Acidic precipitation thought to be caused mainly by the release into the atmosphere of sulphur dioxide (SO2) and oxides of nitrogen (NOx), which dissolve in pure rainwater making it acidic. Sulphur dioxide is formed by the burning of fossil fuels, such as coal, that contain high quantities of sulphur; nitrogen oxides are produced by various industrial activities and are present in car exhaust fumes.
Acidity is measured on the pH scale, where the value of 0 represents liquids and solids that are strongly acidic and 14 represents those that are highly alkaline. Distilled water is neutral and has a pH of 7. Normal rain has a value of 5.6. It is slightly acidic due to the presence of carbonic acid formed by the mixture of CO2 and rainwater. Acid rain has values of 5.6 or less on the pH scale.
Acid deposition occurs not only as wet precipitation (mist, snow, or rain), but also comes out of the atmosphere as dry particles (dry deposition) or is absorbed directly by lakes, plants, and masonry as gases. Acidic gases can travel over 500 km/310 mi a day, so acid rain can be considered an example of transboundary (international) pollution.
Acid rain is linked with damage to and the death of forests and lake organisms in Scandinavia, Europe, and eastern North America. It is increasingly common in countries such as China and India that are industrializing rapidly. It also results in damage to buildings and statues.
Reductions of UK emissions are being sought by using flue-gas desulphurization plants in power stations and by fitting more efficient burners; by using gas instead of coal as a power station fuel; and, with road transport rapidly becoming recognized as the single most important source of air pollution, the compulsory fitting of catalytic converters to all new vehicles.
A 1995 study found Manchester, England, to be the European city worst affected by acid rain. Its rainfall was the most acidic, causing building stone to be destroyed faster there than anywhere else in Europe (other cities affected included Athens, Greece; Copenhagen, Denmark; and Amsterdam, the Netherlands). The other UK test site, at Liphook, Hampshire, England, also fared badly.
Since 1975, emissions of SO2 in the UK have steadily fallen. Between 1980 and 1995 there was a 50% decrease in SO2. This trend continued, with a decrease of almost 50% in the period 1990–2001 in the emission of all chemicals that cause acid rain, mainly in the electricity, gas, and water sectors where the decrease in emissions was 70%.
Effects of acid rain A significant effect of acid rain is to damage the chemical balance of soil. It leaches out important minerals including magnesium and aluminium. Plants living in such soils, particularly conifers, suffer from mineral loss and become more prone to infection. The minerals from the soil pass into lakes and rivers, disturbing aquatic life, for example by damaging the gills of young fish and killing plant life. Lakes affected by acid rain are virtually clear due to the absence of green plankton. Lakes and rivers also suffer more direct damage because they become acidified by rainfall draining directly from their catchment.
Acids in the Environment
Influence of people on weather and climate
Sources and effects of pollutants
Pollution and Eastern Europe
Causes and effects of acid rain
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