Single supersaturated NaClO4 aerosol droplets on a quartz substrate were observed by a high-speed video camera for their morphologic changes with decreasing relative humidity (RH). The supersatu- rated droplets were found to form anhydrous NaClO4 at ~10% RH. Three stages were roughly observed in this process, i.e., the main stem appearance, primary and secondary branches growth, and the deep dehydration. The main stem grew steadily and finished in 120 ms. Affected by both crystal precipitation and water evaporation, primary branches grew for 1200 ms, accompanied by a slower growth of sec- ondary branches. The deep dehydration was more complicated, and lasted for a longer time.
One technique based on the difference spectra was developed to study the state of water in supersaturated Mg(NO3)2 aerosols. The technique could be derived from the observation that the Raman scattering and infrared absorbance cross sections of molecular vibrations of interest remain practically constant from diluted solutions to supersaturated aerosols. The spectra of solvated water were obtained and primarily related to the first hydration layers of solute molecules in supersaturated Mg(NO3)2 aerosols. Based on this investigation, a chain structure was proposed to occur in the supersaturated Mg(NO3)2 aerosols at low relative humidities (RHs).
