As a result of the observed relationship between air pollution and acidification of soils and waters, in 1979 the United Nations Economic Commission for Europe (UNECE) initiated the Convention on Long-range Transboundary Air Pollution (LRTAP). Under this convention a number of working groups were established, to investigate all relevant aspects of air pollution and its effects on ecosystems, crops, human health and materials. The ICP on Modelling and Mapping of Critical Levels and Loads and Air Pollution Effects (ICP M&M) is responsible, inter alia, for the assessment of regional critical loads in Europe. It’s major aim is to develop methodologies and databases of critical loads of sulphur and nitrogen that are used in the assessment of cost-effective emission-abatement alternatives in support of European policies to curb air pollution (Gregor et al., 2001). Following the obligations laid down in the 1988 NOx Protocol (that still proposed flat rate reductions), the 1994 Protocol on further abatement of Sulphur was the first protocol based on computations that provided the most cost-effective measures based on ecosystem vulnerability (expressed by critical loads) and emission abatement costs, optimized in an European framework. In 1999 the so-called multi-effect multi-pollutant protocol (also known as the Gothenburg Protocol) was signed that included sulphur (S), nitrogen oxides (NOx), ammonia (NH3) and volatile organic compounds (VOCs).

The emission reduction protocols have been successful: compared to 1980, the emissions in 2010 in Europe of SOx under the Gothenburg protocol should have been reduced by more than 60%, emissions of NOx by about 40% and those of ammonia by 17%.

Next to critical loads for S and N, also methodologies for critical loads for heavy metals have been developed in Europe. In 1994, a first explorative study was carried out commissioned by the Dutch ministry of Housing, Spatial Planning and the Environment covering emissions, deposition and critical loads for heavy metals in Europe (Van den Hout et al, 1999). This was followed in 2005 by a report of the CCE on critical loads for Cd, Pb and Hg in Europe with contributions of various countries that provided national critical load assessments. In 2006 a report was published to support the review of the Heavy Metal protocol under LRTAP (Hettelingh & Sliggers, 2006), that apart from the ‘priority’ metals also included preliminary critical load assessments for Cu, Zn, As, Cr and Se.

Over the last years, there has been an increased emphasis on dynamic modelling of S and N effects on ecosystems in Europe, to investigate recovery from acidification and eutrophication. Recently, much effort is dedicated to derive critical loads for N, based on biodiversity criteria, thus shifting from abiotic criteria such as a critical N concentration in the soil to criteria like ‘habitat quality index’, based on functions describing probability occurrence of plants as a function of pH, N and climatic variables (Reinds et al, 2012). This work is partly carried out within the EU FPVII Framework project Eclaire.

References:

K. D. Van den Hout, D. J. Bakker, J. J. M. Berdowski, J. A. Van Jaarsveld, G. J. Reinds, J. Bril, A. Breeuwsma, J. E. Groenenberg, W. De Vries, J. A. Van Pagee, M. Villars, C. J. Sliggers,  1999. The Impact of Atmospheric Deposition of Non-Acidifying Substances on the Quality of European Forest Soils and the North Sea. Water, Air, and Soil Pollution, Volume 109, Issue 1-4, pp 357-396.

J. Slootweg, J.-P. Hettelingh, M. Posch (eds.), 2005. Critical Loads of Cadmium, Lead and Mercury in Europe S. Dutchak, I. Ilyin (EMEP/MSC-E) RIVM Report 259101015/2005

J.P. Hettelingh - J. Sliggers (eds.), 2006. Heavy Metal Emissions, Depositions, Critical Loads and Exceedances in Europe. Ministry of Housing, Spatial Planning and the Environment.
Reinds, G.J. L