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Sulfur dioxide (SO2), a colorless, reactive gas, is
produced during the burning of sulfur-containing fuels such as coal
and oil, during metal smelting, and by other industrial processes.
It belongs to a family of gases called sulfur oxides (SOs). Major
sources include power plants, industrial boilers, petroleum refineries,
smelters, iron and steel mills. Generally, the highest concentrations
of sulfur dioxide are found near large fuel combustion sources.
Acid deposition or "acid rain" occurs when SO2
and oxides of nitrogen (NOx) react with water, oxygen, and oxidants
to form acidic compounds. It is deposited in dry form (gas, articles)
or wet form (rain, snow, fog), and can be carried by wind hundreds
of miles across state and national borders. Acid rain harms lakes
and streams, damages trees, crops, historic buildings, and monuments.
The estimated nationwide emissions of SO2 in 1999 were
18.9 million short tons.
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