Acid rain, or acidic precipitation, first attracted scientific attention in the mid 1950s in Scandinavia, with studies focused on acidity and surface waters. Concern in the United States, Europe, and Canada began to rise in the 1960s when researchers noticed that fish populations in remote wilderness lakes were declining for unknown reasons and forests were showing significant leaf damage. Scientists speculated that sulfuric and nitric acid falling to the earth as acid precipitation might be the cause. These acids form high in the clouds when sulfur dioxide (SO2) and nitrogen oxides (NOx)-acid rain precursors emitted by coal-fired electric utilities and other fossil fuel burning sources-react with water, oxygen, and sunlight. Sulfur dioxide (SO2), the main source of acid rain, is a pungent toxic gas produced when sulfur laden coal is burned. The acids formed in the clouds are brought to the earth through rain, snow, or fog, and can also fall directly as particles or gases in a process called dry deposition. Rainfall is naturally slightly acid because it dissolves some of the carbon dioxide found in the atmosphere and produces weak carbonic acid [CO2 + H20 = COOH+ + OH-]. However, the acid rain caused by SO2 and NOx is considered a significant pollution problem.
As public concern about air pollution increased, Congress passed the 1970 Clean Air Act, which required every new power plant that burned fossil fuels to install sulfur controls. As a result, SO2 emissions fell at a rate of 27% per year between 1970 and 1991, even though coal use nearly doubled. But passage of the 1970 law did not diminish public concerns about acid rain. Reports about dying forests and lakes that no longer supported fish continued to grab media and public attention. Canadian government officials stated that 42,000 lakes, mostly small ones, had died from cross border acid rain caused by power plants in the midwestern United States. In Germany, foresters used the term Waldsterben, or forest death, to describe the decline of forests in parts of Germany where prevailing winds blew pollution across the border from coal burning plants in the Eastern bloc.
In the United States, dire predictions were made about acid rain's effects upon the Adirondacks and the Blue Ridge Mountains, where many lakes were found to have a pH of 5.0, which is too acidic for trout and other fish. By the time Ronald Reagan became president in 1981, pressure was mounting from Canada and the U.S. Congress to initiate a major research effort, the National Acid Precipitation Assessment Program, which cost $540 million over 10 years to sample 7,000 lakes and hundreds of woodlands. But even before its results were in, Congress passed the Clean Air Act Amendments of 1990. Title IV of that law called for a 10 million ton reduction in annual SO2 emissions in the United States by the year 2010, an approximately 40% reduction in anthropogenic emissions from 1980 levels.
In 1991, the national assessment program concluded that "there is no evidence of a general or unusual decline of forests in the United States or Canada due to acid rain," and dangerous acidity was detected in only 4% of eastern lakes, not the 50% of lakes widely expected to show serious damage. However, episodic acidification could be possible in one out of every five lakes and streams in the United States, due to a low capacity to neutralize acids. In contrast to the national assessment, the National Surface Water Survey (NSWS) examined 1,000 lakes larger than ten acres and found that 75% of these lakes and 50% of the streams surveyed had been acidified by acid rain.
The national assessment also stated that tree mortality was found in 3% of eastern forests, with high altitude spruce trees suffering the worst effects. Acid rain precursors cause more than 50% of the visibility impairment or "haze" in the eastern United States, and 15% to 30% of haze in the West, the assessment also concluded. While acid rain precursors were being reduced in the United States and other countries, SO2 emissions in Asia were expected to triple from 1990 levels by the year 2010 if the current coal and oil use trends continued.
Aside from acidifying water bodies, damaging forests, and contributing to haze, acid rain is linked to the corrosion of statues and monuments and to other material damages. Sulfate aerosols associated with SO2 emissions are also linked to chronic bronchitis, asthma, and other respiratory health effects.
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