Stationary emissions sources, such as coal-fired and oil-fired power stations, and mobile sources, such as cars, ships and aircraft emit a complex mixture of pollutants, including sulphur dioxide and nitrogen oxides (the precursors to acid rain). It is now well established that this air pollution is transported over hundreds or even thousands of kilometres. Consequently, when acidic pollution is finally deposited, its environmental impacts are felt in areas far removed from their sources. Since this air pollution has no regard for national boundaries, it has been termed transboundary pollution.
Because acidic pollution is transboundary, there is no clear relationship between how much pollution a country emits and how much is deposited there. Throughout Europe, the prevailing wind direction is generally westerly or southwesterly. Consequently, much of the pollution emitted in the UK travels across the North Sea and is deposited in Scandinavia. Whilst the UK emits much more pollution than it receives through acid deposition, Norway and Sweden experience proportionally a much greater amount of acid rain compared to their lower emissions.
To control the spread of transboundary pollution the United Nations Economic Commission for Europe (UNECE) implemented the Convention on Long-Range Transboundary Pollution (1979). Since that time, emissions of sulphur dioxide across Europe have been lowered dramatically, but increases in the volume of traffic have meant that emissions of nitrogen oxides have not fallen as quickly. Consequently, whilst other environmental issues such as global warming and ozone depletion have received more attention in recent years, transboundary acid rain remains a problem today.