Enhancement of aqueous sulfate formation by the coexistence of NO2/NH3 under high ionic strengths in aerosol water

Current air quality models usually underestimate the concentration of ambient air sulfate, but the cause of this underestimation remains unclear. One reason for the underestimation is that the sulfate formation mechanism in the models is incomplete, and does not adequately consider the impact of the synergistic effects of high concentrations of multiple pollutants on sulfate formation. In this work, the roles of gaseous NO2, NH3 and solution ionic strength in the formation of sulfate in the aqueous phase were quantitatively investigated using a glass reactor and a 30 m3 smog chamber, separately. The results showed that sulfate formation was enhanced to different degrees in the presence of gas-phase NO2, NH3 and their coexistence as solutes in both liquid solution and aerosol water. NH3 enhances the aqueous oxidation of SO2 by NO2 mainly by accelerating the uptake of SO2 through increased solubility. More importantly, we found that high ionic strength in aerosol water could significantly accelerate the aqueous oxidation of SO2, resulting in unexpectedly high S(VI) formation rates. We estimate that under severe haze conditions, heterogeneous oxidation of SO2 by NO2 on aerosols may be much shorter than that through gas phase oxidation by OH, aided by high ionic strengths in aerosols. Considering the existence of complex air pollution conditions with high concentrations of NO2, NH3 and aerosol water, as expected in typical urban and suburban settings, the sulfate formation mechanisms revealed in the present work should be incorporated into air quality models to improve the prediction of sulfate concentrations.