Critical loads and critical levels

Critical levels (CLLs) and critical loads (CLs) currently represent the only quantitative methodology for assessing the regional-scale environmental impact of atmospheric pollution on ecosystems both directly and through deposition. The approach can be applied to effects as diverse as the reduction of crop yields due to ozone, damage to ancient monuments by sulphur dioxide, or the deposition of acidifying pollutants on soils. As such, critical levels and critical loads are essential tools in the quest for an objective and cost-effective approach to environmental protection. Critical loads are a fundamental component of the integrated assessment modelling currently being undertaken under the UN-ECE1 Convention on Long-Range Transboundary Air Pollution (CLRTAP) and the EU Acidification and Ozone Strategies. Indeed, they are central to the determination of the levels of emission abatement required by these initiatives. In addition, critical levels form the basis for the air quality limit values being proposed to protect ecosystems under the EU Ambient Air Quality Assessment and Management Directive.