SO₂ This is a colourless gas consisting of a single sulphur atom and two oxygen atoms. sulphur dioxide is a major primary pollutant in the atmosphere originating mostly from coal fired power plants and other fossil fuels combustion. In the atmosphere, sulphur dioxide is usually oxidised by ozone and hydrogen peroxide to form sulphur trioxide, a secondary pollutant. sulphur trioxide, similar to sulphur dioxide, is extremely soluble in water. If these sulphur oxides are present in the atmosphere when condensation occurs, droplets of sulfuric acid (acid rain) are formed. Volcanic eruptions provide a natural source of sulphur dioxide in the atmosphere. However the real problem associated with the production of sulfuric acid in the atmosphere is not with the sulphur dioxide expelled by volcanoes. Anthropogenic production of sulphur dioxide, caused by the burning of fossil fuels, is largely responsible for damage caused by acid rain.