A NATIONAL CRITICAL LOADS FRAMEWORK FOR ATMOSPHERIC DEPOSITION EFFECTS ASSESSMENT .1. METHOD SUMMARY

The United States Environmental Protection Agency (EPA), with the assi stance of the US Department of Energy (DOE) and the National Oceanogra phic and Atmospheric Administration (NOAA) is examining the utility of a critical loads approach for evaluating atmospheric pollutant effect s on sensitive ecosystems. A critical load has been defined as, ''a qu antitative estimate of an exposure to one or more pollutants below whi ch significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge.'' Working in cooperation with the United Nations Economic Community for Europe's (U N-ECE) Long Range Transboundary Air Pollution (LRTAP) Convention, the EPA has developed a flexible, six-step approach for setting critical l oads for a range of ecosystem types. The framework is based on regiona l population characteristics of the ecosystem(s) of concern. The six s teps of the approach are: (1) selection of ecosystem components, indic ators, and characterization of the resource; (2) definition of functio nal subregions; (3) characterization of deposition within each of the subregions; (4) definition of an assessment end point; (5) selection a nd application of models; and (6) mapping projected ecosystem response s. The approach allows for variable ecosystem characteristics and data availability. Specific recognition of data and model uncertainties is an integral part of the process, and the use of multiple models to ob tain ranges of critical loads estimates for each ecosystem component i n a region is encouraged. Through this intercomparison process uncerta inties in critical loads projections can be estimated.