Spatially resolved Raman investigation on phase separations of mixed Na2SO4/MgSO4 droplets

By using spatially resolved Raman spectroscopy together with in situ microscopy, mixed Na2SO4/MgSO4 aerosol particles with molar ratios of 1 : 1 and 2:1 deposited on a quartz substrate were carefully examined in their evaporation processes. Upon decreasing the relative humidity (RH), phase separations were found to occur for these droplets. For Na2SO4/MgSO4 droplets with a molar ratio of 1 : 1, two paths of phase separation were identified, and a large amount of small crystals were observed to disorderly distribute in the residual solutions. By comparisons with the known Raman spectra of crystals, it was concluded that the scattered crystals in the two paths were anhydrous Na2SO4 in metastable phase III and the double salt of Na2SO4 center dot MgSO4 center dot 4H(2)O, respectively. For Na2SO4/MgSO4 aerosol droplets with a molar ratio of 2: 1, only anhydrous Na2SO4 in metastable phase III precipitated with the decrease of RH. Copyright (C) 2008 John Wiley & Sons, Ltd.