Size Dependent Uncatalyzed Sulfite Oxidation in Aqueous Microdroplets

Aqueous aerosols and microdroplets exhibit unique chemical kinetics relative to the bulk phase. Aqueous aerosol and microdroplet chemistry, as it relates to multiphase atmospheric chemistry, also has major impacts on air quality and climate. Here, we have investigated uncatalyzed oxidation of sulfite (SO32−) to sulfate (SO42−) by O2 in aqueous microdroplets deposited on a superhydrophobic substrate, as a function of size, gas-phase composition, and temperature utilizing an environmental cell and in-situ micro-Raman spectroscopy. We show that the uncatalyzed sulfite oxidation driven by O2 in aqueous microdroplets is size-dependent across varying O2 concentrations and temperatures. Sulfite oxidation rates scale with the surface-area to volume ratio (i.e. 1/radius) of the microdroplet and oxidation occurs in the absence of any added catalysts. We use a resistor-based approach to model multiphase mass transfer and reaction in the experimental system and confirm that the observed size-dependent kinetics are consistent with slow bulk kinetics coupled with an efficient reaction at the interface (kII = 9.44×10-3 M-1 s-1, γs,0 = 9.27×10-10 at 298 K and 21% O2). Above a critical droplet radius, bulk kinetics dominate, but for sufficiently small sizes, which are relevant in the atmosphere, rates are accelerated due to the role of 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.