Confocal Raman studies of Mg(NO3)2 aerosol particles deposited on a quartz substrate: supersaturated structures and complicated phase transitions.

Individual Mg(NO3)2 aerosol particles deposited on a quartz substrate were investigated by confocal Raman spectroscopy. With decreasing the relative humidity (RH) from 92.0% to 1.8%, Raman spectra were obtained of Mg(NO3)2 droplets with water-to-solute molar ratios (WSRs) from 43.1 to 5.2, as well as of amorphous particles. At WSR < 6.0, contact ion pairs between Mg2+ and NO3- occurred abundantly, while at RHs of 2.2% and 1.8% with even lower WSRs, amorphous particles appeared with quasi-lattice structures. Two components, one at 3259.0 cm-1 (C1) and the other at ∼3480.0 cm-1 (C2), were resolved for the water O−H stretching envelope through nonlinear curve fittings. The area ratio of C1 to C2, that is, A1/A2, declined with the decrease of WSR, reflecting the breakage of strong hydrogen bonds induced by the hydration of NO3-. Curve fittings were also carried out for the water O−H stretching envelope of NaNO3 droplets. The value of A1/A2 for Mg(NO3)2 droplets was always higher than that for NaNO3 droplets a...