Acid Rain: Biological Effects and Implications

INTRODUCTION Because of the controversy over air pollution that has erupted in newspapers and magazines during the past several years, most people are aware of the potential hazards associated with air pollutants such as sulfur dioxide, nitrogen oxides, ozone, etc. However, there is another, relatively unknown, type of air pollution which is just as serious, if not more serious, than anything of which the public is aware. This pollution goes by the name of "acid rain." Acidity of rainwater is measured in terms of units called pH units. Neutral water, neither acid nor alkaline, has a pH of 7.0. Water of pH 6.0 is ten times as acid as neutral water; water of pH 5.0 is one hundred times as acid as neutral water; and so forth. Each decrease of one pH unit represents a tenfold increase in acidity over the next higher pH value. If neutral water, with a pH of 7, is brought into contact with the atmosphere, it begins to absorb carbon dioxide. The dissolution of carbon dioxide in water produces a weak acid called carbonic acid. Water which has absorbed all of the carbon dioxide it can hold at normal atmospheric temperature and pressure has a pH of about 5.7. And in fact, rainwater collected in areas where pollution is negligible can be expected to have a pH of about this value. In some parts of the world, however, researchers have discovered that rainwater is more acidic than is to be expected from the dissolution of carbon dioxide alone. In the Scandinavian countries and in the northeastern portion of the United States, rainwater consistently has a pH in the range of 3.0 to 4.0. 1 In the United States, values as low as 2.1 have been recorded. 2 The primary component of acid rain appears to be sulfuric acid, formed from sulfur dioxide by a series of complex photochemical and catalytic reactions. 3 Recent studies 4 indicate that nitric acid, formed from the oxidation of nitrogen oxides by ozone, may also be 745 746 ENVIRONMENTAL AFFAIRS significant. Other acids (such as hydrofluoric) may be locally significant if there is an emission source in the area. The acid rain in Scandinavia probably results from industrial emissions in Great Britain and industrialized regions of Europe, 5 while acid rain in New England results from emissions in the heavily industrialized eastern portion of this country. 8 One obvious consequence …