Efficient control of atmospheric sulfate production based on three formation regimes

The formation of sulfate (SO 4 2− ) in the atmosphere is linked chemically to its direct precursor, sulfur dioxide (SO 2 ), through several key oxidation paths for which nitrogen oxides or NO x (NO and NO 2 ) play essential roles. Here we present a coherent description of the dependence of SO 4 2– formation on SO 2 and NO x under haze-fog conditions, in which fog events are accompanied by high aerosol loadings and fog-water pH in the range of 4.7–6.9. Three SO 4 2– formation regimes emerge as defined by the role played by NO x . In the low-NO x regime, NO x act as catalyst for HO x , which is a major oxidant for SO 2 , whereas in the high-NO x regime, NO 2 is a sink for HO x . Moreover, at highly elevated NO x levels, a so-called NO 2 -oxidant regime exists in which aqueous NO 2 serves as the dominant oxidant of SO 2 . This regime also exists under clean fog conditions but is less prominent. Sensitivity calculations using an emission-driven box model show that the reduction of SO 4 2– is comparably sensitive to the reduction of SO 2 and NO x emissions in the NO 2 -oxidant regime, suggesting a co-reduction strategy. Formation of SO 4 2− is relatively insensitive to NO x reduction in the low-NO x regime, whereas reduction of NO x actually leads to increased SO 4 2– production in the intermediate high-NO x regime. Distinct dependence of atmospheric SO 4 2 – formation on NO x levels in haze-fog events is revealed by SO 4 2 – production isopleths that are obtained through simulations of atmospheric chemistry with a box model.