Acid Rain

The average pH of rainwater is 5.6 (pH levels are based on the concentrations of hydrogen ions with respect to pure water). Because water reacts with atmospheric carbon dioxide to form carbonic acid, rain is typically slightly acidic. Acid rain (or acid deposition), however, refers to atmospheric precipitates with pH values below that of rainwater. cid-causing contaminants may originate in locations far from the acid precipitation, and so this issue crosses boundaries, necessitating political solutions.

Acid deposition is linked to numerous negative environmental impacts, affecting both the natural and built environments. Soils become more acidified, rendering them less hospitable to certain plants. Higher acid levels in lakes lead to a reduction in fish populations. Highly acidic precipitation exacerbates the weathering of many buildings, which is a particular problem with historical edifices. Limestone, for example, is weathered and eroded by acid rain. Although natural causes of acid rain exist, most acid deposition results from the by-products of anthropogenic actions, such as discharges from industrial activity. Natural causes such as volcanic eruptions, thunderstorm activity and lightning, and fire can, however, affect the atmosphere, leading to acidic precipitation.

Trees damaged by acid rain and a bark beetle infestation loom above new growth in Germany's Bavarian Forest near Mount Lusen.

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[Page 2]Chemically, sulfur and nitrogen oxides produced by industrial operations enter the atmosphere. Among the largest sources of these oxides is the generation of electrical power, in which coal and oil are burned; vehicular traffic also contributes significantly. The consumption of fossil fuels in both cases sends oxides into the atmosphere. Sulfurous emissions, largely from the production of electricity, combine with atmospheric water and oxygen to produce sulfuric acid (H2SO4). Nitrogen oxides, typically from the combustion of petroleum-derived fuels, react similarly to create nitric acid (HNO3).

Although acid rain may stem from pollution from localized point sources, the ramifications extend regionally. As such, acid rain is truly a regional issue. Atmospheric circulation patterns allow for a relatively efficient transport of pollutants to locales far removed from the original source. For example, acid rain in Scandinavia is often a product of oxides generated in Germany. Similarly, the acid rain falling in the northeastern United States can trace some of its origins to industrial activity in the Midwestern states. In the midlatitudes, prevailing winds are westerly; thus, atmospheric currents can transport these pollutants east of their origins. Mitigation of this problem, then, requires regional understandings and cooperation.