What are "Sulfur Oxides" (SOx)?

Fossil fuels like coal and oil contain sulfur. When fossil fuels are burned, the sulfur forms oxides of sulfur. Of the sulfur compounds emitted, sulfur dioxide is the most important. A colorless gas, it can be detected by taste and odor in the concentration range of 0.3 to 1 ppm. Above 3 ppm, it has a pungent, irritating odor.

After SO2 enters the atmosphere it is chemically transformed by a variety of gas and liquid phase reactions.

Where does it come from?

A large amount of sulfur dioxide is emitted from automobiles, power plants, oil refineries, foundries and steel mills that use coal or oil as fuel. Sulfur oxides are produced when fossil fuels containing appreciable inorganic sufides and organic sulfur are combusted.

In the northeastern US, prevailing winds carry acid rainfall that originates from the midwestern states. Electric power plants that burn high-sulfur coal are responsible for roughly 60-70% of these emissions.

How is our health impacted?

Sulfur dioxide and other oxides of sulfur combine with oxygen to form sulfates and with water vapor to form aerosols of sulfurous and sulfuric acid. These acid mists can irritate the respiratory systems of humans and animals and injure plants. Sensitive populations include asthmatics, individuals with bronchitis or emphysema, children, and the elderly. SO2 produces leaf damage to trees and agricultural crops. Particulate sulfates also reduce visibility.

Are there regulations to protect us?

There are three National Ambient Air Quality Standards for SO2:

Primary standards:

an annual arithmetic mean of 80 micrograms per cubic meter of air (µg/m3), a 24-hour level of 365 µg/m3 and

Secondary standards:

a 3-hour level of 1300 µg/m3.

For an air basin to be classified as having attained the SO2 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. Flue gas scrubbers are required for all new or modified coal-fired utility boilers.

References

Godish, Thad. 1988. Air Quality.

Lewis Publishers, Inc. Chelsea, MI

Legge, A., S. Krupa. 1990. Acidic Deposition:Sulphur and Nitrogen Oxides.

Lewis Publishers, Inc. Chelsea, MI.

Swiss, Martha. 1991. Air Pollution. Air & Waste Management Association.

Pittsburgh, PA.

US Dept. Of Health and Human Services.

1994 NIOSH Pocket Guide to Chemical Hazards.

Hazardous Materials Publishing. Kutztown, PA.