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Over the past 30 years research has indicated that effects of acidic deposition, especially from sulfate ( ) and nitrate ( ), cause serious harm to sensitive ecosystems in North America, Europe and Asia.

In this study Total Daily Maximum Loads (TMDLs) for sulfur and nitrogen from atmospheric deposition were calculated for two watersheds in the United States. The Hubbard Brook Experimental Forest, New Hampshire, is situated in a northern hardwood forest. Acidic deposition at this site is high and mainly due to emissions of and from industrial facilities in the Midwest. Loch Vale, a watershed in the Colorado Front Range, has (sub-) alpine conifer vegetation and receives less inputs of acidic deposition.

To calculate TMDLs the model PnET-BGC was used. PnET-BGC is an integrated dynamic biogeochemical model that simulates chemical transformations of vegetation, soil and drainage water. Using a dynamic model instead of a steady-state model allows estimation of the time needed for an ecosystem to recover after emission reduction strategies are in place and acidic deposition has decreased. Calculations of TMDL values for the two watersheds were conducted to help gain insight in the variation of TMDLs based on differences in vegetation types and levels of acidic deposition.

In comparison, the Critical Loads concept was introduced more than a decade ago, as a basis for the development of air pollution control strategies in Europe. Critical loads are defined as: 'the estimate of exposure to pollutants below which harmful effects on specified sensitive elements of the environment do not occur according to present knowledge'. > Calculated critical load values for sulfur and nitrogen in the two watersheds were used to analyze the differences and similarities between this concept and the TMDL approach.