HO2 Radicals Regulate the Kinetics of Copper-Catalyzed Multiphase Sulfate Formation

Abstract

Sulfate aerosols are produced in urban haze through multiphase SO₂ oxidation catalyzed by transition-metal ions (TMIs). Yet it remains unclear how the catalytic kinetics are regulated by photochemical radicals that also participate in TMIs’ redox cycles. Here, using a kinetic model, we investigate the influence of HO₂ radicals on the kinetics of copper-catalyzed S(IV) oxidation by NO₂─a recently identified reaction contributing to sulfate formation in China’s urban haze. We find that at higher aerosol pH, more HO₂ deprotonates to O₂^–, and the combined HO₂/O₂^– radical pool acts not as an oxidant, but as a reductant that converts Cu²⁺ to Cu⁺. With fewer Cu²⁺ ions available to activate S(IV), sulfate formation is inhibited. This inhibitory effect is enhanced at low-ionic-strength conditions owing to reduced salting-out of aqueous HO₂. As the air pollution in China has been alleviated, SO₂ and secondary aerosol concentration have decreased, leading to generally higher aerosol pH, lower ionic strength, and stronger atmospheric photochemical reactivities. Consequently, more HO₂/O₂^– radicals may participate in the TMIs’ cycles and alter their effectiveness in catalyzing SO₂ conversion. Our findings highlight the importance of TMIs’ cycle through multiple simultaneous heterogeneous reactions across various pH and ionic strength conditions.