The reactions in aerosol phase can play a key role in atmospheric chemistry. The oxidation of SO 2 in aqueous aerosol droplets is one of main sulfate sources in atmosphere, because of the fast kinetics of aqueous-phase sulfur oxidations. Large Criegee intermediates can survive at air-water interfaces, while water is also their major sink, allowing them to react with other species. The aqueous droplets can mediate the efficient production of atmospheric formic acid from gaseous formaldehyde. 9,10 Furthermore, particle-phase oxidation reactions determine the formation, growth and aging of secondary organic aerosols. To better quantify the effects of aerosols on environments, substantial progress has been made in recent years for better understanding these aerosol-phase reactions, including the complicate interplay between their reaction dynamics and physicochemical properties of aerosols. The chemical pathways of aerosols have a complicate interplay with the physicochemical properties of aerosols, which could be affected by the chemical composition of aerosols. Atmospheric aerosols usually contain both inorganic and organic species, which determine the ionic strength and viscosity of aerosols, respectively. These two physicochemical properties have been recognized to play key roles in the reaction kinetics of aerosols. A high ionic strength in aerosols can enhance the reactive uptakes of gaseous reactants, the reaction rate coefficients in aerosol bulks and the rates of aqueous photolysis. Several recent studies also found that high ionic strengths can enhance the reactive uptakes of SO 2 by peroxides and the sulfate formations from the subsequent SO 2 oxidation in deliquesced aerosol particles. Conversely, a high viscosity in organic aerosols can lead to slow diffusion times of molecular species and thus the prolonged timescales of dynamic processes in aerosols, while the relationship between viscosity and diffusivity could also highly depend on phase states of aerosols and molecular interactions. The roles of chemical composition in the heterogeneous reaction kinetics of aerosols are often rationalized in terms of the effects of ionic strength and diffusivity to reactions in aerosol bulks. However, for the reactions at aerosol surfaces or interfaces, it still requires more advanced investigations to clarify the potential role of chemical composition. Such detailed understanding promises to elucidate the mechanism of the accelerated droplet reactions discovered in recent studies. There have been extensive investigations which focused on the effects of ions to adsorption and gradients of concentration and pH at interfaces. The increasing number of ions in bulks can significantly modify the population of anions at interfaces. A very recent kinetics study of nitrite ozonolysis in aerosols investigated the significant dependence of interfacial nitrite concentrations on bulk nitrite concentrations and its potential role in the interfacial kinetics. The pH at surfaces of aerosols can also be different from those in aerosol bulks, and a recent aerosol kinetics study observed the enhanced rates of catalyzed sulfur oxidations in aerosols and attributed this enhancement to the pH gradient at the air-water interfaces of aerosols. In the present work, we choose the oxidation reaction of sodium thiosulfate (STS), Na 2 S 2 O 3 , with O 3 as a model system of sulfur oxidation in aerosols, as such model reaction has been utilized in several studies associated with atmospheric aerosol chemistry. In particular, the present work exploited aerosol optical tweezers (AOT) combined with Raman spectroscopy to characterize the influences of chemical composition to the oxidation reaction kinetics of mixed STS/sucrose aqueous micro-droplets by gaseous O 3 . The present work performed the kinetics measurements of the S 2 O 2- 3 + O 3 reaction in micro-droplets at different relative humidities (RHs), O 3 pressures and stoichiometries of STS and sucrose.