According to EPA, the largest sources of SO 2 emissions are from fossil fuel combustion at power plants (73%) and other industrial facilities (20%).
Smaller sources of SO 2 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 SO 2 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 SO 2 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