Atmospheric emissions of acidifying substances such as sulphur dioxide (SO2) and nitrogen oxides (NOx), mainly from the burning of fossil fuels, can persist in the air for up to a few days and thus can be transported over thousands of kilometres, when they undergo chemical conversion into acids (sulphuric and nitric). The primary pollutants sulphur dioxide, nitrogen dioxide and ammonia (NH3), together with their reaction products, lead after their deposition to changes in the chemical composition of the soil and surface water. This process interferes with ecosystems, leading to what is termed 'acidification'. The decline of forests in Central and Eastern Europe and the many 'dead' lakes in Scandinavia and Canada are examples of damage which are, in part, due to acidification. Modern forestry and agriculture contribute to but can also be affected by acidification. Acidifying substances also play a role in the greenhouse effect (see Chapter 27). Furthermore, nitrogen oxides contribute to the ozone problems (build-up of tropospheric ozone, depletion of stratospheric ozone; see Chapters 32 and 28), and, together with ammonia, contribute to the nitrogen fertilisation of natural terrestrial ecosystems; with phosphate they contribute to eutrophication in water (see Chapter 33).

By the end of the 1970s, acidification was widely recognised as a major threat to the environment. As a result large research programmes were set up to investigate the chain from emission to effects of acidifying substances and to indicate possible policy measures in this field. This has led to a much better understanding and modelling of the processes involved, which in turn has helped to formulate international agreements with explicit objectives for reducing emissions of pollutants leading to acidification.

The effects of these measures are now being evaluated in order to define sustainable releases of acidifying substances. This is based on studies which indicate that there are deposition loadings below which no harmful damage is observed, that is, the concept of 'critical loads'.


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31.1 - The problem

31.2 - The causes

31.3 - The consequences


31.3.1 - Lakes

31.3.2 - Soil

31.3.3 - Forests

31.3.4 - Other effects


31.4 - Goals

31.5 - Strategies


31.5.1 - Legislation

31.5.2 - Possibilities and limitations for emission reductions

31.5.3 - The future



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