Sulfur dioxide (SO2) | Applications

Sulfur dioxide is a heavy, pungent, colorless gas formed
primarily by the combustion of coal, oil, and diesel fuels.
Elevated levels can impair breathing, lead to other respiratory
symptoms, and at very high levels aggravate heart disease.
People with asthma are most at risk. Sulfur dioxide also
contributes to acid rain, which can damage plants, lakes and
buildings.

Glossary Term Category

General Glossary

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Characteristics: Sulfur dioxide is a colorless
gas that can be detected by taste and odor in a concentration as
low as 0.3 ppm. Above 3 ppm, SO2 has a pungent,
irritating odor similar to a struck match. SO2 is
found at appreciable levels in the lower atmosphere
(troposphere). Once in the atmosphere, SO2 is
oxidized to SO3. Due to a high affinity for water, SO3 is
rapidly converted to sulfuric acid (H2SO4), a colorless, mildly
corrosive acid. Sulfuric acid droplets and other sulfates
account for up to 20 percent of suspended particulate matter in
urban air. They can be transported long distances and return to
the earth as a major component of acid rain.

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Sources: A variety of sulfur compounds are
released to the atmosphere from both natural and anthropogenic
sources. Ninety-five percent of sulfur emitted to the air is
released as sulfur dioxide. SO2 is emitted mainly
from stationary sources that use fossil fuels containing sulfur
compounds (coal and oil). The major human sources of
SO2 are coal-burning electrical utilities, refineries,
and ore smelters. The amount of SO2 released depends
on the sulfur content of coal; high-sulfur coal may contain as
much as six percent sulfur by weight.

Concentration: Background levels of
SO2 in the ambient air are as low as 1 part per
billion (ppb). In urban areas, maximum hourly concentrations
have been reported as high as 1.5 to 2.3 ppm. Since the early
1970s and the implementation of significant SO2
control measures, the ambient SO2 concentration in
urban areas has been reduced significantly.

Health effects: In addition to having a bad
odor, high concentrations of sulfur dioxide can affect breathing,
cause respiratory illnesses and aggravate existing respiratory
and cardiovascular diseases. Because of its solubility,
SO2 is almost entirely removed from the mouth, throat,
and nose through breathing. Therefore, the principal effect of
SO2 is to alter the mechanical function of the upper
respiratory system. Exposure to SO2 at very low
levels (0.25 - 0.5 ppm) can cause bronchitis, and at
concentrations lower than 6 ppm can irritate the lungs and
throat. A long period of exposure to concentrations above 6 ppm
impairs the respiratory system's defenses against bacteria and
foreign particles.

The health consequences of sulfur dioxide are more serious when
combined with particulate matter and ozone. Since SO2
and particulate matter may come from a common source, such as the
combustion of coal, high SO2 levels are often
associated with high particulate-matter levels.

Populations most susceptible to the effects of SO2 are
children, the elderly, asthmatics and those with chronic lung
disease such as bronchitis and emphysema.

Ecological effects: The corrosiveness of
SO2 and its acidic derivatives is enough to damage
metals, stone, paint, leather, paper and various fibers, as well
as electrical equipment. SO2 is a major precursor to
acid deposition, and its transformation into sulfurous and
sulfuric acids contribute to acid rain. Acid rain has the
potential to acidify soils and lakes and damage plants and
aquatic life.

SO2 can also directly injure plants. Visible effects
include changes in leaf structure such as chlorosis (chlorophyll
destruction), necrosis (plant tissue death), and pigment
formation. Such visible symptoms are the result of both acute
exposure (short-term exposure to high levels) and chronic
exposure (long-term exposure to low levels). More subtle effects
will not cause visible injury but may include growth reduction
after multiple generations.

Different plant species and varieties vary in their sensitivity
to SO2. The most sensitive plants include pine,
legumes, red and black oak, white ash, alfalfa and blackberry.
Visible injury can occur when sensitive plants are exposed to
SO2 levels as low as 0.12 ppm for eight hours. Plants
with less sensitivity show similar effects when exposed to
SO2 at higher concentrations (0.30 ppm for eight
hours). Alfalfa is the most sensitive agricultural species, with
acute effects observed at a level of 1 ppm for one hour. Because
of its hypersensitivity, alfalfa has been used as a bio-indicator
of SO2 phytotoxicity (plant toxicity) in the ambient
atmosphere.

Trends in Sulfur Dioxide
(SO2)

Sulfur Dioxide Emissions

Sulfur dioxide (SO2) is a colorless gas with a heavy
pungent odor at high concentrations. SO2 is emitted
into the atmosphere from combustion of fuel with high sulfur
content, such as coal and oil. Particulate sulfate compounds can
be formed as well. Once emitted into the atmosphere,
SO2 can be further oxidized to sulfuric acid, a
component of acid rain. Other sources emitting SO2
include steel mills, pulp and paper mills, refineries and
non-ferrous smelters.

In 1994, statewide SO2 emissions were estimated at
about 142,000 tons. Figure 1 is a pie chart of the five major
source categories of 1994 SO2 emissions. This chart
shows that fuel combustion, mainly by electric utilities, was the
major contributor to SO2 emissions in Minnesota.
<ul>
<li>Fuel combustion accounted for an estimated 87 percent
(117,000 tons) of Minnesota's SO2 emissions (142,000
tons). Within this category, electric utilities were the
dominant source, accounting for about 62.3 percent (89,000 tons)
of total SO2 emissions. Ninety-nine percent of
electric utility emissions are attributed to coal combustion.

<li>The second-largest source was industrial fuel combustion,
which produced 17.4 percent (25,000 tons) of total
SO2 emissions. Coal and oil combustion are the major
contributors to SO2 emissions from industrial fuel
combustion.

<li>Fuel combustion activities other than industrial and electric
utilities (i.e., commercial and institutional coal and oil)
contributed about 6.6 percent (9,000 tons) of total
SO2 emissions.

<li>Manufacturing industries, including inorganic chemical
manufacturing, metal processing and petroleum and related
industries (refineries) contributed about nine percent (13,000
tons) of total SO2 emissions.

<li>Transportation, mainly highway vehicles, accounted for 4.7
percent (6,700 tons) of total SO2 emissions.
</ul>

In 1994, point source emissions constituted about 85 percent
(130,000 tons) of total SO2 emissions in Minnesota.

Figure 1. 1994 Minnesota SO2
emissions by principal source categories

Data derived from EPA's Office of Air Quality,
Planning and Standards, Research Triangle Park, NC.

Sulfur Dioxide Emission Trends 

From 1985 to 1994, total SO2 emissions in Minnesota
decreased by 7.3 percent, or 12,000 tons. Figure 2 shows that
153,000 tons of SO2 were emitted in 1985, compared to
142,000 tons in 1994. A comparison of emissions from each major
source shows that the reduction is due to lower emissions from
electric utilities and transportation sources (mainly highway
vehicles). In one year, between 1993 and 1994, total
SO2 emissions decreased by two percent.

From 1985 to 1994, SO2 emissions from electric
utilities decreased from 99,000 tons to 89,000 tons, a 10-percent
decline. Several factors contributed to this. First, EPA
developed stringent standards for new coal-fired utility boilers.
Secondly, the MPCA's 1985 acid deposition control plan--the
first of its kind in the United States--established limits on
release of SO2 from large electric generating plants.
Emissions from these and other sources have declined since 1985
and are well under limits set by the plan.

Finally, Title IV of the 1990 CAAA established a national cap and
trade system to reduce acid rain. This program focused on
emissions of SO2 and NOx from power plants. Emissions
of SO2 are to be reduced by about 10 million tons
annually (about 50 percent reduction from 1980 emissions levels).
These reductions are to be accomplished in two phases. Phase I
was implemented in 1995 and Phase II will be implemented in the
year 2000. For further information on this program, refer to the
MPCA publication entitled, Sulfur Dioxide Emission Credits:
Report to the Minnesota Legislature on the Market and Effect of
the Clean Air Act Emissions Credit Trading Program
(February, 1996).

Transportation, including highway vehicles and off-highway
sources, is another factor contributing to the drop in
SO2 emissions. In 1990, EPA published regulations
that govern desulfurization of diesel motor fuel., indicating
that after 1993, all major diesel motor vehicles had to meet a
sulfur standard of 0.05 percent by weight. EPA expects a 75%
reduction in SO2 emissions from diesel motor vehicles
as a result. As shown in Figure 7.2.3, SO2 emissions
from transportation have fallen from about 11,000 tons in 1985 to
6,700 tons in 1994, a 39 percent decline.

Over the same period, emissions from industrial fuel combustion,
other fuel combustion and manufacturing have risen by 5.6
percent, 0.7 percent, and 12 percent, respectively.

Figure 2. SO2 emission trends
in Minnesota: 1985-1994

Data derived EPA's Office of Air Quality,
Planning and Standards, Research Triangle Park, NC

Sulfur Dioxide Ambient Air Concentrations

Long-Term Sulfur Dioxide Concentration Trends: 
Sulfur dioxide is regulated under two primary (health-related)
national ambient air quality standards (NAAQS). The first is the
annual arithmetic mean of 0.03 ppm (80 mg/m3). The second is a
short-term, 24-hour standard where concentrations may not exceed
0.14 ppm (365 mg/m3) more than once a year. In order to protect
the public welfare and environment, the secondary national
ambient air-quality standard for sulfur dioxide was set at a
three-hour average concentration of 0.5 ppm (1300 mg/m3), not to
be exceeded more than once a year.

In addition to NAAQS for SO2, there are two unique
state standards: a three-hour standard and a one-hour standard,
both with an average concentration of 0.5 ppm (1300 mg/m3).
There are also three secondary state ambient air quality
standards. The first is an annual standard of an arithmetic mean
of 0.02 ppm (60 mg/m3); the second a 24-hour standard of 0.14 ppm
(365 mg/m3); and the third a three-hour standard with different
values for different Air Quality Control Regions (AQCRs). For
the AQCRs of 128, 131 and 133, the standard is 0.5 ppm (1300
mg/m3), which is the same as the secondary national standard.
For the AQCRs of 127, 129, 130 and 132, the three-hour standard
is 0.35 ppm (915 mg/m3). Standards for both one-hour and
three-hour averages are not to be exceeded more than once a year.

The ambient air concentration trends of SO2 are
compared to the two primary (health-related) NAAQS. Thus, in
addition to the annual average SO2 concentration, we
examine the second-highest daily (24-hour) average SO2
concentrations per year. If the annual average or the
second-highest values are greater than the standards, the site is
out of compliance for that year.

The 1984-1995 trends for annual average SO2
concentrations are shown in Figures 3. Trends for the
second-highest 24-hour SO2 concentrations are shown in
Figure 4. Six out of nine SO2 monitoring sites met
the criteria of data completeness for long-term trend analysis,
including: downtown/Minneapolis; Portland and 18th/ Minneapolis;
downtown/St. Paul; St. Paul Park; Pine Bend/Rosemount and
downtown/Rochester. Annual values at each site are connected
with a dashed line. The average of the annual values (by year,
across sites) is plotted as a heavy solid line. Horizontal lines
show the annual and 24-hour standards.

As shown in Figure3, the annual average at each site and the
average of the annual values were lower than state and national
standards. Similarly, the second-highest 24-hour values at each
site and their average were also lower than state and national
standards (Figure 4). For the entire time period, therefore, all
six sites were in compliance, including the Pine Bend and St.
Paul Park sites, which are located near refineries and report
substantially higher SO2 levels than other sites. As
illustrated in Figure 3 the average of the annual values
decreased 66 percent from 1984 to 1995, from 0.0066 ppm to 0.0022
ppm. From 1994 to 1995, the average of the annual values
decreased 27 percent, from .003 ppm to .0022 ppm. The average of
the second-highest daily values (Figure 4) decreased 39 percent
from 1984 to 1995, from 0.048 ppm to 0.029 ppm. From 1994 to
1995, the average of the second-highest daily values decreased by
12 percent, from 0.033 ppm to 0.029 ppm. In conclusion,
long-term trends in both ambient air concentrations and total
SO2, emissions in Minnesota indicate steady improvement.

Figure 3. SO2 trends for annual
average ambient air concentrations in Minnesota:
1984-1995

<img src="../../artwork/air/so2-fig3.gif" alt="so2-fig3.gif -
7.54 K" width=733 height=299 border=0>

Figure 4. SO2 trends for
second-highest 24-hour average ambient air concentrations in
Minnesota: 1984-995

<img src="../../artwork/air/so2-fig4.gif" alt="so2-fig4.gif -
7.68 K" width=783 height=346 border=0>

Summary: Long-term trends in Minnesota's
SO2 emissions and ambient air concentrations indicate
steady improvement. From 1985 to 1994, SO2 emissions
decreased by 7.3 percent, while ambient air concentrations of
SO2 decreased more than 66 percent. The differences
between concentrations and emission trends is likely the result
of the ambient monitoring site locations. Monitoring sites are
generally located in populated urban areas. However, fuel
combustion sources that comprise 87 percent of statewide
SO2 emissions tend to be located in less-populated areas.

Over the past several years, the number of SO2
nonattainment areas dropped. Since 1990, four areas have been
designated nonattainment: the Twin Cities, Pine Bend, Ashland,
and city of Rochester. Presently, only the city of Rochester and
Ashland are still nonattainment; EPA redesignated the status of
the Twin Cities and Pine Bend areas to "attainment." The MPCA
continues to work with EPA on redesignating the two remaining
nonattainment areas.

Continued progress in reducing ambient SO2
concentrations has been possible because new large utility plants
have installed sulfur-removal equipment; and utility, commercial,
residential and industrial users continue to shift to
lower-sulfur fuels. One additional factor contributing to lower
SO2 concentrations is the lower sulfur content in
today's diesel motor fuels.

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