The bimolecular reaction rate coefficient for the S 2 O 2- 3 + O 3 reaction (R1) is equal to 3.7 × 10 8 M -1 s -1 (at zero ionic strength), and that for the HSO - 3 + O 3 reaction (R4) is equal to The equilibrium constants for reactions R2 and R3 are ∼1.23 M atm -1 and 1.39 × 10 -2 M, respectively. Figure shows the representative Raman spectra of single aqueous STS droplets exposed to O 3 at two distinct RHs, and the observed molecular Raman bands can be assigned to the reactant S 2 O 2- 3 or products, such as SO 2- 4 and HSO - 4 (reactions R1a and R4). On the other hand, this work did not observe any distinguishable Raman feature of S 2 O 2- 6 (reaction R1b), which was reported to exhibit large and sharp peaks at both 1095 cm -1 and 705 cm -1 , 65 implying that any produced S 2 O 2- 6 may be subsequently oxidized to sulfates by O 3 . Besides identifying molecular species in droplets, Raman spectroscopy of single aerosol particles can also be utilized to determine several physicochemical properties of each trapped droplet required for investigating the aerosol reaction kinetics, such as radius, concentrations of solutes and ionic strength. The present work utilized the measured Raman intensity ratios of solute and water to estimate the absolute concentrations of STS, SO 2- 4 and HSO - 4 inside each droplet, as well as aerosol ionic strength, and they are provided in Tables S1 and S5. The estimated droplet STS concentrations prior reaction, [STS] b,0 , agree with the predictions of E-AIM model, as shown in Figure , justifying the methodology of the concentration determination employed in this work. In addition to spontaneous Raman scatterings, the optically trapped micro-droplets could also yield cavity-enhanced Raman scattering (CERS), which are WGMs amplified by stimulated Raman scatterings, as shown in Figure . Analyzing these CERS signals allow