Probe molecular diffusivity in single ternary inorganic–organic microdroplets via interfacial ozonolysis of thiosulfate†

Abstract

In this study, aqueous sodium thiosulfate microdroplets mixed with glucose or sucrose are used as a model system of ternary inorganic–organic aerosols. The interfacial ozone oxidation of thiosulfate, which has been characterized in our previous work [J. Phys. Chem. C, 2023, 127, 6248], is exploited via aerosol optical tweezers to determine the bulk diffusivity of thiosulfate in such inorganic–organic microdroplets under variable conditions of RH and inorganic–organic mass ratio. A kinetic multilayer model of aerosol surface and bulk chemistry (KM-SUB) is also utilized to retrieve the bulk diffusivity of thiosulfate from the kinetics measurement results. The kinetics results at relatively high RHs show that the observed reaction time scale increases when lowering RH, and the magnitude of thiosulfate diffusion coefficients is between Stokes–Einstein predictions for binary sodium thiosulfate–water systems and binary organic–water systems, indicating the dominant diffusion kinetics of thiosulfate in viscous fluid matrices of homogeneously mixed inorganics and organics. However, when RH is below 30% for glucose or 40% for sucrose, the kinetics results exhibit incomplete thiosulfate depletion upon prolonged ozone exposure, indicating the co-existence of two distinctly fast and slow diffusion components of thiosulfate. The diffusion coefficients of undepleted thiosulfates become similar to the SE predictions of binary organic–water systems, and they are a few orders of magnitude smaller than those of rapidly depleted thiosulfates. According to the literature, such a diffusion limitation may be attributed to an ion–molecule effect which may lead to the formation of inorganic–organic microgels in aerosols at atmospherically relevant RHs. The results of this work suggest that the cooperative effects of inorganics and organics can play a potential role in the reaction kinetics of atmospheric inorganic–organic aerosols.