Simulations of Sulfate Production on Surfaces of Acidic Aerosol Particles during the Severe Haze Episode in December 2015 over Northern China

Sulfate has a significant influence on earth climate system, air quality, visibility and public health. However, current air quality models, which only include gas phase and in-cloud oxidation of SO₂, cannot explain the rapid growth of sulfate during haze episodes in China. A recent laboratory study proposed that SO₂ oxidation on surfaces of acidic aerosol particles could be an efficient sulfate formation pathway. In this study, we added this heterogeneous reaction in Weather Research and Forecasting model coupled to chemistry (WRF-Chem) to evaluate the importance of sulfate formation on surfaces of acidic aerosol particles during a severe haze episode in December 2015 over North China Plain (NCP). Results showed that the model not only captured the magnitude and temporal variation of sulfate, but also reproduced the rapid increase of SO₂ oxidation rates from clean to polluted days. We also found that contributions of advection transport, vertical mixing coupled with dry deposition, and chemical processes to sulfate are dominant. For chemical processes, gas phase and heterogeneous oxidation contribute to 89.0% and 11.0% in non-heavy polluted days, and 0.5% and 99.5% in heavy polluted days, respectively, indicating the enhanced role of heterogeneous reaction in sulfate formation during the haze episode.