United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-88/009 Nov. 1988
v°/EPA Project Summary
Comparison of Historic SO2 and
NOX Emission Data Sets
Gerhard Gschwandtner, Janice K. Wagner, and Rudolf B. Husar
During the past few years, several
research projects have been conducted
to reconstruct historic air pollution emis-
sion trends in the U.S. This report com-
pares in detail the emission estimates
derived by Gschwandtner et al. and
Husar and attempts to determine the
methodoltgical differences that result in
different emission estimates. In addition,
the U.S. EPA historical trends, the elec-
tric utilities emissions estimates, and the
NAPAP monthly trends inventory are
compared to Gschwandtner's inventory.
The inventories' differing objectives and
reasons for the slightly varying emis-
sions estimates are explained. On the
national level, all estimates are in
reasonable agreement. One objective of
this study was to determine a basis for
changing any of the existing estimates
in order to minimize or eliminate un-
necessary differences in reported na-
tional total emissions for common years.
A revised set of NOX and SO2 emission
estimates, based primarily on
Gschwandtner and EPA trends, is recom-
mended for use in future analyses utiliz-
ing historic emission estimates.
This Project Summary was developed
by EPA's Air and Energy Engineering
Research Laboratory, Research Triangle
Park, NC, to announce key findings of the
research project that is fully documented
in a separate report of the same title (see
Project Report ordering information at
back).
Introduction
The National Acid Precipitation Assess-
ment Program (NAPAP) was established by
Congress to conduct research on the
causes and effects of acid precipitation. As
part of this program, NAPAP's Emissions
and Controls Task Group was formed to
provide information on anthropogenic
sources of air pollution emissions. Several
types of emission inventories have been
produced to meet the various research
needs of other task groups studying dif-
ferent aspects of the acid precipitation
phenomenon. These include an extensive
inventory for 1980 and historic inventories
that span various periods of time. The
historic inventories developed under
NAPAP and the EPA support are sum-
marized in Table 1. In addition, an inven-
tory of historic SOZ and NO* emissions
was prepared independently by Husar for
the National Academy of Sciences.
Each of these inventories was developed
to meet a different research objective. Each
was based on a methodology most ap-
propriate to achieving that objective. As a
result, the methods and estimating techni-
ques are slightly different, yet they yield
similar estimating techniques are slightly
different, yet they yield similar estimates of
emission trends. This report attempts to
identify the reasons for the differences and
to resolve them.
Different Historic Sets
The major objective of each set of historic
emissions estimates is described below.
G. Gschwandtner
The main objective of this inventory was
to produce historic state level emission
estimates of SO2 and NOX in a consistent
manner that could be used as a common
data set by various research groups in
NAPAP.
R.B. Husar
The main objective was to develop
methods for determining the trend in
historic sulfur emissions and to explore the
uncertainty of the emission estimates us-
ing various alternative methods and
information.
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Table 1. Available Historic Emissions Estimates
Pollutant
SOz, NOX
SOa NOX
All Criteria
Pollutants
SOz, NOX
SO* A/Ox
Categories
All
All
All
Electric
Utilities
Utilities
Non-Utilities
Period
1900-1980
1890-1981
1940-1985
1975-1983
1975-1984
Temporal
Resolution
Annual
Annual
Annual'
Annual
Monthly
Geographic
Resolution
State Level
State Level
* National
Plant Level
State Level
Principal
Author
G. Gschwandtner
R.B. Husar
U.S. EPA
E.H. Pechan
D.A. Knudson
'Available for 1940, 1950, 1960, 1965, and yearly from 1970 to 1985.
US. EPA
The main objective is to update the na-
tional emission trend for all criteria
pollutants on an annual basis for previous
years.
E.H. Pechan
The main objective of this file is to pro-
vide accurate SO2 and NOX emission
estimates for individual power plants.
D.A. Knudson
The objective of this inventory is to pro-
vide monthly, state level emission trends of
SO2 and NOX.
Given different objectives, different
technical approaches were developed.
Each approach necessarily required dif-
ferent input information. As a result, the
total aggregated emissions estimated by
each study are similar, but not identical.
Figures 1 and 2 compare the total national
SO2 and NOX emissions reported by the
different studies. Figure 3 presents the
revised emission estimates of Gschwand-
tner which reflect the EPA trends data.
This report examines the differences bet-
ween the various historic emission
estimates in detail, in order to understand
and explain the differences. In addition, an
attempt has been made to estimate the
uncertainty of historic national emissions
estimates.
Conclusions
1. The approaches used by Husar and
Gschwandtner to develop historic SO2
and NOX emissions are comparable,
and their estimates are similar when ad-
justed for differences in source category
accounting.
2. Sulfur content of coal mined and ship-
ped is about 20% higher than the sulfur
of coal consumed, according to informa-
tion reported by the U.S. Department of
Energy. This discrepancy could not be
resolved in this study. Further investiga-
tion is needed to determine if, in fact,
Husar tGschwandtner ? EPA t Knudson
1880 90 1900 1O 20 30 40 50 60 70
/ear
Figure 1. Comparison of total SOs emissions estimated by different studies.
more sulfur has been emitted than is ac-
counted for by consumption-based
sulfur data.
3. The uncertainty methodology and the in-
put data required for uncertainty estima-
tion should be refined for application to
the historic emissions inventory.
4. An analysis of the uncertainty associated
with total national SO2 emissions sug-
gests that the uncertainty is primarily af-
fected by uncertainty in coal sulfur con-
tent. Applying this preliminary uncertain-
ty analysis to the methods used by
Husar and Gschwandtner, as an il-
lustrative example of uncertainty estima-
tion, results in a calculated range of 31
to 45% uncertainty in these historic SO2
emission estimates.
6.
A similar analysis of the uncertainty
associated with total national NOX emis-
sions suggests that this uncertainty is
dominated by the uncertainty in historic
NOX emission factors. Applying this
preliminary uncertainty analysis to the
methods used by Husar and Gschwand-
tner, as an illustrative example of uncer-
tainty estimation, results in a calculated
range of 16 to 38% uncertainty in these
historic NOX emission estimated.
Because differing historic emission
estimates have caused confusion
among NAPAP researchers, the
historical state-level emission estimates
of Gschwandtner have been adjusted so
that the aggregated total is identical to
the U.S. EPA national emission
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— Husar A- Gschwandtner y EPA A Knudson
90 1900 10 20 30 40 50 60 70
Year
Figure 2. Comparison of total /VOX emissions estimated by different studies.
estimates, less states and territories out-
side the contiguous U.S.
7. For 1986, the U.S. EPA trends
methodology was changed so that it ac-
counts for the same FGD removal as the
Pechan electric utility emision estimates.
The revised methodology will also be ap-
plied to future years. For years before
1986, the national emission factors ob-
tained from the Pechan inventories were
incorporated in the trends calculations.
As a result, the U.S. EPA trends for elec-
tric utilities and the Pechan estimates
are identical.
8. The users of historic emissions data
should understand the differing pur-
poses of the various inventories and
select the best inventory.
As a result of the present study, all emis-
sion inventories compared are in agree-
ment regarding the total national emissions.
While some differences have existed in the
past regarding the exact quantity of total na-
tional emissions, the differences have been
studied and adjustments have been made
to yield a single annual national total emis-
sion estimate for NAPAP. As a result of this
comparitive study, the adjusted inventory
presented in report Appendix A is recom-
mended for use by NAPAP for the best
estimates of long-range (back to 1900) an-
nual emission trends.
Year
Figure 3. Revised SO? and /V0« historical emission estimates.
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Gerhard Gschwandtner and Janice K. Wagner are with E. H. Pechan and
Associates, Inc., Durham, NC 27707; and Rudolf B. Husar is with Washington
University, St. Louis, MO 63130.
J. David Mobiey is the EPA Project Officer (see below).
The complete report, entitled "Comparison of Historic SO2 and /VOX Emission
Data Sets." (Order No. PB 89-103964/AS; Cost: $19.95, subject to change);
and a related diskette (Order No. PB 89-103956/AS; Cost $75.00, subject
to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
' '•' u,
Official Business
Penalty for Private Use $300
EPA/600/S7-88/009
0000529 PS
CHICAGO
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