Sulfur Dioxide

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Sulfur dioxide (SO2) is a gas composed of sulfur and oxygen. The chemical formula SOx is used collectively to describe sulfur oxide, SO2, and other sulfur oxides.

emissions and sources. One of the primary sources of sulfur dioxide is the combustion of fossil fuels containing sulfur. Coal (particularly high-sulfur coal common to the eastern United States) and oil are the major fuel sources associated with SO2. Power plants have historically been the main source of SO2 emissions. Some industrial processes and metal smelting also cause SO2 to form.

From 1940 to 1970 SO2 emissions increased as a result of the growing use of fossil fuels, especially coal, in industry and power plants. Since 1970 total SO2 emissions have dropped because of greater reliance on cleaner fuels with lower sulfur content and the increased use of pollution control devices, such as scrubbers, to clean emissions. Between 1970 and 2004, SO2 emissions decreased by 51%, according to the EPA. (See Table 2.2.)

Fuel combustion in power plants has traditionally produced the majority of sulfur dioxide emissions. In 2003 this source accounted for 68% of SO2 emissions.

Sulfur dioxide (SO2) air quality, 1983-2002

[Based on annual arithmetic average]

Sulfur dioxide (SO2) air quality, 1983-2002

[Based on annual arithmetic average]

1983-02: 54% decrease 1993-02: 39% decrease

*National Ambient Air Quality Standards source: "SO2 Air Quality, 1983-2002," in Latest Findings on National Air Quality: 2002 Status and Trends, U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, Washington, DC, August 2003

Air pollutants, health risks, and contributing sources


Health risks

Contributing sources

Particulate matter

(PM-10) Carbon monoxide (CO)

Sulphur dioxide

Nitrogen dioxide (NO2)

Asthma, reduced respiratory function, eye irritation Bronchitis, cancer, lung damage

Blood oxygen carrying capacity reduction, cardiovascular and nervous system impairments Respiratory tract impairment, destruction of lung tissue Retardation and brain damage, esp. children

Lung damage and respiratory illness

Cars, refineries, dry cleaners Dust, pesticides

Cars, power plants, wood stoves

Power plants, paper mills Cars, nonferrous smelters, battery plants Power plants, cars, trucks

*Ozone refers to tropospheric ozone, which is hazardous to human health.

source: Fred Seitz and Christine Plepys, "Table 1. Criteria Air Pollutants, Health Risks, and Sources," in Monitoring Air Quality in Healthy People 2000, Healthy People 2000—Statistical Notes, Centers for Disease Control and Prevention, National Center for Health Statistics, Hyattsville, MD, September 1995

Fuel combustion at industrial facilities contributed another 14%. All other sources were minor contributors (

air quality. Trends in air quality concentrations of SO2 are shown in Figure 2.12. The average concentration fell by 54% between 1983 and 2002. As of April 2005 the EPA reported seventeen nonattainment areas around the country for SO2. These included locations in Montana, Utah, New Jersey, West Virginia, Pennsylvania, Kentucky, Ohio, Arizona, and Indiana.

SO2 is a major contributor to acid rain, haze, and particulate matter. Acid rain is of particular concern because acid deposition harms aquatic life by lowering the pH (level of acidity; a lower value indicates more acid) of surface waters, impairs the growth of forests, causes depletion of natural soil nutrients, and corrodes buildings, cars, and monuments. Acid rain is largely associated with the eastern United States because eastern coal tends to be higher in sulfur content than coal mined in the western United States.

In 1990 the U.S. Congress established the Acid Rain Program under Title IV of the 1990 Clean Air Act Amendments. The program called for major reductions in SO2 and NOx emissions from certain coal-fired power plants and other combustion units generating electricity around the country. The program set two emissions goals:

• Reduce SO2 emissions by approximately half by 2010 compared to the emissions released in 1980 (i.e., from 17.3 million tons per year to 8.95 million tons per year) and maintain a cap of 8.95 million tons per year after 2010

• Achieve a two million ton reduction in NOx emissions compared to the NOx emissions projected for 2000 if the program had not been implemented (i.e., from 8.1 million tons per year to 6.1 million tons per year)

The program expects to meets its goals by tightening annual emission limits on thousands of power plants around the country.

adverse health effects. Inhaling sulfur dioxide in polluted air can impair breathing in those with asthma or even in healthy adults who are active outdoors. As with other air pollutants, children, the elderly, and those with preexisting respiratory and cardiovascular diseases and conditions are most susceptible to adverse effects from breathing this gas. Table 2.5 summarizes the main sources and health risks of all the priority pollutants.

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