Sulfur dioxide (SO2) is a colorless, reactive gas produced when sulfur-containing fuels such as coal and oil are burned. Major sources include power plants and industrial boilers. Generally, air concentrations of SO2 are highest near large fuel-burning industrial facilities.
According to EPA, the largest sources of SO2 emissions are from fossil fuel combustion at power plants (73%) and other industrial facilities (20%).
Smaller sources of SO2 emissions include industrial processes such as extracting metal from ore, and the burning of high sulfur containing fuels by locomotives, large ships, and non-road equipment. Recent federal measures to limit sulfur in fuels have reduced SO2 emissions from on-road and off-road mobile sources such as cars, trucks and construction equipment.
Sulfur Dioxide is part of a larger category of related chemicals, often referred to as 'sulfur oxides' or SOX.
The current primary National Ambient Air Quality Standard (NAAQS) for SO2 is 75 parts per billion (ppb) averaged over 1-hour. The secondary standard is 0.5 parts per million (ppm) averaged over 3 hours.
More information about Sulfur Dioxide can be found at: http://www.epa.gov/airquality/sulfurdioxide/index.html