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In the early 1980s, the problem of acidification was acknowledged on a governmental level in
the Netherlands as posing a major environmental threat. The concern for damage to forests
and ecosystems, and materials and monuments, as well as for acidification of soils and
moorland pools stimulated research and policy development. Acid deposition was used as a
characterisation of the atmospheric input leading to these effects. As such, wet and dry
deposition of acids, and dry deposition of acid precursors as sulphur dioxide and nitrogen
oxides, were considered. At the same time, the important role of ammonia in the acidification
of soils and moorland pools was acknowledged. Ammonia, the most important acid
neutralising component in the atmosphere, was shown to be nitrified to an important extent in
soil into nitric acid, thereby contributing to acidification. This led to the concept of potential
acid deposition, in which the deposition of actual acid (H +) was added to that of potential acid
(ammonia + ammonium). Moreover, the role of atmospheric nitrogen input was recognised as
a disturbing factor for natural ecosystems and forests growing on poor soils.
In Europe critical loads have been widely accepted as a basis for control strategies for regional
air pollution. The use of critical loads for ecosystem studies and abatement strategies requires
relevant deposition data to describe the atmospheric input to the ecosystem. The knowledge of
deposition processes and the possibility of making relevant and accurate deposition estimates
are important issues in the work of long-range transport modelling in Europe. Large-scale
dispersion and deposition of air pollution emitted in different countries above Europe needs to
be assessed correctly to serve as a relevant basis for abatement strategies. On the local scale,
the large variations in deposition to landscape, and in sensitivity between landscape elements,
make it essential to compare the actual deposition with the site-specific critical load to
determine the exceedance value. This is even more important for those areas where high
sensitivity is linked to high inputs. |