EPA-600/3-77-054
June 1977
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields
The nine series are:
1 Environmental Health Effects Research
2. Environmental Protection Technology
3 Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6 Scientific and Technical Assessment Reports (STAR)
7 Interagency Energy-Environment Research and Development
8 "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ECOLOGICAL RESEARCH series. This series
describes research on the effects of pollution on humans, plant and animal spe-
cies, and materials Problems are assessed for their long- and short-term influ-
ences. Investigations include formation, transport, and pathway studies to deter-
mine the fate of pollutants and their effects This work provides the technical basis
for setting standards to minimize undesirable changes in living organisms in the
aquatic, terrestrial, and atmospheric environments.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/3-77-054
June 1977
REGIONAL TRANSPORT AND TRANSFORMATION OF
SULFUR DIOXIDE TO SULFATES IN
THE UNITED STATES
by
Aubrey P. Altshuller
Environmental Sciences Research Laboratory
Research Triangle Park, N.C. 27711
ENVIRONMENTAL SCIENCES RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
RESEARCH TRIANGLE PARK, N.C. 27711
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DISCLAIMER
This report has been reviewed by the Environmental Sciences Research
Laboratory, U. S. Environmental Protection Agency, and approved for
publication. Mention of trade names or commercial products does not con-
stitute endorsement or recommendation for use.
11
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ABSTRACT
The trends in and relationships between ambient air sulfur dioxide
and sulfate concentrations at 48 urban and 27 nonurban sites throughout the
United States between 1963 and 1974 have been analyzed. Large decreases in
sulfur dioxide concentrations at urban sites in the eastern and midwestern
United States have been accompanied by modest decreases in sulfate concen-
trations. Large variations in sulfur dioxide emissions among air quality
control regions also result in much smaller variations in sulfate concen-
trations. Large changes in the patterns of sulfur oxide emissions have little
impact on sulfate concentrations in most air quality regions. Comparisons of
air quality regions with similar sulfur dioxide emission levels and patterns
of emissions in the eastern and western United States and of sulfur dioxide,
sulfate, and vanadium relationships between urban-suburban and urban nonurban
sites lead to the same conclusion. Long-distance sulfur oxide transport with
chemical conversion of sulfur dioxide to sulfates over ranges of hundreds of
kilometers or more provides a consistent explanation for all of the observed
results. This conclusion has been suggested earlier, and the present analysis
strongly supports previous discussions.
Reduction of sulfate concentration levels will require strenuous efforts
to control sulfur oxides not only locally but throughout large geographical
regions. Also, large new additions to utility capacity in western areas may
lead to significant increases in western sulfate concentration levels. The
types of research activities required to quantitate crucial experimental
parameters are discussed.
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CONTENTS
Abstract i i i
Tables vi
1. Introduction 1
Background 1
Procedures 2
Characteristics of sulfate particles 3
2. Relationships for Sulfur Dioxide and Sulfate at
Urban Si tes 6
Trends in sulfate concentrations 6
Sulfur dioxide emissions and sulfate concentrations 8
Effect of isolated compared to nonisolated regional
conditions on sulfates 9
Effects of shifts in sulfur dioxide emissio.
patterns on sulfate concentrations 11
3. Relationships for Sulfur Dioxide and Sulfate at
Nonurban Sites 14
4. Relationships Between Urban and Nonurban Sites 17
Relationships between urban and suburban sites on
the east coast 17
Relationships between urban and nonurban sites on
the east coast 18
5. General Discussion 23
References 28
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TABLES
Number Page
1 Three-Year Running Averages (ug/m ) for Sulfur Dioxide
Sulfate, and Vanadium at East Coast Sites 32
2
2 Three-Year Running Averages (ug/m ) for Sulfur Dioxide
and Sulfates at Midwestern Sites East of Mississippi 35
2
3 Three-Year Running Averages (ug/m ) for Sulfur Dioxide
and Sul fates at Southeast Si tes 38
2
4 Three-Year Running Averages (ug/m ) for Sulfur Dioxide and
Sulfates, at Midwestern Sites West of the Mississippi 40
2
5 Three-Year Running Averages (ug/m ) for Sulfur Dioxide
and Sulfates at Western Sites 41
6 Three-Year Running Averages (ug/m ) for Sulfur Dioxide
and Sulfates at West Coast Sites 43
7 Three-Year Running Averages (ug/m ) Sulfur Dioxide,
Sulfates, and Vanadium at Nonurban Sites 44
2
8 Three-Year Running Averages (ug/m ) for Sulfur Dioxide,
Sulfates, and Vanadium at Sites in Philadelphia-Camden
and Surrounding Suburban Sites 49
VI
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SECTION 1
INTRODUCTION
BACKGROUND
Previous discussions have been published on the distribution of concen-
1
1 2
trations of sulfur dioxide and sulfate in the United States. ' A relation-
ship between annual sulfur dioxide and sulfate concentrations was suggested.
It was pointed out that elevated sulfate concentrations appear to be widely
distributed throughout entire regions in the eastern and midwestern United
1 2
States. ' These uniformly high sulfate levels were attributed to long-range
transport, with transformation of sulfur dioxide to sulfate well downwind of
1 2
urban areas. ' An anthropogenic background was suggested as present through-
1 2
out the eastern United States and a portion of tt,^ mic'vestern United States.
This position has received acceptance in the recent National Academy of
Sciences report on the sulfur oxide and nitrogen oxide pollution from
3
combustion sources. However, some aspects of the contribution of transport
and transformation to urban and nonurban concentrations are still being
debated. ' The need for continuing analysis of the available measurements
of sulfate and sulfur dioxide is "''ear. Decisions on fuel usage patterns and
placement of fossil-fuel power plai.ts may be influenced by improved under-
standing of the processes of sulfate formation, transport, and removal.
Additional sulfate and sulfur dioxide measurements now are available
1 2
beyond data bases used earler. ' Sulfate measurements are available at
many sites into the 1970's. Detailed sulfur dioxide emission inventories
and fuel usage patterns are available for recent years. It is essential
to utilize this additional data base in determining evidence bearing on the
regional distribution patterns for sulfates.
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PROCEDURES
Sulfur dioxide and sulfate concentrations were obtained by the National
Air surveillance Networks from the early 1960's into the 1970's using a 24-hour
average sampling period once every 2 weeks during each year. Results were
utilized for sites having as a minimum several years of concurrent measure-
ments for sulfur dioxide and sulfate (probably including sulfite and sul-
1278
fide, if present). In addition to the previous cited sources, ' ' ' several
912
new publications have been utilized. Methods of analysis have been
reported previously. Three-year running averages were computed as the best
available means of smoothing out the effects on sulfur dioxide emissions and
air quality of varying severities of winter or summer seasons.
In addition to sites located well within urban areas, a number of sites
in nonurban locations exist. Measurements at such sites usually start in
1965, but began earlier at a few locations. These nonurban sites vary from
suburban-rural locations in the eastern United States to rather remote sites
at some western locations. These sites were not originally selected to
necessarily represent the most remote sites geographically available.
Logistical requirements as to available sites, power, volunteer services for
replacement of filters, and related operational needs had to be considered
in the site selections. In the eastern United States, it is difficult to
find sites that are well removed from all power plants, petroleum activities,
paper mills, or some form of industrial activity. Also, the primary intent
was not to find sites for measurements of geophysical significance, but to
represent the concentrations of pollutants people in suburbs, towns,
villages, or rural environments would be exposed to by all surrounding
anthropogenic activities. Some local influences do exist; however, this is
the only body of measurements ever carried out in the United States that is
available to evaluate the potential for regional transport and transformation
of sulfur oxides to sulfates in the United States.
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Concern has been expressed about conversion of some of the sulfur
dioxide present to sulfate on the glass fiber filters used because of the
alkaline nature of the filters. However, this problem usually is over-
simplified. Simulated laboratory measurements are of limited use in
estimating whether or not significant conversion has occurred. To re-
construct the circumstances historically at each of many sites with respect
to both filter and matrix is a very difficult, if not impossible, task.
A few comments can be made based on the observed measurement results
for sulfur dioxide and sulfate. If conversion of sulfur dioxide to sulfate
on the filter substrate were a significant factor, the conversion was not
at all in proportion to sulfur dioxide concentration. This type of artifact
might be expected to result in large decreases in sulfate concentrations with
large decreases in sulfur dioxide concentrations. Such decreases have not
been observed. In fact, high sulfate concentrations occurred in nonurban
sites where sulfur dioxide concentrations were very l">w and also where
possible catalytic metal species also are at very low concentrations.
A recent investigation of sampling and analysis of sulfates involved
collection in Los Angeles, Calif., St. Louis, Mo., and Durham, N. C., of
both high volume samples on glass fiber filters and of low volume samples
on fluoropore filters. There is no indication of sulfur dioxide to sulfate
conversion on fluoropore filters. The analytical results usinp the methyl
thymol blue method on the two types of filter samples from the three sites
gave ratios of concentration ranging between 0.97 and 1.04 to 1. Such
results indicate that an insignificant percentage of the sulfate measured
originated from sampling artifacts on 24-hour high volume samples on glass
fiber filters.
CHARACTERISTICS OF SULFATE PARTICLES
The amount of sulfate available to undergo transport depends on the
characteristics of sulfate particles, the rates of transformation of sulfur
dioxide to sulfate, and the removal processes for sulfur dioxide and sulfate,
3
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Particle size distribution measurements for sulfates or particulate sulfur
are available at sites in Philadelphia, Pa., Cincinnati, Ohio, Chicago,
111., St. Louis, Mo., Los Angeles, Calif., and San Francisco, Calif.
The mass median diameters were computed to range from 0.2 to 1 urn, with most
of the MMD values between 0.2 and 0.6 urn. From 80 to over 90 percent of the
sulfate or particulate sulfur was found to occur in the size range below
2 urn. Analysis for ammonium ion at the same sites in Philadelphia, Cincinnati,
and Chicago indicated that the ammonium ion has almost the same particle size
19
range characteristics as sulfate. Measurements at sites in Los Angeles,
Calif., New York, H. Y., and Columbus, Ohio, and Dayton, Ohio, tend to
90 ?3
indicate that either (Nh'4)2S04 or NH4HS04 may be the predominant species.
Whatever the chemical form of sulfur-containing particles, their particle
size characteristics will result in a very slow rate of removal by dry
deposition or uptake by foliage. Sulfur dioxide would be much more rapidly
removed by foliage and other surfaces. Except during periods of precipitation,
the conditions of emission and surface removal of sulfur dioxide should be
critical. Conversion rates of sulfur dioxide to sulfate in urban and power
24
plant plumes are reported in the 1 to 20 percent per hour range. The rate
25-27
of dry deposition appears to be in the same range However, the rates
for these two major competing processes may vary considerably, with one or
the other process dominating in the same or different areas depending on a
number of emission and atmospheric parameters.
It is not the purpose of this analysis to discuss the characteristics
of the finely divided particulate fraction of the total suspended particulates
in the atmosphere. However, some facts with respect to sulfates are
appropriate to consider briefly. First, sulfates constitute the following
average percentage of total suspended particulates at urban sites in
various regions of the United States: east coast, 15 percent; midwest
(east of Mississippi), 10 percent; southeast, 8 percent; midwest (west of
Mississippi), 6 percent; mountain states, 4 percent; southwest, 5 percent;
west coast, 9 percent. These results suggest that sulfates constitute a
significantly more important fraction of the total suspended fraction in
-------
the eastern United States than in the western United States. At nonurban
sites, sulfates show the same relative regional patterns, except that sul-
fates constitute a larger portion of total suspended particulates at
nonurban compared to urban sites for most regions of the United States.
The average percentage of sulfate in the finely divided particulate (sum
of sulfate, benzene-soluble organics, nitrates, and lead) by region at
urban sites in 1968 thru 1970 were as follows: east coast, 61 percent;
midwest (east of Mississippi) 59 percent; southeast, 50 percent; midwest
(west of Mississippi), 47 percent; mountain states, 36 percent; southwest,
40 percent; west coast, 39 percent. At nonurban sites, the same relative
regional patterns were observed, except that sulfates constituted a
significantly larger portion of the finely divided species at nonurban
compared to urban sites in all regions of the United States.
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SECTION 2
RELATIONSHIPS FOR SULFUR DIOXIDE AND SULFATES
AT URBAN SITES
Sufficient annual average sulfur dioxide and sulfate concentration data
existed at 48 urban sites to permit computation of 3-year running averages
for the period 1963 through 1974. However, even at these sites, the moni-
toring results were incomplete during portions of the periods, particularly
for sulfur dioxide concentrations. Vanadium measurements are available, and
they can be associated with the sulfur-containing emission products from
oil-fired combustion units, so these results are also included for east coast
sites. The available results are listed in Tables 1 to 6. These sites are
grouped on a regional basis as follows: east coast (north of Virginia),
midwest (east of Mississippi), southeast, midwest (west of Mississippi),
interior western states including the mountain states and southwestern
states, and sites on the west coast.
TRENDS IN SULFATE CONCENTRATIONS
The overall averages by region for sulfur dioxide and sulfate show
consistent trends (Tables 1-6). Throughout the 1963 to 1974 period, or the
portion thereof for which sufficient measurements were available, the order
of decreasing sulfur dioxide and sulfate concentrations were as follows:
(1) east coast, and midwest (east of Mississippi), (2) southeast, (3) west
coast, (4) midwest (west of Mississippi), (5) western states. Sites in
western states or in midwestern states west of the Mississippi have had
sulfur dioxide concentrations averaging 10 to 20 percent of the concen-
trations at sites on the east coast. The range of sulfate concentrations
is smaller. Sulfates averaged over western sites have ranged from 30 to
70 percent of the sulfate concentrations at east coast and midwestern (E)
sites.
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Sulfur dioxide concentrations definitely trended downward between the
mid-1960's and early 1970's at east coast and at many midwest urban sites
east of the Mississippi (Tables 1,2). These downward trends in ambient air
sulfur dioxide concentrations are consistent with the decreases in sulfur
dioxide emissions in these areas. A smaller downward trend in sulfur
dioxide may have occurred at southeastern urban sites and urban sites west
of the Mississippi, but the sulfur dioxide results at these sites are too
incomplete or the concentrations too low to be certain as to the trends
(Tables 3-6). Sulfate concentrations did not show proportional downward
trends with sulfur dioxide concentrations in any of the regions. While
the average sulfur dioxide concentrations decreased by 58 percent at
eest coast urban sites between 1963-65 and 1969-71, the sulfate concentrations
decreased by 15 percent (Table 1). At midwestern urban sites (east of
Mississippi), the average sulfur dioxide concentrations decreased by 30
percent between 1965-67 and 1969-71; the sulfate concentrations fluctuated
somewhat but show no significant trends on the average at urban sites. As
e result by the 1970's the average regional concertrat'ons of sulfate in
the midwest (E), exceeded those on the east coast (Table 1,2) at urban sites.
Along the east coast from Providence, R. I. to Baltimore, Md., there
was a slow decrease in sulfate concentrations. Comparing the 1972-74 period
with 1964-66 sulfate concentrations decreased by from 18 to 42 percent. The
largest decreases were around New York city, M. Y. and Philadelphia, Pa.
Therefore, there was a consistent J°crease in sulfate concentretions in most
east coast urban sites.
In most of the other regions no trend in sulfate concentrations was
evident or the data was too limited o discern such trends. The exception
was for sulfates in the southwest and mountain states where an upward trend
in sulfate occurred at a number of the urban sites.
It appeared possible that, while annual average sulfate concentrations
did not decrease in proportion to changes in annual average sulfur dioxide
concentrations, a different relationship would be obtained using higher
percentile sulfate concentrations. Various such comparisons were computed.
-------
The most favorable relationship was obtained using 80th percentile sulfate
concentrations, but the improvement was modest. For example, sites in six
cities -- New York, N. Y., Newark, N. J.s Baltimore, Md., Pittsburgh,
Pa., Indianapolis, Ind., and St. Louis, Mo. -- over the 1963-65 to 1970-72
period showed an averaged decrease in annual average sulfur dioxide concen-
trations of 63 percent and a range from 39 percent to 74 percent decrease
in sulfur dioxide. For the same sites and time period, the annual average
sulfates and 80th percentile sulfates showed an average decrease of 10 and
18 percent, respectively, and a range from 20 percent increase to 37 per-
cent decrease and from a 3 percent increase to a 45 percent decrease,
respectively. Therefore, the choice of statistical measures of sulfate
concentration other than arithmetic annual averages also does not result
in anything like a proportional relationship between sulfur dioxide and
sulfates.
SULFUR DIOXIDE EMISSIONS AND SULFATE CONCENTRATIONS
The influence of variations in sulfur dioxide emissions on sulfate con-
centrations can be examined by use of available results in several air
quality control regions. In the midwest, some air quality control regions,
including Pittsburgh, Pa., Detroit, Mich., Cleveland, Ohio, Chicago, 111.,
and St. Louis, Mo., had annual sulfur dioxide emissions (1972) from
700,000 to 1,200,000 tons/year. The annual average sulfate concentrations
(1970-72 or 1969-71) in these regions (Table 2) ranged from 14 to 20
o o
ug/m and averaged 16.7 ug/m . Other air quality control regions in the
midwest, including Columbus, Ohio, Dayton, Ohio, and Indianapolis, Ind.,
had annual sulfur dioxide emissions of 100,000 to 200,000 tons/year. At
these sites annual average sulfate concentrations (1970-72) ranged from
12 to 14 ug/m3 and averaged 13.0 ug/m3. Therefore, the air quality control
regions with sulfur dioxide emissions five to ten times higher than
regions with the lower sulfur dioxide emissions had sulfate concentration
levels only 28 percent higher. These results demonstrate the low
sensitivity of sulfate concentrations to large differences in sulfur
dioxide emissions originating within the same air quality control region
at least in the midwestern United States.
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EFFECT OF ISOLATED COMPARED TO NONISOLATED REGIONAL CONDITIONS ON SULFATES
Another comparison that can be made is between urban sites within air
quality control regions that are relatively isolated and those that are not
isolated. The term isolated is used in the context of a region surrounded
by other regions with relatively low sulfur dioxide emissions from utility or
other emissions sources. The areas including Minneapolis-St. Paul, Minn., and
Kansas City, Mo., are examples of relatively isolated air quality regions.
The sulfur dioxide emissions within these regions is in the range of 200,000
tons/year (1972). Utility sources contribute 72 and 81 percent of the sulfur
dioxide emissions, while area sources contribute 9 and 8 percent respectively,
5
in these two regions. A number of air quality regions east of the Mississippi
also have sulfur dioxide emissions in the 200,000-ton/year range. These
regions are surrounded within 100 miles or less by other air quality regions
emitting sulfur dioxide in amounts equal to or rn-'ch >rger than 200,000 tons/
year. Such air quality regions include Providence, R.i., Hartford-New Haven-
Springfield, Conn., Baltimore, Md., Washington, D. C., and Indianapolis, Ind.
In this latter group of air quality control regions, utility sources contri-
buted an average of 62 percent (range 51 percent to 76 percent) of the sulfur
dioxide emissions, while area sources contributed an average of 21 percent
(range 9 to 35 percent of sulfur dioxide emissions.) In the two isolated
regions, annual average sulfate concentrations (Table 4) were 7 to 8 ug/m
(1S70-72), while for the nonisolated regions annual average su.fate con-
centrations (Tables 1, 2) were 12 tc 18
concentrations in the isolated regions.
o
centrations (Tables 1, 2) were 12 tc 18 ug/m , about twice the sulfate
It might be suggested that differences in sulfur dioxide emission
patterns within air quality regions might have a significant effect on
ambient air sulfate concentrations. For example, the lower quantities of
near-surface area emissions of sulfur dioxide in the two isolated regions,
rather than the isolation of the regions, might be responsible for the lower
ambient air sulfate concentrations. However, this hypothesis is not
supported by comparisons of the annual average sulfate concentrations
9
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within the five nonisolated regions discussed above. Despite a range of
9 to 35 percent in the near-surface area emissions within these regions,
there are no significant differences in the averaged annual sulfate con-
centrations. The Indianapolis and Baltimore air quality regions are the
two nonisolated regions with comparable area sulfur dioxide emissions to
the Minneapolis-St. Paul and Kansas City air quality regions. The averaged
annual sulfate concentrations in the two nonisolated regions averaged 14
o
ug/m , about twice the sulfate concentrations in the isolated regions.
Therefore, comparisons of sulfate concentrations at sites in isolated and
nonisolated regions with comparable area sulfur dioxide emissions support
isolation as the dominant factor in accounting for differences in regional
sulfate concentration levels.
It also might be suggested that some other factors of a meteorological
nature, such as differences in temperature, solar radiation, mixing heights,
wind speed, or precipitation, might be significant in explaining differences
in these sites in sulfate concentrations. Differences in solar radiation
and wind speeds are small among sites in most of the eastern and midwestern
United States and are somewhat higher for the east coast sites than midwest
sites, but the mean annual afternoon mixing heights are nearly the same
for all of these sites. The two isolated air quality regions being used
for comparison have similar wind speed and mixing height characteristics
compared to other midwestern sites. However, the temperatures throughout
the year in Minneapolis are lower and the annual precipitation in the form of
rain is lower than at sites further east. Kansas City does have a
climatological pattern nearly the same as a number of nonisolated mid-
western sites. The sulfate concentrations at the site in Kansas City
o
were consistently less than 1 ug/m greater than at the site in Minneapolis.
Therefore, if these climatological factors have an influence on sulfates,
the effect is very small.
With isolation as the dominant factor, it follows that at least half of
the sulfate concentrations measured at eastern urban sites can be attributed
to sulfur oxide emissions being transformed to sulfate during transport from
adjacent regions. This contribution can be an interregional contribution
10
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of sulfates. This contribution actually may exceed 50 percent because some
small interregional contribution to sulfates also may occur into the
Minneapolis-St. Paul and Kansas City regions.
EFFECTS OF SHIFTS IN SULFUR DIOXIDE EMISSION PATTERNS ON SULFATE CONCENTRATIONS
Although there already has been some discussion of shifts of sulfur
dioxide emission patterns on sulfate concentrations, additional comparisons
among regions should be considered. One comparison that can be made involved
selecting air quality control regions with the same annual sulfur oxide
emissions, but significantly different contributions to the total emissions
from area sources. The regions compared also should either be all nonisolated
or all isolated from large extraregional sources. Among the appropriate regions
for comparison were four nonisolated regions Providence, R.I., and Hartford-
New Haven-Springfield, Conn., with higher area sources and Baltimore, Md., and
r
Indianapolis, Ind., with lower area emissions. The total sulfur oxide emissions,
utility emissions, and area emissions in 1972 foi Pro* 'dence and Hartford-New Haven-
Springfield averaged 216,000 tons/year, 123,000 tons/year, and 67,000 tons/year,
respectively. The total sulfur oxide emissions, utility emissions, and area
emissions in 1972 for Baltimore and Indianapolis averaged 218,000 tons/year,
5
131,000 tons/year, and 24,000 tons/year, respectively. Therefore, the former
two regions had almost three times the loadings of area sulfur oxide emissions
compared to the latter two regions. The 1970-72 averaged sulfate surface con-
3
centrations were 13.6 ug/m for Providence and Hartford-New Havan-Springfield
o
and 14.2 ug/m for Baltimore and Indianapolis (Table 2). Examination of other
pairs of regions with about the same sulfur oxide emission but different
proportions of area-wide emissions also did not show higher sulfates in
5
regions with higher near-surface-leve^ area sulfur oxide emission sources.
Therefore, the available results do m,t appear to demonstrate that near-surface
area sulfur oxide emissions contribute any more per ton of emissions than
point source emissions to urban surface sulfate concentrations. This con-
clusion applies to the northeastern and midwestern United States.
Most western urban sites have sulfate concentrations two to three times
lower than midwestern sites east of the Mississippi and in the northeastern
United States. Many of the western regions in which these sites are located
11
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have had negligible sulfur oxide emissions from utility sources because of the
use of gas or hydroelectric power to generate electricity. Often industrial
emissions from the primary metal industries or from the petroleum and petro-
chemical industries are the predominate sources of sulfur oxides. Therefore,
in addition to such regions usually being largely isolated from extraregional
sulfur oxide emissions, they also have drastic differences in sulfur oxide
emission patterns compared to eastern sites. Regions such as the ones
including Houston, Texas, and Seattle, Wash., are examples of such types of
regions. Since these western regions have nearly the same sulfate concen-
trations as regions such as Minneapolis-St. Paul and Kansas City, which have
emission patterns more typical of eastern regions, isolation of these regions
from extraregional sulfur oxide emissions again appears to be substantially
more important than the shifts in the sulfur oxide emission patterns within the
region.
A possible exception to the above discussion of the influence of sulfur
oxide emission patterns on sulfate concentrations may occur in the Southern
California air quality region. Sulfate concentrations in this region are
significantly higher than in other western regions. Although utility sources
are more significant in the Southern California air quality region than in
most other western regions, the utility contribution was only 17 percent in
1972. Area sources contributed 45 percent, with transportation sources
alone contributing 11 percent to sulfur oxide emissions. This region had
about the highest proportion of area-wide sulfur dioxide emissions in the
United States and an atypically high transportation component. The sulfate
concentrations measured in this region are in the lower end of the range
of sulfate concentrations in the eastern United States. The southern
California air quality region is relatively well isolated from sulfur
oxide emissions from other regions.
Several factors could contribute to the higher sulfate concentrations
for a western area -- in the Southern California air quality region (Table 6).
These factors include high photochemical activity for conversion of sulfur
dioxide to sulfate, poorer ventilation, and the higher near-surface area
emissions.
12
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There is a gradient in sulfur oxide emissions from near the Pacific
coast, through the western portion of the Los Angeles basin, to the eastern
portion of the basin. All sources of sulfur oxide emissions are denser in
the western portion of the region -- utility, industrial, and transportation
sources. Much lower sulfur dioxide concentrations at sites in the eastern
portion of the region support the existence of such a gradient. Nevertheless,
the sulfate concentration levels are almost the same in the western and
eastern portions of the region (Table 6). This observation is consistent
with a flux of sulfur dioxide being transported, with rapid transformation
to sulfate from the western to eastern portions of the region. These facts
indicate that photochemistry and local meteorology are likely to be the
important factors in explaining the higher sulfate concentrations. The
high surface sulfur oxide emissions in this region are probably an interacting
factor because these surface emissions of sulfur oxides can mix rapidly
with the hydrocarbon and nitrogen oxide precursors to photochemical reactions
that cause conversion of sulfur dioxide to sulfates. It also is reasonable
that the relatively arid conditions in this region resjlt in lower rates of
removal by deposition to vegetation of near-surface area sulfur dioxide
emissions compared to eastern regions. This circumstance results in a larger
fraction of sulfur dioxide being available for conversion to sulfate. Therefore,
the Southern California air quality region may be a very special case with
respect to the significance of area compared to point emission contributions
to sulfur oxide emissions.
13
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SECTION 3
RELATIONSHIPS FOR SULFUR DIOXIDE AND SULFATE AT NONURBAN SITES
Sufficient annual average sulfate concentration data existed at 27
nonurban sites to permit computation of 3-year running averages for the
period 1965 through 1972. There were incomplete results at a number of sites.
Sulfur dioxide concentrations were occasionally available for several years
in the late 1960's and early 1970's at some sites. Vanadium concentrations
were available during periods of several years at the non-urban sites on the
east coast. These results are tabulated in Table 7.
By region, the order of decreasing sulfate concentrations were as follows:
(1) east coast and midwest (east of Mississippi), (2) southeast, (3) south-
west, (4) midwest (west of Mississippi) and west coast, (5) mountain states.
Sulfate concentrations in mountain state sites are only 15 to 25 percent of
the sulfate concentrations at east coast or midwestern (east of Mississippi)
sites.
Upward trends or no trends in sulfates occurred at most nonurban sites
on or near the east coast (Table 7). This is in contrast to the downward
trends at most urban sites in this same geographical area.
There also was an upperward trend in sulfate concentrations at the non-
urban sites in the midwest (E) from the 1960's into the 1970's. This trend
would be consistent with overall increases in sulfur oxide emissions from
utility sources in this region during this period. Upward trends in sulfate
concentrations were apparent for many western nonurban sites.
Sulfur dioxide concentrations were low at all nonurban sites. No clear
trends could be observed at these low concentration levels.
14
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Three-year running averages for nonurban sulfate on the east coast and in
the midwest east of the Mississippi River equal or exceed the corresponding
averaged sulfate concentrations at urban sites in regions west of the
Mississippi (Tables 4-6), The sulfates in the Los Angeles basin area do
exceed levels at eastern and midwestern nonurban sites, but sulfate con-
centrations at other west coast urban sites do not exceed the nonurban
sulfate concentrations at eastern sites.
The possibility for a natural or background biogenic contribution to
sulfate concentrations should be considered. At most sites, there is little
or no evidence for such a contribution being significant. One possible
exception is the very isolated nonurban site on Cape Hatteras. This site
might be expected to be influenced only slightly by transport of sulfur
oxides from urban areas, since it is more isolated from urban contributions
than other eastern nonurban sites. Nevertheless, the sulfate concentrations
are substantially elevated at Cape Hatteras. The surrounding area to the
west of Cape Hatteras for 100 miles and more consists cf shallow coastal
waters, with much swamp and marsh land and only a small number of small
towns and light industrialization. Therefore, it can be hypothesized that
natural sulfur gases are being oxidized in this area in appreciable amounts
to sulfate. However, measurable vanadium concentrations at this site
indicate the possibility for transport even to the site from the coastal
areas of Virginia where appreciable quantities of high vanadium content oil
are utilized.
If a tropospheric or continental concentration background exists for
species formed directly or indirectly from natural sources or for long-lived
species emitted from anthropogenic scjrces, a minimum concentration will be
detectable. If such a minimum exists, it should be evident in an examination
of the distribution of minimum 24-hour sulfate concentrations at nonurban
sites over a period of years.
An examination of the data base at nonurban sites was made for the years
3
1965 to 1972. Minimum 24-hour sulfate concentrations at or below 1 ug/m
occurred for eastern nonurban sites. At western nonurban sites, minimum 24-
15
-------
hour sulfate concentrations at or below 1 ug/m were frequent. Since the
detection limit for sulfate in the analyses was 0.6 ug/m , concentration
values at or below this level have no numerical significance, although a
substantial number of such very low sulfate values do appear as minimum
concentrations at western sites. If a natural background of sulfates does
q
exist, this data base indicates that it probably occurs at less than 1 ug/m .
Such a background is far too small to influence the distribution patterns of
sulfate over the United States. Special situations where sites are downwind
of strong natural sources should be evaluated on an individual basis.
16
-------
SECTION 4
RELATIONSHIPS BETWEEN URBAN AND NONURBAN SITES
RELATIONSHIPS BETWEEN URBAN AND SUBURBAN SITES ON THE EAST COAST
A comparison of measurements in the core area of a metropolitan area with
those in surrounding suburban towns and villages can be made. Measurements
are available for sulfur dioxide, sulfates, and vanadium at two sites within
Philadelphia and Camden and at seven sites in surrounding towns and villages
in Pennsylvania and New Jersey. The measurements were made over the 1964 to
1971 period at the sites in Philadelphia, Camden, and Glassboro, N.J., from
1965 or 1966 to either 1970 or 1971 at sites in Warmister, Pa., West Chester,
Pa., and Burlington County, N. J. (fragmentary results are available also
for sites in Marlton, Pemberton, and Bridgeton, N. J.). The results at the
two urban and four suburban sites have been used to compute 3-year running
averages for sulfur dioxide, sulfates, and vanadvm a^ each site (Table 8).
The ratio of urban to suburban sulfur dioxide concentrations indicates
about a four to one greater dilution of sulfur dioxide at the suburban sites.
The ratio of urban to suburban sulfate concentration indicates almost a two to
one greater dilution of sulfates at the suburban sites. There is a slight
trend towards a greater spread between these two urban to suburban ratios
with time. The ratio of urban to suburban vanadium concentrations increased
from 2.8:1 to 5.7:1, with the incre se in urban vanadium concentrations
reflecting increased urban use of hign-vanadiurn-content residual oil of
lower sulfur content. The site in G\issboro is the only suburban site
showing a consistent increase in vantdium concentration along with the urban
sites in Philadelphia and Camden.
The high sulfate concentrations in the suburban sites are of particular
interest and concern. By the late 1960's, the average sulfur dioxide to
sulfate ratio at these suburban sites was 2:1. The sulfate concentrations
at these suburban sites in the late 1960's exceeded half of the sulfate
17
-------
concentrations within Philadelphia and Camden in 1965-67 when the sulfur
dioxide to sulfate ratios were as high as 6:1 or 7:1. Even more striking
were the sulfur dioxide to sulfate ratios of 1:1 in Glassboro in 1968-70 and
1969-71, with the sulfate concentration in Glassboro in the 1969-71 period
still over half of the average sulfate concentrations in Phi lade!phia-Camden,
o
while the sulfur dioxide concentration of 7 ug/m was only 5 percent of the
1965-67 sulfur dioxide concentration in Philadelphia-Camden.
It is very difficult to explain such high sulfate levels in a number of
suburban sites as associated dominately with local sources of sulfur dioxide
emissions. The more reasonable explanation for the high sulfate concentrations
at such widely scattered suburban sites around Philadelphia-Camden would be
regional scale transport of sulfur dioxide with transformation of sulfur dioxide
to sulfate. The combination of dilution, conversion of part of the sulfur
dioxide to sulfate, and dry deposition of the remainder could account for the
low sulfur dioxide to sulfate ratios observed along with the low sulfur dioxide
concentrations. This discussion is not meant to suggest that there cannot be
some local contributions to the sulfur content at any particular site.
RELATIONSHIPS BETWEEN URBAN AND NONURBAN SITES ON THE EAST COAST
Measurements for sulfur dioxide, sulfate, and vanadium are available for
comparison of averaged concentrations at 11 urban and 6 nonurban sites east
of the Appalachian and north of Virginia. In this region, high vanadium
residual oil is an important utility and industrial fuel in New England,
eastern New York, New Jersey, eastern Pennsylvania, Delaware, and Maryland.
While the relative use of oil compared to coal decreases from New England,
New York-New Jersey, down to Washington, D. C., it appears that vanadium
can be used as a tracer for averaged utility and industrial combustion
emissions through this region. The ratios of urban to nonurban concentrations
computed from Table 1 and Table 7 are as follows:
18
-------
Year S02
1965-67
1966-68
1967-69
1968-70
1969-71
1970-72
1971-73
1972-74
__
--
__
5:1
5:1
__
2.2:1
2.1:1
2.0:1
2.3:1
2.2:1
1.9:1
1.8:1
1.8:1
12:1
7:1
14:1
12:1
10:1
The average sulfur dioxide to sulfate ratios at the urban sites ranged
downward from 8:1 in the early 1960's to just over 4:1 by 1970. At the
nonurban sites the sulfur dioxide to sulfate ratio was just over 1:1.
The urban to nonurban sulfate ratio is just a little higher than the
previously discussed urban to suburban sulfate ratios. The urban to non-
urban sulfur dioxide and vanadium ratios are two to four times greater than
the corresponding urban to suburban ratios.
The high sulfate concentrations and low sulfur dioxide to sulfate ratios
at the nonurban sites are especially in need of explanation. Several possible
contributions to the overall concentrations at the nonurban sites need to be
considered: (1) local emissions around the sites, (2) transport of species
formed in eastern urban areas without further reaction to these nonurban sites,
(3) transport of species formed in eastern urban areas with additional reaction
during transport to these nonurban sites.
Local emissions of sulfates imply either natural emissions or manmade
emissions produced locally. Natural emissions have already been discussed.
It was concluded that the sulfate background under most conditions at most sites
was 1 ug/m or less. Since the annual average concentrations at these sites
3 3
ordinarily ranged from 6 to 10 ug/m , a natural background of 1 ug/m or less
will not explain the observed relationships. Manmade local emissions
would be expected to reflect ratios of sulfur dioxide to sulfate in flue
gases or in plumes near their source. In plumes from coal-fired sources,
28
such ratios are usually 50:1 to 100:1 with a few values as low as 20:1.
19
-------
In plumes from oil fired sources ratios as low as 10:1 might occur downwind,
28-31
but the experimental data available is limited. However, none of
these ratios will account (even assuming frequent fumigation of the sites) for
ratios of sulfur dioxide to sulfate near 1:1. There also is no experimental
evidence that would suggest a much more rapid conversion of sulfur dioxide to
sulfate in a plume from a nearby emission source in a nonurban environment
than in an urban environment. This discussion is not meant to preclude local
contributions to the concentrations at nonurban sites, but such contributions
do not appear capable of accounting for the observed sulfur dioxide and sulfate
concentration relationships.
Vanadium ratios between urban and nonurban sites should serve as a tracer
for direct transport without further reaction of sulfur oxides from urban
combustion sources in this part of the United States. The average sulfate
concentrations near the surface in this part of the United States were 16
o
to 18 ug/m during the 1960's. These are the only types of measurements
available to use to represent the concentrations in the urban plume initially
before travelling downwind. Using these concentrations and the vanadium
ratios of 7:1 up to 13:1 lead to amounts of transported sulfate to the non-
3
urban sites from within the urban areas of 1.2 to 2.6 ug/m . If there were
any vanadium contributed enroute or in the vicinity of the nonurban sites,
these vanadium ratios should be adjusted upward and the sulfate contribution
downward.
The above computation also assumes that vanadium serves equally well as
a tracer for coal-fired as for oil-fired combustion sources within the urban
areas. Most of the nonutility sulfur oxide sources would be burning residual
oil, with use of small amounts of distillate oil, and there would be a very
5
small contribution from vehicular sources. The oil-fired to coal-fired fuel
usage ratio in utility sources was complex and changing. For the 1969-1973
period, during which detailed utility emission inventory information was
available, the oil to coal sulfur oxide ratio over the entire region was
1:1. However, almost three times as much sulfur oxide was associated with
20
-------
coal as oil in the Philadelphia-Baltimore-Washington areas. It has been
assumed that these varying gradients in oil to coal utility combustion-
derived sulfur oxides will not greatly influence the averaged computations
over the entire eastern region under consideration.
An averaged local contribution can be estimated with the use of
several assumptions. It is assumed that the measured sulfur dioxide con-
centrations at the nonurban sites are largely of local origin. Involved
in this assumption is the additional assumption that most of sulfur dioxide
from distant large urban or utility plumes would be removed by conversion to
sulfate or by dry deposition or precipitation before reaching these sites.
It also must be assumed that the local contributions of sulfate can be
estimated from use of urban sulfur dioxide to sulfate ratios. These urban
sulfur dioxide to sulfate ratios by the late 1960's and early 1970's
averaged near 4:1 at eastern sites. However, this ratio is not corrected
for the portion of the sulfate measured at the surface which is not of
local urban orgin. Such an estimate can be made from the earlier discussions
3
of isolated and nonisolated air quality control regions as in the 6-8-ug/m
range. If these amounts of sulfate are subtracted, the resulting ratios would
be 6:1 to 8:1. Using the sulfur dioxide concentrations at the nonurban sites
3
with such ratios would lead to the estimate that about 1 ug/m of sulfate was
of local origin. Since part of the local sulfur dioxide could have originated
3
from naturally produced hydrogen sulfide, this estimate of about 1 ug/m
would include any natural contribution. Some small amounts of sulfur dioxide
from distant sources actually are likely to be transported to these non-
urban sites, particularly the sulfur dioxide originating from elevated sources.
Therefore, it is unlikely that the average local contribution of sulfate could
be much higher than estimated here.
The midrange value for advected sulfate from distant sources of 1.5
ug/m added to 1 ug/m for local sulfate, accounts for about one-third of
the measured sulfate averaged over these nonurban sites. The remaining
available source must be sulfate formed by chemical conversions of sulfur
dioxide during dispersion and transport of the sulfur dioxide from the
21
-------
array of urban sources upwind of these nonurban sites. Based on the above
estimates, about two-thirds of the sulfate measured at these nonurban sites
can be attributed to sulfate formed by chemical reactions converting sulfur
dioxide to sulfate during transport of pollutants from urban areas to
nonurban locations. The contribution of urban sulfur dioxide area emissions
would be expected to be less than elevated utility sources ton per ton because
of the larger removal of sulfur dioxide by dry deposition from near-surface
area sources of sulfur dioxide.
22
-------
SECTION 5
GENERAL DISCUSSION
The urban and nonurban sulfate concentrations unlike sulfur dioxide
and vanadium concentrations are relatively uniform throughout large portions
of the eastern and midwestern United States east of the Mississippi River.
Sulfur dioxide and vanadium concentrations show gradients of 3:1 to 6:1
between urban to suburban sites (Table 8) and gradients of from 5:1 to 14:1
between the urban and nonurban sites in the eastern United States (Tables
1 and 7). In contrast, sulfate concentrations show gradients of less than
2:1 between urban and suburban sites (Table 8) and of slightly more or less
than 2:1 between urban and nonurban sites in the eastern United States
(Tables 1 and 7).
The significantly less than proportional relationship between urban
ambient sulfate concentrations in particular air quality control regions
and sulfur oxide emissions in the same regions is demonstrated by two sets
of results. The large reductions in urban sulfur dioxide emissions in air
quality control regions between lower New England and Baltimore, Md. caused
two to four-fold decreases in ambient sulfur dioxide concentrations while
urban sulfate concentrations decreased by 20 to 40 percent. Comparisons of
various air quality control regions having five- to ten-fold variations in
sulfur oxide emissions showed only about a 30 percent variation in ambient
sulfate concentrations.
How can this lack of proportionality be explained? A number of
reasonable possibilities exist which may singly or in combination provide
an adequate explanation. These factors are as follows: (2) the sulfate
measured is not of local origin but is formed during transport from adjacent
air quality control regions or more distant air quality control regions
within the same geographical region of the United States, (3) the sulfate is
not of local origin but is formed during transport from distant source in
23
-------
another geographical region of the United States, (4) the sulfate is directly
emitted within the same air quality region but the emission rate for sulfate
is not directly proportional to the sulfur content of the fuel used (1) the
mechanisms of formation in the atmosphere are such that on an annual average
basis they do not result in sulfate formation being proportional to sulfur
dioxide concentration.
The experimental results discussed in this report relate to factors
(2) and (3), but the results cannot quantitatively define the distances
over which transport occur. Therefore, the relative contributions from
nearest neighbor air quality control regions, next nearest neighbor air
quality control regions and more distant sources cannot be differentiated.
However, experimental studies on the St. Louis, Mo. urban plume on individual
experiment days demonstrate formation and transport of sulfate distances out
32 33
to several hundred kilometers. ' Measurements of the sulfur budget in
a large power plant plume demonstrated 13% of the sulfur in particulate
form at 40 km with the total sulfur mass in the plume conserved out to 50 to
100 km. Analysis of 1974-75 sulfate episodes in the midwestern United
States have been interpreted to be consistent with a contribution of isolated
05
sources on a scale of 200 to 300 km. Removal rates of sulfur oxides from
urban and industrial plumes of 20 percent per hour have been reported
leading to 1/e distances of less than 100 km over agricultural terrain in
the midwestern United States. Transport of sulfur containing particles
over distances of 1000 to 2000 km over water have been reported in Northern
Europe.36*37
These results strongly support the conclusions of this report and earlier
1 2
analysis of monitoring data ' that sources outside of a given air quality
control region contribute significantly to the total sulfate loading within
that region. The results suggest over land effective transport ranges of
at least several hundred kilometers. Experimental results are lacking as
to whether or not longer range transport can occur over land masses. If
such longer range transport does not occur it must follow that rates of
removal of sulfate particles over land32"35 are substantially greater
24
-------
than over water. *
The 4th factor suggests a lack of proportionality between sulfur in
fuel and sulfate emissions. Recent measurements on various oil-fired boiler-
fuel combinations indicate such a lack of proportionality with percentage
of total sulfur as sulfate increasing as fuel sulfur content decreases.
The increases in sulfate concentration or lack of trends at nonurban
sites on or near the east coast of the United States are difficult to explain.
There are consistent results demonstrating large decreases in ground level
sulfur dioxide concentrations within urban areas. Total utility sulfur
oxides emissions were decreasing substantially in almost all of the air
quality control regions on or near the east coast from the late 1960's
into the 1970's (1968-1973).6 The air quality control regions in which
several of these sites are located (Arcadia Natl. Park, Maine; Coos County,
N. H.; Orange County, Vt.) have very small sulfur oxide emissions and the
immediate surrounding areas very low population densities for the eastern
United States. At the Orange County, Vt. site there was an increase in
vanadium concentration between 1965-67 and 1970-72 which suggests increased
importance of sources burning high vanadium fuel oil. However, the trend
also suggests the possibility of contributions by long range transport
from United States sulfur oxide sources which had increasing emissions
in the midwestern United States or perhaps from sources outside of the
United States.
The trends at the midwestern non-urban United States sites are
reasonably consistent with the increases, particularly in utility sulfur
oxide emissions in the air quality control regions in which the sites
are located or from adjacent air quality control regions. The increases
in sulfur oxide emissions in a significant number of midwestern air
quality control regions east of the Mississippi River was quite sub-
stantial.
25
-------
The rates of conversion of sulfur oxides appear highly variable.
However, slower rates of 1 to 2 percent per hour are sufficient to produce
substantial sulfate concentrations during extraregional multiday transport
periods. The stability of sulfates compared to sulfur dioxide with respect
to further chemical reaction and dry deposition ensures appreciable transport
without appreciable removal, particularly from elevated sources of emissions.
These conclusions have significance with respect to the capability to
achieve reductions in ambient air sulfate concentrations. Substantial re-
ductions of sulfur dioxide emissions within the presently constituted air
quality control regions (AQCR) have resulted in only modest reductions in the
concentrations of sulfates in the same AQCR. The analysis of available
results indicates that substantial reductions of sulfur oxides emissions
throughout large regions will be essential to achieving substantial re-
ductions in the concentrations of ambient air sulfates in the eastern United
States. Long-range transport and chemical transformation of sulfur oxide to
sulfates appears responsible for such a requirement. The size of these
regions needs to be defined, but probably these regions would be at least
as large as the geographical regions used in this paper.
The lack of substantial variations of sulfate concentrations with large
changes in patterns of sulfur oxide emissions has been discussed. Since
many of these changes occur in v/estern areas that differ with respect to
meteorological and terrain aspects, the lack of large variations among such
areas might be attributed to fortuitous balances among various parameters.
However, the consistently low sulfate concentrations at almost all western
sites strongly indicates that such low concentrations are most likely to be
associated with low levels of sulfur oxide emissions through large adjacent
regions or other factors leading to isolation.
The installation of new large clusters of utility capacity in western
areas, if such installations emit substantial amounts of sulfur oxides, may
lead to increases in sulfate concentrations at locations that are long
distances downwind of such clusters. Detailed monitoring of the trend in
26
-------
sulfates at a substantial number of selected western sites appears essential.
The present discussion is not meant to imply that precise quantitative
experimental results are presently available to define several critical
aspects of the sulfate problem. The rates of conversion of sulfur dioxide to
sulfates within utility plumes, plumes from large nonutility point sources,
and urban plumes need continuing experimental measurement. The relative
contributions of utility sources, nonutility point sources, and area sources
to surface-level concentrations of sulfate require much improved quantitation.
Large-scale transport of sulfates in various geographical regions in the
United States as a function of precipitation and other removal parameters has
not been investigated, but urgently needs consideration. The impact of sul-
fates on visibility reduction, materials deterioration, water quality, and
soil chemistry should be evaluated. Finally, the measurement instrumentation
and other experimental techniques essential to the success of such field
investigations must be improved.
27
-------
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31
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-------
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
REPORT NO.
EPA-600/3-77-054
2.
4. TITLE AND SUBTITLE
REGIONAL TRANSPORT AdD TRANSFORMATION OF SULFUR
DIOXIDE TO SULFATES IN THE UNITED STATES
7. AUTHOR(S)
Aubrey P. Altshuller
3. RECIPIENT'S ACCESSION-NO.
5. REPORT DATE
June 1977
6. PERFORMING ORGANIZATION CODE
8. PERFORMING ORGANIZATION REPORT NO.
9 PERFORMING ORGANIZATION NAME AND ADDRESS
Environmental Sciences Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, N. C. 27711
1O. PROGRAM ELEMENT NO.
1AA603 (AH-14)
11. CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
Environmental Sciences Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, N. C. 27711
13. TYPE OF REPORT AND PERIOD COVERED
In-house
14. SPONSORING AGENCY CODE
EPA/600/09
15. SUPPLEMENTARY NOTES
16.ABSTRACT Trends in "and relationships between ambient air S02 and sulfate concentra-
t'ons at 48 urban and 27 nonurban sites throughout the Unitea States between 1963 and
"U74 have been analyzed. Large decreases in S02 concentrations at urban sites in the
eastern and midwestern United States have been accompanied by modest decreases in sul-
fate concentrations. Large variations in S02 emissions among air quality control
regions also result in much smaller variations in sulfate concentrations. Large
changes in the patterns of S02 emissions have little impact on sulfate concentrations
in most air quality regions. Comparisons of air quality regions with similar S02
emission levels and patterns of emissions in the eastern and western United States and
of S0?, sulfate, and vanadium relationships between urban-suburban and urban nonurban
sites lead to the same conclusion. Long-distance SO, transport with chemical con-
version of S0? to sulfates over ranges of hundreds of kilometers or more provides
a consistent explanation for all of the observed results. This conclusion has been
suggested earlier, and the present analysis strongly supports previous discussions.
Reduction of sulfate concentration levels will require strenuous efforts to control
S0?. Also, large new additions to utility capacity in western areas may lead to
significant increases in western sulfate concentration levels. The types of research
activities required to quantitate crucial experimental parameters are discussed.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS C. COS AT I Field/Group
Air Pollution
Sulfur Dioxide
Sulfates
Chemical Reactions
Transport Properties
Trends
United States
13B
07B
07D
12A
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (ThisReport)
UNCLASSIFIED
21. NO. OF PAGES
57
20. SECURITY CLASS (This page)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (9-73)
51
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