The formation of sulfate (SO42−) in the atmosphere is linked chemically to its direct precursor, sulfur dioxide (SO2), through several key oxidation paths for which nitrogen oxides or NOx (NO and NO2) play essential roles. Here we present a coherent description of the dependence of SO42– formation on SO2 and NOx 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 SO42– formation regimes emerge as defined by the role played by NOx. In the low-NOx regime, NOx act as catalyst for HOx, which is a major oxidant for SO2, whereas in the high-NOx regime, NO2 is a sink for HOx. Moreover, at highly elevated NOx levels, a so-called NO2-oxidant regime exists in which aqueous NO2 serves as the dominant oxidant of SO2. 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 SO42– is comparably sensitive to the reduction of SO2 and NOx emissions in the NO2-oxidant regime, suggesting a co-reduction strategy. Formation of SO42− is relatively insensitive to NOx reduction in the low-NOx regime, whereas reduction of NOx actually leads to increased SO42– production in the intermediate high-NOx regime.
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The datasets generated during and/or analysed during the current study are available at https://github.com/xuejianust/NGsulfate/blob/master/NG.zip.
The computer code used to generate the results in this manuscript is available from the corresponding authors on request.
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This study was supported in part by the Research Grants Council of Hong Kong (grant nos. 615406 and 16122017). We acknowledge air quality data from the Atmospheric Research Center, Institute of Environment at HKUST.
The authors declare no competing interests.
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Xue, J., Yu, X., Yuan, Z. et al. Efficient control of atmospheric sulfate production based on three formation regimes. Nat. Geosci. 12, 977–982 (2019). https://doi.org/10.1038/s41561-019-0485-5
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