EPA-450/2-75-007
September 1975
POSITION PAPER ON REGULATION
OF ATMOSPHERIC SULFATES
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Air and Waste Management
Office of Air Quality Planning and Standards
Research Triangle Park, North Carolina 27711
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EPA-450/2-75-007
POSITION PAPER ON REGULATION
OF
ATMOSPHERIC SULFATES
Strategies and Air Standards Division
U. S. ENVIRONMENTAL PROTECTION AGENCY
Office of Air and Waste Management
Office of Air Quality Planning and Standards
Research Triangle Park, North Carolina 27711
September 1975
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This report has been reviewed by the Strategies and Air Standards Division,
Office of Air Quality Planning and Standards, Office of Air and Waste
Management, Environmental Protection Agency, and approved for publication.
Mention of company or product names does not constitute endorsement by
EPA. Copies are available free of charge to Federal employees, current
contractors and grantees, and non-profit organizations - as supplies
permit - from the Air Pollution Technical Information Center, Environmental
Protection Agency, Research Triangle Park, North Carolina, or may be
obtained, for a nominal cost, from the National Technical Information
Service, 5285 Port Royal Road, Springfield, Virginia 22161.
Depicted on the cover is the 16th Century symbol for vitriol,
a term used to represent sulfuric acid and certain other sulfates.
Publication No. EPA-450/2-75-007
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ACKNOWLEDGMENT
This report is the product of the Strategies and Air Standards
Division (SASD) of the EPA Office of Air Quality Planning and Standards
(OAQPS). John Bachmann coordinated the effort with assistance from
Michael Fisher and James Wei gold. The report incorporates comments
from other divisions of OAQPS, and from the Office of Research and
Development (ORD) the Office of Planning and Evaluation, and the
Office of Policy Analysis.
Significant contributions to the section on Atmospheric Chemistry,
Trends, and Transport were made by Paul Altshuller and William Wilson
of the ORD Environmental Research Center at Research Triangle Park,
North Carolina (ERC/RTP). John Knelson and Glen Fairchild of ERC/RTP
provided guidance to the discussion of epidemiological and toxicological
studies. Other inputs were provided by David Shearer, James Smith, and
John Smith of ERC/RTP, and Robert Papetti and Arnold Goldberg of ORD/
Headquarters.
Technical support for the analyses included in this paper was provided
by the following: John Crenshaw, Neil Frank, Jerry Saunders, Dennis
Ludwig, Christopher Davis, Walter Stevenson, Francis Bunyard, and John
McGinnity of OAQPS, and Gerald Akland of ERC/RTP.
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TABLE OF CONTENTS
List of Figures v
List of Tables vi
Executive Summary vii
Introduction 1
1. Review of Current Knowledge 2
1.1 Health and Welfare Effects .' 3
1.1.1 Toxicological Results 3
1.1.2 Epidemiological Results 6
1.1.3 Welfare Effects and Acid Rain n
1.2 Emission and Concentration Patterns 13
1.3 Atmospheric Chemistry and Transport 20
1.3.1 Formation and Mechanisms for Removal 22
1.3.2 Relationship Between Sulfur Dioxide and
Sulfate Trends 29
1.3.3 Long Range Transport 35
1.4 Control Alternatives 41
1.4.1 Precursor Control 41
1.4.2 Alternatives for S0? Emission Control 44
1.4.3 Use of Naturally Clean Fuels 45
1.4.4 Flue Gas Desulfurization (FGD) Technology ... 47
1.4.5 Fuel Pretreatment 47
1.4.6 Alternative Coal Combustion Systems 49
1.4.7 Intermittent Control Systems 50
1.4.8 Employment of New Energy Sources 51
1.4.9 Energy Conservation 52
1.4.10 Applicability and Availability 53
1.5 Information Gaps and Research Needs for Sulfate
Regulation 53
1.5.1 Monitoring 56
1.5.2 Health and Welfare Effects 56
1.5.3 Atmospheric Chemistry and Transport 57
1.5.4 Improve Control Technologies for Sulfur Dioxide
and Sulfates 59
2. Potential Regulatory Strategies 60
2.1 Potential Scope of the Problem 61
2.2 Regulatory Options Under the Clean Air Act 63
2.2.1 National Ambient Air Quality Standards 63
2.2.2 New Source Performance Standards 65
2.2.3 National Emission Standards for Hazardous
Pollutants 67
2.2.4 Emission Standards for Mobile Sources 67
2.2.5 Other Clean Air Act Options 67
2.2.6 Alternate Regulatory Options 69
2.2.6.1 Regional Emission Control 69
2.2.6.2 Emissions Tax as Component of Sulfates
Control Strategy 69
3. Policy Implications 71
References 81
iv
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FIGURES
Pac
Figure 1. Rainfall Acidity 1965-1966 12
Figure 2. Projected Nationwide Sulfur Oxides
Emission Trends, 1950-1990 15
Figure 3. Fossil Steam Generating Capacity, 1970 16
Figure 4. Nationwide Geographic Variation in Annual
S02 Emission Density 17
Figure 5. 1970 Annual Urban Sulfate Concentrations 19
Figure 6. 1970 Annual Nonurban Average Sulfate Concentration . . 19
Figure 7. NASN Sulfate Data, 1972, 1973 21
Figure 8. Bimodal Distribution of Atmospheric Particles 26
Figure 9. Velocity of Deposition for Unit Density Aerosol
as a Function of Diameter 28
Figure 10. NASN Urban and Nonurban Sulfate Trends 31
Figure 11. NASN Urban Sulfur Dioxide Trends 31
Figure 12. Nonurban Sulfate Trends in the Northeast (12 Sites). . . 36
Figure 13. Nonurban Sulfate Trends in the Northeast (8 Sites) ... 36
Figure 14. Aircraft Measurements of Sulfates and SOp
Concentrations 39
Figure 15. Forty-eight-hour Back Trajectories for Air Parcels Arriving
during Sampling shown in Figure 14 39
Figure 16. Maximum Availability of S02 Control Technologies .... 55
Figure 17. 24-State Region with High Sulfate Levels 72
Figure 18. Projected SO Emissions from 24-State Region,
1970-1985 • x- 79
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LIST OF TABLES
Page
Table 1. SO Compounds Toxic to Man and/or Lower Animals 3
A
Table 2. Results of Epidemiological Studies 10
Table 3. U. S. Manmade S02 Emissions, 1972 14
Table 4. Mechanisms that Convert Sulfur Dioxide to Sulfates .... 23
Table 5. Comparison of Sulfur Dioxide and Sulfate Trends
for Selected Urban and Nonurban Sites 33
Table 6. Environmental Inventory of Sulfate Precursor
Agents: Preliminary View 42
Table 7. Summary of Control Alternatives for S02 46
Table 8. Summary Description of Major Flue Gas Desulfuri-
zation Process 48
Table 9. Electric Energy Conservation Savings, 1980 52
Table 10. Partial Summary of Power Plant Technical Control
Options - Removal and Energy Efficiency, Costs,
Timing 54
Table 11. Diffusion Model Sensitivity of Sulfate Formation
to S02 Oxidation Rate for 1500-MW Power Plant 62
Table 12. Summary of Assumptions Used for SO Emissions
Projections of Table 13 76
Table 13. 24-State Region SOY Emissions 76
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EXECUTIVE SUMMARY
The suspected adverse health effects of atmospheric suspended
sulfates* have been of growing concern to the Environmental Protection
Agency (EPA). The sulfate issue is beginning to have a significant
influence on EPA policies and programs that affect levels of sulfur
oxides emissions: the power plant intermittent emission control policy,
the power plant tall stack emissions policy,the energy-related program
for conversion of power plants from oil or gas to coal operation,the
automotive emissions control program,and general policy regarding the
need for additional sulfur oxides regulation. In view of the importance
of these current regulatory efforts and the potential need for a general
sulfate control program, EPA has conducted an extensive review of cur-
rently available information regarding sulfates. Also, the National
Academy of Sciences (NAS), at the request of the Senate Public Works
Committee, and the EPA Science Advisory Board (SAB), at the request
of the EPA Administrator, have recently completed independent studies
pertaining to sulfates. This report summarizes current scientific and
technical information concerning sulfates, and identifies needs for
research and development. The report also discusses the implications of our
current knowledge for present and long-term regulatory control of sulfur
oxides, and presents and evaluates a policy for sulfates.
Health and Welfare Effects
Health-related research indicates that the transformation products
of sulfur dioxide (S02) in ambient air, principally sulfates, are more
likely than sulfur dioxide alone to be responsible for many of the
adverse health effects typically associated with sulfur oxides. Toxicolo-
gical (animal) studies provide evidence that SCL, in the absence of
other pollutants such as ozone or particulates, is a mild respiratory
*"Sulfates," as used in this report, is defined by the measurement
method used in health studies and the National Air Surveillance Network,
i.e., material collected on a high-volume sampler filter and analyzed
as water soluble sulfates. These can include acid-sulfates (e.g., sulfuric
acid, ammonium bisulfate), neutral metallic sulfates, adsorbed SCL, and
sulfites.
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irritant, while certain specific sulfate compounds, especially submicron-
sized sulfuric acid aerosol, are more severe respiratory irritants.
Epidemiological studies* conducted in several U.S. cities suggest that
high daily or annual sulfate levels are associated with increased attack
frequency in asthmatics, worsened symptoms in cardio-pulmonary patients,
decreased ventilatory function in school children, and symptoms of
acute and chronic respiratory diseases in children and adults. The
association of these health indicators with sulfates was stronger than
that for S02. Accurate quantification of effects levels** must await
future research. When viewed together, the results of the toxicological
and epidemiological studies suggest that specific sulf'ate compounds may
also be responsible for the observed excess mortality associated with
high SOp concentrations. However, an association between sulfates and
mortality has not yet been tested by field studies.
Considerable research is necessary before the complex relationship
between sulfates and health effects can be well understood. The health
effects associated with given sulfate levels can be expected to vary
with the chemical form and physical size of sulfates, the presence of
other pollutants, temperature, and other environmental factors. As both
the NAS and SAB reports state, due to the inadequacy of present moni-
toring capabilities and the current incomplete understanding of the
influence of other variables on the sulfates/health relationship, the
preliminary sulfate health effects studies should only be considered as
indicative of the potential health impact of sulfates. Until more
specific information regarding the sulfates/health effects relationship
is available, EPA considers total sulfate measurements to be an imper-
fect but useful indicator of the presence of the toxic sulfate components.
Although cautioning that such measurements will contain varying propor-
tions of toxic and relatively non-toxic sulfate compounds, the NAS
reports appears to support the EPA position,. However, the SAB considers
*Principally conducted through the EPA Community Health and Environ-
mental Surveillance System program (CHESS).
**Best judgement sulfate levels tentatively associated with adverse
health effects in the preliminary epidemiological,studies were as low as
6 to 10 yg/m (24 hour average) and 10 to 15 yg/m (annual) average.
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measurements of water soluble sulfate alone to be unreliable indicators
of the presence of toxic components in what they term the "sulfur oxides/
particulate complex," and, instead, would view high concentrations of
total suspended particulate and/or S02 as better indicators of areas for
concern with respect to the health impact of acid sulfates and other
toxic sulfate components. These divergent opinions can be resolved only
by further monitoring and health effects research.
Economic welfare effects associated with sulfates are ecological
and agricultural damage, materials damage, and visibility degradation.
Sulfates appear to be a major factor in producing acid rain in a large
portion of the Eastern U.S. Research into the significance of these
effects is limited and, for the most part, such effects cannot now be
quantitatively related to levels of sulfates in the air.
Emission and Concentration Distribution
Emissions of sulfur compounds, both natural and manmade, are the
principal source of atmospheric sulfates. Although manmade S0? emissions
represent only half of the total sulfur emissions in the Northern hemisphere,
these manmade emissions are concentrated in the relatively small industrialized
areas where they far outweigh natural production. Power plant emissions,
which currently account for about 55% of manmade SCL emissions in the
U. S., have been a rapidly growing component of the SOp emission complex.
While total manmade emissions increased by 45% between 1960 and 1970,
power plant emissions increased by 90%.
Currently, about 1% of sulfur oxides emissions come from automobiles,
emitted predominantly as S02« Although the contribution to total
sulfur oxides loading is not likely to change, emissions from new cars
equipped with the catalytic converter are partially in the form of
highly toxic submicron sulfuric acid aerosol. Since automotive emissions
do not contribute significantly to current sulfate levels and originate
from mobile sources, control strategies differ from those for stationary
sources. The automotive sulfuric acid issue is not considered in this
discussion of the more general sulfate problem.
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Based on National Air Surveillance Network data, a large portion of
the eastern United States has recorded sulfate concentrations signifi-
cantly higher than concentrations generally observed in other sections of
the country. Urban levels range from 10 to 24 micrograms per cubic meter
3 "•!
(yg/m ) and nonurban levels range from 8 to 14 yg/m (annual average) in
a 24-state region east of the Mississippi, roughly bounded by Illinois
and Massachusetts to the north and Tennessee and North Carolina to the
south. In this 24-state region, the 1972 average of nonurban concentrations
3
exceeded 10 yg/m (annual average) with an urban concentration average
3
of about 13.6 yg/m . The high sulfate levels in the 24-state area
appear to be spatially correlated with high S0~ emission density, high
rainfall acidity patterns, and a high density of power plant locations.
The remainder of the country does not exhibit similar sulfate concentrations
on a regional scale. The 1972 urban average outside the 24-state north-
eastern region was 7.9 yg/m ; whereas, the nonurban annual average was
3
4.4 yg/m . There are some areas, however, such as the Southern California
Coastal Basin, in which high sulfate levels are observed. The 11 stations
in this area measured an annual average sulfate concentration of 11.1
yg/m . Similarly, Tampa, Florida, recorded an annual sulfate level of
3
11.9 yg/m in 1972, although nonurban stations in this region averaged
3
4.9 yg/m for the same period. Thus, while these areas do not exhibit
the regional concentration problems characteristic to the northeastern
U.S., they do have high local sulfate concentrations.
Atmospheric Chemistry and Transport
Sulfur dioxide is oxidized to sulfuric acid and other sulfates by
several mechanisms, most involving reactive agents such as photochemical
smog, ammonia, catalytic metals, and fine particulates. Temperature and
humidity also influence the reaction. These agents can complicate the
relationship between S02 and sulfates; for example, reductions or increases
in S02 concentrations may not result in proportional reductions or
increases in sulfate levels because of the presence of other agents that
affect the formation reaction. Inadequate knowledge concerning formation
mechanisms currently precludes quantitative assessment of catalytic
agent influences.
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regional sulfate increases that, on balance, offset the local decreases.
This explanation is supported by the apparent increase in manmade
sulfates at a limited number of eastern nonurban sites for which data
are available. This increase roughly parallels the increase in overall
SQ,, emissions during that time. Although, in aggregate, urban sulfate
levels showed little change, variable trends were observed for different
cities. Variations in both the spatial distribution of sulfur oxides
emissions and in atmospheric chemistry could affect the relative magni-
tude of local versus imported sulfates and account for the variable
trends for individual cities.
Both EPA and NAS consider the above stated hypotheses plausible;
however, the SAB suggests that the impact of nonurban sources on urban
areas is likely to be minor and prefers to explain the observed trends
in terms of precursor-limited sulfate formation mechanisms, primary
emissions of sulfates, and errors in the measurement methods. The
SAB bases their conclusions regarding transport on the assumption that
dilution and removal would reduce sulfates to negligible levels during
transport and not on the evidence of any transport models or measurements.
In the opinion of the SAB. the available evidence does not substantiate the
validity of their assumptions or any of the explanations offered. These
divergent views underscore the need for additional research.
Despite the uncertainties concerning the relationship between SCL
emissions and ambient sulfate concentrations, EPA believes that the
available evidence suggests that further increases in SC^ emissions are
likely to produce increases in regional sulfate levels. Sulfate in-
creases are not likely to be proportional to the total S02 emissions
increase because of the spatial distribution of the important sources
and the complex formation mechanisms.
Control Alternatives
Although control of catalytic agents such as particulates and oxidants
may eventually prove to be an important component of a sulfate control
strategy, an examination of current emission trends and pollutant formation
information indicates that control of atmospheric sulfates will probably
depend primarily on control of SO,, emissions. A variety of systems and
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concepts for SOp control that are applicable through 1990 have been
examined to provide better definition of research and development objec-
tives. The primary SCL control technologies for sulfate control and
their applicability are discussed:
1. Naturally occurring low sulfur fuels will continue to be
important in controlling SO^ emissions, but are not likely to
provide a total solution to the problem of SO^ and sulfates
control due to their limited supply, at least in the near-
term.
2. Clean fuel allocation and redistribution, fuel switching,
intermittent control, and tall stacks may contribute to
achieving present SOp standards; however, the regional trans-
port theory suggests that these methods would have minor
impact on preventing increases in regional atmospheric sulfate
levels.
3. Flue gas desulfurization will probably be the principal large
stationary source control technology available for at least
the next 10 years. Physical coal desulfurization can also
contribute to sulfur oxides control during this same time
interval.
4. Advanced fuel pretreatment technologies (liquefaction, gasifi-
cation) have a significant potential for reducing S02 emissions,
especially for small point and area sources, but will probably
not have a major impact before 1985.
5. Ultimately, alternate energy supply systems (solar energy,
thermonuclear fusion), improved combustion technologies
(fluidized bed combustion), and general improvements in energy
utilization efficiency should provide more effective use of
energy resources with less environmental degradation.
Information Gaps and Research Needs for Sulfate Regulation
As evidenced by the previous discussion of health effects and
transport mechanisms, considerable uncertainty exists in interpreting
the limited scientific data base on sulfates. Both the SAB and the NAS
place a high priority on the initiation of comprehensive research programs
that are needed before major control strategy decisions can be made.
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An EPA analysis of major research needs indicates that development
of the data and information necessary for a sulfate regulatory program
would require 3 to 5 years. In this regard, if EPA were to set a National
Ambient Air Quality Standard (NAAQS) for sulfates, it could not realisti-
cally be proposed before 1980 or 1981. Important research needs are
summarized below:
Monitoring
A critical path in the research effort is the need to develop
advanced monitoring methods. Important components of the epidemiological,
toxicological, and atmospheric formation studies depend on the characteri-
zation of sulfates by such monitoring methods. The development of these
techniques is expected to take 1 to 3 years, thus extending the com-
pletion of some aspects of all research areas.
Health and Welfare Effects
Appropriate dose-response curves should be developed. The
chemical composition and physical characteristics of the harmful sulfur
compounds should be identified. The program to accomplish this will
include toxicological work, clinical studies of human response to
specific sulfates, and continued epidemiologic studies. Sulfuric acid
and other specific sulfate monitors are needed for more precise quanti-
fication of exposures. Dose-response functions are needed to assess the
effects of atmospheric sulfates on ecological systems and structural
materials. These effects include economic losses to materials and crops
caused by sulfates and/or acid rain.
Physical/Chemical Transformation and Transport
The oxidation rates of SCL to sulfate in ambient air as well
as in power plant and ground-level urban plumes must be determined. The
role of precursors and catalysts in sulfate formation should be further
assessed. Models must be developed to estimate both local concentrations
and long-range transport phenomena for sulfates. Research in this area
is critical to the development of control strategies.
Improved Control Technologies for Sulfur Dioxide and Sulfates
If stringent control of large S02 point sources is required as
part of a sulfate control strategy, current SCL scrubbing technologies
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may not have adequate removal efficiencies. Demonstration projects
designed to achieve greater than 90% removal efficiency, particularly
for new sources, will be evaluated. Also, further development of alter-
native combustion technologies, such as fluidized bed systems, is needed.
Potential Regulatory Strategies
Although the information required to determine and support a control
program for sulfates may not be available for several years, a preliminary
analysis of regulatory options has been made to aid planning and to help
focus research and development programs.
Regulatory approaches currently available under the Clean Air Act
(CAA) were examined to assess their potential utility in a sulfate
control program. Options such as hazardous standards (CAA Section 112),
emergency powers (CAA Section 303), and abatement conferences (CAA Section
115) do not appear appropriate as primary approaches for regulating
sulfates because of their inherently limited scope.
Sulfates exhibit those characteristics* identified in the Clean Air
Act for pollutants to be controlled through National Ambient Air Quality
Standards (NAAQS). Despite the legal rationale for this approach,
potential implementation problems exist. NAAQS implementation generally
has required the use of a pollutant emissions/ambient air quality
relationship to develop emission regulations that apply within currently
established Air Quality Control Regions. Due to the complex sulfate
formation mechanisms and the apparent pollutant transport through
multistate regions, such a relationship may be impossible to develop.
Another problem with the NAAQS approach would be the definition of a
sulfate "threshold" level below which sulfate-associated health effects
are insignificant.
Although New Source Performance Standards (NSPS) can be a valuable
tool for ensuring minimal emissions of S0? from new major sources, this
approach may not provide adequate control to deal fully with the sulfate
problem.
*The pollutant must have "an adverse effect on health and welfare"
and result from emissions from "numerous or diverse11 sources.
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Since long-term implementation difficulties are possible with an
NAAQS and resulting state implementation plans, alternative regulatory
options must be explored over the next several years. These options
include regional emission limitations for S0?, increased control of
sulfate precursors, and economic incentives.
Pol icy Imp! ications
In view of the available data, it is the judgment of EPA that an
air quality standard or other major regulatory program for sulfates is
not supportable at this time. Additional research is needed in order
to fill the information gaps described earlier. The EPA research effort
will focus on improving monitoring capability to permit identification
of particle size and chemical form of toxic sulfates, developing more
comprehensive health effects data, and characterizing the long range
transport and transformation mechanisms. This resea^cn program will
require several years to complete; consequently, it is doubtful that a
comprehensive regulatory program specifically for sulfates could be
initiated before the end of the decade.
Nevertheless, until further research makes a comprehensive regu-
latory program possible, EPA must respond to the potential sulfate
problem suggested by the preliminary sul fata/health effects information
cited earlier. Although considerable uncertainty exists concerning the
relationship between measured ambient sulfafce concentre tions and adverse
health effects, the preliminary health effects information can be useful
in identifying areas of potential health concern.
As described previously, a large portion of the northeastern United
States is experiencing relatively high annual sulfate concentrations,
Nonurban concentrations have averaged in excess of 9 i^g/nT throughout
this 24-state region. The average of urban concentrations has been
about 13 ug/m (annual average). As mentioned earlier, this area of
high sulfate concentrations correlates spatially with high SOg emission
density, high rainfall acidity patterns, and a high density of power
plant locations. Furthermore, the region exhibits widespread violation
of the national primary ambient air quality standard for suspended
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participate matter, a potential precursor agent in the formation of
sulfates. High sulfate concentrations have also been observed in
several isolated urban areas through the remainder of the country.
Given the significant potential for sulfate-related health risk due
to the multiple influence of high sulfate concentrations, high precursor
concentrations, and high SCL emission density, prudence dictates that
EPA adopt a policy of avoiding aggravation of existing conditions by
minimizing further increases in the relatively high sulfate levels in the
northeastern United States and other more localized problem areas. In
addition, close attention must be paid to sulfate trends in areas of lower
sulfate concentrations. Although this goal of avoiding sulfate increases
will primarily be achieved by minimizing SCL emission increases, existing
programs for control of pollutants such as oxidants and particulates may
provide some measure of sulfate control by limiting sulfate formation
processes.
Opinions differ over where, and to what extent, SOp emissions
should be limited to adequately address the potential sulfate problem.
There is general agreement that SO^ emissions increases should be avoided
in or near urban areas where ambient concentrations of sulfates, SCL, or
total suspended particulates are high. The NAS placed high priority on
abating SCL emissions from sources located in or near urban areas with
high concentrations of sulfur dioxide and sulfates. In addition, the
NAS is concerned about the effects of area-wide increases in S02 emissions
on regional sulfate levels. The SAB, however, does not share the same degree
of concern for the impact of area-wide SO^ emissions. Rather, the SAB
suggests that increased S02 and other sulfate precursor emissions may
have primarily a local impact on sulfate formation, and "that increases
in exposure to sulfur oxides or particulates in localities where the
sulfur dioxide and/or total suspended particulates exceed primary stand-
ards should be viewed with grave concern."
EPA considers the points addressed in both reports as important and
essentially compatible with the Agency's assessments. Current efforts
to attain the primary standards for the criteria pollutants are responsive
to the SAB's concerns. In addition, EPA believes that the area-wide
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concern must be addressed by minimizing increases in S0? in the areas of
maximum sulfate impact. A strategy of minimizing regional and local
increases in SC^ emissions can be implemented through existing regu-
latory options such as State Implementation Plans (SIPs) and New Source
Performance Standards (NSPS).
A policy of minimizing S02 emission increases is generally consistent
with other Agency policies previously announced. These policies include
the Clean Fuels Policy, the limited application of intermittent control
systems (ICS), and the significant risk aspect of oil-to-coal conversions.
The EPA Clean Fuels Policy is intended to make it unnecessary for
plants to switch to lower sulfur fuels to comply with state regulations
where such compliance is not needed for attainment and maintenance of
the national health-related standards for S02- By revising SIPs appro-
priately, plants currently in areas meeting primary air quality standards
could continue to burn currently available fuels; no switch to higher
sulfur fuel is intended. Therefore, sulfur emissions from these sources
should not increase.
With respect to EPA policy on intermittent control systems {ICS),
a limited number of isolated power plants may be permitted to use inter-
mittent emission control to meet air quality standards, temporarily
deferring expenditures for costly continuous emission controls. Eligible
plants are already burning coal and, under ICS, will continue to burn
existing fuel except during adverse meteorological conditions, at which
point they will reduce emissions by switching to a lower sulfur fuel or
shifting generation load. Again, total sulfur emissions from these
sources should not increase and may actually be slightly reduced.
The Energy Supply and Environmental Coordination Act of 1974
(ESECA) provides the Federal Energy Administration with the authority to
prohibit a power plant from burning oil or natural gas subject to certi-
fication by EPA of the plant's ability to burn coal in compliance with
certain environmental requirements. Two requirements relate to a plant's
ability to burn coal without contributing to a violation of primary
standards for total suspended particulate or sulfur dioxide. In this
regard, if certain legal criteria are satisfied (regional limitation),
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EPA may specify alternate emission requirements (primary standard con-
ditions) to be met temporarily by converting plants. An additional
requirement of ESECA states that conversions cannot result in an in-
crease in the emission of unregulated pollutants or pollutant precursors
to levels that may result in a significant risk to public health.
Based on currently available health effects information, EPA has decided
to apply this "significant risk" provision only with respect to sulfates.
Under the significant risk provision, EPA plans to restrict emissions of
sulfate precursor pollutants—sulfur dioxide or particulate matter -- if
a converting plant is located in an area with high sulfate concentrations
and with concentrations of particulate or sulfur dioxide in excess of
primary standards.
Though currently available information does not now permit the
establishment of a comprehensive sulfate regulatory program, the in-
formation does suggest a need to minimize increases in regional sulfur
oxides emissions as a means of preventing increased levels of atmos-
pheric sulfates. In the interim, prior to the initiation of any com-
prehensive control program, existing regulatory options can be effective
in limiting increases in sulfate concentrations. The "significant risk"
policy for converting power plants, the vigorous enforcement of state
implementation plans for the control of sulfur dioxide and particulates,
and the increasing application of new source performance standards to
power generating facilities are vital components of an overall
strategy that should limit growth of ambient sulfate levels. EPA
analysis indicates that these regulations and policies should prevent
major S0? emission increases through 1980 in the regions of maximum
sulfate impact. Until information is available to support the enforce-
ment of a more rigorous sulfate regulatory program, the use of these
currently applicable regulatory measures should provide reasonable
protection against increased health risk from sulfates.
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POSITION PAPER ON REGULATION
OF ATMOSPHERIC SULFATES
INTRODUCTION
Atmospheric suspended sulfates have been of growing concern to the
Environmental Protection Agency (EPA) in recent months for a number of
reasons. CHESS* epidemiological results of 1970-71 gave preliminary
indications that health effects previously associated with sulfur
dioxide ($02) and particulates were related more strongly to sulfate
concentrations. Other studies, including toxicological work, have also
indicated that sulfuric acid aerosol (HpSOJ and certain other sulfates
are stronger irritants than sulfur dioxide. Recently published pre-
cipitation studies suggest that the acidity of United States rainfall
(which may be related to acid sulfate formation) is at levels that may
produce serious ecological consequences. Although current control strategies
have been successful in reducing ambient concentrations of S02 and
particulates in urban areas, sulfate levels in some nonurban areas
appear to have increased between 1962 and 1972, and if sulfur dioxide emis-
sions were to increase due to the projected rise in fossil fuel consumption,
there is concern that sulfate levels in urban areas could increase.
Thus, EPA concerns about permanent use of tall stacks and Intermittent
Control Systems (ICS) have been predicated, in part, on the conviction that
widespread use of these measures probably would not prevent increases
in sulfate levels. This concern also influences EPA policy regarding
*EPA's Community Health and Environmental Surveillance System.
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the energy-related program for conversion of power plants from oil or
gas to coal combustion. In addition, information concerning the emission
of sulfuric acid aerosols from automotive catalysts has prompted altera-
tions in the automotive emissions control program.
Although the scientific understanding of health and welfare effects,
atmospheric chemistry, and transport of sulfates is incomplete, a pre-
liminary assessment of possible control strategies is needed so that a
coherent Agency approach to the sulfate problem can be developed.
Recognizing the importance of these issues, the EPA Administrator requested
an external review by the Science Advisory Board (SAB) of the scientific
and technical issues relating to sulfates. In addition, the National
2
Academy of Sciences (NAS), has prepared a comprehensive review largely
devoted to the sulfate question. This report will provide a brief
review of the scientific information developed to date, discuss technical
and regulatory alternatives for sulfate control, and present and evaluate
a policy for sulfates.
Unless otherwise specified, "sulfates", as used in this report, are
defined by the measurement method used in the preliminary health studies
and the National Air Surveillance Network. The method measures material
that is collected on glass fiber filters of high-volume samplers and
analyzed as water-soluble sulfate. This can include sulfuric acid,
soluble sulfate salts, adsorbed sulfur dioxide, sulfite salts, and
sulfates formed due to sampling and analytical artifacts.
1. REVIEW OF CURRENT KNOWLEDGE
It is not within the scope of this report to present a complete review
of the scientific and technical information regarding health effects,
atmospheric chemistry and transport, monitoring, and control of sulfur
oxides. Several comprehensive reviews of these subjects have been published
recently. This section summarizes the major areas relevant to
development of control strategies and policy. An earlier draft has been
reviewed by members of the Science Advisory Board, and their comments
have been considered in development of this version. There seems to be
general agreement among EPA, SAB, and the NAS report with the interpre-
tations presented here, although some exceptions are noted in the body
of the report.
-------
1.1 Health and Welfare Effects
The current Air Quality Standard for S09 is based upon studies
R loc
summarized in the sulfur oxides criteria document. Recent reviews ' '
of the scientific literature indicate that studies conducted since the
criteria document was published have tended to confirm the association
of adverse human health effects with ambient S09 concentrations at or
33
near the primary standard (80 yg/m annual average, 365 yg/m 24-hr
average). Although S02 may cause health effects in combination with
ozone and/or respirable particles, it has long been suspected that the
transformation products of SCL may have more significant health effects
than SCU alone. Studies of polluted air have demonstrated that sulfur
dioxide undergoes atmospheric oxidation leading to the formation of
3
sulfuric acid aerosol and other fine-particulate sulfates. Recent
evidence from both toxicological and epidemiological studies suggests
that these compounds are more likely than sulfur dioxide alone to be
responsible for observed adverse human health effects.
1.1.1 Toxicological Results
A variety of toxicologic studies of sulfur oxide inhalation have
been conducted on lower animals and, to a limited extent, on humans.
Most of the studies have dealt with the effects of S02- Only a few in-
vestigations of sulfuric acid (FLSO,) and inorganic particulate sulfates
have been conducted. The sulfur oxides that have been investigated are
shown in Table 1. Most studies of sulfur oxides have used acute respiratory
Table 1. SO COMPOUNDS TOXIC TO MAN AND/OR LOWER ANIMALS
s\
1. Sulfur dioxide
Q
2. Specific sulfate aerosols (decreasing irritant toxicity)
sulfuric acid, zinc ammonium sulfate, ferric
sulfate, zinc sulfate, ammonium sulfate, ammonium
bisulfate, cupric sulfate, ferrous sulfate, manganese
sulfate
3. Pollutant mixtures
e.g. sulfur dioxide/ozone, sulfur dioxide/sodium
chloride aerosol, sulfur dioxide/vanadium aerosol
-------
physiologic and pathologic effects as indices of response. A few bio-
chemical studies and one investigation of the cocarcinogenic potential
of sulfur dioxide have been conducted. These studies have been extensively
reviewed by Amdur et al. and Goldstein. The results of the toxicologic
studies indicate that sulfur dioxide, by itself, is a mild respiratory
irritant; whereas,sulfuric acid aerosol and certain specific inorganic
sulfates are greater respiratory irritants.
The extent and type of biological response to exposure to sulfur
oxides are complex due to this interaction of many parameters. The particle
size, physico-chemical composition, and presence of interacting factors
as well as mass concentration, are of paramount importance as factors
that affect both the site of deposition of sulfur oxides in the respiratory
tract and the magnitude of the response. Sulfur dioxide is thought to
be a mild irritant because, as a gas, most of it is absorbed in the nose or
mouth and very little penetrates into the lung. By contrast, the reason that
H2S(L and several other particulate sulfur oxides are greater respiratory
irritants is thought to be because they are intrinsically more toxic and/or
because, as small particles, they are deposited deeper in the respiratory
tract than gaseous S02- By a similar mechanism, the response to S02 is
enhanced when it can be adsorbed onto such small particles. The effective
size is apparently important even for particles generally considered to
be in the respirable range. Sulfuric acid aerosol of 0.7 ym size pro-
duces a four-fold greater irritant response than 2.5 ym particles of
;e
6
5
the same compound. A similar size/response relationship exists for fine
particulate zinc ammonium sulfate aerosols.
The specific chemical composition of particulate sulfur/oxide may
have an important bearing on its toxicity. One line of investigation
examined respiratory physiologic responses to a variety of sulfates of
similar aerosol size and mass concentrations. H2S04 was found to be the
greatest respiratory irritant, followed by metallic and ammonium-
containing sulfates in the order listed in Table 1. Although copper,
ferrous, and manganese sulfates were the least toxic of the compounds
tested, they were found to enhance irritant response when mixed with S02-8
The differences in the inhalation response of some of these sulfates was
-------
small. Although these data do^not constitute an adequate basis for a
determination of the comparative toxicity for specific inorganic sulfates,
the data do suggest that the toxicological evaluation of particulate
sulfur oxides must consider the cation as well as the anion of the
molecule, and that aerosol acidity is of great importance.
These studies were based upon a sensitive respiratory physiologic
response, primarily increased pulmonary airflow resistance in guinea pigs.
This response results from narrowing of the airways within the respiratory
system. A similar response has been observed in men exposed to sulfur
dioxide and H^SO. aerosol. This physiological response is a generally
accepted, sensitive measure of airway irritation.
A crucial question arises as to the pathophysiologic significance
of subtle increases in pulmonary airflow resistance. A recent investiga-
Q
tion in guinea pigs demonstrated that the total respiratory deposition
rate of inhaled particles and the pattern of regional respiratory deposition
of these particles was altered by H7SOA inhalation. These effects were
3
noted at H2S04 concentrations as low as 30 yg/m , particle size < 1 urn,
for 1 hour. This response was probably associated with increased pul-
monary airflow resistance. Increased pulmonary airflow resistance is
the principal physiologic response in uncomplicated asthma. It has
been hypothesized, therefore, that rUSCL inhalation may act to increase
the incidence of asthma attacks through increased deposition of inhaled
particles and/or a shift in the principal site of deposition of inhaled
particles to airway regions where asthma can be triggered. On a qualita-
tive basis, the toxicologic data support the epidemiologic evidence of
a better correlation of health responses with particulate sulfur oxides
than with gaseous sulfur dioxide.
Experimental investigations have not yet dealt adequately with the
contribution of covariate factors on the health response to sulfur oxides.
McJilton et al. have recently shown that high relative humidity is
important in potentiating the physiologic response of guinea pigs to
SO -Nad (sodium chloride) mixtures. High relative humidity (^0%)
served to deliquesce the NaCl particles, resulting in particle growth.
An acid aerosol developed, apparently through S02 adsorption on the droplets.
-------
Analysis indicated that the aerosol may have contained acid sulfite.
Under these circumstances, a physiologic response was elicited which did not
occur at low (^40%) relative humidity. Mixtures of SCL and ozone also produce
greater response than either pollutant alone. It is thought that the
reaction of those gases in the respiratory tract to produce sulfuric
acid may be responsible for this synergism. These examples demonstrate
the need not only for further investigation of the effects of specific
sulfates, but also for studies of the biologic response to gas-aerosol
mixtures under conditions that can better simulate the complex physico-
chemical environment to which mankind is exposed.
In summary, much of the limited toxicological work supports the widely
held contention that the neutralized sulfate ion is not innately toxic, and
that the observed associations between health effects and sulfates and SCL
are more likely to be caused by small particulate acid sulfates and
possibly sulfites. They also highlight the need for physical and chemical
characterization of the material currently measured as particulate sulfates.
1.1.2 Epidemiological Results
Epidemiological studies of air pollution give evidence of associations
that must be replicated under a variety of conditions to be accepted as
cause effect relationships. The goal of such studies is to provide data
indicative of dose-response relationships. However, since these are field
studies, it is often very difficult to distinguish the effects of a specific
variable from the multitude of other environmental factors. Unfortunately,
dose measurements in the majority of epidemiological studies involving
sulfur oxides have been of total suspended particles and sulfur dioxide.
No studies have included measurements of respirable-sized sulfuric acid
or other specific sulfate particles. In addition, assessment of response,
whether acute (resulting from daily variations in pollutants) or chronic
(resulting from long term exposures), is difficult.
Although epidemiological work to date is not yet sufficient to provide
accurate dose-response relationships, recent studies (primarily CHESS)
have been conducted using water soluble sulfate measurements as an
indicator of acid sulfates and other potentially toxic sulfur oxides.
-------
The studies were preliminary in nature and interpretation of the results
g
is hampered by a number of factors that are discussed by the authors as
nor
well as reviewers. ' ' Shortcomings existed in determination of pollutant
exposures which, in long term studies, had to be based on historical monitoring
data of uncertain quality; in some cases, past sulfate levels were imputed
from available S'CL and particulate data. Furthermore, as noted above,
water soluble sulfate measurements had to be used as a proxy for fine
particulate acid sulfates. The studies differed in locale and response
rate of study subjects, and the methods of ascertainment of illness used
by CHESS differ from those used by other investigators. These diffi-
culties are being corrected in current and future replicative studies.
9 1
Reviewers such as the NAS and the SAB consider the CHESS studies to
be of significant value and agree that, taken in total, they point out
the potentially important role of certain sulfates in air-pollution-re-
lated health effects. The preliminary studies suggest that human respira-
tory disease is more closely associated with relatively low levels of
sulfates than with sulfur dioxide or total suspended particulate concen-
trations. Adverse effects observed included an increase in attack
frequency in asthmatics, worsening of symptoms in cardio-pulmonary
patients, and decrements in ventilatory function in school children. In
addition, studies, in some cases using reconstructed past pollutant
trends, indicate increased incidence of acute and chronic respiratory
diseases in children and adults.
Daily reports from two panels of asthmatics, one in New York and
one in the Salt Lake Basin, and from a panel of cardiopulmonary subjects
in New York were used in studies of daily exposures of sulfates.
For asthmatics, the frequency of attacks was analyzed; the response for
the cardiopulmonary panel was aggravation of one or more specific
symptoms. In the New York asthma study, there was a 9 percent increase
in risk of attack associated with 24-hour sulfate levels of 8.1 to 10.0
yg/m when the minimum temperature ranged between 30 and 50°F, and a 10
3
percent increase in risk with 24-hour levels greater than 10 ug/m when
the minimum temperature exceeded 50°F. In the Salt Lake study, a
slight decrease in risk was noted with increased sulfate levels between
-------
6 and 10 yg/m when temperatures ranged between 30 and 50°F. However,
when temperatures exceeded 50°F, a 17 percent increase in attack risk
was associated with 24-hour average sulfate levels between 6.1 and 8.0
yg/m . The New York cardiopulmonary study indicated a worsening of
symptoms, such as shortness of breath, cough, and increased production
of phlegm, was associated with sulfates in the 24-hour average concentration
3
range of 6 to 10 yg/m . Total suspended particulates were also correlated
with aggravation of symptoms.
Studies of school children in a moderately polluted industrial
valley of Cincinnati indicated that ventilatory function was significantly
lower than that of children living in a clean area of the metropolitan
region. Differences in sulfate levels were found to be closely associated
with differences in ventilatory response. A similar study in New York
indicated that older children (9 to 13 years) who had been exposed to
elevated levels of sulfur oxides and particulates in their early years
had decreased ventilatory function relative to unexposed children. No
consistent effect was noted for younger children (5 to 8 years).
Other studies observing effects of long-term exposures involved
reconstructing some past sulfate exposures from emission and/or other
pollutant data. These studies are suggestive of a relationship between
sulfates and (1) excess acute respiratory disease in children and families,
and (2) excess risk for chronic bronchitis in adults. The authors suggest
that these effects may occur as a result of long-term (up to 10 years)
3
exposures to sulfates at annual average levels of 10 to 15 yg/m .
12
An earlier study by Dohan found a significant association between
mean annual ambient sulfate concentration and numbers of absences caused by
illness for female employees of a large, multicity U.S. corporation.
Sulfate concentrations in the four areas with the highest illness absence
3
rates were 13.2 - 19.8 yg/m annual average.
Although no studies of pollutant relationships with excess mortality
have involved direct sulfate measurements, a significant association
between ambient sulfur dioxide levels in the presence of particulates
5713
and mortality has been noted in several studies. ' Since some
evidence suggests that effects linked with sulfur dioxide are more
-------
closely associated with sulfates, Finklea et al. have suggested that
increased mortality may be associated with high sulfate levels. Studies
relating mortality to actual sulfate measurements are needed to confirm
this suggestion.
The results of the preliminary CHESS epidemiological studies are
summarized in Table 2. Concentration ranges at or above which adverse
effects were noted are listed. The difference between 24-hour and
annual average times should be noted. A location that experiences
3
several days a year in which 24-hour sulfate levels exceed 10 yg/m is
likely to have an annual average considerably below 10 yg/m . This may be
of great importance in development of possible future control strategies,
which must be directed toward the lowest level at which health effects
are observed.
Because health effects can be expected to vary depending upon the
chemical form and particle size of sulfates, and upon synergisms with
other pollutants such as acid nitrates, levels of total sulfates at
which effects are observed might be expected to vary for different
cities, regions, or changing atmospheric conditions within given regions.
Quantification of the effects of these variables is not now possible.
Therefore, the numbers presented in Table 2 are only indicative of the
relative magnitude of the concentrations that are likely to be associated
2
with adverse effects. As pointed out by NAS , these first approximations
are subject to considerable uncertainty.
In summary, the CHESS studies show that sulfates may be a better indi-
cator of sulfur oxide pollution than S0? and point out that sulfates may
play an important role in the causation of air-pollution-related health
effects. Until improved monitoring capabilities and further health
research permit more accurate dose-response functions, EPA believes that
the consistency of the preliminary studies justifies consideration of total
sulfate measurements as an imperfect but useful indicator of the presence
of acid sulfates and other toxic sulfur-containing components. It is
recognized that in some cases, the bulk of specific sulfates collected may
2
not be of concern to health. The NAS report appears to support use of sul-
fates as an indicator, cautioning that such measurements will contain changing
-------
Table 2. RESULTS OF EPIDEMIOLOGICAL STUDIES11'12*14
Adverse health effect
Increased mortality
Aggravation of symptoms
in elderly
Aggravation of asthma
Decreased lung function
in children
Increased acute lower
respiratory disease
in families
Increased prevalence of
chronic bronchitis
Increased acute respiratory
disease in families
Increased respiratory disease
related illness absences in
female workers
Primary standard
Primary standard
Concentration9 at which effect
was observed
SQ2,
yg/md (ppm)
300-400 (0.11-0.15)
365 (0.14)
180-250 (0.07-0.09)
220 (0.075)
90-100 (0.034-0.037)
95 (0.035)
106 (0.039)
NAb
365
80
Sul fates,
yg/m3
NAb
8-10
6-10
11
9
14
15
13
-
-
Averaging
time
24 hr
24 hr
24 hr
Annual mean
Annual mean
Annual mean
Annual mean
Annual mean
24 hr
Annual mean
aEffects levels are best judgment estimates based on a synthesis of several
studies.
NA = not available.
10
-------
amounts of components with variable toxicities. The likelihood of
variable composition has caused the SAB to consider such measurements
as unreliable indicators of the toxic components in what they term the
"sulfur oxides/particulate complex." The SAB considers elevated levels
of total suspended particulates and/or SCL indicative of areas of con-
cern about acid sulfates and the other toxic components. Further moni-
toring and health effects research is urgently needed to clear up these
uncertainties. Important areas for further research are discussed below
in Section 1.5
1.1.3 Welfare Effects and Acid Rain
Sulfate-related welfare effects include ecological damage, materials
damage, visibility deterioration, and possible climatological alterations.
Of principal concern is the suspected role of sulfur oxides in causing acid
rain. Acid rain, observed in regions of high sulfate concentrations in both
Europe and the United States, can lower the pH of soils and natural waters,
cause mineral leaching, and damage vegetation. " Although the effects are
not always deleterious, the potential exists for serious ecological disruption
15
in some areas of the country. A recent study suggests that acid pre-
cipitation and dry deposition of acid aerosols and gases may be causing
depletion of fish populations in lakes in the Adirondack Mountains of
18
New York. Figure 1 illustrates the pattern of acid rainfall over the
Eastern U.S. in 1965-66. The pH of normal rainfall is about 5.7.
Rainfall of pH 4.7 is ten times more acidic because the scale is logarith-
15
mic. Values as low as pH 2.1 have been reported in individual storms.
In a comprehensive review of the literature on sulfates and acid
1 q
rain, Nisbet indicates that both the acidity of precipitation and the
rate of deposition of sulfates in precipitation has increased in the
northeastern U.S. in recent years. The distribution of acid rainfall is
closely related to observed ambient levels of suspended sulfates. Acid
rain, and dry deposition of acid-sulfate aerosols, as well as SCL in
combination with particulates can cause materials damage. The role of
sulfates in forming acid rain must be considered in relation to other
19
possible acid formers such as nitrates. Nisbet estimates that nitrates
account for about 24% of observed acidity.
11
-------
4.20
4.44 y 4.34
4.48 A 4.22
MILES
0 50 100 200
300
Figure 1. Rainfall acidity, 1965-1966 (pH units).
:*„» 16
12
-------
Sulfates, as a major component of atmospheric fine participate
20
material, have a significant role in visibility reduction. Because some
sulfates have a water absorbing capacity, their impact on visibility is
greatly increased at high humidities. Widespread atmospheric hazes
observed throughout the eastern U.S. are apparently increasing with SO,
21
emissions. Sulfates may comprise two-thirds of all fine particles in this
22
section of the country. Existing information, however, does not
permit adequate quantification of these effects, and considerable work
is needed to provide damage estimates.
1.2 Emission and Concentration Distributions
Most sulfates are not emitted directly, but are secondarily formed
from other sulfur compounds. In the Northern Hemisphere approximately
half of the total annual sulfur emissions are from widely dispersed
natural sources, principally biogenic decomposition processes and sea
23
salt spray. Anthropogenic contributions, however, are concentrated in
nations of high industrial density such as the United States, where they
23 24
can far outweigh the natural impact. ' Approximately 95 percent of
the total pollution related sulfur emissions is in the form of sulfur
dioxide, most of the remainder being hydrogen sulfide, sulfur trioxide,
and a small amount of directly emitted sulfates. Hydrogen sulfide
oxidizes to sulfur dioxide, ultimately forming sulfate, and sulfur
trioxide rapidly reacts with water vapor to form sulfuric acid. When
describing source distributions for sulfates, it is necessary to present
emissions patterns for sulfur dioxide.
A breakdown of 1972 United States manmade sulfur oxide emissions
expressed as S02 is presented in Table 3. Significantly, 55 percent of
the total emissions in 1972 were from electrical production in coal-
fired and oil-fired power plants and occurred largely in the eastern
half of the country. Figure 2 shows past and projected trends in total
and power plant S02 emissions, and it can be seen that most of the 45%
increase in total S02 emissions shown in Figure 2 for 1960 to 1970 was
accounted for by the increase in power plant emissions. Figures 3 and 4
illustrate the geographical distribution of power plant generating
13
-------
Table 3. U. S. MANMADE S02 EMISSIONS, 1972*
Source
Stationary fuel combustion
Electric utilities'3
Coal
Oil
Indus trial /commercial
(point sources)
Coal
Oil
Other
Area sources
Industrial processes
Transportation
Automotive
Other
Solid waste
Miscellaneous
Total, all sources
Emissions,
106 tons
25.1
18.0
16.5
1.5
3.6
2.5
1.0
0.1
3.5
6.8
0.6
0.2
0.4
0.1
0.1
32.7
% of
total emissions
77
55
11
11
21
1.8
0.6
1.2
0.3
0.3
100
aNEDS data.25
bSASD data file.
-------
60
50
40
30
x
O
V)
20
10
ALL SOURCES
POWER PL ANTS
PROJECTIONS ASSUMING
NO CONTROL, 5 percent/yr
GROWTH
PROJECTIONS ASSUMING
SIP AND NSPS IN 1978
NOTE: DISCONTINUITY IN EMISSIONS CAUSED BY CHANGE IN DATA BASE.
1950
1960
1970 1980
YEAR
1990
2000
Figure 2. Projected nationwide sulfur oxides emission trends, 1950-1990 '
15
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capacity and of total S02 emission density, respectively. It is apparent
that power plants account for a large share of SCL emissions and are
concentrated in the northeast quadrant. Many of the industrial process
emissions of SC^ result from the smelting of sulfur-bearing ores at
plants located in the western portion of the nation.
Although automotive sources are responsible for only about 1 per-
cent of total sulfur emissions, they may become increasingly important
with the introduction of catalytic converters. These devices will cause
some of the sulfur in gasoline that was formerly emitted as SOu to be
po £
directly emitted as fine particulate sulfuric acid. Because automotive
emissions are not currently an important component of urban sulfate
levels and because both technical and regulatory control strategies for
vehicles must differ from those for stationary sources, the automotive
sulfate issue will not be examined in any detail in this report.
The best available data base on national ambient concentrations of
water-soluble sulfates in the United States is provided by the National
Air Surveillance Network (NASN). Approximately 250 monitoring sites are
distributed in both urban and nonurban locations across the country. Urban
sites were placed in center city areas; nonurban sites were chosen to
be representative of general background areas in their respective
regions. These sites monitor total suspended particulates once every 2
weeks. The samples are later analyzed for water soluble sulfates. This
is the major available data base from which to examine the extent of
spatial/temporal distributions of suspended sulfate levels and to determine
the degree of correlation between measured ambient sulfate concentrations
and its major precursor, S02- Because urban sites are usually located
in center city areas, little information is obtainable for micro scale
analysis of metropolitan areas.
Figures 5 and 6 illustrate the urban and nonurban annual average
sulfate concentrations for 1970 as measured by NASN. The available data
are limited and represent approximately 25 samples taken at each of 164
urban and 25 nonurban sites. As such, the figures can be considered as
being only indicative of the actual spatial concentration distributions.
Throughout the eastern portion of the country, nonurban concentrations
18
-------
URBAN SiTE
NONURBANSITE
7.0-13.0 jug/m3
[£§ >13.0 /jg/m3
Figure 5. 1970 annual urban sulfate concentrations. 14
Figure 6. 1970 annual nonurban average sulfate concentrations.^
19
-------
reached high levels and only in portions the west were they so low as to
o
approximate the global background of 1 to 2 yg/m (annual average).
Both figures point out the distinct differences between the sulfate
levels in various regions of the country, with a 24-state region (outlined
in Figure 6) east of the Mississippi and north of South Carolina having
significantly higher concentrations at both urban and nonurban sites.
o
In this 24-state region, urban levels ranged from 10 to 27 yg/m and at
3 3
nonurban sites levels generally exceeded 7 yg/m , averaging 9 yg/m
(annual average). In the remainder of the country, annual concentrations
3 3
averaged about 6 yg/m for urban stations and about 4 yg/m for non-
urban locations.
The distribution of sulfate levels in 1972 indicates that the general
patterns have not changed greatly since 1970. In 1972, concentrations
3
at the urban sites in the 24-state region averaged 13.6 yg/m , and the
3
average at the nonurban sites was 10.2 yg/m . Outside of this region,
urban levels averaged 7.9 yg/m and nonurban levels 4.4 ng/m in 1972.
Representative sulfate data for 1972 as well as available 1973 data are
presented in Figure 7. As shown in the figure, although concentrations
are highest in the eastern section of the nation, some locations outside
this region, such as Southern California, also exhibit high concentrations.
Comparison of Figures 5 and 6 with SOo emission density patterns
(Figure 4), and power plant locations (Figure 3), indicates that observed
distribution of high sulfate concentrations are geographically related
to S02 emissions distribution. These patterns as well as complementary
evidence for acid rain (Figure 1) and incidence of atmospheric hazes
suggest that sulfate levels are higher than natural background throughout
the northeast quadrant of the nation.
1.3 Atmospheric Chemistry, Trends, and Transport
Once emitted, sulfur compounds are ultimately removed from the
atmosphere, primarily by precipitation and dry deposition on the ground and
on vegetation. Most of the sulfur dioxide is converted to sulfate, either
before or during the removal process. Scavenging rates for S0£ and
sulfates are not well quantified, but global mass balance estimates of
20
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atmospheric residence times for sulfur compounds range between 1 and
8 days, ' suggesting the possibility of long-range transport. These
processes, which must be understood for development of a relationship
between sulfur dioxide emissions and sulfates in the ambient air, are
discussed more fully below.
1.3.1 Formation and Removal Mechanisms
The mechanisms by which sulfur dioxide is oxidized to sulfates are
not well understood but are important because they determine the formation
rate and, to some extent, the final form of sulfate. Atmospheric sulfur
dioxide may be oxidized to SO, and converted to sulfuric acid aerosol, or
it may form sulfite ions that are then oxidized to sulfate. Subsequent to
the oxidation, sulfuric acid or sulfate may interact with other materials
to form other sulfate compounds. The most important sulfate formation
mechanisms identified to date are summarized in Table 4.,
The rates of oxidation associated with these mechanisms can vary with
4
humidity, temperature, and concentrations of the reactants. At ambient
concentrations, Mechanism 1 is slow and probably important only for very
long range transport of low pollutant levels on a global scale. Rates
for Mechanisms 2 through 5 are thought to range between 1% and 20% of the
sulfur dioxide oxidized per hour, which could make them of significance
in urban or other areas near important emissions sources. Sulfate formation
rates are usually enhanced by increases in humidity. Some idea of the
pseudo first order reaction rates for the oxidation of SCL in the
ambient air is available for Los Angeles and St. Louis, two cities
with different atmospheric characteristics. For Los Angeles, with photo-
chemical smog and carbonaceous aerosol, the observed rate ranged between
on
1 and 13% per hour. For St. Louis, which has less photochemical smog and
31
perhaps more metal-containing aerosol, the rate was from 1 to 2% per hour.
Conversion rates for sulfur dioxide formation in plumes are even less
well defined. Rates between 1 and 50% per hour have been reported.
A review of the reports published before 1975, however, indicates that all
of these studies of plume conversion rates are likely to be subject to
substantial errors due to inadequate sampling capability. Preliminary
22
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results from recent studies indicate that for distances of up to 30
miles downwind of the source, power plant plume S09 oxidation rates were
32
1 to 2% per hour for coal combustion and 10 to 20% per hour for oil
33
combustion. The more rapid rate in oil-fired plumes is thought to be
due to the lack of particulate control, which permits vanadium or other
catalytic particulates to accelerate S02 oxidation in the plume. Further
studies must be completed before firm conclusions can be drawn. However,
coal-fired plant plumes contain higher concentrations of S02, which can
convert to more sulfate over wide areas downwind than plumes from low-
sulfur-oil-fired plants.
Current EPA studies suggest that S02 oxidation rates in coal fired
power plant plumes are initially inhibited by high concentrations of
nitric oxide (NO) with conversion rates increasing miles downwind as the
plume encounters ambient oxidants and perhaps ammonia from rural back-
ground or urban plumes. The observation by Davis of conditions
favorable to photochemical oxidant formation in a power plant plume 25
miles downwind of the source supports this conclusion. Near the source,
NO levels were sufficient to depress preexisting oxidant concentrations;
whereas, farther downwind the NO was depleted and increases in oxidant
levels were observed. A consideration of possible reaction mechanisms
suggests that S02 conversion will be slow during the early stages of
plume dispersion, but will increase as more ambient air mixes with the
plume. Background oxidants that are entrained into the plume will
preferentially oxidize NO to NO,,. However, after some NO is converted
to N02, HNOo can form and be dissociated by sunlight to yield OH, which
proceeds to oxidize SOp by Mechanism 2. Entrained oxidants can react
directly with S02 dissolved in liquid droplets or wet aerosol particles
after NO is converted to N02- The amount of S02 dissolving in the
liquid (droplet or film) is governed by the pH of the solution. As the
liquid becomes acidic, S02 dissolution is inhibited. Ammonia, present
in the ambient air, probably controls this process (Mechanism 3). Thus,
the EPA observation of a slow initial rate of conversion in plumes,
increasing as the plume disperses, is mechanistically reasonable.
Because the extent of oxidation for Mechanisms 2 to 5 may be more
dependent on concentrations of precursors other than SO-, a reduction in
ambient S0? concentration may not produce a corresponding decrease in
sulfate production if these precursors are limiting factors. The actual
24
-------
oxidation rates and controlling mechanisms under realistic environmental
conditions are poorly understood, thus precluding a quantitative assess-
ment of the role of such precursors.
The ultimate chemical and physical forms of sulfate are influenced
by the formation mechanisms and associated precursors. These properties
are of paramount importance in determining the effects associated with
sulfates. Although most sulfate measurements available to date do not
provide species information, chemical forms of sulfate in the atmosphere
are thought to be (roughly in order of suspected abundance) ammonium and
ammonium acid sulfates, sulfuric acid aerosol, various metallic sulfates,
4 24
and organic sulfates. ' Electron Spectroscopy for Chemical Analysis
has identified seven distinct species of particulate-related sulfur in
or or
California aerosols. This and complementary evidence suggest that
some forms of sulfites may exist in the atmosphere. The relative abun-
dance of these particulate sulfur oxide species can be expected to vary
with both location and time. It is therefore important to be able to
monitor specific compounds of greatest toxicity.
37
Studies over the past 5 years of the size distribution of both
sulfate aerosol and the general atmospheric aerosol have led to impor-
tant changes in our understanding of the behavior of particles or aerosols
such as sulfates in the ambient atmosphere. A schematic diagram of a
typical atmospheric aerosol size distribution is shown in Figure 8. The
plot is constructed so that the area under any section of a curve is
proportional to the concentration in that size range. The number con-
centration is dominated by particles in the 0.002- to 0.02- ym size range,
and the surface area is characterized by particles in the 0.05- to 0.5- ym
size range. The volume and mass, however, show a bimodal distribution.
Within the past 2 years, this bimodal distribution has been confirmed by
a variety of studies in many locations that determined particle number
37
and particle mass as a function of size. The size range between 0.1
and 1.0 ym is called the "accumulation mode." Particles in the accumulation
mode are formed by physical and chemical processes that convert gases
to particles, including metal fumes from high temperature sources,
sulfates, nitrates, organic materials, etc. On the other hand, coarse
25
-------
a
cs
o
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CD
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u
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0.01
0.1 1
PARTICLE DIAMETER, microns
10
100
;«0 © © 0 O
SUB-
RANGES
COAGULATION AND CONDENSATION
SEDIMENTATION
Figure 8. Bimodal distribution of atmospheric particles. •*'
26
-------
particles are formed by mechanical processes such as grinding or rubbing--
for example soil, street dust, and rubber tire wear. Sulfates formed by
the conversion of SO,, are found in the accumulation mode; whereas, MgSO.
from sea salt, and CaSCL from gypsum are found in the "coarse particle
mode". Typically, 80% of atmospheric sulfate is found in the accumu-
lation mode. These small sulfate particles, formed by homogeneous
nucleation, grow by condensation of vapors into the accumulation mode.
However, they do not grow into the coarse mode because there are so many
more very small particles that dominate the condensation growth process.
It is important to understand that most sulfate aerosol is secondary,
i.e., formed by gas-to-particle conversion, and occurs in the accumulation
mode. Particles in this size range have very long life-times, the only
efficient removal mechanism being cloud formation processes; they are
also most effective in scattering light and therefore control visibility
reduction. Most importantly, these particles are within the respirable
size range and can pass into the lower respiratory tract. Sulfate
aerosols can absorb water and increase in size; FUSO. and NH.HSO. do
so at all humidities, and (NHA)9SO,, does so at relative humidities above
?n
about 65%. As a result, visibility reduction caused by aerosols can
be much greater than might be expected on a mass basis, and the lung
deposition cannot be predicted on the basis of existing models for non-
hygroscopic, non-deliquescent substances.
Some confusion has existed regarding the relative removal of SO-
and sulfate aerosol by ground and other surface dry deposition. As
shown in Figure 9, the deposition velocity of aerosols depends on the
particle size and reaches a minimum at the accumulation mode size range
between 0.1 and 1.0 um. The deposition velocity of sulfate aerosol is
theoretically, therefore, 100 to 1000 times smaller than for gaseous
31
SOp. It should be clear then that,although S02 is removed fairly
rapidly by dry deposition on soil, vegetation, and other surfaces, once
the SOp has been converted to sulfate, the removal processes become very
slow. Therefore, sulfates, or any secondary accumulation-mode aerosol
(fine particulate matter) can travel long distances before removal by
fallout or rainout.
27
-------
TERMINAL SETTLING VELOCITY
10-3
10-2
10-1 i
AEROSOL DIAMETER, /u
10
Figure 9. Velocity of deposition for unit density aerosol as a function of diameter.
28
-------
These aspects of reaction mechanisms, aerosol size, and deposition
rates are critical to an understanding of the possible effects of tall
stacks. When SO,, is emitted near the ground, as from home heating
units, the SC^ can be removed by surface removal mechanisms (dry deposition)
When S02 is emitted higher in the air, as from the tall stacks of fossil
fuel-fired electric power plants, the SCL is diluted before it reaches
the ground, and the surface removal rates are reduced. Emissions may be
trapped above the inversion layer and remain trapped for hours. Thus,
elevated stacks theoretically permit a longer residence time in the
ambient atmosphere for S0~ and promote fine particulate sulfate for-
mation by the mechanisms discussed previously. However, they also
provide increased dilution of the sulfate and SCL and reduce impacts in
the vicinity of the source.
1.3.2 Relationship Between Sulfur Dioxide and Sulfate Trends
The existence of a number of mechanisms by which SCL is transformed
into sulfates, and the possibility of transport of these fine particulate
compounds into areas with low SCL concentrations complicates the relation-
ship between ambient SCL concentrations and ambient sulfate levels.
However, a good association between these two pollutants would be expected
in source areas relatively isolated from the influence of other major
emission sources and not subject to high levels of substances that
influence sulfate formation. Monitoring in such areas has shown a
reasonably good correlation between SCL and sulfates. Four CHESS sites
monitored SCL and sulfates in the vicinity of a large, relatively
isolated smelter in the Salt Lake City, Utah, area. The sites were
located about 5, 10, 15, and 40 miles from the smelter. Ambient S09 and
OQ ^
sulfate levels were significantly associated at all sites. During
July 1971, a strike closed down the smelter. Sulfate levels, which had
•3
averaged from 13 to 6 yg/m at the four sites (decreasing with distance
3
from the source), dropped to about 4 yg/m at all sites during the
strike. The effect of the S0? emissions on sulfates apparently extended
39
at least 40 miles from the smelter. A similar study was conducted
during a nationwide steel strike in 1956. Although ambient S02 measure-
ments were not made, sulfate levels in four cities with large steel
29
-------
mills were markedly lower during the 2-week strike period than they
were afterwards. In two cities in which the steel industry was the dominant
air pollution source, sulfates were about twice as high after the plants
were put back into operation than during the strike. In this strike,
emissions of both SCL and catalytic metals were affected. These studies
illustrate that (1) ambient S02 concentrations and sulfate levels can be
significantly associated when complicating factors such as high levels
of background sulfates and substantial amounts of sulfate precursors are
not present and (2) a clear relationship exists between S02 emissions
and ambient sulfate levels in the vicinity of large point sources in the
absence of such complicating factors.
The relationship between S02 and sulfate measurements from the NASN
sites is not as apparent as that in the above examples. Seasonal and
long term trends for national average rural and urban sulfates and
sulfur dioxide for 1960 to 1970 are shown in Figures 10 and 11. Any
actual trends are somewhat obscured by the fact that the number of NASN
sites with valid data changes from year to year. Before 1965, sulfates
show the same high-winter, low-summer peaks that characterize S0?
emissions. In contrast, after this time, nonurban, and to some extent
urban, average sulfate levels begin to reach maximum values in the
summer months. This is the opposite result that would be expected if
sulfates were principally derived from area and other low elevation S02
emission sources located near the monitors, because these sources are
generally responsible for winter peaks in S02- Although correlations
between ambient concentrations of S02 and sulfates at NASN sites were
not high, they were significantly better before 1967 than during the
period 1967 to 1970. Figures 10 and 11 show that, as S02 concentrations
trended downward, no long-range trends were observed for sulfates.
27
A comprehensive analysis of urban S02 and sulfate trends by Frank
concluded that if the data can be taken as representative of actual
trends, no simple relationship exists between ambient S02 levels and
sulfate concentrations.
Although trends analyses for post 1970 NASN data have not been
completed, the data available for 1970 to 1973 suggest that mixed urban
sulfate trends continued during a period of continued decreases in S02
30
-------
1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970
YEAR
Figure 10. NASN urban and nonurban sulfate trends.4
200
CO
•SEASONAL
100
O
CO
1964 1965 1966 1967 1968 1969 1970
YEAR
Figure 11. NASN urban sulfur dioxide trends.4
31
-------
concentrations in urban areas. Table 5 indicates concentration trends
for sulfates and S02 for several urban and nonurban NASN sites. Although
S02 concentrations in most of these cities have decreased since the mid-
1960's, sulfate levels have increased in some areas and decreased in
others. When both trends are downward, the decrease in SCL is often
greater than the decrease in sulfates.
Although the lack of proportional response in long term and seasonal
sulfate concentrations is not completely understood, several tentative
explanations have been offered to account for the data. One inter-
pretation suggests that if the potential error in sulfate sampling that
results from S02 being captured on the filter could be subtracted from
the ambient sulfate levels, changes in S09 levels would be better cor-
2
related with sulfates. This is unlikely to be a satisfactory explanation
because it does not account for elevated nonurban averages where S02
levels are insufficient to produce such an error, or areas where sulfates
increased when SOp levels dropped. Unfortunately, the effect may not be
consistent and has not been adequately quantified.
Other explanations offered suggest that local sulfate levels have
been increased by switching from high sulfur fuels to vanadium con-
40
taining, low-sulfur oil and/or increased production of "natural"
41
sulfates from biogenically reduced sulfur compounds produced in pol-
luted waters and marshy areas without increasing S02 levels. Although
these interpretations may have merit in specific locations, they are
probably insufficient to explain the observed trends throughout the
entire eastern U.S. As noted above, a more plausible explanation for at
least part of the lack of trends is that some of the atmospheric mechanisms
described previously for converting SOp to sulfate are more dependent on
the presence of precursor agents such as photochemical smog than on S02
concentrations. A drop in sulfur dioxide concentrations would, therefore,
not produce a corresponding drop in sulfates if unusually high levels of
such precursor agents existed. This complicating effect may well cause
part of the observed lack of correlations. For example, the greater
levels of photochemical oxidants present in the summer may partially
explain the observation that peak sulfate concentrations also occur at
32
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this time. This suggestion is supported by a study that found an
association between peak 24-hr sulfate concentrations and air masses
that contained high oxidant levels.
o on
According to EPA scientists as well as others, ' much of the
observed lack of correlation between long term ambient SCL and sulfate
trends is explainable by long-range transport of fine particulate
sulfates coupled with a change in the spatial distribution of emissions.
According to this hypothesis, urban sulfates are thought to consist of
two components: (1) locally produced sulfates that are related to SCL
emissions and concentrations in the immediate vicinity of the city, and
(2) imported sulfates, which are formed from S02 emissions upwind of the
area and transported into the city. From the mid-1960's to the present,
urban emissions of S02 have decreased from area and industrial sources.
This is reflected in the observed decline in SCL concentrations. During
that same time period, a change in the spatial distribution of SCL
emissions was taking place because much of the growth in the electrical
utility industry was occurring in nonurban areas upwind of. many major
urban centers. As previously shown in Figure 2, the growth in emissions
of power plants through 1970 more than counter-balanced the urban
emissions decreases and caused the total sulfur dioxide emissions to
rise. It is hypothesized that these emissions in nonurban areas were
transported over long distances forming fine particulate sulfates that
increased regional and urban concentrations of sulfates. Although it is
reasonable to suggest that locally produced urban sulfate levels may
have declined due to decreased urban sulfur dioxide emissions, the
increase in regional backgrounds obscured this effect in many cities,
resulting in either no observable trends or actual increases.
Finklea et al. tested this explanation of trends for a number of
U.S. cities. The procedure involved comparing trends in sulfate levels
with trends in (1) S02 emissions from sources in the city (assumed
proportional to ambient S02 trend) and (2) trends in power plant emissions
in large areas surrounding the city. Contributions of the two source
areas to sulfate levels were assumed to be equal. Usually, SOp emissions
in cities declined and contiguous area emissions increased. The net
-------
trends in S02 emissions predicted the sulfate trends with a reasonable
degree of consistency particularly in larger eastern cities. Predictions
were less good in areas with high photochemical oxidants or catalytic
metal concentrations. The net trends in regional emissions predicted
increases in sulfate levels in the few southeastern cities for which data
was available and increased levels were observed. This analysis tends to
confirm the transport explanation of obscured SCL and sulfate trends
during the 1960's. It also suggests that as regional S02 emissions
increase, increases in urban sulfate levels can be expected.
1.3.3 Long Range Transport
The hypothesis that long-range transport of sulfates from power
plants is influencing urban sulfate levels is supported by the limited
data base on emission and concentration trends. Comparisons of SO-
emission density (Figure 4), power plant locations (Figure 3), ambient
sulfate concentrations (Figures 5 and 6), acid rainfall patterns (Figure
20
1), and incidence of atmospheric hazes all show a somewhat similar
distribution pattern concentrated in the northeastern and north central
United States. According to the hypothesis, because SO* emissions have
been increasing, regional sulfate levels should also be increasing. This
is not apparent from a cursory look at national nonurban trends as
presented in Figure 10. The national trends, however, are based on data
from changing numbers and locations of sites from year to year and
include areas in the west where sulfates are not regionally elevated.
Unfortunately, only a limited number of nonurban sites exists from which
to determine long-term trends. Furthermore, because "nonurban" is a
relative term in some sections of the densely populated east, any regional
sulfate increases due to transport over great distances may be partially
obsured by the influence of nearer source areas. Figure 12 shows the
trends in nonurban sulfates from 12 sites in the northeastern area
outlined in Figure 6. Not all of these sites reported valid data
every year. Figure 13 presents trends for those 8 sites for which
continuous data from 1965 to 1970 are available. Data from 1972, available
for only 7 of these sites is also included. Three-year moving means are also
35
-------
10
a.
Co 7
• = 3-YEAR MEAN
• = ANNUAL MEAN
61
62
63
64
65
66 67
YEAR
68 69
70
71
Figure 12. Nonurban sulfate trends in the northeast (12 sites).
11
10
a
C/9
ff 7
= 3-YEAR MEAN
= ANNUAL MEAN
65 66 67 68 69 70 71 72
YEAR
Figure 13. Nonurban sulfate trends in the northeast (8 sites).
72
36
-------
plotted to smooth out variations due to fluctuations in weather patterns
and the limited number of samples taken each year (20 to 26). The sites
used in these figures are the same as those used by Trijonis in a
similar analysis, with the addition of the Cape Hatteras, N.C., site.
Both figures suggest a general increase in nonurban sulfate concen-
trations in the 24-state northeast quadrant outlined in Figure 6.
The apparent increase in average nonurban sulfate levels is not
evenly distributed throughout this region. Eight monitoring sites that
had generally higher levels in 1970 and 1972 than in prior years are
located in the northern New York-New Hampshire-Vermont, midwest, and
southern portions of the 24-state region. Four sites at which concentrations
appear to show no increases are located near the east coast from Maryland
to Maine. Any increases in sulfates at these sites due to long-range
transport may have been obscured by the decreases in urban S02 emissions
in this heavily populated region. It is of note that none of the NASN
nonurban sites located in the 24-state region of Figure 6 reported
sulfate concentrations below 8 yg/m , and 10 had levels in excess of
9 yg/m in 1972 or the latest year of record.
Due to the fluctuations in annual averages and to the limited number
and distribution of existing monitoring sites, these trends do not provide
44
definitive verification or quantification of transport. However, Trijonis
points out that if the average trend lines were accepted as valid, sub-
traction of 1 to 4 yg/m due to natural background indicates that anthro-
pogenic sulfates may have increased by 40 to 100% in eastern nonurban
areas during the 1960's. The increase in suspended sulfates is parallelled
by the apparent rise in the sulfate ion content in precipitation noted
earlier. These results are qualitatively in agreement with the increases
in S02 emissions shown in Figure 2. The spatial distribution of the
sulfate increases is not in conflict with the siting of many new power
plants in the midwest and Ohio river valley area and the inception of
stringent sulfur emission regulations in the urbanized east coast area.
Based on the increases in SO,, emissions past 1970 indicated in Figure 2,
nonurban sulfate levels would not be expected to have increased markedly
through 1975. Nonurban data for 1973 are only available at 3 sites in
this region. Concentrations at these sites remained in excess of 10 yg/m .
37
-------
Evidence from particle characterization and plume chemistry studies
presented previously is also supportive of regional transport. Although
very little field work on this subject has been done in the United
States, European studies have indicated S02 and sulfate transport cover
distances of hundreds of kilometers. Illustrative of this work are
the results of an aircraft sampling run over the North Sea conducted as
part of a transport model validation effort. Observed sulfate and S02
concentrations are shown in Figure 14 and back trajectories (850 millibar)
are plotted in Figure 15. Air from large emission sources in central
Europe was transported towards England across the Netherlands, and the
southern North Sea. Air from relatively clean areas in central Scandanavia
was transported towards the southern parts of Norway. As the aircraft
moved southward, a gradual increase of sulfates and S09 was measured.
3
High sulfate levels (20 yg/m ) were noted at a spot some 100 miles from
the nearest land mass. The peak concentrations were noted west of the
Netherlands, as expected from the trajectories and source distribution.
44
The results of a summer 1974 study of sulfates and oxidants in New
York strongly suggest that high sulfate levels are associated with
certain air masses. Sulfate levels at 11 to 13 urban and one nonurban
sites located throughout the state were noted to have high or low con-
centrations on the same days. The authors concluded that "The high
concentrations which occasionally existed simultaneously over the entire
state suggests that there is a sulfate level associated with certain air
44 16
masses probably due to long range transport." A separate study
found that acid precipitation events in central New York were strongly
associated with air masses that had passed through areas of high SO- and
N02 emission densities 1 to 2 days earlier. Rainfall associated with
air parcels from lower emission density areas was less acidic.
Both EPA scientists and the NAS consider long range transport to be
an important factor that may explain much of the differences in trends
between ambient sulfate and SOp concentrations observed during the
1960's. Using various assumptions based on current understanding of
2
formation and transport, the NAS presents estimates of the impact of
large emission sources on downwind sulfate concentrations. Their analysis
38
-------
Figure 14. Aircraft measurements of sulfates and
sulfur dioxide concentrations (jug/m3).47
Figure 15. Forty-eight-hour back
trajectories for air parcels arriving
during sampling shown in Figure 14.
39
-------
suggests appreciable impacts on sulfate levels at distances of 300
miles downwind. The NAS report states that "Within a large region such
as the northeastern United States, particulate sulfate concentrations in
the atmosphere are related to regional emissions of sulfur dioxide which
is converted to sulfates after emissions." As the report points out,
the relative impact of distant versus local S02 emissions on sulfate
levels will depend on regional characteristics such as spatial distribution
of emissions and the presence of precursors involved in sulfate formation
mechanisms. Accordingly, regional sulfate levels would not be expected
to increase as rapidly as regional S02 emissions.
Although significant long range transport is considered plausible
2 1
by both EPA and NAS , the SAB suggests that the impact of nonurban
sources on urban areas is likely to be minor. The SAB prefers to explain
the observed disparity in ambient SOo and sulfate trends discussed in
the previous section wholly in terms of some of the alternate explanations
noted above. In particular, the SAB believes that the sulfate levels
extant in an urban area are almost completely the result of local
sulfur emissions. The lack of relation between SO- and sulfates is then
explained by the lack of proportionality expected from complex formation
mechanisms, consistently inflated monitoring data, and significant
direct emissions of sulfur trioxide and sulfates. The SAB based its
assessment of the impact of transport on the assumption that dilution
and removal would reduce sulfates to negligible levels during transport
and did not analyze transport models or measurements. The SAB believes
that the available evidence is insufficient to confirm the validity of
their assumptions or any of the explanations offered to date for the
observed urban sulfate trends. This only highlights the need for
additional research in these areas. EPA recognizes the uncertainties
involved and the fact that a quantifiable relationship between S02
emissions and sulfate air quality must await further study of formation
mechanisms, oxidation rates, and long range transport. Even so, once
applicable emission limits have been met by all sources in urban areas
thus reducing locally produced sulfates, EPA believes that, based on the
available evidence concerning transport, further increases in regional
-------
and urban sulfates can be expected if nonurban SOp emissions from power
plants and other sources continue to rise. Given the general levels of
sulfates, other fine particles, and sulfur oxides in the northeast, the
Agency's assessment of the preliminary health data suggests that such
increases should be viewed with concern.
1.4 Control Alternatives
As the previous section has indicated, sulfate formation is a
complex phenomenon that involves many factors. Some parameters such as
humidity, temperature, and natural production of hydrogen sulfide and
ammonia are obviously not controllable. This limits the available
technical control options to control of anthropogenic emissions of
precursors that accelerate the formation of sulfate from SCL or control
of the major precursor SCL. These alternatives are discussed below.
1.4.1 Precursor* Control
Considering sulfate formation mechanisms and the lack of a perceptible
relationship between SOp concentrations and sulfate trends, control of
non-SOp precursors, such as fine particulates and nitrogen oxides, might
be necessary. Such a control strategy is currently being applied to
oxidants. Table 6 lists the principal agents involved in the conversion
mechanisms shown in Table 4 that affect sulfate formation and presents
expected trends and control feasibility.
As pointed out in the table, control of those precursors already
included in other control strategies should be dictated in large part by
considerations other than sulfate formation. For example, if the
critical hydrocarbon/nitrogen oxides ratio necessary to reduce sulfate
formation differs from that for oxidants, the oxidant strategy should
prevail. Similarly, fine particulates should be controlled to prevent
intrinsic health and welfare effects as well as the synergistic effects
possible when they combine with sulfur oxides.
*A1though SOp is the most essential sulfate "precursor," the term
as used here refers to those chemical agents (e.g. oxidants) that are
essential in the previously outlined formation mechanisms.
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The effectiveness of controlling sulfate levels by the control
of pollutant precursors is impossible to predict with currently
available information. Ideally, elimination of precursors would slow
the sulfate formation rate, reducing sulfate concentrations in the
vicinity of SCL emissions and increasing the percentage of S0? that
will be removed from the atmosphere by rainout or dry deposition before
forming sulfates. However, if the relatively low oxidation rate tentatively
measured in St. Louis is typical of eastern cities, ambient^ precursor
control in urban areas might do little to reduce local urban sulfate
concentrations. In addition, favorable meteorological conditions and
natural backgrounds of humidity, oxidant-related hydrocarbons, and
ammonia may be sufficient under some conditions to allow significant
regional formation of sulfates from urban and rural SC^ emissions even
in the absence of urban precursors. Thus, reduction of precursors involved
in a given mechanism may only result in a shift to another formation
mechanism without greatly affecting sulfate levels. Control of pre-
cursors such as catalytic vanadium particulates, which are released
simultaneously with SCL, may slow oxidation in plumes and be a more
successful strategy than control of ambient precursors for reducing
local sulfates if plume oxidation rates without such controls are rapid.
Even if control of catalytic particulates would reduce local sulfate
formation in the plume, rural ambient conditions may still produce
regional sulfate formation and transport. Thus, given current infor-
mation although catalytic particulates and photochemical oxidant control
may be of importance in a sulfate strategy, control of such precursors
without also restricting sulfur dioxide emissions might be of limited
value.
Reliance on tall stacks and precursor control to reduce local
sulfates could result in increased rainfall acidity. As discussed pre-
viously, SCL emissions from tall stacks can travel long distances before
deposition. In addition to allowing more time for sulfate formation,
the lengthy residence time increases the dependence on precipitation
48
scavenging for removal. Recent evidence suggests that this would only
aggravate the already serious acid rain problems described in Section 1.1.3.
-------
Furthermore, for air masses that travel large distances with no rain or
that are stagnant for several days, sulfate formation could be extensive,
even with precursor control at source areas and with slow oxidation.
Based on the current understanding of atmospheric chemistry, trans-
port, and health effects, the most feasible approach to sulfate control
is to control atmospheric emissions of sulfur compounds and pursue
precursors primarily through existing programs aimed at their control.
However, the role of precursors, including those which are uncontrollable,
should be considered before implementing any sulfate-related strategy;
for example, more stringent SCL controls might be necessary in areas
with high smog levels, high emissions of catalytic metals, or high
humidity to prevent excessive levels of acid-sulfates caused by rapid
conversion. Due to the combination of precursor-limited conversion
mechanisms and to the influx of sulfates from distant sources, reduction
of S(L emissions and concentrations in a given urban area may not produce
proportional reductions in sulfates. Furthermore, depending on the type
and concentration of precursors present, reductions in total sulfate
concentrations may not produce comparable reductions in the sjpecific
acid-sulfates or other sulfur oxides thought most responsible for
adverse health effects. Current knowledge does suggest that reduction
in S0? emission density over a large region would reduce regional sulfate
backgrounds; under these conditions, decreases in locally produced urban
sulfates would be expected to become more directly related to reductions
in urban SO- emissions.
1.4.2 Alternatives for SOp Emission Control
The principal approach to sulfate control is likely to be sulfur
dioxide control. Primary sulfate emission control for stationary sources
involves widely utilized conventional particulate control devices.
Control of sulfuric acid emissions from catalyst-equipped motor vehicles
may include desulfurization of gasoline, as yet undeveloped exhaust
particulate control, or switching to alternative exhaust hydrocarbon and
carbon monoxide control systems or alternative engines. As discussed in
Section 1.4.1, control of sulfate precursor agents such as catalytic
-------
metals is unlikely as a principal control strategy. Control of SO-
precursors such as H-S, cs*, and COS is commonly practiced in the
petroleum industry and involves a variety of standard and developing
technologies that will not be discussed here. Application of these
technologies wil.1 become more important with the increase of precombustion
removal of sulfur from fossil fuels.
Consideration of the magnitude of total emissions and the relative
removal rates of SO^ released from low versus high level sources and
resultant sulfate formation potential indicates that permanent control
of large industrial and utility emitters will probably be an important
element of any sulfate control program.
Selection of S02 control technology is dependent upon the characteristics
of the emission sources to be controlled. In the evaluation of a SCL
control alternatives for use in a sulfate control strategy, the following
criteria should be used:
1. Sulfur reduction potential.
2. Relative energy efficiency or penalty.
3. Cost of sulfur removal or emission reduction.
4. Overall environmental impact.
5. Commercial availability of technology.
6. Applicability of approach to scope and/or numbers of emission
sources.
Table 7 presents several major alternative SO^ control technologies
together with a partial listing of their characteristics according to
the above criteria. A summary of the major control alternatives that
might be included in a sulfate strategy follows.
AQ
1.4.3 Use of Naturally Clean Fuels *
This approach is constrained by the availability of "clean fuels":
natural gas, and low-sulfur coal and oil. Due to a growing shortage,
natural gas is already being allocated to high-priority uses. There-
fore, until 1990 the principal natural clean fuel options involve use of
low-sulfur coal and, secondarily, low sulfur oil.
Options include redistribution of clean fossil fuels to large coal
consumers in those regions in which the SO- ambient air quality standard
45
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ments in certain areas, and by expanding and developing clean fuel
extraction and production techniques. Apart from the uncertainty associated
with attempts to regulate the distribution of fuels, this approach may
merely transfer, spread out, or actually increase the sulfur emission
burden and may Aggravate long-term sulfate or acid rain problems.
1.4.4 Flue Gas Desulfurization (FGD) Technology
Stack gas desulfurization systems can be classified into two general
categories: (1) nonregenerable processes in which the sulfur materials
are disposed of as wastes, and (2) regenerable processes from which by-
products such as sulfuric acid and elemental sulfur can be marketed.
Many of these engineering control systems have been studied at the pilot
plant level, some have been tested at the intermediate prototype scale
(10 to 15 MW), and a growing number have been incorporated into full
scale commercial power systems (100 MW or greater). In the United
States, the majority of utilities planning full-scale system instal-
lation through 1980 will likely choose the nonregenerable FGD systems
due to their advanced state of development compared to regenerable
systems. In the 1980's, however, this trend may be reversed as the
regenerable systems, which produce saleable by-products, are proven to
be operationally and economically competitive.
It should be noted that scrubbers are a possible source of localized
primary sulfate emissions, but such emissions are thought to be of
small potential import due to the relatively small amounts emitted from
controlled sources as opposed to uncontrolled sources, and the non-
acidic chemical form of the sulfates from controlled sources. In addition,
primary emissions of S07 may be actually reduced by scrubbers and, where
O
necessary, very effective mist eliminators are available and are being
used on flue gas desulfurization units.
A summary description of the major flue gas desulfurization pro-
cesses is presented in Table 8.
1.4.5 Fuel Pretreatment49'50
Major coal pretreatment methods are (1) mechanical coal cleaning,
(2) coal liquefaction, and (3) coal gasification.
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1. Mechanical coal cleaning: Mechanical coal cleaning is a pro-
cess in which coal is crushed, screened for size, and subjected to techniques
based upon specific gravity and surface characteristics to separate coal
from its heavier impurities including pyritic sulfur. Because only about
30 percent (10 to 47%) sulfur removal is routinely possible, mechanical
cleaning cannot generally be used to attain very low sulfur coal from
high-sulfur coals.
2. Coal liquefaction processes: The most promising liquefaction
technology for short-term commercial application through 1990 is solvent
refining under hydrogen pressure to dissolve the organic components of
the coal, producing a solvent refined coal (SRC). The solution is
filtered to remove the ash and insoluble organic materials, and fractionated
to recover the solvent. The refined coal remaining is solid at room
temperature. Final fuel output contains 0.6 to 0.7 % sulfur.
3. Coal gasification: Almost all processes for gaseous fuel from
coal fall into one of three groups: (1) production of low-Btu gas, typically
around 100-200 Btu/ft3, (2) production of medium-Btu gas, 300-500 Btu/ft ,
and (3) production of pipeline quality synthetic natural gas (SNG) around
1000 Btu/ft (high-Btu gas). Greater than 95% sulfur removal is possible,
although the gasification plant can become a significant sulfur emission
source.
49
1.4.6 Alternative Coal Combustion Systems
The pulverized-coal boiler and steam turbine systems currently
dominate the power generation field. There are alternate approaches
available, however, that have proponents within the energy engineering
field. Two such systems are fluidized bed combustion and magneto-
hydrodynamics; both systems decrease coal usage through increased
overall conversion efficiency.
1. Fluidized-bed coal combustion: In fluidized-bed operations,
coal less than 1 centimeter in size is suspended in a vertically rising
flow of combustion air, which gives the appearance of a boiling fluid. The
various processes have a number of engineering advantages, among them
smaller size and cost, the ability to handle even high-ash coals without
-------
particulate controls (the ash agglomerates in the fluidized bed and is
collected in a hopper), and sulfur removal by inclusion of lime or
limestone in the combustion bed. Because lower operating temperatures are
possible, this system also reduces NO emissions. In addition, the
/\
systems can be engineered to provide hot gases to drive a gas turbine
and steam generator, raising the overall thermal efficiency of the
plant.
It would appear that fludized-bed combustion with SO cleaning
/>
should be considered, especially with respect to low-sulfur coals with
high ash resistivity. It does not, however, seem promising except in
new plant application.
2. Magnetohydrodynamics (MHD): These power systems, currently in
the research and pilot stage, operate on different physical principles
than a conventional steam-electric plant, promising thermal efficiencies
of 50 to 60 percent.
Though energy efficiency is a major promise, MHD also offers some
direct S02 emissions reduction. To aid the ionization of the combustion
gases, potassium or other alkaline metals are added; these react with
the sulfur and are then collected for recycling. Near-term availability
does not appear feasible.
1.4.7 Intermittent Control Systems (ICS)
Itermittent or supplementary control systems are a category of
sulfur oxide management techniques that attempts to avoid large localized
ground level SO concentrations by means of very tall stacks, fuel
X
switching, load shifting, and curtailment of operation when short-term
or seasonal meteorology forecasts indicate poor dispersion. As discussed
in Section 1.3.1, tall stacks may actually increase potential sulfate
formation by extending the time available for conversion before ground
removal of S02 is possible. This possibility has not yet been tested by
comparing plume studies of very tall stacks. Extensive use of ICS would
likely maintain or increase current high levels of sulfates and acid rain,
because growth in S02 emissions from new sources would be comparatively greater
and this growth would not be balanced by corresponding emissions decreases
50
-------
p cr\
from existing sources. In two separately prepared reviews ' of con-
trol options, the NAS points out that intermittent control is likely to
be only a temporary short-term measure in the next 5 to 10 years because
it does not reduce regional atmospheric loading of sulfur oxides nor
2
does it curtail secondary sulfate pollutant formation and transport.
The strong possibility that increases in sulfates and acid rain might
cause significant adverse health and welfare effects precludes permanent
use of such techniques as currently applied in a sulfur oxides and fine-
particle control strategy. Based on current knowledge, intermittent
control will probably be an acceptable technique in the long term only in
special cases where constant emission reduction measures are unavailable
and/or if later information indicates that certain sources are located
in such a manner that SCL and SO/~ are removed by natural mechanisms
without adversely impacting on health or welfare.
49 54
1.4.8 Employment of New Energy Sources '
Today, the major portion of our electric energy requirements is sup-
plied by coal combustion. Therefore, the substitution of alternative basic
energy sources represents an equivalent reduction of coal combustion SO
.A
emissions. The most readily available alternate energy sources are
solar energy, nuclear fission, geothermal energy, and solid waste com-
bustion.
Solar energy is unlikely to be of significant impact through 1990,
but is potentially an attractive major future energy source. Nuclear
fission is already scheduled 'to have a large impact on energy generation
through 1990 and beyond. Further fission development as a sulfur emission
reduction alternative must be balanced by environmental and economic
concerns associated with planned development. Geothermal energy develop-
ment is of limited capability through 1990, especially in the northeast
U.S. where sulfate reduction is most needed. In addition, sulfur control
must be applied to this energy source to prevent natural !-LS emissions.
Combustion of solid waste could displace as much as 5% of utility coal
in the 1980's with a sulfur content equivalent of about 0.3%. Consideration
should be given to the relative energy and environmental impact of this
method of solid waste disposal compared with recycling of the material.
51
-------
1-4-9 Energy Conservation
Any reduction in demand for electric power, whether through economic
or conservation incentives, can result in a decrease in SO emissions
A
from power plants. Detailed discussions of measures such as efficient
pricing, modification of demand, and conservation are presented by
Kahn and Chapman et al. Kahn concludes that more efficient pricing
reflecting the higher costs of supplying electricity during peak hours
could make a significant contribution to demand reduction. He also
notes that reductions in the use of oil and natural gas due to the
energy crisis may lead to increased use of electricity. This could
partially offset reduced demands due to costs.
Conservation measures could have a significant impact on electrical
demand. Table 9 shows that potential savings in 1980 amount to about
400 billion kilowatt-hours. Projected demand in 1980 is expected to
range between 2,200 and 3,200 billion kWh. Estimated reductions in S0?
emissions from this savings are as large as 1.8 to nearly 4.0 million
tons/yr of sulfur oxides nationwide.
Table 9. ELECTRIC ENERGY CONSERVATION SAVINGS, I96062
(109 kWh)
Tvoe
Substitution of natural gas for electricity
in selected household uses
Lighting levels
Aopliance efficiency, general
Air conditioning efficiency
Aluminum production and use in cars
Home insulation
Savings
94
183
23
51
20
21
392
52
-------
1.4.10 Applicability and Availability
It is important to note that the impact of the above technologies
must be considered in reference to the total energy production system in
which they are utilized. For example, utilization of synthetic natural
gas in power plants entails severe energy and cost penalties, but is
competitive with electricity when used for space heating. Table 10
gives a total system evaluation of several S02 control technology
alternatives potentially available to the utility sector. The table
clearly shows the difference in S02 emissions, energy efficiency, and
costs when various control technologies are used.
The maximum potential availability of the major S02 control tech-
nologies for use in a national sulfate control strategy through 1990 is
presented in Figure 16. It should be pointed out that the estimates of
Figure 16 do not indicate actual application of technologies, but the
maximum amount of capacity possible, given economic and technological
constraints. Applicability of the systems would depend on the other
factors outlined previously and included in Table 7.
It is important to note that flue gas desulfurization is the chief
technology available for stationary source control through 1985. Given
the cost and energy penalties of gasification and liquefaction, FGD is
likely to have continued application through 1990.
1.5 Information Gaps and Research Needs for Sulfate Regulation
Decisions to regulate a pollutant require that adequate information
be available to facilitate assessment of health and welfare effects and
design of a workable control strategy. As evidenced by the previous
discussion of health effects and transport, considerable uncertainty
exists in interpreting the limited scientific data base on sulfates.
Both the SAB and the NAS place high priority on initiation of comprehensive
research programs that are needed before a major regulatory control
program is possible. This section outlines some of the more significant
information gaps relevant to control strategy development and indicates
important research needs. A detailed analysis of these needs conducted
64
by the Agency indicates that development of data and information
53
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a>
(O
LU
-------
900
1975
1980
1985
1990
YEAR
Figure 16. Maximum availability of S02 control technologies/*9,53,54,63
55
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necessary for a sulfate regulatory program would require 3 to 5 years.
In this regard, if EPA were to set a National Ambient Air Quality Standard
for sulfates it could not realistically be proposed before 1980 to 1981.
Important research needs are summarized below.
1.5.1 Monitoring
One of the reasons that such a lengthy period is needed for the
research program is that many of the projects cannot begin until after
successful completion of other program elements. The most prominent
example of this interdependency is the development of reliable monitoring
methods for total sulfates, sulfuric acid, and specific sulfates. Such
methods may take 1 to 3 years to develop. Important components of
epidemiological, toxicological, clinical, and characterization studies
are dependent on the use of such monitors. This extends the completion
date of the entire effort.
1.5.2 Health and Welfare Effects
Although toxicological data have provided qualitative information
regarding the relative effects of sulfur dioxide and specific sulfates,
this work must be expanded to provide more quantitative data using
additional response indicators. Additional human studies are especially
necessary. Animal and human experiments have been performed in static
gaseous or particulate atmospheres with no great potential for interaction.
Toxicological and clinical studies of the biologic response to gas-
aerosol mixtures, which can better simulate the complex physico-chemical
environment to which mankind is exposed, are greatly needed.
Despite the accumulation of epidemiological evidence supporting the
role of sulfates, there is a paucity of information concerning the role
of sulfates in the production of, or exacerbation of, chronic pulmonary
disease. Although preliminary epidemiological data support the occurrence
of such effects, more comprehensive population studies are needed to
substantiate preliminary findings. The etiology of such disease is
undoubtedly complex, probably being involved with primary and secondary
factors including sulfur oxides, respirable particles, oxidants, smoking,
56
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temperature or humidity variation, and the presence of biological agents
such as bacteria and viruses. The specific sulfates responsible
for observed health effects should be identified. Characterization
should include information on particle size. No studies of sufficient
sophistication to fully resolve these complex issues have yet been
undertaken.
Studies using available exposure and response assessment techniques
should continue, but development of more definitive dose-response informa-
tion must await the production of instrumentation required to adequately
measure ambient pollution both qualitively and quantitatively.
Although sulfates are suspected to cause significant welfare effects,
damage or effect functions have not yet been developed. The role of
sulfates in forming acid rain must be determined. The effects of acid
rain and dry deposition of sulfate aerosols (particularly sulfuric acid)
on crops, terrestrial and aquatic ecosystems, and materials are incompletely
understood and remain largely unquantified. Although such information
is not needed for health related regulations, the potential magnitude of
the effects is such that a significant impact on the outcome of cost-
benefit analyses is possible.
1.5.3 Atmospheric Chemistry and Transport
More work is needed to develop sulfate-precursor relations to the
extent that they can be used to determine the need for control of precursors
other than SCL- This kind of work should be pursued as soon as possible
because the conclusions that may be drawn will govern the directions of
other control technology and strategy development programs that may be
in progress.
As indicated above, although part of the work that has been done to
date on the physics, chemistry, and transport of sulfates has been in
the area of sulfate-S02 conversion rates for power plant plumes and
urban areas, the current status of knowledge is deficient in several
respects. Some of the open questions are as follows:
1. It is possible that the plume from a nonurban coal-fired power
plant crossing an urban area mixes with precursor agents that signifi-
cantly accelerate the rate of conversion of sulfur dioxide to sulfate.
57
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Assessment of this possibility is difficult because plume breakup is more
likely to occur in the city, and the sulfur dioxide from the plume
cannot be distinguished from the sulfur dioxide from low-level com-
bustion sources. However, certain experimental approaches may allow an
estimate of the relative contribution of plumes to the overall urban
sulfate levels on a day-by-day basis. Several study sites are needed to
evaluate photochemical effects as well as effects of particulate com-
position on rates of conversion of sulfur dioxide to sulfate.
2. Limited experimental results indicate that within some types of
plumes from oil-fired power plants, sulfur dioxide can be rapidly converted to
sulfates. Plumes from several additional oil-fired plants need to be
studied, and the influence of particulate control should be investigated.
3. Little experimental work is available on the low level urban
plume. Because power plant plume contributions from upwind break up in the
city, downwind of the city the urban plume containing the sulfur dioxide
and sulfate from all emission sources may be a problem. We cannot fully
assess the relative rates of conversion of sulfur dioxide to sulfate
compared to the rate of removal of the sulfur dioxide by dry deposition
or precipitation. Possible sinks for sulfates in urban and nonurban
areas need to be identified.
4. The build-up of regional sulfates from numerous rural and urban
sources under stagnating conditions, although possible, has not been
investigated. Estimates of the impact natural sources of hydrogen
sulfide and organic sulfides exact need to be quantified because under
certain conditions these might also be rapidly converted to sulfur
dioxide and then to sulfate. Also unassessed is the influence of natural
hydrocarbons and nitrogen oxides in sulfate formation during periods of
high photochemical activities.
These areas must be addressed in order to develop models that can
be used to estimate both local concentrations and long-range transport
phenomena for those components of sulfates which are important in pro-
ducing adverse effects. Such models are essential in the evaluation of
control strategies.
58
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1.5.4 Improve Control Technologies for Sulfur Dioxide and Sulfates
If, as has been implied, a high degree of control for large point
sources of sulfur dioxide is needed to adequately control sulfates in
some parts of the country, the efficiency required of currently demon-
strated scrubbing technologies might be greater than 90% removal. Modification
of current technologies to demonstrate greater than 95% removal efficiency
could be accomplished by 1978. Alternative combustion technologies showing
promise of high efficiency, such as fluidized bed systems, should be
evaluated and encouraged. Advanced precombustion technologies and
control for small point and area sources of SCL may be necessary and
should be developed. Sulfur traps or alternate hydrocarbon and carbon
monoxide controls may be needed for post-1978 automobiles to control
emissions of sulfuric acid mist.
59
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2. POTENTIAL REGULATORY STRATEGIES
The review of current scientific and technical knowledge indicates
that although a major regulatory initiative for sulfates may not be
possible for several years, the strategies that are ultimately developed
probably must take into account health and welfare effects at low concentration
levels, emissions/concentration relationships complicated by complex
formation mechanisms, and long-range transport. Although scientists and
engineers proceed with research directed toward filling significant
information gaps, strategy analysts must investigate the regulatory
options that, based on current information, appear likely to be needed.
This section outlines the considerations that must be examined in the
context of available and possible regulatory alternatives. It is not to
be construed as an Agency regulatory policy for sulfates. Policy impli-
cations are discussed later in the report.
2.1 Potential Scope of the Problem
The goal of any regulatory approach to sulfate control should be
the protection of health and welfare from significant harm. Because the
acute and chronic effects levels from the health studies listed in Table
2 differ considerably with regard to the degree of control necessary for
achievement, strategy decisions may have to consider the benefits derived
from attainments of various levels. In the long term, no segment of
society should suffer the effects of illness unnecessarily. However, 24-hr
maximum sulfate concentrations at western sites (where annual
2
levels approximate the global background of 1 to 3 yg/m ) can on occasion
be in the range tentatively associated with sensitive health indicators.
Thus, protecting the most sensitive portion of the population could
ultimately involve SCL control in excess of that required to meet current
SCL standards. Any attempt to achieve such levels of control in the
short term (3 to 5 years after initiation of sulfate regulatory control)
could result in failure or unacceptable social and economic consequences.
A recent analysis of strategies assumed proportionality between
SCL emissions and regional sulfate levels to determine the maximum
degree of control that might be needed to meet various presumed effects
60
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levels. The study concluded that continuous control of all major SO,,
sources might be insufficient to achieve sulfate levels presumed needed
to protect the most sensitive portions of the population in some areas.
In addition, controls somewhat in excess of those anticipated for SCL
standards could be needed to prevent excursions possibly related to
mortality and to protect the general population from increased respiratory
ailments. The cost of the incremental control technology assumed necessary
to achieve this level of control in the northeastern U. S. totaled
nearly $30 billion over a 10-year period from 1981 to 1990.
If economic and social factors are to be considered in development
of a sulfate control strategy, a phased reduction of sulfate concentrations
over an extended period of time is likely to be necessary. The degree
of S02 control applied within a region at a given time depends upon the
following factors: (1) efficiency, availability, and cost of S02 control tech-
nologies for existing and new sources, (2) S02 emission density characteristics,
(3) chemical and physical characteristics of existing sulfate levels,
(4) influence of primary sulfuric acid emissions by catalyst equipped
vehicles, (5) levels of other precursors, (6) population at risk from
existing levels, (7) meteorology, and (8) significant welfare effects.
Because current knowledge precludes adequate assessment of the above
factors, evaluation of preliminary sulfate control strategy development
must include a sensitivity analysis of the significant unknowns. Studies
done to date indicate that among the most sensitive parameters relative
to strategy development is the uncertainty regarding the oxidation rate of S02
in plumes and under varying ambient conditions. Estimates of oxidation
rates for power plant plumes range from negligible to 20% or more per
hour. The sensitivity of diffusion model estimates for sulfate increments
from a typical power plant to the plume oxidation rates is shown in
Table 11. The difference between a 1% and a 5% oxidation rate produces
significant differences in incremental sulfates at distances of 5 to
over 50 km downwind. Because a low oxidation rate increases the removal
potential of S09 before it can form sulfates, estimates of the degree of
65
long range transport can be affected. North analyzed the sensitivity
of an oxidation rate of 0.5 to 2%/hour. The differences between the
-------
rates resulted in a tripling in predicted sulfate increments over a wide
front nearly 500 km downwind. Sound decisions as to the degree of
control required for such a power plant might depend upon whether the
plant was located in a rural or urban situation and could not be made without
further information on oxidation rates and long-range transport even if
health effects information were well developed.
Table 11. DIFFUSION MODEL SENSITIVITY OF SULFATE
FORMATION TO S02 OXIDATION RATE
FOR 1500-MW POWER PLANT3
(24-hr sulfate concentration, yg/m3)
Distance, km
5
10
20
30
40
50
Conversion rate, %/hr
1
1
1
1
1
1
1
5
2
4
6
6
7
7
40
21
38
50
54
54
54
Gaussian dispersion model applied to a typical
large power plant under conditions of poor dis-
dispersion.
Only qualitative conclusions can be drawn concerning the degree of
S02 control required for a possible sulfate strategy. Based on extrap-
olations of current knowledge, it is possible that sulfate control
would require greater S02 reductions from all sources than is required
to achieve S02 standards. In addition, distinct differences exist
between various regions with respect to the eight criteria referred to
above as necessary assessments of degree of control. The western United
States clearly requires less sulfate control than does the east. Regions,
subregions, and even urban areas within the east also differ in their
need for control. Sulfate control strategies might recognize these
differences by developing sulfate control regions based on emission,
concentration, and transport patterns, and by prioritizing allocation of
62
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2
limited control resources. The NAS recognized these considerations in
presenting a preliminary methodology for prioritizing major sources
according to their need for control. This technique was useful in
illustrating the crucial scientific information gaps and providing the
general conclusion that power plants located in or near urban areas with
elevated sulfates or SCL should be given high priority with regard to
control. Beyond these general considerations however, the technique is
of limited utility for decision-making until significant new research is
completed.
2.2 Regulatory Options Under the Clean Air Act
The previous section presented a preliminary assessment of some
important considerations associated with development of technically
feasible control strategies for sulfates. In this section, the im-
plication of these considerations with respect to available regulatory
approaches is examined.
2.2.1 National Ambient Air Quality Standards
Under the Clean Air Act of 1970 the Administrator of EPA may establish
a National Ambient Air Quality Standards (NAAQS) for "any air pollutant
which in his judgment has an adverse effect on public health and welfare,11
and results from emissions of "numerous or diverse mobile or stationary
sources." Simultaneous with the publication of health and welfare
criteria, primary and secondary NAAQS are set, respectively, to protect
public health with an adequate margin of safety and to prevent known or
anticipated adverse welfare effects. The Clean Air Act does not allow
consideration of costs of achieving the standard or benefits derived
from the standard to influence the determination of the maximum permissible
exposure level for a pollutant. As indicated in Section 1.5, if an air
quality standard for sulfate was determined to be necessary, it could
not be set before 1980-81.
Once standards are set, a time phased sequence begins. The standards
are promulgated 90 days after publication in the Federal Register.
Within 9 months, each state would be required to submit a state implementa-
tion plan (SIP) that would provide for implementation, maintenance,
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and enforcement of the NAAQS for each air quality control region (AQCR)
within that State. Guidelines issued at the time of promulgation would
specify emission reductions required to achieve the prescribed ambient
pollutant level. Each state implementation plan must be approved or
disapproved by EPA within 4 months of submission. Achievement of the
primary standard must occur within 3 to 5 years after SIP approval.
Although sulfates exhibit those characteristics described above for
pollutants to be controlled through NAAQS, some implementation difficulties
associated with this approach are anticipated. Because an air quality
standard requires that implementation plans include measures that will
ensure attainment of the standards within each AQCR, NAAQS implementation
generally has required the use of a pollutant air quality/emission
control relationship within that AQCR's boundary. Complex sulfate
precursor formation mechanisms and transport across multistate regions,
however, are likely to make such a relationship impossible within current
AQCR boundaries. AQCRs with essentially no SCL emissions could exceed
the standard because of sulfates imported from other regions. Areas
with high S(L emissions and low imported sulfate levels might apply
limited control to meet local air quality goals at the expense of downwind
areas. The present AQCR boundaries were fixed by the Clean Air Act and
are, in general, far too small to permit implementation of a NAAQS for
sulfates in areas where long-range transport is significant.
Another possible problem is the definition of a "threshold" level
for sulfates below which no serious health or welfare effects occur.
Preliminary health studies have identified possible effects at near
background levels, suggesting that the threshold concept may be inappropriate.
In addition, achievement of very stringent control requirements might
require compliance extensions beyond the 3- to 5-year period required by
the act.
The effectiveness of a sulfate NAAQS for protecting health would
depend in part upon the degree of discrimination possible in the health
effects criteria and precursor mechanisms. As more information about
specific sulfate effects becomes available, alternative standards for
sulfuric acid emissions, or combined standards that take into account
-------
synergism between sulfates and other pollutants, such as nitrates, might
be set. A better understanding of complex precursor relationships is
needed before emission-ambient guidelines similar in approach to those
for oxidants can be established. If the above considerations cannot be
resolved, evenhanded implementation of a sulfate standard in all areas
of the country would produce uneven levels of protection from adverse
effects, and in some cases unnecessarily stringent controls.
2.2.2 New Source Performance Standards
The Clean Air Act requires that EPA set emission standards for new
or modified sources that may "contribute significantly to air pollution
which causes or contributes to the endangerment of public health or
welfare." The emission standard for a given pollutant must reflect the
best degree of available control technology, considering cost, that the
Administrator believes has been adequately demonstrated. If a NAAQS has
been set for the pollutant, only new sources are affected. For a non-
NAAQS pollutant, existing sources of the same type may be required to
meet emission standards through a SIP process similar to that described
for NAAQS. If the state plan is approved, existing sources must be in
compliance within 3 to 5 years. If the state plan is unacceptable, EPA
must enforce the New Source Performance Standards (NSPS).
Although sulfate is a non-criteria pollutant, NSPS for the principal
precursor, SCL, already exist for steam generating plants and sulfuric
acid production plants. Strengthening existing NSPS for large, fossil -
fuel- fired boilers would affect only new plants and could be used as
part of an interim approach until more comprehensive action is possible.
Conceptually, a NSPS for total sulfur emissions as measured by a technique
that would collect all SO , might allow retrofit of existing sources.
X
If this approach were possible for sulfates, retrofit of existing sources
would be implemented by the states, subject to EPA approval. The current
NSPS for SCL for steam generators is 1.2 Ib/million Btu heat input and
reflects the 70 to 80% removal efficiencies currently demonstrated.
Given the less stringent need for health criteria, an emissions
standard under the NSPS section of the Act might be set 1 to 3 years
65
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earlier than a NAAQS, or about 1978. According to the sulfate research
program outlined previously, greater than 95% control efficiency with
flue gas desulfurization probably could be demonstrated by that time. A
revised NSPS for 502 at thl's time cou^cl ^e usec* as an interim approach
to limit growth in emissions past 1981. Although it is unlikely that
the NSPS section of the Act could be used as the primary regulatory
mechanism for sulfate control, use of this approach could avoid some of
the implementation difficulties associated with a NAAQS and preclude the
necessity for major statutory changes. Because costs are considered in
setting a NSPS, the consequences of plant shut downs and excessive
expenses associated with installing devices with control efficiencies
greater than 95% could be considered. The timing associated with con-
trolling existing sources may still present some problems, but the
degree of control required can be adjusted to existing technological
capabilities rather than to meeting specified ambient levels. The size
of AQCR's and effect of precursors or imported sulfates would not cause
SIP development difficulties.
More serious troubles with the use of the NSPS approach as the
primary regulatory control method occur in the effectiveness with which
health and welfare is protected. A national NSPS for given sources
would disregard the regionality of the sulfate problem and result in
application of stringent control measures in areas where sulfates were
not a problem. The standard for new sources would have to be lenient to
the extent that the best demonstrated control techniques must be generally
adaptable everywhere. For example, if 95% sulfur reduction is the
maximum control possible for generally available coals, a new electric
plant that could economically scrub and use medium sulfur coal for 98%
control would not be required to do so, because this would not be generally
feasible at all new plants. Although cost-benefit considerations would
suggest tighter standards in regions with more serious sulfate problems,
a uniform standard would ignore this. Conversely, areas with minimal
sulfate problems would be overcontrolling both new and existing plants,
diverting needed control resources that might better be applied in other
regions. Because use of a "total sulfur" NSPS approach for sulfates
66
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control could preclude major statutory changes that otherwise might be
needed,however, mechanisms for implementing this alternative should be
investigated.
2.2.3 National Emission Standards for Hazardous Pollutants
A non-criteria pollutant that in EPA's judgment "may cause, or
contribute to, an increase in mortality or an increase in serious irreversible,
or incapacitating reversible, illness," may be controlled by National
Emission Standards for Hazardous Pollutants (NESHAP).
This option is best used for relatively rapid control of hazardous
pollutants without regard to cost or available technology. This is an
unlikely regulatory path for sulfates.
2.2.4 Emission Standards for Mobile Sources
The Act requires the Administrator to set emission standards for
any air pollutant coming from motor vehicles if the pollutant is harmful
to public health and welfare. This can be done by requiring installation
of emission control devices or regulation of fuels content.
The Administrator has determined that the health risk from pre-
dicted incremental concentrations of fine particulate sulfuric acid from
catalytic converters represents a serious health risk and has indicated
his intent to propose an Emission Standards for Mobile Sources (ESMS)
for sulfuric acid. Desulfurization of gasoline, use of alternative
hydrocarbons and nitrogen oxide controls, and application of exhaust
particulate control devices are possible options for sulfate control
under ESMS. EPA is currently encouraging the development of alternative
HC and NO controls. Implementation would be uniform nationally except
/\
for California, which generally has stricter controls mandated by the
state.
2.2.5 Other Clean Air Act Options
Although application of Abatement Conferences and to a lesser
extent emergency powers is possible for sulfates, such actions are
anticipated to be of minor significance in a national sulfate control
67
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strategy. However, Abatement Conferences could be of use in dealing
with the international long range transport of sulfates and acid rain
between Canada and the United States.
The Clean Air Act was amended by the Energy Supply and Environ-
mental Coordination Act of 1974 (ESECA). Under Section 119(d), a
federally ordered prohibition resulting in the conversion of a fuel
burning stationary source from gas or oil to coal may be suspended if
resultant emissions of a non-regulated pollutant "will result in an
increase which causes (or materially contributes to) a significant risk
to health. . ." This provision is clearly applicable to sulfates.
Implementation of this section of the Clean Air Act is currently being
reviewed and will involve limiting increases in SCv, emissions from
converting plants in areas where elevated sulfate levels pose a public
health concern.
2.2.6 Alternate Regulatory Options
Because implementation problems in applying existing Clean Air Act
options to sulfate regulation are anticipated in the next few years
considerations should be given to modifications to the current approaches.
The regulatory alternatives should attempt to allow efficient imple-
mentation of the most effective control strategies available without
unnecessary adverse social and economic consequences. Also, some means
for recognizing the impact of long range transport of sulfate is required.
Consideration of two potential alternative regulatory mechanisms is
presented in this section.
2.2.6.1 Regional Emission Control
The implementation problems outlined above might be resolved by
dividing the nation into sulfate control regions based on the severity
of the sulfate problem and emissions transport patterns and by proposing
regionally specific SCL emission standards for new and existing sources.
Limited control resources could be used first in the regions with the
most serious acid-sulfate problems to ensure greatest possible reduction
of sulfur oxide emissions where it is most needed.
68
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A regional sulfate control program incorporating the features
outlined above could be implemented by a statutory mandate of S0?
emission reductions similar to the approach taken for hydrocarbon automotive
reductions. For example, the 24-state region east of the Mississippi
with the highest nonurban sulfate levels (Figure 6) could be required
to install stringent continuous SO- emissions control by 1982. In
regions with less serious sulfate problems, control might be deferred 3
to 6 years later. The degree of control could reflect regional variations
in transport, formation, and emissions density considerations. Mandated
reductions might take the form of best available control technology
(BACT) considering costs for both new and existing sources. Ultimately,
as the effectiveness of control options (such as gasification and solar
energy) increased, SOV emissions in all regions would trend downwards as
X
older sources with poorer control were phased out. Assuming requirements
for health criteria are less stringent than for a NAAQS, a regional
emissions reduction program might begin in 3 to 4 years.
In the short term, regional control differences might encourage
expansion of sulfur oxide producing sources in areas where requirements
are less strict. Even though revised NSPS would restrict this growth,
regulations to prevent significant deterioration from sulfates might be
necessary.
2.2.6.2 Emissions Tax as Component of Sulfates Control Strategy
The concept of an emission tax has often been discussed as an
efficient means of controlling sulfur oxides, although it has usually
been dismissed as impractical. The unique characteristics of the
sulfate problem previously discussed make such a tax disincentive worthy
of serious consideration.
Generally speaking, an SO emissions tax might be useful in implementing
X
an overall sulfate control strategy such as regional emission control that
allows for a somewhat lengthy and flexible time for reduction of ambient
concentrations. A sulfur oxides emissions tax would be desirable in
this context because it would promote an orderly industry response to
the availability of SOV control technology. The tax would provide the
X
69
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economic incentive necessary for voluntary implementation of available
control technologies. In a practical vein, the tax would eliminate some
of the problems EPA has faced in convincing utilities to undertake
research on and installation of flue gas desulfurization equipment. With the
tax provision, tactics to delay compliance would no longer be cost free.
Probably the most cogent argument for use of an SO emissions tax
X
for sulfates control lies in its cost efficiency. An emissions tax, if
given sufficient time to work, should result in an aggregate level of
emissions reduction at uniform marginal control cost and, consequently,
minimum total cost. The cost involved in achieving levels of sulfate
control approaching a low "threshold" health effects standard makes the
attribute of economic efficiency very important. A further argument for
the efficiency of the tax derives from the regionality of the sulfate
problem. In the case of SCL abatement, the objective of specific control
strategies is to control localized ground level concentrations. This
localized feature of the SCL problem would tend to reduce the value of a
uniform regional SO emissions tax because the "regions" of control
A
should be quite small, with source parameters playing a major role in
tax determination. An optimum S02 control tax would be determined on a
source specific basis. Assuming long-range transport, however, the wide
area nature of the sulfates problem should reduce the "error" on a
uniform tax structure.
Problems with this approach include the possibility that utilities
would pass the tax on the costs to consumers without installing controls,
the increased costs of electrical production above that required for
control, the political palatability of an added tax, and the difficulty
in altering tax rates to reflect changing cost-benefit information.
To summarize, an emissions tax sulfate control strategy could provide
a relatively cost-efficient means to gradually reduce sulfate concentrations
through orderly development and introduction of SO controls and should
x 2
be investigated as a possible strategy. The National Academy of Sciences
also recommends that this approach be the subject of further study.
70
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3. POLICY IMPLICATIONS
In view of the available data, it is the judgment of EPA that an air
quality standard or other major regulatory program for sulfates is not
supportable at this time. In order to fill the information gaps described
earlier, EPA is expanding its sulfate research effort. The research will
focus on improving monitoring capability to permit identification of
particle size and chemical form of toxic sulfates, developing more com-
prehensive health effects data, and characterizing the long-range transport
and transformation mechanisms. The research program will require several
years to complete; consequently, it is doubtful that a comprehensive
regulatory program specifically for sulfates could be initiated before
the end of the decade. The recent reports by the National Academy of
2 1
Sciences and the Science Advisory Board support EPA's position that
considerable research and data development must precede such a regulatory
program for sulfates.
Nevertheless, until further research makes a comprehensive regulatory
program possible, EPA must respond to the potential sulfate problem
suggested by the preliminary sulfate/health effects information cited
earlier. Although considerable uncertainty exists concerning the
relationship between ambient sulfate concentrations and adverse health
effects, the preliminary health effects information can be useful in
identifying areas of potential health concern.
As described previously, a large portion of the northeastern
United States is experiencing annual sulfate concentrations that
are relatively higher than concentrations observed throughout the
remainder of the country. Figure 17 illustrates a 24-state region
in which nonurban concentrations have averaged in excess of 10 yg/m .
3
The average of urban concentrations has been about 14 yg/m (annual
average). The boundary of this 24-state region has been drawn on the
basis of observed sulfate patterns, S02 emission density, and likely
prevailing transport patterns. This area of high sulfate concentrations
also correlates spatially with high rainfall acidity patterns and a high
density of power plant locations. Furthermore, the region exhibits
widespread violation of the national primary ambient air quality standard
for suspended particulate matter, a potential precursor agent in the
71
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CD
OJ
•l-i
CO
O)
ic
c
01
4-»
CO
4-*
;/)
<3-
CNI
r<
CD
i_
•3
D)
u_
72
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formation of sulfates. The relatively high levels of nonurban sulfate
and rainfall acidity in this region suggest the existence of a general
sulfates problem. In addition, these high nonurban concentrations are
potentially indicative of the background sulfate levels entering urban
areas. Recognizing that these more populous urban areas add
locally formed sulfate to this general background gives further support
to the concern for the health impact of sulfates in this 24 state region.
High sulfate concentrations have also been observed in several isolated
urban areas in the remainder of the country.
Given the significant potential for sulfate related health risk due
to the multiple influence of high sulfate concentrations, high precursor
concentrations, and high SCL emission density, prudence dictates that
EPA adopt a policy of avoiding aggravation of existing conditions by
minimizing increases in the relatively high sulfate levels in the northeastern
United States and other more local ized problem areas. In addition,
close attention must oe paid to sulfate trends in areas of lower sulfate
concentration. Although the goal of avoiding sulfate increases will
primarily be achieved b> nn'nimizing S0? emission increases, existing programs
for control of pollutants such as oxidants and particulates may provide
seme measure of sulfate control by limiting formation processes.
Opinions differ over where, and to what extent, S0? emissions
should be limited to adequately address the potential sulfdte problem.
There is general agreement that SO- emissions increases should De
avoided in or near urban areas where ambient concentrations of sulfates,
p
SO-, or total suspended particulates are high. The NAS" places high
priority on abating S0? emissions from sources located in or near urban
areas with high concentrations of sulfur dioxide and sulfates- In
2
addition, the NAS is concerned about the effects of area-wide S09
60
emissions increases on regional sulfate levels ana in a separate report
cautions "further increases in the ambient concentration of these pollutants
through the relaxation of standards of emissions and air quality would
pose an unacceptable risk for the population." The SAB, however, does
not share this same concern for the impact of area-wide SO- emissions.
Rather, the SAB suggests that increased SCL and other sulfate precursor
emissions may have primarily a local impact on sulfate formation, and
"that increases in exposure to sulfur oxides or particulates in localities
73
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where sulfur dioxide and/or total suspended particulates exceed primary
standards would be viewed with grave concern."
EPA considers the points addressed in both reports as important and
essentially compatible with the Agency's assessments. Current efforts
to attain the primary standard for the criteria pollutants are directly
responsive to the SAB's primary concerns. In addition, EPA believes
that the area-wide concern must be addressed by minimizing increases in
S02 and other sulfate precursor emissions in the areas of maximum sulfate
impact. A strategy of minimizing regional and local increases in S02
emissions can be implemented through existing regulatory options such as
State Implementation Plans (SIPs) and New Source Performance Standards
(NSPS).
A policy of minimizing S0? emission increases is generally consistent
with other Agency policies previously announced. These policies include
the Clean Fuels Policy, the application of intermittent control systems
(ICS), the significant risk aspect of oil-to-coal conversions, and the
recent deferral of more restrictive hydrocarbon/carbon monoxide vehicle
emission standards.
The EPA Clean Fuels Policy is intended to make it unnecessary for
plants to switch to lower sulfur fuels to comply with state regulations
where such compliance is not needed for attainment and maintenance of
the national health-related standards for S02. By revising SIPs appropriately,
plants currently in areas meeting primary air quality standards could
continue to burn currently available fuels; no switch to higher sulfur
fuel is intended. Therefore, sulfur emissions from these sources should
not increase.
With respect to EPA policy on intermittent controls systems (ICS),
a limited number of isolated power plants may be permitted to use intermittent
emission control to meet air quality standards, temporarily deferring
installation of costly continuous emission controls. Eligible iplants
are already burning coal and, under ICS, will continue to burn existing
fuel except during adverse meteorological conditions when they will
reduce emissions by switching to a lower sulfur fuel or shifting generation
load. Again, total sulfur emissions from these sources should not
increase and may actually be slightly reduced.
The Energy Supply and Environment"! Ce oral nation Ac" of 19)4 (ESECA)
provides the Fedora1 Energy Ad^inistraii:;n with th£ authority t:> .r-'ohihit
-------
a power plant from burning oil or natural gas subject to certification by
EPA of the power plant's ability to burn coal in compliance with certain
environmental requirements. Two requirements relate to a plant's ability
to burn coal without contributing to a violation of primary standards
for total suspended particulate or sulfur dioxide. In this regard, if
certain legal criteria are satisfied (regional limitation), EPA may
specify alternate emission requirements (primary standard conditions) to
be met temporarily by converting plants. An additional requirement of
ESECA states that conversions cannot result in an increase in the emission
of unregulated pollutants or pollutant precursors at levels that may
result in a significant risk to public health. Based on currently
available health effects information, EPA has decided to apply this
"significant risk" provision only with respect to sulfates. Under the
significant risk provision, EPA plans to restrict emissions of sulfate
precursor pollutants—sulfur dioxide or particulate matter--if a con-
verting plant is located in an area with high sulfate concentrations and
with concentrations of sulfur dioxide or particulate in excess of primary
standards. This policy recognizes the concerns of both the NAS and the
SAB and is believed to provide adequate protection against increased health
risk due to sulfates by significantly limiting potential S0? emission
increases.
EPA has analyzed the effect of existing regulations, such as SIPs
and NSPS, and the policies described above on sulfur dioxide emissions
in the 24-state region. Table 12 summarizes the assumptions made in the
analysis. Emission levels have been evaluated for 1972, the most recent
year for which area sulfate information is available, and 1980, the time
at which a sulfate regulatory control program might begin. Table 13
lists the regional emissions for 1972, broken down into utility and non-
utility sources, and illustrates the effect of each regulatory option on
emissions in 1980. Each subsequent option listed assumes that each of the
previous options was implemented for the relevant sources.
During the period 1975 to 1980, it is assumed that both utility and
non-utility sources will move to compliance with SIP regulations. Because
most non-utility point and area sources are located in urban areas,
these sources are assumed to comply with SIPs by continuous emission
75
-------
Table 12. SUMMARY OF ASSUMPTIONS USED FOR SOX EMISSIONS
PROJECTIONS OF TABLE 13
1972 Emissions from NEDS25 and SASD files
Growth:
Utilities**' - Federal Power Commission Data
Non-utilities - Based on state-specific OBERS68 population
and economic activity projections
Effect of SIPs on 1975 non-utility emissions determined using
state-specific fuel use and SIP requirements
Effect of SIPs on utility emissions determined using 1980 plant-
specific SIPs. Future revisions in SIP limit were assumed for
some plants in Illinois, Indiana, Kentucky, and Ohio."7
NSPS in effect on utility growth after 1976
ICS Case:
Certain isolated plants continue to burn existing coal meeting
.^2 -s-t«mfards by use of ICS if considered enforceable according
to a range of criteria [29 plants (25,500 MW) to 63 plants
(54,500 MW) qualify under the assumed criteria]
All other sources meet SIPs
• Oil to coal conversions:
standard conditions
48 plants convert and meet primary
Table 13. 24-STATE REGION SOX EMISSIONS
(106 tons/yr)
1972 Emissions
1980 Emissions
Regulatory options:
Enforcement of SIPs, NSPS
ICS (29 to 63 plants)
Conversions3 (48 plants)
Utilities
14.3
10.9
12.5 - 14.9
12.5 - 14.9
Non-
utilities
8.1
7.7
7.7
7.7
Total
22.4
18.6
20.2 - 22.6
20.2 - 22.6
The conversion program requires all candidates to meet SIP before
1980. Thus the maximum impact of the program (0.7 million tons S02/
yr) is assumed to last from 1976 to 1979 and is not apparent in 1980.
76
-------
control methods and thus are not affected by the other policies. Even
when accounting for source growth, SCL emissions from these sources are
reduced by 0.4 million tons/yr in 1980 through compliance with SIPs.
Similarly, if all utilities were to meet applicable SIPs and NSPS,
substantial emissions reductions (about 5.9 million tons/yr) would result.
However, because control options in the near term are limited and costly,
the Clean Fuels Policy has been developed to allow existing power plants
to continue the use of currently available coal supplies while still
meeting primary standards. If affected states revise their SIP regulations
to conform with this policy, emissions would still be reduced by 3.4 million
tons/yr in 1980. If additional SIP revisions were to occur in this region,
part or all of the scheduled reductions in power plant emissions could
be eliminated. SIP revisions do not affect the requirement that new
power plants meet NSPS, which will significantly limit emissions growth
past 1975-1976.
With regard to the analysis of the intermittent control system
option, the majority of existing fossil-fuel-fired power plants in the
24-state region (about 360 plants representing 160,000 MW in 1975) are
located in urban or other areas in which continuous emission control is
required to meet SOp standards and SIPs. However, a small number of
coal-fired power plants are in isolated locations and could defer the
use of limited and costly continuous emission controls by using intermittent
emission control systems (ICS) to meet S0? ambient standards. Assuming
that such plants would continue to burn current fuels except during
periods of poor atmospheric dispersion, SO emissions should not increase
X
from these plants. Using a preliminary screening procedure, 29 plants
(25,500 MW) to 63 (54,500 MW) in the 24-state region have been identified
as possible candidates for the use of ICS. The range of emissions result-
ing if all of the possible candidates used ICS is presented in Table 13.
Under these assumptions, total emissions in 1980 would still remain
at or below 1972 levels.
Concerning the analysis of the potential SO,, emission increase due
to the ESECA program for conversion of power plants from oil or gas to
coal operation, 49 plants, representing 18,300 MW of generating
capacity, are assumed to convert by 1980 in the 24-state region. Because
technical and economic considerations may limit the number of conversions
77
-------
30
25
20
to
o
E 15
in
t/3
x
O
0
1970
1975
1980
1985
YEAR
Figure 18. Projected SOX emissions from 24-state region, 1970-1985.
79
-------
increases in atmospheric sulfate levels. Present EPA policies that
affect sulfur dioxide emissions are consistent with a sulfate-related
policy of minimizing sulfur dioxide emissions increases in high sulfate
regions. The "significant risk" policy for converting power plants, the
vigorous enforcement of state implementation plans for the control of
sulfur dioxide and particulate, and the increasing application of new
source performance standards to power generating facilities are the
vital components of a broad strategy that should limit growth of ambient
sulfate levels. As the previous EPA analysis indicates, these regulations
and policies should prevent major SO^ emission increases through 1980 in
regions of maximum sulfate impact. Until information is available to
support the enforcement of a more rigorous sulfate regulatory program,
the use of these currently applicable regulatory measures should provide
reasonable protection against increased health risk from sulfates.
80
-------
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86
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TECHNICAL REPORT DATA
l/'kasi read luitnicnuns on tnc rc\cn^ bt.ion i
1 REPORT NO
EPA-450/2-75-007
|3 RECIPIENT'S ACCESSiO^NO
! TITLE AND SUBTITLE
Position Paper on Regulation of Atmospheric Sulfates
6. PERFORMING ORGANIZATION CODE
7 AUTHORlS)
9 PERFORMING ORGANIZATION NAME AND ADDRESS
U. S. Environmental Protection Agency
Office of Air and Waste Management
Office of Air Quality Planning and Standards
Research Triangle Park, North Carolina 27711
5 REPORT DAT[-
September 1975
8 PERFORMING ORGANIZATION REPORT NO.
10 PROGRAM ELEMENT NO
11 CONTRACT GRANT NO
12 SPONSORING AGENCY NAME AND ADDRESS
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
15 SUPPLEMENTARY NOTES
16. ABSTRACT
Atmospheric sulfates as measured include a variety of chemical entities. Toxi-
cological evidence indicates that certain sulfates, particularly fine particulate
acid sulfates, are more potent respiratory irritants than sulfur dioxide alone.
Preliminary epidemiological studies suggest that measured sulfates are associated
with a variety of health indicators. Sulfates may also be related to damage to the
environment by direct deposition or by formation of acid rain and can cause visibili-
ty deterioration. Although natural sulfur emissions are important on a global scale,
sulfates in industrialized regions are largely produced by atmospheric reactions of
manmade sulfur oxides emissions. Sulfates may be transported long distances from
source areas and result in high ambient levels over broad regions. This is appar-
ently the case in a 24 state region in the eastern U. S. Considerations of chemistry
and transport suggest that reductions in regional S02 emissions would produce reduc-
tion in sulfates, although the reductions would be less than one to one.
Considering the uncertainties in scientific information, it will take 3 to 5
years of additional research before a major sulfate regulatory program is possible.
Until then, present concerns dictate that EPA follow a policy of avoiding increases
in S02 emissions and consequent sulfate increases in the 24 state region and other
areas subject to high sulfate levels. This policy can be implemented through the
existing authority of the Clean Air Act.
17. KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b. IDENTIFIERS/OPEN ENDEDTERMS
Sulfates
Transport
Acid Rain
Sulfuric Acid
Air Pollution
Air Pollution Control
Stationary Sources
COSATI Held/Group
13 DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS /This Report/
Unclassified
21 NO OF PAGES
108
20 SECURITY CLASS /Tins page/
Unclassified
22 PRICE
EPA Form 2220-1 (9-73)
87
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