The acidic precursors of acid precipitation are mainly produced by fossil fuel combustion in power plants, by smelting industries and from motor vehicle exhausts. Their effects are felt not only in the neighbourhood of the sources, but also at distances of hundreds of kilometres (because emissions were 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.
Other air pollutants, not strictly contributing to acid rain but often included under the heading are volatile hydrocarbons, low-level ozone and ammonia. Toxic substances like DDT, lindane, toxaphene, eldrin, dieldrin, benzene, toluene, napthalene and polychlorinated biphenols (PCBs) are also being transmitted through the atmosphere, [ie] toxic rain.
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.
A significant proportion of hydrocarbons in the atmosphere is emitted by cars and oil refineries, and during production and use of solvents. They combine with nitrogen oxides in the presence of sunlight, a photochemical reaction, to form photo-oxidants, of which ozone is the most harmful at heights below the stratosphere. Ammonia is a combination of nitrogen and hydrogen and is produced by both industry and agriculture. Sources include nitrogen fertilizer factories and intensive livestock farming.
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..