Critical loads and the relevance of ammonia to an effects-based nitrogen protocol

With the signing of the Second Sulphur Protocol in Oslo in 1994, many countries indicated their willingness to adopt an effects-based approach to reducing pollutant emissions. Current activities under the UNECE Convention on Long-Range Transboundary Air Pollution have now moved to revision of the NOx Protocol. Because of the complex interaction of NOx with other pollutants, a “multi-pollutant, multi-effect” approach is envisaged, taking into account emissions of total reactive N (as NOx and NH3), and considering acidification, nutrient N and photochemical oxidant effects. The magnitude of NH3 emissions and deposition of NHx at an international scale have necessitated that NH3 be considered. Adoption of an effects-based approach for NH3 demands much information regarding the quantification of emissions and deposition, the understanding of physicochemical and biological processes for estimating critical loads, and the assessment of effectiveness and costs of emission abatement measures. In addition, methodology is needed to enable abatement strategies to be developed for SO2, NOx and NH3 at the same time. For the Second S Protocol, an approach defining the “S fraction” (Sf) of acidity critical loads was used to distinguish S and N effects. Besides being difficult to justify scientifically, the dependence of Sf on “current” deposition results in changing critical loads as deposition is reduced. Current work under the Convention uses a “critical loads function”, which considers S and N deposition simultaneously, for both acidification and eutrophication. This approach, together with the integrated assessment models approach used for the S Protocol, is achieving some of the multi-pollutant goals, though because of the link of emissions to environmental impacts, ongoing scientific underpinning and improved data remain essential.