Size-Dependent Reaction Pathways in Multiphase SO2 Oxidation Chemistry

Abstract The paradigm for atmospheric multiphase SO2 chemistry, traditionally based on bulk aqueous-phase studies, involves a multistep process of SO2 uptake followed by oxidation. Here, we decouple these steps at the single-droplet level by comparing SO2 uptake and reported dissolved S(IV)(aq) oxidation in aqueous microdroplets of varying sizes. Our results reveal that SO2 uptake into microdroplets to form S(IV)(aq) is suppressed as the droplet radius (r) decreases, in contrast to our previous finding that S(IV)(aq) oxidation is enhanced at smaller radii. Fitting these opposing size-dependent processes shows that SO2 uptake is 1–2 orders of magnitude slower than S(IV)(aq) oxidation for typical atmospheric aerosols (r ≲ 5 μm). Thus, larger cloud and fog microdroplets follow the conventional bulk-phase pathway while smaller aqueous aerosols become uptake-limited and favor direct oxidation at the surface. Mn(II)-catalyzed SO2 oxidation in Na2SO3 microdroplets with contrasting radii (r = 5.9 and 94.0 μm) further supports this size-dependent mechanistic transition. Overall, by distinguishing SO2 uptake from subsequent S(IV)(aq) oxidation, we show that size can modulate the rate-limiting step, leading to different multiphase SO2 oxidation mechanisms. This step-specific perspective highlights the critical and multiple roles of the air–water interface and emphasizes how different size dependencies in individual steps within a multistep mechanism can further our understanding of atmospheric multiphase chemistry.