A NATIONAL CRITICAL LOADS FRAMEWORK FOR ATMOSPHERIC DEPOSITION EFFECTS ASSESSMENT .3. DEPOSITION CHARACTERIZATION

Methods are discussed for describing patterns of current wet and dry d eposition under various scenarios. It is proposed that total depositio n data across an area of interest are the most relevant in the context of critical loads of acidic deposition, and that the total (i.e., wet plus dry) deposition will vary greatly with the location, the season, and the characteristics of individual subregions. Wet and dry deposit ion are proposed to differ in such fundamental ways that they must be considered separately. Both wet and dry deposition rates are controlle d by the presence of the chemical species in question in the air (at a ltitudes of typically several kilometers in the case of wet deposition , and in air near the surface for dry). The great differences in the p rocesses involved lead to the conclusion that it is better to measure wet and dry deposition separately and combine these quantifications to produce ''total deposition'' estimates than to attempt to derive tota l deposition directly. A number of options for making estimates of tot al deposition to be used in critical loads assessment scenarios are di scussed for wet deposition (buckets and source receptor models) and fo r dry deposition (throughfall, micrometeorology, surrogate surfaces an d collection vessels, inference from concentrations, dry-wet ratios, a nd source-receptor models).