Rain water is naturally acidic as a result of carbon dioxide dissolved in water and from volcanic emissions of sulphur. However, it is the chemical conversion of sulphur and nitrogen emissions from power stations, factories, vehicles and homes, where fossil fuels are burnt, that we call acid rain. These waste gases are carried by the wind, sometimes over long distances, and can in time be converted into sulphuric and nitric acids.
Natural sources of sulphur dioxide (SO2) include releases from volcanoes, oceans, biological decay and forest fires. Actual amounts released from natural sources in the world are difficult to quantify; in 1983 the United Nations Environment Programme estimated a figure of between 80 million and 288 million tonnes of sulphur oxides per year. Man-made sulphur dioxide emissions result from combustion or burning of fossil fuels, due to varying amounts of sulphur being present in these fuels. Worldwide emissions of SO2 are thought to be around 69 million tonnes per year.
Levels of sulphur dioxide from combustion sources in the UK have declined in recent decades. Between 1970 and 1999, UK sulphur dioxide emissions fell by 82% due to recession, restructuring of industry, substitution of fuels (for example natural gas) and air pollution control technology. Power station emissions fell by 73% over the same period, but the percentage of UK emissions from power stations has actually increased to 65% of the 1999 total compared to 45% of the total in 1970.
Natural sources of nitrogen oxides (NOx) include volcanoes, lightening strikes and biological decay. Estimates range from between 20 million and 90 million tonnes per year NOx released from natural sources, compared to around 24 million tonnes from human sources world-wide. Nitrogen oxides are produced when fossil fuels are burned. The major sources of NOx in the UK in 1999 were road transport (44%), power stations (21%) and industry (including iron and steel, and refineries) (12%). Emissions of nitrogen oxides from road transport increased during the 1970s and 1980s before decreasing again during the 1990s. For example, in 1970, emissions of NOx from road transport in the UK were 0.769 million tonnes by in 1990 they had risen to over 1.31 million tonnes NOx. Since then, however, emissions from transport have been declining due to improvements in vehicle technology, such as the use of catalytic converters, and the use of cleaner fuels. In 1999 they were 0.714 million tonnes, lower than in 1970.
The geographical distribution of human acidic emission sources is not even. Nitrogen and sulphur emission sources are heavily concentrated in the Northern Hemisphere, particularly in Europe and North America. As a result, precipitation is generally more acidic in these countries, with an acidity in the range pH 4.1 to pH 5.1. Normal or unpolluted rainfall has a pH of 5.6.