Impact of the oxidation of SO2 by NO2 on regional sulfate concentrations over the North China Plain

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We coupled the reaction for the oxidation of water-soluble SO2 by NO2 into the WRF-Chem model and considered the impact of adding this reaction to SO42− concentrations. A comparison of observations and simulation results showed that the model can reproduce the temporal variation and spatial distribution of meteorological elements and concentrations of pollutants over the North China Plain. After adding the reaction, the simulated SO42− concentrations increased by 16%, and the normalized mean bias between the simulated and observed SO42− concentrations decreased by 10% from December 1 to 8, 2018, at an urban site in Beijing. For the horizontal distribution, SO42− concentrations after adding the reaction increased by approximately 15%–25% in Tianjin, at the junction of Hebei and Shanxi provinces, and parts of Inner Mongolia, and by approximately 10%–15% in Beijing and other regions in Hebei Province during this period. The impact of the oxidation of SO2 by NO2 on SO42− concentrations showed clear seasonal variations, with the SO42− concentrations increasing most in winter by a maximum of 20% in northwestern Hebei and southern Inner Mongolia, 12%–16% in Beijing, 8%–12% in other areas of Hebei and Tianjin, and 2%–6% in other regions. The SO42− generated by the reaction was significantly higher in winter and autumn than in other seasons because lower temperatures meant that more SO2 and NO2 were in the liquid phase.