The present work exploited the ozone oxidation of binary aqueous STS micro-droplets as a proxy of sulfur oxidation in inorganic aerosols. The kinetics measurement results of this system show that the ozone oxidation rates of S 2 O 2- 3 in micro-droplets at low RHs (∼ 30%) are significantly faster than those at high RHs (> 80%), and such acceleration is attributed to high droplet ionic strengths enriched at low RHs (∼ 50 M at ∼ 30% RH). According