A NATIONAL CRITICAL LOADS FRAMEWORK FOR ATMOSPHERIC DEPOSITION EFFECTS ASSESSMENT .2. DEFINING ASSESSMENT END-POINTS, INDICATORS, AND FUNCTIONAL SUBREGIONS

The United States Environment Protection Agency, with support from the US Department of Energy and the National Oceanographic and Atmospheri c Administration, has been evaluating the feasibility of an effects-ba sed (critical loads) approach to atmospheric pollutant regulation and abatement. The rationale used to develop three of the six steps in a f lexible assessment framework (Strickland and others, 1992) is presente d along with a discussion of a variety of implementation approaches an d their ramifications. The rationale proposes that it is necessary to provide an explicit statement of the condition of the resource that is considered valuable (assessment end point) because: (1) individual ec osystem components may be more or less sensitive to deposition, (2) it is necessary to select indicators of ecosystem condition that can be objectively measured and that reflect changes in the quality of the as sessment end point, and (3) acceptable status (i.e., value of indicato r and quality of assessment end point at critical load) must be define d. The rationale also stresses the importance of defining the assessme nt regions and subregions to improve the analysis and understanding of the indicator response to deposition. Subregional definition can be b ased on a variety of criteria, including informed judgment or quantita tive procedures. It also depends on the geographic scale at which expo sure and effects models are accurate and on data availability, resolut ion, and quality.