Sulfur oxides (SOx) are compounds formed when materials containing sulfur reach elevated temperatures. These compounds are produced by volcanoes and in various industrial processes. Coal and oil contain a small amount of sulfur and when we burn them to make electricity or heat our homes, the sulfur combines with oxygen in the air to produce sulfur oxides.

The most abundant sulfur oxide gas is sulfur dioxide (SO₂), a colorless gas often described as “smelling of burning” sulfur. Another sulfur oxide gas is sulfur trioxide (SO₃). These compounds help contribute to air pollution and are, collectively, referred to as sulfur oxides.

	Two views of a molecule of sulfur dioxide. Sulfur is yellow and oxygen is red. SO3 would have three oxygen molecules. Picture from the following site.

Sulfur in Coal and Oil
Coal and oil, or fossil fuels, contain small amounts of sulfur. Fossil fuels were formed by ancient plants that layered the bottom of swamps when they died. They formed peat, which later turned to coal through compression of other materials on top of them. Burning, or combustion, of these materials produces sulfur oxides in proportion to their sulfur content. Most of the sulfur oxide emissions (about 95%) are SO₂. A portion of the SO₂ can then react with water in the atmosphere to form SO₃, one of the components in acid rain.

Although best known as an air pollutant, SO₂ is also used in several commercial applications. It is used as a preservative in alcoholic drinks and dried fruits – although it only preserves appearance and does not actually prevent rotting. It is also used in the production of sulfuric acid and as a bleaching agent for paper and cloth. Presently, millions of tons of SOx emissions are produced annually within the U.S., with a large portion originating from electric utilities. Other sulfur oxides-producing activities include refining petroleum, combustion of coal and oil for heat, and operating diesel equipment like trains and large ships.