Acid Rain: Effects

Acid Rain, the wet or dry deposition of acidic substances and their precursors on the Earth's surface. Wet deposition refers to rain, snow, hail, drizzle and other familiar forms of visible precipitation. Dry deposition, mostly invisible, occurs through gravitational settling of large particles and uptake of gases and small particles at the Earth's surface. Rain and other precipitation may be defined as acidic or alkaline depending on chemical composition. The degree of acidity is usually measured on the pH scale, a logarithmic measure of the concentration of hydrogen ions (H+) in precipitation. On the pH scale, 7 represents a neutral solution. Acidic solutions have values below 7 and alkaline solutions have values above 7. For each change of one pH unit, the hydrogen ion content changes by a factor of 10. A clean water sample in equilibrium with atmospheric carbon dioxide will have a value of 5.6 and this is often used as a definition of "clean" rain. When values are different from this, it means that other substances, either natural or manmade, are present in the rain.

Current annual measurements of the average pH of precipitation in the northern hemisphere range from about 4.0 to 7.0. The lower, highly acidic values occur primarily over and immediately downwind of industrialized areas in northeastern North America and Europe. Higher pH values are found over less industrialized regions where the atmosphere contains larger amounts of alkaline dust. The primary cause of low pH in precipitation over northeastern North America is sulphuric acid (H₂SO₄) resulting from industrial and urban emissions of sulphur dioxide (SO₂). Nitric acid (HNO₃) generated from emissions of nitrogen oxides (NOx) is a significant contributing factor in this region. Current annual emissions of SO₂ amount to about 20 million t in the US and about 2 million t in Canada. Coal-fired thermal electric power plants produce about 70% of US emissions and about 20% of Canadian emissions. Nonferrous smelters, producing such metals as nickel and copper, account for about 50% of Canada's SO₂ emissions. The acid rain precursors, SO₂ and NOx, can be transported thousands of kilometres through the atmosphere, returning to earth as dry deposition or in wet acid form.

Acidity Scale The pH scale measures the degree of acidity (artwork by Michael Lee).

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