Size-Dependent Reaction Pathways in Multiphase SO₂ Oxidation Chemistry

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.