Sulfur Dioxide - This is a colorless gas consisting of a single sulfur atom and two oxygen atoms. Sulfur dioxide is a major primary pollutant in the atmosphere originating mostly from coal fired power plants and other fossil fuels combustion. In the atmosphere, sulfur dioxide is usually oxidized by ozone and hydrogen peroxide to form sulfur trioxide, a secondary pollutant. Sulfur trioxide, similar to sulfur dioxide, is extremely soluble in water. If these sulfur oxides are present in the atmosphere when condensation occurs, droplets of sulfuric acid (acid rain) are formed. Volcanic eruptions provide a natural source of sulfur dioxide in the atmosphere. However the real problem associated with the production of sulfuric acid in the atmosphere is not with the sulfur dioxide expelled by volcanoes. Anthropogenic production of sulfur dioxide, caused by the burning of fossil fuels, is largely responsible for damage caused by acid rain.
[Science; v265; 497-9; 1994.]
[Nature; v366; 327-9; 1993.] 
Atmospheric Chemistry Glossary

Диоксид серы образуется при сгорании серы или обжиге серной руды в воздухе.
http://www.epa.gov/oar/aqtrnd95/s02.html

Six Principal Pollutants - Sulfur Dioxide (SO₂)

Nature and Sources of the Pollutant: Sulfur dioxide belongs to the family of gases called sulfur oxides (SO_x ). These gases are formed when fuel containing sulfur (mainly coal and oil) is burned, and during metal smelting and other industrial processes.

Health and Environmental Effects: The major health concerns associated with exposure to high concentrations of SO2 include effects on breathing, respiratory illness, alterations in pulmonary defenses, and aggravation of existing cardiovascular disease. Children, the elderly, and people with asthma, cardiovascular disease or chronic lung disease (such as bronchitis or emphysema), are most susceptible to adverse health effects associated with exposure to SO₂ . EPA's health-based national air quality standard for SO₂ is 0.03 ppm (measured on an annual arithmetic mean concentration) and 0.14 ppm (measured over 24 hours). SO₂ is a precursor to sulfates, which are associated with acidification of lakes and streams, accelerated corrosion of buildings and monuments, reduced visibility, and adverse health effects.

Trends in SO₂ Levels: Between 1986 and 1995, national SO2 concentrations decreased 37 percent and SO₂ emissions decreased 18 percent. Between 1994 and 1995, national SO₂ concentrations decreased 17 percent and SO₂ emissions decreased 13 percent. These significant decreases in concentrations and emissions reflect the success of the first year of the Acid Rain Program. While national SO₂ air quality levels have improved, EPA remains concerned about short-term peak SO₂ concentrations. As a result, EPA proposed a program for States, industry, and communities to use in evaluating and addressing peak concentrations that could occur near some industrial sources.

High concentrations of sulfur dioxide (SO₂) affect breathing and may aggravate existing respiratory and cardiovascular disease. Sensitive populations include asthmatics, individuals with bronchitis or emphysema, children and the elderly. SO₂ is also a primary contributor to acid deposition, or acid rain, which causes acidification of lakes and streams and can damage trees, crops, historic buildings and statues. In addition, sulfur compounds in the air contribute to visibility impairment in large parts of the country. This is especially noticeable in national parks.

Ambient SO₂ results largely from stationary sources such as coal and oil combustion, steel mills, refineries, pulp and paper mills and from nonferrous smelters. There are three NAAQS for SO₂:

• an annual arithmetic mean of 0.03 ppm (80 ug/m³);
• a 24-hour level of 0.14 ppm (365 ug/m³); and
• a 3-hour level of 0.50 ppm (1300 ug/m³).

The first two standards are primary (health-related) standards, while the 3-hour NAAQS is a secondary (welfare-related) standard. The annual mean standard is not to be exceeded, while the short-term standards are not to be exceeded more than once per year.

Title 40, Part 50 of the Code of the Federal Regulations lists the ambient air quality standard for sulfur dioxide.