Continuous Single-Droplet Tracking Reveals Scaling Laws of Surface-Confined Mn(II)-Catalyzed SO2 Oxidation

Size-dependent kinetics in microdroplets are often used to infer whether reactions occur in the bulk liquid or at the air-water interface, but ensemble measurements can obscure the underlying scaling law. Here, we continuously tracked the radius of one optically levitated droplet to determine reaction location directly from its growth trajectory of the same evolving microdroplet. Benchmark SO2 autoxidation experiments recovered distinct scaling behaviors for bulk reaction at pH 5.0 and interfacial reaction at pH 3.8. Applying the same analysis to Mn(II)-catalyzed SO2 oxidation showed that the catalytically effective Mn inventory is diluted with droplet surface area rather than volume, providing direct dynamic evidence for surface-specific Mn catalysis during sulfate-driven hygroscopic growth. Broadband light-scattering aerosol optical tweezers enabled week-long tracking with nanometer-scale radius precision over a broad size range. The central advance is therefore the continuous one-droplet scaling analysis, which resolves how reaction kinetics evolve together with droplet growth, compositional evolution, and catalyst dilution.