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 approximately 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 at the same WSR, indicating a much stronger "structure-making" effect of Mg2+ than of Na+. In the efflorescence process, aerosol particles followed different paths of phase transition from droplets to Mg(NO3)2.6H2O or amorphous states. Reversing somewhat the phase transitions in the efflorescence process, aerosol particles dissolved into droplets with the increase of RH in the deliquescence process. Heterogeneous particles prepared by dehydrating Mg(NO3)2.6H2O were investigated by the depth profiling technique. About 15 h later, the main body of particles changed into Mg(NO3)2.2H2O, a small quantity of Mg(NO3)2.6H2O scattered around particle edges, and some particles were in amorphous states. About 10 days later, a new solid phase occurred on particle surfaces, while the interiors were still Mg(NO3)2.2H2O. With increasing the RH to approximately 11%, significant Mg(NO3)2.6H2O formed on particle surfaces, covering the interior Mg(NO3)2.2H2O.