Suppression of NaNO₃ crystal nucleation by glycerol: Micro-raman observation on the efflorescence process of mixed glycerol/NaNO ₃/water droplets

Although the hygroscopicity of a NaNO₃/water microdroplet and a polyalcohol/water microdroplet, two of the most important aerosols in atmosphere, has been widely studied, little is known about the relationship between the hygroscopic behavior of mixed NaNO₃/polyalcohol/water droplets and their structures on the molecular level. In this study, the hygroscopicity of mixed glycerol/NaNO₃/water droplets deposited on a hydrophobic substrate was studied by micro-Raman spectroscopy with organic-to-inorganic molar ratios (OIRs) of 0.5, 1, and 2. In the mixed glycerol/NaNO₃/water droplets, glycerol molecules tended to combine with Na⁺ and NO₃^- ions by electrostatic interaction and hydrogen bonding, respectively. On the basis of the analyses of the changes of symmetric stretching (v_s-CH₂), asymmetric stretching (v_a-CH₂), their area ratio (Av _a-CH₂/Av_s-CH₂) of glycerol, and symmetric stretching band of NO₃^- (ν₁- NO₃^-) with relative humidity (RH), it was found that the conformation of glycerol was transformed from αα mainly to γγ and partly to αγ with a decreasing RH in the mixed droplets, contrary to the case in the glycerol/water droplet. In addition, the glycerol with γγ and αγ conformation had strong interaction with Na⁺ and NO₃^- respectively, which suppressed the formation of contact of ions and delayed the efflorescence relative humidity (ERH) for the mixed droplets compared to the NaNO ₃/water droplet.