A strategy for single supersaturated droplet analysis: confocal Raman investigations on the complicated hygroscopic properties of individual MgSO4 droplets on the quartz substrate.

We report a new strategy for single supersaturated droplet analysis, i.e., the complicated hygroscopic properties of MgSO4 aerosols under supersaturated state were studied through the micro-Raman observation on an individual MgSO4 droplet deposited on a quartz substrate in a relative-humidity-controlled chamber. Upon reduction of the ambient relative humidity (RH), MgSO4 droplets with tiny volume lost water but did not effloresce. Thus, a detailed spectral evolution of the symmetric stretching vibration band (v1-SO4(2-)) from free ions (at approximately 983 cm(-1)) to monodentate (approximately 995 cm(-1)) and then to bidentate contact ion pairs (CIPs) or more complex chain-structural compositions (approximately 1021 cm(-1)) was observed with the high signal-to-noise (S/N) confocal Raman spectra of the droplet with a diameter of approximately 80 microns. Such a transition process could be well-described by the changes of relative intensity at 983, 995, and 1021 cm(-1). Four steps, i.e., concentrated step, monodentate CIPs step, bidentate CIPs step, and gel step, were roughly observed in the dehumidifying-humidifying cycle according to the intensity ratios of I995/I983 and I1021/I983. Even though the area ratio of the O-H stretching band of water molecules to the v1-SO4(2-) band seemed reversible in the dehumidifying and humidifying processes, the intensity ratios of I995/I983 and I1021/I983 showed a hysteresis in the decomposition of CIPs in the humidifying process with the RH < 40%. The O-H stretching envelope of the MgSO4 droplet was also observed to be sensitive to the structural changes of the hydrogen bonding of water molecules in the four steps. The intensity ratio of Raman scattering for the components with strong hydrogen bonds to those with weak ones, i.e., I3224/I3431, was used to understand the effects of CIPs on the water structures of the first hydration layer of Mg2+. Good consistency on the hysteresis in the humidifying process was also observed from the ratio of I3224/I3431 changing with RH.