Critical Loads Development and Use in Europe: Applicable to USA?

Jean-Paul Hettelingh, Max Posch and Jaap Slootweg
Coordination Centre for Effects (CCE)

Critical loads are developed as early warning indicators for excessive regional deposition of air pollutants that ultimately can affect the sustainability of a good ecological state and of ecosystem services. Critical loads have been used to assess the effectiveness of policy alternatives to abate air pollution. Critical loads for acidification, eutrophication and heavy metals have been modeled and mapped for European terrestrial ecosystems and surface waters under the Convention on Long-range Transboundary Air Pollution (LRTAP Convention). About 29 Parties to the LRTAP Convention participate in the development of the European critical load database under the International Cooperative Programme on Modelling and Mapping. They all have a clearing house institute, which is not always officially established as “National Focal Centre” (NFC) with the secretariat to the Convention. The CCE European background database is used to compute critical loads for the 18 remaining parties within the EMEP modeling domain.The critical loads database includes both empirical and computed critical loads on varying spatial scales. In 2008, National Focal Centres together reported computed critical loads for eutrophication with about 700,000 data points to represent the sensitivity of about 3 million km2 of natural areas. Empirical critical loads of nutrient nitrogen are also reported with about 700,000 data points covering 1.5 million km2. For acidification, about 1 million data points are used to represent about 5.5 million km2. The resolution of critical loads is higher than that of computed depositions within 50x50 km2 grid cells. The latter comes with three numbers. This difference in resolution is not relevant when developing critical loads.

It can be argued that many of the development-elements of the European critical load approach can also be applied to natural systems in the USA. These could be designed in a collaborative effort under a Pilot NFC project. In addition to the compilation of existing critical loads data for natural areas in the USA, it is recommended to also draw a framework of a generic method allowing for the computation, or expert-estimation, of tentative critical loads for other areas. The development of such a framework may benefit from approaches that have led to the European background database.

For the use-elements other requirements need to be identified, such as the availability of operational models for the simulation of atmospheric dispersion of air pollutants and best applicable (local and regional) spatial scales for the assessment of exceedances for natural areas in the USA.

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