A strategy for single supersaturated droplet analysis: confocal Raman investigations on the complicated hygroscopic properties of individual MgS[O.sub.4] droplets on the quartz substrate

We report a new strategy for single supersaturated droplet analysis, i.e., the complicated hygroscopic properties of MgS[O.sub.4] aerosols under supersaturated state were studied through the micro-Raman observation on an individual MgS[O.sub.4] droplet deposited on a quartz substrate in a relative-humidity-controlled chamber. Upon reduction of the ambient relative humidity (RH), MgS[O.sub.4] droplets with tiny volume lost water but did not effloresce. Thus, a detailed spectral evolution of the symmetric stretching vibration band ([v.sub.1]-S[O.sub.4.sup.2-]) from free ions (at ~983 [cm.sup.-1]) to monodentate (~995 [cm.sup.-1]) and then to bidentate contact ion pairs (CIPs) or more complex chain-structural compositions (~1021 [cm.sup.-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.sup.-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 [I.sub.995] /[I.sub.983] and [I.sub.1021]/[I.sub.983]. Even though the area ratio of the O-H stretching band of water molecules to the [v.sub.1] S[[O.sub.4.]sup.2-] band seemed reversible in the dehumidifying and humidifying processes, the intensity ratios of [I.sub.995]/ [I.sub.983] and [I.sub.1021]/[I.sub.983] showed a hysteresis in the decomposition of CIPs in the humidifying process with the RH < 40%. The O-H stretching envelope of the MgS[O.sub.4] 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.sub.3224]/[I.sub.3431], was used to understand the effects of CIPs on the water structures of the first hydration layer of [Mg.sup.2+]. Good consistency on the hysteresis in the humidifying process was also observed from the ratio of [I.sub.3224]/[I.sub.3431] changing with RH.