Size Dependent Uncatalyzed Sulfite Oxidation in Aqueous Microdroplets

Aqueous aerosols and microdroplets exhibit unique chemical kinetics relative to the bulk phase, impacting air quality and climate via multiphase atmospheric processes. Here, we investigate uncatalyzed oxidation of sulfite to sulfate by O₂ in aqueous microdroplets deposited on a superhydrophobic substrate, as a function of size, gas-phase composition, and temperature utilizing in situ micro-Raman spectroscopy. We show that the uncatalyzed sulfite oxidation is size-dependent across varying O₂ concentrations and temperatures. Reaction rates scale with the surface-area to volume ratio (1/radius) of the microdroplet. We use a resistor-based approach to model multiphase mass transfer and reactions in the experimental system, confirming that the observed size-dependent kinetics reflect slow bulk kinetics coupled with an efficient reaction at the interface─k₂ = 9.43 × 10^-3 (+2.26 × 10^-3/-2.38 × 10^-3) M^-1 s^-1, γ_s,0 = 9.27 × 10^-10 (+9.30 × 10^-11/-1.70 × 10^-10) at 298 K and 21% O₂. Above a critical droplet radius, bulk kinetics dominate, but for sufficiently small atmospherically relevant sizes, rates are accelerated due to the role of the interfacial reaction. These results provide insights into chemistry in microcompartments and provide an outlook for improved representations of sulfate formation in atmospheric droplets and aerosols in large-scale atmospheric chemistry models.