Overview of Sulfur Dioxide (SO2) Air Quality in the United States
Updated: June 09, 2022
1. Introduction
The overall purpose of this document is to maintain an up-to-date graphical summary of air quality information that
supports the review of the National Ambient Air Quality Standards (NAAQS) for sulfur dioxide (SO2). In previous reviews
of the SO2 NAAQS, this type of information has generally been included in atmospheric sections of the Integrated Science
Assessment (ISA) and Policy Assessment (PA) for Sulfur Oxides. This stand-alone document will either replace or complement
the air quality emissions and monitoring data in the atmospheric sections of future SO2 NAAQS documents, and will be
updated at regular intervals as new data becomes available.
The content of past NAAQS documents' atmospheric sections has included major sections on emissions and concentration
trends utilizing maps and data from the EPA's National Emissions Inventory (NEI) and the EPA's Air Quality System
(AQS) database. In past NAAQS reviews, this often involved adaptation of figures and tables prepared for other reports, or
development of new figures and tables using data analysis and mapping software. Additionally, the release of updated emission
inventories and ambient monitoring data may not coincide with the schedule for the development of NAAQS documents. As a
result, data access and resources can limit the availability of the most recent information for inclusion in NAAQS documents.
This stand-alone document allows the content to be updated as soon as new data becomes available, rather than pulling
from whatever is available at the time of publication. It also ensures that the public will have access to a consistent set of
maps and figures for each NAAQS pollutant that are updated on a routine basis, rather than separated by several years
following the disparate schedules of the various NAAQS reviews for each pollutant. Moreover, a stand-alone document can be
expanded to include new air quality analyses as they are completed, rather than following the timeline for the public release
of the NAAQS documents. Finally, this document takes advantage of a more flexible digital format for the routinely prepared
maps and trends figures, with an end product that more strongly emphasizes visual presentation of data and reduces the
amount text, while also creating a more interactive presentation of the information through the use of external links.
This document follows an organization similar to the structure of the atmospheric sections of past SO2 NAAQS documents.
The subsequent sections are as follows: 2. Atmospheric Chemistry; 3. Sources and Emissions of SO2; 4. Ambient Air
Monitoring Requirements and Monitoring Networks; 5. Data Handling Conventions and Computations for Determining
Whether the Standards are Met; and 6. SO2 Concentrations Measured at Ambient Air Monitoring Sites Across the U.S.
These sections are broad enough in scope to handle changes in what is known about SO2 atmospheric science as it advances
but specific enough that NAAQS-relevant information will be able to be quickly retrieved by users of the document.
2. Atmospheric Chemistry
Sulfur oxides are a group of closely related sulfur-containing gas-phase compounds [e.g., sulfur dioxide (SO2), sulfur
monoxide (SO), disulfur monoxide (S2O), and sulfur trioxide (SO3)]. Sulfur oxides also appear in the particle phase, as
components of particulate matter (PM). The NAAQS for sulfur oxides are currently set using SO2 as the indicator, because
of the sulfur oxides, SO2 is the most abundant in the atmosphere, the most important in atmospheric chemistry, and the
one most clearly linked to human health effects. Sulfur dioxide is both a primary gas-phase pollutant (when formed during
fuel combustion) and a secondary pollutant (when formed as the product of atmospheric gas- or aqueous-phase oxidation of
reduced sulfur compounds, called sulfides).
The important gas-phase sulfur oxides in the troposphere are SO2 and H2SO4. SO3 is known to be present in the emissions
of coal-fired power plants, factories, and refineries, but it reacts with water vapor in the stacks or immediately after release
into the atmosphere within seconds to form H2SO4, which makes it difficult to detect in the ambient atmosphere. Gas-phase
H2SO4, the product of both SO2 and SO3 oxidation, quickly condenses onto existing atmospheric particles or participates
in new particle formation. Of these species, only SO2 is present at concentrations in the gas phase that are relevant for
chemistry in the atmospheric boundary layer and troposphere, and for human exposures. Other sulfur oxides, including
both S(IV) and S(VI) compounds, appear in the atmosphere due to direct emissions and as the products of the oxidation of
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more reduced forms of sulfur. Gas-phase precursors to SO2 include sulfides and partially oxidized sulfur-containing organic
compounds.
The global atmospheric lifetime of SO2 with respect to reactions with the OH radical in the troposphere is 7.2 days. The
rate constant for the reaction between SO2 and NO3 radical is too small to be important in lowering SO2 concentrations
at urban or regional scales. The same is true for the reaction between SO2 and the hydroperoxyl (HO2) radical. In the
stepwise oxidation of SO2 by OH, SO2 is oxidized to form SO3, taking the sulfur atom from the S(IV) to S(VI) oxidation
state, producing the bisulfite radical (HSO3): SO2 + OH + M -> HSO3 + M, where M is an unreactive gas molecule that
absorbs excess destabilizing energy from the SO2-OH transition state. This reaction is followed by HSO3 + O2 -> SO3 +
HO2. An alternative route involves a stabilized Criegee intermediate (sGI): SO2 + sGI -> SO3 + products. The SO3 that is
generated by either oxidation mechanism (i.e., reaction with OH or via the Criegee reaction) is highly reactive. Water vapor
is sufficiently abundant in the troposphere to ensure that SO3 is quickly converted to gas-phase sulfuric acid: SO3 + H2O +
M -> H2SO4 + M. Because H2SO4 is extremely water soluble, gaseous H2SO4 will rapidly dissolve into the aqueous phase
of aerosol particles and cloud droplets. Conversion from SO2 to H2SO4 increases with increasing relative humidity and O3
levels.
The basic mechanism of the aqueous-phase oxidation of SO2 involves dissolution of SO2 followed by the formation and
dissociation of sulfurous acid (H2SO3). Additionally, in environments where ambient ammonia (NH3) is abundant, SO2 is
subject to fast removal by cloud and fog droplets and ultimately forms ammonium sulfate [(NH4)2S04]. In the same way
that it is removed from the gas phase by dissolving into cloud droplets, SO2 can be removed by dry deposition onto wet
surfaces. Scavenging by rain (wet deposition) serves as another removal route. Modeling studies have shown that slightly
more than half of SO2 is lost by gas- and aqueous-phase oxidation, with the remainder of SO2 loss accounted for by wet and
dry deposition.
Sulfur dioxide is also known to adhere to and then react on dust particles. For some mineral compositions, SO2 uptake
on dust particles is sensitive to relative humidity, the mineral composition of the particle, and the availability of H2O2,
the relevant oxidant. Once SO2 is oxidized to H2SO4 on the particle surface, glyoxal, one of the most prevalent organic
compounds in the atmosphere, will adhere to the surface and react to form oligomers and organosulfate compounds. This
process is enhanced under high humidity conditions.
Sources: Integrated Science Assessment for Sulfur Oxides - Health Criteria, December 2017 (Chapter 2)
Policy Assessment for the Review of the Primary NAAQS for Sulfur Oxides, May 2018 (Chapter 2)
3. Sources and Emissions of S02
Sulfur is present to some degree in all fossil fuels, especially coal, and occurs as reduced organosulfur compounds. Coal
also contains sulfur in mineral form (pyrite or other metallo-sulfur minerals) and in elemental form. Of the most common
types of coal (anthracite, bituminous, subbituminous, and lignite), sulfur content varies between 0.4 and 4% by mass. Sulfur
in fossil fuels is almost entirely converted to SO2 during combustion, making accurate estimates of SO2 combustion emissions
possible based on fuel composition and combustion rates.
Fossil fuel combustion is the main anthropogenic source of primary SO2, while volcanoes and wildfires are the main natural
sources of primary SO2. Anthropogenic SO2 emissions originate primarily from point sources, including coal-fired electricity
generating units (EGUs) and other industrial facilities. Industrial chemical and pulp and paper production, smelter and steel
mill operations, natural biological activity (plants, fungi, and prokaryotes), and volcanoes are among many sources of reduced
sulfur compounds that contribute, through various oxidation reactions in the atmosphere, to the formation of secondary SO2.
In addition to volcanic and other geologic SO2 emissions, naturally occurring SO2 is derived from the oxidation of sulfides
emitted by low flux "area" sources, such as the oceans and moist soils. The mass of sulfur released into the environment by
anthropogenic sources is comparable to natural sources.
Other anthropogenic sources of SO2 emissions include industrial fuel combustion, other industrial processes, commercial
marine vessels, and agricultural and prescribed fires (Figure 1). While electricity generation is the dominant industry sector
contributing to SO2 emissions on a national scale, other sectors can also have a significant influence on local air quality.
Large emissions facilities other than EGUs that may substantially impact local air quality include copper smelters, pulp and
paper mills, cement plants, iron and steel mills, petroleum refineries, and chemical processing plants.
Figure 2 illustrates the national SO2 emissions trends from 2002 to 2021. Declines in SO2 emissions are likely related to
the implementation of national control programs developed under the Clean Air Act Amendments of 1990, including Phase I
and II of the Acid Rain Program, the Clean Air Interstate Rule, the Cross-State Air Pollution Rule, and the adoption of low
sulfur diesel fuel standards. An additional factor is changes in market conditions, such as the reduction of the use of coal in
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energy generation. These changes have resulted in a 87% decrease in SO2 emissions from 2002 to 2021, including reductions
of 91% in emissions from EGUs and 96% in emissions from mobile sources.
The National Emissions Inventory (NEI) is a comprehensive and detailed estimate of air emissions of criteria pollutants,
precursors to criteria pollutants, and hazardous air pollutants from air emissions sources. The NEI is released every three
years based primarily upon data provided by State, Local, and Tribal air agencies for sources in their jurisdictions and
supplemented by data developed by the US EPA. The NEI is built using the EPA's Emissions Inventory System (EIS) first
to collect the data from State, Local, and Tribal air agencies and then to blend that data with other data sources.
Accuracy in an emissions inventory reflects the extent to which the inventory represents the actual emissions that occurred.
Anthropogenic emissions of air pollutants result from a variety of sources such as power plants, industrial sources, motor
vehicles and agriculture. The emissions from any individual source typically varies in both time and space. For the thousands
of sources that make up the NEI, there is uncertainty in one or both of these factors. For some sources, such as power plants,
direct emission measurements enable the emission factors derived from them to be more certain than sources without such
direct measurements. However, it is not practically possible to directly monitor each of the emission sources individually and,
therefore, emission inventories necessarily contain assumptions, interpolation and extrapolation from a limited set of sample
data.
S02 Emissions (2,715 kTon/year)
Stationar;
Combustion: Coa
Stati
Combustion: Other 7%
Industrial Processes 19%
Agricultural &
Prescribed Fires 3%
Mobile Sources 8%
Wildfires 5%
Other 1%
Figure 1. U.S. SO2 emissions (tons/year) by sector. Source: 2017 NEI.
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15000
Stationary Fuel Combustion
~ Industrial and Other Processes
~ Transportation
CO
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>*
£
o
'
E
LU
CM
O
(1.1
*12000 -
9000 -
6000
3000
Inventory Year
Figure 2. U.S. anthropogenic SO2 emissions trend, 2002-2021. Source: EPA's Air Pollutant Emissions Trends Data
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Figure 3 below shows the SO2 emissions density in tons/mi2/year for each U.S. county based on the 2017 NEI. The
majority of SO2 emissions tend to be located near large point sources such as coal-fired EGUs or large industrial facilities.
Counties near urban areas also tend to have higher SO2 emissions due to the higher concentration of industrial facilities.
Counties in rural areas may also have higher emissions due to oil and gas extraction or fires.
Sulfur Dioxide Emissions Density in tons/year/miA2 (# Counties)
~ 0.05-0.19 (1140) ~ 0.2-0.99 (690) ¦ 1-4.99 (304)
Figure 3. U.S. county-level SO2 emissions density estimates in tons/year/mi2. Source: 2017 NEI
4. Ambient Air Monitoring Requirements and Monitoring Networks
Ambient SO2 concentrations are measured by monitoring networks operated by state, local, and tribal air agencies,
which are typically funded in part by the EPA. Measurements are made using ultraviolet fluorescence (UVF) instruments,
which are designated as federal reference methods (FRMs) or federal equivalent methods (FEMs) and the data are reported
to EPA as hourly concentrations and either the maximum 5-minute concentration for each hour or twelve 5-minute average
concentrations for each hour. There were 505 monitoring sites reporting hourly SO2 concentration data to the EPA during
the 2019-2021 period. The locations of these monitoring sites are shown in Figure 4.
The main network of monitors providing ambient data for use in implementation activities related to the NAAQS is
the State and Local Air Monitoring Stations (SLAMS) network, which comprises over 75% of all SO2 monitoring sites. An
important subset of SLAMS sites is the NCore multipollutant monitoring network, which was designed to collect consistent
measurements of criteria pollutants for trends and NAAQS compliance purposes. The NCore was fully operational as of 2011
and consists of approximately 60 urban monitoring stations and 20 rural monitoring stations. Each state is required to have
at least one NCore station.
In 2015, the EPA finalized the Data Requirements Rule (DRR), which required states to monitor or model ambient SO2
levels in areas with stationary sources of SO2 emissions of over 2,000 tons per year. The EPA identified over 300 sources
meeting these criteria, and the states chose to set up ambient monitoring sites to assess compliance with the SO2 NAAQS
near 71 of these sources. These monitors were required to begin operating by January 1, 2017 and collect data through
the end of 2019 to show compliance with the SO2 NAAQS. Some of these monitors are operated by the states as SLAMS
monitors, while others are operated by the industrial sources, which are shown as yellow dots on the map in Figure 4.
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Finally, there are also a number of Special Purpose Monitoring Stations (SPMs), which are not required but are often
operated by air agencies for short periods of time (i.e., less than 3 years) to collect data for human health and welfare studies,
as well as other types of monitoring sites, including monitors operated by federal agencies and tribal governments. The SPMs
are typically not used to assess compliance with the NAAQS.
• SLAMS (310) • NCORE(78) O INDUSTRIAL (59) • SPM/OTHER (58)
Figure 4: Map of U.S. SO2 monitoring sites reporting data to the EPA during the 2019-2021 period. Source: AQS.
5. Data Handling Conventions and Computations for Determining Whether the Standards
are Met
To assess whether a monitoring site or geographic area (usually a county or urban area) meets or exceeds a NAAQS, the
monitoring data are analyzed consistent with the established regulatory requirements for the handling of monitoring data
for the purposes of deriving a design value. A design value summarizes ambient air concentrations for an area in terms of
the indicator, averaging time and form for a given standard such that its comparison to the level of the standard indicates
whether the area meets or exceeds the standard. The primary SO2 NAAQS is the 1-hour NAAQS, which was established in
2010. The procedures for calculating design values for the primary SO2 NAAQS are detailed in Appendix T to 40 GFR Part
50 and are summarized below.
Hourly SO2 measurement data collected at an ambient air monitoring site using Federal Reference or Equivalent Methods,
meeting all applicable requirements in 40 GFR Part 58, and reported to AQS in parts per billion (ppb) with decimal digits
after the first decimal place truncated are used in design value calculations. If multiple monitors collect measurements at the
same site, one monitor is designated as the primary monitor. Measurement data collected with the primary monitor are used
to calculate the design value, and may be supplemented with data from collocated monitors whenever data is not available
for the primary monitor.
First, the maximum hourly concentration is determined for each day (i.e., the "daily maximum value") in a given 3-year
period. For each year, the 99th percentile of the daily maximum values is determined, and the design value is the average of
the three annual 99th percentile values, rounded to the nearest integer in ppb. The primary SO2 NAAQS are met when the
design value is less than or equal to 75 ppb, the level of the NAAQS.
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In addition, the design value must meet data completeness requirements in order to be considered valid. Specifically,
a sample day is considered complete when at least 18 hourly measurements are reported. For each calendar quarter (i.e.,
Jan-Max, Apr-Jun, Jul-Sep, Oct-Dec), the quarter is considered complete if at least 75% of the days in the quarter have
complete data. The primary SO2 design value is considered complete when all 12 calendar quarters in the 3-year period have
complete data. In addition, there are two data substitution tests specified in Appendix T to 40 GFR Part 50 which may be
used to yield a valid design value above or below the NAAQS, respectively, in the event that a site falls short of the minimum
data completeness requirement.
6. S02 Concentrations Measured at Ambient Air Monitoring Sites Across the U.S.
Table 1 below presents summary statistics based on two daily SO2 metrics, the daily maximum 1-hour (MDA1) metric, and
the daily 24-hour average (DA24) metric. These statistics are presented for year-round and each season (winter=Dec/Jan/Feb,
spring=Mar/Apr/May, summer=Jun/Jul/Aug, autumn=Sep/Oct/Nov) based on data reported to AQS for 2019-2021.
Table 1. National distribution of SO2 concentrations in ppb by season for 2019-2021. Source: AQS.
metric
season
N.sites
N.obs
mean
SD
min
Pi
p5
plO
p25
p50
p75
p90
p95
p98
p99
max
max.site
MDA1
all
400
427,647
3.0
13.1
-3.7
-0.1
0
0
0.3
0.9
2.0
5.0
11.0
24.2
39.4
2,732.4
171193007
MDA1
winter
398
105,176
3.0
13.8
-2.0
-0.1
0
0
0.3
1.0
2.0
4.6
10.0
23.5
40.2
1,980.6
150012016
MDA1
spring
399
108,253
3.1
12.6
-3.4
-0.1
0
0
0.3
0.9
2.0
5.2
12.0
26.2
42.0
564.5
120570112
MDA1
summer
396
106,979
2.9
13.2
-2.7
-0.1
0
0
0.3
0.9
2.0
5.4
11.0
22.8
36.0
2,732.4
171193007
MDA1
autumn
393
104,679
3.1
12.8
-3.7
-0.2
0
0
0.3
1.0
2.0
5.1
11.2
24.9
40.4
881.0
270370423
DA24
all
400
427,647
0.9
3.7
-3.7
-0.4
0
0
0.1
0.4
0.9
1.8
2.7
4.8
7.7
395.0
150012020
DA24
winter
398
105,176
0.9
3.9
-2.4
-0.4
0
0
0.1
0.4
1.0
1.8
2.6
5.1
8.7
395.0
150012016
DA24
spring
399
108,253
0.9
3.9
-3.5
-0.4
0
0
0.1
0.4
0.9
1.8
2.6
4.9
7.8
222.3
291439002
DA24
summer
396
106,979
0.8
3.0
-2.9
-0.4
0
0
0.1
0.4
0.9
1.8
2.6
4.3
6.6
212.3
171193007
DA24
autumn
393
104,679
0.9
3.8
-3.7
-0.5
0
0
0.1
0.4
1.0
1.8
2.9
4.9
7.9
296.1
290999007
N.sites = number of sites; N.obs = number of observations; SD = standard deviation; min = minimum; pi, p5, plO, p25,
p50, p90, p95, p98, p99 = 1st, 5th, 10th, 25th, 50th, 90th, 95th, 98th, 99th percentiles; max = maximum; max.site = AQS ID
number for the monitoring site corresponding to the observation in the max column, winter = December/January/February;
spring = March/April/May; summer = June/July/August; autumn = September/October/November.
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Figure 5 below shows a map of the 1-hour SO2 design values at U.S. ambient air monitoring sites based on data the
2019-2021 period. There were only 15 sites with design values exceeding the NAAQS. The maximum design value was 376
ppb at a monitoring site near a power plant in Missouri. The sites with design values exceeding the NAAQS in Hawaii are
due to recurring eruptions from the Kilauea volcano.
• 0 - 25 ppb (270 sites) O 51 - 75 ppb (19 sites) • 101 - 250 ppb (6 sites)
O 26 - 50 ppb (45 sites) O 76 - 100 ppb (7 sites) • 251 - 376 ppb (2 sites)
Figure 5: SO2 design values in ppb for the 2019-2021 period. Source: AQS.
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Figure 6 below shows a map of the site-level trends in the 1-hour SO2 design values at U.S. monitoring sites having valid
design value in at least 15 of the 20 3-year periods from 2000 through 2021. The trends were computed using the Thiel-Sen
estimator, and tests for significance were computed using the Mann-Kendall test. From these figures it is apparent that SO2
concentrations have been decreasing at nearly all sites in the U.S, and there were no sites with significant increasing trends.
V Decreasing < 3 ppb/yr (73 sites)
Figure 6: Site-level trends in 1-hour SO2 design values based on data from 2000 through 2021. Source: AQS, trends
computed using R statistical software.
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Figure 7 below shows the national trends in the 1-hour SO2 design values based on the 175 sites shown in Figure 6. The
national median of the design values has decreased by 90% from about 69 ppb in 2000 to about 7 ppb in 2021.
10th/90th Percentile DV
Median DV
S02 NAAQS Level
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