A colorless gas consisting of a single sulfur atom and two oxygenatoms. Sulfur dioxide is a major primary pollutant in the atmosphere originating mostly from coal fired power plants and otherfossil fuels combustion. In the atmosphere, sulfur dioxide is usually oxidized by ozone and hydrogen peroxide to form sulfur trioxide, a secondary pollutant. Sulfur trioxide, similar to sulfurdioxide, is extremely soluble in water. If these sulfur oxides are present in the atmosphere when condensation occurs, dropletsof sulfuric acid (acid rain) are formed. Volcanic eruptions providea natural source of sulfur dioxide in the atmosphere. Howeverthe real problem associated with the production of sulfuric acidin the atmosphere is not with the sulfur dioxide expelled by volcanoes. Anthropogenic production of sulfur dioxide, caused bythe burning of fossil fuels, is largely responsible for damagecaused by acid rain.