United States Office of Air and Radiation
Environmental Protection Washington D C 20460
Agency
EPA452/R-97-002
August 1997
Air
&EPA Nitrogen Oxides: Impacts on
Public Health and the Environment
Stratospheric
Ozone
Depletion
Global
Warming
Ozone,
Participate
Matter, and
Nitrogen
Dioxide
Acid
Deposition
Visibility, Drinking Water and
Ecosystem Protection
Toxic Products
Eutrophication
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Nitrogen Oxides:
Impacts On Public Health and the
Environment
Office of Air and Radiation
United States Environmental Protection Agency
August 1997
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Acknowledgments
This report was prepared by a team within the United States
Environmental Protection Agency's (EPA) Office of Air and
Radiation and was coordinated with EPA's Office of Water. The
principal author of this report was Doug Grano, Office of Air
Quality Planning and Standards (OAQPS), Air Quality Strategies
and Standards Division, Research Triangle Park NC 27711. Authors
of specific sections were:
Doris Price and Rona Birnbaum
Office of Atmospheric Programs
Acid Rain Division, EPA
401 M Street S.W.
Washington D.C. 20460
Acid Deposition section;
Richard Batiuk,
Chesapeake Bay Program Office, EPA
410 Severn Avenue, Suite 109
Annapolis MD 21403
and
Melissa McCullough,
OAQPS, EPA
Research Triangle Park NC 27711
Eutrophication section; and
Dr. Roy Smith,
OAQPS, EPA
Research Triangle Park NC 27711
Toxic Products section.
Additional contributors to this report were as follows: John
Ackermann (EPA, OAQPS); John Bachmann (EPA, OAQPS); Dr. M. Robins
Church (EPA, National Health and Environmental Effects Research
Lab, Corvallis, OR); Stephen Clark (EPA, Office of Ground Water
and Drinking Water); Ted Creekmore (EPA, OAQPS); Robin Dunkins
(EPA, OAQPS); Chybryll Edwards (EPA, OAQPS); Barry Gilbert (EPA,
OAQPS); Margaret Kerchner (National Oceanic and Atmospheric
Administration, Chesapeake Bay Office); Arnold Kuzmack, (EPA,
Office of Science and Technology); Chris Lindhjem (EPA, Office of
Mobile Sources); Ned Meyer (EPA, OAQPS); Roberta Parry (EPA,
Office of Policy Planning and Evaluation); Bruce Polkowsky (EPA,
OAQPS); Dr. Doug Ryan (United States Forest Service) and Eric
Slaughter (EPA, Oceans and Coastal Protection Division, Office of
Wetlands, Oceans, and Watersheds).
This document was reviewed by the following scientific
experts: Dr. Ellis Cowling (North Carolina State University), Dr.
Rudolph Husar (Washington University at St. Louis, MO), Dr.
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Harvey Jefferies (University of North Carolina, Chapel Hill), Dr.
Hans Paerl (University of North Carolina, Chapel Hill), Dr.
Richard Stolarski (National Aeronautics and Space
Administration), and Dr. Mark Utell (University of Rochester
Medical Center, NY). Their constructive comments provided
critical assistance to improving the scientific perspective of
the final report.
Finally, public comments were invited on the November 1996
draft of the document; discussion of the document and invitation
to comment were provided through the Ozone Transport Assessment
Group's Implementation Strategies and Issues Workgroup and at the
December 5, 1996 Clean Air Act Advisory Committee meeting. The
final document includes consideration by the authors of the
comments received from the public. Comments were received from:
Colorado State University, Desert Research Institute, Houston
Lighting & Power, Pacific Enterprises, Southern Company, Henry L.
Stadler, and United States Forest Service.
Copies of this report may be downloaded from the Office of
Air and Radiation Policy and Guidance website at
http://www.epa.gov/ttn. Questions regarding this report should
be directed to Doug Grano at (919) 541-3292.
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Nitrogen Oxides:
Impacts on Public Health and the Environment
Table of Contents
Executive Summary 1
I. Introduction/Overview 8
II. Clean Air Act Programs Involving Decreases
in Nitrogen Oxides (NOX) Emissions
A. Acid Deposition 15
B. Nitrogen Dioxide 31
C. Ozone 36
D. Particulate Matter 52
E. Visibility Protection 65
III. Additional Public Health and Environmental
Impacts from NOX Emissions
A. Drinking Water 75
B. Eutrophication 79
C. Global Warming 93
D. Stratospheric Ozone Depletion 100
E. Terrestrial Ecosystems 104
F. Toxic Products 109
IV. Interprogram Issues
A. Local and Regional NOX Controls 112
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B. Timing of NOX Emissions Reductions:
Seasonal or Year-Round 120
C. Interface with Other Control Programs:
Three Examples of Secondary Emissions,
EPA's Clean Air Power Initiative, and
New Standards for Ozone and
Particulate Matter 126
Appendices
A. Mobile Source Programs 134
B. Stationary Source Programs 140
C. Sources and Sinks of Atmospheric
Nitrogen 146
D. Acronyms and Abbreviations 155
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Figures and Tables
Page
10
Figure 1-1 National Anthropogenic NOX Emissions by
Source Category for 1994 and 2000
Figure 1-2 National Total NOX Emissions by Source
Category for 1990 11
Figure 1-3 Trends in NOX Emissions for the Period
1940 to 1994 13
Figure II-1 Map of Counties Potentially Not Meeting
the 8-Hour Ozone Standard 39
Figure II-2 Map of Areas Not Meeting the 1-Hour
Ozone Standard 40
Figure II-3 Map of Areas Not Meeting the Particulate
Matter (PM10) Standard 54
Figure II-4 Yearly Average Absolute and Relative
Concentrations for Sulfate and Nitrate 59
Figure III-l Greenhouse Gas Emissions 94
Figure III-2 Nitrous Oxide Emission Sources 96
Figure C-l Plants in 1990 with Greater Than 1,000 Tons
Per Year of NOX Emissions 147
Figure C-2 Density Map of 1994 NOX Emissions 148
Figure C-3 Trends in On-Road NOX Emissions, Vehicle
Miles Traveled, Fuel Use, and Gasoline Price 150
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Table II-1 Utility Boiler Types and Emission Limits 17
Table II-2 Constituents of Atmospheric Fine Particles
Less Than 2.5 Microns and Their Major Sources 61
Table II-3 Constituents of Atmospheric Particles Greater
Than 2.5 Microns and Their Major Sources 62
Table II-4 Annual Averages (March 1988-February 1991)
of Reconstructed Light Extinction (Mm"1) for
19 Regions of the IMPROVE Network 72
Table III-l Estimated Reductions in Nitrogen Loadings to
Chesapeake Bay and Water Quality Response
Under Several Control Scenarios 89
Table III-2 Estimated global sources of Nitrous Oxide 97
Table IV-1 Seasonal Emissions of NOX, 1985 through 1994 124
Table C-l 1994 National NOX Emissions by Source Category 149
Table C-2 Total National Emissions of NOX, 1940 through
1994 151
Table C-3 Estimated Global Emissions of NOX Typical of the
Last Decade 152
Table C-4 Estimated Global Emissions of ML,
153
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Nitrogen Oxides:
Impacts On Public Health and the Environment
Executive Summary
Overview:
Over the past two decades, great progress has been made at the local, state and national
levels in controlling emissions from many sources of air pollution. However, pollutant levels
remain unacceptably high in many areas across the country. The Clean Air Act (CAA) specifies
deadlines for attainment of the ozone (O3) standards, yet continued industrial growth and
expansion of motor vehicle usage threaten to reverse past achievements. An abundance of O3
near the earth's surface results in damaging effects on human health, agricultural crops,
ornamental plants, forests, and materials.
For many years, control of volatile organic compounds (VOCs) was the main strategy
employed in efforts to decrease ground-level O3. More recently, it has become clearer that
decreases in emissions of nitrogen oxides (NOX) may be needed in many areas, especially in areas
where O3 concentrations are high over a large region (as in the Midwest, Northeast, and
Southeast). The 1991 National Academy of Sciences report entitled Rethinking the Ozone
Problem in Urban and Regional Air Pollution recommends that "To substantially reduce O3
concentrations in many urban, suburban, and rural areas of the United States, the control of NOX
emissions will probably be necessary in addition to, or instead of, the control of VOCs."
In addition to attainment of the public health standards for O3, decreases in emissions of
NOX are helpful to several other efforts to improve the environment. On a national scale,
decreases in NOX emissions will also decrease acid deposition, nitrates in drinking water, excessive
nitrogen loadings to aquatic and terrestrial ecosystems, and ambient concentrations of nitrogen
dioxide, particulate matter and toxics. On a global scale, decreases in NOX emissions will, to
some degree, reduce greenhouse gases and stratospheric O3 depletion. Thus, management of air
emissions is essential to both air quality and watershed protection on national and global scales.
In view of the need for NOX emissions decreases in the O3 program and the multiple
environmental benefits that would follow, EPA's Office of Air and Radiation, in coordination
with EPA's Office of Water, has begun implementing an integrated approach to achieve
substantial decreases in the emissions of NOX from mobile and stationary sources. In particular,
EPA's Offices of Air Quality Planning and Standards, Atmospheric Programs, and Mobile
Sources are implementing this strategy by taking a balanced approach to decreasing NOX
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emissions among several categories of mobile and stationary sources, considering costs,
effectiveness, alternatives, and opportunities for market incentives. This integrated approach
involves increased interaction among the air and water programs that are affected by various
forms of atmospheric nitrogen. This interaction is needed so that implementation of the NOX
emissions decreases occurs in a manner that best achieves the multiple public health and
environmental goals. Thus, policy decisions regarding the control of NOX emissions are being
made in the context of the many environmental effects associated with NOX emissions.
Multiple Public Health and Environmental Benefits Flow from NOX Emissions Decreases
The impact of NOX emissions on O3 concentrations is complex. Although NOX emissions
are necessary for the formation of O3 in the lower atmosphere, a local decrease in NOX emissions
can, in some cases, increase local O3 concentrations. This effect of NOX emissions decreases must
be carefully weighed against the multiple benefits than can be associated with decreasing NOX
emissions, including lowering regional O3 concentrations. It should be noted that, with EPA's
July 18, 1997 promulgation of the new O3 standards, greater emphasis might be needed on
regional-scale NOX emissions decreases to reach attainment because the new standards result in
more areas and larger areas with monitoring data indicating nonattainment. Specifically, NOX
emissions also contribute to adverse impacts to public health and the environment in the following
areas:
Acid Deposition: Sulfur dioxide and NOX are the two key air pollutants that cause acid
deposition (wet and dry particles and gases) and result in the adverse effects on aquatic
and terrestrial ecosystems, materials, visibility, and public health. Nitric acid deposition
plays a dominant role in the acid pulses associated with the fish kills observed during the
springtime melt of the snowpack in sensitive watersheds and recently has also been
identified as a major contributor to chronic acidification of certain sensitive surface waters.
Drinking Water Nitrate: High levels of nitrate in drinking water is a health hazard,
especially for infants. Atmospheric nitrogen deposition in sensitive watersheds can
increase stream water nitrate concentrations; the added nitrate can remain in the water and
be transported long distances downstream.
Eutrophication: NOX emissions contribute directly to the widespread accelerated
eutrophication of United States coastal waters and estuaries. Atmospheric nitrogen
deposition onto surface waters and deposition to watershed and subsequent transport into
the tidal waters has been documented to contribute from 12 to 44 percent of the total
nitrogen loadings to United States coastal waterbodies. Nitrogen is the nutrient limiting
growth of algae in most coastal waters and estuaries. Thus, addition of nitrogen results in
accelerated algae and aquatic plant growth causing adverse ecological effects and
economic impacts that range from nuisance algal blooms to oxygen depletion and fish
kills.
Global Warming: Nitrous oxide (N2O) is a greenhouse gas. Anthropogenic N2O
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emissions in the United States contribute about 2 percent of the greenhouse effect, relative
to total United States, anthropogenic emissions of greenhouse gases. In addition,
emissions of NOX lead to the formation of tropospheric O3, which is another greenhouse
gas.
Nitrogen Dioxide (NO2): Exposure to NO2 is associated with a variety of acute and
chronic health effects. The health effects of most concern at ambient or near-ambient
concentrations of NO2 include mild changes in airway responsiveness and pulmonary
function in individuals with pre-existing respiratory illnesses and increases in respiratory
illnesses in children. Currently, all areas of the United States monitoring NO2 are below
EPA's threshold for health effects.
Nitrogen Saturation of Terrestrial Ecosystems: Nitrogen accumulates in watersheds
with high atmospheric nitrogen deposition. Because most North American terrestrial
ecosystems are nitrogen limited, nitrogen deposition often has a fertilizing effect,
accelerating plant growth. Although this effect is often considered beneficial, nitrogen
deposition is causing important adverse changes in some terrestrial ecosystems, including
shifts in plant species composition and decreases in species diversity or undesirable nitrate
leaching to surface and ground water and decreased plant growth.
Particulate Matter (PM): NOX compounds react with other compounds in the
atmosphere to form nitrate particles and acid aerosols. Because of their small size nitrate
particles have a relatively long atmospheric lifetime; these small particles can also
penetrate deeply into the lungs. PM has a wide range of adverse health effects.
Stratospheric O3 Depletion: A layer of O3 located in the upper atmosphere (stratosphere)
protects people, plants, and animals on the surface of the earth (troposphere) from
excessive ultraviolet radiation. N2O, which is very stable in the troposphere, slowly
migrates to the stratosphere. In the stratosphere, solar radiation breaks it into nitric oxide
(NO) and nitrogen (N). The NO reacts with O3 to form NO2 and molecular oxygen.
Thus, additional N2O emissions would result in some decrease in stratospheric O3.
Toxic Products: Airborne particles derived from NOX emissions react in the atmosphere
to form various nitrogen containing compounds, some of which may be mutagenic.
Examples of transformation products thought to contribute to increased mutagenicity
include the nitrate radical, peroxyacetyl nitrates, nitroarenes, and nitrosamines.
Visibility and Regional Haze: NOX emissions lead to the formation of compounds that
can interfere with the transmission of light, limiting visual range and color discrimination.
Most visibility and regional haze problems can be traced to airborne particles in the
atmosphere that include carbon compounds, nitrate and sulfate aerosols, and soil dust.
The major cause of visibility impairment in the eastern United States is sulfates, while in
the West the other particle types play a greater role.
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O3 Formation and Accumulation
Although O3 formation and accumulation in the atmosphere involves complex nonlinear
processes, a very simplified description of the process is offered here. In short, NO is formed
during high temperature combustion involving air (air being largely N2 and 02). The NO is
converted to NO2 by reacting with either inorganic or organic radicals formed from oxidized
VOCs or by reacting with O3. The NO2 then photolyzes, leading to the formation of O3 and NO.
A reaction path that converts NO to NO2 without consuming a molecule of O3 allows O3 to
accumulate; such a path is provided by inorganic and organic radicals that arise from VOC
reactions.
The formation and accumulation of O3 is further complicated by the transport of O3
itself and O3 precursors (including NOX). This transport factor results in interactions between
distant sources in urban or rural areas and local ambient O3 concentrations. The transport of O3
and precursor pollutants over hundreds of kilometers (or hundreds of miles) can be a significant
factor in the accumulation of O3 in certain areas. Another important complicating factor is the
influence of meteorological factors on O3 formation, including temperature, wind direction, and
wind speed.
In the 1990 amendments to the CAA, Congress recognized the importance of NOX
emissions reductions and, especially in the Northeast, the need for regional scale control programs
to achieve the O3 standard. In section 184 of the CAA, Congress established the Northeast Ozone
Transport Commission to address interstate transport of O3 pollution among 12 northeastern
States and the District of Columbia. Further, Congress required large stationary sources located
in the Northeast Ozone Transport region and in moderate, serious, severe and extreme O3
nonattainment areas throughout the country to decrease NOX emissions.
The extent of local controls that will be needed to attain and maintain the O3 national
ambient air quality standards (NAAQS) in and near seriously polluted cities is sensitive both to the
amount of O3 and O3 precursors transported into the local area and to the specific photochemistry
of the area. In some cases, preliminary local modeling performed by the states for the 1-hour O3
standard indicates that it may not be feasible to find sufficient local control measures for individual
nonattainment areas unless transport into the areas is significantly lowered. The EPA has also
conducted preliminary analyses for the new 8-hour O3 standard which indicate that regional NOX
emissions decreases would be effective in helping many areas attain that standard. These
modeling studies suggest that decreasing NOX emissions on a regional basis is effective in
decreasing O3 over large geographic areas.
NOX Emissions Sources and Trends
Emissions of NOX result from fuel combustion at high temperature, which occurs
principally in fossil fuel-fired electric utility and industrial boilers and in motor vehicle internal
combustion engines. Electric utility and motor vehicle emissions each represent about one-third
of the total 1994 NOX emissions. About 85 percent of the total NOX emissions from electric
utilities are attributed to utilities burning coal.
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From 1940 through 1970, annual NOX emissions increased by a factor of three (from
7 million to 21 million tons). Since 1980, annual national NOX emissions leveled off at about 23
million tons. Data show that national NOX emissions slightly increased from 1990-1993. In the
mid-1990s, NOX emissions are expected to decrease somewhat as stationary source NOX controls
and light-duty and heavy-duty tailpipe standards are implemented and enhanced vehicle inspection
and maintenance (I/M) programs begin in some O3 nonattainment areas. Electric utility NOX
emissions are expected to decline after 1999 as the phase II acid deposition standards become
effective. Despite increases in vehicle miles traveled, total on-road vehicle emissions will likely
continue to decline through 2005 as per vehicle NOX emissions decrease due to tighter tailpipe
standards, phase II reformulated gasoline is implemented, and I/M requirements are met. Soon
after the year 2002, overall NOX emissions are projected to begin to increase and continue to
increase in the foreseeable future due to increased economic activity.
General Conclusions and Implications for Future NOX Management Strategies
It has become clearer that controls of NOX emissions may be needed in many areas,
especially in areas of the United States where O3 concentrations are high over a large region (as in
the Midwest, Northeast, and Southeast). In addition to helping attain the NAAQS for O3,
decreases in NOX emissions will also likely help improve the environment by decreasing the
adverse impacts of acid deposition, drinking water nitrate exposure, eutrophication of
waterbodies, global warming, NO2 exposure, nitrogen saturation of terrestrial ecosystems, PM
formation, stratospheric O3 depletion, toxics exposure, and visibility impairment.
Although total NOX emissions will decline from current levels by the year 2000 because of
mandatory CAA programs, NOX emissions will, soon after the year 2002, begin to gradually
increase. Both mobile, including non-road, and stationary sources are significant contributors to
the NOX problem on a nationwide basis. Thus, new initiatives will be necessary to achieve
reductions in NOX emissions that may be needed over much of the nation, especially to help attain
the O3 standards.
The EPA has begun implementing an integrated approach to achieve reductions in
emissions of NOX. This integrated approach involves increased interaction among the air and
water programs that are affected by various forms of atmospheric nitrogen and addresses several
categories of mobile and stationary sources. Policy decisions regarding the control of NOX
emissions are being made in the context of the many environmental effects associated with NOX
emissions. The EPA continues to work under its own authority and in coordination with a wide
range of stakeholders to develop and implement new mobile and stationary source control
programs at the federal, state, and local levels to decrease emissions of NOX. The following are
the key aspects of this strategy:
Mobile Sources
Since the 1970's EPA has required motor vehicle manufacturers to decrease
significantly emissions of NOX from light duty on-road vehicles. The most recent light
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duty vehicle requirements were phased-in over the 1994-96 model years. The EPA
continues to work with state officials, auto manufacturers, oil industry and others to
develop even cleaner cars, known as the National Low Emission Vehicles program.
Reduction in NOX emission levels from heavy-duty vehicles is expected from lower tailpipe
standards for engines produced after 1991 and further reductions are expected with the
1998 and 2004 model year engines. In 1995 cities with the worst smog problems in the
nation began using cleaner reformulated gasoline; a second phase of that program will
reduce emissions of NOX beginning in the year 2000. In addition, EPA is working on
several non-road programs to decrease NOX emissions from large marine, aircraft,
locomotive, and general purpose engines like those used in agriculture, construction, and
general industrial equipment.
Stationary Sources
To help control acid deposition, EPA established a two phased program to reduce
emissions of NOX from coal-fired electric utility generation units. This program is
expected to decrease NOX emissions by about 2 million tons annually by the year 2000.
States are also requiring controls on large sources of NOX that are located in areas of the
country that fail to meet the NAAQS for ground-level O3. To help decrease ground-level
O3, twelve northeastern states and the District of Columbia developed a memorandum of
understanding to reduce emissions of NOX from large boilers by 55-75 percent from 1990
levels. As a means of achieving these reductions with the least cost, EPA is working with
these states to develop an emissions trading program.
Ozone Transport Assessment Group (OTAG)
Over a 2 year period EPA worked with the OTAG, which was chartered by the
Environmental Council of States for the purpose of evaluating O3 transport and
recommending strategies for mitigating interstate pollution. The OTAG was a
consultative process among 37 eastern states which included examination of the extent
that NOX emissions from hundreds of kilometers away are contributing to smog problems
in downwind cities in the eastern half of the country, such as Atlanta, Boston, and
Chicago. The OTAG completed its work in June 1997 and on July 8, 1997 forwarded its
recommendations to EPA for achieving additional cost-effective emissions reduction
programs to decrease ground-level O3 throughout the eastern United States. In its
recommendations OTAG stated that it recognizes that NOX controls for O3 reduction
purposes have collateral public health and environmental benefits, including reductions in
acid deposition, eutrophication, nitrification, fine particle pollution, and regional haze.
Based on these recommendations and additional information, EPA will complete a
rulemaking action requiring States in the OTAG region that are significantly contributing
to O3 nonattainment in downwind States to revise their State implementation plans to
include new rules to reduce their emissions of NOX.
Emerging Technologies
Since passage of the 1970 CAA amendments, air pollution control and prevention
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7
technologies have continuously improved. Technologies such as selective catalytic
reduction and gas reburn systems are in place and successfully performing today that were
only on the drawing board ten years ago. As the demand for more innovative and cost-
effective or cost-saving technologies increases—due to the above new initiatives, for
example-new technologies such as ultra low-NOx gas-fired burners and vacuum insulated
catalytic converters will move from the research and development or pilot program phase
to commercial availability. Thus, it is likely that many new technologies will be available
in the next ten to fifteen years to employ in air pollution control and prevention strategies.
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Nitrogen Oxides Impacts
On Public Health and the Environment
I. Introduction/Overview
Purpose
The purpose of this document is to describe the multiple impacts on human health and
welfare that result from emissions of nitrogen oxides (NOX). Emissions of NOX result in an
unusually broad range of detrimental effects to human health and the environment. In addition,
this document states EPA's intent to consider the multiple environmental impacts of NOX
emissions when making policy decisions regarding regulation of NOX emissions.
Atmospheric nitrogen (N) compounds
Atmospheric N compounds include many forms of N, both inorganic and organic, in
gaseous and paniculate states. One form of N compound—diatomic N gas (N2)~makes up 78
percent of the atmosphere; however, it is inert and, thus, does not readily react with other
compounds in the atmosphere. As described below, many important N compounds can be
classified as oxidized N or reduced N. Other forms of N compounds are highly reactive and also
play a role in the formation and accumulation of various gases and particles in the atmosphere
which lead to harmful effects on human health and welfare.
Seven oxides of nitrogen are known to occur in the atmosphere: NO, NO2, NO3, N2O,
N2O3, N2O4, and N2O5. "NOX " is a symbol for the sum of nitric oxide (NO) and nitrogen dioxide
(NO2); these compounds are generally transformed and cycled within the atmosphere through
nitrate radical (NO3), organic nitrates, and dinitrogen pentoxide (N2O5), eventually forming nitric
acid (NRC, 1991). N2O is not formed as part of this atmospheric chemistry of NOX. Although
not reactive in the lower atmosphere, N2O is a significant greenhouse gas which is reactive once it
diffuses into the stratosphere. The various forms of oxides of nitrogen—NOX, N2O, nitrates, etc.~
are discussed separately in this document with respect to specific human health and environmental
impacts.
Reduced N compounds-ammonia (NH3) and ammonium (NH4+)-are also important to
many of the public health and environmental impacts associated with atmospheric N compounds.
Additional information on emissions of NH3 are contained in Appendix C of this document. The
emphasis of this report, however, is on oxides of nitrogen—their sources, impacts, and an
integrated strategy to decrease their emissions.
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Anthropogenic NOX Emissions Sources
Emissions of NOX are produced primarily by combustion processes during which oxygen
reacts with nitrogen at temperatures above about 2200 degrees Celsius. Both the molecular N
(N2) in the atmosphere and the chemically bound N in materials being burned (fuel N) can react
with oxygen to form NOX. Such combustion occurs principally in fossil fuel-fired electric utility
and industrial boilers and in motor vehicle internal combustion engines. As shown in the
following chart of anthropogenic emissions (EPA, 1995), electric utility and on-road vehicle
emissions each represent about one-third of the total 1994 NOX emissions (figure 1-1). In the year
2000, the percentage of utility emissions is projected to decline as the CAA phase II acid
deposition controls are implemented. About 85 percent of the NOX emissions estimated for
electric utilities are attributed to combustion of coal. The non-road emissions category includes
marine, aircraft, locomotive and construction equipment. Appendix C contains additional
information on anthropogenic NOX emissions.
Biogenic NOX Emissions Sources
Natural sources of NOX include lightning, soils, wildfires, stratospheric intrusion, and the
oceans. Of these, lightning and soils are the major contributors. Lightning produces high enough
temperatures to allow N2 and O2 in the atmosphere to be converted to NO. NO is the principal
NOX species emitted from soils, with emission rates depending mainly on fertilization amounts and
soil temperature; highest emissions occur in the summer. The United States 1990 annual biogenic
emissions of NOX are estimated to be 1.69 million tons (EPA, 1995); using the Biogenic
Emissions Inventory System—Version 2. As shown in figure 1-2, biogenic emissions are about 7
percent of the total NOX emissions in 1990 (EPA, 1995).
In areas with extensive agricultural production, such as the Southeast, biogenic emissions
from soil treated with nitrate-rich fertilizer can represent a measurable portion of total NOX
emissions. Much of the spatial difference in biogenic NOX emissions across the United States can
be attributed to variations in land use. Relatively high densities of NOX in the midwestern United
States are associated with areas of fertilized crop land.
Soil emissions of NO result from two major microbial processes: nitrification and
denitrifications. Nitrification is the process by which microbes in the soil oxidize the ammonium
ion to produce nitrites and nitrates. During the intermediate stages of this process, NO is formed
and subsequently diffuses through the soil into the atmosphere. By contrast, denitrification is an
anaerobic process where nitrate is converted to N2 and N2O; but once again, NO is formed in an
intermediate stage and diffuses to the atmosphere. Once in the atmosphere, NO begins to
participate in atmospheric chemical reactions. Within a time of tens to hundreds of seconds, a
substantial portion of NO has reacted with atmospheric O3 to produce NO2 (Aneja, 1994).
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Figure 1-1. National Anthropogenic NOx Emissions
by Source Category for 1994 and 2000
32.0%
13.0%
33.0%
31.7%
8.0%
14.0%
15.8%
28.7%
5.0%
18.8%
1994
2000
Utilities • Vehicles • Non-Road • Industrial D Other
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Figure 1-2. National Total NOx Emissions
by Source Category for 1990
Vehicles 30.3%
Non-Road 11.1%
Utilities 30.3%
Biogenic 7.1%
Other 5.1%
Industrial 16.2%
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Trends in Anthropogenic NOX Emissions
From 1940 through 1970, NOX emissions increased by a factor of three (from 7 million to
21 million tons). Since 1980, annual national NOX emissions have leveled off at about 23 million
tons. NOX emissions slightly increased from 1990-1993. NOX emissions from electric utilities and
on-road vehicles currently contribute about one third each to the national total (approximately 8
million tons each).
In the mid-1990s, NOX emissions are expected to decrease somewhat as stationary source
NOX controls and light-duty and heavy-duty tailpipe standards are implemented and enhanced
vehicle inspection and maintenance (I/M) programs begin in some O3 nonattainment areas.
Electric utility emissions are expected to decline after 1999 as the phase II acid deposition
standards become effective. Total NOX emissions will decline about 6 percent from current levels
by the year 2000. Despite increases in vehicle miles traveled, total on-road vehicle emissions will
likely continue to decline through 2005 as per vehicle emissions decrease due to tighter tailpipe
standards, phase II reformulated gasoline is implemented, and I/M requirements are met. Shortly
after the year 2002, overall NOX emissions are projected to begin to increase and continue to
increase in the foreseeable future due to increased economic activity, unless new NOX emissions
reduction initiatives are implemented (EPA, 1995).
In general, the per capita NOX emissions show a much smaller increase during the 1940 to
1978 period than did the total NOX emissions trend. Per capita NOX emissions have declined since
1978. NOX emissions normalized by real Gross Domestic Product (GDP) declined and then
increased during the 1940s but declined thereafter, an indication that fewer NOX emissions are
released per dollar of real GDP. These points are illustrated in figure 1-3 below (EPA, 1995).
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30,000
25,000
20,000
15,000
Emissions
10,000
5,000
Figure I-3. Trends in NOx Emissions
for the Period 1940 to 1994
1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990
Year
NOx Emissions NOx Emissions Normalized by GDP Per Capita NOx Emissions
NOx emissions reflect thousands of short tons.
Emissions normalized by GDP are shown as short tons per billion dollars of real GDP (stated in coraott 1987 prices).
Per capita emissions are short tons per 100,000 persons.
Source: U.S. Department of Commerce, population data, GDP^data 3'4
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Organization of this Document
This document is organized in 5 major sections: Introduction/Overview, Clean Air Act
Programs Involving Decreases in NOX Emissions, Additional Public Health and Environmental
Impacts from NOX Emissions, Interprogram Issues, and Appendices. The introduction/overview
section outlines the purpose of the document and provides information on atmospheric N
compounds, sources of NOX emissions, and trends in emissions of NOX. The Programs section
covers the impact of NOX emissions in each of the following subjects: acid deposition, NO2, O3,
PM, and visibility protection. Drinking water, eutrophication, global warming, stratospheric O3
depletion, terrestrial ecosystems, and toxics products are covered under the additional public
health and environmental impacts section. A subsequent section covers specific issues stemming
from interaction among the various programs, including local and regional NOX concerns, seasonal
controls, interface with the VOCs control program, EPA's Clean Air Power Initiative, and cross-
cutting issues related to the new standards for O3 and PM. Finally a set of appendices provides
some detail on the EPA activities within the various programs that impact NOX emissions,
information on sources and sinks of NOX emissions, and a listing of acronyms and abbreviations
References
Aneja, Viney P., "Workshop on the Intercomparison of Methodologies for Soil NOX Emissions:
Summary of Discussion and Research Recommendations," Journal of Air and Waste
Management Association., vol. 44, August 1994.
Chameides, W.L. and E.B. Cowling, The State of the Southern Oxidant Study (SOS): Policy-
Relevant Findings in Ozone Pollution Research, 1988-1994. North Carolina State University,
April 1995.
National Research Council, Committee on Tropospheric Ozone Formation and Measurement,
Rethinking the Ozone Problem in Urban and Regional Air Pollution, National Academy Press,
1991.
Schlesinger, W. H. and A. E. Hartley, "A Global Budget for Atmospheric NH3," Biochemistry 15:
191-211, 1992.
U.S. Environmental Protection Agency, Control Techniques for Nitrogen Oxides Emissions from
Stationary Sources—Revised Second Edition., EPA-450/3-83-002, January 1983.
U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, National
Air Pollutant Emission Trends, 1900-1994, October 1995.
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II. Clean Air Act Programs Involving Decreases in Nitrogen
Oxides (NOX) Emissions
A. Acid Deposition
1. Goals of the Program
The primary goal of the Acid Deposition NOX Emission Reduction Program is to decrease
the multiple adverse environmental and human health effects of NOX, a principal acid deposition
precursor that contributes to air and water pollution, by substantially decreasing annual emissions
from coal-fired power plants. Electric utilities are a major contributor to NOX emissions
nationwide: in 1980, they accounted for 30 percent of total NOX emissions and, from 1980 to
1990, their contribution rose to 32 percent of total NOX emissions. Approximately 85 percent of
electric utility NOX emissions comes from coal-fired plants.
"Acid deposition" occurs when airborne acidic or acidifying compounds, principally
sulfates (SO42") and nitrates (NO3"), which can be transported over long distances, return to the
earth through rain or snow ("wet deposition"), through fog or cloud water ("cloud deposition"),
or through transfer of gases or particles ("dry deposition"). While the severity of the damage
depends on the sensitivity of the receptor, acid deposition, according to section 401(a)(l) of the
CAA, "represents a threat to natural resources, ecosystems, visibility, materials, and public
health."
Since NOX emissions from the burning of fossil fuels at electric utility power plants
contribute to the formation of ground-level O3 and nitrate PM in the air, ambient levels of NO2
and peroxyacetal nitrate (PAN) gases, and atmospheric N deposition, the Acid Deposition NOX
Emission Reduction Program will also mitigate the negative health and welfare effects described
in the other sections of this document. Benefits associated with NOX emissions decreases under
the Acid Deposition Program include lowering excessive N loadings to N sensitive estuarine or
coastal water systems ranging from the Gulf of Maine to North Carolina's Albemarle Pamlico
Sound to Florida's Tampa and Sarasota Bays, decreasing O3 transported into and within O3
nonattainment areas, decreasing inhalable fine particles, and improving visibility, as well as
reducing acid deposition damage to lakes and streams, forests and vegetation, and sensitive
materials and structures.
2. Status of the Program
Title IV (Acid Deposition Control) of the CAA specifies a two-stage program for
decreasing NOX emissions from existing coal-fired electric utility power plants. Analogous to the
national allowance program for decreasing sulfur dioxide (SO2) emissions, this program is to be
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implemented in two phases. Phase I affected units (277 boilers)1 are required to meet the
applicable annual emission rates beginning with calendar year 1996; Phase II affected units (775
boilers) are required to meet the applicable annual emission rates beginning with calendar year
2000. Implementation of the first stage of the program, promulgated April 13, 1995 (60 FR
18751), will decrease annual NOX emissions in the United States by over 400,000 tons per year
between 1996 and 1999 (Phase I) and by approximately 1.17 million tons per year beginning in
2000 (Phase II). These reductions are achieved by applying low NOX burner (LNB) technology to
dry bottom wall-fired boilers and tangentially fired boilers (Group 1).
The second stage of the program, promulgated December 19, 1996 (61 FR 67112),
provides for additional annual NOX emissions reductions in the United States of approximately
890,000 tons per year beginning in the year 2000 (Phase II). Taken together, the two stages
provide for an overall decrease in annual NOX emissions in the United States of approximately
2.06 million tons per year beginning in the year 2000. In the second stage of the title IV Program
EPA has: (1) determined that more effective low NOX burner (LNB) technology is available to
establish more stringent standards for Phase II, Group 1 boilers than those established for Phase I;
and (2) established limitations for other boilers known as Group 2 (wet bottom boilers, cyclones,
cell burner boilers, and vertically fired boilers), based on NOX control technologies that are
comparable in cost to LNBs.
170 Phase I units known as Table 1 units and 107 Phase II units that have become substitution units. (The
170 Table 1 units are coal-fired units with Group 1 boilers listed in Table 1 of 40 CFR 73.10 (a) of the Acid Rain
Program Regulations.)
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The following table presents the boiler types affected by this rule, their population, and the NOX
emission limitations:
Table II-l. Utility Boiler Types and Emission Limits
Boiler Types
Number of Boilers
Phase II Emission Limits
Phase II, Group 1 Boilers
dry bottom wall-fired
tangential
308
299
(Revised)
dry bottom wall-fired: 0.46 Ib/mmBtu
tangential: 0.40 Ib/mmBtu
Group 2 Boilers
cell burners
cy clones > 155MW
wet bottoms > 65 MW
vertically fired
(New)
36
55
26
28
cell burners: 0.68 Ib/mmBtu
cyclones: 0.86 Ib/mmBtu
wet bottoms: 0.84 Ib/mmBtu
verticals: 0.80 Ib/mmBtu
Utilities can choose to comply with the rule in one of three ways: (1) meet the standard annual
emission limitations, (2) average the emissions rates of two or more boilers, which allows utilities
to over-control at units where it is technically easier and less expensive to control emissions, or
(3) if a utility cannot meet the standard emission limit, it can apply for a less stringent alternative
emission limit if it uses the appropriate NOX emissions control technology on which the applicable
emissions limit is based. Although emission limitations for the Acid Deposition NOX Emission
Reduction Program are based on "the degree of reduction achievable through the retrofit
application of the best system of continuous emission reduction" (section 407(b)(l) of the CAA),
the annual averaging period affords sources the flexibility of selecting either continuous or
seasonal controls.
In addition, the current rule allows utilities to "early elect" Phase II units with Group 1
boilers into the Phase I program, provided the units demonstrate compliance with the applicable
annual emission rate on or before January 1, 1997. As an incentive for early reductions, the rule
affects early election units from revisions to the emission limits promulgated in 61 FR 67112
through 2007. EPA has received early election applications for over 250 Phase II units
(corresponding to about 43 percent of the Phase II affected Group 1 boiler population). The early
election and emissions averaging provisions of the Acid Deposition NOX Emission Reduction
Program offer flexibility, promote technology development and competition, and provide
opportunities to reduce the cost of control.
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3 . Science of NOX and Acid Deposition
The burning of fossil fuels is a major contributor to the formation of NOX and thus to
atmospheric N deposition. "Atmospheric N deposition" is the process by which N in airborne or
atmospheric N compounds is transferred to water, soil, vegetation, and other materials (e.g.
buildings, statues, automobiles, etc.) on the earth. While some amount of N deposition can be
beneficial for growth of crops and forests, deposition in excess of plant and microbial demand can
disturb the soil and water N cycle and can result in acidification of lakes, streams, and soils as well
as eutrophication of estuarine and coastal waters bodies (Paerl, 1993) and, more rarely,
freshwater ecosystems (Church, 1997:17; Vitousek et al, 1997:10). Eutrophication of estuarine
and coastal waters is addressed in section III.B of this document.
As mentioned previously, "acid deposition" involves acidic and acidifying sulfur and N
compounds, which can be transported over short and long distances, thus affecting natural
resources and materials up to hundreds of kilometers from the sources of precursor emissions
(SO2 and NOX). As with NOX emissions and O3 formation, the relationship between precursor
emissions and acidity in the atmosphere is complex (NAPAP, 1993:23). Some of these acidic and
acidifying compounds are not emitted directly during the burning of fossil fuels; they are formed
by chemical conversions in the atmosphere of SO2 and NOX gases released during combustion.
a. Acidification
Acidification effects are related to increases in the acidity of water and soil in ecosystems.
Increases in water acidity can impair the ability of certain types offish and other biota to grow,
reproduce, and hence, survive. In some acidified lakes and streams, entire populations offish
species have disappeared. For example, many lakes in the higher Adirondack mountains of New
York and many streams in the Appalachian mountain region have experienced loss of trout and
other biodiversity losses due to high acidity levels in the water (NAPAP, 1993:76). Increases in
soil acidity can impair the ability of some types of trees to grow and resist disease. For example,
growth reductions and injury to red spruce on high elevation ridges of the Appalachian mountains
from Maine to Georgia have been linked to nutrient leaching caused by high soil acidity and
deposition and primarily linked to a predisposition to frost damage from highly acidic cloud water
(Johnson et al, 1992). The effects of acid deposition on forested ecosystems is an important
research issue primarily because the observational data are inclusive (i.e., trees react very slowly
to damaging influences).
/'. Lakes, Streams, and Watershed Ecosystems
Recent scientific studies indicate the amount of N that can be sequestered and retained in
certain watersheds by biological processes is limited (US EPA, 1995:11). As these watersheds
move towards N saturation, nitrate and, to a lesser extent, nitrite can begin to leach into surface
waters, accelerating the process of long-term chronic acidification. Adding N to freshwater
ecosystems that are rich in phosphorus can eutrophy as well as acidify the waters. Eutrophication
also leads to decreased diversity of both plant and animal species (Vitousek et al, 1997:10).
Atmospheric deposition of N compounds plays a significant role in short-term episodic
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acidification, which occurs when pulses of highly acidic water enter lakes and streams during
storm flow, spring snowmelt or autumn rains after prolonged summer drought. Acidic episodes
can expose aquatic organisms (e.g., fish, amphibians) to "acid pulses" containing high
concentrations of inorganic monomeric aluminum (Alim), which is highly toxic to fish, often during
the spawning season in the Spring. Episodic acidification can affect poorly buffered surface
waters in many regions, including high elevation areas in the Mid-Atlantic and the West, as well as
the Northeast (US EPA, 1995:14, 25).
The relative contributions of N and sulfur compounds, primarily NO3" and SO42", to the
problem of surface water and soil acidification differs among regions and sites. The relative
contributions depend not only on external differences in the deposition rates of these chemicals,
but also on differences among the capacities of receptor watersheds to retain N and sulfur and to
"buffer" against pH changes (i.e. alkalinity or hardness). Many areas in the West are more
affected by N deposition, particularly dry deposition, than by sulfur deposition (US EPA,
1995:56).
Acidified watershed ecosystems can show signs of recovery following decreases in acid
deposition rates. According to the Acid Deposition Standard Feasibility Study (US EPA, 1995),
in watersheds where atmospheric deposition of sulfur has been and will continue to be decreased
(commensurate with decreases in SO2 emissions under Title IV of the CAA), environmental
modeling has projected a range of benefits (i.e., fewer acidic ecosystems) in sensitive ecosystems.
The number of acidic systems are substantially fewer than the model projects without the SO2
emissions reductions in Title IV. Recovery rates depend primarily on the rates of pollutant
decreases, ecosystem N retention processes, time lags caused by long-term biological process
responses, and other possible changes in soil chemistry. Although watershed N saturation is
widely accepted in the research community, it is also broadly recognized that there are
uncertainties associated with the rate at which a watershed may become N saturated. However,
additional NOX emissions reductions would likely produce a two-fold benefit by decreasing acid
deposition rates and lengthening the average time before watersheds reach N saturation.
/'/'. Forests and Vegetation
Past assessments of the impacts of acid deposition on forests and vegetation have focused
primarily on SO2 and sulfur deposition, largely because N is an essential nutrient for many
biological processes (Atkinson, 1993; Sommerville et al, 1989). Because N is commonly used as a
fertilizer, it was thought that any atmospherically deposited N would be quickly and beneficially
incorporated into plant and tree organisms (US EPA, 1995:11). Like aquatic ecosystems, the
biological demand for N in forest ecosystems and other vegetation varies across geographical
areas and by season. It is also highly dependent on factors such as tree/plant species (e.g.,
deciduous- species trees tend to have greater demand for N per unit biomass than coniferous-
species trees), soil type, forest age, prevalence of disease and other stresses such as extreme cold
or drought, and land management practices (e.g., use of fertilizers, liming, or other cultivation
methods) (US EPA, 1995:11).
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Acidification effects on health and productivity of forests and other vegetation are divided
into two types: (1) direct effects on foliar organs and (2) indirect or soil-mediated effects resulting
from acidification and physical/chemical alteration of the soil. Direct acidification effects might
include foliar damage, erosion of leaf cuticle waxes, and changes in the physiology of tree leaves
(Society of American Foresters, 1984). Soil-mediated acidification effects include toxic effects on
roots as well as possible changes in nutrient availability, reproductive and regenerative processes.
Increasing evidence reveals that dry deposition is usually a significant portion of total
atmospheric deposition (wet + cloud + dry) of both sulfur and N. For example, across all sites
included in a recent review, dry deposition ranged from 9 to 59 percent of total deposition for
sulfur (S), 25 to 70 percent for nitrate, and 2 to 33 percent for ammonia (Lovett, 1994; 629-
650). Thus, in many areas N is taken up by foliage primarily in dry chemical form (e.g., as nitric
acid vapor), rather than with deposition in precipitation. The response of forest ecosystems to
direct effects of atmospheric deposition of both sulfur and N depend on the nature and timing of
the deposition as well as the type of vegetation exposed. Some species appear less tolerant than
others (i.e., spruce-fir ecosystems appear to be the most sensitive) and younger trees appear more
vulnerable than mature trees.
Considerably more but still limited research has been performed on soil-mediated
acidification effects since soils, together with climate, determine the productivity of terrestrial
ecosystems. These studies have focused primarily on decreases in available base cation plant
nutrients below amounts required for plant growth; and increased mobilization and availability of
toxic aluminum (Al) and other metal ions (Brandt, 1993:14, 31). In certain soils, N deposition
can deplete nutrients by leaching calcium (Ca), magnesium (Mg), and potassium (K). These
important cations are often replaced by hydrogen ions (FT) which, together with increased
mobilization of aluminum, can greatly increase soil acidification. Significant increases in sulfate
and/or nitrate concentration will lead to preferential mobilization, availability, and toxicity of
aluminum over base cations (e.g., Ca2+, Mg2+, K+) in soils with low base saturation, such as the
soils commonly found in high-elevation sites in the Northeast and Southeast (Turchenek et al.,
1987; Turner et al., 1986). Increased concentrations and mobility of aluminum are linked with
root damage and limited uptake of root calcium and magnesium. (Shortle and Smith, 1988).
The timing of aluminum concentration peaks is also important. Toxic aluminum peaks
related to nitrate fluctuations commonly occur in late summer or early fall when soil temperatures
and root growth are usually high (Joslin et al., 1992). It has been estimated that up to 3 percent
of forested soils in the eastern United States could have toxic levels of trace elements in solution
or could act as a source of high levels of acidity to surface waters, thus contributing to the
acidification of watershed ecosystems discussed previously. Further, up to 40 percent of eastern
soils may be sensitive to changes in nutrient status that could result in reduced forest growth or
additional acidification of surface waters (Turner et al., 1986).
Forest ecosystems and other vegetated regions (e.g. crop and grasslands) are also
susceptible to adverse excess N loading effects analogous to eutrophication in aquatic ecosystems.
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These N loading effects result from deposited N of all forms (i.e., including forms other than
acidic nitrate such as ammonia and dissolved organic N) and tend to occur when the demand by
plants and heterotrophic soil organisms for N has been substantially satisfied (i.e., the ecosystem is
approaching N saturation). N deposition to forest ecosystems can affect competitive relationships
across tree/plant species and can therefore change species composition and/or diversity. Other
potential adverse N loading effects include decreased uptake of nutrients from soil, increased
susceptibility to insect and disease attack, and altered reproductive or regenerative processes (US
EPA, 1991).
Evidence has accumulated suggesting N availability in certain forest ecosystems are in
excess of plant and microbial demand. Early indicators of N saturation have implications to forest
ecosystems over large geographic areas. Possible effects include elevated concentrations of
nitrate, aluminum, and hydrogen in streams, which would decrease water quality, increase
susceptibility to frost damage or other disruptions of physiological function that would lower
productivity in certain forest types, increased cation [nutrient] leaching from soils and nitrate
losses that would lead to lower soil fertility and increased acidity (Aber, et al., 1989).
Additionally, recent research conducted in the Colorado Front Range demonstrates that high
elevation (alpine and subalpine) ecosystems may be nearly N saturated at current levels of N
deposition. The results suggest that the Colorado Front Range may be an early warning indicator
of N saturation for other high-elevation catchments in the Rocky Mountains and the western
United States and an indicator for disruption of N cycling in forested ecosystems at lower
elevations as well. (Williams, et al., 1996)
Results of twelve years of experimental N addition to grassland plots in Minnesota have
shown reductions in grassland biodiversity associated with N loadings. N added to research plots
resulted in the loss of almost all native prairie grass species and to dominance by a weedy
quackgrass. These results indicate that N loading can be a major threat to grassland ecosystems,
causing loss of diversity, increased abundance of nonnative species, and the disruption of
ecosystem functioning. (Wedin, et al., 1996)
Finally, NOX is a primary O3 precursor and the damaging effects of O3 on forest
ecosystems have been studied more comprehensively than those related to excess N loading and
acidification. O3 is the most destructive pollutant in forest ecosystems (deSteiguer et al., 1990).
The injurious effects of O3 on plants include visible damage to foliage, decreased growth of roots
and shoots, decreased yield, changes in quality of harvest, and changes in susceptibility to other
stresses. (US EPA, 1993).
b. Materials and Structures
The role of atmospheric N deposition in metals corrosion has not been completely
resolved; some suggest, on the basis of laboratory evidence, thatNOx appreciably increases the
corrosive effect of SO2 (NAPAP, 1993:93-94). It has been estimated that 31-78 percent of the
dissolution of galvanized steel and copper is attributable to wet and dry acid deposition (NAPAP,
1993:93). Deposited acids corrode and dissolve the protective zinc coatings on these surfaces,
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and as a result, the metal underneath rusts. The specific role of N-based acids in the process has
not yet been established. In the late 1980s, NAPAP and the Economic Commission for Europe
initiated several projects in the United States and Europe to clarify the scientific foundation
linking acid deposition and materials damage (NAPAP, 1993:93). These projects include research
to investigate the mechanisms by which N deposition directly impacts or works with other
pollutants to damage structural and other materials.
Acid deposition also damages exterior paints applied to wood and metal substrates.
Special paint formulations involving different organic and inorganic binders, pigments, and
additives have been developed to resist corrosion and spotting from acid deposition (NAPAP,
1993:96). To maximize durability, these special finishes are applied under factory-controlled
conditions. The costs to automotive manufacturers for including acid-resistant features have been
estimated by the EPA and NAPAP to be as high as $400 million annually (US EPA, 1995:97).
Acid deposition can also accelerate the deterioration of stone through processes of erosion,
solubilization, blackening of the stone surface, and cracking (US EPA, 1995:96-97). Acidic-
related damage to cultural and historic buildings, monuments, and structures increases annual
maintenance and reparation costs, which can be extensive. Thus, potentially large economic
benefits could be associated with lessened physical materials damage achieved, in part, through
additional NOX emissions reductions.
4. How much reduction is needed?
Our current knowledge of the science of NOX and acid deposition does not support
quantitative assessments of the tons of NOX needed beyond the CAA nationally or by region to
protect sensitive aquatic and forest ecosystems or to reduce acidic-related damage to materials,
structures, and cultural or historic resources. Nonetheless, model projections from EPA's recent
Acid Deposition Standard Feasibility Study (October 1995) indicate that N deposition may play
an important role in ongoing and future acidification of sensitive watershed ecosystems, and may
equal or exceed the effects of sulfur deposition. The extent of potential future effects depends on
how rapidly atmospheric N deposition moves watersheds toward a state of N saturation, i.e.,
where input of N exceeds biological uptake of N on an annual basis. The time to watershed N
saturation will vary depending on forest age, historic and future rates of N deposition, future
changes in ambient temperatures, water stress, land use as well as other variables.
The United States Congress directed EPA, in Section 404 (Acid Deposition Standards
Study) of the CAA, to provide a report on the feasibility and effectiveness of an acid deposition
standard or standards to protect sensitive and critically sensitive aquatic and terrestrial resources.
The EPA's Acid Deposition Standard Feasibility Study: Report to Congress (US EPA, 1995)
fulfills this requirement by integrating state-of-the-art ecological effects research, emissions and
source receptor modeling, and evaluation of implementation and cost issues related to the
feasibility of establishing and implementing an acid deposition standard or standards.
An acid deposition standard is a rate of deposition (most likely in units of kilograms of
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pollutant per hectare2 per year) that provides a predetermined amount of protection to specific
ecological resources. Aquatic systems are the natural resources most at risk from acid deposition
and those most amenable to quantitative assessment. Other ecological resources such as high-
elevation red spruce forests in the eastern United States and Canada may also be at risk, but less is
known about the effects process, and the rate and extent of impacts on those resources. Target
populations of Adirondack lakes, Mid-Atlantic streams, and Southern Blue Ridge streams were
selected as case studies for detailed analysis in the Acid Deposition Standard Feasibility Study
because they represent ecosystems that receive fairly high amounts of acid deposition, are
sensitive to acid deposition, have the best historical data, and have been the focus of scientific
studies. While many surface waters in western North America are as sensitive as, or more
sensitive than, aquatic systems in the East, acid deposition rates in the West are currently
sufficiently low that the risk of chronic (long-term) acidification to resources in the West is low
and is expected to remain low for the next 50 years. Episodic acidification from spring snow
melts, which adversely affects some eastern surface waters, also affects high elevation western
surface waters. (US EPA, 1995:xiv).
For the Acid Deposition Standard Feasibility Study, EPA scientists modeled the potential
combined effects of atmospheric deposition of both sulfur and N on the chemistry of acid-
sensitive lakes and streams in the regions selected for in-depth study: Adirondacks, Mid-
Appalachian Region, and Southern Blue Ridge Province. Model simulations projected water
chemistry responses out to the year 2040. Projections of sulfur and N deposition rates were
based on results expected from implementation of the 1990 CAA amendments as well as other
more restrictive deposition reduction scenarios using EPA's Regional Acid Deposition Model
(RADM). The modeling incorporated a decision-model based estimate of SO2 emission
allowance trading and the Canadian SO2 control program. Explicit watershed models and data to
estimate the times required for watersheds to reach N saturation were unavailable at the time of
the Study therefore, EPA scientists assumed an encompassing range of times (50 years, 100 years,
250 years, and never) to watershed N saturation and then estimated the potential consequent
effects on surface water Acid Neutralizing Capacity (ANC). (ANC is a commonly used measure
of the concentration of dissolved compounds [e.g., carbonate, bicarbonate, borates, and silicates]
in fresh water which collectively tend to create less acidic conditions. Surface waters with higher
ANC are generally more resistant to acidification.) This innovative modeling component of the
Acid Deposition Standard Feasibility Study is referred to as the Nitrogen Bounding Study (NBS)
in that, given the uncertainties associated with the time when a watershed may reach N saturation,
the results effectively bounded the range of possible water chemistry outcomes . The NBS
received external technical peer review and the entire Feasibility Study has been peer-reviewed by
EPA's Science Advisory Board (US EPA, Appendix D, 1995).
Although model projections in the Acid Deposition Standard Feasibility Study are for
three specific target populations, i.e., groups of lakes or streams with watersheds of similar size,
land, and other characteristics, not even for all watersheds in the study regions, they signal a
A hectare is a unit of surface measure equal to 10,000 square meters.
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direction of probable need for substantial additional reductions in year-round NOX emissions in the
Eastern United States For example, it was estimated that a 40-50 percent decrease in SO2 and
NOX emissions beyond the CAA may be required to keep the number of chronically acidified lakes
in the Adirondacks at 1984 proportions, if these watersheds move towards N saturation in 100
years3 (US EPA, 1995:xvi). Without additional emissions reductions, the model projects the
number of acidic lakes in the Adirondacks could increase by almost 40 percent by 2040, if these
watersheds move towards N saturation in 100 years. As described, the modeling effort
encompasses a range of responses based on time to N watershed saturation. For example, in the
case in which saturation never occurs in the Adirondacks, the number of acidified lakes is lowered
by 40 percent due to the SO2 emissions reduction in the CAA. The effects on episodic
acidification of lakes and streams would be even more pronounced as it is now understood that
high nitrate levels are largely responsible for acidic episodes during snowmelt and high stream
flow periods in the Northeast and probably high-elevation areas in other regions of the United
States (Wigington et al., 1996; US EPA, 1995).
Recent results from the Bear Brook Watershed Manipulation Experiment illustrate the
rapidity with which forested watersheds in the Northeast may reach N saturation in response to
increased atmospheric N deposition (Scofield, 1995; Norton et al., 1994). :. Increased leaching of
nitrate from forested catchments into streams or lakes could lead to increases in surface water
acidification in some areas that could offset increases in ANC (i.e., reductions in acidity) expected
from decreases in SO2 emissions under the CAA.
5. How much reduction will be achieved with current and
projected Title IV programs?
Under the current rule for the Acid Deposition NOX Emission Reduction Program (40
CFR Part 76; FR 18751, April 13, 1995), NOX emissions from existing coal-fired electric utility
power plants will be decreased by over 400,000 tons per year between 1996 and 1999 (Phase I)
and by over 1.5 million tons per year beginning in 2000 (Phase II). These decreases are achieved
by 857 dry bottom wall-fired and tangentially fired boilers (Group 1). The annual cost of this
regulation to the electric utility industry is estimated as $267 million (in 1990 dollars), resulting in
an overall cost-effectiveness of $227 per ton of NOX removed. The nationwide cost impact on
electricity consumers is an average increase in electricity rates of approximately 0.21 percent,
beginning in 2000 (61 FR 1442).
The Phase II Acid Deposition NOX Emission Reduction Program will achieve an additional
reduction of 890,000 tons of NOX per year from existing coal-fired electric utility power plants
beginning in 2000. One hundred twenty thousand (120,000) tons would come from lowering the
Observational data (currently being collected and analyzed by the New York State Department of
Environmental Conservation) which compare the amount of nitrate falling on several Adirondack watersheds with the
amount of nitrate leaving these watersheds in stream water indicate the watersheds may be nearing N saturation
(Simonin, 1996; Evans et al., 1996).
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emission limits for 580 Group 1 boilers affected in Phase II; 77 percent of these boilers are
located in the Eastern United States, defined by the 37 states adjacent to and east of the
Mississippi River. The additional tons would come from establishing emission limits for 190 high-
emitting Group 2 boilers (i.e., cell burners, cyclones, wet bottom boilers, and vertically fired
boilers); 89 percent of these boilers are located in the Eastern United States.
The Phase II Acid Deposition NOX Emission Reduction Program appears to represent a
singular regulatory opportunity for controlling high-emitting Group 2 boilers, which typically emit
NOX at rates in excess of 1.0 Ib/mmBtu. The majority of these coal-fired boilers are located
outside designated O3 nonattainment areas in the states of Illinois, Indiana, Kentucky, Michigan,
Missouri, Ohio, and West Virginia. EPA modeling analyses show that transport of O3 and O3
precursors (primarily NOX) from upwind areas in the Eastern United States contributes
significantly to O3 exceedances in virtually all nonattainment areas in the Northeast Ozone
Transport Region (60 FR 45583). Further, simulations on EPA's Regional Acid Deposition
Model (RADM) indicate not only that utility sources of N contribute the majority of deposits on
the western side of the Chesapeake Bay, but also that the areal extent of the Chesapeake Bay
airshed (which encompasses all or parts of Indiana, Kentucky, Ohio, and West Virginia as well as
10 other states) underestimates the areas contributing atmospheric sources of N deposition
entering the Bay (Dennis, 1995).
The average cost-effectiveness of utility NOX controls under the rule compares favorably
to many of the other pollution control measures being considered by states to mitigate persistent
O3 nonattainment and/or N-based eutrophication water quality problems. For example, decreases
in NOX emissions from coal-fired power plants are comparable or less expensive to implement
than certain alternative controls for reducing N loadings to the Chesapeake Bay from area sources
(farms, forests), even without counting the "clean air" benefits associated with the NOX emission
reductions. Such alternatives, as well as others in the mobile source sector, are presently being
considered by Maryland, Virginia, Pennsylvania, and the District of Columbia to achieve the 40
percent-decrease in controllable nutrient supplies to the Bay, to which these jurisdictions have
committed. The average cost-effectiveness of these other controls are: chemical addition or
biological removal of N from wastewater processing ($4,000 to over $20,000/ton N removed)
and "management practices" to decrease N from fertilizers, animal waste, and other nonpoint
sources ($1,000 to over $100,000/ton of N removed) (Camacho, 1993; Shuyler, 1992). While it
is recognized that to address the Bay's excessive nutrient loading problem in the most efficient
manner requires pursuing an integrated strategy of air, water, and agricultural pollution control
practices, these relative cost-effectiveness ratios and modeling analyses suggest the additional
NOX emissions reductions from coal-fired power plants in the acid deposition rule are a critical
component of this strategy.
6. Summary
The primary goal of the Acid Deposition NOX Emission Reduction Program is to reduce
the multiple adverse effects of NOX, a principal acid deposition precursor that contributes to air
and water pollution, by substantially decreasing annual emissions from existing coal-fired power
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plants. Acid deposition occurs when airborne acidic and acidifying compounds, principally sulfates
(SO42") and nitrates (NO3"), which can be transported over long distances, return to the earth
through rain or snow ("wet deposition"), through fog or cloud water ("cloud deposition"), or
through transfer of gases or particles ("dry deposition"). According to section 401(a)(l) of the
CAA, acid deposition "represents a threat to natural resources, ecosystems, visibility, materials,
and public health." Since NOX emissions from the burning of fossil fuels at power plants also
contribute to the formation of ground-level O3 and nitrate PM in the air, ambient levels of NO2,
and excessive N loadings to the Chesapeake Bay and other estuaries, decreases in NOX emissions
under the Acid Deposition Program are expected to have multiple and synergistic beneficial
impacts on public health and welfare.
The Acid Deposition NOX Emission Reduction Program consists of a two-stage program
which, analogous to the Acid Deposition allowance program for SO2 emission reduction, is
implemented in two phases. The first stage of the program, authorized by section 407(b)(l) of
the CAA and implemented pursuant to 40 CFR Part 76, promulgated in April 1995, will decrease
annual NOX emissions by over 400,000 tons per year, beginning in 1996, from 170 Phase I
affected units4 with Group 1 boilers (i.e., dry bottom wall-fired boilers and tangentially fired
boilers). An additional NOX emissions reduction of over 200,000 tons per year will probably be
realized, beginning in 1997, from about 250 "early election" Phase II units with Group 1 boilers
which voluntarily opted into the Phase I program. The total NOX emissions reduction that would
be achieved by applying LNB technology under the April 13, 1995 rule is estimated at about 1.2
million tons per year, beginning in 2000.
In December 1996, EPA promulgated regulations for implementing the second stage of
the program, authorized by section 407(b)(2) of the CAA. Compliance with the rule would
achieve an additional NOX emissions reduction of 890,000 tons per year, beginning in 2000, from
existing coal-fired units affected in Phase II. Seventy-seven percent of the Group 1, Phase II
boilers and 89 percent of the Group 2 boilers are located in states adjacent to or east of the
Mississippi River. EPA modeling analyses show that utility sources of N in this 37-state region
contribute significantly to the acidification of certain watershed ecosystems (US EPA, 1995),
excess N deposits to the Chesapeake Bay (Dennis, 1995), and O3 exceedances in virtually all
nonattainment areas in the densely populated Northeast Ozone Transport Region (60 FR 45583).
The total NOX emissions decrease under the statutory authority of Title IV (Acid Deposition
Control) is estimated at about 2 million tons per year, beginning in 2000.
Such emissions decreases may not be adequate, however, to protect sensitive watershed
ecosystems in the Northeast and Mid-Atlantic regions as well as in high-elevation areas in the
West and other regions. Recent model projections from EPA's Acid Deposition Standard
Feasibility Study: Report to Congress (US EPA, 1995) signal a direction of probable need for
Known as Table 1 units. These 170 units represent the coal-fired units with Group 1 boilers that are listed in
Table 1 of 40 CFR 73.10 (a) of the Acid Ram Program Regulations and are subject to 40 CFR Part 76, Acid Ram NOX
Emission Reduction Rule.
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substantial additional reductions in year-round NOX emissions. For example, it was estimated that
a 40-50 percent decrease in SO2 and NOX emissions beyond the CAA may be required to keep the
number of chronically acidified lakes in the Adirondacks at 1984 proportions, if the time to N
saturation in these watersheds is 100 years or less. Without additional emissions reductions, the
model projects the number of acidic lakes in the Adirondacks could increase by almost 40 percent
by 2040, assuming N saturation in 100 years. There are uncertainties associated with determining
the rate at which a watershed may reach N saturation and therefore the EPA's Study provides a
range of possible responses. However, the magnitude and direction of projected responses point
towards a need for further emissions reductions to protect sensitive ecosystems. The negative
effects of no additional emissions reductions could be even more pronounced on episodic
acidification of lakes and streams in the Northeast (and potentially high-elevation areas in other
regions) where high nitrate levels are largely responsible for acidic episodes during snowmelt and
high stream flow periods (Wigington et al., 1996; US EPA, 1995). Thus, wintertime NOX
emissions reductions are especially important to lessening the incidence and severity of acidic
episodes in certain areas. Continuous year-round NOX controls appear to be the most beneficial
for decreasing acid deposition damage to natural resources.
References
Aber, John D., Knute J. Nadelhoffer, Paul Steudler, and Jerry M. Melillo. 1989. "Nitrogen
Saturation in Northern Forest Ecosystems." BioScience Vol. 39 No. 6. June.
Atkinson, Giles. 1993. "The Impact of Acid Rain on Crops." Reprint from Center for Social and
Economic Research on the Global Environment, University College London and University of
East Anglia, UK.
Brandt, C. Jeffrey. 1994. Acidic Deposition and Forest Soils: Potential Changes in Nutrient
Cycles and Effects on Tree Growth. Battelle Pacific Northwest Laboratory. EPA 600/R-94/153
ERL-COR817. May.
Camacho, R. 1993. "Financial Cost-Effectiveness of Point and Nonpoint Source Nutrient
Reduction Technologies in the Chesapeake Bay Basin." Report No. 8 of the Chesapeake Bay
Program Nutrient Reduction Strategy Reevaluation. Washington, DC. U.S. Environmental
Protection Agency, February.
Church, M.R. 1997. "Hydrochemistry of forested catchments." Annual Reviews of Earth and
Planetary Sciences. Vol. 25 (preprint).
de Steiguer, J.E., J.M. Pye, and C.S. Love. 1990. "Air pollution damage to U.S. forests."
Journal of Forestry, Vol. 88, No. 8, p. 17. August.
Evans, C.D., T.D. Davies, P.J. Wigington, M. Tranter, and W.A. Kreiser. 1996 (in press). "Use
of factor analysis to investigate processes controlling the chemical composition of four streams in
the Adirondack mountains," New York. Journal of Hydrology.
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Federal Register. 1995. 40 CFR Part 76. "Acid Rain Program: Nitrogen Oxides Emission
Reduction Program." Vol.60, No. 71, pp. 18751. Thursday, April 13, 1995. Direct final rule.
Federal Register. 1995. 40 CFR, Parts 80, 86 and 89. "Control of Air Pollution From Heavy-
Duty Engines." Vol. 60, No. 169, pp. 45580. Thursday, August 31, 1995. Proposed rule.
Federal Register. 1996. 40 CFR Part 76. "Acid Rain Program: Nitrogen Oxides Emission
Reduction Program." Vol.61, No. 13, pp. 1442. Friday, January 16, 1996. Proposed rule .
Johnson, A.H., S.B. McLaughlin, M.B. Adams, E.R. Cook, D.H. DeHayes, C. Eager, I.J.
Fernandez, D.W. Johnson, R.J. Kohut, V.A. Mohnen, NM.S. Nicholas, D.R. Peart, G, A. Schier,
and P.S. White. 1992. "Synthesis and Conclusions from Epidemiological and Mechanistic
Studies of Red Spruce Decline." Pp. 385-412. In C. Eager and M.B. Adams, eds. Ecology and
Decline of Red Spruce in the Eastern United States. Springer Verlag, New York.
Joslin, J.D., J.M. Kelly, and H. Van Meigroet. 1992. "Soil chemistry and nutrition of North
American spruce-fir stands: evidence for recent change." J. Environ. Qual. 21:12-30.
Lovett, G.M. 1994. "Atmospheric deposition and pollutants in North America: an ecological
perspective." Environmental Applications 4:629-650.
National Acid Precipitation Assessment Program (NAPAP). 1990. Integrated Assessment.
External Review Draft.
"National Acid Precipitation Assessment Program (NAPAP)." 1992 Report to Congress. June
1993.
Norton, S.A., J.S. Kahl, I.J. Fernandez, L.E. Rustad, J.P. Scofield and T.A. Haines. 1994.
"Response of the West Bear Brook Watershed, Maine, USA, to the addition of (NH4)2SO4: 3-
year results." Forest Ecology and Management. 68:61-73.
Paerl, H.W. 1993. "The Emerging Role of Atmospheric Nitrogen Deposition in Coastal
Eutrophi cation: A Biogeochemical and Trophic Perspective." Canadian Journal of Fisheries and
Aquatic Sciences. 50:2254-2269.
Scofield, J.P. 1995. "Annual Progress Report for Cooperative Agreement CR-816261.
Watershed Manipulation Project: Maine Site Group."
Shortle, W.C. and K.T. Smith. 1988. "Aluminum-induced calcium deficiency syndrome in
declining red spruce." Science 240:1017-1018.
Shuyler, L.R. 1992. "Cost Analysis for Nonpoint Source Control Categories in the Chesapeake
Basin." Annapolis, MD: U.S. Environmental Protection Agency . Chesapeake Bay Program.
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March.
Simonin, H.A. 1996. Unpublished data.
Society of American Foresters. 1984. "Acidic Deposition and Forests." Bethesda, MD.
Sommerville, M.C., S.E. Spruill, J.O. Rawling, and V.M. Lesser. 1989. "Impact of Ozone and
Sulfur Dioxide on the Yield of Agricultural Crops." Technical Bulletin 292. North Carolina
Agricultural Research Service, North Carolina State University. November.
Taylor, G.E., D.W. Johnson, and C.P. Anderson. 1994. "Air pollution and forest ecosystems: a
regional to global perspective." Ecological Applications. 4:662-689.
Turchenek, L.W., S.A. Abboud, CJ. Tomas, RJ. Fessenden, andN. Holowaychuck. 1987.
"Effects of acid deposition on soils in Alberta." Acid Deposition Research Program, Calgary,
Alberta, Canada.
Turner, R.S., RJ. Olson, and C.C. Brandt. 1986. "Areas having soils characteristics that may
indicate sensitivity to acidic deposition under alternative forest damage hypotheses." ORNL/TM-
9917. Environmental Sciences Division. Pub. No. 2720. Oak Ridge National Laboratory .
US EPA. 1991. "Air Quality Criteria for Oxides of Nitrogen: External Review Draft," EPA
600/8-91/049bA, Environmental Criteria and Assessment Office, Office of Health and
Environmental Assessment, Research Triangle Park, NC. August.
US EPA. 1993. Air Quality Criteria for Ozone and Related Photochemical Oxidants. Volume
II of III. EPA 600/Ap-93/004d. April.
US EPA. 1995. Acid Deposition Standard Feasibility Study: Report to Congress. EPA430-R-
95-00la. October.
Vitousek, P.M., J. Aber, R.W. Howarth, G.E. Likens, P.A. Matson, D.W. Schindler, W.H.
Schlesinger, and G.D. Tilman, "Human Alteration of the Global Nitrogen Cycle: Causes and
Consequences," Issues in Ecology, Number 1, Spring 1997.
Wedin, David A. and David Tilman. 1996. "Influence of Nitrogen Loading and Species
Composition on the Carbon Balance of Grasslands." Science, Vol 274, December 6, 1996.
Wigington, J.P, D.R. DeWalle, P.S. Murdoch, W.A. Kretser, H.A. Simonin, J. Van Sickle, and
J.P. Baker. 1996 (in press). "Episodic acidification of small streams in the Northeastern United
States II. Ionic controls of episodes." Ecological Applications 6(2).
Williams, Mark W., Jill S. Baron, Nel Caine, Richard Sommerfield and Robert Sanford, Jr. 1996.
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"Nitrogen Saturation in the Rocky Mountains." Environmental Science & Technology Vol 30 No
2.
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B. Nitrogen Dioxide
1. Goal of the Program
The EPA has established national ambient air quality standards (NAAQS) for nitrogen
dioxide (NO2) designed to protect public health and welfare. The goal of the program is to first
achieve these clean air standards throughout the country and then to maintain the standards.
Control of NOX emissions is needed locally in some areas to continue to maintain the NO2
NAAQS.
2. Status of the Program
Section 109 of the CAA directs the EPA Administrator to propose and promulgate
primary and secondary NAAQS for pollutants identified under section 108. Section 109 defines a
primary standard as that necessary to protect the public health, allowing an adequate margin of
safety. A secondary standard, as defined in section 109, must specify an air quality concentration
needed to protect the public welfare from any known or anticipated adverse effects associated
with the presence of the pollutant in the ambient air. Welfare effects, as defined in section 302(h)
of the CAA include, but are not limited to, effects on soils, water, crops, vegetation, materials,
animals, wildlife, weather, visibility and climate, damage to and deterioration of property, and
hazards to transportation, as well as effects on economic values and on personal comfort and
well-being.
States are primarily responsible for ensuring attainment and maintenance of the NAAQS.
Under title I of the CAA, States are to submit, for EPA approval, State implementation plans
(SIPs) that provide for the attainment and maintenance of such standards through control
programs directed to sources of the pollutants involved. In addition, Federal programs provide
for nationwide reductions in emissions of air pollutants through, for example, the Federal Motor
Vehicle Control Program, which involves controls for automobile, truck, bus, motorcycle, and
aircraft emissions.
The primary and secondary NAAQS for NO2 is 0.053 parts per million (ppm) (100
micrograms per meter cubed) annual arithmetic average. In selecting the concentration for the
current standard, the Administrator made judgments regarding the lowest concentrations at which
effects were observed, sensitive populations, nature and severity of public health effects, and
margin of safety. After assessing the evidence, the Administrator concluded that the annual
standard of 0.053 ppm adequately protected against adverse health effects associated with
long-term exposures and provided some measure of protection against possible short-term health
effects. The June 19, 1985 Federal Register notice (50 FR 25532) provides a detailed discussion
of the bases for the existing standard.
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Currently, all areas of the United States are in attainment of the annual NO2 NAAQS of
0.053 ppm (EPA, 1994). Los Angeles is the only city in the United States to record violations of
the annual average NO2 NAAQS during the past decade. In 1992, Los Angeles reported air
quality measurements which met the NO2 NAAQS for the first time.
In accordance with the provisions of sections 108 and 109 of the CAA, as amended, the
EPA conducted a review of the criteria upon which the existing NAAQS for NO2 are based. The
revised criteria were published simultaneously with the issuance of the October 11, 1995 Federal
Register notice of proposed rulemaking on the NO2 NAAQS (60 FR 52874). After evaluating the
revised health and welfare criteria under section 109(d)(l) of the Act, and considering public
comment, the Administrator published a final rulemaking notice on October 8, 1996 (61 FR
52852) which concludes that it is not appropriate to propose any revisions to the primary and
secondary NAAQS for NO2 at this time. As described in the proposed and final rulemaking
notices, EPA determined that a 0.053 ppm annual standard would keep annual NO2
concentrations considerably below the long-term levels for which serious chronic effects have
been observed in animals. Retaining the existing standard also provides protection against
short-term peak NO2 concentrations at the levels associated with mild changes in pulmonary
function and airway responsiveness observed in controlled human studies.
3. Science of N02
NO2
NO2 is a brownish, highly reactive gas that is formed in the ambient air through the
oxidation of NO. As described in the "Ozone" section, emissions of NO play a major role in the
formation of O3 in the lower atmosphere through a complex series of reactions with VOCs.
Emissions of NO are rapidly oxidized in the atmosphere to NO2. NOX refers to the sum of NO2
and NO. Sources of NO (NOX) emissions are described in the "Introduction/Overview" section
and Appendix C. The major sources of anthropogenic NOX emissions are on-road vehicles and
electric utilities.
Health Effects of Concern.
Based on the health effects information contained in EPA's Criteria Document (EPA,
1993), which evaluates key studies published through early 1993 and the Staff Paper (EPA,
1995), EPA has concluded that NO2 is the only nitrogen oxide sufficiently widespread and
commonly found in ambient air at high enough concentrations to be a matter of public health
concern. Exposure to NO2 is associated with a variety of acute and chronic health effects. Two
general groups in the population may be more susceptible to the effects of NO2 exposure than
other individuals. These groups include persons with pre-existing respiratory disease and children
5 to 12 years old. Individuals in these groups appear to be affected by lower levels of NO2 than
individuals in the rest of the population. The health effects of most concern at ambient or
near-ambient concentrations of NO2 include mild changes in airway responsiveness and pulmonary
function in individuals with pre-existing respiratory illnesses and increases in respiratory illnesses
in children (5-12 years old).
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Regarding a short-term NO2 standard, EPA concluded that, while short-term effects from
NO2 are documented in the scientific literature, the available information is insufficient to provide
an adequate scientific basis for establishing any specific short-term standard. However, the EPA
has analyzed the relationship between short-term exceedances of NO2 concentrations and the
annual NO2 mean to determine whether the annual standard would be protective of the acute
effects being observed. In 1994, EPA analyzed air quality data from the period 1988-1992 to
determine the estimated number of exceedances of various NO2 short-term air quality indicators
which would occur given attainment of a range of annual averages. The annual averages analyzed
ranged from 0.02 to 0.06 ppm and included the current NO2 NAAQS of 0.053 ppm. The 1-hour
and daily concentration levels chosen for analyses were 0.15, 0.20, 0.25, and 0.30 ppm. The
results of this analysis are reported in "Analysis of High 1 HrNO2 Values and Associated Annual
Averages Using 1988-1992 Data" (McCurdy, 1994). It was concluded that areas attaining the
current annual NO2 NAAQS reported few, if any, 1 hour or daily exceedances above 0.15 ppm.
Based on the analyses of this air quality data, it was concluded that the existing annual standard
provides adequate protection against potential changes in pulmonary function or airway
responsiveness (which most experts would characterize as mild responses occurring in the range
of 0.2 to 0.5 ppm NO2). The adequacy of the existing annual standard to protect against potential
pulmonary effects is further supported by the absence of documented effects in some studies at
higher (3 to 4 ppm) concentrations of NO2 (EPA, 1995, p. 43).
Welfare Effects Associated with Exposure to NO2
NO2 and other N compounds have been associated with a wide range of effects on public
welfare. The principal effects associated with N deposition include acidification and
eutrophication of aquatic systems. Both processes can sufficiently lower water quality making it
unfit as a habitat for most aquatic organisms and/or human consumption. Acidification of lakes
from N deposition may also increase leaching and methylation of mercury in aquatic systems.
Atmospheric N can enter aquatic systems either as direct deposition to water surfaces or as N
deposition to the watershed.
The principal effects on soils and vegetation associated with excess N inputs include: (1)
Soil acidification and mobilization of aluminum, (2) increase in plant susceptibility to natural
stresses, and (3)modification of inter-plant competition. Atmospheric deposition of N can
accelerate the acidification of soils and increase aluminum mobilization if the total supply of N to
the system (including deposition and internal supply) exceeds plant and microbial demand.
4. How much reduction is needed to maintain the current
standard?
As noted above, all areas of the United States are currently in attainment of the annual
NO2 NAAQS of 0.053 ppm. Los Angeles, the last city in the United States to record violations of
the annual average NO2 NAAQS, has reported air quality measurements which met the NO2
NAAQS since 1992. The Los Angeles and New York areas generally have recorded the highest
annual NO2 ambient concentrations in the nation. These two areas are expected to decrease NOX
emissions in the future to meet the O3 and/or PM standards. The November 1994 SIP submittal
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for the Los Angeles area includes a 59 percent decrease in NOX emissions. The New York area
will benefit from significant NOX emissions reductions throughout the Northeast Ozone Transport
Region through implementation of the September 1994 NOX Memorandum of Understanding.
Also helpful in maintaining the NO2 standard are the broad scale NOX emissions reductions from
implementation of the acid deposition requirements.
Given the implementation of NOX emissions reductions needed to achieve the various
goals of the CAA, it appears that the NO2 standard will continue to be attained throughout the
nation in the foreseeable future. Additional local and regional NOX emissions decreases in areas
with relatively high NO2 concentrations are planned to meet the O3 and PM NAAQS. Thus,
implementation of NOX emissions reductions for O3, PM, and acid deposition is likely to assure
maintenance of the NO2 standard.
5. How much reduction will be achieved with current and
projected programs?
As described in Appendices A and B, substantial decreases in NOX emissions will be
achieved through implementation of several on-going CAA programs. These decreases will
benefit the NO2 program.
6. Summary
Exposure to NO2 is associated with a variety of acute and chronic health effects. The
health effects of most concern at ambient or near-ambient concentrations of NO2 include changes
in airway responsiveness and pulmonary function in individuals with pre-existing respiratory
illnesses and increases in respiratory illnesses in children. Currently, all areas of the United States
are in attainment of the annual NO2 NAAQS of 0.053 ppm. Through implementation of NOX
emissions reductions related to acid deposition and attainment of the O3 and PM NAAQS, it is
likely that the NO2 standard will continue to be attained throughout the nation in the foreseeable
future.
References
McCurdy, T. R., "Analysis of high 1 hour NO2 values and associated annual averages using 1988-
1992 data," Report of the Office of Air Quality Planning and Standards, Durham, NC., 1994.
U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, revised
criteria document, Air Quality Criteria for Oxides of Nitrogen, three volumes,
EPA-600/8-91/049aF-cF, August 1993.
U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, final
revised OAQPS Staff Paper, Review of the National Ambient Air Quality Standards for Nitrogen
Oxides: Assessment of Scientific and Technical Information, EPA-452/R-95-005, September
1995.
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U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, National
Air Quality and Emissions Trends Report, 1993, EPA-454/R-94-026, October 1994.
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C. Ozone
1. Goals of the Program
The EPA has established health and welfare standards for ground level5 O3, which is the
major component of summertime "smog." The goal of the program is to achieve and maintain
these clean air standards throughout the country. As described below, reactions of the emissions
of NOX and VOCs result in the formation of O3 which can adversely affect public health and
welfare in many areas. Decreases in NOX emissions may be needed locally in some areas to attain
the O3 NAAQS. In other areas, regional scale NOX emissions reductions may be needed to help
attain the O3 NAAQS in some downwind areas and/or to help maintain O3 levels below the
standard in some attainment areas.
2. Status of the Program
The NAAQS
Section 109 of the CAA directs the EPA Administrator to propose and promulgate
primary and secondary NAAQS for pollutants identified under section 108. Section 109 defines a
primary standard as that necessary to protect the public health, allowing an adequate margin of
safety. A secondary standard, as defined in section 109, must specify an air quality concentration
needed to protect the public welfare from any known or anticipated adverse effects associated
with the presence of the pollutant in the ambient air. Welfare effects, as defined in section 302(h)
of the CAA include, but are not limited to, effects on soils, water, crops, vegetation, materials,
animals, wildlife, weather, visibility and climate, damage to and deterioration of property, and
hazards to transportation, as well as effects on economic values and on personal comfort and
well-being.
States are primarily responsible for ensuring attainment and maintenance of the NAAQS.
Under title I of the CAA, States are to submit, for EPA approval, State implementation plans
(SIPs) that provide for the attainment and maintenance of such standards through control
programs directed to sources of the pollutants involved. In addition, Federal programs provide
for nationwide reductions in emissions of air pollutants through, for example, the Federal Motor
Vehicle Control Program under title II of the Act, which involves controls for automobile, truck,
bus, motorcycle, and aircraft emissions.
History of NAAQS Reviews
Ground level (tropospheric) O3 refers to O3 occurring from the ground level through about 15 kilometers;
stratospheric O3, which occurs between about 15-50 kilometers altitude is discussed in section III.D of this document.
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Establishment ofNAAQS for Photochemical Oxidants
On April 30, 1971, the EPA promulgated NAAQS for photochemical oxidants under
section 109 of the Act (36 FR 8186). Identical primary and secondary NAAQS were set at an
hourly average of 80 parts per billion (ppb) total photochemical oxidants not to be exceeded
more than 1 hour per year.
Review and Revision ofNAAQS for Photochemical Oxidants
EPA published proposed revisions to the original NAAQS in 1978 (43 FR 16962) and
final revisions in 1979 (44 FR 8202). The primary standard was revised from 80 to 120 ppb;
the secondary standard was set identical to the primary standard; the chemical designation of
the standards was changed from photochemical oxidants to O3; and the form of the standards
was revised from a deterministic form to a statistical form, which defined attainment of the
standards as occurring when the expected number of days per calendar year with maximum
hourly average concentrations greater than 120 ppb is equal to or less than one.
Subsequent Review ofO3 NAAQS
In 1982 (47 FR 11561), the EPA announced plans to revise the 1978 Criteria
Document. On August 10, 1992 (57 FR 35542), the EPA published a proposed decision
under section 109(d)(l) that revisions to the existing primary and secondary standards were
not appropriate at that time. On March 9, 1993 (58 FR 13008), the EPA published a final
decision concluding that revisions to the current primary and secondary NAAQS for 03 were
not appropriate at that time. Given the potential importance of new studies and the EPA's
continuing concern about the health and welfare effects of 03, the March 9, 1993 notice
emphasized the Administrator's intention to complete the next review of the NAAQS as
rapidly as possible and, if appropriate, to propose revisions of the standards at the earliest
possible date.
Most Recent Review ofO3 NAAQS
A series of peer-review workshops was held on draft chapters of the revised Criteria
Document in July and September 1993, and a first external review draft was made available
for the Clean Air Scientific Advisory Committee6 and public review on January 31, 1994.
The EPA review includes analysis of the following alternative primary standards: the current
1-hr standard of 120 ppb, with a maximum expected exceedance rate of one per year
(averaged over 3 years); an 8-hr standard in the range of 70-90 ppb, with a maximum
expected exceedance rate of one per year (averaged over 3 years);and an 8-hr standard in the
range of 70-90 ppb, with a maximum expected exceedance rate of five per year. Further
information on this subject was published in the advance notice of proposed rulemaking on
"National Ambient Air Quality Standards for Ozone and Particulate Matter" published in the
June 12, 1996 Federal Register.
On December 13, 1996, EPA proposed in the Federal Register to change the O3
A standing committee of EPA's Science Advisory Board.
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standards. The proposed revised standards would provide protection for children and other at-
risk populations against a wide range of O3 induced health effects. As described in detail in that
notice, EPA proposed to change the current standard in several respects: (1) attainment would
be based on 8-hour averages of O3, not 1-hour averages; (2) the acceptable concentration
would be lowered from 120 ppb O3 to 80 ppb.
EPA then conducted an extensive public comment process, receiving approximately 57,000
comments at public hearings held across the country and through written, telephone and computer
messages on the O3 and particulate standards proposal. The proposed standards were also
subjected to an intensive inter-agency review process. A court order required EPA to finalize a
PM standard by mid-July of this year, and EPA committed to a court to do the same for O3.
The final air quality standards for O3 and PM were published in the Federal Register of
July 18, 1997 (62 FR 38856). The O3 standards are the same as those proposed in 1996, with one
significant change: the final standard is set at the average fourth highest concentration instead of
the third; this should provide greater stability in the standard for businesses and communities by
requiring more "bad air" days before an area is found to be out of attainment.
It should be noted that, with EPA's July 18, 1997 adoption of the new O3 and PM
standards, greater emphasis might be needed on regional-scale NOX emissions reductions to reach
attainment of the new standard(s). The new standards result in more areas and larger areas with
monitoring data indicating nonattainment (Figure II-1).
O3 Nonattainment Areas
There are over 700 sites maintained by the States and local air pollution control
agencies that measure ground level hourly 03 concentrations (EPA, 1995). Most of these
monitoring sites are located in urban and suburban area locations, with far less density of sites
in rural areas. Peak 03 concentrations typically occur during hot, dry, stagnant summertime
conditions. Year-to-year meteorological fluctuations and long-term trends in the frequency and
magnitude of peak O3 concentrations have a significant influence on an area's compliance status.
In 1991 EPA designated areas attainment and nonattainment for the 1-hour O3 standard
(November 6, 1991 Federal Register at 56 FR 56694). At that time 98 areas were designated
as not in attainment of the NAAQS for O3 (not including transitional and incomplete/no data
areas). Over the last several years, many of these areas achieved the NAAQS for O3 and were
redesignated to attainment, leaving a total of 60 O3 nonattainment areas (as of July 28, 1997)
shown in Figure II-2. More than one hundred million people live in areas designated as not
attaining the 1-hour O3 standard.
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Counties Potentially not Meeting Ozone Standard
(0.08 ppm, 8-hour, 4th Maximum Concentration)
This map is based on measured air quality data from 1993 to 1995. When designations of nonattainment are made in two
to three years they will be made on the most recent three years of quality assured data available for each area,
Figure II-l
map_9395 pre
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Classified Ozone Nonattainment Areas
As of July 28, 1997
Classifications
| Extreme & Severe
Serious
jjjjjljll Moderate
LiD Marginal
Figure II-2
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3. Science of NOX and 03
Health and Welfare Effects
In the lower levels of the troposphere O3 can have adverse effects. Concentrations of O3
near the earth's surface can result in damaging effects on human health, agricultural crops,
ornamental plants, forests, and materials. A summary of these effects is provided below; for
further information, see EPA's December 13, 1997 notice of proposed rulemaking and
references cited therein, such as "Review of National Ambient Air Quality Standards for
Ozone, Assessment of Scientific and Technical Information," OAQPS Staff Paper, June 1996.
A wide array of health effects has been attributed to short-term (1 to 3 hrs), prolonged
(6 to 8 hrs), and long-term (months to years) exposures to elevated 03levels. Those acute
health effects induced by short-term exposures to 03 as low as 120 ppb, generally while
engaged in heavy exercise, such as running, include: transient pulmonary function responses,
transient respiratory symptoms and effects on exercise performance, increased airway
responsiveness, transient pulmonary inflammation, increased susceptibility to respiratory
infection, and increased hospital admissions and emergency room visits. Similar health effects
have been observed following prolonged exposures to 03 as low as 80 ppb and at lower levels
of exercise than for short-term exposures.
Welfare effects addressed by a secondary O3 NAAQS include, but are not limited to,
effects on soils, water, crops, vegetation, man-made materials, animals, wildlife, weather, visibility
and climate, damage to and deterioration of property, and hazards to transportation, as well as
effects on economic values and on personal comfort and well-being. Of these welfare effects
categories, the effects of O3 on crops and vegetation are of most concern at concentrations
typically occurring in the United States. By affecting crops and vegetation, O3 also directly and
indirectly affects soils, water, animals, wildlife, and economic values, as well as aesthetic values,
genetic resources, and natural ecosystems. Thus, providing protection for crops and vegetation
would increase the protection afforded to these other related public welfare categories.
O3 Background
O3 is a naturally occurring, trace constituent of the atmosphere. 03 concentrations vary
by altitude, geographic location, and time. The natural component originates from three
sources: (1) stratospheric O3 which is transported down to the troposphere, (2) O3 formed
from the photochemically-initiated oxidation of biogenic and geogenic methane and carbon
monoxide (CO) with biogenic NOX, and (3) O3 formed from the photochemically-initiated
oxidation of biogenic VOCs with biogenic NOX. Lightning and soils are the major biogenic
sources of NOX emissions and play an important role in the oxidation of methane, carbon
monoxide, and biogenic VOCs, though the magnitude of this natural part cannot be precisely
determined (EPA, 1996). In remote locations--!.e., areas thought to be unaffected by
anthropogenic sources~O3 concentrations tend to be quite low, typically ranging from 20-40
ppb (NRC, 1991). It is reasonable to assume that, in the absence of anthropogenic emissions,
the average summertime O3 concentration in the eastern half of the United States would be
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about 30-40 ppb (OTAG-AQA, 1997).
O3 Formation
While O3 formation in the atmosphere involves complex nonlinear processes, a simplified
description of the process is offered here (Science and Technical Support Work Group, 1997).7
Combustion sources that use air as an oxidizer will produce NOX when temperatures are above
about 2200 degrees Celsius. In addition, incomplete combustion results in the emission of raw
components and oxygenated organic components from the fuels. In sunlight these are sources of
free radicals (e.g., OH, HO2, RO, RO2) that oxidize VOCs to carbonyls, CO, and carbon dioxide,
while simultaneously oxidizing NO to NO2 and recreating the free radical. Each free radical is
cycled up to 5 times. The NO2 reacts with sunlight to recreate NO and to produce O3. After the
first oxidation of NO to NO2, every subsequent operation of the cycle produces an O3 molecule
with an efficiency of greater than 90 percent. In current chemical reaction mechanisms, a typical
nitrogen is cycled 3 to 5 times. Some of the O3 produced reacts with organics and with sunlight to
produce more free radicals to maintain the cyclic oxidation process. This represents a powerful
positive feedback process on the formation of more O3, given available NOX.
The carbonyls produced in the organic oxidation also react with sunlight to produce more
free radicals. As the cycle operates, NO2 is converted into inorganic and organic nitrates; this
form of nitrogen cannot cycle. This also removes free radicals. A system that converts all NOX to
nitrogen products cannot create any more O3. NO2 reacts rapidly with free radicals and in
situations that have a limited supply of radicals, NO2 can compete with the VOCs for the limited
free radicals. This results in virtually no production of O3. Large amounts of emitted NO relative
to the radical sources prevents radical and NO cycling because the reaction between emitted NO
and existing O3 removes O3 (a radical source), and the large amount of NO2 formed competes
effectively with the VOCs for the other available radicals, thus leading to an overall suppression
of O3 in such rich situations.
Different mixtures of VOCs and NOX, therefore, can result in different O3 levels such that
the total system is non-linear. That is, large amounts of VOCs and small amounts of NOX make
O3 rapidly but are quickly limited by removal of the NOX. Decreases of VOCs under these
circumstances show little effect on O3. Large amounts of NOX and small amounts of VOCs
(which usually implies smaller radical source strengths) result in the formation of inorganic
nitrates, but little O3. In these cases, decreases in NOX emissions result in an increase in O3
concentrations. Some combination of VOCs and NOX is optimum at producing O3.
The preceding is a static description. In the atmosphere, physical processes compete with
chemical processes and change the outcomes in complex ways. The existence of feedback and
non-linearity in the transformation system confound the description. Competing processes
determine the ambient concentration and there are an infinite set of process magnitudes that can
give rise to the same ambient concentrations and changes in concentrations. Lack of any direct
See, for example, NRC, 1991 for more information on O3 chemistry.
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measurement of process magnitudes result in the need to use inferential methods to confirm any
one explanation of a particular O3 concentration.
Regional Scale of the O3 Problem
As described in the preceding section, the impact of NOX emissions on O3 concentrations
is complex. While NOX emissions are necessary for the formation of O3 in the lower atmosphere,
a local decrease in NOX emissions can, in some cases, increase local O3 concentrations. This
effect of NOX emissions reductions is further discussed in section IV. A of this document, Local
and Regional NOX Controls.
The formation of O3 is further complicated by biogenic emissions, meteorology, and
transport of O3 and O3 precursors. The contribution of O3 precursor emissions from biogenic
sources to local ambient O3 concentrations can be significant. This is especially true for
emissions of biogenic VOCs. Important meteorological factors include temperature and wind
direction and speed. Long-range transport of O3, reactive N compounds, and partially oxidized
organics such as aldehydes (which are excellent radical sources) can result in interactions
between distant sources in urban or rural areas and local ambient O3. Peroxyacetyl nitrate
(PAN), formed from reaction of radicals with NO2, can transport NOX over relatively large
distances through the atmosphere. Its rate of decomposition slows significantly with decreases in
temperature, so that it can be formed near the surface in NOX rich areas, advected aloft to cooler
conditions higher in the atmosphere, transported by the higher wind speeds aloft, and then be
brought down to the warmer surface air to decompose and deliver NO2 to downwind areas (NRC,
1991).
Typically, O3 episodes (periods including high O3 concentrations) last from 3-4 days on
average, occur as many as 7-10 times a year, and often are of large spatial scale; in the eastern
United States, high concentrations of O3 in urban, suburban, and rural areas tend to occur
concurrently on scales of over 1000 kilometers (NRC, 1991). Maximum values of non-urban O3
commonly exceed 90 ppb during these episodes, compared with average daily maximum values
of 60 ppb in summer. Thus, an urban area need contribute an increment of only 30 ppb over the
regional background during a high O3 episode to cause a violation of the 1-hour O3 NAAQS of
120 ppb (NRC, 1991).
The precursors to O3 and O3 itself are transported long distances under some commonly
occurring meteorological conditions. The transport of O3 and precursor pollutants over hundreds
of kilometers can be a significant factor in the accumulation of O3 in any given area. Few urban
areas in the United States can be treated as isolated cities unaffected by regional sources of O3
(NRC, 1991). As described below, there is a growing body of evidence that decreasing regional
O3 levels by decreasing regional NOX emissions holds the key to the ability of a number of the
most seriously polluted nonattainment areas in the Eastern United States to attain and maintain
the O3 NAAQS.
4. How much reduction in NOX is needed nationally to
achieve the 03 standard?
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Summary: National, Regional, and Local Scale NOX Emissions Reductions Are Needed
As noted below, studies of the South, the Northeast Ozone Transport Region, and the
states bordering Lake Michigan indicate that O3 and O3 precursors transported from attainment
areas both within the regions and outside of the regions contribute to O3 nonattainment within
the regions. The extent of local controls that will be needed to attain and maintain the O3 NAAQS
in and near seriously polluted cities is sensitive both to the amount of O3 and precursors
transported into the local area and to the specific photochemistry of the area. In some cases,
preliminary local modeling with respect to the 1-hour standard performed by the states indicates
that it may not be feasible to find sufficient local control measures for individual nonattainment
areas unless transport into the areas is significantly decreased. The EPA has also conducted
preliminary analyses for the new 8-hour O3 standard which indicate that regional NOX emissions
decreases would be effective in helping many areas attain that standard. These preliminary
modeling studies consistently suggest that decreasing NOX emissions on a regional basis may be
the most effective approach for decreasing O3 over large geographic areas, even though local
NOX controls may be detrimental in urban centers of selected nonattainment areas on some days.
Thus, large decreases in NOX emissions, in combination with other local controls, may be needed
over much of the nation if all areas are to attain the O3 standard, as summarized below.
California
NOX emissions reductions from 25-60 percent are needed in specific nonattainment areas.
The State of California adopted their O3 SIP on November 15, 1994. The SIP covers
most of the populated portion of the state and relies on both NOX and VOC emissions reductions
for most California nonattainment areas to demonstrate compliance with the NAAQS.
Specifically, the revised SIP projects that the following NOX emissions reductions are needed
(from a 1990 baseline): South Coast, 59 percent; Sacramento, 40 percent; Ventura, 51 percent;
San Diego, 26 percent; and San Joaquin Valley, 49 percent.
The South Coast's control strategy for attainment of the O3 standard specifies a 59
percent decrease in NOX emissions. The design of this strategy took into account the need to
decrease NOX as a precursor of PM, as described in the SIP submittal. This represents a decrease
of over 800 tons of NOX per day. The emissions reductions are to be achieved from a
combination of national, state, and local control measures.
The Sacramento Metropolitan area's control strategy for attainment of the O3 standard
specifies a 40 percent decrease in NOX emissions. Modeling results indicate that NOX emissions
reductions in this urban area are more effective than VOC emissions reductions on a tonnage basis
in lowering ambient O3 concentrations. The decreases are to be achieved from a combination of
national, state, and local control measures, especially mobile source measures such as standards
for heavy duty vehicles and off-road engines.
The Lake Michigan Area
Regional NOX emissions reductions are needed.
Modeling and monitoring studies performed to date for the states surrounding Lake Michigan
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(Wisconsin, Illinois, Indiana, and Michigan) indicate that decreasing O3 and O3 precursors
transported into the nonattainment areas would have a significant effect on the number and
stringency of local control measures to meet the 1-hour O3 NAAQS. In many cases boundary
conditions appear to contribute significantly to peak O3 concentrations. The O3 and O3
precursors flowing into a metropolitan area can greatly influence the peak O3 concentration
experienced in the metropolitan area. For example, the 1991 Lake Michigan Ozone Study found
that transported O3 concentrations entering the region were 40-60 percent of the peak O3
concentrations in some of the metropolitan areas. The air mass entering the study area was
measured by aircraft at 70-110 ppb on episode days (Roberts et al, 1994). In the Lake Michigan
case, a 30 percent reduction in boundary conditions was shown by modeling to be as effective at
lowering peak O3 concentrations as a 60 percent decrease in local VOC emissions (LADCO
1994).
These studies suggest that without such region-wide emissions reductions, the necessary
degree of local control will be very difficult to achieve, even with very stringent local controls.
The EPA Matrix study (Chu and Cox, 1995) reinforces that regional NOX control will be effective
in lowering O3 across the Midwest region. Taken together, the information available to date
suggests that additional reductions in regional NOX emissions will probably be necessary in
meeting the NAAQS in the Chicago/Gary/Milwaukee area and downwind (including western
Michigan), even though currently available modeling shows that there may be a detrimental effect
on some days from applying NOX controls locally in and near the major nonattainment areas.
New York Study
Regional NOx emissions reductions of 75 percent are needed.
New York State's recent Urban Airshed Model (UAM) studies show that substantial
decreases in the O3 transported from other regions would be necessary if several areas within the
UAM domain are to achieve O3 attainment (John et al, 1994a,b). This UAM study demonstrates
the potential effectiveness of a regional NOX emissions reduction strategy in combination with a
local VOC emissions reduction strategy. The New York study showed that the combination of a
regional strategy reflecting a 25 percent decrease in VOCs and a 75 percent decrease in NOX
with a local strategy reflecting a 75 percent decrease in VOCs and a 25 percent decrease in NOX
would be necessary for all areas throughout the New York UAM domain to lower predicted O3
levels to 120 ppb or less during adverse meteorological conditions.
Northeast Ozone Transport Region
Regional NOx emissions need to be decreased 50-75 percent.
In its analysis supporting the approval of a Low Emission Vehicle program in the mid-
Atlantic and Northeast states comprising the Northeast Ozone Transport Region (OTR),8 EPA
reviewed existing work and performed analyses to evaluate in detail the degree to which NOX
controls are needed (EPA, 60 FR 48673). These studies indicated that NOX emissions must be
The Northeast Ozone Transport Region (OTR) is comprised of the states of Maine, New Hampshire,
Vermont, Massachusetts, Rhode Island, Connecticut, New York, New Jersey, Pennsylvania, Delaware, Maryland, and
the Consolidated Metropolitan Statistical Area that includes the District of Columbia and northern Virginia.
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decreased by 50 to 75 percent from 1990 levels to obtain predicted O3 levels of 120 ppb or less
throughout the OTR. In addition, the EPA Matrix Study indicates that regional NOX emissions
reductions are more effective at lowering high O3 concentrations than regional VOC emissions
reductions.
More recent studies have confirmed these conclusions (Kuruville et al, 1994; Cox et al,
1993). Additional modeling simulations suggest that region-wide NOx controls coupled with
urban-specific VOC controls would be needed for O3 attainment in the northeastern United States
(Rao et al, 1995). Taken together, these studies point to the need to decrease NOX emissions in
the range of 50 to 75 percent throughout the OTR and that VOC emissions must also be
decreased by the same amount in and near the Northeast urban corridor to reach and maintain
predicted hourly maximum O3 levels of 120 ppb or less.
Southeast
NOX emissions reductions up to 90 percent will be needed for the Atlanta area to attain.
In the South, relatively high concentrations of O3 are measured in both rural and urban
areas (Chameides and Cowling, 1995). Analyses of monitored data by Southern Oxidant Study
investigators suggest that the background O3 levels are likely to be more responsive to decreases
in NOX emissions than VOC emissions. Modeling to-date indicates that, in the absence of regional
control measures, NOX emissions reductions on the order of ninety percent may be needed for the
Atlanta area to attain the O3 standard during worst case weather conditions (Chameides and
Cowling, 1995).
Ozone Transport Assessment Group (OTAG)
The EPA supported a consultative process among 37 eastern states which included
examination of the extent to which NOX emissions from as far as hundreds of kilometers away are
contributing to smog problems in downwind cities in the eastern United States. Known as the
OTAG and chaired by the State of Illinois, this group looked into ways of achieving additional
cost-effective programs to further lower ground-level O3 throughout the eastern United States.
The OTAG's modeling workgroup reported several key findings as a result of modeling analyses
they conducted (OTAG-RUSM, 1997):
* NOX emissions reductions are more effective than VOC emissions reductions in
lowering regional O3 concentrations; NOX reductions decrease O3 domainwide,
while VOC reductions decrease O3 only in urban areas.
* Elevated and low-level NOX emission reductions are both effective in lowering
regional O3 concentrations.
* More NOX emission reductions result in more O3 benefit.
* Emission reductions in a given area mostly affect O3 in that same area.
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* Emission reductions in a given area also affect O3 in downwind areas.
* Emission reductions will be effective in lowering 1-hour and 8-hour O3.
* Regional reductions in NOX emissions are necessary to help provide for attainment
and maintenance of the O3 NAAQS in the eastern United States.
The OTAG's air quality analysis workgroup also reported several important findings as a result of
analyses they conducted (OTAG-AQA, 1997):
* The distances of O3 impact deduced from multiple types of analysis range from 150
to 500 miles.
* The direct influence of specific urban areas can be directly traced to some 150-200
miles before merging indistinguishably into the regional O3 pattern.
* O3 transported at night can have a significant impact hundreds of miles downwind
the next day.
Integrated Strategies for Implementing the O3 and PM Standards
Common Factors
As described in the sections on "Ozone" and "Particulate Matter," EPA published
revisions to the O3 and PM NAAQS on July 18, 1997 (62 FR 38856). As part of the revisions
process, EPA initiated action to address strategies for the implementation of the new NAAQS.
These ongoing reviews and related implementation strategy activities to date have brought out
important common factors between O3 and PM. Similarities in pollutant sources, formation, and
control exist between O3 and PM, in particular the fine fraction of particles. These similarities
provide opportunities for optimizing technical analysis tools (i.e., monitoring networks, emissions
inventories, air quality models) and integrated emissions reduction strategies to yield important
cross-cutting benefits across various air quality management programs. This integration could
result in a net reduction of the regulatory burden on some source category sectors that would
otherwise be impacted separately by O3, PM, and visibility protection control strategies.
Federal Advisory Committee Act (FACA) Process
The EPA initiated a process designed to provide for significant stakeholder involvement in
the development of integrated implementation strategies for the new/revised O3 and PM NAAQS
and a new regional haze program. As described below, this process involves a new subcommittee
of the Agency's Clean Air Act Advisory Committee (CAAAC), established in accordance with the
FACA (5 U.S.C. App.2). The CAAAC was established to provide independent advice and
counsel to the EPA on policy and technical issues associated with the implementation of the Act.
The CAAAC advises EPA on the development, implementation, and enforcement of several of the
new and expanded regulatory and market-based programs required by the Act.
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The CAAAC advises on issues that cut across several program areas. A new
subcommittee of the CAAAC, the Subcommittee for Ozone, Particulate Matter, and Regional
Haze Implementation Programs (the Subcommittee), was established in August 1995 to address
integrated strategies for the implementation of the new O3 and PM NAAQS, as well as a regional
haze program. The focus of the Subcommittee will be on assisting EPA in developing
implementation control strategies, preparing supporting analyses, and identifying and resolving
impediments to the adoption of the resulting programs. The Subcommittee is composed of
representatives selected from among state, local, and tribal organizations; environmental groups;
industry; consultants; science/academia; and federal agencies. Recommendations made by the
Subcommittee will be submitted to EPA through CAAAC.
5 . How much reduction in NOX emissions will be
achieved with current and projected NOX programs?
Substantial emissions reductions are currently being achieved through implementation of
the CAA measures for mobile and stationary sources. These measures include the retrofit of
reasonably available control technology on existing major stationary sources of NOX and
implementation of enhanced vehicle inspection and maintenance (I/M) programs under Title I,
new tailpipe standards for new motor vehicles under Title II, and controls on certain coal-fired
electric power plants under Title IV, Phase I. Total NOX emissions will decline about 6 percent
from current levels by the year 2000. Despite increases in vehicle miles traveled, total on-road
vehicle emissions will likely continue to decline through 2005 as per vehicle emissions decrease
due to tighter tailpipe standards, phase II reformulated gasoline is implemented, and I/M
requirements are met.
Shortly after the year 2002, overall NOX emissions are projected to begin to increase and
continue to increase in the foreseeable future due to increased economic activity, unless new NOX
emissions reduction initiatives are implemented (EPA, 1995). It is clear that new controls will be
needed to approach the decreases in NOX emissions of 25-90 percent which are projected as being
needed over large portions of the nation to attain the O3 standard. As described in Appendices A
and B, several such new initiatives to decrease emissions of NOX are planned or underway.
6. Summary
Emissions of NOX result in the formation of O3 that can contribute to local O3
nonattainment problems in some cases and/or, through long-range transport, contribute to
nonattainment in downwind areas. High O3 concentrations occur over large portions of the
Eastern United States on some days during the summer. The transport of high O3 concentrations
into certain urban nonattainment areas makes it impractical for these urban areas to attain the
NAAQS based on local controls alone. Thus, decreases in NOX emissions are needed locally in
some areas to attain the O3 NAAQS while, in other areas, NOX emissions reductions may be
needed to help attain the O3 NAAQS in downwind areas or to help maintain O3 levels below the
standard in attainment areas.
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Control strategies must consider efforts to decrease regional scale emissions as well as
local emissions. In general, NOX emissions reductions in upwind, rural areas coupled with VOC
emissions reductions in urban nonattainment areas appears to be an effective strategy. In some
cases however, the urban nonattainment area is also upwind of another urban nonattainment area
or contains substantial biogenic VOC emissions. In these cases, NOX emissions reductions may
be needed in addition to, or instead of, VOC emissions reductions for purposes of O3 attainment.
In both cases, decreases in precursor emissions in the upwind areas will help the downwind
metropolitan areas attain and maintain the O3 standard. Thus, effective O3 control will require an
integrated strategy that combines cost-effective emissions reductions in emissions at the local,
state, regional, and national levels.
References
Chameides, W.L. and E.B. Cowling, The State of the Southern Oxidant Study (SOS): Policy-
Relevant Findings in Ozone Pollution Research, 1988-1994. North Carolina State University,
April 1995.
Chu, Shao-Hung and W.M. Cox, "Effects of Emissions Reductions on Ozone Predictions by the
Regional Oxidant Model during the July 1988 Episode, Journal of Applied Meteorology, Vol.
34, Nol 3, March 1995.
Cox, William M. and Chu, Shao-Hung, "Meteorologically Adjusted Ozone Trends in Urban
Areas: A Probabilistic Approach," Atmospheric Environment, Vol. 27B, No. 4, pp 425-434,
1993.
Finlayson-Pitts, BJ. and J.N. Pitts, Jr., "Atmospheric Chemistry of Tropospheric Ozone
Formation: Scientific and Regulatory Implications," Air and Waste Management Association,
Vol. 43, August 1993.
Georgia Environmental Protection Division, "The 1994 State Implementation Plan for the Atlanta
Ozone Nonatttainment Area," 1994.
John, K., S.T. Rao, G.Sistla, W.Hao, and N.Zhou, I994a, Modeling Analyses of the Ozone
Problem in the Northeast, EPA-230-R-94-018, 1994.
John, K., S.T. Rao, G.Sistla, N.Zhou, W.Hao, K. Schere, S. Roselle, N.Possiel, R.Scheffe,
1994b, "Examination of the Efficacy of VOC and NOx Emissions Reductions on Ozone
Improvement in the New York Metropolitan Area," printed \n_Air Pollution Modeling and Its
Application, Plenum Press, NY, 1994.
Kuruville, John et al., Modeling Analyses of Ozone Problem in the Northeast, prepared for EPA,
EPA Document No. EPA-230-R-94-108, 1994.
Lake Michigan Air Directors Consortium, Lake Michigan Ozone Study—Evaluation of the UAM-
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V Photochemical Grid Model in the Lake Michigan Region., 1994.
National Research Council, Committee on Tropospheric Ozone Formation and Measurement,
Rethinking the Ozone Problem in Urban and Regional Air Pollution, 1991.
Nichols, Mary D., Assistant Administrator for Air and Radiation, "Ozone Attainment
Demonstrations," memorandum to EPA Regional Administrators, March 2, 1995.
Ozone Transport Assessment Group, "Summary of Modeling Data," prepared by Mike Koerber,
Co-Chair Regional and Urban Scale Modeling Workgroup, for the June 2-3, 1997 Policy Group
meeting.
Ozone Transport Assessment Group, "Summary of Air Quality Data," prepared by Dave
Guinnup, Co-Chair Air Quality Analysis Workgroup, for the June 2-3, 1997 Policy Group
meeting.
Rao, S.T., G. Sistla, W. Hao, K. John and J. Biswas, "On the Assessment of Ozone Control
Policies for the Northeastern United States," presented at the 21st NATO/CCMS International
Technical Meeting on Air Pollution Modeling and Its Application, Nov. 6-10, 1995.
Roberts, P.T., T.S. Dye, M.E. Korc, H.H. Main, Air Quality Data Analysis for the 1991 Lake
Michigan Ozone Study, final report, STI-92022-1410-FR, Sonoma Technology, 1994.
Science and Technical Support Work Group, Ozone and Particulate Matter National Ambient Air
Quality Standards, Federal Advisory Committee Act, Harvey Jeffries and Tom Helms, co-chairs,
"Conceptual Model for Ozone, Particulate Matter and Regional Haze," February 19, 1997.
Seinfeld, John H., "Urban Air Pollution: State of the Science," February 10, 1989 vol., Science.
U.S. Environmental Protection Agency, Low Emission Vehicle Program for Northeast Ozone
Transport Region; Final Rule, January 24, 1995 Federal Register. 60 FR 48673.
U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, National
Air Pollutant Emission Trends, 1900-1994," October 1995.
U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, Review of
National Ambient Air Quality Standards for Ozone, Assessment of Scientific and Technical
Information, OAQPS Staff Paper, EPA-452/R-96-007, June 1996.
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D. Particulate Matter
1. Goals of the Program
The EPA has established health and welfare standards for particulate matter (PM). The
goals of the program are to achieve and maintain these clean air standards throughout the country.
As described below, emissions of NOX can result in the formation of particulate nitrates that can
contribute to PM nonattainment in some areas. Decreases in NOX emissions might be needed in
some areas to attain the PM NAAQS. In other areas, NOX emissions reductions may not be
needed to attain the PM NAAQS, but could help maintain PM levels below the standard in
attainment areas.
2. Status of the Programs
The NAAQS
Section 109 of the CAA directs the EPA Administrator to propose and promulgate
primary and secondary NAAQS for pollutants identified under section 108. Section 109 defines a
primary standard as that necessary to protect the public health, allowing an adequate margin of
safety. A secondary standard, as defined in section 109, must specify an air quality concentration
needed to protect the public welfare from any known or anticipated adverse effects associated
with the presence of the pollutant in the ambient air. Welfare effects, as defined in section 302(h)
of the CAA include, but are not limited to, effects on soils, water, crops, vegetation, materials,
animals, wildlife, weather, visibility and climate, damage to and deterioration of property, and
hazards to transportation, as well as effects on economic values and on personal comfort and
well-being.
States are primarily responsible for ensuring attainment and maintenance of the NAAQS.
Under title I of the CAA, States are to submit, for EPA approval, SIPs that provide for the
attainment and maintenance of such standards through control programs directed to sources of
the pollutants involved. In addition, Federal programs provide for nationwide reductions in
emissions of air pollutants through, for example, the New Source Performance Standards program
under title I of the Act, which involves controls for major stationary sources.
PM
The term PM refers to a solid or liquid material that is suspended in the atmosphere. PM
includes materials of both organic and inorganic composition, and generally can also be divided
into a primary component and secondary component. Primary PM consists of solid particles,
aerosols, and fumes emitted directly as particles or droplets from various sources. Secondary PM
is produced from gaseous pollutants, mainly SO2, NOX, ammonia, and some VOCs. These
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precursor gases react with one another and with oxygen and water in the atmosphere to form
particles or condensible compounds. The chemical and physical properties of PM vary greatly
with time, region, meteorology, and source category, thus complicating their understanding and
control.
The PMNAAQS
The PM NAAQS include PM2 5 standards and PM10 standards. The PM2 5 standards are
set at 15 micrograms per cubic meter, annual mean, and 65 micrograms per cubic meter, 24-hour
average. The PM10 standards are set at 50 micrograms per cubic meter, annual average, and
150 micrograms per cubic meter, 24-hour average. (For more details see the "Most Recent
Review of the Particulate Matter NAAQS" section below).
Areas That Do Not Meet the PM10 NAAQS
In 1990 EPA designated 70 areas as moderate nonattainment for PM10, and 13
additional areas were added in 1994 for a total of 83 PM10 nonattainment areas. Five of the
initial areas have been reclassified to serious nonattainment areas. Based on air quality data
for 1992 to 1994, 37 of these (but none of the serious areas) were determined to have met the
PM10 NAAQS by their December 31, 1994 attainment date. The current 46 nonattainment
areas are shown in Figure II-3 below.
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AREAS DESIGNATED NONATTAINMENT
FOR PM10 PARTICULATES
Figure II-3
CIRCLE DIAMETER
INDICATES RELATIVE SIZE
OF AFFECTED POPULATION
OAQPS. AQSSD
PGOUARY, 20. 1906
Ln
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Establishment of the PM NAAQS and Subsequent Reviews
Establishment of the NAAQS for PM
NAAQS for PM were first established in 1971 (April 30, 1971 Federal Register). The
reference method specified for determining attainment of the original standards was the high-
volume sampler, which collects PM up to a nominal size of 25 to 45 microns (so-called total
suspended particulate or TSP). The primary standards (measured by the indicator TSP) were
260 micrograms per cubic meter, 24-hour average, not to be exceeded more than once per
year, and 75 micrograms per cubic meter, annual geometric mean. The secondary standard
(measured as TSP) was 150 micrograms per cubic meter, 24-hour average not to be exceeded
more than once per year.
First Review of NAAQS for PM
In October 1979 (44 FR 56731), EPA announced the first review of the criteria
document and NAAQS for PM and, after a lengthy and elaborate process, promulgated
significant revisions of the original standards in 1987 (52 FR 24854, July 1, 1987). In that
decision, EPA changed the indicator for particles from TSP to PM10, the latter referring to
particles with a mean aerodynamic diameter less than or equal to 10 microns.9 EPA also
revised the acceptable concentration and form of the primary standards by 1) replacing the 24-
hour TSP standard with a 24-hour PM10 standard of 150 micrograms per cubic meter with no
more than one expected exceedance per year averaged over 3 years and 2) replacing the annual
TSP standard with a PM10 standard of 50 micrograms per cubic meter, expected annual
arithmetic mean. The secondary standard was revised by replacing it with 24-hour and annual
standards identical in all respects to the primary standards. The revisions also included a new
reference method for the measurement of PM10 in the ambient air and rules for determining
attainment of the new standards.
Most Recent Review of the PM NAAQS
To initiate its most recent review, EPA analyzed thousands of peer-reviewed scientific
studies. These studies were then synthesized, along with a recommendation on whether the
existing standards were adequately protective, and presented to an independent scientific advisory
body ("CASAC"), as required by the CAA. After holding more than 125 hours of public
discussion, and based upon 250 of the most relevant studies, CASAC concluded that EPA's
current O3 and particulate standards should be strengthened. This review took several years to
complete.
On December 13, 1996, EPA proposed in the Federal Register to change the PM
standard (61 FR 65638). As described in detail in that notice, EPA proposed to change the
current standards by adding two new primary PM2i5 standards set at 15 micrograms per cubic
The more precise term is 50 percent cut point or 50 percent diameter. This is the aerodynamic particle
diameter for which the efficiency of particle collection is 50 percent. Larger particles are not excluded altogether,
but are collected with substantially decreasing efficiency and smaller particles are collected with increasing (up to
100 percent) efficiency. Ambient samplers with this cut point provide a reliable estimate of the total mass of
suspended particulate matter of aerodynamic size less than or equal to 10 microns.
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meter, annual mean, and 50 micrograms per cubic meter, 24-hour average. The revisions would
provide increased protection against a wide range of potential PM-related health effects. The
proposed annual PM2 5 standard would be based on the 3-year average of the annual arithmetic
mean PM2 5 concentrations, spatially averaged across an area. The proposed 24-hour PM2 5
standard would be based on the 3-year average of the 98th percentile of 24-hour PM2 5
concentrations at each monitor within an area. The EPA proposed to revise the current 24-
hour PM10 standard of 150 micrograms per cubic meter by replacing the 1-expected-
exceedance form with a 98th percentile form, averaged over 3 years at each monitor within an
area. The EPA proposed to retain the current annual primary PM10 standard of 50
micrograms per cubic meter. In addition, EPA proposed to revise the current secondary
standards by making them identical to the suite of proposed primary standards.
EPA then conducted an extensive public comment process, receiving approximately
57,000 comments at public hearings held across the country and through written, telephone and
computer messages. The proposed standards were also subjected to an intensive inter-agency
review process. A court order required EPA to finalize a PM standard by mid-July of this year,
and EPA committed to a court to do the same for O3.
EPA's final air quality standards for O3 and PM were published in the Federal Register of
July 18, 1997 (62 FR 38856). With respect to PM, the final standards include one significant
change from EPA's 1996 proposal: the final standard set the 24-hour limit at 65 micrograms per
cubic meter, instead of 50 micrograms (as proposed), to provide maximum flexibility for local
areas and sources, while still retaining the public health protections of the proposal that are
incorporated into the annual standard.
3 . Science of NOX and PM
Health and Welfare Effects
Exposure to airborne PM has a wide range of adverse health effects. The damages caused
by PM vary depending on its concentration, composition, and the sizes of the constituent
particles. A summary of these effects is provided below; for further information, see EPA's
notice of proposed rulemaking on " National Ambient Air Quality Standards for Ozone and
Particulate Matter" published in the December 13, 1997 Federal Register and relevant
documents referenced in that notice.
As discussed in EPA's Criteria Document (EPA, April 1996) and Staff Paper (EPA, July
1996) and summarized in the December 13, 1996 proposal notice, the key health effects
associated with PM include: 1) premature mortality; 2) aggravation of respiratory and
cardiovascular disease (as indicated by increased hospital admissions and emergency room visits,
school absences, work loss days, and restricted activity days); 3) changes in lung function and
increased respiratory symptoms; 4) changes to lung tissues and structure; and 5) altered
respiratory defense mechanisms. Most of these effects have been consistently associated with
ambient PM concentrations, which have been used as a measure of population exposure, in a
number of community epidemiological studies. Although mechanisms by which particles cause
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effects have not been elucidated, there is general agreement that the cardio-respiratory system is
the major target of PM effects.
The EPA revised the secondary (welfare-based) PM NAAQS by making them identical to
the primary standards. The EPA believes that the PM2 5 and PM10 standards, combined with the
CAA required regional haze program, will provide protection against the major PM-related
welfare effects. These welfare effects include visibility impairment, soiling, and materials damage.
The Administrator of EPA signed the proposed rulemaking notice for the regional haze rules on
July 18, 1997.
Size of Particles
The health and environmental effects of PM are strongly related to the size of the
particles (EPA Staff Paper, 1996). The aerodynamic size and associated composition of
particles determines their behavior in the respiratory system (i.e., how far the particles are
able to penetrate, where particles are deposited, and how effective the body's clearance
mechanisms are in removing them). Furthermore, particle size is one of the most important
parameters in determining atmospheric lifetime of particles, which is a key consideration in
assessing health effects information because of its relationship to exposure. The total surface
area and number of particles, chemical composition, water solubility, formation process, and
emission sources all vary with particle size. Particle size is also a determinant of visibility
impairment, a welfare consideration linked to fine particle concentrations. Thus, size is an
important parameter in characterizing PM, and particle diameter has been used to define the
present standards.
Atmospheric Behavior of Fine and Coarse Particles
Sulfates, nitrates, and some organic particles as well as their precursors can remain in
the atmosphere for several days and can be carried hundreds or even thousands of kilometers
from their sources to remote locations such as national parks and wilderness areas (NRC,
1993). Fine particles are small enough that gravitational forces are largely overcome by the
random forces from collisions with gas molecules. Thus fine particles tend to follow air streams
and are difficult to remove by impaction on surfaces. Therefore, fine particles have very long
lifetimes in the atmosphere, travel long distances, and tend to be more uniformly distributed over
larger geographic areas than coarse particles (EPA, 1996). The atmospheric lifetimes of fine
particles with respect to dry deposition is on the order of weeks. Removal of fine particles occurs
when the particles absorb water, grow into cloud droplets, grow further to rain drops, and fall out
as rain. This process lowers the atmospheric lifetime of fine particles to on the order of several
days.
In contrast, coarse particles are large enough so that the force of gravity exceeds the
buoyancy forces of the surrounding air currents leading to their settling out to the earth's surface.
Coarse particles are in the 2.5 to 10 micron size range. These larger particles tend to fall rapidly
out of the air, with atmospheric lifetimes of only minutes to hours depending on their size. Coarse
particles are also too large to follow air streams, such that they tend to be easily removed by
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impaction on surfaces. Coarse particles are primarily composed of crustal elements (silicon,
aluminum, iron and potassium); biological materials (bacteria, pollen, and spores) also appear in
the coarse mode.
Emission Sources and Formation Processes of Particles
In most locations, a variety of diverse activities contribute significantly to PM
concentrations, including fuel combustion (from vehicles, power generation, and industrial
facilities), residential fireplaces, agricultural and silvicultural burning, and atmospheric
formation from gaseous precursors (largely produced from fuel combustion). Other sources
include construction and demolition activities, wind blown dust, and road dust. From these
diverse sources come the mix of substances that comprise PM. The major chemical
constituents of PM10 are sulfates, nitrates, carbonaceous compounds (both elemental and
organic carbon compounds), acids, ammonium ions, metal compounds, water, and crustal
materials. The amounts of these components vary from place to place and over time.
Coarse particles are primarily the result of crushing or grinding processes. Fine
particles result from (1) direct emissions, (2) gaseous emissions which condense in the
atmosphere without any other chemical reactions, and (3) precursor gases that later chemically
react to form fine particles. Particles formed as a result of chemical reaction of gases are
termed secondary particles because the direct emissions from a source is a gas (e.g., S02 or
NO) that is subsequently converted to a low vapor pressure substance in the atmosphere.
Sources of fine and coarse particles are summarized in Tables II-2 and II-3 (EPA, 1996). The
fraction of fine particulate due to sulfate is greater in the East, and the nitrate fraction is larger
in the West (see figure II-4; EPA, April 1996).
Transformation from gases to particles requires substantial interaction in the
atmosphere. Such transformation can take place locally, during prolonged stagnations, or
during transport over long distances. Moisture, sunlight, temperature, and the presence or
absence of fogs and clouds affect transformation. In general, particles formed from these
types of secondary processes will be more uniform in space and time than those that result
from primary emissions.
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% NO
Yearly average absolute and relative concentrations for sulfate and nitrate.
Source: Sisler et al. (1993) and Malm et al. (1994b).
Figure II-4
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A large fraction of the mass in the fine size fraction is derived from material that has
been volatilized in combustion chambers and then recondensed to form primary fine PM, or
has been formed in the atmosphere from precursor gases as secondary PM. Since precursor
gases and fine PM are capable of traveling great distances, it is difficult to identify precisely
the contribution of the individual sources. Sulfuric acid, which is the source of particle strong
acidity and sulfates, is formed from the atmospheric reaction of S02 which is formed during
combustion of sulfur compounds contained in fossil fuels. As noted below, nitrates are
formed by atmospheric reactions of NOX which are generated during combustion or other high
temperature processes. Ammonia, which neutralizes sulfuric and nitric acid to form sulfates
and nitrates, has a variety of sources, the most important being emissions from animal waste
and fertilizers.
PM may be formed from emissions of NO which are converted to N02 which then
participates in various reactions to form other substances, including O3 and PM. Nitrate
airborne particles can be produced by several mechanisms. One major mechanism of nitrate
formation involves nitric acid vapor which has a much higher vapor pressure than sulfuric acid
and tends to stay more in the gas phase. Nitric acid (HN03) is mostly formed in the gas-phase
reaction of NO2 with the hydroxyl radical. The gaseous nitric acid can react with ammonia to
form ammonium nitrate or at airborne particle surfaces to form nitrate salts, such as sodium
nitrate. Thus, nitrate size distributions depend, in part, on the size distributions of the
particles on which they react. Conditions that favor aerosol nitrate formation include high
nitric acid concentrations, high ammonia (gas phase) or salt particle concentrations, low
temperatures, and high relative humidity. If the air parcel carrying the aerosol nitrate
experiences a temperature increase and/or decrease in humidity, the concentration of the
aerosol nitrate would be expected to decline as the nitric acid or ammonia returns to gas phase.
Fine particle nitrate concentrations near 100 micrograms per cubic meter over 24-hour
averaging times have been observed in the eastern end of the South Coast Air Basin that
surrounds Los Angeles during late October (Science and Technical Support Work Group,
1997).
Visibility-Impairing Particles
As described in the "Visibility Protection" section of this document, fine particles are
effective in impairing visibility by scattering or absorbing light. Different types of particles
have varying efficiencies in causing visibility impairment. The fine particles principally
responsible for visibility impairment are sulfates, nitrates, organic matter, elemental carbon
(soot), and soil dust. Coarse particles also impair visibility, although less efficiently than fine
particles.
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TABLE II-2 CONSTITUENTS OF ATMOSPHERIC FINE PARTICLES LESS THAN
2.5 MICRONS AND THEIR MAJOR SOURCES
Sources
Primary PM
Aerosol
species Natural Anthropogenic
SO4= Sea spray Fossil fuel
combustion
Secondary PM
Natural
Oxidation of
reduced sulfur gases
emitted by the
oceans and
wetlands; and SO2
and H2S emitted by
volcanism and forest
fires
Anthropogenic
Oxidation of SO2
emitted from fossil
fuel combustion
NO,
Minerals
Erosion,
re-entrainment
NFL
Organic
carbon
Wild fires
Elemental
carbon
Metals
Bioaerosols
Wild fires
Volcanic
activity
Viruses,
bacteria
Motor vehicle
exhaust
Fugitive dust;
paved, unpaved
roads; agriculture
and forestry
Motor vehicle
exhaust
Open burning, wood
burning, cooking,
motor vehicle
exhaust, tire wear
Motor vehicle
exhaust, wood
burning, cooking
Fossil fuel
combustion,
smelting, brake wear
Oxidation of NOX
produced by soils,
forest fires, and
lighting
Oxidation of NOX
emitted from fossil
fuel combustion; and
in motor vehicle
exhaust
Emissions of NH3
from wild animals,
undisturbed soil
Oxidation of
hydrocarbons
emitted by
vegetation,
(terpenes, waxes);
wild fires
Emissions of NH3
from animal
husbandry, sewage,
fertilized land
Oxidation of
hydrocarbons emitted
by motor vehicles,
open burning, wood
burning
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TABLE II-3 CONSTITUENTS OF ATMOSPHERIC PARTICLES
GREATER THAN 2.5 MICRONS AND THEIR MAJOR SOURCES
Sources
Aerosol species
Minerals
Metals
Miscellaneous
ions
Organic carbon
Organic debris
Bioaerosols
Primary
Natural
Erosion,
re-entrainment
Erosion,
re-entrainment,
organic debris
Sea spray
—
Plant, insect
fragments
Pollen, fungal
spores, bacterial
agglomerates
Anthropogenic
Fugitive dust; paved,
unpaved road dust,
agriculture and forestry
—
Road salting
Tire and asphalt wear
—
—
Secondary
Natural Anthropogenic
— —
— —
— —
— —
— —
— —
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4. How much reduction is needed?
Implementing the PM10 Standards
As shown in figure II-3, there are still several PM10 nonattainment areas in the country.
Some of these areas may need to consider decreases of NOX emissions as part of their
attainment planning. The importance of NOX as a PM10 precursor varies significantly from
place-to-place.
Integrated Strategies for Implementing the O3 and PM Standards
Common Factors
As noted above, EPA published revisions to the O3 and PM NAAQS on July 18, 1997. As
part of the revisions process, EPA initiated action to address strategies for the implementation of
the new NAAQS. These ongoing reviews and related implementation strategy activities to date
have brought out important common factors between O3 and PM. Similarities in pollutant
sources, formation, and control exist between O3 and PM, in particular the fine fraction of
particles. These similarities provide opportunities for optimizing technical analysis tools (i.e.,
monitoring networks, emissions inventories, air quality models) and integrated emissions
reduction strategies to yield important cross-cutting benefits across various air quality
management programs. This integration could result in a net reduction of the regulatory burden
on some source category sectors that would otherwise be impacted separately by O3, PM, and
visibility protection control strategies.
Federal Advisory Committee Act (FACA) Process
The EPA initiated a process designed to provide for significant stakeholder involvement in
the development of integrated implementation strategies for the new/revised O3 and PM NAAQS
and a new regional haze program. As described below, this process involves a new subcommittee
of the Agency's Clean Air Act Advisory Committee (CAAAC), established in accordance with the
FACA (5 U.S.C. App.2). The CAAAC was established to provide independent advice and
counsel to the EPA on policy and technical issues associated with the implementation of the Act.
The CAAAC advises EPA on the development, implementation, and enforcement of several of the
new and expanded regulatory and market-based programs required by the Act.
The CAAAC advises on issues that cut across several program areas. A new
subcommittee of the CAAAC, the Subcommittee for Ozone, Paniculate Matter, and Regional
Haze Implementation Programs (the Subcommittee), was established in August 1995 to address
integrated strategies for the implementation of the new O3 and PM NAAQS, as well as a regional
haze program. The focus of the Subcommittee will be on assisting EPA in developing
implementation control strategies, preparing supporting analyses, and identifying and resolving
impediments to the adoption of the resulting programs. The Subcommittee is composed of
representatives selected from among state, local, and tribal organizations; environmental groups;
industry; consultants; science/academia; and federal agencies. Recommendations made by the
Subcommittee will be submitted to EPA through CAAAC.
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5. How much reduction will be achieved with current
and projected programs?
As described in detail in the Appendices, several current and future programs will achieve
decreases in NOX emissions. In addition, some States are planning or have underway PM10
attainment plans which specifically call for NOX emissions reductions in certain areas, including,
for example, the South Coast Air Basin in California. Additional NOX emissions reductions
might be needed to attain the new PM2 5 standards.
6. Summary
Emissions of NOX result in the formation of particulate nitrates which contribute to PM10
nonattainment in some areas. Decreases in NOX emissions are needed in some areas to attain the
PM10 NAAQS, including the Los Angeles area. In other areas, NOX emissions reductions may be
needed to attain the PM2 5 NAAQS and/or help maintain concentrations below the PM NAAQS in
attainment areas.
References
National Research Council, Committee on Haze in National Parks and Wilderness Areas
Protecting Visibility in National Parks and Wilderness Areas. National Academy Press,
Washington, D.C., 1993.
Science and Technical Support Work Group, Ozone and Particulate Matter National Ambient Air
Quality Standards, Federal Advisory Committee Act, Harvey Jeffries and Tom Helms, co-chairs,
"Conceptual Model for Ozone, Particulate Matter and Regional Haze," February 19, 1997.
U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards Air Quality
Criteria for Particulate Matter (Criteria Document) (three volumes, EPA/600/P-95-001aF thru
EPA/600/P-95-001cF, April 1996, NTIS # PB-96-168224.
U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, Review of
the national ambient air quality standards for paniculate matter: policy assessment of scientific
and technical information, (Staff Paper), EPA-452/R-96-013, July 1996, NTIS # PB-97-
115406.
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E. Visibility Protection
1. Goals of the Program
Visibility is an air quality related value essential to the enjoyment of national parks,
wilderness areas, and other scenic areas throughout our country. In section 169A of the 1977
amendments to the CAA, Congress recognized that visibility was an important aspect of public
welfare that should be protected. It established as a national goal "the prevention of any future,
and the remedying of any existing, impairment of visibility in mandatory class I Federal areas
which impairment results from man-made air pollution." The CAA also calls for the development
of programs to ensure reasonable progress toward the national goal, including the
establishment of a new regional haze program for the protection of visibility in mandatory Federal
Class I areas across the country. These programs are to be implemented by the States and can
be regionally specific.
Regional haze and other visibility impairment is primarily caused by fine particles in the air
which scatter or absorb light. These particles include, elemental carbon (soot), nitrates, organic
matter, soil dust, and sulfates. The major cause of visibility impairment in the Eastern United
States is sulfate, formed primarily from SO2 emitted from coal combustion by electric utility
boilers, while in the West the other four particle types play a greater role. Emissions of NOX lead
to the formation of particulate nitrates. Thus, the decreases in emissions of NOX will help improve
visibility and make progress toward the national goal.
2. Status of the Program
CAA Visibility Requirements
The CAA includes two emissions control programs specifically concerned with visibility in
national parks and wilderness areas. One of these, the Prevention of Significant Deterioration
program is directed mainly at new sources, as noted below. The other, a visibility protection
program, is aimed largely at existing sources.
Prevention of Significant Deterioration (PSD)
The PSD program requires each new or expanded major emitting facility locating in a
clean air area to install the best available control technology and meet increments that limit the
cumulative increases in pollution in clean air areas. Because all areas of the country are clean with
respect to the NO2 standard, the PSD program applies to major sources of NOX throughout the
nation.
The PSD program has protected visibility to some extent by decreasing the growth of
emissions of pollutants that contribute to regional haze, including SO2 and NOX. The PSD
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program includes a special air quality related values (AQRV) test for evaluating a major emitting
facility that might affect a Class I area. Many large national parks and wilderness areas are
designated as Class I areas and therefore are subject to the most stringent increments. The federal
land manager has the responsibility to protect the AQRV. The EPA's PSD regulations are found
in the Code of Federal Regulations at 40 CFR 52.21.
Visibility Rules
The EPA issued visibility rules in 1980 (See 40 CFR 51.300-307 and 45 FR 80084)
requiring states containing mandatory class I areas to: (1) develop a program to assess and
remedy visibility impairment from new and existing sources; (2) develop a long-term strategy to
assure progress toward the national goal; (3) develop a visibility monitoring strategy; (4) consider
"integral vistas" outside of class I areas in all aspects of visibility protection; and (5) notify Federal
land managers (FLM) of proposed new major stationary sources and consider visibility analyses
conducted by FLMs in their permitting decisions (40 CFR 51 subpart P). These visibility rules lay
out a process for visibility impacts from a single source or small group of sources which may
reasonably be anticipated to cause or contribute to visibility impairment in a Federal Class I area.
The process is initiated by a certification of impairment by a FLM. The State determines whether
the impairment is attributable to the source(s) and, if so, requires controls to reduce impairment.
The 1980 visibility regulations required 36 States containing mandatory Class I areas to
submit strategies for monitoring visibility. The EPA completed in 1985 Federal regulations to
establish a national visibility monitoring network which would be cooperatively managed by EPA,
Federal land management agencies, and State air agency representatives. This network is now
known as IMPROVE (Interagency Monitoring of Protected Visual Environments). Due to
resource limitations, IMPROVE monitors could not be placed in all 156 mandatory class I areas.
Instead, the IMPROVE Steering Committee has selected a set of priority sites. Data are currently
being collected at more than 40 Class I locations. The IMPROVE monitoring protocol specifies
aerosol, photographic, and optical (light extinction) measurements twice a week.
At that time of the rulemaking, EPA also expressed its intention to regulate regional haze
at some future date when monitoring techniques are improved and the relationship between air
pollutants and visibility impairment is better established. Much progress has been made in
technical areas important to the successful implementation of a regional haze program,
including areas such as visibility monitoring, regional scale modeling, and scientific
knowledge of the regional effects of particles on visibility. As described below, EPA plans to
begin a new regional haze program for the protection of visibility in mandatory Federal Class I
areas10 across the country.
Grand Canyon Visibility Transport Commission
Section 169B of the 1990 CAA amendments required the establishment of the Grand
Areas designated as mandatory class I areas are those national parks exceeding 6000 acres, wilderness areas
and memorial parks exceeding 5000 areas, and all international parks which were in existence on August 7, 1977.
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Canyon Visibility Transport Commission (GCVTC11). The Commission was charged with
assessing adverse impacts on visibility from projected growth in the region, and requires the
Commission to recommend measures to EPA for addressing adverse impacts to visibility in the
region. The Commission formally adopted its report to the EPA on June 10, 1996. The EPA is
to use the Commission's recommendations as guidance for developing national strategies and/or
rulemaking. Implementation of specific program components will be the responsibility of tribes,
states, and, in some cases, federal agencies. The primary recommendations in the Commission's
report include: air pollution prevention measures such as energy conservation and increased
energy efficiency; tracking of emissions growth in clean air corridors; development of a plan for
an emissions cap and trading program for stationary sources; and establishing a regional emissions
budget for mobile sources.
Regional Haze Rules
The Administrator of EPA signed the notice of proposed rulemaking for the regional
haze rules on July 18, 1997. These rules are a continuation of the 1980 rules. From the time
the GCVTC report is received, section 169B requires EPA to issue rules within 18 months to
assure reasonable progress toward remedying adverse impacts due to regional haze. After
establishment of a regional haze program, States affected by these rules are required, under
section 169B(e)(l), to revise their state implementation plans (SIPs) within 12 months to include
such emission limits, schedules of compliance, and other measures as may be necessary for
program implementation.
Relation to NO2 NAAQS Review
As described in the final rulemaking notice of October 8, 1996 (61 FR 52852) on revision
to the NO2 NAAQS, EPA determined that establishment of a secondary NO2 standard to protect
visibility is not appropriate. While NO2 can contribute to brown haze, there is no established
relationship between ground level NO2 concentrations at a given point and visibility impairment
due to a plume or regional haze. Furthermore, regional scale NO2 light extinction is much less
than aerosol extinction. These considerations helped lead to the conclusion that establishment of
a secondary NO2 standard to protect visibility would not be appropriate.
Relation to PM Standard Review
In reviewing the NAAQS for PM, EPA also considered the appropriateness of a
secondary standard to address a number of welfare effects, specifically including visibility.
Because of regional variations in visibility conditions created by background concentrations of
fine particles, annual average humidity, pollutant mix, and resulting total light extinction,
however, a regional haze program under the regulatory authority in section 169A may be
preferable to setting a secondary NAAQS. As described in the July 18, 1997 Federal Register.
The Commission consists of the Governors, or their designees, from the States of Arizona, California, Colorado,
Nevada, New Mexico, Oregon, Utah, and Wyoming; the President of the Navajo Nation; the Chairman of the Hopi Tribe;
the Governor of the Pueblo of Acoma; the Chairman of the Hualapai Tribe; and ex-officio members from EPA, Bureau of
Land Management, National Park Service, United States Forest Service, United States Fish and Wildlife Service, and the
Columbia River Inter-Tribal Fish Commission.
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EPA set the secondary PM standards identical to the primary standards which, in conjunction
with a regional haze program under sections 169A and 169B of the Act, EPA believes is the most
appropriate and effective means of addressing the welfare effects associated with visibility
impairment. Together, the two programs and associated control strategies should provide
appropriate protection against the effects of PM on visibility and allow all regions of the country
to make reasonable progress toward the national visibility goal.
3. Science of NOX and Visibility
Visibility Basics
For an object to be seen against a background, there must be sufficient contrast between
the object and its background. That is, the light from the object and the background must be
sufficiently different in apparent brightness or color to make the object stand out against the
background. Light from objects and their background viewed through the atmosphere from a
distance are modified by the particles and gases in the atmosphere (primarily clean air).
Light as it traverses the atmosphere is scattered (i.e., redirected in directions) and
absorbed (i.e., converted from light to heat) by the particles and gases in the atmosphere. This
affects the appearance of scenes in two ways. The image-forming light from scenic features is
diminished since a fraction of the light is scattered or absorbed; non-image-forming light is
scattered into the sight path. Both of these effects lower the contrast between object and
background, and cause the scene to be more obscured. This decrease in contrast is further
decreased with distance to the scenic feature being viewed, and with increased concentration of
airborne particles.
Many studies have been published on visibility conditions and related aerosol
concentrations. The NAPAP report (1991) lists 33 aerosol and visibility databases. From these
studies, the major contributors to visibility impairment from natural and man-made sources are
sulfate particles, organic particles, elemental carbon, suspended dust, and nitrate particles.
Regional haze is primarily caused by fine particles in the air, typically less than 2.5 microns
in diameter, which scatter and absorb light. These particles include sulfates, nitrates, organic
matter, elemental carbon (soot), and soil dust (NRC, 1993). The fate of regional haze is a
function of meteorological and chemical processes, sometimes causing fine particle loadings to
remain suspended in the atmosphere for long periods of time (3-5 days) and to be transported
long distances (thousands of kilometers) from their sources. A large fraction of anthropogenic
airborne particles (sulfates, nitrates, and some organic particles) accumulates in the 0.1-1.0
micron diameter range. These particles can survive in the atmosphere for several days and can
be transported hundreds or even thousands of kilometers from their sources to remote
locations, such as national parks and wilderness areas (NRC, 1993). During transport, the
emissions from many different sources can become mixed, making it difficult to assess the
effects of individual sources on visibility.
Types of Visibility Impairment
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Visibility effects are manifested in two principal ways: (1) as local impairment (e.g.,
plumes and localized hazes) and (2) as regional haze. Local-scale impairment is defined as
impairment that is "reasonably attributable" to a single source or group of sources. Visibility
impairment in some urban areas can be dominated by local rather than regional sources,
particularly in mountain valleys in the winter and in meteorologically stagnant conditions.
The second category of impairment, regional haze, is produced from a multitude of
sources combined over many days. Regional haze impairs visibility in every direction over a
large area. Objects on the horizon are obscured and the texture of nearby objects is reduced.
In some cases, the haze may be elevated and appear as layers of discoloration.
The contribution by particles from both natural and man-made sources are highly variable.
Natural particle sources such as wildfires, windblown dust, salts from ocean spray, etc. are highly
variable across time and space with the result that natural background levels of visibility are highly
variable. Concentrations of man-made and natural particles also vary because of the influence of
variable meteorology responsible for atmospheric transport and dispersion. (For further
information on visibility, see the 1979 Report and the 1990 NAPAP Report "Acidic Deposition:
State of Science and Technology, Volume III, Report Number 24.)
Visibility Metrics
Visual range, which is defined as the greatest distance that a large dark object can be seen
against the background sky, is the oldest and most commonly used visibility metric. Visual range
was developed and continues to function well as an aid in military operations and transportation
safety. Airport observations of visual range have been made since 1919, and have been computer
archived since the late 1940's. Daylight observations involve viewing preselected visibility
markers (large dark objects) at known distances from the observation point to determine the most
distant marker that is visible.
Another traditional visibility metric is extinction coefficient, which is the attenuation of
light per unit distance due to scattering and absorption by gases and particle in the atmosphere.
Extinction coefficient is expressed in inverse length units (e.g., km"1) and is used primarily by
scientists studying the causes of reduced visibility. Direct relationships exist between
concentrations of atmospheric constituents and their contribution to extinction coefficient.
Apportioning extinction coefficient to atmospheric constituents provides a method to estimate
change in visibility caused by change in constituent concentrations. Calculation of the extinction
coefficients from air quality models can be used to estimate the expected visibility changes from
emission changes.
Visibility Impairing Particles
Light scattering, and to a lesser degree, light absorption by suspended particles are the
most important contributors to visibility degradation. The influence of particles depends on the
concentration, composition, and the size of the particles. Particles composed of materials such as
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sulfates and nitrates absorb under high relative humidity conditions. Since the solution drops are
larger than the dry particles, visibility impairment by sulfate and nitrate particles increases during
high humidity conditions.
Sulfate, nitrate, and organic carbon are major contributors to visibility degradation. These
particles begin as gaseous emissions and undergo chemical transformation in the atmosphere.
These particles are mostly in a size range from about 0.1 to 1.0 microns diameter which scatters
more light for the same mass concentration than smaller or larger particles. That is, the particles
that scatter light most efficiently per unit mass are those of approximately the same size as
wavelengths of visible light (0.4-0.7 microns). Coarse particles (i.e., those in the 2.5 to 10
micron size range) also impair visibility, although less efficiently than fine particles. Black
carbon, primarily from incomplete combustion such as in diesel exhaust or wood smoke, is the
principal cause of light absorption in the atmosphere. Sulfates and nitrates readily absorb water
from the atmosphere and grow in size in a nonlinear fashion as relative humidity levels
increase. Because humidity varies seasonally, the visibility impacts of the sulfate and nitrate
particles also varies by season.
NOX Emissions
As described in the "Ozone" section of this document, NO is formed during combustion or
any high temperature process involving air. The NO is converted to NO2 by O3 or other
atmospheric oxidants. The NO2 then participates in various reactions to form other substances,
including O3 and PM.
Nitrate Particulates
Nitrate airborne particles can be produced by several mechanisms. One major mechanism
of nitrate formation involves the gas-phase reaction of NO2 with OH to produce nitric acid,
HN03. The gaseous nitric acid can react at airborne particle surfaces to form nitrate salts. For
example, a particle containing calcium carbonate can neutralize the nitric acid to produce calcium
nitrate; it follows that nitrate size distributions depend, in part, on the size distributions of
particles on which they react (NRC, 1993). Ammonia gas (NH3) is important in the generation of
sulfate and nitrate particles through the neutralization of sulfuric and nitric acid. When
concentrations of NH3 and HN03 are sufficiently high, ammonium nitrate (NH4N03) can be
formed. Ammonium nitrate is often found in submicron particles in locations such as Denver or
Los Angeles. However, little submicron ammonium nitrate is typically found in parts of the nation
where ammonia concentrations are low and acid sulfate concentrations are high (NRC, 1993).
(See additional discussion in the "Particulate Matter, Emissions Sources and Formation Processes
of Particles" section of this document.)
NO 2
The only gas that absorbs visible light to any appreciable extent at concentrations expected
in the atmosphere is NO2. NO2 is a strong absorber of visible and ultraviolet light and can thereby
contribute to haze. In addition, NO2 has a broad absorption band at the blue end of the spectrum;
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consequently, when NO2 concentrations are high, the atmosphere has a distinct brownish color.
However, because of its high reactivity and relatively short lifetime, NO2 does not normally
contribute significantly to haze in remote areas; it is a problem only in areas close to sources
(NRC, 1993).
The most significant optical effect of NO2 involves discoloration (EPA, 1996). NO2
appears as a yellow to reddish-brown gas because it strongly absorbs blue light, allowing red
wavelengths to reach the eye. The extent to which NO2 filters out blue light is determined by the
integral of NO2 concentration along the sight path. In regard to regional haze, because the effect
of NO2 depends on the product of the pollution concentration and the viewing path length, the
coloration of 0.05 ppm NO2 over 10 km is the same as 0.5 ppm over 1 km. When NO2 is
dispersed over a large area, as in the case of urban emissions, ground level concentrations at
individual points may be less than a national standard but because an observer views the entire
NO2 mass, the urban plume appears of brownish color.
Significance of Anthropogenic Sources of Fine Particles
The concentrations of background fine particles are generally small when compared
with concentrations of fine particles from anthropogenic sources. The same relationship holds
true when one compares light extinction due to background fine particles with light extinction
due to anthropogenic fine particles. Anthropogenic contributions account for about one-third
of the average extinction coefficient in the rural West and more than 80 percent in the rural
East (NAPAP, 1991). In the eastern United States, sulfates dominate fine particle
concentrations, stemming from regional SO2 emissions. In contrast, nitrate plays a small role
in the East but is significant in areas of the West; for example, nitrates dominate the overall
light extinction in the mountainous areas just outside Los Angeles, with most of the nitrate
formation in this area coming from NOX emissions within the urban area (EPA, 1996).
Monitoring Data
While the amount of total light extinction varies significantly across the country, so
does the mix of visibility-impairing pollutants from region to region. As described in Table II-
4, IMPROVE monitoring data were used to establish annual apportionment of current aerosol
components to the total visibility impairment for class I areas. This gives an indication of the
relative contribution to visibility impairment due to nitrates in a variety of areas. Nitrates are
the largest contributor to light extinction in National Parks and Wilderness Areas in Southern
California.
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Table II-4. Annual Averages (March 1988-February 1991) of Reconstructed Light Extinction
(Mm'1) for 19 Regions of the IMPROVE Network.
(Source: IMPROVE, CIRA Report, Feb. 1993)
REGION
ALASKA
APPALACHIAN
BOUNDARY
WATERS
CASCADES
CENTRAL
ROCKIES
CENTRAL
CALIFORNIA
COAST
COLORADO
PLATEAU
FLORIDA
GREAT BASIN
HAWAII
NORTHEAST
NORTHERN
GREAT PLAINS
NORTHERN
ROCKIES
SIERRA
NEVADA
SIERRA
HUMBOLDT
SONORAN
DESERT
SOUTHERN
CALIFORNIA
WASHINGTON,
D.C.
WEST TEXAS
Total
Extinction
25.4
112.2
68.2
58.8
28.1
56.3
27.1
87.5
23.4
53.2
71.3
39.7
54.3
33.4
28.0
31.3
63.5
164.3
36.7
Aerosol
Extinction
15.4
102.2
58.2
48.8
18.8
46.3
17.1
77.5
13.4
43.2
61.3
29.7
44.3
24.4
18.0
21.3
53.5
154.3
26.7
Sulfate
6.7
69.7
29.8
19.0
5.8
15.4
6.0
42.4
3.4
31.5
38.3
13.1
12.4
5.7
4.4
8.1
7.7
75.6
12.2
Nitrate
0.7
6.9
8.4
3.3
1.3
12.1
1.4
9.5
0.9
1.0
5.1
3.3
4.0
3.6
1.4
1.3
23.8
24.6
1.4
Organics
4.6
16.7
14.1
19.2
6.1
10.6
4.7
15.4
4.6
5.0
11.0
7.3
19.6
8.1
7.7
5.5
9.7
25.0
5.7
Elemental
carbon
0.5
4.6
2.2
4.9
1.3
2.7
1.5
3.6
0.6
0.7
4.0
1.4
4.3
2.5
1.8
1.8
4.8
18.4
1.5
Soil and
Coarse
2.6
4.3
3.8
2.4
3.6
5.6
3.5
6.7
4.0
5.1
2.9
4.7
3.9
3.4
2.7
4.5
7.5
10.6
5.9
4. How much reduction is needed regionally; nationally?
While the answers to these questions are not clear at this time, it is clear that the type and
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amount of emissions reductions needed varies from area to area. Visibility conditions vary
regionally, as a function of background concentrations of fine particles, average relative humidity
levels, and anthropogenic particle loadings, all of which are generally higher in the East than in the
West. It is important to note that even in areas with relatively low concentrations of
anthropogenic fine particles, such as the Colorado plateau, small increases in emissions can lead to
significant decreases in visual range. This is one reason for the emphasis on protecting visual air
quality in the highly valued national parks and wilderness areas in the Colorado plateau region. In
areas with relatively higher fine particle concentrations, such as the Great Smoky Mountains
National Park, it takes a greater reduction in ambient concentration to make an equivalent
improvement in visual range.
Most visibility impairment in national parks and wilderness areas results from the
transport by winds of emissions and secondary airborne particles over great distances (typically
hundreds of kilometers). Consequently, visibility impairment is usually a regional problem, not a
local one. Progress towards the national goal of remedying and preventing anthropogenic
visibility impairment in Class I areas will require regional programs that operate over large
geographic areas; strategies should be adopted that consider many sources simultaneously on a
regional basis (NRC 1993). The outcome of the GCVTC report and EPA's subsequent
rulemaking will help define the needed reductions to meet the national goals.
5. How much reduction will be achieved with current and
projected programs?
Implementation of the CAA will achieve substantial decreases in NOX emissions. The PSD
program has managed atmospheric loadings from new sources and has safeguarded some large
parklands from excessive emissions from new sources. Programs that may achieve additional NOX
emissions reductions are described in the Appendices to this document. Further, the revisions to
the PM and O3 NAAQS could lead to additional emissions reductions.
6. Summary
Visibility impairment can occur due to local plumes or widespread regional haze. The
sources of locally visible plumes are easy to identify, for example, the smoke from a power plant
stack or from a burning field. However, when plumes are carried by winds, they become more
diffuse, and the sources are identified less readily. In regions with many sources, the plumes can
merge and become mixed with the emissions from many sources, such as motor vehicles, power
plants, and industrial operations. The result is a widespread haze in which individual contributions
from the various sources are virtually indistinguishable.
In most cases, visibility degradation is caused by five kinds of paniculate substances:
sulfates, nitrates, organic matter, elemental carbon, and soil dust. Regional haze is produced
from a multitude of sources and impairs visibility in every direction over a large area, such as
an urban area, or possibly over several states. Multiple sources may combine over many days
to produce haze, which is often regional in scale. The fate of regional haze is a function of
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meteorological and chemical processes, sometimes causing fine particle loadings to remain
suspended in the atmosphere for long periods of time and to be transported long distances
from their sources. The major cause of visibility impairment in the East is sulfate, formed
primarily from SO2 emitted from coal combustion by electric utility boilers, while in the West the
other four particle types play a greater role. Emissions of NOX lead to the formation of particulate
nitrates; thus, the reduction in emissions of NOX will help improve visibility and make progress
toward the national goal.
References
Grand Canyon Visibility Transport Commission, Recommendations for Improving Western
Vistas, June 10, 1996 report of the Grand Cany on Visibility Transport Commission to the United
States Environmental Protection Agency, June 10, 1996.
National Acid Precipitation Assessment Program (NAPAP) 1979 and the 1990 Reports Acidic
Deposition: State of Science and Technology, Volume III, Report Number 24, entitled
"Visibility: Existing and Historical Conditions - Causes and Effects."
National Acid Precipitation Assessment Program (NAPAP), (1991). Office of the Director,
Acid Deposition: State of Science and Technology. Report 24, "Visibility: Existing and
Historical Conditions - Causes and Effects." Washington, D.C.
National Research Council, Committee on Haze in National Parks and Wilderness Areas
Protecting Visibility in National Parks and Wilderness Areas. National Academy Press,
Washington, D.C., 1993.
Science and Technical Support Work Group, Ozone and Particulate Matter National Ambient Air
Quality Standards, Federal Advisory Committee Act, Harvey Jeffries and Tom Helms, co-chairs,
"Conceptual Model for Ozone, Particulate Matter and Regional Haze," February 19, 1997.
U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards,
Effects of the 1990 Clean Air Act Amendments on Visibility in Class I Areas: An EPA Report to
Congress. Research Triangle Park, N.C., 1993.
U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, Review of
the national ambient air quality standards for particulate matter: policy assessment of scientific
and technical information, (Staff Paper), EPA-452/R-96-013, July 1996, NTIS # PB-97-
115406.
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III. Additional Public Health and Environmental Impacts from NOX
Emissions
A. Drinking Water
As described in the section on "Acid Deposition," nitrate and nitrite leaching aggravates
the effects of acidification, both long term (e.g. cation leaching) and episodically (e.g. Al peaks).
Adding inorganic N to freshwater ecosystems can eutrophy as well as acidify the waters when
they are already rich in phosphorus or, more rarely, when N is the limiting nutrient (Vitousek et
al, 1997:10; Church, 1997:17).
Most freshwater ecosystems will not be eutrophied by additional nitrate or nitrite leaching
because phosphorous is the limiting nutrient. (Because the direct contribution of nitrite is
typically very small compared to nitrate, sometimes this process is called simply "nitrate
leaching.") Nitrate leaching has implications that go beyond its eutrophying and acidifying
effects. Because primary producers (plants) in most freshwater systems do not assimilate added
nitrate, this ion can remain in the water and be transported long distances downstream. By
contrast, acidity may be neutralized by natural processes before it reaches large streams, lakes or
estuaries. This characteristic leads to two impacts that can occur downstream: elevated levels of
nitrate in drinking water supplies (discussed below) and eutrophication of estuaries and coastal
waters (discussed in the following section). These downstream impacts can occur in hydrological
systems even where acidification is not a problem.
Under the Safe Drinking Water Act, EPA has set maximum contaminant levels (MCLs)
for nitrate and nitrite in drinking water to protect human health. These levels are established at 10
milligrams per liter (mg/L) of nitrate N and 1 mg/L of nitrite N (40 CFR 141.62). The primary
adverse human health effect associated with exposure to nitrate or nitrite is methemoglobinemia
(National Research Council, 1995:2). To cause methemoglobinemia, nitrate must be converted
into nitrite. Methemoglobinemia occurs when nitrite oxidizes iron (Fe2+) in blood into FE3+, a
form that does not allow oxygen transport, and can cause brain damage or death (Vitousek et al,
1997:9). This condition in adults is rare (National Research Council, 1995:2), but is a significant
concern for infants because microbes in an infant's stomach may convert high levels of nitrate to
nitrite (Vitousek et al, 1997:9). Insufficient oxygenation of the blood is characterized by bluish
skin and lips ("blue baby syndrome"). The National Research Council concluded, in 1995, that
"results from epidemiological studies are inadequate to support an association between nitrate or
nitrite exposure from drinking water in the United States and increased cancer rates in humans"
(National Research Council, 1995:2). However, an epidemiological study, published in 1996, of
rural populations using community water supplies in Nebraska, concluded that "long-term
exposure to elevated nitrate levels in drinking water may contribute to the risk for Non-Hodgkin's
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Lymphoma (NHL)" (Ward et al, 1996:465).
The contribution of atmospheric N deposition to elevated levels of nitrate in drinking
water supplies is an evolving impact area. The Ecological Society of America (ESA) has included
discussion of this impact in a recent major review of causes and consequences of human alteration
of the global N cycle in its Issues in Ecology series (Vitousek et al, 1997:9). This series is
designed to report, in language understandable by non-scientists, the consensus of a panel of
scientific experts on issues relevant to the environment. Issues in Ecology is supported by the
Pew Scholars in Conservation Biology Program and ESA, a national professional society founded
in 1915.
For decades, N concentrations in major rivers and drinking water supplies have been
monitored in the United States, Europe, and other developed regions of the world. Monitoring
data from public water systems in the United States show that surface water sources of drinking
water do not exceed MCLs for nitrate and nitrite, with very rare exceptions unrelated to airborne
deposition. On the other hand, analysis of these data confirms a substantial rise of N levels in
surface waters, which are highly correlated with human-generated inputs of N to their watersheds.
These N inputs are dominated by fertilizers and atmospheric deposition (Vitousek et al, 1997:9).
Nitrate levels in the Mississippi River have more than doubled since 1965; they have risen in
major rivers of the northeastern U.S. by three- to ten-fold since 1900 (Vitousek et al, 1997:9).
Agricultural sources dominate human-generated inputs of N in regions with intensive
farming, including the Mississippi River basin and Texas. In other areas, such as the northeastern
United States, NOX emissions from industrial origin are the major factor in river export of N to
lakes, major rivers, and estuaries (Howarth et al, 1996). Relatively low nitrate water from
forested watersheds can serve to dilute higher nitrate water from urban or agricultural watersheds
for many towns and cities. As water from forested watersheds increases in nitrate, therefore, its
dilution capability diminishes and thus can threaten community water supplies even before actual
nitrate concentrations exceed the drinking water standard (Ryan, 1996). Nitrate levels tend to be
higher in private wells than community water supplies due to lack of regular testing, simpler
construction, and shallow depth (Ward et al, 1996:165). High levels of nitrate in private well
water typically indicate that pollution is seeping in from septic tanks, animal wastes, fertilizers,
municipal landfills, or other nonpoint sources (EPA, 1996).
Increases in atmospheric N deposition to sensitive forested watersheds approaching N
saturation would be expected to result in increased nitrate concentrations in stream water. Only
one incidence of this phenomenon in the United States has been documented in peer-reviewed
literature, although it has been well-established for areas in Germany and the Netherlands (Riggan
et al, 1985:786). High nitrate concentrations from chaparral watersheds have been found in
stream water in the South Coast (Los Angeles County, CA) Air Basin (Riggan et al, 1985, 1994).
Stream water nitrate concentrations in watersheds subject to chronic air pollution were two to
three orders of magnitude greater than in chaparral regions outside the air basin. Within the San
Gabriel Mountains, nitrate concentrations were greatest, as high as 7.0 mg/L, where watersheds
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adjoin the urbanized basin and may contribute to existing groundwater nitrate pollution (Riggan et
al, 1985:786, 781).
Some high nitrate concentrations reported in this study were measured after a storm
washed ash from burned landscape into the stream water. Wildfires in chaparral subject to
chronic air pollution may cause inordinately high stream water nitrate loading which, when
released through high stream discharge and channel scouring, could saturate the aquatic system
and seriously pollute downstream waters (Riggan et al, 1985:788). These chaparral watersheds
are specialized ecosystems and probably are not typical of United States watersheds in general.
Nonetheless, high nitrate concentrations have also been observed in forest soils in the vicinity of
the Asheville Watershed in western North Carolina; the amounts in soil solution suggest elevated
levels of nitrate in groundwater (Smithson, 1997).
While observational evidence directly linking atmospheric deposition to elevated levels of
nitrate in drinking water supplies is very limited, atmospheric deposition can supply N to
ecosystems in a manner not dissimilar to fertilizer application (Vitousek et al, 1996:7-8). In fact,
researchers have noted that some amount of atmospheric N deposition can be beneficial to
agriculture, as fertilizer, in some areas, depending on the soil, crops harvested, biological uptake
processes, and other factors (Acid Rain Program, 1995:36). Thus, one would expect that
decreases in atmospheric N deposition to N-sensitive watersheds resulting from decreases in local
or transported NOX or ammonia N emissions would lessen the contribution of airborne deposition
to elevated levels of nitrate in drinking water supplies.
References
Acid Rain Program. 1995. Adverse Effects of Nitrogen Oxides (NOJ and Benefits of Reduction,
draft staff assessment paper (Air Docket A-95-28, Item II-A-13). U. S. Environmental Protection
Agency; Washington, DC. December.
Devesa, S.S. and T. Fears. 1992. "Non-Hodgkin's Lymphoma Time Trends: United States and
International Data" Cancer Res. 1992; 52(suppl): 5432S-5449S.
Howarth, R.W., G.Billen, D.Swanwy, A. Townsend, N. Jaworski, K. Lajtha, J.A.Downing,
R.Elmgren, N. Caraco, T. Joddan, F. Berendse, J. Freney, V. Kudeyarov, P. Murdoch, Zhu
Zhao-Liang. 1996. "Regional nitrogen budgets and riverine N &P fluxes for the drainages to the
North Atlantic Ocean: Natural and human influences." Biogeochemistry (in press).
National Research Council. 1995. Nitrate andNitrite in Drinking Water. Washington, DC:
National Academy Press.
Riggan, P.J., R.N. Lockwood, and E.N. Lopez, "Deposition and Processing of Airborne Nitrogen
Pollutants in Mediterranean-Type Ecosystems of Southern California" Environmental Science
and Technology, vol. 19 (p. 781-789), 1985.
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Riggan, P.J., R.N. Lockwood, P.M.Jacks, C.G.Colver,F.Weirich, L.F.DeBano, and J.A.Brass,
"Effects of Fire Severity on Nitrate Mobilization in Watersheds Subject to Chronic Atmospheric
Deposition" Environmental Science and Technology., vol. 28 (p. 369-375), 1994.
Ryan, Douglas F., USD A, Forest Service, Forest Environment Research, personal communication
November 7, 1996.
Smithson, Paul. 1997. Unpublished doctoral dissertation, North Carolina State University,
Department of Soil Science (p. 95-96, 115, 122, 130), 1997.
U.S. Environmental Protection Agency, "Environmental Indicators of Water Quality in the United
States: Fact Sheets". EPA-841-F-96-001, 1996.
Vitousek, Peter M., John Aber, Robert W. Howarth, Gene E. Likens, et al. 1997. Human
Alteration of the Global Nitrogen Cycle: Causes and Consequences. Issues in Ecology.
Published by Ecological Society of America, Number 1, Spring 1997.
Ward, M.H., S.D. Mark, K.P. Cantor, D. D. Weisenburger, A. Correa-Villasenor, and S.H.
Zahm. 1996. "Drinking Water Nitrate and the Risk of Non-Hodgkin's Lymphoma"
Epidemiology 7:465-471.
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B. Eutrophication
1. Goals of the Program
Eutrophication has been identified as the most serious pollution problem facing the
estuarine waters of the United States (NRC, 1993). In fact, 54 percent of impacted square miles
of estuaries that have been assessed have been shown to be impaired by nutrients (EPA 1994).
N is the limiting nutrient controlling eutrophication in most temperate estuaries (Nixon, 1986,
Swain et al, 1994) and some limited percentage of freshwater lakes (EPA, 1993). Studies of N
loadings to estuarine and coastal systems within the United States and worldwide have shown
that N deposited from the atmosphere is a significant portion of the total N loadings, ranging
from 10 to 70 percent, with 27 percent of the N loadings delivered to the Chesapeake Bay
originating from atmospheric sources (Linker et al, 1993; Paerl. 1993, Valigura et al, 1995). N
has been shown to be the nutrient controlling the baywide reductions of bottom water dissolved
oxygen in Chesapeake Bay (Thomann et al. 1994).
The EPA Office of Air Quality Planning and Standards' program addressing the issue of
N deposition and resultant eutrophication impacts is the Great Waters Program. The charge of
the Great Waters program, under section 112(m) of the CAA amendments, is to evaluate the
deposition of hazardous air pollutants (and other pollutants at the discretion of the
Administrator) to the Great Lakes, Lake Champlain, Chesapeake Bay and other estuarine and
coastal waters. Given the discretion for additional pollutants, and the lack of attention to the
NOx/eutrophication link, EPA chose to use this program as a vehicle to address this N
deposition/eutrophication issue. The Great Waters Program provides information in biennial
reports to Congress regarding: the adverse effects of the deposition on human health and the
environment; the proportion of the loading which comes from the atmosphere; the sources of the
pollution; evaluations of the effectiveness of existing regulatory programs in addressing these
problems; and, ultimately, what changes to regulations are needed to prevent the identified
adverse effects. The overall goal of the Great Waters Program is to identify and prevent adverse
effects due to air pollutants deposited to aquatic ecosystems.
The Great Waters program works cooperatively with a number of other EPA offices, the
National Oceanic and Atmospheric Administration (NOAA), and state agencies with respect to
the N deposition/eutrophication issue, including the Chesapeake Bay Program Office, the Office
of Water, various National Estuary Programs (e.g., Tampa Bay), and NOAA's Air Resources
Laboratory and Coastal Ocean Program Office. The goals of these programs are complementary
to and intersect with the Great Waters goal of characterizing and addressing the water quality
problems due to atmospheric deposition of N compounds.
2. Status of the Program
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Great Waters Program
Because of established monitoring data bases, monitoring methodology research,
enhanced monitoring networks, and linked airshed-watershed-water quality models available or
under development, the Great Waters Program has been focused on further advancing efforts in
specific geographic areas. For estuarine issues, that work has focused on the Chesapeake Bay.
Participants in the Great Waters Program believe that providing a comprehensive evaluation in a
well-studied geographic area provides an invaluable understanding of the total scope of the
problem, causes, processes, and the range of feasible control/reduction/prevention management
options — an understanding that would be impossible to develop with a number of disparate,
less-intensive studies. The Great Waters Program has already found that the information
developed through the Chesapeake Bay Program is directly applicable to other east coast
estuarine systems with adjustments needed for the respective waterbody's watershed
characteristics. This direct transferability of technologies and technical findings will save
similar financial investments and years of effort by the many other place-based estuarine and
coastal management programs along the Atlantic and Gulf coastlines.
Since the First Report to Congress on the atmospheric deposition of pollutants to the
Great Waters, studies of other coastal waters, at National Estuary Program waters in particular,
have investigated the significance of atmospheric deposition of N compounds to their waters. To
improve understanding and reduction of N deposition to Chesapeake Bay and other coastal
waters, the Chesapeake Bay Program, various National Estuary Programs, and the Gulf of
Mexico Program continue to develop and refine modeling and monitoring efforts by addressing
uncertainties such as N retention in watersheds, the differences in transport and fate of various N
compounds, and the contribution of near shore ocean waters to the N inputs to estuaries.
The second report to Congress on the atmospheric deposition of pollutants to the Great
Waters was completed in June 1997 (EPA, 1997). The report continues to find N compounds a
pollutant of concern. Atmospheric deposition of N compounds can contribute significantly to
eutrophication in coastal waters, where plant productivity is usually limited by N availability.
Accelerated eutrophi cation and its subsequent effects such as nuisance algal blooms and reduced
oxygen levels pose significant problems for Chesapeake Bay and many other estuaries. The
report also indicates that substantial progress has been made in addressing N contamination issues
in Chesapeake Bay, the largest United States estuary. A strategy has been developed by the
Chesapeake Bay Program for reducing the N load to the Bay. Part of this process includes the
large-scale modeling and understanding of the type and geographic origin of airborne N to the
Bay. Significant data also have been collected on rates and amounts of N deposition (including
comparison of direct and indirect deposition and of wet and dry deposition), and models have
been developed to evaluate the impact of several N reduction scenarios on the Bay's water quality.
Chesapeake Bay Program
The Chesapeake Bay, largest of the 130 estuaries in the United States, was the first in the
nation to be targeted for restoration as an integrated watershed, airshed, and ecosystem. The
64,000 square mile drainage basin covers parts of six states including New York, Pennsylvania,
Maryland, Delaware, West Virginia, and Virginia and includes more than 150 tributaries. Now
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in its thirteenth year, the Chesapeake Bay Program is a unique, regional, federal-state-local
partnership that has directed and coordinated Chesapeake Bay restoration since the signing of the
historic 1983 Chesapeake Bay Agreement. The Chesapeake Bay Program includes the state of
Maryland, the Commonwealths of Virginia and Pennsylvania, the District of Columbia, the
Chesapeake Bay Commission (representing the three state legislatures), and EPA on behalf of
more than 25 participating federal departments and agencies.
Building on an expanded understanding of the Bay system and increasing experience
with on-the-ground implementation within the cooperative basinwide partnership, the 1987
Chesapeake Bay Agreement set forth a comprehensive array of goals, objectives and
commitments addressing living resources, water quality, population growth and development,
public information, and governance (Chesapeake Executive Council 1987). The centerpiece of
the agreement was a commitment to achieve a 40 percent decrease of controllable N and
phosphorus entering Chesapeake Bay by the year 2000. Atmospheric deposition was considered
part of the uncontrollable portion of nutrient loadings to the Bay tidal waters under the 1987 Bay
Agreement. Through amendments to the Bay Agreement, the signatories have since committed
to "quantify the impacts and identify the sources of atmospheric inputs on the Bay system" and
incorporate atmospheric deposition as an integral component of the tributary basin-specific
nutrient decrease strategies (Chesapeake Executive Council 1992, 1993).
A irshed- Watershed Models
While the current Chesapeake Bay models can simulate the relative effects of
atmospheric N deposition on water quality, the ultimate goal is to link airshed emissions directly
to ecological responses within a single model simulation framework. To provide for this
predictive capacity, the Chesapeake Bay Program is configuring the Bay Watershed Model to
accept daily atmospheric loadings to land use categories—forest, pasture, cropland, and urban
(Chesapeake Bay Program 1995). The model can then simulate increased or decreased
atmospheric loadings to the Bay tidal waters along with nutrients from other land-based point
and nonpoint sources. The estuary model is being upgraded to simulate basic ecosystem
processes of submersed aquatic vegetation, benthic microorganisms, and major zooplankton
groups. With these refinements, the overall loads of controllable and uncontrollable N from the
surrounding airshed and watershed, and the impact of these loads on the ecosystem can be
simulated and evaluated. In parallel, Chesapeake Bay Program state and federal managers are
developing the tools and information necessary to assess the efficacy of atmospheric source
control options for basin-wide and tributary nutrient decrease strategies. The integrated Bay
airshed-watershed-estuary-ecosystem models will be completed in mid-1997.
N Deposition Workshops
The Chesapeake Bay Program sponsored a coastal shared resources initiative which
focuses on airsheds, the coastal ocean, migratory waterfowl and neotropical birds, and migratory
fish. As part of this initiative, two workshops have been held to address atmospheric N
deposition to estuarine and coastal waters. The first workshop was held at the Airlie House in
Warrenton, Virginia, in October 1995 (the "Airlie workshop"). There, leading scientists and key
policy and regulatory officials assembled to explore mechanisms by which air and water
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pollution control programs can work together to help protect eastern coastal ecosystems11. The
focus of the workshop was on atmospheric N compounds, but many of the conclusions would
apply equally well to other pollutants occurring in the air, such as toxic chemicals, trace metals,
and persistent organic compounds. In all such instances, the atmosphere constitutes a resource
that is shared among many different coastal jurisdictions, as well as between state and federal air
and water regulatory agencies.
The workshop concluded that there is need for (1) a better understanding of how all
atmospheric N species affect coastal ecosystems, and of the related policy options, (2) a cross-
media approach to the atmospheric deposition and loading problem, and (3) a coalition of
interested parties extending from the north to the south of the potentially affected eastern coast
of the continental United States, including terrestrial, atmospheric, and aquatic aspects as equals.
The work that is needed is essentially multi-media, requiring attention by a consortium of
workers. The workshop resulted in a call for more cooperation across different issues, estuaries
and bays, scientific disciplines, and state and federal agencies. It was determined that outreach
to state and federal agencies, non-government organizations, industry, and the public at large, is
critically needed.
To structure this effort, an action plan is included as the final section of the workshop
report. The focus of short term actions is on effectively conveying the importance of the N
deposition issue to both public citizens and officials in a unified voice. Mid-term, the emphasis
shifts to improving cooperation across disciplines, estuaries and coastal waters, and agencies.
And finally, the long-term actions deal with applying cross-media practices of all environmental
issues and concerns detailed in the workshop report (East Coast Atmospheric Resource Alliance,
1996). (A copy of the report may be obtained from the NOAA Chesapeake Bay Office.)
The second workshop was held in Raleigh, North Carolina in March, 1997 (the "Raleigh
workshop"). There were three broad objectives for this workshop: (1) to determine the essential
connections between issues, programs, agencies, organizations and jurisdictions which could
advance the ability to address the atmospheric N issue; (2) to identify and/or create new
platforms for discussion of solutions to these problems; and (3) to identify management issues
around which additional research and policy work are needed to advance the understanding of
the ecosystem impacts for both airsheds and watersheds. Many of the questions addressed at the
Airlie workshop were carried forward to the Raleigh workshop. Participants at the Raleigh
workshop also endorsed a list of practical studies developed and prioritized at an earlier
workshop in 1994 (described below). A report on the proceedings of the Raleigh workshop will
be made available.
This workshop was convened as part of a larger east coast alliance of national estuary programs and
coastal management programs under the Chesapeake Bay Program sponsored coastal shared resources initiative. The
shared resource issues of focus include airsheds, the coastal ocean, migratory waterfowl and neotropical birds, and
migratory fish.
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NOAA Air Resources Laboratory
The NOAA Air Resources Laboratory conducts work in collaboration with academic
researchers and EPA and includes air and deposition monitoring, as well as modeling and
modeling support. Relevant activities include the following.
I. Strategic Scientific Planning: In an effort to coordinate scientific programs to reduce
existing uncertainties in atmospheric loadings estimates, NOAA Air Resources
Laboratory, in coordination with the Chesapeake Bay Program convened a workshop in
June 1994, inviting scientists and managers with expertise and experience in
understanding or managing atmospheric deposition. The challenge given to the
workshop was simple-to construct a list of practical studies that would make the
greatest impact on reducing the current uncertainty in estimates of the contribution of
atmospheric deposition to declining coastal aquatic ecosystem health. The listing that
resulted is summarized below and substantiated in the workshop proceedings report
Atmospheric Loadings to Coastal Areas: Resolving Existing Uncertainties (Chesapeake
Bay Program, 1995).
Priority 1 — Conduct intensive, coordinated integrated monitoring at special locations
within the watershed, with wet deposition, dry deposition, and local catchment
area characterizations — to provide quality integrated monitoring data for
evaluating model performance.
Priority 2 — Work to improve existing atmospheric models — to address limitations of
current models, especially limited resolution and the inability to handle
orographic and chemical factors that are likely to be of critical importance.
Priority 3 - Improve biogeochemical watershed models — to characterize watershed
chemical processing and retention better.
Priority 4 — Improve emissions inventories and projections — to provide more accurate
inputs for deposition models, with better spatial resolution.
Priority 5 - Conduct process-oriented measurements to extend vertical and spatial
meteorological and chemical concentration and deposition coverage and to
quantify representativeness — to provide more advanced input data as models
evolve and input data requirements increase.
Priority 6 — Establish an extensive array of less intensive measurements — (this item
follows from Priority 1) to provide a nested network with a small number of
Priority 1 intensive stations supporting a denser array of simple stations designed
to provide improved spatial resolution for some selected variables.
II. Dry Deposition Inferential Method Network (DDIM): Dry deposition is a
component that is not well characterized, because it is difficult to measure and to emulate
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deposition to water. Data from Wye, Md. have revealed deposition rates which were
similar to those reported from other DDIM stations in the region, and that dry deposition
of nitrate (HNO3/NO3) was approximately 46 percent of total nitrate deposition.
However, further analysis indicated that some of the data may be suspect due to artifacts
associated with sampler design affected by high ammonia and sea salt concentrations
over the Eastern Shore. Future field studies will work to determine the validity of using
the DDIM system in coastal areas.
III. Nitrogen Photochemistry: A collaborative research effort with the University of
Maryland focused on nitrogen photochemistry and transport of anthropogenic precursors
from local and regional sources. Results from this project indicate that highest NOX
concentrations occur when the source air mass has passed over the industrial midwest
and/or the Washington-Baltimore area. Furthermore, it appears that urban airshed
models may underestimate NOX concentrations in near-urban environments. These
results need to be verified and reinforced with further studies.
IV. Precipitation Chemistry: A daily precipitation chemistry site was established in the
lower Chesapeake Bay, on an island chosen to emulate precipitation deposition directly
to a water body. The data from this site will allow (for the first time) the evaluation of
deposition model outputs such as those given by RADM.
V. NO Off-gassing: Efforts are in process to investigate soil emissions of NO at Wye,
MD.
VI. Meteorological Measurement: NOAA provided meteorological information support
for the Atmospheric Exchange Over Lakes and Oceans experiment in August 1995. In
addition, in late March 1992, the first Chesapeake Bay Observing System (CBOS) buoy
was placed by the University of Maryland in the North Bay, Maryland, the first of a
proposed system of up to 11 similar buoys to record physical measurements. Given
certain assumptions, the meteorological measurements made from the buoy allow the
estimation of dry deposition velocities for nitric acid to the Bay surface. These
measurements are being used to evaluate the ability of mesoscale atmospheric models to
calculate deposition rates for N chemicals.
VII. Modeling: Operational forecasts and hindcasts are being performed and archived
twice per day with the Regional Atmospheric Modeling System (RAMS) model. Work
is currently underway to compare the RAMS predictions for over-water physical
parameters with that measured by the CBOS buoys, and to reduce the RAMS resolution
to characterize the Baltimore plume better.
Other Estuary Programs
The Chesapeake information is serving as the base for work in the Tampa Bay, where the
Great Waters Program has begun leveraged support for the Tampa Bay National Estuaries
Program for NOX, organic N, and ammonia deposition sampling, as well as the sampling needed
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to characterize the linkage between atmospheric deposition and urban storm water runoff. This
effort began in 1995 and is expected to be complete in 1998. Further work in estuaries will be
targeted in cooperation with the EPA Office of Water and the National Estuary Programs.
3 . Science of NOX and Eutrophication
Though the availability of N normally limits biological productivity in coastal waters,
over-abundance of N is of concern in areas which have developed nutrient enrichment problems,
known as eutrophication. In addition to increasing primary productivity, nutrient enrichment
generally alters the normal ratios of N to phosphorus and to other elements, such as silicon and
iron. This alteration may induce changes to phytoplankton community structure. Species which
normally occur in low abundance may be favored, and in some cases, toxic and/or noxious algal
blooms may result. For the New England coast, for example, the number of red and brown tides
and shellfish problems from nuisance and toxic plankton blooms have increased over the past
two decades. In coastal areas with poor or stratified circulation patterns (e.g., Chesapeake Bay,
Long Island Sound, Albemarle-Pamlico Sound) the "overproduction" of algae tends to sink to
the bottom and decay, using all (anoxia) or most (hypoxia) of the available oxygen in the
process, causing fish kills and loss of critical habitat. In some cases, the increase in suspended
matter due to overproduction decreases light infiltration or algae grows directly on submerged
living organisms, in turn causing a loss of submersed aquatic vegetation and coral communities.
Atmospheric deposition of N species is recognized by all east-coast estuarine programs
as either a significant contributor to estuarine eutrophi cation or a mechanism of possible concern
(East Coast Atmospheric Resource Alliance, 1996). The region from which the atmospheric N
pollution arises is much larger than the water surface that is potentially affected, and even much
larger that the watershed that drains into it. The extent of "airsheds" are now starting to be
recognized. The Chesapeake Bay airshed (defined as the source region for 75 percent of
deposited emissions) extends upwind of and borders the water body itself, reaching to New
York, Ontario, Ohio, Tennessee and South Carolina (Dennis 1996). Emissions from within the
Chesapeake Bay airshed may also affect estuaries along the coast from the Carolinas to New
York. Similarly, the airsheds of other estuaries will overlap. Thus, airsheds constitute an
important "shared resource" which must be recognized. Reductions in emissions in airsheds
benefit downwind N-sensitive ecosystems, and assessments of the benefit of such decreases must
take all benefitting water bodies into account, and not just one single ecosystem that is especially
favored.
The uncertainties in studies to date make it important that a better understanding be
obtained of the processes that transport and deposit N to estuaries and coastal zones. Current
estimates are that emissions of NOX are the largest contributor to atmospheric N loads to coastal
waters of eastern North America (40-60 percent), with ammonia (20-40 percent) and organic N
(about 20 percent) also contributing significant amounts on an annual basis (Paerl, 1993, 1995).
The relative contribution of atmospheric N deposition to total new N inputs to estuarine, coastal,
and offshore waters around the world ranges from less than 10 percent up to 70 percent (Paerl,
1993, 1995; Valigura et al, 1995). Studies performed on several other major East Coast
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estuaries from Albemarle-Pamlico Sound to the Gulf of Maine have provided atmospheric N
loading estimates that range between 18 percent and 39 percent of the total N load (East Coast
Atmospheric Resource Alliance, 1996).
The most recent estimate of a 27 percent contribution of atmospheric deposition to total
N loadings to the Chesapeake Bay falls within the range reported for other estuaries (Linker et
al, 1993). Relative atmospheric contributions to coastal seas and open oceans may be higher,
ranging up to 60-70 percent. However, a key factor is the actual amount (or total load) of N
being considered. For example, the atmospheric contribution to the open ocean may be 70
percent compared to 27 percent in Chesapeake Bay, but the amount deposited (kg/ha) is small
relative to that delivered to the Chesapeake Bay system (Paerl, 1995). In the coming decades,
the atmosphere will become an even more significant source of N loadings to the Chesapeake
Bay and other east coast estuaries when anticipated increases in population and land
development result in increases in mobile source and power plant emissions. (Fisher, et al.,
1988;Pechan, 1991).
Seasonal Impacts
Simulated water quality responses to year round vs seasonal nutrient decreases were
conducted as a 1992 reevaluation of the bay wide Tributary Nutrient Reduction Strategy by the
Chesapeake Bay Program and indicated the need for year round controls on phosphorus and N
loadings (Thomann et al 1994). The seasonal nutrient reductions scenarios, largely focused on
assessing seasonal vs. year round biological nutrient removal at wastewater treatment facilities,
indicated that winter N inputs contribute to summer eutrophication events. Further work is
planned in the 1997 timeframe to more specifically explore the effects of seasonal controls from
air emission sources.
4. Needed Reductions - Regionally, Nationally
Obviously, characterizing the reduction of emissions needed to eliminate or ameliorate
the atmospheric contribution to eutrophication of the nation's estuarine and coastal waters
depends on a number of factors. These include: (1) amount of N in an estuarine/coastal
ecosystem which constitutes a problem (i.e., the threshold at which nutrient inputs lead to
eutrophication-related impacts); (2) relative contribution of specific atmospheric N compounds
(NOX, NH4+, Organic N) to the total N loading and biotic impacts on the individual ecosystem;
and (3) contribution of emission sources, or source categories, to deposition in specific estuarine
or coastal waters.
The specific answers to these questions are going to differ for any given estuary, due to
conditions of trophic status, hydrology, non-atmospheric N (and atmospheric ammonia and
organic N) inputs, location and meteorology relative to NOX and other N sources, development
within the surrounding watershed, and other variables. Responses to these questions are being
pursued by the EPA and NOAA and, to a growing degree, by Atlantic and Gulf national estuary
programs as a result of the 1995 Shared Resources Workshop described previously. There are
no complete answers yet, but insights into all three questions are emerging from work on
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Chesapeake Bay. Some Bay efforts to provide information are described below.
Emission Source Characterization
From the major Bay influencing states—Maryland, Virginia, Pennsylvania, West
Virginia, Ohio, New York, and New Jersey—utility sources (e.g., power plants) contribute 37
percent of the total NOX emissions, mobile sources (e.g., cars, trucks) contribute 35 percent,
other area sources (e.g., ships, boats, lawn equipment) contribute 21 percent, and other point
sources (e.g., industries) contribute 6 percent (Dennis, 1996). These source emissions
contributions are roughly equivalent to the source contributions to deposited ions of N from
utility and mobile sources to the Chesapeake Bay and its watershed.
Atmospheric Deposition Modeling
Most assessments involving atmospheric deposition to the Chesapeake Bay have made
use of the Regional Acid Deposition Model (RADM) (Dennis, 1995), which contains advanced
descriptions of atmospheric chemistry and state-of-the-art formulations of atmospheric transport
and dispersion. The model has been tested in considerable detail (Dennis, 1995). In practice,
RADM is used in conjunction with the most advanced large scale watershed model available to
yield not only estimates of deposition to the Chesapeake Bay watershed but also loadings to the
Chesapeake Bay tidal waters.
Arced Delineation
Bounded by a set of decision rules, a series of scenarios run on the RADM were used to
delineate the airshed of the Chesapeake Bay. The Chesapeake Bay airshed, roughly 350,000
square miles in size or more than 5 and a half times larger than the watershed, includes all of
Maryland, Virginia, Pennsylvania, Delaware, the District of Columbia, West Virginia, and Ohio,
most of New York, half of New Jersey, North Carolina, and Kentucky, and parts of Tennessee,
South Carolina, Michigan, Ontario, and Quebec, including Lake Ontario and Lake Erie. The
airshed is defined as that contiguous region of air emissions that contributes the majority of the
deposition (75 percent) to the Bay and its watershed.
As defined, the Chesapeake Bay airshed, containing 30 percent of Eastern United States
and Canadian emissions, accounts for 75 percent of the atmospheric N deposited onto the Bay
and its watershed. The remaining 25 percent originates from emission sources outside the
airshed. These remaining sources were located beyond a predefined point of diminishing return;
that is, when a 50 percent reduction in emissions from large source regions would be expected to
produce less than a 10 percent reduction in deposition onto the Bay watershed (Dennis, 1996).
Therefore, the areal extent of the Chesapeake Bay airshed as defined here is an underestimate of
the actual areas of the United States and Canada that contain sources that contribute to N
deposition to the Bay and its watershed. A still undefined portion of the airshed is that portion
which contributes to the atmospheric N deposition on coastal waters which, in turn, contributes
to the influx of N from coastal waters into the southern Chesapeake Bay (Chesapeake Bay
Program, 1994).
Relative Source Emission Contributions to Loadings
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The pattern of N deposition from these difference sources were simulated using the
RADM. These model simulations suggest that N emissions from utilities contribute a majority
of the N which deposits on the western side of the Bay watershed, with a decreasing trend from
the western to eastern portion of the watershed. The RADM runs further suggest that mobile
sources (associated with the Boston-Washington megalopolis) contribute a majority of the N
which deposits along the Delmarva Peninsula, the mainstem Chesapeake Bay, and the lower
portions of the western shore tidal tributaries, with a decreasing trend from the eastern to
western portion of the basin.
5. Reductions with Current and Projected Programs
Reduction scenarios are part of an effort to determine how to achieve the goal of a 40
percent reduction in nutrient loadings to the Chesapeake Bay by 2000 (from a 1985 baseline).
Land-based nonpoint source and wastewater treatment facility-based point source reduction
actions, planned for implementation in many Chesapeake Bay tributary watersheds, are
approaching the limits of technology. Options for reductions in air emissions are being explored
for maintaining the targeted 60 percent nutrient loadings cap beyond the year 2000 in the face of
a growing population and resultant development in the watershed.
Anticipated reductions in NOX emissions, and resultant N loadings to estuarine waters,
due to a range of CAA and Ozone Transport Commission regulatory and non-regulatory actions
are currently being simulated through the linked Chesapeake Bay airshed-watershed-estuarine
water quality ecosystem processes models. Present model estimates are that over one quarter of
the total N loading to the Bay system comes from the atmosphere. However, significant
refinements to the Bay Watershed Model are being conducted and final results are expected in
mid-1997. Table III-l shows the estimated changes in that loading that would result from two
scenarios of NOX control — first, full implementation of the CAA amendments of 199012, and
second, implementation of the more intensive controls advocated by the Ozone Transport
Commission13.
Table III-l. Estimated Reductions in N Loadings to Chesapeake Bay and Water Quality
Response Under Several Control Scenarios.
The Clean Air Act Scenario estimates the atmospheric load reductions expected under full
implementation of the Clear Air Act titles I, II, and IV.
The Ozone Transport Commission Scenario evaluates the effects of conditions found in Clean Air Act
titles I, II, and IV, as well as additional N decreases to lower ground level O3 in mid-Atlantic and New England
metropolitan regions as called for by the Ozone Transport Commission.
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Control
Scenario
Bay
Agreement
(no air
controls)
Clean Air
Act
Ozone
Transport
Commission
Nitrogen
Atmo-
spheric
Deposition
Reduction
to Bay
0%
9%
12%
Nitrogen
Atmospheric
Deposition
Reduction to
Watershed
0%
14%
21%
Nitrogen Load
Reduction to
Bay (millions
of Ibs per year)
75 (target)
14
24
Water Quality
Improvement*
20%
4% **
6% ***
* The water quality improvements are quantified in terms of estimated reductions in the volume of Bay bottom with no
dissolved oxygen (i.e., reduction in Chesapeake Bay "dead waters"); decreased nitrogen loadings will also result in decreased water
column nitrogen concentrations which will, in turn, decrease algae growth and so improve light penetration, necessary to support
the critically important underwater Bay grasses (Batiuk et al., 1992; Chesapeake Bay Program, 1994d; Dennison et al., 1993;
Thomann et al, 1994).
** Determined by the difference between the Bay Agreement scenario alone and the Bay Agreement plus Clean Air Act
scenario, Response of the Chesapeake Bay Water Quality Model to Loading Scenarios, Thomann, et al. 1994.
*** Extrapolation of Bay Agreement plus CAA scenario to load reductions under Ozone Transport Commission
controls.
6 . Summary
One of the goals of the Great Waters program is to evaluate, and determine an effective
means to address, the impacts of atmospheric N deposition to estuarine, and other N-limited
waters. This goal is shared with the Chesapeake Bay Program, as well as many of the eastern
National Estuary Programs. The efforts of these programs have been focused to date largely on
the Chesapeake, using a variety of state-of-the-art models and monitoring data to evaluate the
airshed, source categories of emissions, and ecosystem impacts. Work thus far has shown the
relative contribution of the atmosphere to total N loadings to be significant. The airshed of the
Chesapeake reaches from New York to Ontario, Kentucky, and South Carolina. Other affected
estuaries are going to have similar, and overlapping airsheds.
The impacts of excess N, in these N-limited systems, is contributing to the most serious
water quality problem in eastern estuaries at the present, eutrophication. The impacts of
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eutrophication range from increased turbidity shading out beneficial submersed aquatic
vegetation habitats, to the exacerbation of noxious algae blooms, to the creation of low or no-
oxygen conditions (hypoxia or anoxia) which negatively affect fish populations.
In the Chesapeake, the affected parties have agreed that a 40 percent reduction in N
inputs is needed for the Bay's health. The implementation of CAA provisions could account for
about a fifth to a third of that. These same actions would be expected to provide similar benefits
to yet unstudied estuaries with eutrophication problems.
References
Chesapeake Bay Program. 1995. Atmospheric Loadings to Coastal Areas: Resolving Existing
Uncertainties. A report of the Atmospheric Loadings Workshop, Baltimore, Maryland June 29-
30, 1994, organized and lead by the Air Quality Coordination Group and the Scientific and
Technical Advisory Committee of the Chesapeake Bay Program.
Chesapeake Executive Council. 1987. 1987 Chesapeake Bay Agreement. Annapolis, Maryland.
Chesapeake Executive Council. 1992. Chesapeake Bay Agreement 1992 Amendments.
Annapolis, Maryland.
Chesapeake Executive Council. 1993. Directive 93-1 Joint Tributary Strategy Statement.
Annapolis, Maryland.
Dennis, R.L. 1996. Using the Regional Acid Deposition Model to determine the nitrogen
deposition airshed of the Chesapeake Bay watershed. In: J.E. Baker (ed.). Atmospheric
Deposition of Contaminants to the Great Lakes and Coastal Waters. SET AC Press, Pensacola,
Florida.
East Coast Atmospheric Resource Alliance. 1996. Airsheds and Watersheds - the Role of
Atmospheric Nitrogen Deposition. A report of the Shared Resources Workshop, October 11-12,
1995, Warrenton, Virginia.
Fisher, D.C., J. Ceraso, T. Mathew, and M. Oppenheimer. 1988. Polluted coastal waters: the role
of acid rain. Environmental Defense Fund, New York, NewYork.
Linker, L.C., R.L.Dennis, and D.Y.Alegre. 1993. Inpact of the Clean Air Act on Chesapeake
Bay Water Quality. International Conference on Environmental Management of Enclosed
Coastal Seas. Baltimore, MD.
National Research Council. 1993. Report of the Committee on Wastewater Management for
Coastal Urban Areas, Water, Science, and Technology Board. Washington, D.C.
Nixon, S.W. 1986. "Nutrient Dynamics and the Productivity of Marine Coastal Waters". R.
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Halwagy, D. Clayton, and M. Behbehani (eds): CoastalEutrophication. pp 97-115. The Alden
Press, Oxford.
Paerl, H.W. 1993. "The Emerging Role of Atmospheric Nitrogen Deposition in Coastal
Eutrophication: A Biogeochemical and Trophic Perspective." Canadian Journal of Fisheries
and Aquatic Sciences. 50:2254-2269.
Paerl, H.W. 1995. "Coastal Eutrophication in Relation to Atmospheric Nitrogen Deposition:
Current Perspectives." Ophelia. 41:237-259.
Pechan, E.H. 1991. Background documentation on SO2 and NOX forecasts. E.H. Pechan and
Associates, Springfield, VA. EPA Contract No. 68-D00120.
Swain, W., T. Colborn, C. Bason, R. Howarth, L. Lamey, B. Palmer, and D. Swackhamer. 1994.
Exposure and Effects of Airborne Contamination. U.S. Environmental Protection Agency. EPA-
453/R-94-085.
Thomann, R.V., J.R. Collier, A. Butt, E. Gasman, and L.C. Linker. 1994. Response of the
Chesapeake Bay Water Quality Model to Loading Scenarios. Chesapeake Bay Program Office,
Annapolis, Maryland. CBP/TRS 101/94.
U.S. Environmental Protection Agency. 1994. National Water Quality Inventory: 1992 Report to
Congress. Washington, D.C.
U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, revised
criteria document, Air Quality Criteria for Oxides of Nitrogen (three volumes,
EPA-600/8-91/049aF-cF, August 1993.
U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards,
Deposition of Air Pollutants to the Great Waters., Second Report to Congress, EPA-453/R-97-
011, June 1997.
Valigura, R.A., I.E. Baker, J. R. Scudlark, and L.L. McConnell. 1995. Atmospheric Deposition
of Nitrogen and Contaminants to Chesapeake Bay and its Watershed. In: Perspectives on
Chesapeake Bay, 1994: Advances in Estuarine Science. S. Nelson and P. Elliott, eds.
Chesapeake Research Consortium, Edgewater, Maryland.
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C. Global Warming
Global Warming
The "greenhouse effect" is the name for the physical process whereby energy from the sun
passes through the atmosphere relatively freely, while heat radiating from the earth is partially
blocked or absorbed by water vapor and other radiatively important gases in the atmosphere.
Because the sun is hotter than the earth, its energy is radiated at a higher frequency which is not
absorbed well by gases such as carbon dioxide or water vapor. In contrast, these types of gases
are effective absorbers of the lower-frequency energy radiated by the earth. Since the
"greenhouse" gases responsible for this selective absorption make up only about one percent of
the atmosphere, they are also known as "trace" gases. The energy absorbed by the different trace
gases can be calculated accurately. When the concentration of a trace gas increases, this
additional absorption warms the planet, if there are no other changes in the climate system.
Greenhouse Gases
Naturally occurring greenhouse gases include water vapor, carbon dioxide (CO2), methane
(CH4), nitrous oxide (N2O), and O3 (EPA, November 1995). In addition, other photochemically
important gases—such as carbon monoxide, NO, NO2 and VOCs—are not greenhouse gases, but
contribute indirectly to the greenhouse effect because they influence the rate at which O3 and
other gases are created and destroyed in the atmosphere. That is, emissions of NOxlead to the
formation of tropospheric O3, which is also a greenhouse gas. As described below, some
important sources of NOX are also emitters of N2O. Since 1800, atmospheric concentrations of
carbon dioxide have increased by more than 25 percent, methane concentrations have doubled,
and N2O concentrations have risen approximately 8 percent; this recent atmospheric buildup
appears to be largely the result of anthropogenic activities (EPA, October 1995).
Figure III-l illustrates the relative contribution of the greenhouse gases to total United
States anthropogenic emissions in 1994 (EPA, November 1995). Due largely to fossil fuel
consumption, carbon dioxide emissions accounted for the largest share of United States emissions
on a carbon equivalent basis14—almost 85 percent. Methane emissions accounted for 11 percent
and N2O emissions comprise about 2 percent of the global warming potential of all the United
States greenhouse gases (EPA, November 1995). On a global basis,N2O is estimated to
contribute 6 percent of the total global warming atmospheric gases (Kramlich and Linak, 1994).
14In order to compare the ability of a greenhouse gas over time to trap heat in the atmosphere relative
to another gas, the concept of global warming potential (GWP) is used. The GWP uses carbon dioxide as the
reference gas with a GWP of 1. Thus, emissions of greenhouse gases may be reported in terms of million
metric tons of carbon equivalent over a century. N2O, with a GWP of 320, has a much larger ability to trap
heat than carbon dioxide on a gram for gram basis.
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Figure III-l Greenhouse Gas Emissions
(U.S. 1994, Million Metric Tons, Carbon Equivalent)
1600 -n
1400
1200
1000
800
600
400
200
,/
/^ '
x xl
7 / , ' , , ' /
X X I . X xl x 1 X
F ' I \r J \ IX x
C02
CH4
N20
Other
MMTCE
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N2O
N2O is a chemically and radiatively active greenhouse gas that is produced naturally from a
wide variety of biological sources in soil and water, as well as from various anthropogenic
sources. With an atmospheric lifetime of 150 years, N2O is extremely long-lived and very stable
in the troposphere (Kramlich and Linak, 1994). While actual emissions of N2O are much smaller
than carbon dioxide emissions, N2O is approximately 320 times more powerful than carbon
dioxide at trapping heat in the atmosphere over a 100-year time horizon (EPA, November 1995).
N2O Anthropogenic Emissions Sources
N2O is produced naturally in soils through the microbial processes of denitrification and
nitrification15. A number of anthropogenic activities add N to soils, thereby increasing the
amount of N available for nitrification and denitrification, and ultimately the amount of N2O
emitted. Fertilizer use accounts for approximately 45 percent of total United States emissions of
N2O. Anthropogenic emissions of N2O in the United States have increased over 10 percent from
1990 to 1994 primarily for two reasons: increased fertilizer use and general economic growth
(EPA, October 1995). The major United States anthropogenic emissions sources are summarized
in the Figure III-2 (EPA, November 1995).
Other important anthropogenic activities producing N2O are fossil fuel combustion from
mobile and stationary sources, adipic acid production, and nitric acid production, which
contribute 31, 13, and 10 percent, respectively, of the total United States emissions of N2O. N2O
is a product of the reaction that occurs between nitrogen and oxygen during fossil fuel
combustion. Total emissions are estimated at approximately one half million tons or 41 million
metric tons carbon equivalent (MMTCE)16 (EPA, November 1995). Mobile emissions totaled
9.3 MMTCE in 1994, with road transport accounting for approximately 95 percent of these N2O
emissions. N2O emissions from stationary fossil fuel combustion sources were 3.2 MMTCE in
1994. Also with respect to 1994, the production of adipic acid (used to produce nylon)
generated 5.4 MMTCE of N2O. There are currently four plants in the United States that produce
adipic acid. Since 1990, two of the plants have employed emissions control measures destroying
about 98 percent of the N2O before its release into the atmosphere. By 1996, all adipic acid
production plants will have N2O emissions controls in place as a result of a voluntary agreement
among producers (EPA, November 1995). Production of nitric acid is another industrial source
of N2O emissions. Nitric acid is a raw material used primarily to make synthetic commercial
fertilizer: N9O emissions from this source were about 3.8 MMTCE in 1994.
Denitrification is the process by which nitrates or nitrites are reduced by bacteria, which results in the escape
of nitrogen into the air. Nitrification is the process by which bacteria and other microorganisms oxidize ammonium salts
to nitrites, and further oxidize nitrites to nitrates.
MMTCE is a method of comparing the global warming potential of various greenhouse gases. Carbon
dioxide was chosen as the reference greenhouse gas.
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Figure III-2
Nitrous Oxide Emission Sources
(1994 Million Metric Tons, Carbon Equivalent)
15
10
5-
Agriculture Fossil Fuel Acid/Prod. Other
MMTCE
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N2O Global Emissions Sources
N2O is emitted by several sources, which have large uncertainties, and its global atmo-
spheric budget is difficult to reconcile. The updated global budget is presented in Table III-2
below, expressed in million metric tons of N (WMO, 1994). Since the mid-1970s, systematic
tropospheric measurements of N2O have been made at locations worldwide. These data show the
atmospheric concentration of N2O to be increasing at an average rate of approximately 0.3
percent per year (Khalil and Rasmussen, 1992; Vitousek et al, 1997).
TABLE III-2. Estimated global sources of N2O (million metric tons N per year)
Natural
Oceans
Tropical Soils
Wet Forests
Dry Savannas
Temperate Soils
Forests
Grasslands
Anthropogenic
Cultivated Soils
Animal Waste
Biomass Burning
Stationary Combustion
Mobile Sources
Acid Production
1.4-5.2
2.2-3.7
0.5-2.0
0.5-2.0
?
1-3
0.2-0.5
0.2-1.0
0.1-0.3
0.1-0.6
0.5-0.9
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03
O3 is an important greenhouse gas present in both the stratosphere and troposphere.
Although representing only 10 percent of the total O3 column, tropospheric O3 is important
because it can influence climate, as it is a greenhouse gas itself, and because its photolysis by UV
radiation in the presence of water vapor is the primary source for hydroxyl radicals (WMO,
1994). Hydroxyl radicals are responsible for the oxidative removal of many trace gases, such as
methane, hydrofluorocarbons, and hydrocholorfluorocarbons that influence climate and/or are
important for the stratospheric O3 layer.
Observations show that free tropospheric O3 has increased above many locations in the
Northern Hemisphere over the last 30 years. Model simulations and the limited observations
together suggest that tropospheric O3 may have doubled in the Northern Hemisphere since pre-
industrial times. Such changes in O3 have potentially important consequences for warming,
although detailed quantification is not possible due to uncertainties in the size and distribution of
the O3 change (Houghton, 1994).
Emissions of NOxlead to the formation of tropospheric O3. The increases in tropospheric
O3 will be regional in nature and so will the associated effects noted above. Because changes in
the tropospheric O3 are highly spatially variable, both regionally and vertically, assessment of
global long-term trends is extremely difficult. To the extent reductions in emissions of NOX would
lower tropospheric O3 concentrations, the global warming effects of tropospheric O3 would also
be lowered.
Emissions Control Programs
While the goal of many NOX control procedures for mobile and stationary sources is to
convert NO into N2, some NOX control programs strive to prevent NOX emissions in the first
place. For example, efforts to lower miles traveled by vehicles or agricultural methods to
decrease the amount of N fertilizer applied will decrease both NOX and N2O. Other control
programs are directed at N2O emissions, such as in adipic acid production. However, many NOX
control procedures for mobile and stationary sources convert NO into N2 and, in that process, a
portion of the reactions will form N2O. Therefore, the development of an emissions control
program to achieve a specific environmental goal may need to take into account the impact on
both NOX and N2O emissions and the resultant environmental impacts that may go beyond the
specific goal.
The amount of N2O emitted is generally small compared to the NOX emissions reductions
and varies, depending upon fuel, technology type, and pollution control device. Emissions also
vary with the size and vintage of the combustion technology, as well as maintenance and
operation practices. Staged combustion, including low-NOx burners, and reburn technologies
have only a small influence on N2O. In selective non-catalytic reduction, use of NH3 results in less
than 5 percent of the NO reduced being converted to N2O; use of urea converts more than 10
percent of the NO. The application of selective catalytic reduction suggests that N2O emissions
are negligible from vanadium catalysts; noble metal catalysts, however, may convert significant
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quantities of NO into N2O (Kramlich and Linak, 1994). Regarding mobile sources, as catalytic
converter-equipped vehicles have increased in the United States motor vehicle fleet, emissions of
N2O from this source have also increased (EPA, 1995).
References:
Houghton, J. T., et al, Climate Change 1994, "Radiative Forcing of Climate Change," reports of
working groups I and III of the Intergovernmental Panel on Climate Change, Cambridge
University Press, section 6.2.
Khalil, M.A. and Rasmussen, R.A., "The Global Sources of Nitrous Oxide," J. Geophys. Res.
97, 14651-14660, 1992.
Kramlich, J.C. and Linak, W.P, "Nitrous Oxide Behavior in the Atmosphere, and in Combustion
and Industrial Systems," Prog. Energy Combust. Sci., Vol 20, 1994.
U.S. Environmental Protection Agency, Office of Policy, Planning and Evaluation, Inventory of
U.S. Greenhouse Gas Emissions and Sinks: 1990-1994, November 1995, EPA-230-R-96-006.
U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, National
Air Pollutant Emission Trends, 1900-1994, October 1995, EPA-454/R-95-011.
usek, P.M., J. Aber, R.W. Howarth, G.E. Likens, P.A. Matson, D.W. Schindler, W.H.
Schlesinger, and G.D. Tilman, "Human Alteration of the Global Nitrogen Cycle: Causes and
Consequences," Issues in Ecology, Number 1, Spring 1997.
World Meteorological Organization, Scientific Assessment of Ozone Depletion: 1994, Global
Ozone Research and Monitoring Project—Report No. 37.
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D. Stratospheric Ozone Depletion
Stratospheric O3
O3 is mainly found in two regions of the Earth's atmosphere. Over 90 percent of
atmospheric O3 resides in the stratosphere, a layer between approximately 15 and 50 km (about
10-30 miles) above the Earth's surface (Kramlich and Linak, 1994). This stratospheric O3 is
commonly known as the "ozone layer." The remaining O3 is in the lower region of the
atmosphere, the troposphere, which extends from the Earth's surface up to about 10 km.
Concentrations of O3 in the stratosphere are maintained by the balance between photochemical
production of O3 (by photolysis of molecular oxygen, O2) and destruction of O3. Stratospheric O3
is produced by the photolysis of O2, which gives oxygen atoms (O), which then react with O2 to
form O3. Photochemical reactions associated with hydrogen oxides (HOX), NOX, and halogen
oxides (C1OX and BrOx) destroy O3. The relative contribution of each to O3 destruction varies
with such factors as altitude, latitude, and season (WMO, 1994).
Stratospheric O3 plays a beneficial role by absorbing most of the biologically damaging
ultraviolet sunlight called UV-B, allowing only a small amount to reach the Earth's surface.
Many experimental studies of plants and animals, and clinical studies of humans, have shown the
harmful effects of excessive exposure to UV-B radiation. In contrast, at the planet's surface, high
concentrations of O3 are toxic to living systems and can damage the tissues of plants and animals.
The ground-level O3 concentrations in the smoggiest cities are very much smaller than the
concentrations routinely found in the stratosphere (WMO, 1994).
Stratospheric O3 Depletion
Decreases in stratospheric O3 have occurred since the 1970s. The most obvious feature is
the annual appearance of the Antarctic O3 hole in September and October. The October average
total O3 values over Antarctica are 50-70 percent lower than those observed in the 1960s; this
phenomenon has come to be known as the Antarctic "ozone hole." The O3 loss occurs at
altitudes between about 14 and 24 km. Smaller, but still significant losses in global total-column
O3 have also been observed in the more populated mid-latitudes (30-60 degrees) of both
hemispheres (WMO, 1994). For example, in the middle latitudes of the Northern Hemisphere,
downward trends of about 6 percent per decade over 1979-1994 were observed in winter and
spring and about 3 percent per decade were observed in summer and fall.
Polar Regions
The principal cause of O3 loss in the polar regions is photochemistry involving the halogen
species, chlorine and bromine (WMO, 1994). Long-lived halogen species, primarily
chlorofluorocarbons, are released in the troposphere from human activities. The photochemical
degradation of these organic source molecules in the stratosphere leads to the formation of
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inorganic halogen species. The release of chlorine from these species occurs in high-latitude
winter in reactions on surfaces of stratospheric aerosol particles. The formation and reactivity of
these particles are enhanced at the low temperatures characteristic of the interior of the polar
vortices. The removal of reactive nitrogen, especially nitric acid, by aerosol particle
sedimentation in the vortex strongly regulates the rate of recovery by controlling the availability of
active chlorine. In the Antarctic, weather patterns result in the annual polar vortex, which
prevents the transport of O3 from the southern hemisphere mid-latitudes to the polar regions,
primarily in September and October.
Mid-latitudes
As noted above, stratospheric O3 concentrations are lowered by photochemical reactions
associated with the hydrogen, N, chlorine and bromine radicals. The concentrations of these
radical species are maintained by photodegredation of the corresponding source gases: H2O and
methane for HOX, N2O for NOX, and halogen source gases for C1OX and BrOx. Increases in radical
concentrations (e.g., increases in C1OX due to chloroflurocarbons emitted at the Earth's surface,
and increases in NOX due to N2O emitted at the ground and stratospheric injection of NOX by
aircraft) lead to changes in O3 (WMO, 1994).
In the low stratosphere (10-22 km), reactions involving HOX dominate the O3 loss rate,
while between 23 and 40 km, NOX cycles dominate (WMO, 1994). Decreases in NOX above
about 22 km, where it represents the dominant photochemical loss mechanism, would result in
local stratospheric O3 increases (Tie et al, 1994). The broad picture is of reactions involving HO2
being responsible for over half the photochemical destruction of O3 in the low stratosphere at mid-
latitudes, while halogen (chlorine and bromine) chemistry accounts for a further third. Although
catalytic destruction by NOX accounts for less than 20 percent of the photochemical O3 loss in the
low stratosphere at mid-latitudes, NO and NO2 are vital in regulating the abundance of hydrogen
and halogen radicals and thus the total photochemical O3 destruction rate (WMO, 1994).
It should also be noted that there is observational evidence that tropospheric O3 (about 10
percent of the total-column O3) has increased in the Northern Hemisphere (north of 20 degrees N)
over the past 3 decades (WMO, 1994). The upward trends are highly regional. Tropospheric O3
(and aerosols) can decrease global UV-B irradiances. However, recent trends in tropospheric
pollution probably had only minor effects on UV trends relative to the effect of stratospheric O3
decreases (WMO, 1994).
N2O
The major sources of N2O emissions are described in the "Global Warming" section of this
document. As noted previously, N2O has an atmospheric lifetime of 150 years. The major sink of
N2O is photo dissociation following diffusion into the stratosphere (WMO, 1994). Its products of
dissociation are the major source of stratospheric NOX, which are important in regulating
stratospheric O3. The dissociation produces NO, which leads to the subsequent chemical
destruction of stratospheric O3 (Kramlich and Linak, 1994). Formation of NO in the stratosphere
is the result of photolysis of N2O and reaction with an oxygen atom. The NO formed
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subsequently reacts with stratospheric O3, forming oxygen (O2). The net effect is that increased
global concentrations of N2O contribute to the thinning of the stratospheric O3 layer (Vitousek,
1997).
N2O is emitted by natural and anthropogenic sources, which are listed in Table III-2 and
have large uncertainties. Stratospheric O3 destruction is further complicated by potential
interaction between chlorine monoxide and NO2, which may decrease stratospheric O3 destruction
by interfering with atomic chlorine formation (Kramlich and Linak, 1994). Since the mid-1970s,
systematic tropospheric measurements of N2O have been made at locations worldwide. These
data show the atmospheric concentration of N2O to be increasing at an average rate of
approximately 0.3 percent per year (Khalil and Rasmussen, 1992; Vitousek et al, 1997)).
NO,
Emissions from aircraft are a relatively small source of NOX The impact of the aircraft
emissions depend on the altitude as well as the amount of the emissions. Models indicate that the
NOX emissions from the current subsonic fleet produce upper-tropospheric O3 increases as much
as several percent, maximizing at northern midlatitudes. In contrast, projected fleets of
supersonic aircraft may decrease total-column O3 concentrations by 0.3-1.8 percent for the
Northern Hemisphere (WMO, 1994).
Emissions Control Programs
As noted in the previous section concerning global warming, the development of an
emissions control program to achieve a specific environmental goal may need to take into account
the impact on both NOX and N2O emissions and the resultant environmental impacts that may go
beyond the specific goal. Some NOX control programs strive to prevent NOX emissions in the first
place. Other control programs are directed at N2O emissions, such as in adipic acid production.
However, many NOX control procedures for mobile and stationary sources convert NO into N2
and, in that process, a portion of the reactions will form N2O.
References
Kramlich, J.C. and Linak, W.L, "Nitrous Oxide Behavior in the Atmosphere, and in Combustion
and Industrial Systems," Prog. Energy Combust. Sci., Vol 20, 1994.
Levine, J.S., "The Global Atmospheric Budget of Nitrous Oxide, and Supplement: Global
Change: Atmospheric and Climatic," 5th International Workshop on Nitrous Oxide Emissions,
Tsukuba, Japan, 1992.
Khalil, M.A. and Rasmussen, R.A., "The Global Sources of Nitrous Oxide," J. Geophys. Res.
97, 14651-14660, 1992.
Tie, X.X., G.P. Brasseur, B. Briegleb, and C. Grainier, "Two-Dimensional Simulation of
Pinatubo Aerosol and Its Effect on Stratospheric Ozone," J. Geophys. Res., 99, 20545-20562,
1994.
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Vitousek, P.M., J. Aber, R.W. Howarth, G.E. Likens, P.A. Matson, D.W. Schindler, W.H.
Schlesinger, and G.D. Tilman, "Human Alteration of the Global Nitrogen Cycle: Causes and
Consequences," Issues in Ecology, Number 1, Spring 1997.
U.S. Environmental Protection Agency, Office of Policy, Planning and Evaluation, Inventory of
U.S. Greenhouse Gas Emissions and Sinks: 1990-1994, November 1995, EPA-230-R-96-006.
World Meteorological Organization, Scientific Assessment of Ozone Depletion: 1994, Global
Ozone Research and Monitoring Project—Report No. 37.
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E. Terrestrial Ecosystems
N Deposition
Over the last 40 years, the rate atmospheric deposition of N compounds has increased
more than 10 times in eastern North America (Vitousek, 1994). N accumulates in watersheds
with high N deposition. The primary sources of N (NOX and ammonia, NH3) deposition are the
combustion of fossil fuels, the manufacture and use of fertilizers, livestock, and burning of
biomass as a result of changing land use practices (Townsend, et al, 1996; Schlesinger and
Hartley, 1992). Because North American terrestrial ecosystems are generally considered to be N-
limited, N deposition often has a fertilizing effect, accelerating plant growth. While this effect is
often considered beneficial, N deposition is causing important adverse changes in some terrestrial
ecosystems, including shifts in plant species composition and decreases in species diversity or
undesirable nitrate leaching to surface and ground water and decreased plant growth.
The N Cycle
All organisms require N to live; it is an essential component of cholorphyll, genetic
material, and proteins. In many ecosystems on land and sea, the supply of N is a key factor
controlling the nature and diversity of plant life, the population dynamics of both grazing animals
and their predators, and vital ecological processes such as plant productivity and the cycling of
carbon and soil minerals (Vitousek et al, 1997).
Although the earth's atmosphere is 78 percent N gas (in the form of the N2 molecule),
most plants and animals cannot use the atmospheric N2 directly. Before plants can use N, it must
be bonded ("fixed") into inorganic compounds, mainly ammonium, NH4+, and nitrate, NO3".
There are both natural and anthropogenic processes that "fix" atmospheric N2 into these inorganic
compounds.
The main natural source of N fixation is N fixing organisms. N fixing organisms include
algae and bacteria. The most important ones are bacteria that form symbiotic relationships with
higher plants, especially legumes. These bacteria manufacture an enzyme that enables them to
convert atmospheric N2 directly into plant-usable forms. Lightning is also a natural source of N
fixation as conditions of high pressure and temperature allow N2 and O2 to combine and form into
nitrates.
During the past century, human activities have at least doubled the rate of transfer of
atmospheric N2 to biologically usable forms (Vitousek et al, 1997; Schlesinger, 1992). The major
anthropogenic sources include industrial processes that produce N fertilizers, the combustion of
fossil fuels, and the cultivation of legumes. Furthermore, biomass burning, drainage of wetlands
and land clearing are important activities that contribute to biologically available N. Most of this
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N is deposited by precipitation over land, where it enters biogeochemical cycles (Schlesinger,
1992).
N Deposition Effects on Forest Ecosystems
Rates of growth in North American forest ecosystems have traditionally been considered
limited by N availability and, thus, N inputs are usually considered beneficial. In N-limited forests,
most N is bound in soil organic matter and only becomes available for biotic uptake when
decomposition of soil organic matter releases inorganic nitrate and ammonium ions. Plants and
microbes promptly absorb inorganic N, maintaining minimal available N pools in the soil and
thereby limiting leaching losses. Although N limitation is still widespread, recent findings in
North America and Europe suggest that, because of chronic N deposition from air pollution, some
forests are no longer N-limited and that this condition may increase tree mortality and alter water
quality (Aber et al, 1996; Sullivan, 1993; Vitousek et al, 1997).
N saturation has a complex cascade of damaging effects for ecosystems (Vitousek et al,
1997). As NH4+ builds up in the soil, it is increasingly converted to nitrate, a process that releases
hydrogen ions and helps acidify the soil. The buildup of nitrate may result in leaching of nitrate
into streams or groundwater. As negatively charged nitrates leach away, they carry with them
positively charged alkaline minerals, thus decreasing soil fertility. As calcium is depleted and the
soil acidified, aluminum ions are mobilized, eventually reaching toxic concentrations that can
damage tree roots or kill fish if the aluminum washes into streams.
The impacts of N saturation first became apparent in Europe almost two decades ago
when scientists observed significant increases in nitrate concentrations in some lakes and streams
and also extensive yellowing and loss of needles in spruce and other conifer forests subjected to
heavy N deposition (Vitousek et al, 1997). In Southern California, elevated nitrate and NO fluxes
from soil, indicators of N saturation, have recently been reported for chaparral watersheds in the
San Gabriel Mountains as well as in portions of the mixed conifer forest in the San Bernardino
Mountains (Fenn et al, 1996). While there are important differences in the magnitude of N
deposition, vegetation and soil cover, areas of concern include the Great Smoky Mountains in
North Carolina, northeastern United States, southern California, and the Colorado Front Range of
the Rocky Mountains (Johnson et al, 1991; van Miegroet et al, 1993; Fenn et al, 1996; Williams
et al, 1996; Vitousek et al, 1997). In some cases excess N availability can lead to decreased tree
growth and increased mortality (McNulty et al.1996). Nutrient imbalances, with signs such as
depressed Ca: Al and Mg:N ratios in foliage, may act with other stresses to produce these effects
(Aberetal. 1996).
N saturation has been defined in several ways, including, as a condition in which (1)
available N is frequently in excess of total biotic demand, (2) vegetation within an ecosystem no
longer exhibits a positive growth response to N addition, even through other growth factors are
not growth limiting, and (3) sustained N losses approximate or exceed N inputs—the N retention
capacity of the system has been exceeded (Fenn, et al, 1996). Recent research suggests that
because N-limited forests retain most N inputs, N deposition above relatively low amounts to
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these systems is largely cumulative (Ryan, 1996). Moreover forests respond nonlinearly to
cumulative N inputs if elevated N deposition continues over long periods. That is, N inputs to N-
limited forests (with little accumulated N) stimulate their growth and those inputs are strongly
retained with little nitrate leaching. After N inputs have accumulated (to cause N saturation), the
response of the forest changes so that N deposition increases nitrate leaching and may decrease
forest growth. In order to predict how an ecosystem will respond to N deposition one needs to
know where the ecosystem is on this response curve to cumulative N inputs. Predictions are
uncertain about how much cumulative N inputs it will take over what time period to bring about
N saturation in particular forest ecosystems because processes controlling N accumulation and
loss are not completely understood and because data on the current stage of N saturation are
lacking for most forests.
N Deposition Effects on Grassland Ecosystems
Reduced Species Diversity
Most natural ecosystems have been N limited. Accordingly, native plant species have
adapted to this environmental constraint. With an increase in N deposition, many of the native
plant species may no longer be able to compete with other species that are adapted to high N
conditions.
N deposition in grassland ecosystems has been shown to (1) alter the composition of the
grassland species in the affected area, (2) decrease species diversity, and (3) increase the above
ground productivity (Wedin and Tilman, 1996). In a 12 year experimental study of N deposition
on Minnesota grasslands, plots dominated by native grasses shifted to low-diversity mixtures at all
but the lowest N addition rates (Wedin and Tilman, 1996). While prairie grasses can thrive were
N availability is limited, as N availability is increased, competing, non-native species begin
invading the prairie plots. After 12 years of N addition, species richness (number of plant species
per area) declined by more than 50 percent, with the greatest losses at levels spanning current
atmospheric deposition rates in eastern North America (Wedin and Tilman, 1996). That is, the
native grasses with supplemental N deposition showed an impaired ability to compete with non-
native species. In England, N fertilizers applied to experimental grasslands led to increased
dominance by a few N responsive grasses and loss of many other plant species; at the highest
fertilization rate, the number of plant species declined more than fivefold (Vitousek et al, 1997).
Global Warming Impacts
Because plants use atmospheric carbon dioxide to fix carbon in their tissues, over their
lifetime they are a sink for atmospheric carbon dioxide, the primary global warming gas. As the
plants die and decompose, a portion of the carbon returns to the atmosphere and a portion may be
incorporated into the soils, resulting in a net sink of carbon dioxide. With increased N inputs and
resulting increased plant growth, it has been hypothesized that the amount of carbon removed
from the atmosphere might increase.
There is considerable uncertainty over both the magnitude and persistence of any N-
derived carbon sink. Different ecosystem types vary greatly in their potential for carbon storage.
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Much of the area experiencing high N deposition is covered by grasslands or cultivated areas. In
general, N deposition that stimulates wood production will cause a relatively large and long-term
removal of carbon from the atmosphere. In contrast, foliar biomasses in forests and grasslands
have much more limited capacity for carbon storage and deposition onto cultivated areas is not
likely to contribute to any terrestrial sink (Townsend, et al, 1996).
In the case of the native prairie grasses, the decomposition is slow, resulting in the storage
of carbon in the earth. In contrast, the non-native plants which thrive on N (and replace native
plants under increased N deposition), decompose rapidly due to their high N content and, thus,
return most of their carbon to the atmosphere, where it can contribute to global warming. The 12
year study indicates that the carbon storage, in some plots, was decreased by 50 percent and
concludes that N-caused shifts in species composition limit the ability of temperate grasslands to
serve as significant long-term carbon stores (Wedin and Tilman, 1996). Thus, N deposition
shifted the mix of plants toward the faster growing non-native species which decreased the area's
ability to move carbon dioxide from the air and store it in the soil.
References:
Aber, John D., Knute J. Nadelhoffer, Paul Steudler, and Jerry M. Melillo. 1989. Nitrogen
Saturation in Northern Forest Ecosystems. BioScience Vol. 39 No. 6. June, 378-386 .
Aber JD, Melillo JM, Nadelhoffer KJ, Pastor J, Boone RD. 1991. "Factors controlling nitrogen
cycling and nitrogen saturation in northern temperate forest ecosystems." Ecol. Appl. 1:305-315
Aber, J.D., A. McGill, S. G. McNulty, R.D. Boone, K.J. Nadelhoffer, M. Downs,
R. Hallett. "Forest biogeochemistry and primary production altered by
nitrogen saturation." Water, Air, & Soil Pollution^yolume 85 (p. 1665-1670), 1995.
Agren GI, Bosatta E. 1988. Nitrogen saturation of terrestrial ecosystems .Environ. Pollut.
54:185-97.
Christ M, Zhang Y, Likens GE, Driscoll CT. 1995. Nitrogen retention capacity of a northern
hardwood forest soil under ammonium sulfate additions. Ecol. Appl. 5:802-12.
Fenn, M., M. A. Poth, D.W. Johnson. "Evidence for nitrogen saturation in the San Bernardino
Mountains in Southern California," Forest Ecology and Management 82 ( p. 211-230), 1996.
McNulty, S.G., J.D. Aber, S.D. Newman. 1996. Nitrogen saturation in a high
elevation New England spruce-fir stand. Forest Ecology and Management
84:109-121.
Ryan, Douglas F., J.D. Aber, J.S. Baron, B.T. Bormann, C.T. Driscoll, M.E. Fenn, D.W.
Johnson, A.D. Lemly, S.G. McNulty, and R. Stottlemyer, "Nitrogen Deposition from Air
Pollution: Implications for Forest Ecosystems in North America" draft 1996.
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Schlesinger, W.H., Biogeochemistry: An Analysis of Global Change., 1991.
Schlesinger, W.H. and Hartley, A.E., "A Global Budget for Atmospheric NH3", Biogeochemistry
15: 191-211, 1992.
Stoddard JL. 1994. Long-term changes in watershed retention of nitrogen: Its causes and
aquatic consequences. In: Environmental Chemistry of Lakes and Reservoirs, Advances in
Chemistry Series No. 237, ed. LA Baker, pp. 223-284, Washington, DC: Amer. Chem. Soc.
Sullivan,!.J. 1993. "Whole ecosystem nitrogen effects research in Europe." Environmental
Science & Technology 37:1482-1486.
Townsend, A.R., B.H.Braswell, E.A.Holland, and J.E.Penner, "Spatial and Temporal Patterns in
Terrestrial Carbon Storage Due to Deposition of Fossil Fuel Nitrogen," Ecological Applications,
6(3), 1996pp. 806-814, 1996.
Van Sickle, J. and M.R. Church. 1995. Methods for estimating the relative effects of sulfur and
nitrogen deposition on surface water chemistry. EPA Rep. no. EPA/600/R-95/172.
Washington, DC US Environ. Protect. Agency. 121 pp.
Vitousek, P.M., J. Aber, R.W. Howarth, G.E. Likens, P.A. Matson, D.W. Schindler, W.H.
Schlesinger, and G.D. Tilman, "Human Alteration of the Global Nitrogen Cycle: Causes and
Consequences," Issues in Ecology, Number 1, Spring 1997.
Wedin, David A. and David Tilman, "Influence of Nitrogen Loading and Species Composition on
the Carbon Balance of Grasslands." Science, Volume 274, December 6, 1996.
Williams, Mark W., Jill S. Baron, Nel Caine, Richard Sommerfield and Robert Sanford, Jr. 1996.
"Nitrogen Saturation in the Rocky Mountains." Environmental Science & Technology Vol 30 No
2.
Vitousek, P.M., Ecology, Volume 75 (1861),1994.
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F. Toxic Products
Introduction
In the atmosphere, NOX reacts with common hydrocarbons and O3 to form a wide variety
of toxic compounds. Shepson et al. (1987) reported that photochemical reactions in a laboratory
smog chamber substantially increased the mutagenicity of such mixtures. This added
mutagenicity, measured by Ames assays, was time-dependent and associated mostly with the gas
phase. The mutagenic transformation products were also refractory, accumulating and persisting
in the chamber for hours after their production.
The accumulation of new mutagens by photochemically-mediated nitrate reactions raises
significant public health concerns. Chemicals that are able to alter bacterial genes (DNA) also
have the potential to cause mutations in higher organisms, and some fraction of these mutations
may initiate carcinogenesis. Examples of transformation products thought to contribute to
increased mutagenicity include the nitrate radical, peroxyacetyl nitrate, nitroarenes, and
nitrosamines (Shepson et al., 1987; CARB, 1986). Toxicological data on many of these
compounds are limited, making it difficult to discuss effects of specific reaction products, but the
overall trend is well-established. Information on formation and toxicity of groups of substances is
summarized below.
Nitrate Radical
The gaseous nitrate radical is a product of the reaction of NO2 with O3 (Finlayson-Pitts
and Pitts, 1993). This compound dissociates rapidly in sunlight, but it is possible that nighttime
concentrations may become significant. Although health effects of the gaseous nitrate radical
have not been described, radicals are generally highly reactive compounds with the potential to
damage complex biological molecules such as proteins, lipids, and nucleic acids. Pitts et al.
(1983) have postulated that the nitrate radical is capable of inducing genetic changes, but this
does not appear to have been tested in the laboratory.
Peroxyacetyl Nitrate (PAN)
PAN (CH3CO3NO2) is an organic-nitrogenous air pollutant formed by complex
photochemical reactions of common aliphatic compounds and NOX. PAN has a relatively long
thermal decomposition lifetime in the absence of NO. When NO is removed from the atmosphere
by reaction with O3, as in the afternoon and above the mixed layer at night, PAN can persist.
PAN is an important lacrimator, thought to contribute much of the eye-stinging effect of urban
smog. Tests for mutagenicity and carcinogenicity of pure PAN have not been reported in the
literature. However, Shepson et al. (1987) concluded that PAN formation accounted for a
substantial part of the increased mutagenicity observed in their smog chamber study.
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Nitroarenes
Butler et al. (1981) demonstrated that polynuclear aromatic hydrocarbons (PAHs) are
nitrated in the laboratory to form nitroarenes when exposed to concentrations of NO2 and nitric
acid similar to those found in ambient air. These results suggest that PAHs (hydrocarbons having
multiple carbon-ring structures, and major constituents of poly cyclic organic matter) may also be
rapidly nitrated in the atmosphere. Rosenkrantz and Mermelstein (1983) reported similar
reactions in PAHs adsorbed on soot particles and other substrates. Many of these compounds are
mutagens and/or carcinogens, even when their parent non-nitrated PAH analogs are not.
Examples from this broad spectrum of substances include nitrated analogs of quinoline, pyrene,
fluorine, and naphthalene. Rosenkrantz and Mermelstein (1983) reported that many nitroarenes
are potent bacterial mutagens and also produce a variety of genetic and genotoxic effects in
mammalian cell assays, including unscheduled DNA synthesis, sister-chromatid exchange,
chromosomal aberrations, gene mutations, and cell transformation. Data from whole animals are
limited, but various nitroarenes have produced skin tumors in mice and bladder tumors in dogs
and monkeys.
Nitrosamines
Gehlert et al. (1979) reported the formation of nitrosamines (nitrated organic amine
compounds) from amines and NOX in the laboratory, and proposed that this reaction also occurs
in the atmosphere. Although nitrosamines are rapidly decomposed by sunlight, ambient
concentrations could rise during the night. Nitrosamines are a major class of powerful chemical
carcinogens, with different compounds sometimes exhibiting high target organ specificity in
animal studies. Sites of cancers induced by nitrosamines in rodents include the liver, bladder,
lung, kidney, and pancreas (Casarett and Doull, 1986). Carcinogenesis occurs through metabolic
activation, followed by methylation of DNA by the electrophilic metabolites.
Summary
In laboratory tests, nitrates react readily with common organic chemicals, and even O3, in
the presence of light to form a wide variety of mutagenic and carcinogenic transformation
products. Although animal inhalation studies of individual compounds formed in this way are
limited, results of bacterial and mammalian cell bioassays indicate clearly that both mixtures and
individual mixture components are able to alter DNA. More research, especially whole animal
studies, would help the EPA understand the potential magnitude of public health impacts of these
transformation products.
References
Butler, J.D. and P. Crossley. Atmospheric Environment. 15:91(1981).
California Air Resources Board. The effects of oxides of nitrogen on California air quality.
March 1986.
Casarett, I. and LJ. Doull. Toxicology. Macmillan Publishing Co., New York. C.D. Klaassen,
M.O. Amdur, and J. Doull (eds.) (1986).
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Finlayson-Pitts, BJ. and J.N. Pitts, Jr., "Atmospheric Chemistry of Tropospheric Ozone
Formation: Scientific and Regulatory Implications," Air and Waste Management Association,
Vol. 43, August 1993.
GehlertandW. Rolle. Experimentia 33:579 (1979).
Pitts, Jr. J.N., A.M. Winer, G.W. Harris, W.P.L. Carter, and B.C. Tuazon. Trace nitrogenous
species in urban atmospheres. Environ. Health Perspectives 52:153-157 (1983).
Rosencrantz, H.S. and R. Mermelstein. Mutagenicity and genotoxicity of nitroarenes: all
nitrogen-containing chemicals were not created equal. Mutation Research 114: 217-267 (1983).
Shepson, P.B., T.E. Kleindienst, and E.O. Edney. The production of mutagenic compounds as a
result of urban photochemistry. EPA 600/3-87/020 (1987).
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IV. Interprogram Issues
A. Local and Regional NOX Requirements
Policy Decisions Should Consider All the Effects ofNOx Emissions
As described in detail in sections II and III of this document, NOX emissions impact the
environment in a number of ways. Some of these are regional scale, adverse impacts. As noted in
the "Ozone" section, decreasing local NOX emissions can, in some cases, increase local O3
concentrations. This effect of NOX emissions reductions must be carefully weighed against the
multiple beneficial effects of NOX emissions reductions with respect to acid deposition, drinking
water nitrate, eutrophication, global warming, NO2, N in forest ecosystems, O3, PM, stratospheric
O3 depletion, toxics and visibility degradation. Further, the effects of emissions of N2O are also of
concern with respect to global warming and stratospheric O3 depletion. The EPA believes that
policy decisions should be made considering the environment as a whole, rather than narrowly
viewing each program, one at a time. As stated by two committees of the National Research
Council:
Changes in VOCs and NOX will, because of their complex chemical interactions, also lead
to changes in a variety of other pollutants associated with O3, such as nitric acid,
peroxyacetyl nitrate, NO2, and aerosol particles. Some of these pollutants have known
harmful effects on human health and welfare. Hence, it is important to recognize that
control strategies implemented for O3 will simultaneously affect other species. (NRC,
1991).
Visibility is just one of many air-quality problems. The pollutants that impair visibility
contribute to other environmental problems, some of which have been or are being
considered as objects of federal, state, or local legislation or regulation. For example,
controls aimed at decreasing acid deposition or lowering ambient concentrations of O3 and
PM10 could improve visibility in Class I areas; conversely, controls aimed at improving
visibility could alleviate other air-quality problems. Policy makers should weigh these
linkages in the design and assessment of possible control strategies. (NRC, 1993).
The remainder of this section primarily describes (1) the effects of NOX emissions
reductions on O3 concentrations, (2) CAA requirements for NOX controls in certain areas, (3)
EPA action to waive these requirements in certain cases, and (4) the need for regional scale NOX
controls in the future for purposes of O3 attainment. Although the focus of the discussion is on
O3, EPA intends to use its discretion wherever possible to assure that policy decisions concerning
O3 are consistent with other environmental goals which may suggest the need for reductions in
NOX emissions. The EPA believes that effective O3 control requires an integrated strategy that
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combines cost-effective reductions in emissions at the local, state, regional, and national levels.
Role ofNOx and VOCs in Urban O3
Great progress has been made over the past two decades, at the local, state and national
levels in controlling emissions from many sources of air pollution. During this period, control of
VOCs was the main strategy employed in efforts to lower ground-level O3. The 1970 and 1977
amendments to the CAA did not explicitly require NOX emissions reductions from stationary
sources for purposes of attainment of the O3 standard. With respect to mobile sources, both VOC
and NOX emissions were decreased substantially. Scientific evidence at the time suggested that
VOC emissions reductions were preferred in most instances (EPA, 1993). The VOC control
approach was reinforced by the fact that NOX emissions decreases could in some cases increase O3
concentrations.
More recently, it has become clearer that NOX controls may be needed in many areas,
especially areas where O3 concentrations continue to be high over a large region (as in the
Midwest and Northeast). In the debate leading up to the 1990 CAA amendments, Congress
included consideration of NOX. A report by the Office of Technology Assessment (U.S.
Congress, 1989) provided support for inclusion of NOX controls in the O3 program. The 1990
amendments changed the statutory framework to place NOX emissions reductions on a more equal
footing with the VOC emissions reductions.
In the process of adding these new NOX requirements, Congress recognized that NOX
emissions reductions would help achieve lower O3 concentrations in some O3 nonattainment areas,
but that "there are some instances in which NOX reductions can be of little benefit in reducing O3
or can be counter-productive, due to the offsetting ability of NOX to 'scavenge' (i.e., react with)
O3 after it forms" (H.R. Rep. No. 490, 101st Congress, 2nd Sess., at 204). The Congress
provided for additional review and study under section 185B of the CAA "to serve as the basis for
the various findings contemplated in the NOX provisions" (H.R. Rep. 490 at 257).
Under section 185 of the CAA, the EPA, in conjunction with the National Academy of
Sciences, conducted a study on the role of O3 precursors in tropospheric O3 formation which
examined the role of NOX and VOC emissions, the extent to which NOX emissions reductions may
contribute or be counterproductive to achieving attainment in different nonattainment areas, the
sensitivity of O3 to the control of NOX, the availability and extent of controls for NOX, the role of
biogenic VOC emissions, and the basic information required for air quality models. The NAS
portion of the study was published in 1991 (NRC, 1991). The section 185B study was completed
and submitted to Congress July 30, 1993 (EPA, 1993).
The 1991 National Academy of Sciences report recommends that "To substantially reduce
O3 concentrations in many urban, suburban, and rural areas of the United States, the control of
NOX emissions will probably be necessary in addition to, or instead of, the control of VOCs."
The section 185B study concludes that the O3 precursor control effort should focus on NOX
controls in many areas and that the analysis of NOX benefits is best conducted through
photochemical grid modeling. The shift to consideration of both NOX and VOCs, explains the
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section 185B study, coincides with improved data bases and modeling techniques that provide the
analytical means to evaluate the effectiveness of O3 precursor control strategies.
The 1990 Clean Air Act Amendments (CAAA)
The 1990 CAAA include substantial new requirements to decrease emissions of NOX from
major stationary sources. To help attain the 1-hour O3 air quality standard in the near-term,
section 182(f) of the CAAA requires certain existing sources to install reasonably available
control technology (RACT) and new sources must install controls representing the lowest
achievable emission rate (EPA, 1992). Section 182(f) also specifies circumstances under which
the new NOX requirements for RACT and NSR would be limited or would not apply. To decrease
acid deposition, the CAAA require coal-fired utility boilers to meet emission limits in two phases.
Further, in a longer-term provision, the CAAA require States to adopt additional control
measures as needed to attain the O3 standard. This requirement supplements the RACT and NSR
requirements. Thus, a State would need to require NOX controls which achieve emissions
reductions greater than the NOX RACT/phase I acid deposition limits where additional reductions
in emissions of NOX are necessary to attain the O3 standard by the attainment deadline.
For O3 nonattainment areas designated as serious, severe, or extreme, state attainment
demonstrations involve the use of photochemical grid modeling for each nonattainment area.
Although these attainment demonstrations were due November 15, 1994, the magnitude of this
modeling task, especially for areas that are significantly affected by transport of O3 and O3
precursors (NOX and VOCs) generated outside of the nonattainment area, has delayed many states
in submitting complete modeling results. Recognizing these challenges, EPA issued guidance on
O3 demonstrations, based on a two-phase approach for the submittal of O3 SIP attainment
demonstrations (Nichols, 1995). The guidance established a 2-phase approach which includes an
intensive modeling effort to address the problem of long distance transport of O3, NOX and VOCs
and submittal of the attainment plans in 1997.
Section 182(f) of the CAA-NOX Waiver
As described in EPA guidance (EPA, December 1993), the CAAA include new provisions
in section 182(f) to control emissions of NOX and specify circumstances under which the new NOX
requirements would be limited or would not apply. Section 182(f)(l) provides that certain new
NOX requirements shall not apply if the Administrator determines that any one of the following
tests is met:
(1) in any area, the net air quality benefits are greater in the absence of NOX reductions
from the sources concerned;
(2) in nonattainment areas not within an O3 transport region, additional NOX
reductions would not contribute to O3 attainment in the area; or
(3) in nonattainment areas within an O3 transport region, additional NOX reductions
would not produce net O3 air quality benefits in the transport region.
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Further, section 182(f)(2) states that the application of the new NOX requirements may be limited
to the extent necessary to avoid excess reductions of NOX as determined by applying tests similar
to tests (l)-(3) above.
Administrative Procedures
Section 182(f)(3) provides that a person may petition the Administrator for a NOX
exemption at any time after the final section 185B report is submitted to Congress. The final
section 185B report was sent to Congress by the Administrator on July 30, 1993. The petition
may be made with respect to any nonattainment area or any O3 transport region. The EPA must
grant or deny a petition within 6 months after its filing.
If EPA grants a petition, the section 182(f)NOx requirements or portions of those
requirements (EPA, 1992), would no longer apply to those sources or areas, as described in
EPA's approval action. However, States remain free to adopt NOX restrictions for other reasons.
For example, a State may determine that NOX emissions reductions are needed for purposes of O3
maintenance planning, O3 attainment in separate downwind nonattainment areas, visibility
protection, PM control strategy, acid deposition program or other environmental protection.
The CAA requires EPA to view NOX waivers in a narrow manner. In general, section
182(f) provides that waivers must be granted if states show that decreasing NOX within a
nonattainment area would not contribute to attainment of the O3 NAAQS within the same
nonattainment area (Seitz, 1995). Only the role of local NOX emissions on local attainment of the
O3 standard is considered in nonattainment areas outside an O3 transport region. The role of NOX
in regional attainment will be addressed separately. As described in the "Ozone" section, NOX
has been shown to be effective in decreasing regionally transported O3.
Status ofNOx Waiver Petitions
In response to State NOX waiver petitions submitted between 1992-1995, EPA granted
NOX waivers under section 182. Most waivers were granted on the basis that the area had already
attained the O3 standard and, thus, additional NOX (or VOC) reductions "would not contribute to
ozone attainment in the area." In some cases, the waivers were granted based on dispersion
modeling which showed that the area would attain just as expeditiously based solely on additional
VOC reductions or that local NOX reductions increased local peak O3 concentrations; this also
meets the above test that additional NOX reductions would not contribute to O3 attainment in the
area.
Specifically, the EPA received petitions for a NOX waiver for 51 O3 nonattainment areas.
Of these petitions, EPA approved (as of July 1997) waivers for 48 nonattainment areas and 3
were pending. Most of the waivers granted (28) were simply based on air quality monitoring data
over a period of 3 or more years indicating the area had attained the O3 standard (and, thus,
additional NOX reductions were not needed for attainment). Several States submitted NOX waiver
petitions (7) accompanied by an attainment plan showing achievement of the O3 standard by the
statutory deadline through additional VOC controls only. None of these 41 nonattainment areas
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with approved NOX waivers have demonstrated or even sought to demonstrate that NOX
reductions might increase O3 concentrations in specific areas. Only in the cases of the Lake
Michigan (9 nonattainment areas), Phoenix AZ, Baton Rouge LA and the Houston/Beaumont TX
areas was information submitted to show that, on some days, local NOX emissions decreases lead
to local increases in peak O3 concentrations in some but not necessarily all portions of these areas.
Even for the 4 areas with modeling information, those analyses were generally considered
preliminary analyses that would be replaced with more complete modeling associated with
attainment plans.
NOX waivers are granted on a temporary or contingent basis
The EPA's approval of any NOX exemption is granted on a contingent basis (Seitz, 1994).
That is, a monitoring-based exemption lasts for only as long as the area's monitoring data continue
to demonstrate attainment. Thus, if a violation is monitored (prior to the area being redesignated
as being in attainment) the exemption would need to be revoked and the requirement to adopt
NOX controls would again apply. Similarly, any modeling-based exemption may need to be
withdrawn if updated modeling analyses reach a different conclusion than the modeling on which
the exemption was based (EPA, 1992).
In the Federal Register notices approving individual waiver petitions, EPA gave notice
that approval of the local petition is on a contingent or temporary basis and stated that additional
local and regional NOX emissions reductions may be needed to reduce the long range transport of
O3. Where such additional NOX reductions are necessary to reduce the long range transport of O3,
EPA stated that authority provided under section 110(a)(2)(D) of the CAA would be used and
that a section 182(f) NOX waiver would, in effect, be superseded for those control requirements
needed to meet the section 110(a)(2)(D) action. Further, EPA noted that States may require
additional NOX reductions in these nonattainment areas for non-O3 purposes, such as attainment of
the PM-10 standard or achieving acid rain reduction goals.
In some cases, despite a potential increase in O3 concentrations in central urban areas,
State and local agencies may need to decrease NOX emissions as part of a larger plan to meet
various environmental goals. For example, the South Coast area of California models this effect,
yet substantial NOX emissions reductions are contained in their attainment plan. The NOX
emissions reductions in the South Coast are needed to attain the PM10 standard and to maintain
the NO2 standard in the same air basin as well as to help lower O3 concentrations in areas
downwind of the basin. In a different situation, NOX emissions reductions in the New York
metropolitan area are needed for downwind areas within the State and in other States to attain the
O3 standard; yet, additional VOC controls may be needed in the metropolitan area to offset the
local impact of NOX emissions reductions. Similarly, NOX emissions reductions in areas upwind of
the Northeast Ozone Transport Region may be needed to help downwind areas attain and
maintain the O3 standard, even though those NOX emissions reductions may not help the upwind
areas lower local O3 concentrations. Models provide a way to test various control strategies so
that the best approach, considering all the environmental goals, can be selected.
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Further, the NOX waiver does not shield an area from the acid deposition requirements of
Title IV of the CAA. Regional and/or local NOX emissions reductions may also be needed to slow
eutrophication in sensitive water bodies, improve visibility and/or decrease PM concentrations.
Furthermore, increases in NOX are not viewed by EPA as a solution to local O3 problems; NOX
emissions reductions are generally needed at least to counter increases in NOX emissions due to
economic growth. Thus, a local NOX waiver should be considered temporary and does not shield
an area from NOX requirements needed for O3 attainment in downwind areas or to meet other
CAA requirements.
Modeling Analyses
The OTAG addressed the complex issue of regional impacts due to transport of NOX and
VOC emissions. The OTAG modeling results indicate that urban NOX reductions produce
widespread decreases in O3 concentrations on high O3 days. In addition, urban NOX reductions
also produce limited increases in O3 concentrations locally, but the magnitude, time, and location
of these increases generally do not cause or contribute to high O3 concentrations. Most urban O3
increases modeled in OTAG occur in areas already below the O3 standard and, thus, in most
cases, urban O3 increases resulting from NOX reductions do not cause exceedance of the O3
standard. There are a few days in a few urban areas where NOX reductions are predicted to
produce O3 increases in portions of an urban area with high O3 concentrations.
In other words, modeling analyses conducted as part of the OTAG process indicated that,
in general, NOX reduction disbenefits are inversely related to O3 concentration. On the low O3
days leading up to an O3 episode (and sometimes the last day or so) the increases are greatest, and
on the high O3 days, the increases are least (or nonexistent); the O3 increases occur on days when
O3 is low and the O3 decreases occur on days when O3 is high. This indicates that, in most cases,
urban O3 increases may not produce detrimental effects. However, OTAG modeling indicates
that at least one area for one day of one episode experienced an increase in O3 on a high O3 day.
Overall, OTAG modeling thus demonstrates that the O3 reduction benefits of NOX control far
outweigh the disbenefits of urban O3 increases in both magnitude of O3 reduction and geographic
scope.
It should also be noted that the modeling analyses completed within the OTAG process
necessarily utilized a larger grid size than States are likely to use in their attainment plans. That is,
future analyses by States will likely use smaller grid sizes. The smaller grid sizes should provide
more precise information on effects such as local NOX emissions reacting with local O3.
Furthermore, new work is on-going to analyze air quality monitoring data, in part, to assess
weekday and weekend patterns that may relate changes in NOX and VOC emissions to changes in
O3 concentrations. These air quality modeling and monitoring studies will provide additional
information that may be important as States develop their attainment plans.
Regional Transport ofO3
The problem of regional transport of O3 and its precursors is widely recognized by the
States. In response to concerns about this problem raised by state environmental commissioners
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comprising the Environmental Council of the States, EPA has worked closely with states in the
OTAG to develop various recommended control measures intended to address the regional nature
of O3. Similarly, State and local air administrators passed a unanimous resolution endorsing
national NOX emission regulations (Becker, 1995).
Control strategies need to consider efforts to decrease regional scale emissions as well as
local emissions. In general, NOX emissions decreases in upwind, rural areas coupled with VOC
emissions reductions in urban nonattainment areas appears to be an effective strategy. In some
cases however, the urban nonattainment area is also upwind of another urban nonattainment area
or biogenic VOC emissions are very high. In these cases, local NOX emissions reductions may be
needed in addition to VOC emissions reductions for purposes of O3 attainment. In both cases,
decreases in precursor emissions in the upwind areas will help the downwind metropolitan areas
attain and maintain the O3 standard. Thus, effective O3 control will require an integrated strategy
that combines cost-effective decreases in emissions at the local, state, regional, and national
levels. Specific regional perspectives on the need for an integrated strategy are described in the
"Ozone" section.
References
Becker, William, Executive Director of STAPPA/ALAPCO, Letter to U.S. EPA Public Docket,
October 13, 1995.
National Research Council, Committee on Haze in National Parks and Wilderness Areas,
Protecting Visibility in National Parks and Wilderness Areas, 1993.
National Research Council, Committee on Tropospheric Ozone Formation and Measurement,
Rethinking the Ozone Problem in Urban and Regional Air Pollution, 1991.
Nichols, Mary D., Assistant Administrator for Air and Radiation, U.S. Environmental Protection
Agency, "Ozone Attainment Demonstrations," memorandum to EPA Regional Administrators,
March 2, 1995.
Ozone Transport Assessment Group, Joint meeting of RUSM & ISI workgroups, "First Round
Strategy Modeling," October 25, 1996.
Seitz, John S., Director, Office of Air Quality Planning and Standards, U.S. Environmental
Protection Agency, "Section 182(f) Nitrogen Oxides (NOX) Exemptions—Revised Process and
Criteria," EPA memorandum to Regional Air Directors, May 27, 1994.
Seitz, John S., Director, Office of Air Quality Planning and Standards, U.S. Environmental
Protection Agency, "Section 182(f) Nitrogen Oxides (NOX) Exemptions- Revised Process and
Criteria," EPA memorandum to Regional Air Directors, February 8, 1995.
US Congress, Office of Technology Assessment, Catching Our Breath: Next Steps for Reducing
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Urban Ozone, Washington DC 1989.
U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, The Role
of Ozone Precursors in Tropospheric Ozone Formation and Control—A Report to Congress.
EPA-454/R-93-024,1993 .
U.S. Environmental Protection Agency, "Nitrogen Oxides Supplement to the General Preamble
for Implementation of Title I of the Clean Air Act," Federal Register of November 25, 1992 (57
FR 55620).
U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, "Guideline
for Determining the Applicability of Nitrogen Oxides Requirements Under Section 182(f),"
December 1993.
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B. Timing of NOX Emissions Reductions: Seasonal or Year-
Round
Regulation Can Affect the Seasonal Distribution ofNOx Emissions
As noted previously, NOX emissions have adverse impacts on the environment in several
ways. In some cases, the timing of the NOX emissions can be important to the subsequent
environmental impacts. As noted below, year-round reductions in NOX emissions are more
helpful than seasonal approaches at minimizing the impacts of acid deposition and
eutrophication, while summertime NOX emissions reductions are most helpful in attaining the O3
standard. In some cases, PM10 nonattainment and visibility impacts are strongly related to
seasonal conditions. Since regulatory programs may be designed to achieve emissions decreases
at a constant, year-round rate or on a seasonal basis, the impacts of the policy decisions on the
timing of the required decreases need to be understood and, to the extent possible, integrated.
NOX Emissions by Season
As shown in the table at the end of this section, total NOX emissions vary somewhat by
season, with summer emissions usually slightly highest (EPA, 1995). Several source categories
emit evenly throughout the year. NOX emissions from electric utilities are highest in the
summer. On-road NOX emissions vary little from season to season, but are slightly higher in the
summer than winter. Non-road engine emissions (non-road diesel and gasoline) are higher in the
summer than in the winter since weather limits the use of these engines for construction, lawn
and garden, recreation, and light commercial purposes.
Seasonal Considerations in Environmental Programs
Acid Deposition
The impacts from acid deposition are both cumulative and short-term. The cumulative
effects are due to long-term chronic acidification of watersheds. In addition, some important
adverse effects are associated with springtime snowmelt. As described in section II. A of this
document, nitric acid deposition plays a dominant role in the acid pulses associated with the fish
kills observed during the springtime melt of the snowpack in sensitive watersheds. Thus,
wintertime NOX emissions reductions are especially important to lessening the incidence and
severity of acidic episodes in certain areas. In addition, the timing of aluminum concentration
peaks is also important. Toxic aluminum peaks related to nitrate fluctuations commonly occur in
late summer or early fall when soil temperatures and root growth are usually high (Joslin et al.,
1992). Continuous year-round NOX emissions reductions appear to be the most beneficial for
decreasing acid deposition damage to natural resources.
Eutrophication
N is the limiting nutrient in most coastal estuaries and many lakes. Thus, as described in
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section III.B of this document, addition of N results in accelerated plant growth in the waterbody
causing adverse ecological effects and economic impacts that range from nuisance algal blooms
to oxygen depletion and fish kills. Simulated water quality responses to year-round vs seasonal
nutrient emissions reductions conducted as a 1992 reevaluation of the baywide Tributary Nutrient
Reduction Strategy by the Chesapeake Bay Program indicated the need for year round controls on
phosphorus and N loadings (Thomann et a. 1994).
Ground-level O3
High ambient concentrations of O3 are associated with periods of elevated temperature
and solar radiation. Thus, in most parts of the country, high O3 episodes occur only during
summer months. Accordingly, the control of NOX emissions on a summer season basis may be
part of some areas' strategies to attain the O3 standard at least cost. It should also be noted that
application of NOX emissions controls that focus emissions reductions in the summer will, in
many cases, also achieve significant emissions reductions on a year-round basis. For example,
efforts to decrease emissions from large boilers will usually include installation of low NOX
burners—which will achieve year-round moderate amounts of emissions reductions—and may
include, in addition, some type of summer season control, such as switching to a cleaner fuel or
post-combustion technology. In some cases, year-round emissions reductions contained in
States' O3 attainment plans are explicitly supplemented by seasonal requirements; e.g., the
Northeast States adopted a strategy which calls for NOX emissions reductions in three phases
over the region with Phase 1 requiring year-round controls and the subsequent phases covering
the May through September timeframe.
NO2
Since the NO2 NAAQS requires ambient concentrations to be averaged over an annual
period, seasonal emissions should not affect this program.
PM
In some cases PM10 nonattainment is related to seasonal emissions. For example, in
some mountain/valley locations, the burning of wood for heating purposes results in wintertime
exceedances of the NAAQS. In other cases, PM10 nonattainment is related to a variety of
sources on a non-seasonal basis.
Visibility and Regional Haze
Visibility is lowest in the summer in the region south of the Great Lakes and east of the
Mississippi; in some locations the light extinction is more than twice as great in the summer as
during the other seasons (NRC, 1993). The most intense regional haze in the US occurs in the
east, where summertime meteorological conditions associated with slow-moving high-pressure
systems create stagnant conditions (NRC, 1993). Thus, in the summer, pollutants from many
different sources can accumulate, causing severe and widespread visibility degradation.
In the Grand Cany on Visibility Transport Commission's June 10, 1996 report it is noted
that seasons influence the relative visibility impacts of regional and local emissions. Both
emission types contribute to visibility impairment much of the year, and either type can be the
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dominant cause of impairment on any particular day regardless of the season.
Example of Consideration of Seasonal Concerns in O3 Policy
O3 Season Emissions Limit
To help achieve the O3 standard, many areas require application of RACT on major
stationary sources of NOX. Typically these controls involve modifications to combustion
equipment, such as installation of low NOX burners. Such controls operate continuously, year-
round.
EPA guidance issued in 1993 (Shapiro, 1993) gives States the option to allow sources to
control NOX emissions by switching to cleaner fuels during the O3 season. The NOX emissions
reductions must be equal to or greater than emissions reductions that would have occurred from
application of typical RACT controls. In general, a fuel-switching program would provide new
flexibility to States and industry in meeting certain Act requirements, including the NOX RACT
requirements. Fuel switching is a viable option for units where natural gas is readily available
since the price of natural gas in the O3 season may be competitive with other fuels.
Fuel-Switching Environmental Considerations
The EPA considered the relative environmental benefits for fuel switching and
presumptive NOX RACT. In terms of the primary purpose of NOX RACT, that is lowering O3
effects in areas of high concentrations, it is clear that the NOX emissions reductions due to
burning a cleaner fuel during the O3 season would be much more effective than lesser emissions
reductions at the presumptive NOX RACT levels, which would be evenly spread over an entire
year. The use of natural gas instead of coal could also substantially decrease annual and
summertime emissions of SO2, carbon dioxide (CO2), PM10, and associated toxic emissions such
as mercury. Further, emissions reductions of these pollutants may be especially effective in the
summer with respect to decreasing regional haze and sulfate-related PM, both of which tend to
peak in the summer. Thus, the potential benefits that go beyond the title I O3 and NOX RACT
goals include helping attain/maintain the NAAQS for SO2 and PM, decreasing mercury and
other air toxic emissions, improving visibility, and cutting emissions of CO2, a global warming
gas.
The EPA also considered evidence suggesting that, for certain ecosystems, decreases in
N deposition that occur only during the summer would be less effective at decreasing acid
deposition and nutrient impacts than emissions reductions that occur more uniformly throughout
the year. It is not possible at this time to determine or fully quantify this relative ecological
impact. Moreover, due to the inherent limits on the amount of fuel switching that can occur and
the required NOX emissions reductions under the CAA, wintertime N deposition would be
projected to decrease in most areas regardless of fuel switching. In contrast, the O3 related
benefits—and many of the additional potential benefits of fuel switching noted above—are well
known and quantifiable. In conclusion, it was EPA's judgment that substantial decreases in O3
concentrations would occur from fuel switching; this benefit and the accompanying
improvements in visibility, PM, air toxics, and global warming that also occur from fuel
switching clearly outweigh the decreased year-round benefits.
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Table IV-1. Seasonal Emissions for Nitrogen Oxides, 1985 through 1994
(thousand short tons)
Source Category
Fuel Comb.-Electric Utility
Winter
Spring
Summer
Fall
Fuel Comb.-lndustrial
Winter
Spring
Summer
Fall
Fuel Comb.-Other
Winter
Spring
Summer
Fall
Chemical & Allied Product Mfg.
Winter
Spring
Summer
Fall
Metals Processing
Winter
Spring
Summer
Fall
Petroleum & Related Industries
Winter
Spring
Summer
Fall
Other Industrial Processes
Winter
Spring
Summer
Fall
Solvent Utilization
Winter
Spring
Summer
Fall
Storage & Transport
Winter
Spring
Summer
Fall
Waste Disposal & Recycling
Winter
Spring
Summer
Fall
On-Road Vehicles
Winter
Spring
Summer
Fall
Non-Road Sources
Winter
Spring
Summer
Fall
Miscellaneous
Winter
Spring
Summer
Fall
Total All Sources
Winter
Spring
Summer
Fall
1985
6,976
1,801
1,603
1,862
1,650
3,209
825
794
785
805
712
343
163
56
149
262
65
72
62
63
87
22
22
21
21
124
31
31
31
31
327
79
83
83
82
2
1
1
1
1
2
1
1
1
1
87
21
22
22
22
8,089
2,063
2,046
1,997
1,983
2,734
587
674
780
693
309
25
122
72
91
22,860
5,861
5,624
5,775
5.599
1986
6,909
1,757
1,568
1,935
1,648
3,065
789
758
749
769
694
328
162
56
148
264
66
72
62
63
80
20
21
20
20
109
27
27
27
27
328
79
83
83
83
3
1
1
1
1
2
1
1
1
1
87
22
22
22
22
7,773
1,962
1,974
1,920
1,918
2,777
596
685
793
703
257
22
120
46
70
22,348
5,668
5,485
5,718
5.476
1987
7,128
1,748
1,627
2,055
1,698
3,063
788
757
748
769
706
332
165
59
151
255
64
70
60
61
75
19
19
18
19
101
25
25
25
25
320
77
81
81
81
3
1
1
1
1
2
1
1
1
1
85
21
21
21
21
7,651
1,938
1,939
1,885
1,889
2,664
574
657
759
674
351
27
125
92
107
22,403
5,615
5,483
5,807
5.499
1988
7,530
1,928
1,679
2,137
1,786
3,187
820
789
779
800
740
349
172
61
158
274
69
75
64
66
82
20
21
20
20
100
25
25
25
25
315
76
80
80
79
3
1
1
1
1
2
1
1
1
1
85
21
21
21
21
7,667
1,949
1,942
1,879
1,892
2,974
625
720
830
738
726
46
146
278
256
23,618
5,928
5,665
6,180
5.845
1989
7,607
1,980
1,760
2,034
1,833
3,209
825
794
784
805
736
349
172
59
155
273
69
74
64
66
83
21
21
20
21
97
24
24
24
24
377
75
79
79
78
3
1
1
1
1
2
1
1
1
1
84
21
21
21
21
7,682
1,956
1,938
1,903
1,885
2,844
610
704
810
720
292
24
123
62
83
23,222
5,955
5,704
5,864
5.700
1990
7,576
1,804
1,729
2,104
1,879
3,256
837
806
796
817
772
334
167
59
151
276
71
75
65
66
87
20
21
20
20
700
25
25
25
25
306
74
77
77
77
2
1
1
1
1
2
1
1
1
1
82
20
21
21
21
7,488
1,881
1,907
1,864
1,836
2,843
610
704
809
720
373
28
127
103
115
23,038
5,747
5,761
5,830
5.700
1991
7,488
1,858
1,722
2,064
1,843
3,775
816
786
111
796
779
338
167
59
155
278
71
75
65
67
78
19
20
19
19
97
24
24
24
24
297
72
75
75
75
2
1
1
1
1
2
1
1
1
1
83
20
21
21
21
7,373
1,863
1,857
1,834
1,820
2,796
600
691
796
709
283
24
123
57
79
22,672
5,705
5,572
5,797
5.597
1992
7,475
1,864
1,768
2,019
1,825
3,276
827
796
786
806
730
344
171
60
156
284
73
77
66
68
80
20
20
20
20
96
24
24
24
24
305
74
77
77
77
3
1
1
1
1
3
1
1
1
1
83
21
21
21
21
7,440
1,873
1,880
1,855
1,833
2,885
618
713
822
732
249
22
121
40
66
22,847
5,757
5,680
5,796
5.613
1993
7,773
1,914
1,779
2,188
1,893
3,797
822
791
782
802
726
341
170
60
155
286
73
77
67
69
87
20
21
20
20
95
23
24
24
24
375
76
80
80
79
3
1
1
1
1
3
1
1
1
1
84
21
21
21
21
7,570
1,907
1,896
1,856
1,851
2,985
638
737
852
758
279
20
120
25
53
23,276
5,856
5,717
5,976
5.725
1994
7,795
1,913
1,772
2,207
1,904
3,206
825
793
784
804
727
343
170
60
154
297
74
79
68
70
84
21
22
21
21
95
23
24
24
24
328
79
83
83
83
3
1
1
1
1
3
1
1
1
1
85
21
21
21
21
7,530
1,925
1,897
1,866
1,841
3,095
660
764
886
786
374
28
128
103
116
23,675
5,910
5,760
6,128
5.817
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References
Grand Canyon Visibility Transport Commission, Recommendations for Improving Western
Vistas, June 10, 1996 report of the Grand Cany on Visibility Transport Commission to the United
States Environmental Protection Agency, June 10, 1996.
National Research Council, Committee on Haze in National Parks and Wilderness Areas,
Protecting Visibility in National Parks and Wilderness Areas, 1993.
Shapiro, Acting Assistant Administrator, Office of Air and Radiation, U.S. Environmental
Protection Agency, "Fuel Switching to Meet the Reasonably Available Control Technology
(RACT) Requirements for Nitrogen Oxides (NOX)" EPA memorandum to Regional Air Directors,
July 30, 1993.
U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, National
Air Pollutant Emission Trends, 1900-1994, October 1995, EPA-454/R-95-011.
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C. Interface with Other Control Programs: Three Examples of
Secondary Emissions, EPA's Clean Air Power Initiative, and
New Standards for 03 and Particulate Matter
1. Synthetic Organic Chemical Manufacturing Industry (SOCMI)
The CAA amendments of 1990 mandate that SIPs for certain O3 nonattainment areas be
revised to require RACT to limit emissions of VOCs from sources for which EPA has published a
control techniques guideline (CTG) document. Each CTG document contains a recommended
"presumptive norm" for RACT for a particular source category, based on the EPA's current
evaluation of capabilities and problems general to the source category. In some cases, controls
for VOCs could increase NOX emissions (secondary emissions). In EPA's development of the
SOCMI CTG for Halogenated and Nonhalogenated Vent Streams (EPA, August 1993), this issue
was addressed. The CTG illustrates eight optional levels of control, with the NOX secondary
emissions listed. In most options the amount of secondary NOX emissions is extremely small
compared to the decrease in VOC emissions. In addition, the CTG addresses the issue of
secondary emissions as follows:
Another important consideration in applying RACT is emission of pollutants such as
carbon monoxide and nitrogen oxides from combustion-based control devices. The
potential consequences of emission from control devices are twofold. First, depending on
the VOCs-to-NOx ratio in the ambient air, NOX emissions from control devices may cause
more O3 to be formed than could be eliminated through VOC emissions reductions.
Second, emissions from control devices may be enough to trigger New Source Review.
(Table 6-1 shows expected national emissions of NOX and, in parentheses, the maximum
annual emissions of NOX at a single facility.) Whether the VOC emission decreases are
worth the increase in other pollutants from the VOCs control device is highly dependent
on air quality and meteorological conditions in each specific geographical area. Therefore,
States may select a less stringent level of control as RACT based on these considerations.
2. Pollution Control Projects
The EPA issued guidance (Seitz, 1994) which addresses issues involving the EPA's new
source review (NSR) rules and guidance concerning the exclusion from major NSR of pollution
control projects at existing sources. The guidance assures that any increase in NOX emissions due
to decreases in VOC emissions would be minimized and that certain PSD requirements are met.
That is, a qualifying add-on control device may be considered a pollution control project and may
be considered for an exclusion from parts of EPA's NSR rules. The permitting agency should: (1)
verify that the NOX increase has been minimized to the extent practicable; (2) confirm (through
modeling or other appropriate means) that the actual significant increase in NOX emissions does
not violate the applicable NAAQS PSD increment, or adversely impact any Class I area; and (3)
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apply all otherwise applicable SIP and minor source permitting requirements, including
opportunity for public notice and comment.
3. Landfill Methane
As the waste in a landfill decomposes, it breaks down to form landfill gases, such as
methane, smog-causing VOCs, and air toxics, pollutants known or suspected of causing cancer
and other serious health effects. Landfills are the largest anthropogenic source of methane
emissions in the United States. Methane is a greenhouse gas that contributes to global warming.
In the March 12, 1996 Federal Register. EPA issued final rules for municipal solid waste landfills
which will achieve significant decreases in emissions of VOCs, air toxics and methane.
This section provides a discussion of the secondary air emissions associated with
municipal solid waste (MSW) landfill control devices such as flares, boilers, gas turbines, and
stationary internal combustion (1C) engines. These control techniques, except flares, use the
energy content of the landfill gas to generate electricity or steam. At the same time, the burning
of the landfill gas produces NOX emissions. Consequently, EPA is concerned about the impact of
these secondary emissions in evaluating the overall benefits of applying landfill air emission
controls. The overall impact on NOX emissions, however, appears to be a decrease (EPA, 1991).
In evaluating the options for control of air emissions at MSW landfills, it is important to
consider the overall impact of the controls. The emission controls involving energy recovery
generally yield electricity or steam. Thus, landfill energy recovery devices such as gas turbines
and 1C engines are expected to decrease local or regional electric utility power generation. The
electricity or steam produced by these controls would otherwise be produced by some other
means. This decrease in utility requirements is likely to result in the reduction of NOX emissions
from coal-fired power plants.
In this analysis, electricity generated from landfill energy recovery techniques is assumed
to displace an equal amount of electricity that would otherwise be generated from coal-fired utility
boilers. Based on current utility fuel costs, this is a reasonable assumption. Therefore, the net
secondary air impacts represent the difference between air emissions generated by the control
equipment and air emissions that would be generated from producing an equivalent amount of
electricity with a coal-fired boiler/steam turbine.
The EPA judged that an analysis of secondary emissions from control techniques at MSW
landfills should consider the differential between emissions from an 1C engine or a gas turbine and
the emissions they might "displace" at a coal-fired utility plant under the rules for coal-fired utility
boilers (40 CFR 60, Subparts D and Da). The emission factors for the energy recovery
techniques were simply compared to the emission factors for the utility boiler to estimate relative
impacts. The EPA analysis (EPA, 1991) found that overall NOX emissions would be decreased at
the following rates in cases where a gas turbine or 1C engine is the control device: 224 and 139
pounds per million standard cubic feet of landfill gas, respectively.
Clean Air Power Initiative
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Summary
The Clean Air Power Initiative (CAPI) is a multi-stakeholder project intended to improve
air pollution control efforts involving the power generating industry. The project's goal is to
develop an integrated regulatory strategy for three major pollutants emitted from electric power
generators; namely, SO2, NOX, and, potentially, mercury. Major decreases in these pollutants are
expected to be needed to reduce the detrimental health effects of ground-level O3, fine particles,
and hazardous air pollutants and reduce the environmental effects of acidification,
eutrophication, ecosystem, crop, and materials damage, and regional haze. CAPI has considered,
where feasible, new approaches to pollution control that recognize the long-range transport of
many air pollutants and the economic benefits of emissions trading. The project was initiated by
EPA's Assistant Administrator for Air and Radiation in 1995. As individual companies develop
and implement strategies to participate in more competitive power markets, they could benefit
from greater certainty in being able to plan for and reduce costs of future environmental
regulations. The EPA is interested in reinventing its regulatory approach to decrease the number,
administrative complexity, and cost of its requirements while improving the likelihood of
achieving environmental results. The air quality improvement scenarios considered under the
CAPI could be implemented through existing CAA authority, however new congressional
authority may be preferable. A strategy, agreed to and supported by multiple stakeholders,
would provide an opportunity to protect public health and the environment at lower cost to the
power generation industry and to taxpayers than traditional regulation.
Background
Emissions from fossil-fuel-fired electric power plants contribute significantly to a number
of important air pollution and multi-media issues. These can be briefly categorized as: 1) adverse
effects on human health from ground level O3, PM, and persistent toxic air contaminants; 2)
environmental impacts such as eutrophication of coastal surface waters, wide-spread regional
haze that decreases visibility, acidification of surface waters from acid deposition, ecosystem and
crop damage from ground level O3, and ecosystem damage from mercury and other persistent
toxic pollutants; and 3) climate change due to greenhouse gases. In 1994, power plants were
responsible for 70 percent of all sulfur oxide (SOX) emissions, 33 percent of all NOX emissions, 23
percent of point source emissions of direct or "primary" PM, 23 percent of anthropogenic
mercury emissions, and 36 percent of all anthropogenic CO2 emissions (EPA, 1995). In addition,
power plants contribute to a range of other environmental impacts due to their water consumption
and disposal of solid wastes.
For purposes of the CAPI, EPA has chosen to focus on the pollutants that are related to
the first two categories of health and environmental effects noted above because these pollutants
are associated with pressing regional health and environmental concerns in North America. In
addition, EPA has clear statutory authority to regulate these pollutants. From a control/emissions
perspective, the pollutants of greatest importance can be grouped into three categories: sulfur
oxides, NOX, and mercury and other directly emitted toxic fine particles. Sulfur and NOX
emissions undergo complex atmospheric transformations that result in the formation of acidic fine
particles and gases, O3 smog, and toxic pollutants. The resultant mix, along with directly emitted
mercury and fine particles, can be transported by weather systems over long distances and affect
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air and water quality in areas far from where they were emitted. Because of this long range
transport and the location of multiple power generation emission sources in the United States, the
resulting atmospheric and deposition problems affect broad multi-state regions.
The CAA as amended in 1990 contains multiple requirements that will affect the power
generating industry well into the future. Regulations based on some of these requirements have
already taken effect and are being implemented, such as the SO2 allowance trading program under
Title IV and NOX RACT (an emissions standard based on RACT) under Title I. However, other
mandated measures and requirements are still under development. One of the most difficult and
important issues in this regard is related to developing O3 attainment plans under Title I. In
addition, the CAA mandates that EPA periodically review standards and conduct studies that
could affect the power generation industry over the next 10 years.
In implementing the CAA requirements, it is important to recognize that the electric
power industry is facing major changes in the way it is structured and the way it generates,
transmits, and distributes electricity. Competition is building in the industry in response to
changes in the law, technology, and markets. In a non-competitive environment, electric utilities
faced with pollution control requirements were allowed by their regulators to pass on
environmental costs to consumers in the form of higher electricity rates. In a competitive market,
utilities that have higher rates because of pollution controls would be at a relative disadvantage,
while those with lower or no pollution control costs could increase market share. From an
environmental perspective, there is concern that, absent appropriate emissions controls and policy
instruments allowing for reduced compliance costs, the overall move towards a competitive
market could result in significant regional environmental degradation (Browner, 1996).
Implementing the multiple requirements in a piecemeal fashion is unlikely to result in
economically optimal results. Many of the individual actions would ultimately result in different
source-specific emissions requirements or specific control technology mandates for the same
pollutants. On this basis alone, it is worth examining coordination of the activities. However, our
understanding of the nature of the environmental effects of power generation emissions suggests
the possibility of significantly more efficient and effective approaches.
As noted above, the current environmental issues associated with power generation are
related to regional scale transport of the emissions and transformation products of three key
pollutants released from hundreds of sources. This decreases the need to be concerned about
single source specific effects that are typically addressed with command and control approaches.
A focus on regional emissions reductions enables us to consider regional market based solutions
that have proven to be considerably more cost-effective. Conceptually, the multiple regional
problems associated with these emissions could be addressed by establishing an emissions budget
or cap at a set amount within a certain geographic area, allocating those emissions to sources, and
allowing sources within that region to trade their emissions with one another ("cap and trade"
approach). While the current experience with this approach has been with the Congressionally
mandated Title IV acid rain program, the States and EPA have already made considerable
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progress in developing emissions budget approaches for implementing regional NOX control
programs. Such approaches could also readily be considered for potential fine particle programs
or regional persistent toxic pollutants such as mercury.
Beyond the consideration of market-based approaches, EPA believes it is highly
desirable to engage key stakeholders in the industry, states, and environmental groups in a
discussion of a more effective way to establish cost-effective implementation approaches for
addressing the key air emissions from the power generating industry. Agreements on desirable
interim emissions reduction targets for fixed time periods could provide regulatory certainty for
the industry, assurances that emissions reductions would be achieved and sustained, more cost-
effective emissions reductions for companies and their customers, and reduce the resources all
parties must expend on issue-by-issue rule development, risk and cost assessments, control
strategy plan development, permitting, monitoring, and litigation.
Goals of the Initiative
CAPI is intended to break the current combative and costly pattern of regulation with a
new collaborative approach that assures the public of the health and environmental protections
promised by the CAA while providing the power generation industry with more certainty of
future regulatory requirements, greater flexibility, and cost savings. EPA believes consideration
of a cap and trade approach for SO2, NOX, and potentially, mercury (with appropriate local
safeguards, such as Title IV provides) is appropriate because these pollutants are transported far
from their source and much of the current health and environmental damage caused by these
pollutants comes from their total loadings in the air and on the ground.
Progress to Date
EPA began CAPI in 1995 by holding a series of small meetings with interested
stakeholders and by developing a model that could analyze the cost and emission implications of
different emissions reduction scenarios for SO2, NOX, and mercury. Detailed information about
the model and the various scenarios analyzed will not be presented in this paper, but are well
documented elsewhere17. In 1996, EPA held public meetings in April, May, and July to continue
and expand the dialogue on CAPI and to hear reactions to various emissions reduction scenarios
that EPA believes are consistent with the requirements of the CAA. As described in EPA's
CAPI report (EPA, October 1996) EPA also believes that cap and trade programs are effective
ways to ensure that environmental goals are maintained in the future without continually
returning to the industry for more emissions reduction actions. EPA will continue to improve the
modeling tool developed for CAPI, test other scenarios and sensitivity cases, explore economic
and environmental impacts, and provide input to the Clean Air Act Advisory Committee
deliberations. EPA would also like to continue to work with the power generation industry to
find cost-effective solutions to our environmental problems
17 See D EPA's Forecast of Electric Power Generation and Air Emissions, and 2) Analysis of Options for Air
Emissions Control Under the Clean Air Power Initiative. Office of Air and Radiation, U.S. EPA April 1996; and 3)
Revised Forecast of Electric Power Generation: 4) Analyzing Electric Power Generation Under the CAAA. July 1996;
and 5) Supporting Analysis for EPA's Clean Air Power Initiative. Office of Air and Radiation, U.S. EPA, October 1996.
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New Standards for O3 andPM
Common Factors for O3 andPM
As described in the sections on "Ozone" and "Particulate Matter," EPA published
revisions to the O3 and PM NAAQS on July 18, 1997 (62 FR 38856). As part of the revisions
process, EPA initiated action to address strategies for the implementation of the new NAAQS.
These ongoing reviews and related implementation strategy activities to date have brought out
important common factors between O3 and PM. Several similar health effects have been
associated with exposure to O3 and PM, including for example aggravation of respiratory disease
(e.g., asthma), increased respiratory symptoms, and increased hospital admissions and emergency
room visits for respiratory causes.
Other similarities in pollutant sources, formation, and control exist between O3 and PM, in
particular the fine fraction of particles. These similarities include: (1) atmospheric residence
times of several days, leading to regional-scale transport of the pollutants; (2) similar gaseous
precursors, including NOX and VOCs, which contribute to the formation of both O3 and PM in the
atmosphere; (3) similar combustion-related source categories, such as coal and oil-fired power
generation and industrial boilers and mobile sources, which emit particles directly as well as
gaseous precursors of particles (e.g., SOX, NOX, VOCs) and O3 (e.g., NOX, VOCs); and (4) similar
atmospheric chemistry driven by the same chemical reactions and intermediate chemical species
which favor both high O3 and fine particle concentrations. High fine particle concentrations are
also associated with significant impairment of visibility on a regional scale. These similarities
provide opportunities for optimizing technical analysis tools (i.e., monitoring networks, emissions
inventories, air quality models) and integrated emission reduction strategies to yield important co-
benefits across various air quality management programs. This integration could result in a net
reduction of the regulatory burden on some source category sectors that would otherwise be
impacted separately by O3, PM, and visibility protection control strategies. In recognition of the
potential benefits of integrating the Agency's approaches to providing for appropriate protection
of public health and welfare from exposure to O3 and PM, the Agency plans to develop associated
implementation strategies under coordinated schedules.
Integrated Implementation of the New O3 and PM Standards
The EPA initiated a process designed to provide for significant stakeholder involvement in
the development of integrated implementation strategies for the new/revised O3 and PM NAAQS
and a new regional haze program. As described below, this process involves a new subcommittee
of the Agency's Clean Air Act Advisory Committee (CAAAC), established in accordance with the
Federal Advisory Committee Act (FACA) (5 U.S.C. App.2).
The FACA was enacted in 1972 to open the advisory committee process to public scrutiny
and to protect against undue influence by special interest groups over government decision
making. Federal Advisory Committees may be established by statute, the President, or by the
head of a Federal Agency. An advisory committee or subcommittee is established under FACA to
obtain advice or recommendations from advisory groups established by or closely tied to the
Federal Government. The CAAAC was established to provide independent advice and counsel to
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the EPA on policy and technical issues associated with the implementation of the Act. The
CAAAC advises EPA on the development, implementation, and enforcement of several of the
new and expanded regulatory and market-based programs required by the Act.
The CAAAC advises on issues that cut across several program areas. The programs
falling under the purview of the CAAAC include those for meeting NAAQS, reducing emissions
from vehicles and vehicle fuels, decreasing air toxic emissions, issuing operating permits and
collecting fees, and carrying out new and expanded compliance authorities. The CAAAC holds
meetings, analyzes issues, conducts reviews, performs studies, produces reports, makes
recommendations, and undertakes other activities necessary to meet its responsibilities.
Comments, evaluations, and recommendations of the CAAAC and responses from the EPA are
made available for public review, in accordance with Section 10 of F AC A.
A new subcommittee of the CAAAC, the Subcommittee for Ozone, Particulate Matter,
and Regional Haze Implementation Programs (the Subcommittee), was established in August
1995 to address integrated strategies for the implementation of potential new O3 and PM
NAAQS, as well as a regional haze program. The Subcommittee is composed of representatives
selected from among state, local, and tribal organizations; environmental groups; industry;
consultants; science/academia; and federal agencies. Recommendations made by the
Subcommittee will be submitted to EPA through CAAAC. To facilitate communication between
the Subcommittee and CAAAC, some members of CAAAC are on the Subcommittee.
The Subcommittee is charged with providing advice and recommendations to EPA on
developing new, integrated approaches for implementing potential revised NAAQS for O3 and
PM, as well as for implementing a new regional haze reduction program. The Subcommittee is
expected to examine key aspects of the implementation programs for O3 and PM, to provide for
more flexible and cost-effective implementation strategies, as well as to provide new approaches
that could integrate broad regional and national control strategies with more localized efforts. In
addition, the Subcommittee will consider new and innovative approaches to implementation
including market-based incentives. The focus of the Subcommittee will be on assisting EPA in
developing implementation control strategies, preparing supporting analyses, and identifying and
resolving impediments to the adoption of the resulting programs.
Issues involved in the revision of the O3 and PM NAAQS, such as the averaging time,
level, and form of any revised standards, were addressed in accordance with the NAAQS review
process described in the above sections, including review by CASAC, and are not within the
Subcommittee's charge. CASAC is charged with providing advice and recommendations to the
Administrator on all matters pertaining to the review of and possible revisions to the NAAQS.
Similarly, selection of the appropriate indicator or units of measurement for quantifiable changes
in visibility are being addressed through an independent, scientific peer-review process and, thus,
will not be a subject for recommendations by the Subcommittee.
References
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Browner, C. M., Administrator, U.S. Environmental Protection Agency, letter to Council on
Environmental Quality, May 13, 1996.
Seitz, John, Director, Office of Air Quality Planning and Standards, U.S. Environmental
Protection Agency, "Pollution Control Projects and New Source Review (NSR) Applicability,"
July 1, 1994 memorandum to Regional Air Directors.
U.S. Environmental Protection Agency, Office of Air and Radiation, "EPA's Clean Air Power
Initiative" October 1996.
U.S. Environmental Protection Agency, Office of Air and Radiation, "EPA's Forecast of Electric
Power Generation and Air Emissions," and "Analyzing Electric Power Generation Under the
CAAA," April 1996.
U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, Control of
Volatile Organic Compound Emissions from Reactor Processes and Distillation Operations
Processes in the Synthetic Organic Chemical Manufacturing Industry., August 1993, EPA-450/4-
91-031.
U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, Air
Emissions from Municipal Solid Waste Landfills, Background Information for Proposed
Standards and Guidelines, EPA-450/3-90-011a, March 1991.
U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, National
Air Pollutant Emission Trends, 1900-1994, October 1995, EPA-454/R-95-011.
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Appendices
Introduction
Appendices A and B provide information on specific programs currently underway to
achieve decreases in emissions of NOX In some cases, these are programs specifically mandated
by the CAA. In other cases, they are programs that are needed to meet more general CAA
requirements. In addition, several related efforts by the States are described. These are control
programs that affect both new and old sources of emissions and are generally based on currently
available control technologies.
Since passage of the 1970 CAA amendments, air pollution control and prevention
technologies have continuously improved. Technologies such as selective catalytic reduction and
gas reburn systems are in place and successfully performing today that were only on the drawing
board ten years ago. As the demand for more innovative and cost-effective or cost-saving
technologies increases—due to the above new initiatives, for example—new technologies such as
ultra low-NOx gas-fired burners and vacuum insulated catalytic converters will move from the
research and development or pilot program phase to commercial availability. Thus, it is likely that
many new technologies will be available in the next ten to fifteen years to employ in air pollution
control and prevention strategies.
Appendix C provides more detailed information with respect to sources of NOX emissions.
In addition, Appendix C describes mechanisms that eventually remove NOX from the atmosphere
and provides some information on emissions of ammonia. Finally, Appendix D lists several
acronyms and abbreviations.
A. Mobile Source Programs
The control of NOX emissions from mobile sources is under much more investigation since
the CAA amendments of 1990. Light-duty vehicles, heavy-duty engines, nonroad engines, and
fuels are all required to produce lower emissions. However, the United States has experienced
tremendous growth in the activity of mobile sources overwhelming much of the emissions
reductions from the introduction of cleaner technologies; vehicle miles traveled has grown
exponentially without signs of leveling since the passage of the initial Clean Air Act.
Light-Duty Vehicles
Tailpipe Standards
Programs that decrease or can decrease NOX emissions from light-duty vehicles are
numerous and involve all facets of the operation of these vehicles. These range from performance
requirements like tailpipe standards to fuel modifications to in-use repairs.
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The tailpipe standards for light-duty vehicles are evolving as a result of the 1990 CAA
amendments and California rulemakings. The CAA established more stringent Tier 1 tailpipe
emission standards, lowering the NOX emissions standard from 1.0 gram per mile to 0.4 grams per
mile. In addition, states were allowed to adopt California Low Emission Vehicle (LEV) standards
which further increased the stringency of the NOX standard. This led to a discussion about a new
emission control program called the National Low Emission Vehicle (NLEV.) program, a
voluntary but enforceable program which included a 0.2 gram per mile NOX standard for light-
duty vehicles. The CAA also requires that EPA conduct a study to determine whether more
stringent tailpipe standards (called "Tier 2" standards) are necessary, technically feasible, and cost
effective. This study is currently underway.
EPA has recently updated the modeling of emission benefits that would result from the
NLEV program. Based on a 1999 model year program start in northeast states and a 2001 model
year start nationwide, as well as on realistic assumptions regarding individual state adoptions of
the California LEV program, EPA estimates that NOX will be reduced by 538 tons per day
nationwide in 2005 and 1699 tons per day when the program is fully implemented in 2015.
Based on a detailed assessment by California in 1994 and updated in April, 1996,
California has estimated the incremental cost of a LEV in California to be $120 per car. EPA
believes that the incremental cost for NLEV will be lower than the California estimate due to
technology advances, harmonization of California and federal programs, economies of scale
resulting from a nationwide program, and historical evidence that suggests that California's own
cost estimates have generally been overstated and that the prices of newly-introduced
technologies often decrease in successive model years.
Tailpipe Standards Summary
- Reductions: Tier 1 sets a standard intended to achieve a 60% NOX emissions reduction in grams per mile
(phasing in 1994-1996); NLEV could achieve an additional 50% NOX emissions reduction in grams per mile.
- Reference: NLEV Proposed Rulemakmg Notice of 10-10-95; Final Rulemakmg 6-7-97 (62 FR 31192).
Onboard Diagnostics
Another requirement of the CAA was the incorporation of On-board Diagnostic systems
to determine the functionality of the emission control devices. This strategy might lower the cost
of in-use compliance by notifying operators and repair personnel of problems with the emission
control devices. The benefits and costs of the program depend in large measure on the emission
standards imposed on the design of new vehicles. It also depends on the willingness or
requirements of owner/operators to repair detected malfunctions. It is expected that NOX
emissions reductions would be derived from the installation of sensors to detect failures of the
engine to meet its emission standards. A notice of proposed rulemaking was issued on May 28,
1997, proposing changes to the existing regulations.
Supplemental Federal Test Procedure
The CAA also required that EPA reexamine the test procedure for light-duty vehicles.
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Following an extensive study of in-use driving behavior and testing of in-use emissions, EPA
completed a rule in October 1996 that supplements the existing test procedure to include higher
speeds, higher acceleration driving behavior, and the operation of the vehicle air conditioner.
EPA found that vehicles produce more NOX emissions when operated under the higher loads,
particularly with the air conditioner operating. Due to the change in the test procedure, vehicle
emissions will need to be decreased to meet the current emissions limits for light-duty vehicles.
That is, the more stringent supplemental test procedure is expected to result in reductions in
VOCs, CO, and NOX emissions; NOX emissions are expected to be lowered by roughly 10%
which, depending upon the future tailpipe standards, can produce NOX emission reductions of
about 125,000 to 400,000 tons. The cost of such a program is uncertain but EPA estimates that
the cost effectiveness of the institution of a new standard is about $1,000 per ton of VOC and
NOX emissions decreased. For further information, EPA published a final rule in the Federal
Register on October 22, 1996 (61 FR 54851).
Supplemental Federal Test Procedure Summary
-Reductions: 125,000-400,000 tons per year or roughly 10%.
- Reference: Final Rulemaking Notice of October 22, 1996.
Clean Fuel Fleets
Provisions of the CAA amendments of 1990 require the establishment of a clean-fuel fleet
program in certain O3 and CO nonattainment areas. This requires that some of the new vehicles
purchased by certain fleet owners meet clean-fuel fleet vehicle exhaust standards. These
requirements apply to light-duty vehicles and trucks and heavy-duty engines. The most recent
rulemaking for this program was published in the Federal Register on September 30, 1994.
Transportation Alternatives
While CAA section 108(f) lists transportation control measures that may lower vehicle
miles traveled (VMT), EPA does not require states or local areas to adopt specific transportation
control measures. The CAA simply requires severe and extreme O3 areas to adopt "transportation
control strategies and transportation control measures to offset any growth in emissions due to
growth in VMT and numbers of vehicle trips," and to implement transportation control measures
as necessary for attainment. EPA has interpreted the offset requirement as only applying to
emissions of VOCs but it might have some effect on NOX emissions. EPA has also issued
guidance for states to use in calculating activity growth in order to determine its scope and trend,
"Section 187, VMT Forecasting and Tracking Guidance", January, 1992. The guidance is meant
to ensure that growth will be properly determined and monitored.
In-Use Initiatives for Light-Duty Vehicles
Due to poor maintenance or deliberate tampering, motor vehicles in use have consistently
emitted pollutants in excess of the established standards. Motor vehicle Inspection and
Maintenance (I/M) programs have been singled out as the primary means to rectify these
problems by identifying vehicles in need of repair. Many areas are required to implement I/M
programs with various stringency to control emissions of CO, VOCs, and NOX. The benefits of
enhanced I/M programs on NOX emissions is estimated to be about 9% with overall tonnage
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reductions dependent upon the stringency of the tailpipe standards, possibly approaching 500,000
tons per year with wide coverage. The cost effectiveness of an I/M program targeted solely for
NOX emission reductions has not been determined. The final rule for I/M programs was published
on November 5, 1992.
Inspection/Maintenance Summary
- Reductions: 9%NOX reduction; up to 500,000 tons per year.
- Reference: Final Rulemakmg Notice of 11-5-92 (57 FR 52950).
Highway Heavy-Duty Vehicles
New heavy-duty engines intended for use in highway vehicles have been required to meet
more stringent NOX emission limits for engines built in 1985, 1990, and 1991 with subsequent
reductions in those limits for 1998. While it is estimated that emissions per engine-mile will be
significantly reduced once fleet turnover fully implements the new technologies, growth in activity
will largely overwhelm these reductions. EPA has proposed new standards to begin with new
engine manufactured in 2004 that would reduce NOX emissions by over 1,000,000 tons by the
year 2020. The cost effectiveness of the program is estimated to be $200 to $500 per ton of NOX
emissions decreased.
Heavy-Duty Vehicles Summary
- Reductions: 50% Reduction from 1998 NOX Levels by 2020.
- Reference: Notice of Proposed Rulemaking on 6-27-96 (61 FR 33469). Final Rulemaking expected in the
fall of 1997.
Nonroad Engines
There are several nonroad engine control programs some of which target VOC emissions,
and some which target NOX emissions. Those controls that target NOX emissions are for large
marine, aircraft, locomotive and general purpose engines like those used in agriculture,
construction, and general industrial equipment.
General Purpose Nonroad Engines
These engines produce the greatest portion of the nonroad NOX emissions. The EPA
finalized an initial rule for compression-ignition (diesel) engines with rated power of over 37
kilowatts on June 17, 1994. The rule sets emission standards for new engines built starting in the
years ranging from 1996 through 2000 depending upon the size of the engine. The cost
effectiveness for this program was estimated at less than $200 per ton of NOX decreased. The
overall NOX emissions decreased once the rule is fully phased in, including engine turnover, is
estimated at approximately 300,000 tons per year.
EPA intends to propose second and third tiers of standards for nonroad diesels in the fall
of 1997. It is estimated that emission reductions for nonroad engines when the new standards are
fully implemented will result in an additional reduction of 1,600,000 tons of NOX per year. EPA
expects this program to be very cost effective, with cost per ton of NOX removed of under $1000.
General Purpose Nonroad Engines Summary
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- Reductions: In Phase I, a 20% reduction will produce about 300,000 tons per year reduction; Phases II and
III could provide about an additional 40% reduction or about 800,000 tons per year.
-Reference: Final Rulemaking Notice of 6-17-94; Advanced Notice of Proposed Rulemaking, August 31,
1995. EPA expects to formally propose emission limits for in the fall of 1997.
Locomotive
The EPA proposed regulations for controlling locomotive emissions in January, 1997.
This category represents the second largest producer of nonroad NOX emissions. The EPA is
under a court-ordered deadline to complete a final rule by December 17, 1997. The rule as
proposed on February 11, 1997 (62 FR 6365) includes stringent emission standards for all
locomotives produced after the effective date of the standards, as well as requiring lesser emission
reductions of most of the existing fleet of locomotives. The EPA estimates that standards under
consideration will produce almost 400,000 tons of NOX emissions reduction per year by 2010,
with ultimate reductions of almost 600,000 tons per year by 2040. The cost effectiveness of the
control strategy is estimated in the proposal to be around $175 per ton of NOX decreased.
Marine
The EPA is working with the International Marine Organization, a subgroup of the United
Nations, to develop an agreement to control emissions from ships on international voyages. Such
an agreement would provide important measures to control emissions from ships for which
national standards could not apply. However at this time, the level and cost effectiveness of
control measures have not been determined nor has there been any agreement.
Aircraft
The EPA is working with several government agencies and governments in the
International Civil Aviation Organization to incorporate NOX control measures for aircraft engines
in an international agreement. However at this time, the level and cost effectiveness of control
measures have not been determined nor has there been any agreement.
Fuels Programs
Gasoline
Reformulated gasoline (RFG) was instituted as a cleaner gasoline and was intended
primarily as a VOCs emissions reduction strategy with a no NOX increase requirement. The
program is implemented in two phases with increasing stringency of the standard. Phase I
provides for a decrease in VOCs and toxic emissions with a small (about 1.5%) decrease in NOX
emissions to provide a compliance margin for the no NOX increase provision at minimal additional
cost. In phase II of the program, EPA increased the stringency of VOCs control and exercised its
discretion by imposing a NOX control limit.
Phase II will include a requirement for a 6.8% NOX control for the summer of the year
2000 in addition to more stringent VOC emissions reductions. The NOX requirement will
probably be achieved by lowering the amount of sulfur in gasoline. The standard was set on the
basis of a marginal cost effectiveness, but the average cost effectiveness for the NOX control
standard is now projected to be less than $2,000 per ton decreased. The overall NOX emissions
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reduction depends upon the amount of fuel that RFG displaces in the field, but for the nine cities
required to implement RFG 22,000 summer tons of NOX per year will be decreased. The program
is mandatory for nine metropolitan areas and optional for other areas. Under certain conditions,
states have the ability to mandate their own clean gasoline programs and are investigating those
options. The final RFG rule was published in the Federal Register on February 16, 1994 (59 FR
7716).
RFG, Phase II Summary
- Reductions: Roughly 7% in areas used reducing nationwide NOX emissions by at least 22,000 tons per
summer season.
- Reference: Final Rulemaking Notice of February 16, 1994 (59 FR 7716).
Other Programs
Ozone Transport Assessment Group
The OTAG was a consultative process among the eastern States and EPA. The OTAG
process assessed national and regional control strategies, using improved modeling techniques.
Significant new modeling analyses were conducted by EPA and other agencies as part of the
OTAG process. The goal of the OTAG process is for EPA and the affected States to reach
consensus on the additional regional and national emission reductions that are needed to help
achieve attainment of the O3 standard. On July 8, 1997 OTAG forwarded its final
recommendations to EPA. Based on the results of the OTAG process, States are expected to
submit in 1997 attainment plans which show attainment through local, regional and national
controls.
OTAG Summary
Reductions: Depends on subsequent SIP revisions.
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B. Stationary Source Programs
Emission Standards for Coal-Fired Power Plants
Title IV (Acid Deposition Control) of the CAA specifies a two-stage program for
decreasing NOX emissions from existing coal-fired electric utility power plants. Analogous to the
national allowance program for decreasing SO2 emissions, this program is to be implemented in
two phases. Phase I affected units (277 boilers) are required to meet the applicable annual
emission rates beginning with calendar year 1996; Phase II affected units (775 boilers) are
required to meet the applicable annual emission rates beginning with calendar year 2000.
Implementation of the first stage of the program, promulgated April 13, 1995 (60 FR 18751),
will decrease annual NOX emissions in the United States by over 400,000 tons per year between
1996 and 1999 (Phase I) and by approximately 1.17 million tons per year beginning in 2000
(Phase II). These decreases are achieved by applying low NOX burner (LNB) technology to dry
bottom wall-fired boilers and tangentially fired boilers (Group 1).
The second stage of the program, promulgated December 19, 1996 (61 FR 67112),
provides for additional annual NOX emissions reductions in the United States of approximately
0.89 million tons per year beginning in the year 2000 (Phase II). Taken together, the two stages
provide for an overall decrease in annual NOX emissions reductions in the United States of
approximately 2.06 million tons per year beginning in the year 2000. In the second stage of the
title IV Program EPA has: (1) determined that more effective low NOX burner (LNB) technology
is available to establish more stringent standards for Phase II, Group 1 boilers than those
established for Phase I; and (2) established limitations for other boilers known as Group 2 (wet
bottom boilers, cyclones, cell burner boilers, and vertically fired boilers), based on NOX
control technologies that are comparable in cost to LNBs.
The total annual cost of this regulation to the electric utility industry is estimated at $267
million, resulting in an overall cost-effectiveness of $227 per ton of NOX removed. The final rule
sets lower Group 1 emission limits and establishes emission limits for several other types of coal-
fired boilers (Group 2) in Phase II. The annual cost of these additional reductions will be
approximately $200 million, at an average cost-effectiveness of $229 per ton of NOX removed.
By the year 2000, the Phase IINOX rule will achieve an additional decrease of 890,000 tons of
NOX annually.
Coal-Fired Power Plants (Group I)
- Reductions: 400,000 tons per year, Phase I
- Reference: Final rule published in the Federal Register of April 13, 1995.
Coal-Fired Power Plants (Phase II, Groups I & II)
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- Reduction: 2,060,000 tons NOX emissions reduction, Phases I & II.
- Reference: Phase II proposed rule published in the Federal Register of December 19, 1996.
Clean Air Power Initiative (CAPI)
CAPI is a multi-stakeholder project intended to improve air pollution control efforts
involving the power generating industry. The project's goal is to develop an integrated regulatory
strategy for three major pollutants emitted from electric power generators; namely, SO2, NOX, and
mercury. Major decreases in these pollutants are needed to decrease the detrimental health effects
of ground-level O3, fine particles, and hazardous air pollutants and decrease the environmental
effects of acidification, eutrophication, ecosystem, crop, and materials damage, and regional haze.
Industrial Combustion Coordinated Rulemaking
The CAA requires regulation of air emissions from several categories of industrial
combustion sources, including boilers, process heaters, waste incinerators, combustion turbines,
and internal combustion engines. These combustion devices are used pervasively for energy
generation and waste disposal in a wide variety of industries and commercial and institutional
establishments. They combust fuels including oil, coal, natural gas, wood, and non-hazardous
wastes. Both hazardous air pollutants and criteria pollutants are emitted. The industrial
combustion regulations will affect thousands of sources nationwide, and will have significant
environmental and health impacts and cost considerations.
The EPA plans to implement an Industrial Combustion Coordinated Rulemaking to develop
recommendations for Federal air emission regulations that address the various combustion source
categories and pollutants. Regulations will be developed under sections 112 and 129 of the CAA,
as well as section 111. The overall goal of the Industrial Combustion Coordinated Rulemaking is
to develop recommendations for a unified set of Federal air regulations that will maximize
environmental and public health benefits in a flexible framework at a reasonable cost of
compliance, within the constraints of the CAA.
International NOX Protocol
The United States signed the Nitrogen Oxides Protocol in Sophia, Bulgaria in 1988. The
Protocol caps national NOX emissions. For the United States, this means a cap at 1984 levels or
about 23.2 million tons per year. The cap is to be achieved by the year 1994. The United States
emissions in 1994 were about 23.6 million tons. Due to emission reductions mandated by the
CAA, NOX emissions are projected (EPA Trends Report, October, 1995) to fall to 20.5 million
tons by the year 2000 and remain below 22 million tons through the year 2010.
NOX Protocol
- Reductions: 0.4 million ton reduction needed from 1994 level.
- Reference: Nitrogen Oxides Protocol in Sophia, Bulgaria in 1988.
Municipal Waste Combusters
Standards of performance for new municipal waste combustor (MWC) units and emission
guidelines for existing MWC's implement sections 111 and 129 of the CAA . The standards and
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guidelines apply to MWC units at plants with aggregate capacities to combust greater than
35 megagrams per day (Mg/day) (approximately 40 tons per day) of municipal solid waste
(MSW) and require sources to achieve emission levels reflecting the maximum degree of
reduction in emissions of air pollutants that the Administrator determined is achievable, taking
into consideration the cost of achieving such emissions reduction, and any non-air-quality health
and environmental impacts and energy requirements. The promulgated standards and guidelines
establish emission levels for MWC organics (dioxins/furans), MWC metals (cadmium (Cd), lead
(Pb), mercury (Hg), PM, and opacity), MWC acid gases [hydrogen chloride (HC1) and SO2],
NOx anc[ MWC fugitive ash emissions.
As explained at proposal (59 FR 48198, September 20, 1994), the combination of SD/FF,
GCP, and SNCR was the basis of the new source MACT floor for NOX. These technologies
remain the basis for the final NOX MACT floor. The final standard (December 19, 1995 Federal
Register) for MWC's at large plants is 180 ppmv (24-hour averaging period) for the first year of
operation, and 150 ppmv (24-hour averaging period) thereafter. The final standards do not
require NOX control for MWC's at small plants.
The EPA intends to amend the MWC standards in August 1997. The amended standards
and guidelines would apply to only MWCs units larger than 250 tons per day capacity. (The
MWC regulations would no longer apply to MWC plants larger than 35 Mg/day. Note the
amendments would change both (1) size (250 tpd vs 35 Mg/day) and (2) unit capacity vs
aggregate plant capacity.)
New Source Performance Standards (NSPS) for Boilers
Pursuant to section 407(c) of the CAA, EPA has reviewed the emission standards for NOX
contained in the standards of performance for new electric utility steam generating units and
industrial-commercial-institutional steam generating units.(this requirement covers three existing
NSPS in 40 CFR part 60: 1) Subpart Da for Utilities, 2) Subpart Db for Industrial boilers and
Subpart DC for small boilers). The proposed changes to the existing standards for NOX emissions
reduce the numerical NOX emission limits for both utility and industrial steam generating units to
reflect the performance of best demonstrated technology. The proposal also changes the format
of the revised NOX emission limit for electric utility steam generating units to an output-based
format to promote energy efficiency and pollution prevention.
The primary environmental impact resulting from the revised NOX standards is reductions in
the quantity of NOX emitted from new steam generating units subject to the proposed revisions to
the NSPS. Estimated baseline NOX emissions from these new steam generating units are 39,500
Mg/year (43,600 tons/year) from utility steam generating units and 58,400 Mg/year (64,400
tons/year) from industrial steam generating units in the 5th year. The revised standards are
projected to reduce baseline NOX emissions by 23,000 Mg/year (25,800 tons/year) from utility
steam generating units and 18,000 Mg/year (20,000 tons/year) from industrial steam generating
units in the 5th year after proposal. This represents an approximate 42 percent reduction in the
growth of NOX emissions from new utility and industrial steam generating units subject to these
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revised standards.
Boilers NSPS
- 25,800 ton NOX emissions reduction annually from utility steam generating units
- 20,000 ton NOX emissions reduction annually from industrial steam generating units
- Reference: Proposed rule in the July 9, 1997 Federal Register
New Source Review
In O3 nonattainment areas (and in an Ozone Transport Region) major new or modified
sources are required to control emissions by the Lowest Achievable Emission Rate and to offset
the new emissions. In attainment areas the sources are required to control emissions by Best
Available Control Technology and are not required to offset the new emissions. These programs
help control emissions due to economic growth.
NO2 NAAQS Rulemaking
Currently, all areas of the United States are in attainment of the annual NO2 NAAQS of
0.053 ppm. Through implementation of NOX emissions reductions related to acid deposition and
attainment of the O3 and PM NAAQS, it is likely that the NO2 standard will continue to be
attained throughout the nation in the foreseeable future.
O3 Attainment Plans
The 1990 CAA amendments provide the framework for action by states and EPA for
national, regional, and local controls to achieve the NAAQS. Under these provisions, states are
expected to submit SIPs demonstrating how each nonattainment area will reach attainment of the
O3 NAAQS. Based on the degree that O3 concentrations in an area exceed the standard, the Act
spells out specific requirements that states must incorporate into their attainment plans and sets
specific dates by which nonattainment areas must reach attainment.
To help attain the 1-hour O3 air quality standard in the near-term, certain existing sources
must install RACT and new sources must install controls representing the lowest achievable
emission rate. To decrease acid deposition, the CAA requires coal-fired utility boilers to meet
emission limits in two phases. Further, in a longer-term provision, the CAA requires States to
adopt additional control measures as needed to attain the O3 standard. This requirement
supplements the RACT and NSR requirements. Thus, a State would need to require NOX
controls which achieve emission reductions greater than the NOX RACT/phase I acid deposition
limits where additional reductions in emissions of NOX are necessary to attain the O3 standard by
the attainment deadline.
The stationary (and mobile) source control measures needed for attainment will vary from
region to region. For example, stationary sources in the South Coast Air Quality Management
District of California are required to comply with Best Available Retrofit Control Technology
(BARCT), which are stricter that Federal requirements. The application of BARCT to industrial
sources generally results in NOX emission rate reductions of 70-90 percent from uncontrolled
levels.
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Attainment Plans
- Reductions: Depending on (future) modeling findings and SIP revisions.
OTAG
Over a 2 year period EPA worked with OTAG, which was chartered by the Environmental
Council of States for the purpose of evaluating O3 transport and recommending strategies for
mitigating interstate pollution. The OTAG was a consultative process among 37 eastern states
which included examination of the extent that NOX emissions from hundreds of kilometers away
are contributing to smog problems in downwind cities in the eastern half of the country, such as
Atlanta, Boston, and Chicago. The OTAG completed its work in June 1997 and on July 8, 1997
forwarded its recommendations to EPA for achieving additional cost-effective emissions
reduction programs to decrease ground-level O3 throughout the eastern United States. Based on
these recommendations and additional information, EPA will complete a rulemaking action
requiring States in the OTAG region that are significantly contributing to O3 nonattainment or
interfering with maintenance of attainment in downwind States to revise their SIPs to include new
rules to reduce their emissions of NOX.
OTAG
- Reductions: Depends on EPA rulemaking and subsequent SIP revisions.
Northeast Ozone Transport Commission (OTC) NOx Memorandum of Understanding (MOU)
Phases II & III
To implement the OTC MOU, the required emissions reductions are applied to a 1990
baseline for NOX emissions in the OTR to create a "cap", or emissions budget for each of the two
target years: 1999 and 2003. The NOX Budget Model Rule provides that once the 1990 baseline is
established, the OTC MOU emissions reduction requirement is applied to create the 1999 and
2003 emissions budgets. The budget would then be allocated as "allowances" to the emission
units subject to the program (budget sources). Budget sources are defined as fossil fuel fired
boilers and indirect heat exchangers of 250 million Btu or greater, and electric generating units of
15 megawatts, or greater. Budget sources are defined on a unit level, meaning that each boiler or
utility generator is considered a separate budget source. There are approximately 465 budget
sources that would be applicable to the NOX Budget Program. The regionwide seasonal NOX
Budget for 1999 (Phase II Target) is approximately 220,000 tons. For Phase III, in 2003, the
target is approximately 143,000 tons of NOX.
OTC NOX MOU
- Reductions: Approximately 272,000 ton seasonal (May through September reductions) reduction in Phase II
(May 1999); and 76,000 additional ton seasonal reduction in Phase III (May 2003).
- Reference: NESCAUM/MARAMA NOX Budget Model Rule, January 31, 1996 and 1990 OTC Nox Baseline
Emission Inventory", Volume 1: Supplemental Material. E.H. Pechan Associates, July 12, 1995.
Reasonably Available Control Technology (RACT)
Certain existing major sources of NOX must purchase and install reasonably available
controls to decrease NOX emissions. The new NOX requirements apply in certain O3
nonattainment areas and in the Northeastern Ozone Transport Region.
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143
RACT
- Reductions: generally a 30-50 percent reduction in NOX emissions.
- Reference:NOx Supplement to the General Preamble, November 25, 1992 (57 FR 55620).
Substitution ofNOJor Required VOC Emissions Reductions
States may choose to decrease NOX emissions instead of required VOC emissions reductions
for certain VOC control programs.
Visibility & Regional Haze
The Administrator of EPA signed the notice of proposed rulemaking for the regional
haze rules on July 18, 1997.
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144
C. Sources and Sinks of Atmospheric Nitrogen
Sources ofNOx
Summary
A summary of national NOX emissions sources and NOX emissions trends is located in the
"Introduction/Overview" section of this document. A map of large stationary sources (figure C-
1) and an emissions density map (figure C-2) are provided (EPA, 1995). Supplemental
information is provided below regarding sources of NOX emissions. In addition, information is
provided regarding sinks of NOX.
Fuel Combustion: Electric Utility, Industrial, and Other
NOX is emitted when fossil fuels are used to generate electricity. Electric utilities account
for a large portion of the total national NOX emissions; about 8 million tons in 1994, accounting
for 33 percent of total NOX emissions in that year as shown in tables C-l and C-2 (EPA, 1995).
Transportation: On-Road Vehicles and Non-Road Sources
Emissions from on-road vehicles peaked in 1978 and have declined since then due to
emissions control programs. Currently, on-road vehicle emissions constitute approximately
32 percent of total NOX emissions. Figure C-3 below displays trends in on-road NOX emissions,
vehicle miles traveled, fuel use, and real gasoline prices for the period 1940 through 1993. NOX
emissions from on-road vehicles increased as VMT and fuel use increased from the period 1940
through 1978 (EPA, 1995). However, NOX emissions begin to decline after 1978 while VMT and
fuel use continued rising. The effects of regulations controlling vehicle emissions accounts for the
declines in NOX emissions occurring after 1978. Although VMT has more than doubled since
1970, NOX emissions from on-road vehicles are nearly equal to their 1970 levels.
In contrast to the on-road vehicle NOX emissions trends, emissions from non-road sources
increased over the entire period of 1940 to 1994. Emissions control measures for selected non-
road engine categories are scheduled to begin in 1996. Significant emissions reductions are not
expected, however, until after the year 2000.
-------
Plants in 1990 with Greater than 1,000 tons per year of
NITROGEN OXIDE Emissions
Figure C-l
-------
Density Map of 1994 NITROGEN OXIDE Emissions
Emission Density
HHigh
B Above Average
D Average
D Below Average
D Low
oo
-------
147
Table C-1. 1994 National NOX Emissions by Source Category
(thousand short tons)
Source Category
FUEL COMB. -ELECTRIC UTILITY
FUEL COMB. -INDUSTRIAL
FUELCOMB.-OTHER
CHEMICAL & ALLIED PRODUCT MFC
METALS PROCESSING
PETROLEUM & RELATED INDUSTRIES
OTHER INDUSTRIAL PROCESSES
SOLVENT UTILIZATION
STORAGE & TRANSPORT
WASTE DISPOSAL & RECYCLING
ON-ROAD VEHICLES
NON-ROAD SOURCES
MISCELLANEOUS*
Total
Emissions (percent)
Source Category
FUEL COMB. -ELECTRIC UTILITY
FUEL COMB. -INDUSTRIAL
FUELCOMB.-OTHER
CHEMICAL & ALLIED PRODUCT MFC
METALS PROCESSING
PETROLEUM & RELATED INDUSTRIES
OTHER INDUSTRIAL PROCESSES
SOLVENT UTILIZATION
STORAGE & TRANSPORT
WASTE DISPOSAL & RECYCLING
ON-ROAD VEHICLES
NON-ROAD SOURCES
MISCELLANEOUS*
Total
Point
7,795
1,891
100
291
84
95
324
3
3
20
0
0
0
70,604
Point
73.51
17.83
0.94
2.74
0.80
0.89
3.05
0.02
0.02
0.19
0.00
0.00
0.00
100
NO*
Area
0
1,315
627
0
0
0
5
0
0
65
7,530
3,095
374
13,011
NO*
Area
0.00
10.11
4.82
0.00
0.00
0.00
0.04
0.00
0.00
0.50
57.87
23.79
2.88
100
Total
7,795
3,206
727
291
84
95
328
3
3
85
7,530
3,095
374
23,615
Total
33.01
13.58
3.08
1.23
0.36
0.40
1.39
0.01
0.01
0.36
31.88
13.10
1.59
100
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148
Figure C-3. Trends in On-Road Nitrogen Oxides (NOx) Emissions,
Vehicle Miles Traveled (VMT), Fuel Use, and Gasoline Price
200
Percent
150
100
50
m
age of 1970 Value
•
—
-
—
- j
ll
J
II
ll
ll
_ n
I 1
m
i
n
f
hill
1
ri J
-i
1
!i F
1
i
Jrlnr
j
•
[ [ [ f
1 i i
i
i
r
i
r
1
-1
1940 1960
71 1973 1975 1977 1979 1981 1983 1985 1987 1
Year
1 NOx Emissions Fuel Use 1
M VMT Q Gasoline Price
1989 1991
1993
Gasoline prices are stated in constant 1987 price levels.
Source: Energy Statistics Sourcebook
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149
Table C-2. Total National Emissions of Nitrogen Oxides, 1940 through 1994
(thousand short tons)
Source Category
FUEL COMB.-ELEC. UTIL.
Coal
bituminous
subbituminous
Oil
FUEL COMB.-INDUSTRIAL
Coal
bituminous
Gas
natural
FUEL COMB.-OTHER
Residential Other
CHEMICAL & ALLIED PRODUCT MFG
METALS PROCESSING
Ferrous Metals Processing
PETROLEUM & RELATED INDUSTRIES
OTHER INDUSTRIAL PROCESSES
Mineral Products
cement mfg
SOLVENT UTILIZATION
STORAGE & TRANSPORT
WASTE DISPOSAL & RECYCLING
ON-ROAD VEHICLES
Light-Duty Gas Vehicles & Motorcycles
light-duty gas vehicles
Light-Duty Gas Trucks
light-duty gas trucks 1
light-duty gas trucks 2
Heavy-Duty Gas Vehicles
Diesels
heavy-duty diesel vehicles
NON-ROAD SOURCES
Non-Road Gasoline
Non-Road Diesel
construction
industrial
farm
airport service
Aircraft
Marine Vessels
Railroads
MISCELLANEOUS
TOTAL ALL SOURCES
Categories displayed below Tier 1 do not sum
1940
660
467
255
125
193
2,543
2,012
1,301
365
337
529
177
6
4
4
105
107
105
32
NA
NA
110
1,330
970
970
204
132
73
155
NA
NA
991
122
103
70
NA
33
NA
0
109
657
990
7,374
to Tier 1
1950
1,316
1,118
584
288
198
3,792
1,076
688
1,756
1,692
647
227
63
110
110
110
93
89
55
NA
NA
215
2,143
1,415
1,415
339
219
120
296
93
93
1,538
249
187
158
NA
29
NA
2
108
992
665
10,093
totals be<
1960
2,536
2,038
1,154
568
498
4,075
782
533
2,954
2,846
760
362
110
110
110
220
131
123
78
NA
NA
331
3,982
2,607
2,606
525
339
186
363
487
487
7,443
312
247
157
40
50
NA
4
108
772
447
14,140
1970
4,900
3,888
2,112
1,041
1,012
4,325
771
532
3,060
3,053
836
439
271
77
77
240
187
169
97
NA
NA
440
7,390
4,158
4,156
1,278
725
553
278
1,676
1,676
7,628
81
941
599
75
166
78
72
40
495
330
20,625
1980
7,024
6,123
3,439
1,694
901
3,555
444
306
2,619
2,469
747
356
276
65
65
72
205
181
98
NA
NA
111
8,621
4,421
4,416
1,408
864
544
300
2,493
2,463
2,423
102
1,374
854
99
280
113
106
110
731
248
23,281
1990
7,576
6,698
4,600
1,692
210
3,256
613
445
1,656
1,436
772
352
276
87
53
700
306
216
121
2
2
82
7,488
3,437
3,425
1,341
780
561
335
2,375
2,332
2,843
124
1,478
944
125
230
144
139
173
929
373
23,038
;ause they are intended to show maioi
1993
7,773
7,008
4,535
2,054
169
3,797
550
399
1,650
1,440
726
363
286
87
54
95
375
222
124
3
3
84
7,570
3,680
3,668
1,420
828
592
315
2,094
2,047
2,985
122
1,433
1,007
131
256
152
147
183
945
279
23,276
1994
7,795
7,007
4,497
2,098
151
3,206
568
412
1,634
1,427
727
364
297
84
56
95
328
234
131
3
3
85
7,530
3,750
3,737
1,432
830
603
333
2,015
1,966
3,095
125
1,494
1,076
136
265
159
153
188
947
374
23,615
r contributors.
1994 emissions estimates are preliminary and will be updated in the next report.
Tier 1 source categories and emissions are shaded.
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150
Estimated Global Emissions ofNOx
The tables provided earlier in this section describe national emissions of NOX. The table
below provides estimates of global emissions of NOX, expressed in million metric tons of TV
(Schlesinger, 1992). The total estimate is 52.5 million metric tons of N.
Table C-3. Estimated Global Emissions of NOX Typical of the Last Decade
(Million Metric Tons N/year)
Fossil Fuel Combustion
Soil Release
Biomass Burning
Lightning
Ammonia Oxidation
Aircraft
Transport from Stratosphere
24
12
8
5
3
0.4
0.1
NOX Sinks: Removal Processes
The main mechanism that removes NOX from the atmosphere is the conversion of NO and
NO2 to nitric acid (HNO3) and the subsequent dry or wet deposition. The reaction of the OH
radical with NO2 is the major gas-phase route to the formation of FDSTO3 and it is the major
homogeneous gas-phase sink for NOX in the troposphere (NRC, 1991). HNO3 is formed in the
daytime reaction of NO2 with the hydroxyl radical (OH) and in nighttime reactions which form
dinitrogen pentoxide which then hydrolyzes to produce HNO3 (Science and Technical Support
Work Group, 1997). Gaseous HNO3, formed from this reaction, undergoes wet and dry
deposition, including, in some cases, combination with gaseous ammonia to form particulate
phase ammonium nitrate. Dry deposition refers to the uptake of gases and particles at the earth's
surface by vegetation, soil, and water. Wet deposition is the major loss route for atmospheric
NOX (Schneider et al, 1982). Wet deposition refers to the removal of gases and particles from the
atmosphere through incorporation into rain, fog, and cloud water, followed by precipitation to the
earth's surface. Additional radical termination reactions include reaction of NO2 with higher
carbon number peroxy radicals to make organic nitrates and with peroxyacyl radicals to make
PAN (Jeffries, 1995).
The oxidations of NOX leading to removal by HNO3 or nitrates take place in hours to days,
during which time the NOX compounds participate in a number of reactions in which they switch
back and forth between the various intermediate NOX (Roberts, 1995; Jeffries, 1995). The
lifetime of NOX (due to chemical conversion to HNO3) is estimated to be 1-2 days (EPA, 1993),
while the residence time for nitrate is estimated to be 1-3 days (Schwartz, 1989). In the summer,
the tropospheric lifetime of NO2 (due to chemical conversion to HNO3) is estimated of the order
of one day at mid-latitudes (Schneider et al, 1982).
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151
Sources of Reduced N
As briefly noted in portions of this document, reduced N compounds—ammonia (NH3) and
ammonium (NH4+)~are also critical to many of the public health and environmental impacts
associated with atmospheric N compounds. Emissions from fertilized agriculture and domestic
animals account for over half the total estimated global NH3 budget of the atmosphere reflecting
the level of direct human impact in this area (Schlesinger and Hartley, 1992). The major sink for
atmospheric NH3 is conversion to NH4+, which is deposited in dry particles or as a dissolved ion in
precipitation.
Estimated Global Emissions ofNH3
The table below provides estimates of global emissions of NH3, expressed in million metric
tons of TV (Schlesinger and Hartley, 1992). The total estimate is 75.2 million metric tons of N per
year.
Table C-4. Estimated Global Emissions of NH3 (Million Metric Tons N/year)
Domestic Animals
Sea Surface
Undisturbed Soils
Fertilizers
Biomass Burning
Human Excrement
Coal Combustion
Automobiles
32
13
10
9
5
4
2
0.2
References
National Research Council, Committee on Tropospheric Ozone Formation and Measurement,
Rethinking the Ozone Problem in Urban and Regional Air Pollution., 1991.
Jeffries, H., "Photochemical Air Pollution" in Composition, Chemistry and Climate of the
Atmosphere, edited by H. Singh, Van Nostrand Reinhold, NY, 1995, ISBN 0-442-01264-0.
Roberts, J.M., "Reactive Odd-Nitrogen (NOy) in the Atmosphere" in Composition, Chemistry
and Climate of the Atmosphere, edited by H. Singh, Van Nostrand Reinhold, NY, 1995, ISBN 0-
442-01264-0.
Schlesinger, W. H. and A. E. Hartley, "A Global Budget for Atmospheric NH3," Biochemistry 15:
191-211, 1992.
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152
Schneider, T., and L. Grant, (Editors) Air Pollution by Nitrogen Oxides, 1982, Elsevier Scientific
Publishing Company, Amsterdam, The Netherlands, p.252.
Schwartz, S.E., "Acid Deposition: unraveling a regional phenomenon," Science, 243:753-763,
1989.
U.S. Environmental Protection Agency, Air Quality Criteria for Ozone and Related
Photochemical Oxidants, external review draft EPA/600/AP-93/004a, 1993.
U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, National
Air Pollutant Emissions Trends, 1900-1994, EPA-454/R-95-011, October 1995.
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153
D.
CAA:
CO2:
HNO3:
N:
NAAQS:
NH3:
NH4+:
N2O:
NO:
NO2:
NOX:
02:
03:
PPB:
PPM:
PM:
-2.5
10
PM
PM
PSD:
SIPs:
SOX:
SO2:
VOCs:
Acronyms and Abbreviations
Clean Air Act
Carbon dioxide
Nitric acid
Nitrogen
National ambient air quality standards
Ammonia
Ammonium
Nitrous oxide
Nitric oxide
Nitrogen dioxide
Sum of NO and NO2
Oxygen
Ozone
Parts per billion
Parts per million
Particulate matter; refers to a solid or liquid material that is suspended in the
atmosphere.
Particles with an aerodynamic diameter less than or equal to 2.5 microns
Particles with an aerodynamic diameter less than or equal to 10 microns
Prevention of Significant Deterioration
State implementation plans
Sulfur oxides
Sulfur dioxide
Volatile organic compounds
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