Formation of Acid Rain

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The process that leads to acid rain begins with the burning of fossil fuels. Burning, or combustion, is a chemical reaction in which oxygen from the air combines with carbon, nitrogen, sulfur, and other elements in the substance being burned. The new compounds formed are gases called oxides. When sulfur and nitrogen are present in the fuel, their reaction with oxygen yields sulfur dioxide and various nitrogen oxide compounds. Nitrogen oxides enter the atmosphere from many sources, with motor vehicles emitting the largest share.

Once in the atmosphere, sulfur dioxide and nitrogen oxides undergo complex reactions with water vapor and other chemicals to yield sulfuric acid, nitric acid, and other pollutants called nitrates and sulfates. The acid compounds are carried by air currents and the wind, sometimes over long distances. When clouds or fog form in acid-laden air, they too are acidic, and so is the rain or snow that falls from them.

Acid pollutants also occur as dry particles and as gases, which may reach the ground without the help of water. When these “dry” acids are washed from ground surfaces by rain, they add to the acids in the rain itself to produce a still more corrosive solution. The combination of acid rain and dry acids is known as acid deposition.

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