Sulfur Dioxide (SO2)

views updated

Sulfur Dioxide (SO2)


Sulfur dioxide is a gas that enters the atmosphere as a result of burning fossil fuels and from volcanic eruptions. In the atmosphere, sulfur dioxide reacts with water vapor, oxygen, and nitrogen oxides to create acid rain. When rain is acidic, it causes considerable damage to forests and to the ecosystems of freshwater bodies. Where sulfur dioxide, water vapor, andnitrogenoxidesare mixedwithveryfine smoke or dust particles, a sulfate aerosol can be produced that influences cloud cover and regional temperature.

Historical Background and Scientific Foundations

Sulfur dioxide, a compound of one sulfur atom and two oxygen atoms (SO2), is a gas with a putrid odor. It is primarily emitted into the atmosphere from burning fossil fuels—with coal the main fossil fuel in use globally. Although a smaller percentage of nitrogen oxides are produced from burning coal than from gasoline and diesel fuel, the production from coal predominates. In the United States, two thirds of all sulfur dioxide emissions are from coal powered electricity plants.

In the United States, sulfur dioxide emissions from vehicles are greatly reduced by emission control standards. Low sulfur fuels are also being produced for use in vehicles in Europe and the United States.

Impacts and Issues

Sulfur dioxide emitted into the atmosphere in one region drifts with the prevailing winds to other regions, frequently beyond the country of origin. Where burning coal is used in power plants in large quantities to meet the growing demand for electricity, and in areas down-wind of the use areas, the problem of acid rain can become a serious international concern.

Since environmentally damaging acid rain is the result of sulfur dioxide reacting with oxygen, water vapor, and nitrogen oxides, there are regulations in place in most developed countries to try and eliminate the emissions of SO2. Filters are often used to remove pollutants.


AEROSOL: Particles of liquid or solid dispersed as a suspension in gas.

NITROGEN OXIDES: Compounds of nitrogen and oxygen such as those that collectively form from burning fossil fuels in vehicles.


The Acid Rain Program was established as part of the Clean Air Act Amendments of 1990. The goal of the program was to reduce the pollutants that cause acid rain, including sulfur dioxide. From 1990 to 1995, the first five years the amendments were in force, annual net emissions of sulfur dioxide by electric utility companies decreased by nearly 25%.

The program targeted electric utility companies, especially those that relied on coal-fired power plants. Sulfur dioxide emissions were permanently capped at 8.95 million tons, half the amount emitted by program targets in 1980. To reduce emissions, utility companies had the option of updating or closing older plants, switching to low-sulfur coal, or installing pollution-control devices called scrubbers. A 2002 study found that most utility companies had overwhelmingly chosen the least expensive options of meeting guidelines. For example, 55% of plants in the study chose to switch to burning lower sulfur coal at $116 per ton compared to 16% that installed scrubbers at $322 per ton. Although researchers acknowledge that cleaner coal-burning technology and initiatives like the Acid Rain Program have dramatically reduced sulfur dioxide emissions and acid rain, critics note that mandating more costly remediation methods like scrubbers and technologically current power plants could further reduce harmful emissions.

The effort to control sulfur dioxide emissions and reduce acid rain is a driving motivation for industrialized countries to change to cleaner fuels or “green” energy sources, such as wind and solar power for electricity. It is also an incentive to develop less-polluting hybrid vehicles for transportation.

See Also Acid Rain; Environmental Pollution; Montreal Protocol.


Web Sites

“Glossary of Climate Change Terms.” U.S. Environmental Protection Agency, November 13, 2006. <> (accessed August 14, 2007).

“What Is Acid Rain?” U.S. Environmental Protection Agency, June 8, 2007. <> (accessed August 14, 2007).