A strategy for single supersaturated droplet analysis: Confocal Raman investigations on the complicated hygroscopic properties of individual MgSO ₄ 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-S0₄^2-) from free ions (at ∼983 cm^-1) to monodentate (∼995 cm^-1) and then to bidentate contact ion pairs (CIPs) or more complex chain-structural compositions (∼1021 cm^-1) was observed with the high signal-to-noise (S/N) confocal Raman spectra of the droplet with a diameter of ∼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 I ₉₉₅/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., I ₃₂₂₄/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.