United States
Protection
Agency
AIR
»EPA
Office of Air Quality
Planning and Standards
Research Triangle Park, NC 27711
EPA-454/R-93-024
JULY 1993
The Role of Ozone Precursors in Tropospheric
Ozone Formation and Control
A Report to Congress
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THE ROLE OF OZONE PRECURSORS IN TROPOSPHERIC
OZONE FORMATION AND CONTROL
A Report to Congress
U.S.. ENVIRONMENTAL PROTECTION AGENCY
Office of Air Quality Planning and Standards
Research Triangle Park, North- Carolina 27711
July 1993 , „
Chicago, IL eCo' J
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DISCLAIMER
This report has been reviewed by the Office of Air Quality
Planning and Standards, U.S. Environmental Protection Agency, and
has been approved for publication. Any mention of trade names or
commercial products is not intended to constitute endorsement or
recommendation for use.
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Table of Contents
page
Section 1 - Introduction 1-1
1.1 1990 Clean Air Act Requirements 1-1
1.2 Report Structure - roles of NAS and EPA 1-1
Section 2 - Key Ozone Control Strategy Issues 2-1
2 .1 Background 2-1
2.2 Ozone Precursor Control Strategies 2-1
2.3 Supporting Data Bases and Air Quality Modeling....2-5
Section 3 - EPA Perspectives on the NAS Findings 3-1
3.1 Ozone in the United States 3-1
3 . 2 Ozone Trends 3-3
3.3 State Implementation Planning 3-6
3 . 4 Anthropogenic VOC Emissions 3-8
3 . 5 Biogenic VOC Emissions 3-12
3 . 6 Ambient Air Quality Measurements 3-15
3 . 7 Air Quality Models 3-18
3 . 8 VOC versus NOx Control 3-23
3 . 9 Alternative Fuels for Motor Vehicles 3-29
3.10 A Research Program on Tropospheric Ozone 3-32
Section 4 - Overview of NOx Control Technologies 4-1
4 .1 Introduction • 4-1
4.2 Source category specific control techniques 4-4
4 . 3 Summary 4-8
Section 5 - Summary 5-1
Section 6 - References 6-1
Attachment 1 - Rethinking the Ozone Problem in Urban and
Regional Air Pollution
Attacnment 2 - The Avaiiaoility and Extent of NOx Controls
Attachment 3 - Summary of Public Comments and EPA Responses
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SECTION 1 - INTRODUCTION
1.1 1990 Clean Air Act Requirements
This Report responds to the requirements of Section 185B of
the 1990 Clean Air Act Amendments (CAAA). In this section, the
Act requires that "The Administrator, in conjunction with the
National Academy of Sciences, shall conduct a study on the role
of ozone precursors in tropospheric ozone formation and control."
Specifically, the Act requires that the EPA submit a Report to
Congress that addresses the following topics:
1. the roles of oxides of nitrogen (NOx) and volatile
organic compounds (VOC) emissions reductions,
2. the extent to which NOx reductions may contribute (or
be counterproductive) to achieving attainment in
different nonattainment areas,
3. the sensitivity of ozone to control of NOx,
4. the availability and extent of controls for NOx,
5. the role of biogenic VOC emissions, and
6. the basic information required for air quality models.
Findings from this Report must be considered in cases where
the Administrator makes determinations regarding the
applicability of Section 182(f) requirements for stationary
source NOx control in approving State Plan provisions. Section
182(f)(3) also provides that a person may petition EPA for such
determinations only after the final Report is submitted to
Congress.
1.2 Report Structure - roles of NAS and EPA
The National Academy of Sciences (NAS) report entitled
Rethinking the Ozone Problem in Urban and Regional Air Pollution
is a comprehensive review of the science underlying the topics
listed under Section 185B, and should be considered as an
integral part of this 185B Report. The NAS report is included as
Attachment 1 of this Report. Attacnment 2 is an SPA report
addressing the availability and extent of NOx controls, a topic
.:ot addressed in tne NAS report. A draft 185B Report was suoject
to a 30-day public review and comment period. Attachment 3
includes a summary of public comments and EPA responses. For
clarification, the complete Report including the three
attachments is referred to herein as either the "Joint Report" or
the "185B Report."
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The Environmental Protection Agency (EPA) urged development
of the NAS study in 1987 and provided partial funding from 1989
through 1991. Other sponsors included the Department of Energy
with additional support from the American Petroleum Institute and
the Motor Vehicle Manufacturers Association. The study was
conducted by the Committee on Tropospheric Ozone Formation and
Measurement established in 1989 by the Board on Environmental
Studies and Toxicology of the National Research Council (NRC) in
collaboration with the NRC's Board on Environmental Sciences and
Climate. The Committee's members are experts in the fields of
atmospheric chemistry, measurement, mathematical modeling,
meteorology, exposure assessment, air-pollution engineering, and
environmental policy. The Committee's report was released in
December, 1991.
The remaining sections of this Report include:
Section 2 - Providing an EPA overview of key ozone control
strategy issues, emphasizing the NOx issues listed in Section
185B of the CAA;
Section 3 - Providing EPA perspectives on the ten main NAS
findings in Attachment I (i.e., the NAS report); and
Section 4 - Providing a summary of Attachment 2, the NOx control
technology report.
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SECTION 2 - KEY OZONE CONTROL STRATEGY ISSUES
2.1 Background
Tropospheric1 ozone pollution, which occurs at ground level
and is the major component of ground-level summertime "smog,"
remains an important environmental and health concern despite
nearly 20 years of regulatory efforts. Based on data collected
from 1988-1990, roughly 100 metropolitan areas were classified
nonattainment (40 CFR,- part 81) of the ozone National Ambient Air
Quality Standard (NAAQS).
Ozone is a "secondary" pollutant formed in the atmosphere by
reactions of volatile organic compounds (VOCs) and oxides of
nitrogen (NOx) in the presence of sunlight. Carbon monoxide (CO)
also plays a role in the formation of ozone. Major sources of
VOCs include exhaust and evaporative emissions from motor
vehicles, emissions from solvent use and emissions from the
chemical and petroleum industries. In addition, there is now a
heightened appreciation of the importance of VOCs emitted by
vegetation (biogenic emissions). NOx and CO come mainly from
combustion; major sources include motor vehicles and electricity
generating stations.
Formation of ozone in the atmosphere involves complex
nonlinear processes, adding to the difficulty of identifying
effective control strategies. Scientific knowledge continues to
evolve at a rapid pace. Recent scientific information has
resulted in increased focus on the role of reducing NOx emissions
in lowering ozone concentrations. Previous scientific studies
and air pollution control programs emphasized VOC reductions as
the primary approach to ozone control. The following EPA
perspectives identify two key components (strategy selection;
modeling and data bases) which must be addressed in resolving the
tropospheric ozone problem.
2.2 Ozone Precursor Control Strategies
2.2.1 Evolving Perspective on NOx and VOC controls
Previous ozone precursor control programs emphasized VOC
reductions to achieve the ozone NAAQS. The best available
scientific evidence at the time suggested that VOC reductions
were preferred in most instances. The VOC control approach was
reinforced Dy the fact chat: NOx reductions couid in some cases
increase ozone.
:The term "tropospheric ozone" refers to ozone occurring from
ground level through the first several kilometers of the atmosphere
and is not to be confused with high altitude (i.e., roughly 15 - 30
km) "stratospheric ozone."
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Several recent studies discussed in this Joint Report
indicate that NOx controls may lead to greater ozone benefits
than previously thought. This apparent shift coincides with
improved data bases and modeling techniques that provide the
analytical means to evaluate the effectiveness of ozone precursor
control strategies. Particularly noteworthy is the important
role of biogenic emissions in control strategy calculations.
This emerging knowledge combined with the recognition that
significant numbers of ozone nonattainment areas remain, implies
a need for using sophisticated approaches to develop effective
control strategies.
2.2.2 Need for Area-Specific Analyses
While the recent shift in attention toward NOx control may
be justified, a synthesis of findings from all studies suggests
that a nationally based control approach - - whether NOx-only or
VOC-only or combined VOC + NOx - - is not likely to be an
efficient means for reducing ozone everywhere. The problem is
complex and ozone response to precursor control can vary greatly
with each area. The following facts present a simplified
description of how ozone responds to changes in VOC or NOx:
NOx change
ozone levels initially can increase or decrease with
respect to NOx controls.
However, in cases where ozone might initially increase
in response to small NOx reductions, ozone levels
eventually will decline if NOx levels are reduced more
substantially.
VOC change
VOC reductions generally reduce ozone, although
conditions exist where the degree of reduction can be
minimal.
ozone levels rarely increase when VOC is reduced.
The body of knowledge based on over 20 years of research and
applied studies (Attachment 1) further suggests:
1. NOx controls generally are more effective than VOC
control in areas where ambient VOC/NOx ratios are
relatively large. For example, rural areas and smaix
to moderate sized urban areas in the eastern U.S.
generally are characterized by such ratios.
2. VOC controls generally are more effective than NOx
controls in areas where ambient VOC/NOx ratios are
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relatively small. For example, the central core
sections of large urban areas (New York and Los
Angeles) generally are characterized by such ratios.
3. From the perspective of simple smog chamber
experiments, combined VOC and NOx reductions are as a
rule less effective than reducing either one alone. In
the real world, however, the complexity of geographical
and temporal variations in meteorology, source
composition, and control feasibility often make
combined VOC and NOx control the option of choice.
4. Ambient data bases generally are not available to
adequately characterize these VOC/NOx ratios, which can
vary substantially with space and time in an urban area
and its surroundings.
5. Ambient VOC/NOx ratios are influenced by emissions,
pollutant inflow from surrounding areas, transformation
through atmospheric chemical reactions, deposition, and
given the non-uniformity of sources in urban areas and
surrounding regions, by meteorology also.
6. Ozone response to precursor changes is influenced by
meteorology and can exhibit day-specific responses.
7. Gridded photochemical air quality simulation models
provide the most comprehensive treatment of processes
responsible for ozone formation and provide a means for
estimating changes in ozone due to hypothesized VOC and
NOx reductions.
8. High quality emissions, air quality and meteorological
data bases are critical for deriving credible model
conclusions.
As one proceeds from a central-city location downwind to the
suburban and rural fringes, a gradual change from relatively low
to high VOC/NOx ratios occurs. This pattern arises, in part,
from a greater concentration of NOx emissions sources (e.g.,
automobile exhaust and various commercial and industrial
activities) in central-core regions and attendant influences
associated with relatively faster depletion of NOx during
downwind transport. Thus, when NOx reductions are applied in
large urban areas, greater ozone reductions are likely to be seen
several kilometers downwind of the central urban-core, relative
-3 tine central-core. This is 30, oecause NOx availability
becomes the limiting factor in further ozone production when the
VOC/NOx ratio is high.
This highly generalized spatial response pattern has
important implications for population exposure to ozone arising
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from NOx controls, and for the long-distance transport of ozone.
Conclusions based on just an analysis of peak hourly ozone may
not always apply to other important measures such as total
population exposed to high concentrations of ozone. Certain
modeling studies have suggested that NOx controls result in less
reduction of total ozone exposure relative to VOC controls in the
New York City and Los Angeles airsheds. However, NOx controls
appear to have a greater effect than do VOC controls in reducing
downwind ozone, thereby lowering the ozone transported to
downwind metropolitan areas. Thus, upwind NOx controls may
result in a reduction of ozone precursor controls needed in the
downwind location.
These observations indicate that comprehensive, objective
analyses should be applied to specific areas to determine an
effective ozone precursor control strategy. Such analyses need
to consider both the regional and smaller urban-scale
implications. Furthermore, multiday, area-specific application
of gridded photochemical models supported by the best available
data bases is the preferred method for developing and evaluating
such strategies. This reasoning is consistent with that
recommended by the NAS report:
"Application of grid-based air quality models to various
cities and regions shows that the relative effectiveness of
controls of volatile organic compounds (VOCs) and oxides of
nitrogen (NOx) in ozone abatement varies widely These
cities share an ozone problem, but differ widely in the
relative contributions of anthropogenic VOCs and NOx and
biogenic emissions. As a result, the optimal set of
controls relying on VOCs, NOx, or, most likely, reductions
of both, will vary from one place to the next."
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2.3 Supporting Data Bases and Air Quality Modeling
2.3.1 Description of the issue
Development and implementation of effective control
strategies relies on air quality models and supporting data. The
NAS report raises the following concerns regarding the adequacy
of data bases and modeling used to date in formulating and
verifying ozone precursor control programs:
1. models need to be subjected to careful, comprehensive
evaluations;
2. emission inventories, particularly mobile source
components, have underestimated VOC emissions, a
problem of consequence both in evaluating and in
applying models;
3. biogenic emissions must be better defined, for the same
reasons, and included in strategy assessment;
4. ambient air monitoring networks have not provided
adequate feedback information on effectiveness of
control programs, or corroboration of emission
inventories;
5. gridded photochemical models should be used in strategy
development; and
6. air quality, emissions, and meteorological data bases
used to drive and evaluate models need improvement.
It is clear that, as the NAS report suggests, past emission
inventories understated VOC emissions. This probably was one of
the reasons that NOx controls were not adequately integrated into
past air quality management programs. Emissions are crucially
important inputs to current photochemical grid models, and less
important, but nevertheless still significant, inputs to the
earlier used EKMA model. If emissions estimates understate VOCs
relative to NOx, the models will wrongly give greater weight to
VOC control in reducing ozone. Failure to consider the role of
biogenic VOC emissions exacerbated this problem. Furthermore, a
more comprehensive ambient measurement network could have
provided basic reality checks needed to implement mid-course
corrections. Greater confidence will result j.f analyses based on
both ambient data and models suggest similar control pathways.
2.3.2 Dealing with the technical issues
The EPA has been sustaining a program development and
research effort to improve data bases and model systems.
Reflecting those needs, Congress added substantial new
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requirements in the 1990 CAAA to foster improvements in data
bases and model applications.
Air Quality Modeling. The EPA has developed an integrated
application program for urban and regional scale gridded
photochemical modeling. Two major modeling initiatives were
undertaken by EPA in the late 1980s and early 1990s to
foster use of advanced gridded photochemical models in the
SIP process: (a) development of application capability to
apply the Regional Oxidant Model (ROM) and (b) updating and
documenting the Urban Airshed Model (UAM) to make it more
accessible for use by State agencies. The ROM provides
simulated boundary air quality concentrations needed to
drive the finer scale UAM. These boundary concentrations
for current year model exercises would be especially
difficult and expensive to characterize through ambient
measurements, and impossible for projected future modeling
years2. This program is consistent with the NAS
recommendation and Title I CAA mandates for the use of
gridded photochemical models in the more seriously polluted
ozone nonattainment areas to demonstrate that control
strategies will result in attainment of the ozone NAAQS.
Virtually every metropolitan area required by the Act to
conduct grid modeling has progressed through the initial
model setup phases or has plans in place to meet the
regulatory modeling requirements. This is a considerable
achievement considering that prior to the 1990 amendments
most gridded photochemical model applications were directed
toward research objectives. However, EPA's ROM and UAM
models have yet to be subjected to the comprehensive
evaluations needed to produce high accuracy and precision in
model predictions. Such evaluation efforts require large
blocks of funds that could not be accommodated to date in
the Agency's limited research budgets. Innovative and
cooperative efforts between public and private sectors must
be implemented to meet this need.
Emission inventories and biogenics. EPA has accelerated and
enhanced the process of upgrading the inventory process by:
1. continued improvement of mobile source emission
factor models, including better
evaporative/running loss factors and accounting of
emissions not included under standard EPA Federal
Test Procedures (FTP).
2. substantial increase in funding to improve
2Regulatory model applications include a "current" year
evaluation based on observed data and a "future" year assessment of
impacts of planned emission control strategies on ozone.
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emission inventories,
3. designation of SIP emission inventories as high
priority for EPA Regional Office and State
implementation programs,
4. requiring inclusion of biogenic emissions in
gridded photochemical model attainment
demonstrations,
5. developing state-of-the-art biogenic emissions
models and supporting research efforts on improved
biogenic emission estimates, and
6. establishing the Emissions Inventory Branch (EIB)
in 1991 within the Office of Air Quality Planning
and Standards (OAQPS) to centralize coordination
of inventory activities, and establishing the
Joint Emissions Inventory Oversight Group (JEIOG)
in 1990 between EPA's Office of Research and
Development and the Office of Air and Radiation to
develop research programs and guidance on high
priority inventory issues.
Air Quality Monitoring. The NAS has pointed out that past
air quality management efforts did not place enough emphasis
on monitoring as a "feedback" mechanism for determining how
well control strategies were implemented. Air quality
models are excellent control strategy design tools which
incorporate the necessary physical and chemical processes to
make objective air quality assessments in the uncertain
future. Despite the best efforts and intentions, control
programs are not always implemented as planned, or are as
effective as desired. Also, the models and supporting data
bases are not perfect. Therefore, subsequent ambient
monitoring of ozone precursors is needed to provide
assurance that planned reductions are actually occurring and
effecting air quality improvements, and to support future
analyses to identify appropriate adjustments to control
programs. These modeling and data collection efforts used
to develop, assess and check control strategies are embodied
in the existing State Implementation Plan (SIP) process.
Section 182 (c) of the CAA contains requirements for enhanced
ozone monitoring. The EPA has proposed regulations and
attendant guidance in 1992 pursuant ~o thi3 section WHICH
would establish a network of Photochemical Assessment
Monitoring Stations (PAMS) in the 23 areas classified as
serious or above. The enhanced ozone monitoring program is
a concerted effort by the EPA and State and local air
agencies to address data deficiencies, including those
identified by the NAS. PAMS data are intended to support:
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1. better assessments of ambient ozone and precursor
concentrations,
2. corroboration of ozone precursor emission
inventories,
3. ozone model applications, and
4. tracking progress of control programs.
Research to Improve Ozone Strategies
The initiatives outlined above are important steps toward
upgrading emissions and monitoring data bases and regulatory
models. Nevertheless, additional intensive data collection and
model application efforts beyond these initiatives will be
needed. Furthermore, additional development of more robust
models which improve the treatment of meteorological and chemical
reaction processes is needed. The advances in our knowledge of
atmospheric chemistry must continue to address uncertainties in
areas that have not been fully tested (e.g., natural and aromatic
VOC emissions effects, and future-year projected atmospheres
arising from the planned implementation of emission strategies).
Strong commitments must be made to ensure decisive improvements
in emissions and monitoring data bases, which are the foundation
for developing and assessing success of ozone precursor control
strategies. A strong supporting research effort is needed to
improve the modeling and monitoring tools which are the basis of
all application efforts.
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SECTION 3.0 - EPA PERSPECTIVES ON THE MAS FINDINGS
The National Academy of Sciences (NAS) produced ten findings
in the Executive Summary of their report entitled Rethinking the
Ozone Problem in Urban and Regional Air Pollution (Attachment 1).
These NAS findings and recommendations are reproduced below and
combined with the following EPA responses.
3.1 Ozone in the United States
3.1.1 NAS finding
Despite the major regulatory and pollution-control programs
of the past 20 years, efforts to attain the National Ambient Air
Quality Standard for ozone largely have failed.
3.1.2 NAS discussion
Since passage of the 1970 Clean Air Act amendments, exten-
sive efforts to control ozone have failed three times to meet
legislated deadlines for complying with the ozone NAAQS.
Congress set 1975 as the first deadline, but 2 years after this
deadline, many areas were still in violation of the NAAQS. The
1977 amendments to the Clean Air Act extended the deadline for
compliance until 1982 and allowed certain areas that could not
meet the 1982 deadline until 1987. For 1987, however, more than
60 areas still exceeded the NAAQS/ the following year, the number
of areas exceeding the NAAQS jumped to 101. In 1990, 98 areas
were in violation of the NAAQS.
EPA has reported a trend toward lower nationwide average
ozone concentrations from 1980 through 1989, with anomalously
high concentrations in 1983 and 1988. Ozone concentrations were
much lower in 1989 than in 1988, possibly the lowest of the de-
cade. However, since the trend analysis covers only a 10-year
period, the high concentrations in 1983 and 1988 cannot be as-
sumed to be true anomalies, nor can the lower concentrations in
1989 be assumed to be evidence of progress. It is likely that
meteorological fluctuations are largely responsible for the highs
in 1983 and 1988 and the low in 1989. Meteorological variability
and its effect on ozone make it difficult to determine from year
to year whether changes in ozone concentrations result from
fluctuations in the weather or from reductions in cne emissions
of precursors of ozone. However, it is clear that orogress
tcwara nationwide attainment of tne ozone NAAQS has been
extremely slow at best, in spite of the substantial regulatory
programs and control efforts of the past 20 years.
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3.1.3 EPA comment
Air quality data clearly attest to a current situation where
a number of U.S. metropolitan areas are nonattainment with
respect to ozone. In large measure, the 1990 amendments to the
Clean Air Act (CAA) were motivated by the extent of nonattainment
in the U.S. Although many areas remain nonattainment, analysis
of 1980-1991 ozone data (EPA, 1992) suggests a downward trend in
peak ozone values. Furthermore, this trend coincides with a
period of substantial population and economic growth throughout
the 1980s. In the absence of effective emission control
programs, the demographic growth would have led to increased
emissions and attendant increases in ozone. Thus, considerable
"progress" has been achieved by averting increases in ozone
concentrations in the face of demographic factors which would
otherwise worsen ozone levels.
In recognizing the current extent of ozone pollution,
perhaps the most important message is that the ozone problem is
complex and historically has been extremely difficult to solve
with state-of-the-art scientific knowledge and technologies. The
knowledge-base concerning ozone continues to evolve rapidly and
provides added insight on methods to counter ozone pollution. To
improve our chances of solving the ozone problem, existing air
quality programs must be flexible enough to enable adoption of
evolving knowledge and emerging methods. Similarly, air programs
must incorporate the extended vision and patience required to
realize long-term air quality improvements which are functions of
future, uncertain values of economic, technological and
meteorological variables. Finally, new research and control
initiatives need to be undertaken.
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3.2 Ozone Trends
3.2.1 HAS finding
The principal measure currently used to assess ozone trends
(i.e., the second-highest daily maximum 1-hour concentration in a
given year) is highly sensitive to meteorological fluctuations
and is not a reliable measure of progress in reducing ozone over
several years for a given area.
3.2.2 NAS recommendation
More statistically robust methods should be developed to
assist in tracking progress in reducing ozone. Such methods
should account for the effects of meteorological fluctuations and
other relevant factors.
3.2.3 EPA comment
The EPA's annual ozone trend assessments focus on
statistical measures closely related to the health-based National
Ambient Air Quality Standards (NAAQS) in order to provide a
convenient frame of reference for the results. The ozone NAAQS
requires that the average number of days with hourly maximum
ozone concentrations above 0.12 ppm not be more than one per
year. Thus, the annual second highest daily maximum (SHDM) value
is used as an ozone trend statistic. This does place an emphasis
on peak values, which can be influenced by year-to-year
variations in meteorology. The EPA's trend reports have
highlighted this point, particularly with respect to the effect
of hot summer weather on ozone (EPA, 1991a).
The EPA recognizes the need for additional trend statistics
that may be less influenced by year-to-year meteorological
fluctuations. Such statistics could be useful indicators of
long-term progress and the EPA will continue to encourage their
development. At the same time, the EPA strongly supports the NAS
caution that these alternatives "should not be mere statistical
entities." Analyses must communicate trends relevant for the
health based NAAQS. Trends in the quality of the air that people
actually breathe can not be discounted in favor of trends in
r.ypocneticai air quaiiry "Chan wouia have occurred unaer
normalized meteorological conditions.
The use of statistical approaches which normalize effects of
meteorological influences are useful for developing insight on
the ^relation between ozone and emission trends. This, in turn,
would provide a clearer view of long-term changes of health-and
environmentally-related measures. Indeed, if the trends were
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adjusted to consider the strong meteorological influence
generated by a hot, dry 1988 summer, as well as effects in other
years with less conducive meteorology, different conclusions
might be reached concerning the rate of progress in reducing
ozone.
The EPA has initiated a program (Cox and Chu, 1991) to
investigate techniques for adjusting ozone trends for
meteorological influences, to address concerns regarding the
apparent lack of robustness exhibited by extreme value indicators
such as the second highest daily maximum. One of the methods
being studied is a statistical model in which the frequency
distribution of ozone concentrations is described as a function
of meteorological parameters such as temperature and wind speed.
The statistical model includes a trend component such that long-
term changes in ozone can be determined that are less dependent
on annual changes in meteorological conditions. The model has
been applied in approximately 25 urban areas using monitored
ozone data collected over the past decade. The results show
promise in that adjusted ozone trend statistics are relatively
smooth compared with the unadjusted ozone data. The EPA is
seeking to review and expand the technical basis for the
methodology under a cooperative agreement with the National
Institute of Statistical Sciences (NISS). Under this agreement,
the NISS also will examine more sophisticated statistical models
that would treat both spatial and temporal components of
meteorological/ozone behavior. Expectations of definitive
results from this work must be tempered by the fact that the
usable ozone record is not much greater than a decade; certainly
less than optimal for such a study.
Separate programs are under development which explicitly
address progress in emissions reductions and the relationships
among emissions, meteorology and air quality. Data from the
developing enhanced ozone monitoring program (discussed in
Section 3.6) potentially will address many of the issues raised
by the NAS; tracking ozone precursor emissions trends is one of
several program objectives.
The NAS committee did not address the definition of the
ozone NAAQS. However, the NAS did raise concerns that are
relevant to the definition and use of ozone design values. Most
of the planning, control strategy development,.implementation and
verification elements of the State Implementation Planning (SIP)
process are basea on the ozone aesign values for specific
metropolitan areas. For example, classification status of ozone
nonattainment areas is oased on ozone design values. Given trie
importance of ozone design values in developing and assessing
ozone strategies, the EPA is conducting an Ozone Design Value
study as required by Section 183(g) of the 1990 CAA to determine
if a different and/or additional ozone statistic should be
incorporated in the planning process.
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In summary, existing reporting procedures of the ozone
trends data provide a view of air quality metrics which closely
reflect the current 1-hour ozone standard. However, the second
highest daily maximum is a difficult metric to use for assessing
long-term trends. The EPA is developing and encouraging the use
of statistical procedures which attempt to isolate meteorological
influences on ozone trends and facilitate interpretation of long
term underlying trends.
Ozone trends data from 1982 through 1991 are shown below.
The recent trends suggest improvement relative to 1988. The
1989-1991 period is too brief to provide conclusive evidence that
control programs are responsible for the downward trend.
Meteorological influences are not extracted form the trends data.
OZONE TREND, 1982-1991
(ANNUAL 2ND DAILY MAX HOUR)
CONCENTRATION, PPM
0.30
0.251
0.20-
0.15-
0.10-
0.05-'
0.00
495 SITES
90% of sites have lower
2nd max 1-hr concentrations
.than this line
Average tor a)) sites
10% of sites have lower
2nd max 1-hr concentrations
than this line
HAAQ&
I I I I I I i I
82 83 84 85 86 87 88 89 90 91
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3.3 State Implementation Planning
3.3.1 HAS finding
The State Implementation Plan (SIP) process, outlined in the
Clean Air Act for developing and implementing ozone reduction
strategies, is fundamentally sound in principle but is seriously
flawed in practice because of the lack of adequate verification
programs.
3.3.2 HAS recommendation
Reliable methods for monitoring progress in reducing
emissions of VOCs and NOX must be established to verify directly
regulatory compliance and the effectiveness associated with
mandated emission controls.
3.3.3 EPA comment
The SIP process was developed to determine the level of
emissions controls required to meet the NAAQS and ensure
implementation and enforcement of those controls. The 1977 CAAA
required that the NAAQS for ozone be met by 1982, with extensions
for certain areas to 1987. Since a number of ozone nonattainment
areas remain, aspects of the SIP process have been criticized.
The NAS found that the basic SIP approach consisting of strategy
development, implementation and enforcement elements was sound.
However, the SIP process was weakened by the lack of an adequate
verification component. A major issue here is the scope of
efforts to track changes in ozone precursor emissions and related
ambient concentrations and the effectiveness of control measures
in reducing precursors and, ultimately, ozone.
Section 182 (c) (1) of the 1990 CAA addresses the need for
ambient verification and mandates promulgation of enhanced ozone
monitoring regulations. A principal objective of developing an
enhanced monitoring program is corroboration of emission trends
through measurements of ozone precursor concentrations (VOCs and
NOx). [A more complete description of this program is included
in Section 3.6.]
Briefly, previous VOC and NOx monitoring programs were
designed principally to support trajectory modeling analyses and
to characterize ambient pollutant trends in major urban centers.
As sucn, measurements focused on VOC and NOx concentrations
observed between 6-9 AM on weekdays, and typically were taken
at aowntown sices. A moire comprehensive set of measurements is.
needed to provide an independent means of verifying that
emissions reductions required by a control strategy are, in fact,
occurring at levels specified in the SIP. Furthermore, the
measurements should be capable of explaining measured trends in
VOC and NOx which are inconsistent with expectations accompanying
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a control strategy.
To meet such an objective, VOC and NOx measurements are
needed at several locations within a nonattainment area several
times a day. Also, VOC measurements need to be resolved into
individual compounds and meteorological data collected in order
to associate specific source categories with ambient data. In
addition to providing data for these basic verification needs,
enhanced monitoring program networks serve other related SIP
functions. The measurements may provide a means for refining
emission estimates, supporting ozone model applications and
tracking future air quality and emissions trends. Recognizing
that the PAMS are intended to serve multiple needs and the
significance of ambient data in the SIP process, careful
assessments of the contemplated PAMS program must be conducted to
insure that the program is capable of meeting prescribed goals.
A PAMS network will be phased-in over time once the program
is promulgated. Regulations and guidance for establishing an
enhanced monitoring system are being set and forthcoming data are
not expected until after 1994. The monitoring program will
provide a feedback/verification mechanism for post-1994 SIP
revisions with attainment targets in the 1999-2010 timeframe.
However, the air quality modeling needed to design initial
control strategies will be completed before November, 1994 and,
consequently, will not benefit from the enhanced ozone monitoring
data.
In addition to verification through ambient monitoring, it
is important to track emissions reductions. Past SIPs have
assumed that regulatory programs for stationary sources would be
implemented with full effectiveness, achieving all of the
intended emission reductions at all times. The EPA's rule-
effectiveness policy (1989) requires SIPs to reflect more
realistic emission reduction scenarios and encourages States to
evaluate actual reductions and incorporate that information into
the SIP.
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3.4 Anthropogenic VOC Emissions
3.4.1 HAS finding
Current emissions inventories significantly underestimate
anthropogenic emissions of VOCs. As a result, past ozone control
strategies may have been misdirected.
3.4.2 HAS recommendation
The methods and protocols used to develop inventories of
ozone precursor emissions must be reviewed and revised. Indepen-
dent tests, including monitoring of ambient VOCs, should be used
by government agencies to assess whether emissions are indeed as
they are represented by emissions inventories.
3.4.3 EPA comment
The NAS raises important concerns shared by the EPA
regarding the adequacy of past emission inventories.
Illustrating the seriousness of this subject is the recent
understanding that mobile source VOC emissions may have been
underestimated by a factor as high as 2 or more. Emission
inventories need to be improved. The EPA acknowledges that past
inventories have understated VOC and CO emissions due to
limitations of data and methodologies. In addition to the studies
cited by the NAS,. more recent analyses (Baugues, 1991; Fujita et
al., 1992) generally support the conclusion that historical
anthropogenic emission estimates have been understated.
The precise extent to which various source categories have
been understated is not clear, and adequate data currently are
not available to revise the emissions models or estimates. The
most likely suspects are mobile source emissions (including non-
road mobile source emissions) , commercial/consumer solvents, and
point source emissions (caused by inappropriate rule
effectiveness assumptions, poorly-characterized fugitive
emissions, and emissions from sources emitting less than
prescribed cutoff limits such as 100 tons/year, which
historically have not received adequate attention). Shortcomings
in emissions estimates have arisen from incomplete scientific
understanding and inadequate emphasis on inventory studies.
The 1990 CAAA put a much higher premium on accurate
_nvem:ory compilations ^han nas been true formerly. As noted in
Section 2, the EPA has accelerated and enhanced its efforts at
upgrading the inventory process and has directed increased
support to States to assist them in utilizing available new
findings in State Implementation Plans due in 1993.
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1. Continued improvement of mobile source emission factor
models.
Concerns about mobile source emission factor models have
provided an ongoing feedback mechanism to improve existing
techniques for several years. Mobile source models are upgraded
periodically to reflect new findings on evaporative and exhaust
emission components in addition to incorporating changes stemming
from combustion technology in new automobiles. Mobile source
models are generated from emission testing of in-use vehicles
using a driving cycle typical of urban driving. However, this
driving cycle may be inadequate in characterizing the full
spectrum of vehicle driving patterns of individual operators such
as heavy acceleration rates. Transient rapid acceleration events
may cause order of magnitude increases in exhaust emissions for
short periods of time. The EPA is evaluating whether other
driving modes should be added to this cycle. Several other
factors such as accounting for frequency of cold and hot starts,
high-emitting older vehicles, use of average speeds for model
inputs, determining traffic flow patterns and other
parameterization considerations present formidable challenges.
Ambient studies which measure emissions over characteristic
traffic areas may provide useful enhancements to mobile source
emissions modeling methods. Meanwhile, the EPA has field
research programs underway focusing on driving cycle effects and
better characterization of higher mileage and earlier vintage
vehicles.
In addition to improving highway vehicle emission estimates,
attention must be directed toward nonroad vehicle emissions
(e.g., diesel engines in large construction operations). A
recent study (EPA, 1991b) suggests that nonroad vehicle emissions
potentially constitute a significant portion of total vehicle
emissions. Accordingly, methodologies are being developed to
provide more accurate estimates of the nonroad emissions
component.
2. Designation of emission inventories as high priority
for EPA and State and local programs.
Emission inventories form the basis for several air quality
programs. The effort to develop the 1985 National Acid
Precipitation Assessment Program (NAPAP) Emission Inventory
proved successful _n providing an adequate inventory of 502 and
NOx emissions from large point sources for the icid rain orogram.
A rocus _3 now on che 51? emission inventories for ozone
nonattainment areas. The EPA has issued several volumes (EPA,
1991 c-g) of Emission Inventory guidance procedures and held
national workshops on preparing inventories. States were
required to submit inventories by November, 1992.
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Steps to upgrade these inventories above previous
inventories include: focus on quality assurance/quality control,
procedures for developing biogenic emissions estimates, guidance
to develop future year emission estimates and vehicle miles
travelled (VMT), improved mobile source models reflecting current
research, training workshops for applying mobile source models,
electronic submittal of inventories to allow for consistency
checking, and the availability of a direct State assistance
system. Considerable fundamental research/testing is being
applied to improve accuracy of stationary source emission
factors, mobile emission procedures, uninventoried sources, and
other emission inventory methodology needs (e.g., continuous
emissions monitoring for VOCs, Fourier transform infrared (FTIR)
integrated measurements for fugitive releases including
speciation, and real-time episodic data on emissions).
As noted in Section 3.6, major objectives of the new
enhanced ozone monitoring program include verification of
inventories and the tracking of inventory changes. This
objective is a central concern shared by the HAS and the EPA.
Emissions are difficult to estimate, and monitoring checks are
needed to affirm the adequacy of estimation procedures, or
uncover as yet unknown systematic errors in the inventory
process. Accordingly, strong support for an expansion of the
enhanced monitoring program should be fostered to develop a
responsive monitoring system which complements the inventory
process.
3. Establishing a Emissions Inventory Branch (EIB) and a
Joint Emissions Inventory Oversight Group (JEIOG).
The EIB resides in the OAQPS and consolidates all OAQPS
inventory activities into a single program function.
Initially, the EIB has taken steps to enhance the consistency and
quality control aspects of the inventory process. Recently,
several workshops have been conducted and guidance developed to
assist States in producing improved inventories.
The JEIOG is a multi-discipline effort structured to develop
consistency among the EPA's various research and operation arms
and incorporate the latest research and technical efforts in the
inventory process. JEIOG members have identified a number of
pressing inventory-related needs and have attempted to prioritize
these in terms of their widespread applicability and impact on
program neeas. Work nas oegun to address several of the highest
priority Issues. These include uncertainty assessment.
estimation tecnniques for emissions from natural sources,
emission factor development, field validation of area source
methodologies, resolution of the difference between emissions
inventories and- ambient measurements, methods for improving
estimates of automotive activity levels, and other tasks.
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Past ozone precursor control approaches have relied on the
best science available. More recent developments on the role of
biogenic emissions and the understatement of anthropogenic
emissions in control strategy analyses cast uncertainty on past
control strategy approaches. The current approach to developing
strategies includes (1) enhancement of the ambient data bases
used to design and check the progress of strategies, (2) a focus
on improving emission inventories, and (3) regional and local
application of the most comprehensive and defensible air quality
models. While the above efforts constitute an important step
toward improvement, other important needs have yet to be
addressed. A need exists for research-grade emission inventory
data bases for at least two (south and northeast) nonattainment
areas for use in evaluating existing and future ozone models and
emission inventory protocols. Reliable emission inventory and
accompanying monitoring data are needed not only for the early
morning hours (6-9 AM), but also for mid-day hours when the
highly uncertain and perhaps critically important biogenic and
auto evaporative VOC emissions occur at peak levels.
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3.5 Biogenic VOC Emissions
3.5.1 MAS finding
The combination of biogenic VOCs with anthropogenic NO,, can
have a significant effect on photochemical ozone formation in
urban and rural regions of the United States.
3.5.2 HAS recommendation
In the future, emissions of biogenic VOCs must be more
adequately assessed to provide a baseline from which the
effectiveness of ozone control strategies can be estimated before
such strategies are applied for a specific urban core or larger
regions. Ambient measurements of concentrations and emission
rates are needed to improve the accuracy of biogenic VOC invento-
ries .
3.5.3 EPA comment
Throughout the late 1970s and mid-1980s there was a
consensus (Altshuller, 1983) in the scientific community that,
while natural emissions may be quite large, they did not play a
significant role in ozone formation and precursor control
assessments. This consensus can be attributed to 1) a focus on
modeling large urban areas where anthropogenic VOCs dominate1,
2) a failure to properly account for rural ozone formation and
transport of ozone and precursors, and 3) the large uncertainties
in biogenic emission rates2. Consequently, biogenic emissions
were not included in air quality modeling analyses.
Chameides et al. (1988) illustrated the need for considering
biogenic emissions when assessing the effectiveness of emission
reduction strategies. That work and other similar follow up
studies (Scheffe .et. aJ,., 1990) showed that for Atlanta, Georgia,
VOC control requirements increase by as much as 50% when model
simulations incorporate biogenics, relative to simulations
without biogenics. The primary reason for this finding was that
after anthropogenic VOCs were reduced substantially in the
simulations, ozone became much more sensitive to further changes
in VOCs. Thus, adding biogenics at this point produced
significant differences in predicted ozone. Recognizing that
biogenic emissions have important implications on ozone precursor
1 Although biogenics probably represent over half of all
.anthropogenic ana natural; 70C emissions on a continental, oasis,
it is important to point out that in most central urban cores,
anthropogenic VOC emissions constitute the larger fraction.
2 Uncertainty ranges of +/- a factor of three typically are
associated with biogenic emission estimates.
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control programs and must be considered in ozone modeling
analyses, the EPA expanded both the 5-City UAM (Morris et al.,
1990; EPA, 1990a) and Regional Ozone Modeling for Northeast
Transport (ROMNET) (EPA, 1991h) studies to verify with grid
modeling and test the geographic robustness of the Chameides et
al. findings. As a result of those efforts, the EPA's gridded
photochemical modeling policy (EPA, 1991i) now requires
incorporation of biogenic emissions.
Model simulations based on the EPA's 5-City UAM study also
suggested that NOx control is more effective in reducing ozone
than VOC control for Atlanta (Scheffe .et. al., 1990), an effect
linked to consideration of biogenics. [The NOx implications from
biogenics are addressed in Section 3.8.] Initially, the EPA
reacted cautiously to these findings by acknowledging that
biogenics may indeed be important in areas, such as Atlanta,
characterized by a large biogenics component and predominantly
residential demographics. Following the Atlanta studies, results
from the EPA's ROMNET study suggested that the importance of
biogenics extended beyond the southeast into much of the
northeastern U.S. as well.
Based on these and other studies, the EPA agrees with NAS
findings stating that biogenic emissions are a significant
component of total VOC loading to the atmosphere. Biogenic
emissions can influence both direction (i.e., VOC or NOx) and
extent of required emissions controls and, therefore, are
necessary inputs to regulatory model applications which assess
various ozone mitigation strategies.
To address the large uncertainty attendant with biogenic
emission estimates (a factor of at least +/- _3_ typically is
associated with biogenic emission estimates), the Biogenic
Emissions Inventory System (BEIS) (Pierce and Waldruff, 1991) has
been modified to incorporate latest model developments and
improve mapping resolution of vegetation groups emitting
biogenics. The BEIS is the EPA's recommended model to develop
biogenic inputs for ozone air quality models, which are used to
assess the effectiveness of emission control strategies.
The Southern Oxidant Study (SOS), a comprehensive air
quality and modeling effort designed to determine, among other
things, the relation between biogenic emissions and ozone
formation is funded partly by the EPA. The SOS is motivated, in
par-, oy the uncertainty ana acknowledged importance of biogenic
emissions. Use of ambient data collection and analvsis
-ec.ir.iques forms the rna^or cnrust in attempting to sned
additional insight on the role of biogenics in ozone generation.
Although the SOS contains a significant natural emissions
component, the difficulties associated with characterizing
biogenic emissions will require additional efforts to improve
emissions measurement techniques for natural systems. The JEIOG
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is funding additional research work to update the national
landuse/biomass data base, improve methodologies for making
environmental corrections of emissions rates (i.e., biogenic
emissions are functions of temperature and sunlight intensity),
and reevaluating the existing biogenic emission factors.
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3.6 Ambient Air Quality Measurements
3.6.1 HAS finding
Ambient air quality measurements now being performed are
inadequate to elucidate the chemistry of atmospheric VOCs or to
assess the contributions of different sources to individual
concentrations of these compounds.
3.6.2 HAS recommendation
New measurement strategies that incorporate more accurate
and precise measurements of the individual trace compounds in-
volved in ozone chemistry should be developed to advance
understanding of the formation of high concentrations of ozone in
the United States and to verify estimates of VOC and NOX
emissions.
3.6.3 EPA comment
Past ambient data bases have not provided enough information
to fully characterize an area's ozone problem. As noted
elsewhere in this report, air quality measurements are of value
for several reasons: understanding ozone formation processes,
providing feedback on the SIP process, verifying both emissions
estimates and effectiveness of emission reduction programs, and
providing support for model application efforts. The linkage
with emissions is particularly noteworthy, as discussed in
Sections 3.3 and 3.4, and worth restating here: Had an effective
ambient data collection been in place, it may have been possible
to (1) determine that anthropogenic emissions were understated,
and (2) effect an attendant shift in control strategy emphasis.
Recent and ongoing studies like the Southern Oxidant Study
(SOS) rely strongly on the use of ambient measurements to assist
in the development of effective control strategies, and enhance
our predictive modeling efforts. Given all the uncertainties in
the modeling process (emissions, meteorology, chemistry, growth
forecasting, etc.), ambient data provide basic reality checks on
our predictive efforts. The EPA in partnership with State and
local Air agencies is developing a national program to enhance
ambient data bases. This enhanced ozone monitoring program
should address several data deficient areas identified by the
!JAS. A brief review of the previous nationaily-basea ozone
precursor monitoring program followed by discussion of the new
er.nancea ozone .nonitoring program provides insignt into the
motivation for expanding ambient data collection efforts.
Implementation of the previous Non-Methane Organic Compound
(NMOC) network was partially motivated by Empirical Kinetic
Modeling Approach (EKMA) data needs. The EKMA relies heavily on
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measured air quality data to drive model simulations. Partly as
a consequence of the modeling requirement for monitoring data,
the first nationally-based VOC (the more descriptive term "NMOC"
is used rather than VOC) monitoring program was established.
The design of the NMOC monitoring networks which supported
EKMA analyses (i.e., unspeciated ambient data collected from 6:00
A.M. - 9:00 A.M. in nonattainment central business districts) did
not provide continuous year-to-year data at given locations.
Thus, the NAS concludes (Attachment 1) that the NMOC data base
did not provide (1) explanations for continued and frequent ozone
exceedances beyond 1987, or (2) an adequate ambient data
monitoring system to serve as a feedback mechanism to track
progress towards attainment and make midcourse corrections as
needed.
Following completion of the regulations and attendant
guidance proposed in 1992 on the enhanced ozone monitoring
program, a network of Photochemical Assessment Monitoring
Stations (PAMS) will be phased in over several years. Such a
program is mandated by Section 182 (c) (1) of the CAA. The
enhanced ozone monitoring program will require States to
establish PAMS as part of their SIP ambient air monitoring
networks in nonattainment areas classified serious, severe, or
extreme. Each affected State air pollution control agency (or
local delegated agency) would install an array of additional
monitors to gather data on ozone, NOx, VOCs (speciated, including
aldehydes) and meteorological parameters. Such stations would be
positioned to sample upwind and downwind (rural/suburban fringe
areas) and at central urban locations. Sampling frequency at
certain sites would allow for the development of diurnal patterns
in observed concentration profiles.
However, routine measurements of the sort envisioned for
PAMS will not provide all of the data needed to completely
characterize air quality. In particular, the PAMS data are not
expected to provide the array of trace atmospheric measurements
recommended by the NAS, or the intensive set of measurements
needed to drive and evaluate gridded photochemical models.
Furthermore, the PAMS locations are limited to highly polluted
areas; yet we have seen that rural/regional effects are important
factors in understanding ozone formation and control.
Data to be collected from the PAMS network will not meet all
concerns raisea oy che NAS. Other programs directed at specific
locations will be needed to address shortcomings associated with
namonaiiy-oasea, routine efforts. Implementation of well-
designed, intensive field studies conducted at specific areas
over a 2 - 6 week period of probable ozone conducive weather are
needed to enhance the PAMS and other data bases. Such studies,
when combined with sophisticated model applications, provide
valuable insight on ozone cause-effect phenomena and reduce the
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uncertainty in conclusions regarding the effectiveness of
emission control strategies. These field studies are expensive
and take time to design, execute and produce results. Several
are needed covering the non-compliance areas. Priority areas
include the northeast corridor and the Texas-Louisiana coastal
region (Houston, Beaumont, Port Arthur, Baton Rouge). Case
examples and additional discussion on intensive studies are
provided in the following section on air quality modeling.
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3.7 Air Quality Models
3.7.1 NAS finding
Although three-dimensional or grid-based ozone air-quality
models are currently the best available for representing the
chemical and physical processes of ozone formation, the models
contain important uncertainties about chemical mechanisms, wind-
field modeling, and removal processes. Moreover, important
uncertainties in input data, such as emissions inventory data,
must be considered when using such models to project the effects
of future emissions controls.
3.7.2 HAS recommendation
Air-quality models are essential in predicting the
anticipated effects of proposed emissions controls on ambient
ozone concentrations. Therefore, the effects of uncertainties on
model predictions, such as uncertainties in the emissions
inventory and in the chemistry incorporated in the models, must
be elucidated as completely as possible. Predictions of the
effects of future VOC and NOX controls should be accompanied by
carefully designed studies of the sensitivity of model results to
these uncertainties.
3.7.3 EPA comment
While advocating the use of gridded photochemical models as
the principal planning tool for developing ozone precursor
emission reduction strategies, the NAS report presents important
caveats associated with model applications: (1) the model's
description of physical and chemical processes is not perfect,
(2) many current data bases which will be used to drive and
evaluate model applications do not contain the desired level of
spatial, temporal and compositional resolution, and (3)
uncertainty in the model estimates is not treated explicitly in
regulatory applications. This last caveat falls under the
general subject of "The use of models in the regulatory process."
In addressing these issues, it is important not to lose sight of
the primary NAS conclusion that grid-based photochemical models
are the best available tools for assessing ozone mitigating
strategies.
Model processes and components: The EPA encourages a systematic
program tc incorporate ^improvements to the moaeis reflecting new
scientific findings and efficient computational methods as they
become available. Two major modeling initiatives were undertaken
by the EPA in the late 1980s and early 1990s to foster use of
advanced gridded photochemical models in the SIP process: (a)
development of the application capability to apply the Regional
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Oxidant model (ROM) and (b) updating and documenting the Urban
Airshed Model (UAM) to make it more accessible for use by State
agencies. Recognizing that ozone often persists for several days
and is transported over large distances, the Regional Ozone
Modeling for Northeast Transport (ROMNET) project was initiated
in 1987. In addition to providing valuable information for
assessing region-wide emission control strategies, the ROMNET
project also produced interfacing software for generating inputs
to the UAM for future SIP applications. This ROM-UAM linkage
should provide a consistent basis for supplying current and
future year boundary conditions for UAM (boundary conditions are
an especially difficult, but important, set of model inputs which
must be specified in using the UAM). The EPA's Regional Oxidant
Modeling program has grown to include a significant applications
component to complement the research and development focus.
Capability now exists to provide ROM-generated information for
use in urban attainment demonstrations east of 99 W longitude.
This covers almost all serious or more severely polluted areas
outside of California.
Recognizing that gridded photochemical modeling is the
preferred tool for understanding ozone problems and assessing
effectiveness of precursor controls, several UAM initiatives were
undertaken by the EPA in the late 1980s: the 5-City UAM study
(EPA, 1990a), placing the upgraded UAM into the public domain,
development of the UAM User's Guides (EPA, 1990b) and guidance
for regulatory application (EPA, 1991i). The 5-City UAM study
provided useful information on emerging policy issues such as
alternative fuels, biogenic emissions, and the relative
effectiveness of VOC and NOx controls. Perhaps more
significantly, the 5-City UAM study served as a precursor for
future UAM SIP applications as the project emphasized control
strategy evaluations and included a technology transfer component
to train participating States to operate the UAM. '
In 1990 the EPA released an updated version of the UAM to
the public domain, reflecting numerous advances in photochemistry
and numerical solution techniques which emerged during the 1980s.
An extensive multi-volume UAM User's Manual was prepared to
facilitate operation of the UAM. Guidance on regulatory
application of the UAM released in 1991 should foster national
consistency among UAM applications. Several efforts are underway
to improve pre and postprocessing UAM capabilities and train the
States in applying the model.
The 1990 CAA reauires the use of gricided models In .Tianv
r.onattainment areas, and over the last three years the EPA has
positioned itself and the States to address this mandate with the
aforementioned ROM and UAM efforts. The NAS reminds us, however,
that current methods are not without flaws and that regulatory
approaches should incorporate advances as they become
operational. Accordingly, the continuation and enhancement of
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research and application efforts addressing the following areas
are needed:
1. improving the ability of chemical mechanisms to
characterize biogenic (and anthropogenic) processes and
treat future, expected changes in the mix of
atmospheric pollutants;
2. integrating day-specific emissions estimates, including
direct emissions measurements, into emissions models
which develop inputs to air quality models;
3. integration of prognostic meteorological models capable
of characterizing complex flow phenomena into air
quality modeling;
4. development of computationally efficient, bi-
directional, variable grid systems offering high
spatial resolution and broad geographical coverage;
5. development of plume-in-grid modeling techniques to
more rigorously treat major point source plumes in
regional and urban ozone models; and
6. evaluation of models using special, research-grade
ambient monitoring and emission inventory data bases.
Applications of more advanced modeling methods such as
variable grid approaches and prognostic meteorological models
which have the capability of characterizing complex flow
phenomena are encouraged. The Lake Michigan Ozone Study (LMOS),
funded partially by the EPA, is applying such techniques.
Although significant advances have occurred over the last decade,
further improvements in the basic chemical mechanism, emissions
and meteorological components and solution techniques must be
made in gridded photochemical systems. A strengthened commitment
to research in these areas is required. Balancing the desire to
use the best tools while fostering national consistency will
continue to pose challenges to regulatory programs.
Model Data Bases and Regulatory Application: Concerns about the
adequacy of available data bases, as well as the topographical
and meteorological complexity of certain areas, have motivated
several intensive field studies to support ongoing modeling
efforts; the South Coast; Air Quality Study, San Joaquin Valley
Study, Lake Michigan Ozone Study, Baton Rouge Study, and the
Southern Oxiaanr. Study (SOS) are examples. A similar effort is
being initiated in the Texas and Louisiana Gulf Coast. Data base
adequacy concerns remain, and additional efforts clearly are
desirable and to be encouraged in other areas. However,
practical constraints of time and resources preclude requiring
multi-million dollar field efforts in all cases. Avenues of
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funding should be explored for this support. Consideration
should be given to increased public sector appropriations, and
the establishment of partnerships among industry, academia and
.government research and regulatory components. The Baton Rouge
application and the SOS program are excellent examples of a
cooperative venture among government, industry, and academia.
Most pressing among such new programs needed, is one addressing
the ozone problem in the northeast.
The 5-city UAM study identified difficulties associated with
using routinely available data bases; however, the study also
suggested that in some areas routine data bases could support
model applications. The NAS recommendation that minimum
aerometric data bases be established for model applications was
considered when national guidance for applying the UAM (EPA,
1991i) was developed. However, the disparate area-specific
requirements for data bases preclude setting of generalized
minimum data base criteria. Furthermore, the CAA provisions
requiring SIP submittals with model attainment demonstrations for
serious and above as well as for certain moderate ozone
nonattainment areas are due by November, 1994. To meet that
deadline, modeling should be near 'completion in the late 1993
timeframe. This timing does not allow for planning and
implementation of specific field studies to support modeling.
However, most urban scale model applications in this immediate
round of SIP demonstrations will benefit from supporting ROM
applications to generate boundary value concentrations, as
discussed above. For future applications (i.e., post 1994), the
PAMS program will enhance routine aerometric data bases and
assist in evaluating model performance and development of model
inputs. However, enhancements to the twice daily, routinely
collected National Weather Service/Federal Aviation Association
(NWS/FAA) vertical profiles of wind speed and direction and
temperature are recommended for those areas performing grid
modeling. Due to the spatial and temporal sparseness of the
NWS/FAA data, the urban-scale three-dimensional gridded model
applications often contain significant uncertainties in their
meteorological fields.
This discussion on supporting data bases underlies a
fundamental concern about uncertainty in model predictions. As
the richness of support data increases, uncertainty in both model
inputs and subsequent predictions decreases. The EPA's guidance
en regulatory application of the UAM incorporates several NAS
recommendations .Including: (1) cne use of multiple meteorological
episodes to test emission strategies, (2) model diagnostic and
performance evaluations, (3) expansion of domains and (4)
execution of multi-day, as opposed to single day, simulation
periods. However, the guidance does not prescribe criteria for
establishing minimum data bases, nor require a formal report on
uncertainty. Model simulations conducted to define the more
sensitive parameters are useful starting points for quantifying
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uncertainty and designing complementary field efforts. These
data issues are best addressed on a case by case basis, because
metropolitan areas exhibit varying climatological, topographical
and demographic features leading to different data priorities.
Concerns about the richness of existing model support data
are motivation for implementing data enhancement programs.
However, such concerns should not be the basis for circumventing
or delaying the modeling analysis required to determine ozone
precursor control requirements. In fact, extensive model
simulations should help in planning the type, number and siting
of additional monitors for enhanced monitoring networks. While
sparse data bases hinder the use of the air quality models in a
highly deterministic manner, their use for directional guidance
on precursor control programs is certainly indicated. Precise
determination of specific control requirements from air quality
models is probably beyond the reach of the models at present,
given the lean and error-prone data bases that are often used.
However, models are critical for structuring and guiding control
strategy development when used with flexibility and reality
checks on the interpretation of modeling results.
Although the models and supporting data bases are not
capable of yielding high precision control strategy requirements,
they remain the most objective tools available for driving
control strategy decisions. Models provide the best planning
approach for air quality management at the present time.
Consequently, the EPA requires use of models in the attainment
demonstration process. Nevertheless, the long-term goal should
be to enhance data bases and the modeling science to improve our
basis for control measure decisions.
In addition to data base considerations underlying model
exercises, the gridded models provide analytical capabilities
that have not been completely tapped. Model analyses can be
expanded to explore multifaceted questions coupling air quality,
population exposure, and economic considerations. Intermediate
years between the current baseline and future attainment years
can be modeled to gain a sense of progress and incorporate
"relative speed toward improvement" as a factor in developing
emission control strategies. Air quality metrics of interest can
be expanded to include averaging times greater than 1 hour, net
domain and selected sub-domain spatial effects, and population
exposure. Also, other pollutants and processes such as
formaldehyde, peroxy acecyi nitrate (PAN), nitric acia, ana
nitrate deposition can be incorporated in model analyses. This
-;>:panaea use of griaaea moaeis potentially can produce the
information needed to explore a wide spectrum of existing and
emerging issues. The vehicle for developing such information is
underway, as we progress through the initial stages of applying
complex models on a routine basis.
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3.8 VOC versus NOx Control
3.8.1 HAS finding
State-of-the-art air-quality models and improved knowledge
of the ambient concentrations of VOCs and NOX indicate that NOX
control is necessary for effective reduction of ozone in many
areas of the United States.
3.8.2 HAS recommendation
To substantially reduce ozone 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.
3.8.3 EPA comment
The EPA agrees that NOx controls in addition to or instead
of VOCs are likely to reduce ozone in many areas. However, in
certain cases NOx controls might not be effective in reducing
ozone. Possible exceptions are not necessarily limited to New
York and Los Angeles urban cores. Application of gridded
photochemical models on a case by case basis is required to
determine the efficacy of NOx controls, because the ozone
response to precursor reductions is area specific. The following
Urban Airshed Model (UAM) and Regional Oxidant Model (ROM)
modeling studies support the general assertion that NOx controls
(1) may be beneficial in many places (e.g., Atlanta and several
parts of the northeastern U.S.) and (2) might not be effective in
reducing ozone in other areas (e.g., Dallas-Fort Worth and New
York). [The following UAM applications for Atlanta and Dallas-
Fort Worth are based on the 5-City UAM study (EPA, 1990a) and are
provided to illustrate that ozone response to controls of VOCs or
NOx is area-specific. The five-city study investigated the
feasibility of low-cost UAM applications, and modeling results
are to be viewed for illustration only. A more complete analysis
examining several sets of meteorological conditions and more
closely scrutinized data bases are required for SIPs or a
demonstration under Section 182 (f) .]
Figures 3-1 through 3-4 illustrate somewhat contrasting
ozone responses from VOC and NOx controls for specific days in
Atlanta and Dallas-Fort Worth. Figures 3-1 and 3-2 depict
relative benefits of VOC and NOx control, respectively, to the
base case (i.e., zero control) throughout the Atlanta domain.
VOC controls 3now ^imiteci oenefits restricted co the center of
the Atlanta domain (Fig. 3-1); whereas NOx controls show more
widespread and more pronounced reductions in peak ozone
predictions throughout most of the domain (Fig. 3-2). The
Dallas-Fort Worth results show greater peak ozone reductions for
VOC controls (Fig. 3-3), and several areas which exhibit ozone
3-23
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.a
O.
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Y
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IK
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1 X
Atlanta domain: base case - 60% VOC control (6/4/84)
Atlanta domain: base case minus 60% NO* control
Figures 3-1 (top) and 3-2. Peak ozone difference plots for
Atlanta showing impacts from 60% VOC and NOx control. Positive
and negative values reflect ozone reductions and increases,
respectively, from the base case. The modeling domain consists
of a 40 by 40 array of 4 km grid cells.
3-24
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a
03
O
C
•
0)
C
O
N
O
Dallas-Fort Worth domain: 1995 base case - 60% ROG control
JO
Q.
O
C
0)
C
O
N
O
Dallas-Fort Worth domain: 1995 base case - 60% NOx control
Figures 3-3 (top) and 3-4. Peak ozone difference plots Dallas-
Fort Worth showing impacts from 60% VOC and NOx control. Plus and
minus values reflect ozone reductions and increases,
respectively, from the base case. The modeling domain consists
of a 40 by 45 array of 5 km grid cells.
3-25
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increases due to NOx controls (Fig. 3-4).
The ROMNET study (EPA, 1991h) extended the limited number of
model simulations which considered biogenics and explicitly
compared effects of strategies emphasizing NOx controls with
those emphasizing reductions in VOC. Some of the important
relevant ROMNET findings include:
I. NOx controls show greater reductions in peak-ozone
throughout most of the ROMNET domain, relative to VOC
controls,
2. NOx controls increase peak ozone in certain areas of
the New York City CMSA, and
3. VOC controls are effective in reducing New York City
CMSA peak ozone,
4. the benefits of NOx controls increase as longer ozone
averaging times (e.g., 8 hours) are used as the
analysis metric, and
5. conclusions regarding relative benefits of NOx or VOC
controls show day-to-day variations for the Baltimore -
Washington D.C. area.
The ROMNET study suggests that NOx controls also are
beneficial in areas outside the southeast U.S. The results
showed rather clear benefits of NOx controls in rural/semi-rural
northeast regions. Furthermore, the benefits may extend beyond
the rural regions since net ozone flowing into cities (i.e.,
transported ozone) is reduced. In light of the NAS assertion
that anthropogenic VOC emissions have been understated, the
ROMNET results could be interpreted as over-optimistic with
respect to the beneficial impacts of VOC controls.
The evidence from these and other studies suggests that
analysis of NOx benefits is best conducted through photochemical
grid modeling, a view supported by the NAS. Less rigorous
approaches are not advised. The EPA investigated the feasibility
and acceptability of applying relatively inexpensive screening
techniques (Langstaff and Scheffe, 1991) to the needs of the
NOx/VOC control issue. Those techniques appeared to generate
more questions than they answered and therefore were not adopted.
The prevailing opinion of a spectrum of government, academic ana
industrial groups is that photochemical grid modeling is needed
zo fuiiy address ~he NOx issue.
These ROM studies, while highly significant, do not provide
complete evidence on the relative efficacies of the VOC control
and NOx control approaches to attainment. First, the ROMNET
study was not designed to address this latter objective. It was
3-26
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intended instead to determine how best to use currently available
VOC control and NOx control technologies for ozone reduction.
Thus, relative impacts of VOC and NOx controls were derived for
only few levels of control and extrapolation of results to other
levels is known not to be valid. Clearly, additional ROM runs
are needed for different levels of VOC and NOx controls before
the relative efficacies of the VOC and NOx control approaches can
be assured reliably. Second, questions are raised because ROM's
spatial resolution is too coarse for adequate simulations of
ozone on the urban scale. The NOx emission sources are spread
across too large a volume in the regional model to properly treat
the urban scale atmospheric chemistry. In general, the regional
model will cause lower VOC/NOx ratios in the grid cells
containing urban areas by introducing NOx emissions in areas that
do not, in reality, contain emission sources. This effect, as
well as the understated-VOC-emissions problem, biases the ROM
results in favor of VOC control over an exaggerated spatial
extent. On the other hand, for those areas in the urban complex
directly affected by NOx plumes, the regional model dilutes the
NOx emissions through too large a volume, thereby raising the
ratio compared to what is really occurring in the plume's
vicinity. Finally, the ROM studies have focused on the northeast
U.S. Comparable studies in other Regions are also needed before
a national picture can emerge regarding the relative
effectiveness of VOC and NOx controls. Of all these issues,
emission inventory accuracy is probably the most important one,
and it is imperative that the issue be resolved before it can be
determined whether NOx control is indeed as beneficial as the
existing evidence suggests and NAS supports.
The statutory requirement to conduct photochemical grid
modeling is accomplished through the SIP process. All ozone
nonattainment areas classified as "serious" or above and
interstate moderate areas are required to submit an attainment
demonstration based on gridded photochemical modeling as part of
the SIP submittal due in November, 1994.
In addition, the grid-based modeling results are amenable to
a wide spectrum of time, spatial, demographic, and economic
analyses. Thus, assessments of various control strategies can
include consideration of population exposure impacts.
Consideration of population exposure effects arising from NOx
control measures may be critical because NOx controls, unless
very large, can increase ozone in certain locations, and decrease
^zone _n other locations within a oroad area. Further, the
relative impacts on population exposure arising from VOC or NOx
controls .~ay oe area specific, rather than exhibiting any general
trend favoring a unilateral precursor control approach.
The important conclusion from this analysis is that, as
pointed out by NAS and agreed by EPA, the latest evidence
suggests that the ozone precursor control effort should focus on
3-27
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NOx controls in many areas. The development and implementation
of control programs should not be hindered by a bias favoring one
control direction over another. This is extremely significant
because it raises questions regarding the effectiveness of the
VOC and NOx control programs mandated by the current CAA.
Because of these implications and the fact that the evidence, as
explained above, has limitations, it is imperative that more
responsive modeling studies of highly polluted areas addressing
the specific issue of relative effectiveness of VOC and NOx
controls receive immediate and highest priority.
3-28
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3.9 Alternative Fuels for Motor Vehicles
3.9.1 HAS finding
The use of alternative fuels has the potential to improve
air quality, especially in urban areas. However, the extent of
the improvement that might result is uncertain and will vary
depending on the location and on the fuels used. Alternative
fuel use, alone, will not solve ozone problems nationwide.
Moreover, it will not necessarily alleviate the most critical
problem associated with motor vehicle emissions—increased
emissions as in-use vehicles age.
3.9.2 NAS recommendation
Because there is uncertainty about the degree to which
alternative fuels would reduce ozone, requiring the widespread
use of any specific fuel would be premature. An exception may be
electric vehicles, which can lead to substantial reductions in
all ozone precursor emissions. Coordinated emissions measurement
and modeling studies should be used to determine which fuels will
work best to control formation of ozone.
3.9.3 EPA comment
Motor vehicle emissions constitute roughly 35% of all man-
made VOC and NOx emissions, and 65% of total CO emissions.
Clearly, efforts towards reducing reactive VOC and CO3 emissions
from this major group of emission sources should yield air
quality benefits. The term "alternative fuels" is interpreted
broadly in this section to include, among others, alcohols and
alcohol/gasoline blends (e.g., methanol and ethanol),
reformulated gasoline, natural gas, liquified petroleum gas,
hydrogen or electricity. Several documents describing the
expected air quality benefits from certain alternative fuels are
available (EPA, 1988; EPA, 1989; EPA, 1990 c,d,e).
Alternative fuel programs represent one facet of an array of
CAAA programs designed to reduce ozone and air toxics pollution.
In particular, the CAAA require the introduction of clean fuel
vehicles in (1) centrally fueled fleets in over 20 areas and (2)
a pilot program for California where these vehicles must
specifically be designed to meet stricter tailpipe emission
standards regardless of what clean fuel is used. Also, the CAAA
require introduction of reformulated gasoline. A sunset of nine
of the worst ozone nonattainment areas are required to have
reformulated gasolene programs in i995. Reformulated gasoline
3Carbon monoxide (CO) plays an important role in ozone
formation and reductions of CO generally lead to reductions in
ozone.
3-29
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use would be required in all gasoline vehicles in these areas.
The reformulated fuel program mandates a 15% VOC and toxics
reduction by 1995 and at least a 20%
reduction in 2000, relative to baseline gasoline use (CAAA,
Section 219).
The following assessment clarifies the benefits to be
expected from use of alternative fuels in highly polluted areas.
The assessment is based on current evidence on the role of the
VOC/NOx ratio factor, relative importance of auto emissions in
current and future atmospheres, and on effectiveness of current
or targeted emission control technologies.
Most alternative fuel programs currently are expected to
result in reduced amounts and/or reactivities of VOC emissions.
Fuel programs mandated by the CAA do not require NOx reductions
although, based on current knowledge, NOx reductions may be
possible. The VOC reductions from the CAAA alternative fuels
programs are expected to produce ozone benefits in urban areas
with relatively low VOC/NOx ratios and lesser benefits in rural
areas and cities with high VOC/NOx ratios. Benefits in future
years are not easy to predict because of uncertainties in growth
of vehicle miles travelled which may vary widely among cities.
Ozone reductions associated with auto vehicle-based emission
controls will stabilize once new standards are fully phased in.
Reductions associated with fuel modification will diminish as the
older technology cars that are most responsive to these
modifications leave the fleet. The beneficial effect of VOG
emission reductions on ozone air quality should increase as the
VOC/NOx ratios diminish due to implementation of future auto
emission standards and other regulations that are in place.
Of the alternative fuels contemplated for future use, the
reformulated gasoline is the only one for which widespread
introduction is required. VOC reductions are expected to reduce
ozone in the nine areas where reformulated fuels are required.
Auto/Oil Air Quality Improvement Research Program (AQIRP) data
clearly support the potential of gasoline reformulation to abate
ozone. Since the CAAA require only that NOx emissions with
reformulated gasolines be no greater than that with conventional
gasolines, less ozone benefit would be expected in areas where
ozone reduction requires reduced NOx emission. However, recent
data suggests there may be NOx side benefits from changes in
gasoline composition (such as reducing fuel sulfur levels) to
achieve VOC reductions. Also, the EPA has general authority for
establishing NOx reduction reauiraments. Most, other a!tsrr.ai:iTTe
fuel programs (e.g., fleet programs, California Clean Car Pilot,
etc.) are, at least initially, relatively small. Although these
programs merely allow for compliance via alternative fuels rather
than specifically requiring them, they may add an economic
advantage to alternative fuels. The intrinsic cleanliness of
electric, compressed natural gas, and alcohol fuels (with respect
3-30
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to evaporative emissions) could provide greater benefits than
standard gasoline if emission control devices fail.
In considering the potential for ozone abatement with
alternative fuels, it is important to recognize that today's
literature simply provides a "snapshot in time". The AQIRP
reformulated gasoline activities have focused on VOC, toxics, and
CO reduction. The data frequently show NOx reductions as well
when certain fuel parameters are changed as they will be with
gasoline reformulations. Thus, it is recognized that there exist
possibilities with alternative fuels for reductions of both VOC
and NOx that do not exist with gasoline. For example, the
relatively high octane characteristics of several alternative
fuels provide greater possibilities for manipulation of spark
timing to manage NOx emissions; many of the alternative fuels
provide the possibility for leaner engine combustion and a new
regime of associated catalyst technologies; alcohol fuels
typically have lower peak combustion temperatures than gasolines,
favoring lower NOx emissions; and many of the alternative fuels
have naturally lower sulfur levels providing for improved long
term catalyst performance (and lower NOx). Because the
alternative vehicle-fuel technologies may have different emission
control approaches, their durabilities with mileage accumulation
in consumer fleets may be different. NOx emissions can be
reduced by alternative modal manipulation of engine air/fuel
ratio, spark timing, valve timing, etc., and change in fuel
composition. Also, stratified charge engines, which are more
feasible with alcohol fuels than with gasoline, can have
significant lower NOx. Some work with CNG and alcohol fueled
vehicles already show newly developed engines with low NOx
compared to conventional gasoline or diesel engines. These,
clearly, are areas for focus of future alternative fuels
research.
In summary, alternative fuels can be part of an ensemble of
contributions from several programs that are needed to make
significant progress in abating ozone pollution.
3-31
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3.10 A Research Program on Tropospheric Ozone
3.10.1 NAS finding
Progress toward reducing ozone concentrations in the United
States has been severely hampered by the lack of a coordinated
national research program directed at elucidating the chemical,
physical, and meteorological processes that control ozone
formation and concentrations over North America.
3.10.2 NAS recommendation
A coherent and focused national program should be
established for the study of tropospheric ozone and related
aspects of air quality in North America. This program should
include coordinated field measurements, laboratory studies, and
numerical modeling that will lead to a better predictive capa-
bility. In particular, the program should elucidate the response
of ambient ozone concentrations to possible regulatory actions or
to natural changes in atmospheric composition or climate. To
avoid conflict between the long-term planning essential for
scientific research and the immediacy of requirements imposed on
regulatory agencies, the research program should be managed
independently from the EPA office that develops regulations under
the Clean Air Act and from other government offices that develop
regulations. The research program must have a long-term
commitment to fund research on tropospheric ozone. The direction
and goals of this fundamental research program should not be
subjected to short-term perturbations or other influences arising
from ongoing debates over policy strategies and regulatory
issues. The program should also be broadly based to draw on the
best atmospheric scientists available in the nation's academic,
government, industrial, and contract research laboratories.
Further, the national program should foster international
exchange and scientific evaluations of global tropospheric ozone
and its importance in atmospheric chemistry and climate change.
The recommended tropospheric ozone research program should be
carefully coordinated with the Global Tropospheric Chemistry
Program currently funded and coordinated by the National Science
Foundation (NSF) and with corresponding global change programs in
the National Aeronautics and Space Administration (NASA), the
National Oceanic and Atmospheric Administration (NOAA), the
Department of Energy fDOS), and other agencies.
2.10.3 3PA icramenc
The NAS finding that a coordinated national research program
is crucially needed was fully endorsed by the Agency and a
commitment was made to commence implementation immediately. The
EPA has begun a serious public/private sector research
coordination effort for base programs; and has completed a joint
3-32
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effort to identify and plan needed additional tropospheric ozone
research. EPA, in conjunction with the National Oceanic and
Atmospheric Administration (NOAA) and the Electric Power Research
Institute (EPRI), has convened an executive level Research
Cooperators Group representing 45 major organizations. This
group of executives has met twice in the past year to explore
research needs, priorities, and opportunities for cooperation,
and will continue to do so.
During the last half of 1992, the EPA convened a working
group of 140 senior scientists representing a cross-section of
disciplines and public and private organizations to design a
national research initiative. They produced the "Coordinated
North American Research Strategy for Tropospheric Ozone." In it
is a strategic framework for the 43 research questions and 139
corresponding research projects which must be undertaken to
produce the key scientific information needed to effectively
address the ozone problem. The strategy offers alternative
options for conducting research on a range of priority issues
that will cost from $600 million to $1 billion in new resources
over the next 10 years. Clearly, no one organization can supply
this level of funding. Cooperation from the participating public
and private organizations is key to the success of the research
strategy.
Many organizations have worked to establish highest
priorities for the research program. The next step is to secure
funding partnerships, through a combination of government and
private funding, to begin as much of the priority research as
possible. EPA has asked that the NAS/NRC extend the function of
its 185B Committee on Tropospheric Ozone Formation and
Measurement to serve in an advisory and peer review capacity in
setting up the coordinated national research program.
Progress towards achieving ozone attainment has been
hampered by numerous factors which all relate to the evolutionary
nature of atmospheric chemistry and a scarcity of observational
data to check and correct our predictive methods and control
implementation programs. The system in place within EPA for
planning, conducting and evaluating research provides for
soliciting and utilizing input from the users of the research
products, but also from the scientific community at large.
Periodic peer reviews of EPA research plans and work are
conducted to provide sufficient input from outside scientists to
ensure maximum credibility and scientific merit of EPA's ozone
research program. The principal focus wichin IPA'3 Office of
Research and Development is on advancing the science; relevance
of the research effort to the regulatory programs also is
considered.
The achievements of EPA's research program demonstrate an
effective effort based on efficient use of limited research
3-33
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budgets. The EPA conducted or supported most of the basic
chemistry and smog chamber studies leading to development of
chemical mechanism models which are the framework for past and
current ozone research and regulatory efforts. Large scale
efforts to develop photochemical models (such as the UAM, ROM,
and RADM) for ozone air quality were initiated and sustained by
EPA. Similarly, EPA research lead to various ozone-related
pollutant measurement and characterization techniques:
chromatographic analysis of ambient organics, remote sensing
methods for ozone and for ozone precursor emissions, and
reactivity-characterization of organic pollutants. In addition,
EPA research played a major role in measuring and characterizing
emissions from natural, stationary and mobile sources. These and
other accomplishments have been recognized by NAS, as reflected
by the frequent use of EPA studies as a basis for the NAS
findings.
Despite past scientific advances, gaps and uncertainties in
knowledge and controversial issues hamper progress toward ozone
control. Significant challenges remain due to the complexity of
the physical and chemical processes underlying ozone formation
and control, as well as the difficulty and cost involved in
obtaining reliable and sufficient ambient monitoring and
emissions data.
The EPA agrees in concept with all NAS recommendations for
new, needed research; such recommendations repeatedly have been
submitted by EPA researchers. Research needs clearly must be
prioritized to effectively utilize available resources. EPA's
current planning efforts include prioritizing tasks emerging from
the list of NAS recommendations. Most NAS recommendations were
motivated by the fact that the historically high emphasis on VOC
controls, rather than NOx, was partly responsible for the current
nonattainment status of several areas. The solution of this
problem is not a simple redirection of the control effort
everywhere. Current belief is that control of VOC will continue
to be the most effective approach to ozone reduction in some
areas, as NOx control or combined VOC+NOx controls would.be in
others. Key to development of successful ozone control
strategies is the availability of reliable area-specific evidence
on the relative efficacies of the VOC control and NOx control
approaches. Accordingly, the development of such evidence for
assessing effectiveness of ozone precursor control strategies is
one of the most important objectives receiving highest priority
in cne Agency's ozone research effort.
Research programs designed to improve model and data base
systems to develop optimum ozone strategies are under way within
the EPA. Observations-Based Modeling (OBM) methods are being
developed within the Southern Oxidants Study (SOS) --a large
field/modeling research program on the photochemical ozone
problems in the Southeast. OBM methods can assess the relative
3-34
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benefits of VOC and NOx controls. Since OEMs are driven by
ambient measurements, they complement the emissions driven grid
models and potentially offer strong corroborative support for
control strategy assessments. The OEM development program was
initiated in 1991 and, assuming current rate of progress
continues, is expected to provide usable methods and conclusive
strategy-related evidence for the South by the late 1990's.
The EPA's base research program continues to develop and
refine the Emission-Based Modeling (EBM) methods required by the
Clean Air Act. Doubt exists regarding the credibility of
predictions based on previous EBM applications. This credibility
problem is due to uncertainties both in the models and in the
requisite emission inventory (El), air quality and meteorological
inputs for operating EBM's. On-going EPA research programs and
plans address these uncertainties, and improvements are expected
to be achieved in the future. The EPA plans to conduct extensive
field, laboratory, emission inventory and modeling studies that
would result in EBMs and associated emission inventories that
could be used with confidence to improve ozone strategies for
southern cities as part of the SOS. Such efforts, contingent on
expected levels of resources, are targeted for completion by 1998
and would be conducted in cooperation with other Federal and
state government agencies and private research institutions. An
analogous program for northern cities is planned for the early
2000's.
Emission inventory research, supported also by the
utilities, auto and oil industries and other governmental
agencies, is focused on improving estimates for three areas of
greatest uncertainty: natural sources, mobile sources, and large
numbers of area-wide sources. Estimation protocols will be
developed for these sources with subsequent field validation of
emission rates. The validated methods will be used to develop
improved emissions inputs for the EBMs by the late 1990's.
Establishment and operation of a first PAMS network in the mid-
1990 's is expected to provide aerometric data to partially
support model evaluations and applications. Enhancement of the
PAMS capability through other intensive programs to fully meet
the modeling needs is not anticipated until several years later.
The EPA plans provide for sustaining a development program
expected to produce the following new generation air quality
concepts by the mid-1990's: (1) Mcdels-3 system of Catalogued
EBMs, (2) OEMs, (3) chemical mechanisms for ozone with improved
irsacment of biogenic and anthropogenic VGC's, particular!-- in
the low VOC, NOX concentration area, and (4) mechanistic concepts
such as the Australian GRS mechanism. These projected
accomplishments will be of partial use in the 1999 round of SIP
revisions, and of full use in subsequent rounds.
3-35
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The NAS also offered recommendations concerning the
management of the EPA's research programs. The comments on
interaction between the research and regulatory components of EPA
and on utilization of outside scientific talent have been
addressed above. The EPA agrees with NAS on the need to foster
international exchange on ozone science, and has been pursuing
such exchanges with Europe, Canada, Mexico, Japan, and Australia.
Environmental Agreements with Germany, Mexico and Japan have
facilitated several interactions. The EPA has been an active
participant in interagency cooperative activities within the
Federal Government, including membership in the Federal
Coordinating Committee on Science, Engineering and Technology,
ties to the NRC Committee on Atmospheric Chemistry, and
interagency agreements with NOAA, NSF, DOE, NASA, and DOC/NIST.
Clearly, a well managed research program, closely coordinated
with other ozone research programs within and outside the Federal
Government and the country, is a prerequisite to efficient use of
the resources available to obtain, disseminate, and utilize the
scientific information needed.
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SECTION 4 - OVERVIEW OF NOX CONTROL TECHNOLOGIES1
4.1 Int r octuc t ion
Nearly all anthropogenic
NOx emissions produced from
stationary or mobile sources
result from combustion of
fossil fuels (Figure 4.1).
Combustion-derived NOx is
formed through two mechanisms,
thermal NOx and fuel-bound
NOx. Thermal NOx occurs at
high flame temperatures
through oxidation of molecular
nitrogen in air and is the
principal component from fuels
which contain little or no
nitrogen (e.g., natural gas or
distillate oil). Fuel NOx
occurs through oxidation of
nitrogen species contained in
the fuel and is the principal
component from higher nitrogen
fuels such as heavy oil and
coal. Noncombustion generated
NOx emissions accompany
chemical manufacturing
processes such as nitric acid
and explosives production.
Industrial
Processes
(3.0%)
Other Solid
(1.5%) Waste
^ (0.5%)
Fuel
Combustion
(56.9%)
Control techniques. NOx
control methods (1) modify the
combustion environment so that
less thermal and/or fuel NOx
is generated, and/or (2)
chemically treat flue gas
emissions to transform NOx to
molecular nitrogen. [Fuel
switching is also considered a NOx control technique.] NOx
generation is enhanced when high flame temperatures and excessive
oxygen are available. Accordingly, combustion-based control
Figure 4-1. Distribution of
1990 NOx emissions by major
source category. Transportation
fraction also reflects fuel
combustion. Data taken from
EPA, 1991a.
LThis section summarizes basic information presented in
Accacnmenc .1 on available NOx control cecnnoiogo.es for scac^onary
sources, and briefly addresses mobile source control technologies.
Since the completion of Attachment 2 in early 1992, several
additional Available Control Technology (ACT) documents addressing
various NOx control techniques have been published by EPA.
4-1
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technologies include an array of modifications to operating
conditions and burner design which achieve lower temperatures and
richer fuel to oxygen ratios (i.e., more fuel relative to
available oxygen). Examples of these modifications are low
excess air (LEA), staged combustion {overfire air (OFA), biased
burner firing (DBF), and burners out of service (BOOS)}, flue gas
recirculation (FGR) and steam/water injection cooling. Low-NOx
burners (LNB) are burners based on staged combustion principles,
which are used in retrofitting operations and new facilities.
Repowering is alternative retrofit approach that involves a
basic change in the type of combustion system and extensive
modification of the power plant. Two examples of repowering
technology are: conversion of the boiler portion of the system
from a pulverized coal-fired burner to a fluidized bed combustor;
or installation of an integrated gasification combined cycle
system (IGCC) requiring the addition of a coal gasifier and a gas
turbine power generator. The normal motivation for repowering is
to expand generation capacity; however, some systems, especially
IGCC, can provide highly effective NOx control.
Flue gas treatment techniques typically are based on the use
of ammonia (NH3) or other reductants like urea to reduce NOx to
molecular nitrogen. The most common techniques are selective
catalytic reduction (SCR) and selective non-catalytic reduction
(SNCR). Various catalysts used in SCR facilitate ammonia-induced
NOx reduction at different temperature ranges and operating
conditions. Thus, SCR systems are considered the most effective
NOx reduction methods available (Gao et al., 1991); however, they
are much more expensive than combustion modificatiion systems.
SNCR use is somewhat restricted by temperature dependence and
reaction time requirements. Non-selective catalytic reductions
(NSCR) do not use ammonia and reduce VOCs and carbon monoxide in
addition to NOx. Table 4.1 summarizes basic features of control
technologies for stationary sources.
Fuel switching can effect substantial NOx emission
reductions by burning a cleaner fuel in the ozone season. For
example, the use of natural gas instead of coal could also reduce
annual and summertime emissions of sulfur dioxide (SO2), carbon
dioxide (CO2), and particulate matter (PM-10). Further, emission
reductions of these pollutants may be especially effective in the
summer with respect to visibility and PM-10.
4-2
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Industrial NOx controls often are applied in combination.
Multiple modifications of operating conditions, as well as
combined combustion modification and flue gas treatment are
viable control approaches.
Conditions which favor NOx reduction might reduce operating
efficiencies. For example, fossil fuels burn more completely
when high temperatures and excessive oxygen prevail. The modest,
less expensive techniques which modify operating conditions may
be accompanied by attendant efficiency reductions and increased
carbon monoxide emissions.
4-3
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Table 4.1 Summary of Basic Control Technologies for Reducing NOx
from Stationary Sources.
Technology
Combustion based
Low Excess Air (LEA)
Staged Combustion
overfire air
(OFA)
biased burner
firing (BBF)
burners out of
service (BOOS)
Low-NOx burner (LNB)
Description
provide less than normal air
to reduce oxidation through
various equipment adjustments
initial combustion in fuel
rich zones followed by leaner
zones to complete combustion
rich burn with additional air
at top of normal combustion
zone
rich burn at low burner rows
with additional air at upper
rows
use of "air only" burners with
total fuel directed to fewer
burners
installation of new burners
incorporating staged design
concepts
Flue gas recirculation
(FGR)
Steam/water injection
cooling
recycling portion of flue gas
back to primary combustion
zone to lower peak flame
temperature and thermal NOx
induces direct cooling in
combustion zone to reduce
thermal NOx
4-4
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Table 4.1 continued
Repowering
fluidized bed
IGCC
Fuel Switching
Flue Gas Treatment
Selective Catalytic
Reduction (SCR)
Nonselective catalytic
reduction (NSCR)
Selective noncatalytic
reduction (SNCR)
replacement of burner/boiler
system with a different
combustion system
lower combustion temperatures
suppress thermal NOx and
design may control fuel NOx
coal gasification system may
remove fuel nitrogen and low
NOx turbine combustor can
suppress thermal NOx
temporary switch to cleaner
burning fuel during summer
ozone season
ammonia-induced reduction of
NOx to N2 aided by a catalyst
reduction of NOx and VOCs by
available carbon monoxide
aided by a catalyst
ammonia-induced reduction of
NOx without a catalyst
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4.2 Source category specific control techniques
Principal combustion related stationary sources accounting
for roughly 57% of all anthropogenic NOx emissions are utility
and industrial boilers, petroleum refinery operations, and
natural gas transmission engines (Figure 4-2).
7 -
6 -
5-
1^
|4-
5
2 -
1 -
™
^
V/
%/
///
W
W/
\
NOx emissions by major source categories
coal-fired
tvxl f'TTI 777
(tons/year)
0 1 1 1 1 1 1 1
utility industrial nat. gas petroleum cement pulp nitric
boilers boilers transmission refin- manufac. paper acid production
13 -
12 -
11 -
10 -
_ 9 -
1 8 "
i ^ -
a
" 5 -
4 -
3 -
2 -
1 -
0-
^
0--X;
\^\\.
¥
coal
i
1
\^
\$
%
Xx
•v^
^
%
,\\
XV
^N\
Number of sources by source category
XV
X\J
v\x
_u
Figure 4-2. NOx emissions by source
category (above) and source distribution
(below) among major stationary sources
greater that 300 tons/year. Daua based
on 1985 NAPAP annualized emissions
estimates.
Transportation sources; including passenger cars, gasoline and'
diesel trucks, off-highway vehicles, and air, rail and marine
vessels, account for roughly 38% of all anthropogenic NOx
emissions (EPA, 1991a). As discussed above, two basic principles
4-6
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(combustion modification and flue gas treatment) underlie most
NOx reduction approaches. Since most NOx emissions are
combustion derived, NOx emissions from several of the major
source categories discussed below are controlled by similar
technologies.
4.2.1 Boilers
NOx control approaches for coal-, oil- and gas-fired
utility boilers in the United States have focused on combustion
modification techniques over the past two decades. Flue gas
treatment techniques are more widely applied in Germany and
Japan. Low excess air (LEA) is easy to implement in existing and
new boilers and is widely applied throughout the U.S.
Coal-fired utility boilers. Generally, LEA is supplemented by
other modifications resulting in total control efficiencies for
coal-fired boilers ranging from 15 to 60% for various forms of
Low NOx Burner Technology (LNBT). When low NOx burners
incorporate overfire air, the potential NOx emission reduction
can range from 40 to 60%. Several examples of LNB applications
are listed in Attachment 2. Repowering technology is being
demonstrated at a number of coal-fired utility power plants as
part of the Department of Energy (DOE) Clean Coal Technology
Program. The potential NOx reductions are projected to range
from 50 to 95%, depending on repowering type and specific design.
SCR applications are widespread in Japan and Germany, and recent
applications have emerged in Austria. U.S. applications have
been limited to a few demonstrations. SCR has been applied to
more than 100 utility boilers in Japan, and a similar number of
SCR units operate in Germany. NOx reductions from 70-90%
typically are associated with those applications. Applications
of SCR in the United States, which include gas turbines, other
industrial combustion sources, and municipal waste combustors,
are typically designed for 80 to 90% removal. Two SNCR
applications on coal-fired boilers have also been running very
successfully on a long term basis since 1989 and 1990.
Oil and gas-fired utility boilers. Similar combustion
modification techniques are applied to oil- and gas-fired
boilers, although staged combustion modifications (BOOS and BBF)
and FGR are more prevalent, relative to coal-fired boilers.
General NOx reductions of about 40% are reported for low-cost
combustion modifications, and reductions to 60% are associated
with LNB systems. Repowering technologies are potentially
applicable to this class of equipment as well. Also, full-scale
applications or 3CR aga^n ars _.i:n:iced co Japan and Germany.
Certain California based utilities plan to implement SCR systems
by the mid-1990's. SCR NOx reduction efficiencies from 75-80%
are reported for oil and gas-fired boilers. Limited SNCR
applications report NOx control efficiencies of about 50 to 60
percent.
4-7
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Other boilers. Control technologies applied to smaller scale,
industrial, commercial and institutional boilers used in
manufacturing processes and heat and electricity generation are
similar in extent and efficiencies to those noted for large scale
utilities. Fluidized bed combustors may be used for
commercial/industrial heat and steam generation; however, these
are normally new, not repowered, systems.
4.2.2 Stationary Internal combustion (1C) engines
Stationary 1C engines which burn natural gas or oil are used
in a number of utility, commercial, industrial and municipal
operations. 1C engines pump natural gas through extensive
pipeline networks. Several types of engines and turbines exist,
many that are 50 or more years old.
Reciprocal engines. Combustion-based control technologies
include pre-ignition chamber combustion, ignition timing
retardation, air/fuel mixture adjustment, and exhaust gas
recirculation (EGR). NOx reductions range from 20 to 80%
depending on engine type and applied control technology.
SCR applications in the U.S. have been limited largely to
natural gas-fired engines in California with NOx reductions
approaching 90%. SCR has been applied to gas- and oil-fired 1C
engines in Japan and Germany. Non-selective catalytic reduction
(NSCR) has been demonstrated to achieve 90% NOx reductions (in
addition to attendant VOC and CO reductions) on numerous tests
performed on rich-burning units in California.
Turbines. Water/steam injection cooling frequently is applied to
gas- and oil-fired turbines resulting in NOx reductions from 70
to 85 percent. Various combustion modification.techniques are
also applied to turbines. SCR is used on over 70 turbines in the
U.S. to supplement reductions from steam/water injection or
combustion modifications. NOx reductions as high as 90% have
been reported.
4.2.3 Industrial processes involving combustion
Petroleum refining and chemical process heaters. Process
heaters are used extensively in a range of refining processes
(e.g., distillation, thermal cracking, and coking) and other
chemical operations. Commonly applied combustion controls are
based around automated LEA and low-NOx burner installations. LNB
^nducsd MOx rad.ucti.cns range from 40 - 70 percent. SCR _LS
applicable to numerous processes and has been installed on more
than 9 refinery process heaters in California, resulting in NOx
reductions up to 90 percent. SNCR also has been installed on
several refinery process heaters in California with NOx
reductions ranging from 35 to 70 percent.
4-8
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Metallurgical processes. Several processes including
pelletizing, sintering, coke ovens, blast furnaces, heat treating
and finishing produce NOx emissions in the iron and steel
industry. Control applications for these operations appear to be
limited to furnace overhaul and LEA, with NOx reductions ranging
from 20 to 70 percent.
Cement manufacturing. Combustion modifications have been
applied to certain cement kilns. Applications have not been
widespread and NOx reductions of about 15 - 30% have been
reported.
4.2.4 Noneombustion industrial processes.
Nitric acid plants are the largest noncombustion generated
source of NOx emissions. Emissions can be controlled on new
plants through advanced process design features such as high
inlet pressure absorption columns or strong acid applications.
Existing plants must rely on flue gas treatment techniques
including extended absorption, SCR and NSCR. Extended absorption
typically is used in retrofit applications by adding a second
absorption tower in series with the primary tower. NOx
reductions of about 95% have been reported. SCR is used
extensively in Japan and Europe. Three U.S. plants apply SCR
with a reported NOx reduction of 97 percent for one plant.
European facilities exhibit NOx reductions from 44 to 87 percent.
Three of the four U.S. adipic acid plants apply extended
absorption or thermal reduction to reduce NOx emissions from 81
to 86 percent. Information concerning existing controls on
explosive manufacturing plants is limited.
4.2.5 Motor vehicles
The EPA's control programs for VOC from mobile sources are
highly visible and well known: vehicle standards, fuel
volatility, reformulated gasoline, etc. The EPA also is
implementing a number of programs to reduce motor vehicle NOx
emissions under Title II. Use of reformulated gasoline is
required to result in no NOx increase; in practice, some
reductions of NOx are expected. Also, Tier I standards for light
duty vehicle NOx emissions have been implemented for 1994, and
final rulema.Ki.ng requiring enhanced inspection/maintenance
programs for a number of areas include a NOx reduction
requirement;. The ZPA implemented, stricter NOx standards for
heavy duty diesels effective with the 1991 model year; NOx
standards for these engines are even lower for the 1998 model
year. Finally, the EPA plans to propose standards to control NOx
from new non-road diesels over 50 horsepower.
4-9
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Motor vehicle NOx from gasoline engines is reduced
catalytically by the 3-way catalyst system with feedback control
to monitor and control the air:fuel mixture with an oxygen
sensor. The feedback control results in stoichiometric operation
'which allows NOx to be reduced to nitrogen (this can occur only
with stoichiometric or rich mixtures; excess air prevents
catalytic NOx reduction) while VOCs and CO are oxidized to H20
and C02 (which can occur with stoichiometric or lean mixtures).
Exhaust gas recirculation also reduces motor vehicle NOx but
frequently is not necessary with the 3-way catalyst. NOx is
controlled from diesel engines by modifications of the combustion
chamber and injection timing.
Just as with gasoline, vehicle NOx levels with alternative
fuels, such as 85-100% methanol, 85-100% ethanol, and Compressed
Natural Gas (CNG), are dependent upon engine design and
calibration as well as emission control design. Peak combustion
flame temperature is lower with alcohol and CNG fuels than with
gasoline which leads to lower NOx. However, other factors such
as engine calibration (e.g., very lean air:fuel ratio, advanced
ignition timing, high compression ratio) with these fuels also
affects NOx. Vehicles or engines using alternative fuels would
have to meet the same NOx standards as those using gasoline (or
diesel) fuel. While most data suggest little overall change in
NOx emissions with alcohol fuels and a potential for increase
with CNG, this work generally has focused on minor adaptations of
engines designed for gasoline, with no priority for NOx control.
Recent work on more fundamentally optimized concepts shows
vehicles can be designed for low NOx with alcohols or CNG. Also,
it is important that available data suggests heavy-duty engines
with alternative fuels could have lower NOx than heavy-duty
diesel engines.
Transformation to clean fuels such as electrified vehicles
produce zero vehicle NOx emissions. However, increased
electricity would need to be generated in order to support
electrified motor vehicle fleets. Some argue (Gao et al., 1991)
that a massive move to electric vehicles could result in reduced
utility NOx emissions since the increased power demand would
force construction of new, efficient, high capacity power plants
to replace certain existing plants.
4.3 Summary
NOx control technologies for a variety of utilicy,
industrial, commercial and other sources involve some form of
combustion modification and/or flue gas treatment or fuel switch.
Table 4-1 summarizes the more common control approaches.
Combustion modifications'and low-NOx burners producing NOx
reduction efficiencies from 15-60 percent have been the prevalent
control approaches on major U.S. utilities, which represent the
4-10
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majority of stationary source NOx emissions. Repowering
technology, which is being demonstrated for coal-fired utility
systems, offers the potential for NOx control from 50-95%, More
advanced and highly efficient SCR applications are widely used in
Japan and Germany with NOx reductions from 70 - 90% or more.
Similar NOx control technologies and attendant efficiencies are
installed on a multitude of other U.S. manufacturing and chemical
industry processes. However, advanced SCR and SNCR are applied
more widely in the chemical industry (limited mostly to select
California sites) than in the utility industry. Fuel switching
approaches during the summer ozone season provide various
environmental benefits in addition to seasonal NOx reduction.
Combustion modification technologies that result in NOx
reductions can decrease operating efficiency and increase other
emissions, particularly carbon monoxide. Repowering is projected
to permit expanded capacity and/or increased generation
efficiency; however, a full assessment of the environmental
impacts of the demonstration system has not been completed. SCR
is extremely effective in reducing NOx emissions but carries
large associated capital and operating and maintenance costs,
relative to combustion modifications. Both the effectiveness and
required maintenance of all technologies depend on several
factors, principally the type of source on which control is
applied. Accordingly, careful formulation of recommended
technologies must consider source type and specific unit
characteristics.
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SECTION 5.0 - SUMMARY
This report provides an assessment of the most significant
issues affecting tropospheric ozone pollution and their
implications for regulatory programs. In addition, the report
presents findings from the NAS report entitled RETHINKING THE
OZONE PROBLEM IN URBAN AND REGIONAL AIR POLLUTION. The EPA
perspectives are then provided on how those findings relate to
previous and existing programs, and the implications they hold
for developing NOx and VOC control strategies. Reductions in NOx
and VOC emissions are likely to reduce ozone in many areas.
However, the latest evidence suggests that NOx controls will be
more effective than VOC controls in many areas. Ozone response
to reductions to VOC or NOx varies from one area to another.
Consequently, control strategy development should be made on a
case-by-case basis through application of gridded photochemical
models. The EPA plans on requiring such applications, based on
the best available data, to meet certain CAAA requirements.
The NAS has questioned the adequacy of existing data bases
to support modeling analyses which are used to design optimal NOx
and VOC emission reduction strategies. The EPA has recognized
many of the same concerns identified by the NAS, and initial
efforts are underway to correct expected deficiencies. These
efforts include (I) a greater emphasis on compilation of high
quality emission inventories, (2) development of an enhanced
ozone and precursor monitoring system to characterize ambient
concentrations and track progress of emissions control
implementation programs, and (3) use of gridded photochemical
modeling for SIP planning.
Despite these initial efforts, several concerns remain which
pose significant challenges to both research and regulatory
programs:
1. Current air quality and meteorological monitoring
networks and emission inventories may not provide
sufficiently comprehensive data sets to reduce the
uncertainty associated with applying photochemical
gridded models. The EPA believes that the models will
provide accurate directional indications of the
relative effectiveness of NOx or VOC control, but the
level of confidence in the precision of control level
estimates generated by the models is compromised by
deficiencies in current routine data bases.
2. Avenues of support for designing and implementing
research grade laboratory and field studies to fully
address the serious concerns raised by the NAS have not
been identified. Aggressive and innovative approaches
fostering private and public sector partnerships are
needed to (I) ensure adequate support of new research,
5-1
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(2) develop concurrence on the interpretation of
current techniques and (3) improve confidence of future
applications.
3. Current programs do not account for model uncertainty.
This inadequacy is serious considering the costliness
of emission control programs. Techniques to consider
model uncertainty should be developed as they will
provide important information for decision makers and
assist in defining needed research efforts for model
and monitoring improvements.
Following is a synopsis of 11 key findings. The first ten
of these are NAS findings followed by EPA's response in italics.
The eleventh finding on NOx control technology was not addressed
by the NAS:
1. Ozone in the United States - Ozone remains a widespread
and important problem despite repeated abatement programs. EPA
agrees that ozone reduction efforts have not achieved previous
goals and that new research and control initiatives will have to
be undertaken.
2. Ozone Trends - The metric used to identify ozone trends
by the EPA is highly influenced by meteorology and unreliable as
a measure of progress. The trends analysis tracks a key
indicator relevant to human and welfare impacts. Other methods
designed to account for meteorological influence on ozone
concentrations are being investigated and encouraged by the EPA.
3. State Implementation Planning - While sound in design,
the SIP process is flawed in practice by the lack of an adequate
verification program. A new monitoring program which is designed
to track progress towards attainment, reductions in emissions,
and provide information needed to make mid-course adjustments is
under development to make the SIP process more complete. The
PAMS program is facing substantial challenges which will require
increased support to successfully fulfill those objectives.
4. Anthropogenic,. YOG _Emiss_ions - Emission inventories
understate anthropogenic VOCs. This may lead to incorrect
development of control strategies. EPA has accelerated and
enhanced its efforts to improve data and emission estimation
techniques to develop an accurate nationwide VOC and NOx
inventory. Additional efforts are needea; particularly pressing
is the need for a true research level inventory addressing-
-episodic conditions to further improve the inventory process
beyond ongoing efforts.
5. Bioqenic VOC Emissions - Biogenic emissions have
significant impacts on ozone formation. The NAS confirms EPA
findings. Based on recent studies, EPA requires that biogenics
5-2
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be included in photochemical grid model applications including
those which assess the effectiveness of emission control
strategies. The large uncertainty in biogenic emission estimates
demands that a high priority be placed on characterizing natural
emissions.
6. Ambient Air Quality Measurements - Routine measurements
do not elucidate relevant chemical processes needed to assess
source contributions. A new monitoring program mandated by the
CAA is under development as an initial step toward applying
advanced monitoring methods routinely in key areas. However,
support for additional methods research and data acquisition is
required to address issues raised by the NAS, particularly the
need for monitoring low-level NOx in rural environments.
7. Air-Quality Models - Grid models are preferred control
strategy assessment tools; however, parts of the model structure
and supporting data bases (particularly emissions) contain
important areas of uncertainty. Lacking pertinent information,
the EPA has not considered model uncertainties in development of
emission control programs. The EPA requires use of the best
available models and data bases within practical resource and
time constraints. Both the NAS and EPA agree that research to
reduce model uncertainties has immediate and highest priority.
8. VOC versus NOx control - NOx control is necessary for
effective reduction of ozone in many areas, and more NOx control
should be emphasized than is allowed under current EPA strategy.
NOx control is expected to provide ozone reductions in many
areas. Area-specific and regional analyses using photochemical
grid models, as required under the 1990 CAA for more seriously
polluted ozone nonattainment areas, will be used to assess the
effectiveness of control strategies. Such objective analyses
should not be biased toward any single precursor.
9. Alternative Fuels for Motor Vehicles - The CAA requires
widespread use of reformulated gasoline and will likely lead to
limited use of other clean fuels in large urban areas with
serious ozone problems. The NAS concluded that alternative fuels
by themselves will not solve the ozone problem. VOC reductions
from the reformulated gasoline program are expected to reduce
ozone in the nine areas where reformulated fuels are required.
There also is the potential for NOx reductions with the use of
reformulated fuel, as well as other alternative fuels such as
alconols and CNG.
10. A.. Researcn Program on Tropospneric Ozone - A coordinated
national research program is needed. The EPA agrees with the
NAS's assessment of the national research effort. The NAS made
numerous recommendations for specific new research needed. The
EPA has commenced implementation of such recommendationsr
starting with a major cooperative, planning effort with other
5-3
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Federal agencies, industry and academia to develop a national,
coordinated research program.
11. Availability and Extent of NQx Control -
Stationary Sources - Combustion modifications are used
extensively in the U.S., relative to flue gas treatment
techniques which are more widely applied in Japan and Germany.
Mobile Sources - The EPA has a wide variety of programs underway
to reduce mobile source NOx. These include stricter Tier I
standards for passenger cars and light duty trucks. Also,
stricter NOx standards are required for heavy duty diesel
engines, and enhanced vehicle inspection and maintenance programs
have been promulgated that will result in NOx benefits from in-
use vehicles. Finally, the EPA plans to propose non-road
emission standards for NOx reductions for diesels over 50 HP.
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SECTION 6.0 - REFERENCES
Altshuller, A.P. 1983. Assessment of Natural Volatile Organic
substances and their effect on air quality in the United
States. EPA-600/D-83-047. Environmental Protection Agency,
Office Research and Development, Research Triangle Park,
N.C.
Baugues, K. 1991. Reconciling Differences Between Ambient and
Emission Inventory Derived NMOC/NOx Ratios: Implications for
Emission Inventories. Presented at the 84th AWMA
Conference, Vancouver, BC.
Chameides, W.L., R.W. Lindsay, J. Richardson and C.S. Kiang.
1990. The role of biogenic hydrocarbons in urban
photochemical smog: Atlanta as a Case Study. Science,
241:1473.
Cox, W.M., and S-H Chu. 1991. Meteorologically adjusted ozone
trends in urban areas: a probability approach. Presented at
the Tropospheric Ozone Specialty Conference, Atlanta, GA.
EPA (Environmental Protection Agency). 1988. Guidance on
estimating motor vehicle emission reductions from the use of
alternative fuels and fuel blends. EPA-AA-TSS-PA-87-4.
Environmental Protection Agency, Office of Mobile Sources,
Ann Arbor, MI.
EPA (Environmental Protection Agency). 1989. Analysis of the
economic and environmental effects of methanol as an
automotive fuel. Special Report. Environmental Protection
Agency, Office of Mobile Sources, Ann Arbor, MI.
EPA (Environmental Protection Agency). 1990a. Urban Airshed Model
Study of Five Cities. EPA-450/4-90-006a-g. Environmental
Protection Agency, Office of Air Quality Planning and
Standards, Research Triangle Park, N.C.
EPA (Environmental Protection Agency). 1990b. User's Guide for
the Urban Airshed Model. Volumes I-VI. EPA-450/4-90-007A-F.
Environmental Protection Agency, Office of Air Quality
Planning and Standards, Research Triangle Park, N.C.
EPA (Environmental Protection Agency). 1990c. Analvsis of the
economic and environmental effects of compressed natural gas
as an automotive fuel. Volume I: Passenger cars and. Licrht
trucks. Special Report. Environmental Protection Agency,
Office of Mobile Sources, Ann Arbor, MI.
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EPA (Environmental Protection Agency). 1990d. Analysis of the
economic and environmental effects of compressed natural gas
as an automotive fuel. Volume II: Heavy-duty vehicles.
Special Report. Environmental Protection Agency, Office of
Mobile Sources, Ann Arbor, MI.
EPA (Environmental Protection Agency). 1990e. Analysis of the
economic and environmental effects of ethanol as an
automotive fuel. Special Report. Environmental Protection
Agency, Office of Mobile Sources, Ann Arbor, MI.
EPA (Environmental Protection Agency). 1991a. National Air
Quality and Emissions Trends Report, 1989. EPA/450/4-91-
003. Environmental Protection Agency, Office of Air Quality
Planning and Standards, Research Triangle Park, N.C.
EPA (Environmental Protection Agency). 1991b. Procedures for
Preparing Emissions Projections. EPA-450/4-91-019.
Environmental Protection Agency, Office of Air Quality
Planning and Standards, Research Triangle Park, N.C.
EPA (Environmental Protection Agency). 1991c. "Guidance for
initiating ozone/CO SIP emission inventories pursuant to the
1990 Clean Air Act Amendments," Environmental Protection
Agency, Office of Air Quality Planning and Standards,
Research Triangle Park, N.C.
EPA (Environmental Protection Agency). 1991d. Emission inventory
requirements for ozone State Implementation Plans," EPA-
450/4-91-010. Environmental Protection Agency, Office of Air
Quality Planning and Standards, Research Triangle Park, N.C.
EPA (Environmental Protection Agency). 1991e. Procedures for the
preparation of emission inventories for carbon monoxide and
precursors of ozone. Volume I: General guidance for
stationary sources," EPA-450/4-91-016. Environmental
Protection Agency, Office of Air Quality Planning and
Standards, Research Triangle Park, N.C.
EPA (Environmental Protection Agency). 1991f. Procedures for the
preparation of emission inventories for carbon monoxide and
precursors of ozone. Volume II: Emission inventory
requirements for photochemical air quality simulation
models," EPA-450/4-91-014. Environmental Protection Agency,
Jffice of Air Quality Planning and Standards, Researcn
Triangle Park, N.C.
•
EPA (Environmental Protection Agency). 1991g. Nonroad engine and
vehicle emission study. 21A-2001. Environmental Protection
Agency, Office of Mobile Sources, Ann Arbor, MI.
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EPA (Environmental Protection Agency). 1991h. Regional Ozone
Modeling for Northeast Transport (ROMNET). EPA-450/4-90-
002a. Environmental Protection Agency, Office of Air Quality
Planning and Standards, Research Triangle Park, N.C.
EPA (Environmental Protection Agency). 1991i. Guideline for
Regulatory Application of the Urban Airshed Model. EPA-
450/4-91-013. Environmental Protection Agency, Office of
Air Quality Planning and Standards, Research Triangle Park,
N.C.
EPA (Environmental Protection Agency). 1992. National Air
Quality and Emissions Trends Report, 1991. 450-R-92-001.
Environmental Protection Agency, Office of Air Quality
Planning and Standards, Research Triangle Park, N.C.
Fujita, E.M., B.E. Croes, C.L. Bennett and D.R. Lawson. 1992.
Comparison of emission inventory and ambient concentration
ratios of CO, NMOG, and NOx in California's South Coast Air
Basin. J. Air and Waste Manage. Assoc. 42:264-276
Gao, D., Milford, J.B., Odman, M.T. and T. Russell. 1991. NOx
Control and the corresponding ozone reductions: Analysis of
the ROMNET Results. Draft report submitted to Environmental
Protection Agency, Office of Air Quality Planning and
Standards, Research Triangle Park, N.C.
Langstaff, J. and R.D. Scheffe. 1991. A screening procedure for
evaluating the effects of nitrogen oxide emissions
reductions on ozone. Presented at the Tropospheric Ozone
Specialty Conference, Atlanta, GA.
Morris, R.E., Myers, T.C., Causley, M.C., Gardner, L and E.L.
Carr, 1990. Urban Airshed Model Study of Five Cities: Low-
Cost Application of the Model to Atlanta and Evaluation of
the Effects of Biogenic Emissions on Emission Control
Strategies. EPA-450/4-90-006D. Environmental Protection
Agency, Office of Air Quality Planning and Standards,
Research Triangle Park, N.C.
Pierce, T. and P. Waldruff. 1991. PC- BEIS: A personal computer
version of the Biogenics Emissions Inventory System. JAWMA,
41:937-941.
Scneffe, R.D., G.L. Gipson and R.E. Morris. 1990. The Influence
of 3iogenic Emissions Estimates on Ozone Precursor Control
Requirements for Atlanta, in Air Pollution Modeling ana Its
Application VIII, NATO:CCMS, Plenum Press, NY.
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TECHNICAL REPORT DATA
(Please read Instructions on reverse before completing!
1 . REPORT NO. 2
EPA-454/R-93-024
t. TFTLE AND SUBTITLE
THE ROLE OF OZONE PRECURSORS IN TROPOSPHERIC OZONE
FORMATION AND CONTROL
'. AUTHORIS)
1. PERFORMING ORGANIZATION NAME AND ADDRESS
J.S. Environmental Protection Agency
Technical Support Division (MD-14)
Dffice of Air Quality Planning and Standards
Research Triangle Park, NC 2771 1
2 SPONSORING AGENCY NAME AND ADDRESS
3. RECIPIENT'S ACCESSION NO
6. REPORT DATE
July 30, 1993
6. PERFORMING ORGANIZATION CODE
8. PERFORMING ORGANIZATION REPORT NO
10. PROGRAM ELEMENT NO
1 1 . CONTRACT/GRANT NO
13. TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY CODE
6 SUPPLEMENTARY NOTES
6. ABSTRACT
This Report is the main component of the CAA Section 185B Report to Congress. The Report addresses the
echnical issues underlying the development of effective ozone precursor strateies, and provides EPA responses
D the National Academy of Sciences report entitled, Rethinking the Ozone Problem in Urban and Regional Air
'Dilution.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
Ozone, oxides of nitrogen
volatile organic compounds
photochemical modeling
control strategies
. DISTRIBUTION STATEMENT
b. IDENTIFIERS/OPEN ENDED TERMS
13. SECURITY CLASS tfapon)
Unclassified
o. COSATI Field/ Group
21 NO OF PAGES
67
20 SECURITY CLASS IPagel
Unclassified
22 PRICE
Form 2220-1 IRev 4-77) PREVIOUS EDITION IS OBSELETE
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