||Sea salt and crustal aerosols are major components of the atmospheric aerosols. The mixtures of K+, Na+, Mg2+, or Ca2+ with sulfates, nitrates, and chlorides, are hygroscopic. An aqueous droplet in atmosphere will respond to the change of ambient relative humidity by evaporation or condensation of water molecules. At equilibrium, the water activity (aw) of an aqueous droplet is equal to the ambient relative humidity (RH) of vapor phase. The water activity of hygroscopic aerosols depends on their chemical compositions. The compositional dependence on water activity of the aqueous droplets in atmosphere is one of the important thermodynamic characteristics of atmospheric aerosols. It affects the air quality, visibility degradation and climate change. The approach of single particle levitation has been used to study water activities of atmospheric aerosols by many researchers (e.g. Tang, 1986, 1997; Chan et al., 1992, 1997; Richardson et al., 1984). In this study, a dynamic experimental method developed by Liang and Chan (1997) was employed to determine the water activities of the solutions studied. Upon a step change of RH in the feed to EDB, the compositional change of a suspended droplet is continuously measured. This experimental approach can reduce the time of a set of water activity measurements from several hours to within an hour. In this research, water activities of pure soluble crustal and sea salts (KCl, KNO3, CaCl2, Ca(NO3)2, MgCl2, Mg(NO3)2, MgSO4) the mixtures of magnesium salts (MgCl2-Mg(NO3)2, MgCl2-MgSO4, Mg(NO3)2-MgSO4, and MgCl2-Mg(NO3)2-MgSO4), the mixtures containing both sodium and magnesium salts (NaCl-MgCl2, NaNO3-Mg(NO3)2, Na2SO4-MgSO4), and the ternary mixtures of sodium salts (NaCl-NaNO3-Na2SO4) are determined by the dynamic measurements using an EDB. The results are compared with the available literature data and are used to evaluate the performance of two commonly used thermodynamic models: the Zdanovskii-Stokes-Robinson (ZSR) and Kusik and Meissner (KM) models for multi-component electrolyte solutions. The ZSR model is found to give more accurate predictions.