Characterization of Critical Load Exceedances in Europe
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The excess of acidic and eutrophying depositions over critical loads (critical load exceedances) is considered a measure for the risk of harmful effects on sensitive elements of the environment. The magnitude and the geographical distribution of critical load exceedances over Europe vary with the extent to which national emissions of sulfur dioxide, nitrogen oxide and ammonia are reduced. The scientific support of negotiations on emission reductions in the framework of the Convention on Long-range Transboundary Air Pollution (LRTAP) of the UN Economic Commission for Europe has been based on the integrated assessment of sources, including abatement costs, and risks to receptors (e.g. forests, lakes) using critical load exceedances. The shift from a single-pollutant (sulfur) protocol in 1994 to a multi-pollutant protocol in 1999 necessitated an extension of the methods by which critical load exceedances are computed and mapped. The focus changed from the protection of the most sensitive ecosystem against excessive deposition of one pollutant, to an assessment of the accumulated exceedance by more pollutants of all ecosystems. This paper presents and compares the different characterisations ("gap-closure methods") used in those negotiations. It is shown that the approach finally used has several appealing features, but treats the exceedance as a linear damage function, thus going beyond the critical load definition as a simple on-off limit value.
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