United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-94/012 March 1994
EPA Project Summary
Comparison of the 1985 NAPAP
Emissions Inventory with the
1985 EPA Trends Estimate for
Industrial SO2 Sources
David Zimmerman and Rebecca Battye
Section 406 of the 1990 Clean Air Act
Amendments (CAAA) requires that the
Administrator of the Environmental Pro-
tection Agency (EPA) transmit a report
to Congress containing a national in-
ventory of annual sulfur dioxide (SO2)
emissions from industrial sources, as
well as emission projections for the
next 20 years, not later than 1995. The
requirement stems from the 5.6 million
ton emissions cap on SO2 from indus-
trial sources that is contained in Title
IV of the CAAA and based on esti-
mated 1985 emissions from the National
Acid Precipitation Assessment Program
(NAPAP). This report presents analy-
ses of 1985 industrial SO2 emissions
from two available data sources: the
NAPAP inventory and the EPA Trends
report. These analyses conclude that
the two data sources estimate compa-
rable emissions in the aggregate, but
estimates for specific categories and
for processes within those categories
vary widely. The Trends method is
limited to source categories that emit
10,000 tonnes per year of SO2. In gen-
eral, the Trends method overestimates
emissions from these source catego-
ries due primarily to the absence of
SO2 control efficiency assumptions.
Overestimation of emissions in the
Trends data set is offset by the inclu-
sion of additional source categories in
the NAPAP inventory with the final ag-
gregate estimates within less than 10%
of each other. (NOTE: Trends method-
ology is being changed for 1993, using
the 1985 NAPAP emissions inventory
as a base.)
This Project Summary was developed
by EPA's Air and Energy Engineering
Research Laboratory, Research Tri-
angle Park, NC, to announce key find-
ings of the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
Section 406 of the 1990 Clean Air Act
Amendments (CAAA) requires that the
Administrator of the Environmental Pro-
tection Agency (EPA) transmit a report to
Congress containing "a national inventory
of annual sulfur dioxide emissions from
industrial sources not later than January
1, 1995 for all years for which data are
available, as well as the likely trend in
SO2 emissions over the following twenty
year period (1995 to 2015)." The CAAA
also establishes an emissions cap of 5.6
million tons per year from industrial
sources; this annual cap is equivalent to
the 1985 industrial SO2 estimate from the
National Acid Precipitation Assessment
Program (NAPAP) inventory. To provide
the analysis mandated by Congress, the
1985 baseline data must first be exam-
ined to identify strengths and weaknesses
in the available emission and supporting
data. The purpose of the overall study
was to understand the similarity and dif-
ferences between existing data sets and
determine which data are suitable to serve
as a baseline for the SO2 emission projec-
tions. This paper presents the initial analy-
sis of two major sources of industrial data
currently available: the 1985 NAPAP emis-
sion inventory and the 1985 national emis-
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sion estimates, referred to as the Trends
emission estimates.
Overview of the Data Sets
The 1985 NAPAP emission inventory
effort supported acid deposition research,
including atmospheric modeling, through
comprehensive, detailed source emission
estimates provided by local and state
agencies. It is a bottom-up inventory and
a 1985 snapshot. The SO2 emission data
for significant (>100 tons per year) sources
were systematically quality assured, with
greater effort expended on larger sources.
The inventory included a unique confirma-
tion step, allowing individual plants emit-
ting at least 2500 tons per year to review
their emission estimates prior to finaliza-
tion.
Prior to 1993, EPA prepared an annual
emissions Trends report representing both
current and historic emissions (1940 to
present). Industrial emission estimates
were derived from national, published ac-
tivity data and standard, process-level
emission factors; historic estimates were
altered based on the most recent activity
data and emission factors to better repre-
sent the most current understanding of
emission processes. For the industrial
sector, activity data were obtained prima-
rily from the Department of Energy, En-
ergy Information Administration; the De-
partment of the Interior, Bureau of Mines;
and the Department of Commerce, Bu-
reau of Census. It is essentially a top-
down approach designed to follow cat-
egory emissions through time and not a
true inventory. It presents a consistently
derived national emission estimate at the
emission category level (e.g., industrial oil
combustion) rather than the source (e.g.,
boiler) level.
Due to findings discussed here, as well
as other factors, the Trends methodology
has been revised as of 1993. References
to Trends in this paper will no longer be
valid for years 1985 and beyond, effective
with the 1993 edition of the Trends report.
The reader is cautioned that comments
herein on the Trends report are valid only
for editions prior to 1993.
Methodology
The 1985 NAPAP and Trends industrial
emission estimates were directly com-
pared. The 1985 NAPAP-published emis-
sion estimates as well as the annual U.S.
point source file were used as the basis
for the analysis. The 1992 report National
Air Pollutant Emission Estimates, 1900-
1991, and the background procedures
(Procedures Document for the Develop-
ment of National, Regional and Prelimi-
nary Air Pollutant Emissions Trends Re-
port) formed the basis of the analysis of
the published Trends estimates. (Note
that the Trends estimates are updated
annually; the Trends procedures them-
selves have also been revised in 1993.
This analysis uses the Trends procedures
used through 1992, but does not include
the new Trends procedures that will be
reflected in the forthcoming 1993 report.)
This study of industrial SO2 emissions veri-
fied emission estimates and activity (i.e.,
throughput) where possible based on the
original source materials.
The comparison of NAPAP and Trends
analyzed the derivation of individual in-
dustrial category estimates. Such analy-
ses were complicated by several factors:
• NAPAP is comprehensive and in-
cludes all reported industrial emission
categories; Trends is limited to cat-
egories thought to emit at least 10,000
tonness SO2 (uncontrolled) annually.
• NAPAP is source and plant specific;
Trends is national and category spe-
cific. No opportunity exists to match
individual data values between the
inventories; in fact, category defini-
tions differ between the two invento-
ries.
• The 1985 NAPAP inventory is a
single-year inventory and is not up-
dated; Trends adjusts historical emis-
sion estimates based on the most
current information.
Results of the Comparison
This research led to a highly detailed
view of the two sets of emission estimates
on a category basis, principally relying on
emission and activity (throughput) data.
The authors attempted to reproduce the
1985 Trends emissions estimates and
noted any irregularities between the cal-
culated and published data. The analy-
ses proved complex, especially when dis-
aggregating data to create comparable
categories between NAPAP and Trends
data sets, and raised a number of ques-
tions.
Table 1 summarizes the differences be-
tween the NAPAP and Trends estimates.
Overall, the two 1985 estimates (NAPAP
and Trends) compare favorably: NAPAP
estimates 5.6 million tons SO2 (as reflected
in the CAAA), and Trends estimates 6.0
million tons. However, Table 1 shows
distinct variability in the category-level es-
timates between the two data sets. The
major industrial SO2 emission producing
categories, industrial combustion, non-fer-
rous metals, iron and steel, petroleum re-
fineries, oil and natural gas production,
pulp and paper, and cement manufactur-
ing are discussed in detail in the report.
Summary
As these two data sets are reviewed
and compared, the genesis and purpose
of each methodology must be considered.
The 1985 NAPAP inventory was a com-
prehensive emissions inventory designed
to reflect actual conditions in 1985, with
great emphasis on the gathering and qual-
ity assurance of the emission estimate at
the plant and process level. It was as-
sembled mainly at the state and local level
by professionals familiar with the individual
sources and with significant review by the
major sources. The Trends methodology
was designed to provide a longitudinal
view of the direction and probable magni-
tude of emission changes. It was as-
sembled from standard emission factors
and national activity data in most cases
and was not intended to reflect actual
conditions for individual plants or source
types at any given time. A de minimus
level of 10,000 tonnes SO2 (uncontrolled)
was established for inclusion of a source
category, so Trends is not intended to be
comprehensive. The Trends methodol-
ogy reviewed and presented here is cur-
rently in transition; subsequent Trends re-
ports will reflect a fundamental shift in the
estimation approach.
The two 1985 estimates, Trends and
NAPAP, provide reasonable agreement,
within less than 10%, when viewed as an
aggregate industrial SO2 estimate. There
is greater divergence when the data sets
are compared at the category and subcat-
egory levels. Because the data sets were
developed from different data sources and
with different purposes, disagreement at
disaggregated category levels primarily
reflects the inherent differences in the
methodologies.
The overall agreement appears to be
due to a balance between Trends overes-
timates where subcategories overlap be-
tween Trends and NAPAP and the inclu-
sion of more categories and more indi-
vidual emission points in NAPAP. The
systematic Trends overestimate, relative
to the NAPAP inventory, is due in part to
the absence of SO2 control efficiency esti-
mates in the methodologies. SO2 control
technologies have been applied to most
of the large industrial SO2 categories
through the promulgation of New Source
Performance Standards (NSPS), issuance
of operating permits, and New Source
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Table 1. Magnitude Differences Between 1985 Trends and NAPAP SO2 Emission Estimates
Source Category *
Coal"
Oilc
Natural Gas d
Wood
Miscellaneous Fuel
Other Fuel Combustion Emissions
Reported through NAPAP
1 ° Copper
1° Lead and Zinc
2° Lead
1° Aluminum
Other Primary and Secondary
Metals Emissions Reported through NAPAP
Iron and Steel
Iron and Steel Foundries
Oil and Natural Gas Production
Pulp and Paper
Cement
Glass
Lime
Sulfuric Acid
Carbon Black
Petroleum Refineries
Other Industrial Process Emissions
Reported through NAPAP
Total
Trends
(tons)
1,840,000
540,000
0
10,000
80,000
650,000
240,000
30,000
70,000
360,000
160,000
250,000
620,000
30,000
30,000
210,000
10,000
830,000
5,960,000
NAPAP
(tons)
1,721,000
713,000
33,000
42,000
14,000
74,000
655,000
106,000
21,000
58,000
42,000
204,000
16,000
332,00
130,000
291,000
23,000
32,000
217,000
28,000
640,000
220,000
5,612,000
Delta
(tons)
119,000
-173,000
-33,000
-32,000
66,000
-74,000
-5,000
134,000
9,000
12,000
-42,000
156,000
-16,000
-172,000
120,000
329,000
7,000
-2,000
-7,000
-18,000
190,000
-220,000
348,000
Delta
(percent)
6.9
-24.3
-100.0
-76.2
471.4
-100.0
-0.8
126.4
42.9
20.7
-100.0
76.5
-100.0
-51.8
92.3
113.1
30.4
-6.3
-3.2
-64.3
29.7
-100.0
6.2
" Except where noted, the emissions for a source category represent process level emissions only and do not include emissions from the combustion of fuel.
b Excludes bitumnous coal and lignite consumed at cement and lime manufacturing facilities.
0 Excludes both distillate and residual oil consumed at cement plants and petroleum refineries and residual oil consumed at iron and steel mills.
d Excludes natural gas consumed in cement manufacturing, petroleum refining, the iron and steel industry, glass manufacture, and at crude petroleum and natural gas production
facilities
Review Permits. The strengths and weak-
nesses revealed in this review are pre-
sented below.
NAPAP Data Set
• The 1985 NAPAP inventory still rep-
resents the most comprehensive and
accurate emissions estimates for 1985
because of its rigorous quality assur-
ance of emissions and bottom-up na-
ture. The inventory accounts for indi-
vidual source operating characteris-
tics, controls and emission factors.
• Activity data in the 1985 NAPAP in-
ventory were not subject to the same
standard of quality assurance or com-
pleteness. Some data were unre-
ported due to confidentiality restric-
tions, and activity data for small
sources (i.e., <100 tons per year)
passed only the grossest quality as-
surance checks. There are known
reporting problems among miscella-
neous fuels and other categories. The
accuracy and representativeness of
activity data in the NAPAP inventory
are best evaluated source by source;
category-level summaries are unreli-
able without adjustments.
• It is still possible to locate question-
able data values in the 1985 NAPAP
emission inventory when examined on
a source by source basis, especially
for smaller emitters.
Trends Data Set
• Some industrial emission categories,
notably iron and steel foundries, are
missing from the Trends method. As
such, the inventory is not compre-
hensive, although few major gaps
were found under close scrutiny.
As a top-down approach, broad as-
sumptions of emission factors and
controls are used across a category.
Frequently, estimates make no ad-
justment for controls. Accommodat-
ing individual source operating char-
acteristics, including emission factors,
is impossible.
The underlying industrial activity data
are largely reliable and probably far
superior to the corresponding NAPAP
estimates at the category level. Any
method for 1995 and beyond should
take advantage of these independent
data sources.
Based on the Trends documentation,
the actual Trends execution contains
minor to moderate errors in calcula-
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tion of activity and emissions. Trends
could also benefit from recently re-
vised standard emission factors and
updated sources of activity data.
Conclusion
The 1985 NAPAP SO2 emission esti-
mates remain the most reliable emission
estimate baseline because that was one
of the intentions of the inventory effort.
Reliable emission projections of the 1985
data, especially to 1995 and beyond, are
difficult because the activity data in the
NAPAP inventory are incomplete due, in
part, to confidentiality and other restric-
tions. Because it is a 1985 inventory,
emission factors and activity data do not
reflect current or future operations for these
source categories.
The Trends methodology reflected in
this review is poorly suited to provide a
baseline estimate for industrial emissions,
fundamentally due to its design and pri-
mary objectives. Its strengths lie in the
identification and use of the underlying
industrial activity data which would pro-
vide a firm foundation for year-to-year pro-
jection of specific industrial activities and
their baseline emissions.
Results using the new Trends method
could not be reviewed in the scope of this
research. Due to findings discussed here,
as well as other factors, the Trends meth-
odology has been revised as of 1993.
References to Trends in this paper will no
longer be valid for years 1985 and be-
yond, effective with the 1993 edition of
the Trends report. The reader is cau-
tioned that comments herein on the Trends
report are valid only for editions prior to
1993.
Several additional sources of data that
may provide more recent data and fill data
gaps for an analysis of industrial SO2 emis-
sions were identified: Information Collec-
tion Requests obtained in support of the
Maximum Achievable Control Technology
(MACT) standards developed under Title
III of the CAAA, SO2 state implementation
plan (SIP) inventories, and ozone/carbon
monoxide SIP inventories. Of these, the
MACT data are the most promising be-
cause many of the significant industrial
SO2 source categories are under MACT
development; the data collected from the
sources have been targeted for and will
therefore be accessible from the
Aerometric Information Retrieval System;
and major sources, production data, and
control equipment information can be iden-
tified from the data collected.
Finally, the 1985 NAPAP data were ana-
lyzed to determine whether individual
source tracking would be a feasible option
to improve the reliability of the emission
estimates and the projections through time.
Such an option would improve and up-
date the baseline estimate and provide
current information on processes and con-
trol equipment. It could also be used with
the industrial activity data available from a
variety of sources to project the remain-
der of the inventory. Figure 1 shows that
relatively few sources (about 130) account
for approximately 50% of the total indus-
trial SO2 emissions. To capture 80% of
the SO2 emissions, approximately 500
sources would need to be inventoried.
5,000,000 _
4,000,000 -
CO
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D. Zimmerman andR. Battye are with EC/R, Inc., University Tower, Suite 404, 3101
Petty Road, Durham, NC 27707.
Charles C. Masser is the EPA Project Officer (see below).
The complete report, entitled "Comparison of the 1985NAPAP Emissions Inventory
with the 1985 EPA Trends Estimate for Industrial SO2 Sources," (Order No.
PB94-152220/AS; Cost: $36.50, 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 Researchlnformation
Cincinnati, OH 45268
Official Business
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