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 obsereed 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 Na ₂SO₄ in metastable phase 111 and the double salt of Na₂SO₄-MgSO₄ · 4H₂O, 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.