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acid rain

  The deposition of sulphuric and nitric acids onto land or water by rainwater. Acid rain is one form of acid precipitation, which also includes acid snow, acid hail, dry deposition and acid fog condensation. On a pH scale of 14, a substance with a pH value of less than 7 is considered acidic, while a pH value greater than 7 is considered alkaline. Rainwater is naturally slightly acidic with a pH value of about 5.6 due to the formation of weak carbonic acids. Acid rain generally has an average pH range of 3-5, although higher levels of acidity have been recorded. Acidity is greatest near the base of clouds, and is diluted by a factor of 0.5 to 1 pH during rainfall (Pickering and Owen, 1994).

The English chemist R.A. Smith discovered a link between industrial pollution and acid rain in Manchester as early as 1852, although it was known in the twelfth century that burning coal caused air pollution (Turco, 1997). Although Smith first used the term acid rain in 1872, it is only since the late 1950s that Smith\'s ideas have been treated seriously. The concern expressed in Sweden, generated through the studies of soil scientist Svente Oden, focused attention on this international issue. In 1972 the Swedish Government presented its case at the United Nations Conference on the Human Environment in Stockholm. The term acid rain has been used extensively in recent decades.

Acid rain is caused primarily by the cumulative release of nitrogen and sulphur from the burning of fossil fuels. These are commonly in the form of coal for power, heating and industry, and petrol in automobiles. While acid rain may occur through natural processes such as volcanic activity, it is the cumulative impact of human activities that have caused a marked increase in acid rain over the past decades.

Acid rain is a major concern in western Europe and North America because of the higher generation rates there and because the process crosses international boundaries (see McCormick, 1997). It is a controversial process because acid rain deposition may occur several hundred kilometres from the source of pollution, especially when tall smokestacks are built to displace pollution from its source area. The areas most affected by acid rain tend to be downwind of dense concentrations of power stations, smelters and cities. They are often upland areas which receive high levels of precipitation. They also tend to be forest areas that are dissected by rivers and lakes (Park, 1987). Acid rain kills forests when acidic particles directly damage leaves and/or when the soil becomes acidified and the metals bound in the soil are freed; the nutrients necessary for plant growth are then leached by the water. Acid rain also lowers the pH value of lakes and other water bodies, which kills fish and other aquatic forms of life and may corrode buildings and other structures. (PM)

References McCormick, J. 1997: Acid earth: the politics of acid pollution, 3rd edn. London: Earth scan. Park, C. 1987: Acid rain: rhetoric and reality. London: Methuen. Pickering, K. and Owen, L. 1994: An introduction to global environmental issues. London and New York: Routledge. Turco, R. 1997: Earth under siege: from air pollution to global change. Oxford and New York: Oxford University Press.



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