ACID RAIN

ACID RAIN, precipitation whose acidity has increased on account of some human activity. Dust storms, volcanic eruptions, and biological decay can affect the acid level of rain or snow, but industrial pollutants may raise the acidity of a region's precipitation more than tenfold. Certain pollutants mix with atmospheric water vapor to form acids, which may then fall to the ground in a process called dry disposition, or fall in combination with rain or snow, called wet disposition.

The term "acid rain" was coined in 1872 by Robert Angus Smith, an English chemist who studied the chemical content of rain near Manchester, England. In retrospect it is clear that U.S. cities such as Chicago, Pittsburgh, and St. Louis, heavy consumers of bituminous coal, also suffered from acid precipitation. Nevertheless the first large-scale effort to monitor the chemistry of precipitation did not occur until the late 1940s with the work of Hans Egner of Sweden. In the 1960s, European researchers began publicizing the effects of acidic precipitation on soils, vegetation, aquatic ecology, and human-made structures. In the 1970s, the discovery that several Canadian lakes had high acid levels (pH levels between 4 and 5) increased public awareness of the issue. By that time problems as diverse as crumbling monuments, fish kills, and dying forests were linked to acid precipitation.

The acid rain issue transcends political boundaries. Power plants in the Midwest of the United States, for example, may create acid rain that falls to the ground in eastern Canada. Indeed pressure from Canada, Sweden, and Norway, net receivers of atmospheric sulfur dioxide, led to a series of international acid rain conferences beginning in 1979. Acid rain debates seriously strained relations between the United States and Canada in the 1980s. The Canadian government expressed anger when the Ronald Reagan administration deferred action pending further study of the issue.

Efforts to abate acid rain have focused on two pollutants, sulfur dioxide, a by-product of burning coal or fuel oil, and nitrogen oxides generated largely by automobiles and power plants. In the United States the federal CLEAN AIR ACT (CAA) of 1970 restricted both pollutants. However, acid rain was not the motivating factor behind the CAA, and studies later suggested the law may have worsened the problem. When monitoring devices were

placed near factories, many firms simply built taller smokestacks to disperse pollutants higher into the atmosphere, away from the monitors. Consequently acid rain spread even wider. In 1977 amendments to the CAA required that utilities install scrubbers in each new coal-fired power plant. Implementation of these and additional amendments in the 1980s are credited for decreasing annual sulfur dioxide emissions in the United States from 26 to 21 million metric tons by 1989. Similarly nitrogen oxide emissions, which peaked at 22 million metric tons in 1981, fell to 19 million tons by 1990.

In 1990 additional amendments to the CAA imposed stricter AIR POLLUTION standards on vehicles and set a cap on national sulfur emissions governed by a market-based system of emission allowances. These regulations, along with a provision allowing eastern utilities to use more low-sulfur western coal, apparently helped reduce acid precipitation in the Northeast by up to 25 percent. But acidified water and soil continued to imperil lake and forest ecosystems. A major study sponsored by the U.S. ENVIRONMENTAL PROTECTION AGENCY released in late 1999 found that sulfate levels had fallen sharply in most lakes of the Northeast and the Midwest, but that acidity levels had not fallen along with them, perhaps because prolonged acid precipitation had weakened the lakes' natural buffering capacity.

ACID RAIN

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