Simultaneous assessment of deposition effects of base cations, sulfur, and nitrogen using an extended critical load function for acidification

Base cations (BC) play an important role to prevent soil acidification. In certain acid sensitive areas, such as China, BC deposition is high and a considerable fraction is of anthropogenic origin. BC deposition might decrease in the future with the implementation of air pollution control measures. The effect of changes in BC deposition, however, has seldom been considered in critical load calculations based on the steady-state mass balance (SSMB) method. In order to better quantify the importance of the BC deposition for acid deposition mitigation policy, an extension of the SSMB method for critical load calculation for soil acidification is presented. The BC deposition is taken into account as a variable along with sulfur (S) and nitrogen (N) deposition, creating an S-N-BC critical load function. As a case study, critical loads of S and N for the Tie Shan Ping catchment in Chongqing in southwest China under variable BC deposition were calculated. Results indicate that abatement of BC deposition has significant impact on the critical loads of S and N. A 75% reduction in BC of assumed anthropogenic origin decreases the critical loads of acids by 58%. The current deposition does not exceed the critical loads, but if BC deposition from anthropogenic sources was controlled, then the exceedance would be considerable. Uncertainty analysis show that the size of the BC deposition of natural origin is the single parameter contributing the most to the steady-state S and N critical load. The extended critical load function can be used by policy makers to set more reasonable acidity control strategies in the future. The method also highlights for policymakers the competition between emission control of particulate matter driven by human health targets and potential increase of net acid load from such measures.