The Role of the Droplet Interface in Controlling the Multiphase Oxidation of Thiosulfate by Ozone

Predicting reaction kinetics in aqueous microdroplets, including aerosols and cloud droplets, is challenging due to the probability that the underlying reaction mechanism can occur both at the surface and in the interior of the droplet. Here, we use a stochastic reaction-diffusion model of thiosulfate oxidation by gas phase ozone to examine how the interface influences the multiphase reaction kinetics measured in levitated microdroplets using mass spectrometry. Building a realistic kinetic model of multiphase reactions requires both a detailed multistep reaction mechanism as well as the surface affinities of all reactants and products. Deep-UV Second Harmonic Generation spectroscopy is used to probe surface affinities of thiosulfate, sulfate, and sulfite, key species in the reaction mechanism. Thiosulfate has an appreciable surface affinity with a measured Gibbs free energy of adsorption of -7.29 ± 2.47 kJ/mol in neutral solution, while sulfate and sulfite exhibit negligible surface propensity. The Gibbs free energy is combined with data from liquid flat jet ambient pressure x-ray photoelectron spectroscopy to constrain the concentration of thiosulfate at the surface in the kinetic model. Kinetic simulations show that the primary reaction between thiosulfate and ozone occurs at the interface and in the bulk, with the contribution of the interface decreasing from ~65% at pH 5 to ~45% at pH 13. Additionally, sulfate, the major product of thiosulfate ozonation and an important species in atmospheric processes, can be produced by two different pathways at pH 5, one with a contribution from the interface of >70% and the other occurring predominantly in the bulk (>98%). Finally, we use the kinetic model to demonstrate the impact of a range of atmospherically relevant droplet sizes and reactant concentrations on product distributions and relative importance of surface and bulk chemistry. The observations in this work have implications for mining wastewater remediation and are likely applicable to other atmospherically-relevant reaction mechanisms, suggesting that future microdroplet/aerosol chemistry studies should carefully consider the role of both interfacial and bulk chemistry.