Sulfate is a major component of the urban PM2.5 in China. The current air-quality models consistently underestimate the particulate sulfate formation during haze episodes, indicating that our understanding on the mechanism of atmospheric SO2 conversion remains incomplete. Recently, the synergistic oxidation of SO2 and NO2 within aerosol microdroplets has been recognized as a key additional pathway for SO2 atmospheric conversion. However, the influence of ambient conditions (such as molar ratio of reactant gases and relative humidity) on the reaction kinetics remains understudied. Here we adopt micro-Fourier transform infrared (micro-FTIR) spectroscopy to measure the time-evolution of SO42- and NO3- concentration within a single MgCl2 microdroplet, under various SO2/NO2 molar ratio. We observed that: The single droplet of MgCl2 reacts with SO2/NO2 to form SO42- and NO3-, and that the formation rate of SO42- is the fastest when the molar ratio of SO2/NO2 is 1:5. Our research will provide reliable kinetic parameters for air-quality model, which will in turn aid the policy makers to device informed PM2.5 containment strategies.