Acid rain is abnormally acidic precipitation, which can fall as snow, mist, dew or dry particles, as well as rain. It arises because sulphur and nitrogen oxides and other acid precursors emitted into the atmosphere from natural and man-made sources react with moisture in the atmosphere and undergo chemical transformations, leading to the formation of sulphurous, sulphuric and nitric acids which return to the earth in precipitation. In sensitive areas, this increases the acidity of water bodies and the soil, and can damage aquatic ecosystems, crops and forests. Acid deposits are absorbed into the soil where they can break down other naturally present minerals and leach away nutrient sources necessary for the health growth of trees, plants and crops.
As groundwater, acid contamination eventually enters nearby streams and bodies of water, often carrying toxic metals such as aluminium that can deform or kill aquatic life (and perhaps contribute to the incidence of Alzheimer's disease). The phosphates, which nourish phytoplankton and other aquatic plants, attach themselves to the aluminium and become less available as a nutrient. As the water gets more acid still, other toxic metals like cadmium, zinc, lead and mercury also become increasingly soluble and may be taken up by water life through food chains. Acidic water in reservoirs, by dissolving lead water pipes, can introduce unhealthy levels of lead in drinking water.
The main source of the acid pollutants is fossil fuels; they contain chemical elements including carbon, hydrocarbon, sulphur, and nitrogen among others. These chemicals are released into the atmosphere as waste products when the fuels are burned. Oxygen combines with the chemicals to produce oxides, such as sulphur dioxide and nitrogen oxides, the main causes of acid rain. Sulphur dioxide is emitted principally by power stations and industrial and commercial installations when burning coal and oil, and by metal smelters when burning iron and other metallic ores. Nitrogen oxides: nitric oxide, nitrous oxide and nitrogen oxide, produced during the burning of coal, oil and petroleum. They are emitted both by stationary sources, eg power stations and smelters, and by motor vehicles. Once emitted, some oxides fall directly onto surfaces of plants, trees, soils, lakes and buildings. This is dry deposition which turns to acid when it becomes wet by the action of dew, rain or falling into bodies of water. Oxygen in the atmosphere transforms the remaining oxides into sulphuric and nitric acids which are deposited as rain, snow, hail or dew. This is wet deposition. Dry deposition generally occurs close to the point of emission. Wet deposition often occurs thousands of kilometres downwind of emission sources. Emissions may be injected at increasingly higher levels in the atmosphere by rising chimney stacks built to minimize local pollution.
Corrosion is accelerated in most materials used in construction of buildings, bridges, dams and industrial equipment. Acid depositions can also severely damage monuments and historic buildings. Remedial action is considerably handicapped by the fact that the population in the source area may have different priorities than that where the acid is deposited, and may place quite different values both on the costs of any damage and on the costs of various control strategies.
British scientist R A Smith first noted the problem of acid rain in Europe in 1872, but it took another century before environmental acidification was widely recognized as a major problem. During that century the acidity of precipitation in Europe increased by at least a factor of 10. The Sulphur Protocol (1985) to the UN/EEC/EUE Convention on Long-Range Transboundary Pollution calls for a 30% reduction in sulphur dioxide emissions from 1980 levels. Partly as a result of this Protocol, the 1980s saw a 23% reduction in European sulphur dioxide emissions. However, emissions from power stations, the main source of sulphur dioxide, have remained roughly stable, indicating that reductions have mainly resulted from a shift in domestic fuel use. Early in 1994, representatives of almost every country in Europe are expected to sign a comprehensive agreement to sharply reduce emissions of sulphur dioxide according to computer modelling of critical loads. Critical load calculations take account of ecosystem vulnerability to acidity, which depends on local conditions, especially soil chemistry; soils derived from limestone, for example, can absorb and neutralize acids, while granitic soils can tolerate very little acid deposition. Other important factors are soil thickness, precipitation, deposition of dust and other acid-neutralizing materials, and the synergistic effects of nitrous / sulphurous acid mixtures. Thawing acidic ice can result in very sudden releases of acid sediments, producing ecosystem shock. The scenario which has been negotiated, once targets are reached, would leave only 7% of European ecosystems receiving sulphur depositions above their critical loads (at 1993 about 30% are above).
Acid rain may also lower the buffering capacity of soils. If soil pH drops below 4.2, naturally-occurring aluminium may be mobilized, posing a threat to forests and water courses.
The Protocol on Nitrous Oxides (1988) stipulates a freezing of emissions from their 1987 levels, but nitrous oxide emission have actually increased over the 1980s. It is unlikely that fuel and engine design changes alone can suffice and that the only effective answer is to decrease motor vehicle usage.
Acidification problems are serious in many areas of North America and western Europe, where an estimated 5 to 10 million sq km acid; although the highest concentrations are evident in eastern Europe and Scandinavia, where the pH value of rain increased by one point (a factor of 10) between 1977 and 1988. The effects occur in countries which are major emitters of the gases and in distant countries receiving the acid deposition as a result of prevailing wind patterns (both sulphur dioxide and nitrous oxides can remain in suspension the atmosphere for many days and be carried up to 1,000 km from the original source before being deposited). 75% of acid deposits in Norway are estimated to originate from other countries. The average northern precipitation today is 100 times more acidic than 180-year old ice cores from Greenland. European rain water should have a pH value between 5 and 6, but over large areas it is now between 4 and 5. In the last 20 to 50 years, forest soils in large areas of Europe have become 5 to 10 times more acid. In 1982, it was estimate that one third of west Germany's forests have suffered damage, including three quarters of fir trees. Fish populations have been eliminated by acid precipitation in lakes of the Scandinavian countries and in Canada. Most fish cannot reproduce in water with a pH of 4.5. In Sweden, damage to fisheries attributed to acidification has been observed in 2,500 lakes, and is assumed to have affected 16,000 of the country's 85,000 lakes and 100,000 kilometres of its rivers.
Until recently, it was thought that acid deposition was a big problem only in the Northern Hemisphere. In 1993, it has become clear that Southeast Asia is on the verge of an acidification problem as widespread and severe as anything seen in Europe and North America. Energy consumption has been doubling every 12 years and with it the emissions of sulphur dioxide and other pollutants. In 1993, researchers of the USA Geological Survey reported that concentrations of sulphate had declined significantly in rainwater collected at 26 of 33 sites in 18 States monitored from 1980 to 1991. Most of the sites were in the eastern half of the country. Findings on nitrates were ambiguous. Significant decreases in acidity occurred at 9 of the sites monitored. The National Acid Precipitation Assessment Program reported that since the Clean Air Act was passed in 1970, sulphur dioxide emissions have declined by about 30% nationwide. Nitrogen oxides emission have declined by about 6% since 1978.
Acid rain remains a problem, with critical loads (the threshold at which acid deposition causes damage) frequently exceeded over large parts of North America, Europe and Southeast Asia (Kuylenstierna, Cinderby and Cambridge 1998).
The Convention on Long Range Transboundary Air Pollution has resulted in significant reductions in emissions of acidifying gases in Europe and North America - for example, between 1985 and 1994, SO2 emissions in Western, Central and Eastern Europe fell by 50 per cent in line with the Convention on Long Range Transboundary Air Pollution protocols (Olendrzynski 1997). However, emissions in other regions, especially in parts of Asia, are a major and growing problem. Impacts have already been observed: for example, the World Bank has estimated China's overall annual forest and crop losses due to acid rain at US$5 000 million (World Bank 1997); in Japan, many monitoring sites recorded annual sulphur dioxide deposition at levels equal to or greater than those in Europe or North America; and in the Republic of Korea winter rain acidity has been nearly as high as pH4 (Shrestha and Iyngararasan 1998).
Acid rain falling into a lake may have no effect on the pH of the water over a decade or longer. Quite suddenly, however, the buffering capacity of the lake may be exhausted, the pH may drop precipitously, heavy metals may be mobilized, and aquatic life may be seriously impaired. The first observed case of rapid acidification after a long time delay was in Big Moose Lake in the Adirondack Mountains, USA..
Compared with 1990 levels, sulphate ions in the atmosphere have dropped considerably, reduced to almost negligible levels at former hotspots. But the problem has not disappeared altogether. Nitrates from sources like agriculturally emitted ammonia released from fertilisers and livestock feed remain a contributor to nitric acid precipitation. And there is concern that acid rain – from both sulphur and nitrogen – is an increasing problem in Asia.