Magnesium sulfate aerosols studied by FTIR spectroscopy: Hygroscopic properties, supersaturated structures, and Implications for seawater aerosols

Supersaturated MgSO₄ aerosols and dilute MgSO₄ solutions were studied by FTIR spectroscopic techniques (i.e., aerosol flow tube (AFT) and attenuated total reflection (ATR)). The hygroscopic properties of MgSO₄ aerosols were investigated with results in good agreement with previous measurements by a scanning electrodynamic balance (SEDB). Well-defined spectral evolutions with changing relative humidity (RH) for the V₃ band of SO₄^2- and the water O-H stretching envelope could be directly related to the observed hygroscopic properties of MgSO₄ aerosols. When the RH decreased from ∼55 to ∼40%, the v₁ band of SO₄^2- in supersaturated MgSO₄ aerosols was observed to transform from a sharp peak at ∼983 cm^-1 into a wide band at ∼1005 cm^-1. The sharp peak at ∼983 cm^-1 was mainly assigned to such associated complexes of Mg²⁺ and SO ₄^2- as double solvent-separated ion pairs (2SIPs), solvent-shared ion pairs (SIPs), and simple contact ion pairs (CIPs) in supersaturated MgSO₄ aerosols, while the wide band at ∼1005 cm^-1 was due to polymeric CIPs chains, probably the main component of gels formed in MgSO₄ aerosols at low RHs. Relating to this v ₁ band transformation, the peak position of the v₃ band was first shown to be a sensitive indicator of CIPs formation, spanning across ∼40 cm^-1 on the formation of polymeric CIPs chains, which could also be supported by aerosol composition analysis in the form of water-to-solute molar ratios (WSR). In the water O-H stretching envelope, the absorbance intensities at 3371 and 3251 cm^-1 were selected to represent contributions from weak and strong hydrogen bonds, respectively. The absorbance intensity ratio changing with RH of 3371 to 3251 cm^-1 could be related to the previous observations with the v₁ and v₃ bands of SO₄^2-. As a result, the formation of CIPs with various structures in large amounts was supposed to significantly weaken hydrogen bonds in supersaturated MgSO₄ aerosols, while 2SIPs and SIPs were not expected to have similar effects even when occurring in abundance. In comparison with MgSO₄ aerosols, the peak positions of the V ₃ band of SO₄^2- in artificial seawater aerosols implied that the MgSO₄ component should be contained as gels or concentrated solutions in the fissures of microcrystals of sea salts for freshly formed seawater aerosols at low RHs.