Size-Dependent Reaction Pathways in Multiphase SO ₂ Oxidation Chemistry

High Resolution Image Download MS PowerPoint Slide The paradigm for atmospheric multiphase SO 2 chemistry, traditionally based on bulk aqueous-phase studies, involves a multistep process of SO 2 uptake followed by oxidation. Here, we decouple these steps at the single-droplet level by comparing SO 2 uptake and reported dissolved S(IV)(aq) oxidation in aqueous microdroplets of varying sizes. Our results reveal that SO 2 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 SO 2 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 SO 2 oxidation in Na 2 SO 3 microdroplets with contrasting radii ( r = 5.9 and 94.0 μm) further supports this size-dependent mechanistic transition. Overall, by distinguishing SO 2 uptake from subsequent S(IV)(aq) oxidation, we show that size can modulate the rate-limiting step, leading to different multiphase SO 2 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.