Abstract. Sulfate aerosols exert profound impacts on climate, ecosystem, visibility, and public health, but the sulfate formation pathway remains elusive. In the present study, a source-oriented WRF-Chem model is applied to simulate a persistent air pollution episode from 04 to 15 July 2015 in Beijing-Tianjin-Hebei (BTH), China to study contributions of four pathways to the sulfate formation. When comparing simulations to measurements in BTH, the index of agreement (IOA) of meteorological parameters, air pollutants and aerosol species generally exceeds 0.6. On average in BTH, the heterogeneous reaction of SO2 involving aerosol water and the SO2 oxidation by OH constitutes the two most important sulfate sources, with a contribution of about 35 %–38 % and 33 %–36 % respectively. The primary emission accounts for around 22 %–24 % of sulfate concentrations due to high SO2 emissions. The SO2 oxidation by stabilized Criegee Intermediates (sCI) also plays an appreciable role in the sulfate formation, with a contribution of around 9 % when an upper limit of the reaction rate constant of sCI with SO2 (κsCI + SO2 = 3.9 × 10−11 cm3 s−1) and a lower limit of the reaction rate constant of sCI with H2O (κsCI + H2O = 1.97 × 10−18 cm3 s−1) are used. Sensitivity studies reveal that there still exist large uncertainties in the sulfate contribution of the SO2 oxidation by sCI. The sulfate contribution of the reaction is decreased to less than 3 % when κsCI + SO2 is decreased to 6.0 × 10−13 cm3 s−1. Furthermore, when κsCI + H2O is increased to 2.38 × 10−15 cm3 s−1 based on the reported ratio of κsCI + SO2 to κsCI + H2O (6.1 × 10−5), the sulfate contribution becomes insignificant, less than 2%. Further studies need to be conducted to better determine κsCI + SO2 and κsCI + H2O to evaluate effects of the sCI chemistry on the sulfate formation.
Effects of stabilized Criegee Intermediates (sCI) on the sulfate formation: A case study during summertime in Beijing-Tianjin-Hebei (BTH), China
