BRIEFING NOTES -
A STATUS REPORT ON SULFUR OXIDES
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
National Environmental Research Center
Research Triangle Park, North Carolina 27711
April 1974
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600S74001
BRIEFING NOTES-
A STATUS REPORT ON SULFUR OXIDES
J.F. Finklea, D.B. Turner, G.G. Akland, R.I. Larsen,
V. Hasselblad, and S.D. Shearer
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
National Environmental Research Center
Research Triangle Park, North Carolina 27711
April 1974
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SUMMARY
Fuel oils and coals contain varying amounts of organic and inorganic
sulfur that is oxidized to sulfur dioxide or sulfur trioxide during
combustion. In the atmosphere these oxides are transformed into acid-
sulfate aerosols including sulfuric acid and sulfate salts that may be
transported for long distances. Acid-sulfate aerosols cause adverse
effects on human health, vegetation, materials and visibility. Acid
aerosols are especially vexing respiratory irritants because their
small particle size allows deep penetration into the lung. Sulfates
of natural origin, from soil or sea spray, are probably larger, less
irritating particles.
Adverse effects on public health include aggravation of asthma,
increases in deaths and aggravations of illness among the elderly and
infirm. Healthy individuals can also be adversely affected as they
will experience substantial increases in the frequency of chronic
bronchitis symptoms and the occurrence of acute lower respiratory
infections.
Recent reductions of sulfur dioxide emissions in major cities
have not controlled urban levels of acid-sulfate aerosols. Local
urban sources of acid-sulfate aerosols may be well controlled but
imported acid-sulfate aerosols arising from increasing emissions of
sulfur oxides from distant power plants that burn high-sulfur fuels
are now intruding into northeastern urban areas.
Massive conversion of urban power plants to high sulfur fuels or
unrestricted use of tall stacks and supplementary control systems can
greatly aggravate the acid-sulfate aerosol problem. Flue gas de-
sulfurization, cleaner fuels or modified combustion processes will be
required to control the acid-sulfate aerosol problem. Expected growth
in coal-fired steam electric power generation must be accompanied by
increased availability of cleaner fuels or widespread use of effluent
controls to avoid further aggravation of the acid-sulfate aerosol
problem. Likewise, equipping the majority of light-duty motor vehicles
with oxidation catalysts would measurably increase human exposure to
acid-sulfate aerosols. Control of these mobile source emissions may
prove mandatory if public health is to be protected.
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TWELVE QUESTIONS ABOUT SULFUR OXIDES
WHY CONTROL SULFUR OXIDES?
HOW ARE EMISSIONS OF SULFUR OXIDES CHANGING?
HOW HAVE SULFUR DIOXIDE AND ACID-SULFATE AEROSOL LEVELS CHANGED?
ARE ACID-SULFATE AEROSOLS TRANSPORTED FROM ONE AIR QUALITY CONTROL REGION
TO ANOTHER?
WHAT ARE THE RELATIONSHIPS BETWEEN AMBIENT LEVELS OF ACID SULFATE AEROSOLS
AND SULFUR DIOXIDE?
WILL CONVERSION OF URBAN POWER PLANTS TO HIGH SULFUR FUELS CHANGE ACID-SULFATE
AEROSOL LEVELS?
WILL GROWTH IN ELECTRICAL POWER DEMANDS CHANGE ACID SULFATE AEROSOL LEVELS?
WHAT IS THE PUBLIC HEALTH SIGNIFICANCE OF*PROJECTED CHANGES IN ACID-SULFATE
AEROSOL LEVELS?
HOW MUCH MUST SULFUR OXIDES EMISSIONS BE CONTROLLED?
WHAT KINDS OF CONTROL TECHNOLOGY ARE BECOMING AVAILABLE?
HOW WILL THE INTRODUCTION OF VEHICLES EQUIPPED WITH OXIDATION CATALYSTS
ALTER EXPOSURE AND CONTROL TECHNOLOGY REQUIREMENTS?
WHAT IMPORTANT CAVEATS MUST BE KEPT IN MIND?
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WHY CONTROL SULFUR OXIDES?
Industrialized nations will utilize increasing amounts of
fossil fuels for at least another decade.
Without controls inorganic and organic sulfur in fossil fuels
will be emitted as sulfur dioxide or sulfur trioxide.
* Sulfur dioxide is transformed in the atmosphere into acid-sulfate
aerosols (acids and sulfate salts).
Acid-sulfate aerosols are fine particulates which have a relatively
long atmospheric residence and which are capable of penetrating
deeply into the respiratory tract.
Acid-sulfate aerosols adversely affect human health, vegetation,
materials, and visibility.
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3.0 r
(Number
.001
.01 .1 1 10
PARTICLE DIAMETER, MICRONS
[" COAGULATION"
V.L
^SEDIMENTATION
Typical particulate distribution
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HOW ARE EMISSIONS OF SULFUR OXIDES CHANGING?
HOW HAVE SULFUR DIOXIDE AND ACID-SULFATE AEROSOL LEVELS CHANGED?
" Between 1960 and 1970 emissions in major cities decreased by
roughly 50%.
* Ambient levels of sulfur dioxide in major cities decreased proportionately.
Suspended sulfates, a proxy for acid-sulfate aerosols, first
decreased a little and then leveled off.
Reductions in urban emissions and ambient levels of sulfur dioxide
were primarily achieved by switching industrial sources and
steam electric power plants to low-sulfur fuels.
* Reduction in sulfur dioxide emissions from strike-bound smelters
outside western cities reduced acid-sulfate aerosol levels in
cities and improved visibility.
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35
30
TOTAL (EPA)
POWER PLANTS (EPA)
A NFS
tn
g
10
1940
Annual sulfur oxides emissions.
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MAXIMUM 24-HOUR CONCENTRATION
QUARTERLY LINE
TREND LINE
24-HOUR PRIMARY STANDARD (365)
'
\/ V"XN
ANNUAL PRIMARY STANDARDS (80)
n , , ,-
5/1/68 COAL = 1'. S
RESIDUAL OIL = lr. 5
LIGHT OIL = 0.7',S
10/1/70 RESIDUAL OIL = fl.5'ปS
LIGHT OIL = 0.4% S
10/1/71 ALLOIL=0*.3'. S
COAL=0.2%S
JPRE-1958 FUEL = 2.5% S|
Comparison ol SOj trends al Bayonne, New Jersey, with regulations governing
percent sulfur content in fuel.
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Nationwide emissions of sulfur oxides increased by 45% because of
increased emissions from steam electric power plants. Light-duty
vehicles contribute less than one percent of national emissions.
Locating steam electric power plants burning high-sulfur fuels
outside major cities allows sulfur oxides to change into acid-
sulfate aerosols before air masses reach major metropolitan areas.
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ARE ACID-SULFATE AEROSOLS TRANSPORTED
FROM ONE AIR QUALITY CONTROL REGION
TO ANOTHER?
Long-distance transport of aerosols is scientifically
plausible, consistent with geographical emissions patterns and
supported by complimentary types of evidence.
' Atmospheric turbidity is greatest over the eastern United States
but increases in turbidity are not limited to the immediate
proximity of cities or sources. Turbidity is largely due to
aerosols.
Acid rainfall shows both localized "hot spots" that correspond to
increased emissions and generalized patterns that indicate long-
distance transport.
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D < 1 ton/kn>2
GJ 1-20 tons/km2
20tons/km2
Nationwide geographic variation in 862 emission density.
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ซar ^.1__ r___J&O
I OMLJMOMA" ^/l
POWER GENERATION SIZE, NIW
O 0-2,000
O 2,001-4,000
4,001-8,000
1- 15,000
SHADED AREA INDICATES TVA POWER SYSTEM
Location of major coal-and oil-fired power units, 1971.
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,
i"<>ซTM7>s;5"ซ"""
*ฐ<--.
7*J--Tllrs/
O /---.
/ :<=<>r<ฃ;i3
SULFURIC ACID PLANT SIZE, T/day
O 0-2,500
2,501-5,000
5,001-8,000
o
8,001-21,500
Sulfuric acid manufacturing capacity, 1970.
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SYMBOL
mil
:.:...:...
Hi
CONCENTRATION
RANGE, ppm $04
0.03 TO 0.865
0.865 TO 1.61
1.61 TO 3.14
3.14 TO 4.67
*
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14
Recent University studies show acid-sulfate aerosols may impact
a large area of the Mississippi Valley in summer.
Urban and non-urban National Air Sampling Network stations show
elevated annual average sulfate levels in the area bounded by
Chicago, Boston, Washington and Cincinnati.
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Non-urban average sulfate concentration. ug/m3.
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URBAN SITE
NONURBAN SITE
7.0 - 13.0 pg/m3
CTl
1970 sulfate concentrations, urban (jjg/rri3).
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17
WHAT ARE THE RELATIONSHIPS BETWEEN AMBIENT
LEVELS OF ACID - SULFATE AEROSOLS AND
SULFUR DIOXIDE?
' Before 1967 sulfur dioxide levels and suspended sulfate levels
at NASN stations were fairly well correlated.
Correlations were best (.5) in western cities without photochemical
problems and in midwestern cities from Chicago south to St. Louis.
* Correlations were somewhat lower (.4) in northeastern cities,
probably due to differences in fuels and imported sulfates from
sources outside the city.
* After 1967 poor correlations were found in cities where sulfur dioxide
emissions were controlled and where acid-sulfate aerosols intruded.
* Correlations remained good in western cities where imported sulfates
are not as important.
* The poorest correlations were found in cities with major photochemical
smog problems.
Correlations between total suspended particulates and suspended
sulfates were good throughout the period of study (1962 - 1970).
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18
WILL CONVERSION OF URBAN POWER
PLANTS TO HIGH-SULFUR FUELS CHANGE
ACID-SULFATE AEROSOL LEVELS?
The Office of Air Quality Planning and Standards of EPA evaluates
power plant conversions to coal on a case by case basis.
Limited variances thus far granted will increase 24-hour levels of
sulfur dioxide by 50 to 200 jjg/irr in the most impacted areas.
Rates for transformation of sulfur dioxide into sulfates in the
atmosphere can vary widely (1-20% per hour).
Assuming a 5% per hour sulfur dioxide to sulfate transformation
rate, a 24-hour increment in sulfates of 9 /ug/nr was estimated
for an arc of communities downwind-of New York-New Jersey power
plants scheduled for conversion to coal. Worrisome sulfate levels
already exist in these communities.
Assuming that conversion to sulfate would take place in the
one kilometer high box covering the metropolitan area, an increase of
up to 15% in sulfates could follow variances already granted. Massive
conversion to coal in urban areas could raise sulfate levels by 15
to 70% depending on how much sulfur dioxide is transformed into
sulfate.
Control of sulfur emissions from power plants and other urban
sources is necessary to control acid-sulfate aerosol exposures.
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19
WILL GROHTH IN ELECTRICAL POWER DEMANDS
CHANGE ACID-SULFATE AEROSOL LEVELS?
Location of new or expanded uncontrolled steam electric power
plants inside major metropolitan areas is most undesirable and
not very likely.
Both urban and non-urban acid-sulfate aerosol levels in ambient air will
be increased at the rate of one to six percent per year if power
plants remain uncontrolled or if they rely upon tall stacks during
periods of good dispersion and fuel switching during periods of
poor dispersion.
Other troublesome alternatives are less reliance on coal and more
reliance on imported low-sulfur fuels and accelerated installation
of nuclear generating capacity.
A more rational course for the next decade is to utilize coal cleaning
and flue gas desulfurization so that the sulfur oxides problem
will not worsen.
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20
WHAT IS THE PUBLIC HEALTH SIGNIFICANCE OF
PROJECTED CHANGES IN ACID-SULFATE
AEROSOL LEVELS, EXCLUSIVE OF EMISSIONS
FROM VEHICLES EQUIPPED WITH OXIDATION CATALYSTS?
Changes in sulfate concentration at New York NASN Stations and at
the New York Residential Monitoring Stations of the EPA CHESS Program
were predicted. Two kinds of increase of exposure will occur.
First, exposures will be increased as a result of conversion
of sulfur dioxide to acid-sulfate aerosols in power plant plumes. In
residential areas sulfate levels might increase from 20 to 29
micrograms per cubic meter for 24 hours.
Secondly, sulfate levels over the larger metropolitan regions would
increase as a result of conversion of urban power plants to coal and
growth in sulfur oxide emissions from more distant steam electric
power plants. The magnitude of this increase is more difficult
to ascertain but upper and lower boundary estimates can be made.
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21
* An upper boundary estimate for sulfate Increases that might
reasonably follow Increases In sulfur dioxide emissions can be calculated
by assuming the following:
First, urban sulfur dioxide emissions will increase 15% with
limited conversion to coal and 50 to 75% with massive conversion to coal.
Second, local emissions are relatively more effective contributors
to urban sulfates than distant emissions sources on a ton for ton emissions
basis. Limited existing data suggest local sources' are twice as effective
as distant sources.
' Third, urban sulfates will Increase at the rate of 2 1/2% per year
from more distant sources through 1980.
* Under the upper boundary estimate, limited conversion to coal would
Increase annual averages of sulfate levels by 2 to 3 jug/m3 while full
conversion would cause increases of 7 to 16 jug/m3. To this would be
added annual increments of 0.5 jjg/nr from distant sources.
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22
A lower boundary estimate assumes that at least half of urban
sulfate (8-12 iig/m3) is imported, that conversion of power plants
to coal would only affect sulfates of local origin and that this increment
will correspond to 5 percent of the predicted increase in ambient sulfur
dioxide over a wide area. Sulfur dioxide increases range from 15
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23
* Upper and lower boundary increments were used to calculate expected
frequency distributions for ambient sulfate levels at central city and
residential stations.
Prediction equations were used to recapitulate sulfate exposures in
older health studies from sulfur dioxide and total suspended
particulate exposures. Dose response relationships for long-term and
short-term sulfate exposures were estimated but these should be viewed as
best judgment not as precise statistical statements.
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Estimated Impact of Coal Conversion in the New York - New Jersey Metropolitan Area*
Coal
Conversion
Limited
Full
Station
Central City
(NY-NASN)
Residential
(NJ)
Cental City
(NY-NASN)
Residential
(NJ)
Estimated
Base (1970)
Lower Boundary
Upper Boundary
Base (1971)
Lower Bounday
Upper Boundary
Base (1970)
Lower Boundary
Upper Boundary
Base (1 971)
Lower Boundary
Upper Boundary
S
Frequency Dist
10
12
12
14
6
6
7
12
13
20
6
7
10
20
12
12
14
7
7
8
12
14
22
7
8
12
30
16
16
18
8
9
10
16
18
28
8
9
15
ulfate Concentration_^ujj
ributfon
40
16
16
19
10
10
11
16
19
29
10
11
17
50
20
20
23
11
11
13
20
23
35
11
12
20
60
21
22
24
13
13
14
21
24
36
13
14
22
70
24
25
28
15
15
17
24
29
43
15
17
25
/m3)
80
31
32
25
18
18
21
31
36
53
18
20
31
90
37
39
33
23
26
30
37
45
65
23
25
39
Max-'
imum
48
52
55
56
58
65
48
66
83
56
62
99
Arith-
metic
Mean ซ
22 '
23
26
13
13
15
22 g
26,
39
13
15
23 -.
*No provision mdde for increases in sulfate due to growth in emissions outside the urban area.
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25
RECAPITULATING 24-HOUR LEVELS OF SUSPENDED SULFATES FROM MEASURED LEVELS
OF SULFUR DIOXIDE
Y = 9 + .03x 1959 - 1960 Nashville
sulfate in sulfur dioxide Study (A= .8)
jjg/m3 in
Y = 9 + .05x 1966 - 1967 NASN data
sulfate in sulfur dioxide from 8 inland cities
jjg/m3 . in jug/m3 (n= .5)
RECAPITULATING ANNUAL AVERAGE SUSPENDED SULFATE LEVELS FROM MEASURED LEVELS
OF SULFUR DIOXIDE
Y = 9 + .04x Pooled NASN data from
sulfate in sulfur dioxide NY City, Chicago and New
in ug/m3 Jersey - 1962 - 1967
(ซ= -6)
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Threshold and Illustrative Health Risks
for Suspended Sulfates and Sulfur Dioxide
Adverse Health
Effect
Threshold Concentra-
tion and Exposure
Duration
Illustrative Health Risk
Definition
Sulfate Level
Sulfur Dioxide
Equivalent
Increase in Daily
Mortality
(4 studies)
Aggravation of Heart
and Lung Disease in
the Elderly
(2 studies)
Aggravation of
Asthma
(4 studies)
Excess Acute Lower
Respiratory Disease
in Children
(4 studies)
Excess Risk for
Chronic Bronchitis
(6 studies)
25yg/m for 24 hours
or longer
9yg/m for 24 hours
or longer
6 to lOyg/m for
24 hours
13yg/m for several
years
10 to 15yg/m for
up to 10 years
2 1/2% increase in
daily mortality
50% increase in
symptom aggravation
75% increase in
frequency of asthma
attacks
50% increase in
frequency
50% increase in
risk
38yg/nr for 24 hours
48yg/m for 24 hours
30p'g/m for 24 hours
20yg/m annual
average
15 to 20yg/m annual
average
600yg/m for 24 hours
750yg/m for 24 hours
450yg/m for 24 hours
ro
en
100 to 250yg/nT
annual average
100 to 250yg/rrT
annual average
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27
* Current urban sulfate exposures are high enough to aggravate existing
respiratory illnesses and to increase mortality on the worst few days
each year.
With limited coal conversion up to 50 days each year would
exceed the mortality threshold. With full conversion up to 150
days would exceed this level.
In residential areas the expected increases in annual average
sulfate exposures over a 10-year period .could cause chronic respiratory
disease to increase 150 percent in non-smokers and as much
as 50 percent in smokers. Lower respiratory disease in children could
increase by 70 percent. Again, these estimates are not precise.
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28
HOW MUCH MUST SULFUR OXIDES EMISSIONS BE CONTROLLED?
Estimates of the degree of control necessary can be made by using
observed natural background levels of sulfates as a base and assuming
that reductions in national sulfur dioxide emissions will be accompanied
by proportional reductions in sulfate emissions.
Fifty percent control of all man-made emissions might protect healthy
individuals while over 90 percent control may prove necessary to protect
the most susceptible individuals.
Current utility plans for flue gas desulfurization would reduce
emissions by roughly ten percent by 1977, but growth in uncontrolled sources
can easily offset this modest gain.
* Since acid-sulfate aerosols, not sulfates of soil and marine origin
are probably of greatest interest, calculations based on "sulfate" measurements
may somewhat overestimate the degree of control required.
* Because some sulfur dioxide to acid-sulfate aerosol transformation
mechanisms are not dependent on sulfur dioxide concentration, the necessary
control may be underestimated.
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29
1000
100
1
3.
O
^-
cc
o
o
ซ*
LJ_
_i
=j
LU
3
=3
o .
cc
10
0.1
1960
10% INCREASE FROM
SELECTIVE SWITCHING
FROM GAS AND OIL
TO COAL
25% DECREASE FROM MORE .EFFICIENT
COAL CLEANING IMPL|MENTED OVER_
A10-Yr PERIOD
15 ug/nT HEALTH OF
HEALTHY INDIVIDUALS
ADVERSELY AFFECTEC
Jil^^LC
^^n?^)^!/!!!^
1968
1 pg/m3, BACKGROUND CONCENTRATION (MEASURED AT NONURBAN
SITES IN IDAHO AND S. DAKOTA).
PARTICULATE SULFATE ARITHMETIC MEAN CONCENTRATION EXPECTED
AT 5TH HIGHEST PERCENTILE NASN URBAN SITE FOR A 3%/yr GROWTH
RATE, WITH COAL CLEANING IMPLEMENTED OVER A 10-yr PERIOD AND
WITH 95% AND 99% CONTROL IMPLEMENTED OVER A 30-yr PERIOD.
1970
1980
1990
2000
2010
2020
YEAR
Future impact of sulfate control strategy based on high-level NASN
24-hour annual maximum concentrations.
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30
1000
100
ro
i,
3.
o
10
10% INCREASE FROM
SELECTIVE SWITCHING
FROM GAS AND OIL TO COAL
25% DECREASE FROM MORE EFFICIENT
COALCLEANING IMPLEMENTED OVER A
V 10ug/m3, ASTHMA AGGRAVATED AND
SYMPTOMS IN ELDERLY AGGRAVATED
2005
4ug/m3, BACKGROUND CONCENTRATION (MEASURED AT NONURBAN
NASN SITES IN IDAHO AND S. DAKOTA).
PARTICULATE SULFATE ANNUAL MAXIMUM CONCENTRATION EXPECTED
AT 5TH HIGHEST PERCENTILE NASN URBAN SITE FOR A 3%/yr GROWTH
RATE WITH COAL CLEANING IMPLEMENTED OVER A 10-yr PERIOD AND
WITH 95% AND 99% CONTROL IMPLEMENTED OVER A 30-yr PERIOD.
0.1
1960
1970
198"
1990
YEAR
2000
2010
2020
Future impact of sulfate control strategy based on high-level NASN
annual average concentrations.
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31
WHAT KINDS OF CONTROL TECHNOLOGY ARE
BECOMING AVAILABLE?
There are three practicable current approaches - control sulfur
content of fuel, remove sulfur oxides from effluent gases, and control
the emission of hydrocarbon precursors of photochemical oxidants which
may facilitate sulfur dioxide to acid-sulfate aerosol conversion in the
atmosphere. All of these courses are being pursued.
Flue gas desulfurization is being further improved.
First, some of the current gas cleaning demonstration could be modified
to provide 90-95% efficient flue gas cleaning sulfur removal for electric
utilities.
Second, new control processes may provide better than 90-95% efficient
flue gas sulfur removal for electric utilities.
Third, flue gas cleaning for industrial combustion and major industrial
process emitters can be developed.
Fourth, sulfate emissions from flue aas desulfurization systems are
thought intrinsically less irritating than acid-sulfate aerosols.
Furthermore, ground level concentrations of these sulfates are likely
to be much less than those observed from uncontrolled plumes.
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32
* Clean fuels are being developed and combustion processes modified to
produce less pollution.
First, more efficient fuel desulfurizatlon techniques are being
sought.
Second, coal conversion systems such as gasification and liquefaction
are being developed.
Third, techniques to utilize lower sulfur fuels such as western coals
are being developed.
Fourth, alternate nonpolluting energy systems such as fuel cells, solar
energy, geothermal systems and advanced combustion systems are being evaluated.
Fifth, strategies to conserve clean fuels and energy are being structured.
Sixth,, techniques for removing pollutants which may catalyze or
participate in secondary sulfate reactions are being developed.
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33
HOW HILL THE INTRODUCTION OF VEHICLES
EQUIPPED WITH OXIDATION CATALYSTS ALTER EXPOSURE
AND CONTROL TECHNOLOGY REQUIREMENTS?"
ฎ Our present cars burn gasoline containing sulfur
and emit sulfur dioxide.
This sulfur dioxide is diluted, dispersed, and
changed rather slowly into ambient sulfuric acid
and sulfates which are much more potent irritants
than sulfur dioxide.
Cars equipped with oxidation catalysts will emit
sulfuric acid or sulfates directly, thus increasing
exposure to these pollutants along busy expressways,
in urban street canyons, and around complex sources
like shopping centers and large airports.
Incremental acid-sulfate aerosol exposures from
catalyst-equipped cars will be added to the worrisome
levels of sulfate already present in most of our
cities.
The magnitude of acid-sulfate aerosol exposures
attributable to catalyst usage can be estimated and
compared to the previous upper and lower boundary estimates
of incremental acid-sulfate aerosol exposures attributable
to conversion of power plants to coal.
Use of catalyst-equipped cars for two model years in
a major metropolitan area such as New York will increase
acid-sulfate aerosol exposures by one-half to twice as
much as limited conversion of power plants to coal or by
oner-sixth to one-half as much as massive conversion of
power plants to coal.
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34
0 Together, these increments could cause up to 90
additional days to exceed the mortality threshold
(25 jjg/m^) with limited conversion to coal. Full
conversion to coal could cause up to 150 additional
days to exceed the mortality threshold.
Annual average sulfate exposures in suburban areas
would also be pushed over the threshold for adverse
health effects and adverse effects in the central
city attributed to long-term exposures would be
exacerbated.
Use of catalyst-equipped vehicles for four model
years in the New York metropolitan area will increase
acid-sulfate aerosol exposures by nine-tenths to
twice as much as limited conversion to coal or by
one-third to one and one-half times as much as
massive conversion of power plants to coal.
Together, these increments could cause up to 145 days
to exceed the mortality threshold (25 yg/m3) with limi-
ted conversion to coal. Massive conversion would cause
up to 70 additional days to exceed this level.
Use of motor vehicles equipped with oxidation catalysts
for more than four model years would further exacerbate
the problem.
Acid-sulfate aerosol exposures associated with use
of catalyst-equipped vehicles will be substantially
less if fuel economy of new cars is improved above
the projected 10 mpg average base, if the sulfur
content of gasoline is reduced, or if 100% of the new
vehicles are not catalyst-equipped but rely on other
technology to meet emissions standards.
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35
Adverse health effects along busy arterial thorough-
fares will be measurable within two to six years
without conversion of power plants to coal.
In November 1973, the Administrator of EPA recognized
the problem of unregulated emissions from catalyst-
equipped vehicles and took the following steps.
First, research was expanded and accelerated to
better define the problem.
Second, programs were instituted to monitor
changes in air quality along traffic corridors.
Third, control options including desulfurization
of gasoline and promulgation of a particulate
emission standard are being considered.
Fourth, an annual report addressing health-related
issues for the Congress will be prepared.
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36
WHAT IMPORTANT CAVEATS MUST BE KEPT IN MIND?
* The answers to the preceding eleven questions are current best judgments
but they are clouded by significant scientific uncertainties involving
many key aspects of the sulfur oxides problem. These have been dealt
with in some detail in previous technical reviews and briefing documents
that are public information.
* Interpretations of historical trends in emissions and air quality
are hampered by a very limited data base. Indeed, our current monitoring
systems for sulfur dioxide, suspended sulfates, strong acids, precipitation
chemistry, trace metals, ammonia, and hydrocarbons are not adequate
to answer pertinent questions about the origin, transformation and removal
of sulfur oxide air pollutants. Simultaneous monitoring in urban, suburban
and rural settings is required.
Current measurements of suspended sulfates serve as a useful proxy
for acid-sulfate aerosols but measurements that delineate particle size and
chemical composition are required for sulfur compounds and other aerosol
components. Aerosols of natural and man-made origins must be
characterized and differentiated.
* The mechanisms and rates for the transformation of sulfur dioxide to
acid-sulfate aerosols in plumes and in the atmosphere are not well understood.
Plumes from controlled and uncontrolled industrial and power plant
combustion sources should be studied.
' Predictive models which will give needed precision to estimates of
long-range transport and the influence of emission height must be developed.
* More soundly based dose-response functions for the adverse effects
on public health and welfare must be developed. Interlocking clinical,
epidemiologic and laboratory animal studies are required to reduce scientific
uncertainties about adverse health effects. Carefully designed studies of
plant damage, material degradation, visibility impairment and climatic
changes are required to develop reasonable damage functions for adverse
effects on the public welfare.
* Control technology research must be enhanced in ways previously
discussed and the societal and environmental impacts of these controls
carefully considered.
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37
Sound societal judgments can be based only on a sound scientific
information base. Failure to acquire the needed information will
lead to needless discord and likely to one or more national economic or
public health tragedies.
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38
MAJOR CONCLUSIONS
Acid-sulfate aerosols represent a complex public health and regulatory
problem.
Control of sulfur oxide emissions from urban and rural power plants will
be required.
It is our best judgment that massive conversion of urban power plants to
high-sulfur fuels and the use of tall stacks with supplementary control
systems in rural power plants will greatly increase sulfate concentrations
and endanger public health.
Equipping vehicles with oxidation catalysts for several model years
will further aggravate the problem unless control measures are instituted.
Uncertainties in our scientific information base will foster major
societal disagreements about the steps necessary to control the
problem.
Scientific uncertainties are being reduced by research programs but
important information gaps will remain for five to ten years.
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