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Published December 2019 | Supplemental Material
Journal Article Open

Efficient control of atmospheric sulfate production based on three formation regimes


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 NO_x (NO and NO₂) play essential roles. Here we present a coherent description of the dependence of SO₄²⁻ formation on SO₂ 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 SO₄²⁻ 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 SO₂, whereas in the high-NO_x regime, NO₂ is a sink for HO_x. Moreover, at highly elevated NO_x 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 NO_x emissions in the NO₂-oxidant regime, suggesting a co-reduction strategy. Formation of SO₄²⁻ is relatively insensitive to NO_x reduction in the low-NO_x regime, whereas reduction of NO_x actually leads to increased SO₄²⁻ production in the intermediate high-NO_x regime.

Additional Information

© 2019 Springer Nature Limited. Received 27 February 2018; Accepted 08 October 2019; Published 18 November 2019. Data availability: The datasets generated during and/or analysed during the current study are available at https://github.com/xuejianust/NGsulfate/blob/master/NG.zip. Code availability: The computer code used to generate the results in this manuscript is available from the corresponding authors on request. 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. Author Contributions: J.X. and X.Y. contributed equally to this work. J.X., X.Y., and J.Z.Y. conceived the regime framework describing SO42– formation. X.Y. and J.X. performed model simulations. A.K.H.L and Z.B.Y. were instrumental in starting the project. J.X., X.Y., Z.B.Y, S.M.G., J.Z.Y. and J.H.S. wrote the manuscript. The authors declare no competing interests.

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