Compiled by Bedrich Moldan
The Convention on Long-Range Transboundary Air Pollution of the UN Economic Commission for Europe provides activities to calculate and map critical loads and critical thresholds. Critical loads may be considered indicators of unsustainability, showing the point at which particular stresses on the environment will result in significant damage to ecology and to human health. Information on these activities is published in regular reports by the Coordination Center for Effects (CCE) at the National Institute of Public Health and the Environment in the Netherlands.
Methodology for calculating and mapping critical loads of acidity have been developed and related to critical loads of sulphur (described in the first CCE report). This approach evolved towards the application of a critical load for both acidity and eutrophication. The critical loads approach became further broadened to include different kinds of impacts; this is important for the scientific support of protocol negotiations aiming to reduce specific pollutants. This is described in the second CCE report.
Current research supporting UN/ECE protocols on long-range transboundary air pollution is moving away from single critical loads towards the future application of multiple critical thresholds. This has led to the creation of so-called protection isolines depicting combinations of sulphur and nitrogen deposition that ensure protection against acidification and/or eutrophication. This is described in the third CCE report.
A new methodology has been designed within the Effects Program as part of preparations for scientific support of negotiations to revise the UN/ECE LRTAP protocol on the reduction of nitrogen. This methodology takes into account multiple effects (e.g. acidification and eutrophication) of multiple pollutants (e.g. sulphur and nitrogen). It replaces the previous approach, which only considered acidification by a single pollutant, such as sulphur (see Hettelingh et al. 1995).
Critical loads are computed for acidification and eutrophication for a large range of ecosystems in every EMEP (a European monitoring and evalution system) grid square (150 km x 150 km). Protection isolines have been derived which reflect the combinations of sulphur and nitrogen deposition at which the protection of ecosystems against acidification or eutrophication does not change. Different isolines correspond to different protection levels: for example, the fifth percentile protection isoline is used to identify grid cells, at which point more than five per cent of the ecosystem is at risk. The use of these protection isolines means that the exceedance of the limit is no longer expressed by the difference between a single critical load and the deposition of a single pollutant, as was done in the research supporting the negotiations of the Second Sulphur Protocol.
By considering two pollutants simultaneously, many combinations of sulphur and nitrogen deposition can be formulated which provide the same protection against acidification and eutrophication. Exceedance of the limit is currently expressed in terms of the proportion of the ecosystems in each EMEP grid cell which is protected, given a certain combination of sulphur and nitrogen deposition onto each grid cell.
When the maximum critical load is exceeded, a mandatory reduction of the appropriate pollutant is required. The median critical loads, which protect only 50% of the ecosystems, yield higher levels of deposition than the pentile critical loads, which protect 95% of the ecosystems. Based on the calculation of critical loads of sulphur and nitrogen, the required deposition reduction in different target years are calculated. Then the emission reduction requirements can be formulated based on model calculations. The use of critical loads as an indicator that is directly tied to policy decisions, allows more effective protection against acidification and eutrofication in Europe.
REFERENCES
Downing, R.J., J.P. Hettelingh, and P.A.M. de Smet (eds.) (1993) Calculation and Mapping of Critical Loads in Europe: Status Report 1993. CCE, RIVM Rep. 259101003, Bilthoven, The Netherlands.
Hettelingh, J.-P., Downing, R.J. and P.A.M. de Smet (eds.) (1991) Mapping Critical Loads for Europe. Technical Report No.1, RIVM Rep. 259101001, Bilthoven, The Netherlands.
Hettelingh, J.-P., Posch, M., de Smet, P.A.M. and R.J. Downing (1995) The Use of Critical Loads in Emission Reduction Agreements in Europe. Water Air Soil Pollut. (in press).
Posch, M., de Smet, P.A.M., Hettelingh, J.-P. , Downing, R.J. (eds.) (1995) Calculation and Mapping of Critical Thresholds in Europe: Status Report 1995. CCE, RIVM Report No. 259101004, Bilthoven, The Netherlands.