HO ₂ Radicals Regulate the Kinetics of Copper-Catalyzed Multiphase Sulfate Formation

Sulfate aerosols are produced in urban haze through multiphase SO 2 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 HO 2 radicals on the kinetics of copper-catalyzed S(IV) oxidation by NO 2 ─a recently identified reaction contributing to sulfate formation in China’s urban haze. We find that at higher aerosol pH, more HO 2 deprotonates to O 2 –, and the combined HO 2 /O 2 – radical pool acts not as an oxidant, but as a reductant that converts Cu 2+ to Cu + . With fewer Cu 2+ 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 HO 2 . As the air pollution in China has been alleviated, SO 2 and secondary aerosol concentration have decreased, leading to generally higher aerosol pH, lower ionic strength, and stronger atmospheric photochemical reactivities. Consequently, more HO 2 /O 2 – radicals may participate in the TMIs’ cycles and alter their effectiveness in catalyzing SO 2 conversion. Our findings highlight the importance of TMIs’ cycle through multiple simultaneous heterogeneous reactions across various pH and ionic strength conditions.