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Precipitation comprises all solid and liquid forms of water that are deposited on the Earth's surface from the atmosphere. It includes rain, snow, hail, dew and sleet. All forms of precipitation are acid in so far as they have a pH of less than 7; in general, precipitation unaffected by human activity has a pH of 5.6. This naturally acidic state of precipitation is caused by the combination of water and carbon dioxide in the atmosphere to produce carbonic acid. However, the term acid precipitation, or acid rain, is usually applied to precipitation characterized by a pH of less than 5.1 (Elsworth 1984) and that contains sulphurous and nitrous acids. The latter are derived from various sources, among which fossil fuels are the most important.

The phenomenon of acid precipitation was first recognized by Robert Angus Smith, a Scottish chemist, in 1852 following a survey of air pollution in Manchester. Smith coined the term 'acid rain', which he associated with sulphur dioxide emissions from fossil fuels burned in local factories. Various observers subsequently noted the impact of acid precipitation on aquatic and terrestrial ecosystems. For example, Gorham (1958) noted that the chemistry of upland lakes in the English Lake District was affected by acid precipitation from air masses that had passed over Britain's industrial heartland. Despite this recognition of its impact, acid precipitation did not emerge as a major environmental issue until the late 1960s. By this time, Scandinavian ecologists were becoming concerned about declining fish stocks; they were also beginning to recognize transboundary transportation of acid precipitation, i.e. the export of acid precipitation from source areas such as the industrial regions of Europe and the UK and its transport to and deposition in far distant areas such as Scandinavia. In this context, acid precipitation became a political as well as an ecological issue. The polluters were unwilling to recognize this, and the polluted demanded mitigation measures. The impact of acid precipitation manifests in many ways. Both terrestrial and aquatic ecosystems may be adversely affected through reductions in pH, which have repercussions for the biota and water quality; human health may be impaired and building materials may be corroded.

Internationally agreed measures to curb acid precipitation are now in operation in Europe and North America, where the problem is most acute. The first of these was established in 1979. This was the Convention on Long Range Transboundary Air Pollution (CLRTAP), a protocol that was adopted in 1985 and that became known as the '30 percent club' because of the agreement between its thirty-five members to reduce sulphur emissions by 30 per cent of 1980 levels by 1993. Britain, Poland, Spain and the USA declined to subscribe to the convention, although eventually all succeeded in reducing sulphurous emissions to a degree. Another protocol was signed in 1994 in Oslo to tailor targets to polluters rather than to reassert overall objectives.

The measures discussed above have been confined to the Northern Hemisphere, where the impact of acid precipitation has been most intense and most extensive. By c. 1750, industrialization and the large-scale burning of fossil fuel were occurring, so there have been nearly 250 years of uncontrolled emissions of sulphurous and nitrous acids. The impact has been particularly severe in areas of acid bedrock (such as granite) that are in receipt of air masses from industrialized regions. While measures to curb acid precipitation have facilitated a degree of ecosystem recovery in parts of the temperate zone of the Northern Hemisphere, the problem is now spreading into the tropics as developing countries industrialize, especially in Southeast Asia and China. Acid precipitation is thus rapidly becoming an environmental issue of global proportions. . .

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