Single Droplet Tweezer Revealing the Reaction Mechanism of Mn(II)-Catalyzed SO₂ Oxidation

Sulfate aerosol is one of the major components of secondary fine particulate matter in urban haze that has crucial impacts on the social economy and public health. Among the atmospheric sulfate sources, Mn(II)-catalyzed SO₂ oxidation on aerosol surfaces has been regarded as a dominating one. In this work, we measured the reaction kinetics of Mn(II)-catalyzed SO₂ oxidation in single droplets using an aerosol optical tweezer. We show that the SO₂ oxidation occurs at the Mn(II)-active sites on the aerosol surface, per a piecewise kinetic formulation, one that is characterized by a threshold surface Mn(II) concentration and gaseous SO₂ concentration. When the surface Mn(II) concentration is lower than the threshold value, the reaction rate is first order with respect to both Mn(II) and SO₂, agreeing with our traditional knowledge. But when surface Mn(II) concentration is above the threshold, the reaction rate becomes independent of Mn(II) concentration, and the reaction order with respect to SO₂ becomes greater than unity. The measured reaction rate can serve as a tool to estimate sulfate formation based on field observation, and our established parametrization corrects these calculations. This framework for reaction kinetics and parametrization holds promising potential for generalization to various heterogeneous reaction pathways.