The sensitivity of sulfur wet deposition to atmospheric oxidants

The wet deposition of sulfate (SO42−) in eastern North America is chemically linked to primary emissions of sulfur dioxide (SO2) via atmospheric oxidants and therefore also to the emissions of nitrogen oxides (NOx) and hydrocarbons (RH) within the region. Despite the major reductions in SO2 sources mandated by the Clean Air Act Amendments of 1990, large sulfate deposition events are still observed in eastern North America. A case study of one such event in June 1996 in central Pennsylvania is used to gain an understanding of the linkages between sulfur deposition and oxidant chemistry and transport. These issues are addressed here using a combination of meteorological and chemical models (HY-SPLIT and OZIPR) in order to interpret air and precipitation chemistry data (AIRMoN) from the Penn State research sites. The large deposition of sulfate resulting from this rain event can be accounted for by the scavenging of ambient sulfate formed prior to the storm. The potential impacts that future NOx controls might have on ambient sulfate, visibility, and acidic deposition depend in part on the resulting changes in total oxidant levels and the competition that naturally exists between the clear-air and in-cloud oxidation pathways. The findings here suggest that lowered levels of NOx, even when they result in reduced production of ozone (O3), may nevertheless lead to enhanced generation of hydrogen peroxide (H2O2) and therefore to the continued occurrence of sulfate episodes and high-deposition events in the future.