Concern regarding the impacts of continued nitrogen and sulfur deposition on ecosystem health has prompted the development of critical acid load assessments for forest soils. A critical acid load is a quantitative estimate of exposure to one or more pollutants at or above which harmful acidification-related effects on sensitive elements of the environment occur. A pollutant load in excess of a critical acid load is termed exceedance. This study combined a simple mass balance equation with national-scale databases to estimate critical acid load and exceedance for forest soils at a 1-km2 spatial resolution across the conterminous US. This study estimated that about 15% of US forest soils are in exceedance of their critical acid load by more than 250 eq ha-1 -,> including much of New England and West Virginia. Very few areas of exceedance were predicted in the western US.
McNulty, Steven G.; Cohen, Erika C.; Myers, Jennifer A. Moore; Sullivan, Timothy J.; Li, Harbin. 2007. Estimates of critical acid loads and exceedances for forest soils across the conterminous United States. Environmental Pollution, Vol. 149: 281-292