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Sulfur Dioxide - Acid Rain

Sulfur dioxide (SO2) is a colorless gas, belonging to the family of gases called sulfur oxides (SOx). It reacts on the surface of a variety of airborne solid particles, is soluble in water and can be oxidized within airborne water droplets. Sulfur dioxide, a product of the combustion of sulfur containing fossil fuels, plays an important role in atmospheric corrosion in urban and industrial type atmospheres. It is adsorbed on metal surfaces, has a high solubility in water and tends to form sulfuric acid (acid rain) in the presence of moisture films. Sulfate ions are formed in the surface moisture layer by the oxidation of sulfur dioxide and their formation is considered to be the main corrosion accelerating effect from sulfur dioxide.

Sulfur dioxide may be expressed either in terms of a deposition rate or an airborne concentration. Either measure is to be made in accordance with ISO 9225. The units used for the sulfur dioxide categories in the ISO 9223 are as sulfate deposition (SD) rate in mg m-2 day-1.

SD <= 10


11 < SD <= 35


36 < SD <= 80


81 < SD <= 200


Natural sources of sulfur dioxide include releases from volcanoes, oceans, biological decay and forest fires. The most important man-made sources of sulfur dioxide are fossil fuel combustion, smelting, manufacture of sulfuric acid, conversion of wood pulp to paper, incineration of refuse and production of elemental sulfur.

Coal burning is the single largest man-made source of sulfur dioxide accounting for about 50% of annual global emissions, with oil burning accounting for a further 25 to 30%. The major health concerns associated with exposure to high concentrations of sulfur dioxide include effects on breathing, respiratory illness, alterations in pulmonary defenses, and aggravation of existing cardiovascular disease. In the atmosphere, sulfur dioxide mixes with water vapor producing sulfuric acid. This acidic pollution can be transported by wind over many hundreds of miles, and deposited as acid rain.

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