotrophic.7 Having the biological capacity to oxidize methane to CO2 and water by metabolism under aerobic
conditions. See aerobic.
Methyl bromide (CH3Br). " An effective pesticide; used to fumigate soil and many agricultural products. Because
it contains bromine, it depletes stratospheric ozone when released to the atmosphere. See ozone depleting
substance.
Metric ton.' Common international measurement for the quantity of greenhouse gas emissions. A metric ton is equal
to 1000 kilograms, 2204.6 pounds, or 1.1023 short tons.
Mineral.7 Any naturally occurring inorganic substance found in the earth's crust as a crystalline solid.
Model year.8 Refers to the "sales" model year; for example, vehicles sold during the period from October 1 to the next
September 31 is considered one model year.
Molecule. 7 Chemical combination of two or more atoms of the same chemical element (such as O2) or different
chemical elements (such as H2O).
Montreal Protocol on Substances that Deplete the Ozone Layer. " The Montreal Protocol and its amendments
control the phaseout of ozone depleting substances production and use. Under the Protocol, several
international organizations report on the science of ozone depletion, implement projects to help move away
from ozone depleting substances, and provide a forum for policy discussions. In the United States, the
Protocol is implemented under the rubric of the Clean Air Act Amendments of 1990. See ozone depleting
substance, ozone layer.
Motor gasoline.2 A complex mixture of relatively volatile hydrocarbons, with or without small quantities of additives,
obtained by blending appropriate refinery streams to form a fuel suitable for use in spark-ignition engines.
Motor gasoline includes both leaded and unleaded grades of finished gasoline, blending components, and
gasohol.
Municipal solid waste (MSW).2 Residential solid waste and some non-hazardous commercial, institutional, and
industrial wastes. This material is generally sent to municipal landfills for disposal. See landfill.
Naphtha.2 A generic term applied to a petroleum fraction with an approximate boiling range between 122 and 400
degrees Fahrenheit.
Natural gas.7 Underground deposits of gases consisting of 50 to 90 percent methane (CH4) and small amounts of
heavier gaseous hydrocarbon compounds such as propane (C3H4) and butane (C4H10).
Natural gas liquids (NGLs).2 Those hydrocarbons in natural gas that are separated as liquids from the gas. Includes
natural gas plant liquids and lease condensate.
Nitrogen cycle.7 Cyclic movement of nitrogen in different chemical forms from the environment, to organisms, and
then back to the environment.
Nitrogen fixation. 7 Conversion of atmospheric nitrogen gas into forms useful to plants and other organisms by
lightning, bacteria, and blue-green algae; it is part of the nitrogen cycle.
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Nitrogen oxides (NOJ.' Gases consisting of one molecule of nitrogen and varying numbers of oxygen molecules.
Nitrogen oxides are produced, for example, by the combustion of fossil fuels in vehicles and electric power
plants. In the atmosphere, nitrogen oxides can contribute to formation of photochemical ozone (smog), impair
visibility, and have health consequences; they are considered pollutants.
Nitrous oxide (N2O).' A powerful greenhouse gas with a global warming potential most recently evaluated at 310.
Major sources of nitrous oxide include soil cultivation practices, especially the use of commercial and organic
fertilizers, fossil fuel combustion, nitric acid production, and biomass burning.
Nonbiodegradable. 7 Substance that cannot be broken down in the environment by natural processes. See
biodegradable.
Nonlinearities.I0 Occur when changes in one variable cause a more than proportionate impact on another variable.
Non-methane volatile organic compounds (NMVOCs).2 Organic compounds, other than methane, that participate
in atmospheric photochemical reactions.
Non-point source. 7 Large land area such as crop fields and urban areas that discharge pollutant into surface and
underground water over a large area. See point source.
Nuclear electric power.3 Electricity generated by an electric power plant whose turbines are driven by steam generated
in a reactor by heat from the fissioning of nuclear fuel.
Nuclear energy.7 Energy released when atomic nuclei undergo a nuclear reaction such as the spontaneous emission
of radioactivity, nuclear fission, or nuclear fusion.
Oil shale.7 Underground formation of a fine-grained sedimentary rock containing varying amounts of kerogen, a solid,
waxy mixture of hydrocarbon compounds. Heating the rock to high temperatures converts the kerogen to a
vapor, which can be condensed to form a slow flowing heavy oil called shale oil. See kerogen, shale oil.
Oil. See crude oil, petroleum.
Ore.7 Mineral deposit containing a high enough concentration of at least one metallic element to permit the metal to
be extracted and sold at a profit.
Organic compound.7 Molecule that contains atoms of the element carbon, usually combined with itself and with
atoms of one or more other element such as hydrogen, oxygen, nitrogen, sulfur, phosphorus, chlorine, or
fluorine. See inorganic compound.
Organic fertilizer.7 Organic material such as manure or compost, applied to cropland as a source of plant nutrients.
Oxidize.2 To chemically transform a substance by combining it with oxygen.
Oxygen cycle.7 Cyclic movement of oxygen in different chemical forms from the environment, to organisms, and then
back to the environment.
Ozone.6 A colorless gas with a pungent odor, having the molecular form of O3, found in two layers of the atmosphere,
the stratosphere and the troposphere. Ozone is a form of oxygen found naturally in the stratosphere that
provides a protective layer shielding the Earth from ultraviolet radiation's harmful health effects on humans
and the environment. In the troposphere, ozone is a chemical oxidant and major component of photochemical
smog. Ozone can seriously affect the human respiratory system. .
Ozone Depleting Substance (ODS). " A family of man-made compounds that includes, but are not limited to,
chlorofluorocarbons (CFCs), bromofluorocarbons (halons), methyl chloroform, carbon tetrachloride, methyl
bromide, and hydrochlorofluorocarbons (HCFCs). These compounds have been shown to deplete stratospheric
ozone, and therefore are typically referred to as ODSs.
Ozone layer. 7 Layer of gaseous ozone (O3) in the stratosphere that protects life on earth by filtering out harmful
ultraviolet radiation from the sun. See stratosphere, ultraviolet radiation.
Ozone precursors.2 Chemical compounds, such as carbon monoxide, methane, non-methane hydrocarbons, and
nitrogen oxides, which in the presence of solar radiation react with other chemical compounds to form ozone,
mainly in the troposphere. See troposphere
T-12 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
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Particulate matter (PM).7 Solid particles or liquid droplets suspended or carried in the air.
Particulates. See paniculate matter.
Parts per billion (ppb). 'Number of parts of a chemical found in one billion parts of a particular gas, liquid, or solid
mixture. See concentration.
Parts per million (ppm).7 Number of parts of a chemical found in one million parts of a particular gas, liquid, or
solid. See concentration.
Pentanes plus.2 A mixture of hydrocarbons! mostly pentanes and heavier fractions, extracted from natural gas.
Perfluorocarbons (PFCs). ' A group of human-made chemicals composed of carbon and fluorine only. These
chemicals (predominantly CF4 and C2F6) were introduced as alternatives, along with hydrofluorocarbons, to
the ozone depleting substances. In addition, PFCs are emitted as by-products of industrial processes and are
also used in manufacturing. PFCs do not harm the stratospheric ozone layer, but they are powerful greenhouse
gases: CF4 has a global warming potential (GWP) of 6,500 and C2F6 has a GWP of 9,200.
Petrochemical feedstock.2 Feedstock derived from petroleum, used principally for the manufacture of chemicals,
synthetic rubber, and a variety of plastics. The categories reported are naphtha (endpoint less than 401 degrees
Fahrenheit) and other oils (endpoint equal to or greater than 401 degrees Fahrenheit).
Petrochemicals.7 Chemicals obtained by refining (i.e., distilling) crude oil. They are used as raw materials in the
manufacture of most industrial chemicals, fertilizers, pesticides, plastics, synthetic fibers, paints, medicines,
and many other products. See crude oil.
Petroleum coke.2 A residue that is the final product of the condensation process in cracking.
Petroleum.2 A generic term applied to oil and oil products in all forms, such as crude oil, lease condensate, unfinished
oils, petroleum products; natural gas plant liquids, and non-hydrocarbon compounds blended into finished
petroleum products. See crude oil.
Photosynthesis.7 Complex process that takes place in living green plant cells. Radiant energy from the sun is used
to combine carbon dioxide (COj) and water (H2O) to produce oxygen (O2) and simple nutrient molecules, such
as glucose (C5HI2O6).
Photovoltaic and solar thermal energy.2 Energy radiated by the sun as electromagnetic waves (electromagnetic
radiation) that is converted into electricity by means of solar (i.e., photovoltaic) cells or useable heat by
concentrating (i.e., focusing) collectors.
Point source.7 A single identifiable source that discharges pollutants into the environment. Examples are smokestack,
sewer, ditch, or pipe. See non-point source.
Pollution.7 A change in the physical, chemical, or biological characteristics of the air, water, or soil that can affect the
health, survival, or activities of humans in an unwanted way. Some expand the term to include harmful effects
on all forms of life.
Poly vinyl chloride (PVC).2 A polymer of vinyl chloride. It is tasteless, odorless and insoluble in most organic
solvents. A member of the family vinyl resin, used in soft flexible films for food packaging and in molded
rigid products, such as pipes, fibers, upholstery, and bristles.
Population.7 Group of individual organisms of the same species living within a particular area.
Prescribed burning.7 Deliberate setting and careful control of surface fires in forests to help prevent more destructive
fires and to kill off unwanted plants that compete with commercial species for plant nutrients; may also be used
on grasslands.
Primary oil recovery.7 Pumping out the crude oil that flows by gravity into the bottom of an oil well. See enhanced
oil recovery, secondary oil recovery.
Quad.8 Quad stands for quadrillion, which is, 1015
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Radiation.' Energy emitted in the form of electromagnetic waves. Radiation has differing characteristics depending
upon the wavelength. Because the radiation from the Sun is relatively energetic, it has a short wavelength (e.g.,
ultraviolet, visible, and near infrared) while energy re-radiated from the Earth's surface and the atmosphere has
a longer wavelength (e.g., infrared radiation) because the Earth is cooler than the Sun. See ultraviolet
radiation, infrared radiation, solar radiation, longwave radiation, terrestrial radiation.
Radiative forcing.' A change in the balance between incoming solar radiation and outgoing infrared (i.e., thermal)
radiation. Without any radiative forcing, solar radiation coming to the Earth would continue to be
approximately equal to the infrared radiation emitted from the Earth. The addition of greenhouse gases to the
atmosphere traps an increased fraction of the infrared radiation, reradiating it back toward the surface of the
Earth and thereby creates a warming influence.
Rail. * Includes "heavy" and "light" transit rail. Heavy transit rail is characterized by exclusive rights-of-way, multi-car
trains, high speed rapid acceleration, sophisticated signaling, and high platform loading. Also known as
subway, elevated railway, or metropolitan railway (metro). Light transit rail may be on exclusive or shared
rights of way, high or low platform, multi-car trains or single cars, automated or manually operated. In generic
usage, light rail includes streetcars, trolley cars, and tramways.
Rangeland.7 Land, mostly grasslands, whose plants can provide food (i.e., forage) for grazing or browsing animals.
Seefeedlot.
Recycling.7 Collecting and reprocessing a resource so it can be used again. An example is collecting aluminum cans,
melting them down, and using the aluminum to make new cans or other aluminum products.
Reforestation.2 Replanting of forests on lands that have recently been harvested.
Renewable energy.2 Energy obtained from sources that are essentially inexhaustible, unlike, for example, the fossil
fuels, of which there is a finite supply. Renewable sources of energy include wood, waste, geothermal, wind,
photovoltaic, and solar thermal energy. See hydropower, photovoltaic.
Residence time.' Average time spent in a reservoir by an individual atom or molecule. Also, this term is used to define
the age of a molecule when it leaves the reservoir. With respect to greenhouse gases, residence time usually
refers to how long a particular molecule remains in the atmosphere. See lifetime.
Residential sector.3 An area or portion consisting only of housing units.
Residual fuel oil.2 The heavier oils that remain after the distillate fuel oils and lighter hydrocarbons are distilled away
in refinery operations and that conform to ASTM Specifications D396 and D975. Included are No. 5, a
residual fuel oil of medium viscosity; Navy Special, for use in steam-powered vessels in government service
and in shore power plants; and No. 6, which includes Bunker C fuel oil and is used for commercial and
industrial heating, electricity generation, and to power ships. Imports of residual fuel oil include imported crude
oil bumed as fuel.
Secondary oil recovery. 7 Injection of water into an oil well after primary oil recovery to force out some of the
remaining thicker crude oil. See enhanced oil recovery, primary oil recovery.
Sector. Division, most commonly used to denote type of energy consumer (e.g., residential) or according to the
Intergovernmental Panel on Climate Change, the type of greenhouse gas emitter (e.g. industrial process). See
Intergovernmental Panel on Climate Change.
Septic tank.7 Underground tank for treatment of wastewater from a home in rural and suburban areas. Bacteria in the
tank decompose organic wastes and the sludge settles to the bottom of the tank. The effluent flows out of the
tank into the ground through a field of drainpipes.
Sewage treatment (primary).7 Mechanical treatment of sewage in which large solids are filtered out by screens and
suspended solids settle out as sludge in a sedimentation tank.
Shale oil.7 Slow-flowing, dark brown, heavy oil obtained when kerogen in oil shale is vaporized at high temperatures
and then condensed. Shale oil can be refined to yield gasoline, heating oil, and other petroleum products. See
kerogen, oil shale.
T-14 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
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Short ton.' Common measurement for a ton in the United States. A short ton is equal to 2,000 Ibs. or 0.907 metric
tons.
Sink.' A reservoir that uptakes a pollutant from another part of its cycle. Soil and trees tend to act as natural sinks for
carbon.
Sludge.7 Gooey solid mixture of bacteria and virus laden organic matter, toxic metals, synthetic organic chemicals,
and solid chemicals removed from wastewater at a sewage treatment plant.
Soil.7 Complex mixture of inorganic minerals (i.e., mostly clay, silt, and sand), decaying organic matter, water, air, and
living organisms.
Soil carbon.9 A major component of the terrestrial biosphere pool in the carbon cycle. The amount of carbon in the
soil is a function of the historical vegetative cover and productivity, which in turn is dependent in part upon
climatic variables.
Solar energy.7 Direct radiant energy from the sun. It also includes indirect forms of energy such as wind, falling or
flowing water (hydropower), ocean thermal gradients, and biomass, which are produced when direct solar
energy interact with the earth. See solar radiation.
Solar radiation.' Energy from the Sun. Also referred to as short-wave radiation. Of importance to the climate system,
solar radiation includes ultra-violet radiation, visible radiation, and infrared radiation.
Source." Any process or activity that releases a greenhouse gas, an aerosol, or a precursor of a greenhouse gas into the
atmosphere.
Special naphtha.2 All finished products within the naphtha boiling range that are used as paint thinners, cleaners, or
solvents. Those products are refined to a specified flash point.
Still gas.2 Any form or mixture of gases produced in refineries by distillation, cracking, reforming, and other processes.
Principal constituents are methane, ethane, ethylene, normal butane, butylene, propane, propylene, etc. Used
as a refinery fuel and as a petrochemical feedstock.
Stratosphere.7 Second layer of the atmosphere, extending from about 19 to 48 kilometers (12 to 30 miles) above the
earth's surface. It contains small amounts of gaseous ozone (O3), which filters out about 99 percent of the
incoming harmful ultraviolet (UV) radiation. Most commercial airline flights operate at a cruising altitude in
the lower stratosphere. See ozone layer, ultraviolet radiation.
Stratospheric ozone. See ozone layer.
Strip mining.7 Cutting deep trenches to remove minerals such as coal and phosphate found near the earth's surface
in flat or rolling terrain. See surface mining.
Subbituminous coal.2 A dull, black coal of rank intermediate between lignite and bituminous coal.
Sulfur cycle.7 Cyclic movement of sulfur in different chemical forms from the environment, to organisms, and then
back to the environment.
Sulfur dioxide (SOz).' A compound composed of one sulfur and two oxygen molecules. Sulfur dioxide emitted into
the atmosphere through natural and anthropogenic processes is changed in a complex series of chemical
reactions in the atmosphere to sulfate aerosols. These aerosols are believed to result in negative radiative
forcing (i.e., tending to cool the Earth's surface) and do result in acid deposition (e.g., acid rain). See aerosols,
radiative forcing, acid deposition, acid rain.
Sulfur hexafluoride (SF6).' A colorless gas soluble in alcohol and ether, slightly soluble in water. A very powerful
greenhouse gas used primarily in electrical transmission and distribution systems and as a dielectric in
electronics. The global warming potential of SF6 is 23,900. See Global Warming Potential.
Surface mining.7 Removal of soil, sub-soil, and other strata and then extracting a mineral deposit found fairly close
to the earth's surface. See strip mining.
Synthetic fertilizer.7 Commercially prepared mixtures of plant nutrients such as nitrates, phosphates, and potassium
applied to the soil to restore fertility and increase crop yields. See organic fertilizer.
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Synthetic natural gas (SNG).3 A manufactured product chemically similar in most respects to natural gas, resulting
from the conversion or reforming of petroleum hydrocarbons. It may easily be substituted for, or interchanged
with, pipeline quality natural gas.
Tailings.7 Rock and other waste materials removed as impurities when minerals are mined and mineral deposits are
processed. These materials are usually dumped on the ground or into ponds.
Tar sand.7 Swamp-like deposit of a mixture of fine clay, sand, water, and variable amounts of tar-like heavy oil known
as bitumen. Bitumen can be extracted from tar sand by heating. It can then be purified and upgraded to
synthetic crude oil. See bitumen.
Temperature.7 Measure of the average speed of motion of the atoms or molecules in a substance or combination of
substances at a given moment. See heat.
Terrestrial.7 Pertaining to land.
Terrestrial radiation.9 The total infrared radiation emitted by the Earth and its atmosphere in the temperature range
of approximately 200 to 300 Kelvin. Terrestrial radiation provides a major part of the potential energy changes
necessary to drive the atmospheric wind system and is responsible for maintaining the surface air temperature
within limits of livability.
Trace gas.' Any one of the less common gases found in the Earth's atmosphere. Nitrogen, oxygen, and argon make
up more than 99 percent of the Earth's atmosphere. Other gases, such as carbon dioxide, water vapor, methane,
oxides of nitrogen, ozone, and ammonia, are considered trace gases. Although relatively unimportant in terms
of their absolute volume, they have significant effects on the Earth's weather and climate.
Transportation sector.8 Consists of private and public passenger and freight transportation, as well as government
transportation, including military operations.
Troposphere.1&7 The lowest layer of the atmosphere and contains about 95 percent of the mass of air in the Earth's
atmosphere. The troposphere extends from the Earth's surface up to about 10 to 15 kilometers. All weather
processes take place in the troposphere. Ozone that is formed in the troposphere plays a significant role in both
the greenhouse gas effect and urban smog. See ozone precursor, stratosphere, atmosphere.
Tropospheric ozone precursor. See ozone precursor.
Tropospheric ozone.' See ozone.
Ultraviolet radiation (UV)." A portion of the electromagnetic spectrum with wavelengths shorted than visible light.
The sun produces UV, which is commonly split into three bands of decreasing wavelength. Shorter wavelength
radiation has a greater potential to cause biological damage on living organisms. The longer wavelength
ultraviolet band, UVA, is not absorbed by ozone in the atmosphere. UVB is mostly absorbed by ozone,
although some reaches the Earth. The shortest wavelength band, UVC, is completely absorbed by ozone and
normal oxygen in the atmosphere.
Unfinished oils.3 All oils requiring further refinery processing, except those requiring only mechanical blending.
Includes naphtha and lighter oils, kerosene and light gas oils, heavy gas oils, and residuum.
United Nations Framework Convention on Climate Change (UNFCCC).' The international treaty unveiled at the
United Nations Conference on Environment and Development (UNCED) in June 1992. The UNFCCC
commits signatory countries to stabilize anthropogenic (i.e. human-induced) greenhouse gas emissions to
"levels that would prevent dangerous anthropogenic interference with the climate system". The UNFCCC also
requires that all signatory parties develop and update national inventories of anthropogenic emissions of all
greenhouse gases not otherwise controlled by the Montreal Protocol. Out of 155 countries that have ratified
this accord, the United States was the first industrialized nation to do so.
Vehicle miles traveled (VMT).8 One vehicle traveling the distance of one mile. Thus, total vehicle miles is the total
mileage traveled by all vehicles.
Volatile organic compounds (VOCs).6 Organic compounds that evaporate readily into the atmosphere at normal
temperatures. VOCs contribute significantly to photochemical smog production and certain health problems.
See non-methane volatile organic compounds.
T-16, Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998
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Wastewater.2 Water that has been used and contains dissolved or suspended waste materials. See sewage treatment.
Water vapor.' The most abundant greenhouse gas; it is the water present in the atmosphere in gaseous form. Water
vapor is an important part of the natural greenhouse effect. While humans are not significantly increasing its
concentration, it contributes to the enhanced greenhouse effect because the warming influence of greenhouse
gases leads to a positive water vapor feedback. In addition to its role as a natural greenhouse gas, water vapor
plays an important role in regulating the temperature of the planet because clouds form when excess water
vapor in the atmosphere condenses to form ice and water droplets and precipitation.
Waxes.2 Solid or semisolid materials derived from petroleum distillates or residues. Light-colored, more or less
translucent crystalline masses, slightly greasy to the touch, consisting of a mixture of solid hydrocarbons in
which the paraffin series predominates. Included are all marketable waxes, whether crude scale or fully
refined. Used primarily as industrial coating for surface protection.
Weather.' Weather is the specific condition of the atmosphere at a particular place and time. It is measured in terms
of such things as wind, temperature, humidity, atmospheric pressure, cloudiness, and precipitation. In most
places, weather can change from hour-to-hour, day-to-day, and season-to-season. Climate is the average of
weather over time and space. A simple way of remembering the difference is that climate is what you expect
(e.g. cold winters) and 'weather' is what you get (e.g. a blizzard). See climate.
Wetland.7 Land that stays flooded all or part of the year with fresh or salt water.
Wetlands.2 Areas regularly saturated by surface or groundwater and subsequently characterized by a prevalence of
vegetation adapted for life in saturated-soil conditions.
Wood energy.2 Wood and wood products used as fuel, including roundwood (i.e., cordwood), limbwood, wood chips,
bark, sawdust, forest residues, and charcoal.
References
1 U.S. Environmental Protection Agency, Global Warming website, http://www.epa.gov/globalwarming. February 26, 1999.
2 Energy Information Administration, Emissions of Greenhouse Gases in the United States 1997, DOE/EIA-0573(97), U.S.
Department of Energy, Washington, DC. October 1998. [See http://www.eia.doe.gov]
3 Energy Information Administration, Annual Energy Review 1997, DOE/EIA-03 87(97), U.S. Department of Energy, Washington,
iJ\-r*y July Lyyo,
4 United Nations Framework Convention on Climate Change. [See http://www.unfccc.de]
5 Intergovernmental Panel on Climate Change, Climate Change 1995: The Science of Climate Change, Cambridge University Press:
New York, 1996
6 Cooper's Comprehensive Environmental Desk Reference, Arthur R. Cooper, Sr., Van Nostrand Reinhold: New York, 1996.
7 Miller, G. Tyler, Jr., Living in the Environment, An Introduction to Environment Science, sixth edition, 1990.
8 Davis, Stacy, Transportation Energy Data Book, Oak Ridge National Laboratory, U.S. Department of Energy, Edition 17, 1997.
'Carbon Dioxide Information Analysis Center, website at http://cdiac.esd.ornl.gov, Oak Ridge National Laboratory, U.S. Department
of Energy, February 26,1999.
10 Resources for the Future, Weathervane website, http://www.weathervane.rff.org/glossary/index.htnil, February 26, 1999.
11 U.S. Environmental Protection Agency, Ozone Depletion Glossary, http://www.epa.gov/ozone/defns.html, February 26, 1999.
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Mobile Air Conditioning: Air conditioning systems in cars; trucks, buses, and trains primarily use the
refrigerant HFC-134a. The quantity of refrigerant in a typical car air conditioner is .small, but because of
millions of air conditioned vehicles on the road, plus the increasing number of miles those vehicles are
being driven, mobile air conditioners are the largest user of HFC-134a in the United States.
Air conditioning equipment: A number of HFCs are used in refrigeration and air conditioning systems,
with the result that emissions of HFCs occur during operation and repair of these systems. HFCs have
global warming potentials (GWPs) that range from 140 to 11,700 times the warming potential of carbon
dioxide. The use of recapture and recycling equipment such as that shown.significantly reduces emis-
sions of these refrigerants into the atmosphere. - -
Refrigeration: Household refrigerators use the refrigerant HFC-134a, and so do many of the processes
that allow cold food to arrive fresh on the way to your home. Cold storage refrigeration, refrigerated
transport, and retail food refrigerators use HFC-134a~or other HFC refrigerant blends. The amount of
refrigerant contained in household refrigerators is typically small, but because of the large number of
households in the United States, household refrigeration is considered an important HFC-134a end-use.
Additionally, HFCs are used in the insulating foam of the refrigerator unit.
Aluminum Production: Primary aluminum smelting from alumina ore results in emissions two PFCs,
CF4 and C2F6. These PFCs have global warming potentials of 6,500 and 9,200 times the warming poten-
tial of carbon dioxide, respectively. PFCs are intermittent by-products of the smelting process, occurring
when the alumina ore content of the electrolytic bath falls below critical levels optimal for the chemical
reactions to take place.
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