The Causes And Forms Of Acid Rain
The principal chemicals that produce acid rain are sulphur dioxide (SO2), nitrogen oxides (NOx) including nitric oxide and nitrogen dioxide. Natural as well as human activities are responsible for the production of these atmospheric pollutants. Natural processes include bacterial action in soils, volcanic eruptions and degassing from oceanic plankton. SO2 from volcanic eruptions can significantly affect the Earth's atmospheric chemistry and climate.
The noxious effects of pollution caused by human activity, particularly the burning of coal, have been recognised for centuries, although the association with acid rain was not realised until the nineteenth century. Most atmospheric pollutants are linked to industrial processes, for example smoke from factories, SO2 and NOx from power stations, pollen from agricultural activities, dust from building construction and asbestos from insulating material. Of these the emissions from factories and power stations have been linked to acid rain. Approximately 120 Mt of S02 is emitted annually around the globe from these principal sources and other industries using oil and coal as fossil fuels. The effects on the environment are amplified as a result of the traditional concentration of industry and power production in certain areas. For example, the concentration of power stations along the Trent valley in Britain has resulted in an annual emission of some 600 000 tonnes of S02 from this area alone. Other notorious areas for S02 emissions include the eastern parts of Germany (4 Mt S02 per year), and Poland (4.9 Mt S02 per year).
The chemical reactions that lead to acid rain begin as energy from sunlight in the form of photons, which hit ozone molecules (O3) to form free oxygen (O2) and single reactive oxygen atoms. These oxygen atoms react with water molecules (H20) to produce electrically charged, negative, hydroxyl radicals (HO). It is these hydroxyl radicals that are responsible for oxidising sulphur dioxide and nitrogen dioxide to produce sulphuric acid and nitric acid. Additionally, the formation of nitric acid can trigger further reactions, which release new hydroxyl radicals to generate more sulphuric acid. The usual way in which these acids reach the ground is in water droplets from clouds as rain. However, sulphuric acid can condense to form microscopic droplets that contribute to the many-recorded acid hazes, mists and fogs experienced over urban districts. Some of the particles will settle to the ground or vegetation can absorb some of the S02 gas directly from the atmosphere. The latter process has been described as dry deposition as distinct from true acid rain or wet deposition.
