Accelerated Sulfur Oxidation by Ozone on Surfaces of Single Optically Trapped Aerosol Particles

The sulfur oxidation in mixed sodium thiosulfate/sucrose/aqueous micro-droplets by gaseous ozone is studied in this work via aerosol optical tweezers coupled with Ra- man spectroscopy, which can simultaneously determine various physicochemical proper- ties and the heterogeneous reaction kinetics of single optically trapped micro-droplets, allowing for elucidating their complicate interplay. According to the kinetics measure- ment results at different relative humidities, ozone concentrations and stoichiometries of inorganic and organic solutes, this work finds that a high aerosol ionic strength can accelerate the ozone oxidation of thiosulfate at air-water interfaces, while a high aerosol viscosity prolongs the reaction timescales due to diffusion-limited kinetics. The kinetic multilayer model of aerosol surface and bulk chemistry (KM-SUB) is utilized to inves- tigate the observed heterogeneous kinetics results and to retrieve the surface reaction rate coefficients. The KM-SUB simulations results indicate that the observed kinet- ics of sulfur oxidation in binary sodium thiosulfate aqueous micro-droplets with high ionic strengths is dominated by interfacial reactions, and the fitted surface reaction rate coefficients increase one order of magnitude when the droplet ionic strength increases around 40 M. Furthermore, this work demonstrated that including the effects of inter- facial ion depletion in the kinetics simulations can lead to an excellent agreement with the experimental results, implying its potential role in the interfacial kinetics.