One of the most serious environmental problems facing many regions of the world is acid rain. This generic term covers a variety of phenomena, including acid fog and acid snow, all of which correspond to atmospheric precipitation of substantial acidity. In this section, the nature of the acids present in precipitation is discussed.
The phenomenon of acid rain was discovered by Angus Smith in Great Britain in the mid-1800s, but then it was essentially forgotten until the 1950s. It refers to precipitation that is significantly more acidic than "natural" (i.e., unpolluted) rain, which itself is often mildly acidic due to the presence in it of dissolved atmospheric carbon dioxide, which forms carbonic acid, H2C03:
C02(g) + H20(aq)^H2C03(aq)
The weak acid H2CO3 then partially ionizes to release a hydrogen ion, H+, with a resultant reduction in the pH of the system:
H2C03(aq) ^^ H+ + HC03~
Because of this source of acidity, the pH of unpolluted, "natural" rain is about 5.6 (see Problem 3-10). Only rain that is appreciably more acidic than this— that is, with a pH of less than 5—is considered to be truly "acid" rain since, because of natural trace amounts of strong acids, the acidity level of rain in clean air can be a little greater than that due to carbon dioxide alone. Strong acids such as hydrochloric acid, HC1, produced by emissions of hydrogen chloride gas by volcanic eruptions, can produce "natural" acid rain temporarily in regions such as Alaska and New Zealand. On the other hand, the pH of unpolluted rain may be somewhat greater than 5.6 due to the presence of weakly basic substances originating with airborne soil particles that have partially dissolved in the droplets.
The two predominant acids in acid rain are sulfuric acid, H2S04, and nitric acid, HN03, both of which are strong acids. Generally speaking, acid rain is precipitated far downwind from the source of the primary pollutants, namely sulfur dioxide, S02, and nitric oxide, NO. The strong acids are created during the transport of the air mass that contains the primary pollutants.
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