Causes of acidification

By far the most dominant cause of the acidification problem is the airborne deposition of sulphur, nitrogen oxides and ammonia. As the deposition of these acidifying air pollutants has been gradually decreasing over the last few decades, in some areas the harvesting of biomass through forestry practices is gaining in importance.

Sulphur deposition, in the form of sulphuric acid, is generally the most important cause of acidification. Most sulphur comes from burning coal and oil, during which the sulphur in the fuel is converted into sulphur dioxide.

Anthropogenic (man-made) emissions of sulphur in Europe rose sharply from the end of the Second World War until the end of the 1970s. Between 1980 and 2010, emissions from land-based sources in Europe fell by 85 per cent - from 53 to 8.1 million tonnes of sulphur dioxide a year. At the same time, however, emissions from international shipping in European waters increased from 1.7 to 2.4 million tonnes a year.

Deposition of airborne nitrogen compounds - nitrogen oxides and ammonia - contribute to acidification and are also the dominant cause of eutrophication of many ecosystems on land and at sea.

Nitrogen oxides are always produced during combustion. Emissions from land-based sources in Europe have fallen from 23 million tonnes a year in 1980 to 13 million tonnes a year in 2010, a reduction of 44 per cent. About half the emissions in Europe come from the transport sector, and most the rest from combustion plants. In the case of nitrogen oxides a large part of the emission reduction from land-based sources has been offset by rising emissions from ships at sea, which have almost doubled since 1980, from 2.4 to 4.0 million tonnes.

The main source of ammonia emissions is agriculture. The amount of ammonia that evaporates depends primarily on how manure is handled during storage and spreading. According to statistics, European emissions have fallen by 34 per cent between 1990 and 2010, from 7.6 to 5.0 million tonnes per year.

Finally the harvesting of biomass, i.e. normal forestry operations, also leads to soil acidification.