Sulfate Formation Apportionment during Winter Haze Events in North China

There is a large gap between the simulated and observed sulfate concentrations during winter haze events in North China. Although multiphase sulfate formation mechanisms have been proposed, they have not been evaluated using chemical transport models. In this study, the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) was used to apportion sulfate formation. It was found that Mn-catalyzed oxidation on aerosol surfaces was the dominant sulfate formation pathway, accounting for 92.3 ± 3.5% of the sulfate formation during haze events. Gas-phase oxidation contributed 3.1 ± 0.5% to the sulfate formation due to the low OH levels. The H₂O₂ oxidation in aerosol water accounted for 4.2 ± 3.6% of the sulfate formation, caused by the rapid consumption of H₂O₂. The contributions of O₃, NO₂ oxidation, and transition metal ion-catalyzed reactions in aerosol water could be negligible owing to the low aerosol water content, low pH, and high ionic strength. The contributions from in-cloud reactions were negligible due to the barrier provided by stable stratification during winter haze events.