FTIR-ATR in situ observation on the efflorescence and deliquescence processes of Mg(NO₃)₂ aerosols

The efflorescence and deliquescence processes of Mg(NO₃) ₂ aerosol particles deposited on ZnSe substrate have been investigated through in situ Fourier transform infrared-attenuated total reflection (FTIR-ATR) technique at the molecular level. At relative humidity (RH) of ∼3%, Mg(NO₃)₂ particles existed as amorphous states. The amorphous Mg(NO₃)₂ particles were transformed into crystalline Mg(NO₃)₂ • nH₂O (n ≤ 5) with slight increasing of RH. Thermodynamically stable Mg(NO ₃)₂•6H₂O crystals were gradually formed on the particle surface and started to be dissolved at the saturation point (∼53% RH). At the same time, a continuous phase transition from Mg(NO ₃)₂ • nH₂O (n≤5) to Mg(NO ₃)₂•6H₂O occurred on the particle surface. This led the solid particles to completely deliquesce at 76% RH, which was much higher than the saturation point of 53% RH. In the efflorescence process, Mg(NO₃)₂ droplets entered into the supersaturated region due to the gradual evaporation of water. Finally, amorphous particles were formed when RH decreased below 5%. In the FTIR-ATR spectra of the supersaturated Mg(NO₃)₂ droplets, the absorbance of the symmetric stretching vibration of NO ₃ ^- (v ₁- NO ₃ ^- ) clearly became stronger. It resulted from the continuous formation of solvent share ion pairs (SIPs), and even the contact ion pairs (CIPs) between Mg²⁺ and NO ₃ ^- .