Effects of stabilized Criegee intermediates (sCIs) on sulfate formation: A sensitivity analysis during summertime in Beijing-Tianjin-Hebei (BTH), China

Sulfate aerosols have profound impacts on the 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 4 to 15 July 2015 in Beijing-Tianjin-Hebei (BTH), China, to study the contributions of four pathways to 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. Primary sulfate emissions account for around 22 %-24% of the total sulfate concentration. SO2 oxidation by stabilized Criegee intermediates (sCIs) also plays an appreciable role in sulfate formation, with a contribution of around 9% when an upper limit of the reaction rate constant of sCIs with SO2 (KsCI + SO₂ = 3.9×10^-11 cm³ s^-1) and a lower limit of the reaction rate constant of sCIs with H2O (KsCI + H2O = 1.97 × 10^-18 cm³ s^-1) are used. Sensitivity studies reveal that there are still large uncertainties in the sulfate contribution of SO2 oxidation by sCIs. The sulfate contribution of the reaction is decreased to less than 3% when KsCI + SO₂ is decreased to 6.0 × 10^-13 cm³ s^-1. Furthermore, when KsCI + H2O is increased to 2.38 × 10^-15 cm³ s^-1 based on the reported ratio of KsCI + H2O to KsCI + So2 (6.1 × 10^-5), the sulfate contribution becomes insignificant at less than 2 %. Further studies need to be conducted to better determine KsCI + SO2 and KsCI + H2O to evaluate the effects of sCI chemistry on sulfate formation.