Acid Rain

Acid rain

Acid rain is rain with a pH (a logarithmic measurement of acidity or alkalinity) of less than 5.7. Acid rain usually results from elevated levels of nitric and sulfuric acids in air pollution. Acidic pollutants that can lead to acid rain are common by-products from burning fossil fuels (e.g., oil, coal , etc.) and are found in high levels in exhaust from internal combustion engines (e.g., automobile exhaust). Acidic precipitation may also occur in other forms such as snow.

Acid rain occurs when polluted gasses become trapped in clouds . The clouds may drift for hundreds, even thousands, of miles before finally releasing acidic precipitation. Trees, lakes , animals, and even buildings are vulnerable to the slow corrosive effects of acid rain, whose damaging components are emitted by power plants and factories, especially those burning low grades of coal and oil.

Acid rain was first recognized in 1872, approximately 100 years after the start of the Industrial Revolution in England, when an English scientist, Robert Angus Smith (1817–1884), pointed out the problem. Almost another century passed, however, before the public became aware of the damaging effects of acid rain. In 1962, the Swedish scientist Svante Oden brought the acid rain quandary to the attention of the press, instead of the less popular scientific journals. He compiled records from the 1950s indicating that acid rain came from air masses moving out of central and western Europe into Scandinavia.

After acid rain was discovered in Europe, scientists began measuring the acidity of rain in North America . Initially, they found that the problem was concentrated in the northeastern states of New York and Pennsylvania because the type of coal burned there was more sulfuric. By 1980, most of the states east of the Mississippi, as well as southeastern Canada, were receiving acidic rainfall. Acid rain falls in the West also, although the problem is not as severe. Acid rain in Los Angeles, California is caused primarily by local traffic emissions. Car emissions contain nitrogen oxide, the second highest problematic gas in acid rain after sulfur dioxide.

Acid rain is measured through pH tests that determine the concentration of hydrogen ions. Pure water has a neutral pH of approximately 7.0. When the pH is greater than 7, the material is thought to be alkaline. At a pH of 5.7, rain is slightly acidic, but when its pH is further reduced, the rain becomes an increasingly stronger acid rain. In the worst cases, acid rain has shown a pH of 2.4 (about as acidic as vinegar). When pH levels are drastically tipped in soil and water, entire lakes and forests are jeopardized. Evergreen trees in high elevations are especially vulnerable. Although the acid rain itself does not kill the trees, it makes them more susceptible to other dangers. High acid levels in soil causes leaching of other valuable minerals such as calcium, magnesium, and potassium. According to the World Watch Institute, in the late 1980s and early 1990s forest damage in Europe ranged from a low of 4% in Portugal to a high of 71% in Czechoslovakia, averaging 35% overall.

Small marine organisms cannot survive in acidic lakes and rivers , and their depletion affects larger fish and ultimately the entire marine life food chain. Snow from acid rain is also damaging; snowmelt has been known to cause massive, instant death for many kinds of fish. Some lakes in Scandinavia, for example, are completely devoid of fish. Acid rain also eats away at buildings and metal structures. From the Acropolis in Greece to Renaissance buildings in Italy, ancient structures are showing signs of slow corrosion from acid rain. In some industrialized parts of Poland, trains cannot exceed 40 miles (65 km) per hour because the iron railway tracks have been weakened from acidic air pollution.

New power plants in the United States are being built with strict emissions standards, but retrofitting older plants is difficult and expensive. Nevertheless, the United States Environmental Protection Agency requires most of the older and dirtier power plants to install electrostatic precipitators and baghouse filters—devices designed to remove solid particulates. Such devices are required in Canada, in industrialized countries in Western Europe, and in Japan. Scrubbers, or flue-gas desulfurization technology, are also being used because of their effectiveness in removing as much as 95% of a power plant's sulfur dioxide emissions. These devices are expensive, however, and there are clauses in pollution control laws that allow older plants to continue operation at higher pollution levels. Another way to reduce acid rain is for power plants to burn cleaner coal in their plants. This does not require retrofitting but it does increase transportation costs since coal containing less sulfur is mined in the western part of the United States, far away from where it is needed in the midwest and eastern part of the country.

See also Atmospheric pollution; Erosion; Global warming; Groundwater; Petroleum, economic uses of; Rate factors in geologic processes; Weathering and weathering series