Hygroscopic properties of CH3SO3Na, CH3SO3NH4, (CH3SO3)2Mg, and (CH3SO3)2Ca particles studied by micro-FTIR spectroscopy.

The hygroscopic behavior of CH(3)SO(3)Na, CH(3)SO(3)NH(4), (CH(3)SO(3))(2)Mg, and (CH(3)SO(3))(2)Ca particles as a function of relative humidity (RH) has been studied using microscopic Fourier transform infrared (micro-FTIR) spectroscopy. The approach used exposure of substrate-deposited, approximately 1 microm dry-size particles to humidified nitrogen followed by micro-FTIR spectroscopy over a selected sample area. The results show that CH(3)SO(3)Na particles undergo characteristic phase transitions at deliquescence relative humidity (DRH) of 71% and efflorescence relative humidity (ERH) of approximately 40%. In contrast, CH(3)SO(3)NH(4), (CH(3)SO(3))(2)Mg, and (CH(3)SO(3))(2)Ca particles do not undergo phase transitions and exhibit continuous, reversible uptake and evaporation of water under the influence of changing RH. The extent of water uptake is quantified and presented as water-to-solute ratios (WSR) in particles as a function of RH. The WSR values are determined from the integrated absorbance of the water and the solute-specific bands in IR spectra recorded at different RH.