Characteristics and major influencing factors of sulfate production via heterogeneous transition-metal-catalyzed oxidation during haze evolution in China

The heterogeneous transition-metal-catalyzed oxidation (hereinafter referred to as TMI-catalyzed pathway) is an important pathway for atmospheric sulfate formation. While most research has focused on in-cloud reactions of sulfate, studies on its heterogeneous reactions on aerosol surfaces remain scarce. To better understand this pathway, we investigated the temporal changes of sulfate heterogeneous production rates of this pathway during haze evolution, and did not find any pattern in different haze stages. Although previous studies have shown a high sensitivity of the TMI-catalyzed pathway to aerosol pH, sulfate production rates of this pathway did not always increase in haze aggravation stages with higher aerosol acidity. We then explored the major influencing factors on this pathway in various haze evolution stages. The results suggest that the highly variable dissolved Fe(III) and Mn(II) concentrations and ionic strength during haze evolution processes would considerably influence the production rate of the TMI-catalyzed pathway. This further implies that the north or northwest air masses, which often carry crustal-source particulates, may have a non-negligible effect on sulfate formation during the generation and/or dissipation stages of haze in Beijing by influencing aerosol pH and transition-metal contents.