Continuous measurements of the water activities of the Mg2+-Ca2+-Na+-Cl-- NO3--SO42-H2O system

Condensational growth and deliquescence are important characteristics of atmospheric aerosols. A scanning electrodynamic balance (SEDB), based on a well-characterized increase of relative humidity (RH) in the vicinity of a levitated particle in an electrodynamic balance, was used to measure the water activity of an equal molar aqueous solution of CaCl2 + Ca(NO3)2, MgCl2 + CaCl2, Mg(NO3)2 + Ca(NO3)2, NaCl + Ca(NO3)2, and NaNO3 + Ca(NO3)2 mixtures in both evaporation and growth modes. These mixtures were selected because of the lack of literature data at high concentrations. CaCl2 + Ca(NO3)2, MgCl2 + CaCl2, and Mg(NO3)2 + Ca(NO3)2 mixtures neither crystalline nor deliquesce because the solutes are nondeliquescent in single particle measurements. The presence of nitrate suppresses the crystallization of NaCl, since efflorescence was not observed even at RH = 20%. The water cycles of NaCl + Ca(NO3)2 (molar ratio = 3:1) and NaNO3 + Ca(NO3)2 (3:1) mixtures were also measured. The results show that the presence of Ca(NO3)2 lowers the deliquescence relative humidity values of NaCl and NaNO3 and alters their hygroscopicity. The Zdanovskii-Stokes-Robinson (ZSR) and Simulating Composition of Atmospheric Particles at Equilibrium (SCAPE) predictions are consistent with the water activity measurements except for the Mg(NO3)2 + Ca(NO3)2 mixtures, for which the predictions show significant deviations from the measurements at low RH. For the Mg(NO3)2 + Ca(NO3)2, NaCl + Ca(NO3)2, and NaNO3 + Ca(NO3)2 mixtures, the ZSR model prediction shows larger deviation when compared with the measurements than does the SCAPE model throughout the range of water activity studied.