for determining the aerosol radius and its time evolution of each optically trapped microdroplets. The present study utilized a Mie theory simulation program mrfit developed by Preston et al. to analyze the spectral positions of CERS signals and obtained fitted radius and refractive index (RI). Following the strategy of our previous work, the CERS peaks were identified by their distinct sharp features (FWHM = a couple of cm -1 ) and the synchronous progress of their wavelengths in measured Raman spectra time series (see Figure ). Such assignment of progressing CERS peaks was also assisted by the predicted positions from the Mie theory simulation, when assuming a continuous change of droplet size and RI and no mode hopping of CERS peaks. The fitted RI values can be further used to retrieve [STS] b,0 , and its values determined by both CERS and spontaneous Raman spectroscopy are also in a good agreement. Since STS is the only ionic species in micro-droplets prior reaction, the droplet ionic strength prior reaction, I, is equal to 3[STS] b,0 . For the case where no obvious CERS signals existed, we determined the droplet radius prior reaction via brightfield imaging. According to Table , the typical radius of droplets studied in this work is about 3 µm. (at 445 cm -1 or 997 cm -1 ), SO 2- 4 (at 980 cm -1 ) and HSO - 4 (1050 cm -1 ) from a trapped aqueous STS micro-droplet exposed to O 3 (∼14 ppm) after 0 second, as well as the representative error bars for droplet radius. See Table (Expt. 11) in the Supporting Information for details of the experimental conditions. ) and (b) ternary aqueous STS:sucrose mixtures in 2:4 mass ratio (Expt. 20-22 in Table ), exposed to O 3 (∼12-15 ppm) at different RHs, as well as KM-SUB model fit results (lines). The data shown here are the integrated Raman band intensities of S 2 O 2- 3 which are normalized to those prior reaction. Each data point represents the mean of 5 consecutive spectra, and for clarity, only the representative error bars are shown here. The KM-SUB input parameters for these fits are provided in Table .