In Europe, the major efforts to decrease the effects of SO2 and nitrogen emissions have been aimed at decreasing transfers to air, soil, and groundwater. Most of the measures in Europe are focused on decreasing human and plant exposure and decreasing ecosystem loads leading to acidification and eutrophication. Countries agreed to decrease air emissions by signing different protocols developed under the Convention on Long-Range Transboundary Air Pollution, including the first and second sulfur protocols, the first NOx protocol, and the NMVOC (Non-Methane Volatile Organic Compounds) protocol. The last protocol, the Goteborg Protocol, is unique in the sense that it requires decreases in emissions of four pollutants with the objective of abating three specific effects—acidification, eutrophication, and the effects from tropospheric ozone on human health and vegetation. The protocol, which has been signed by 29 European countries together with the United States and Canada, is based on a gap-closure method aiming at decreasing the spatial exceedances of critical loads and levels in the most cost-efficient way. Critical loads for each European country are defined on the basis of information developed by each country. The agreed upon decreases in emissions for the European Union (EU) member states are expected to lead to overall decreases in the European (except Russia) emissions of 78% for SO2 and approximately 44% for NOx during the period 1990 to 2010. The corresponding figure for ammonia is 17%.
The protocols have had a major effect on the emission trends in Europe, especially for SO2 (Table I). European emissions reductions are being made with the clear objective that environmental loads, exposures, and effects should be decreased—the so - called ‘‘effects-based approach,’’ which was initiated under the Second Sulphur Protocol. For NOx, the main measure to decrease emissions is exhaust gas regulations introduced in the EU countries in approximately 1990, resulting in the application of three-way catalysts in gasoline cars. Even regulations on heavy-duty vehicles have caused emission reductions of NOx. Furthermore, selective catalytic reduction technologies (SCR) with ammonia or urea as a reductor have been implemented in many combustion plants. In eastern Europe, the main cause of decreases in NOx emissions is the shutdown of a large number of industrial plants. Despite declining emissions rates, total NOx emissions have remained steady or even increased during the same period in North America and Europe due to increases in vehicle kilometers/miles traveled, increases in electricity usage, and the sometimes differing regulatory frameworks applied to various sectors. Decreasing total NOx emissions likely means that continuing technological advances need to be combined with regulatory approaches (e. g., emissions caps).