State of Water in Supersaturated Nitrate Aerosols Disclosed by the Raman Difference Spectra

The hygroscopic properties of supersaturated aerosols as a function of relative humidity (RH) can be determined on the molecular level by the solutes with varied structures, as well as the solvent in the state of solvated water or free water. Although the former has been investigated by FTIR and Raman spectroscopy, the latter has gone mostly unnoticed. In this work, the state of water in supersaturated Mg(NO3)2 and NaNO3 aerosols were investigated through the application of the Raman difference spectra with respect to pure water. This technique could be developed from the observation that the Raman scattering and infrared absorbance cross sections of the molecular vibrations of interest remain practically unchanged from diluted solutions to supersaturated aerosols at low RHs. The results were expressed in terms of the percentage of free water (Wfree) as a function of RH, as well as the solvated water-to-solute ratio (WsolvatedSR) and the free water-tosolute ratio (WfreeSR) as a function of the total water-to-solute ratio (WSR). Solvated water observed in the Raman difference spectra was primarily related to the first hydration layers. In Mg(NO3)2 aerosols, three phases were identified with distinct mechanisms for the transition of the state of water. One unique structure with WsolvatedSR ) 4 was proposed to occur in supersaturated Mg(NO3)2 aerosols at low RHs. In NaNO3 aerosols, it was found that the equality of solvated and free water could not provide a necessary condition for efflorescence, in contrast to the recent investigations by fluorescence spectroscopy. According to this investigation, solvated water could be more abundant than free water not only prior to the efflorescence of supersaturated NaNO3 aerosols, but also in relatively diluted droplets.