||Hygroscopicity, condensational growth and deliquescence are important characteristics of atmospheric aerosols. Apart from the inorganic fraction of atmospheric aerosols, water-soluble organic compounds have recently received much attention because of their ability to absorb water and affect the radiation balance and the climate. Partly due to their relatively high volatility, thermodynamic data on water-soluble organic compounds are scarce. In this thesis, 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 applied to measure the water activity of binary and ternary mixed aerosols containing inorganic and organic solutes. The SEDB enables the measurement of water activity data of evaporating droplets within an hour, which can potentially be used to measure volatile species. We first applied the SEDB to study the hygroscopicity of non-volatile aerosols containing sodium and calcium salts. We then confirmed that the SEDB can be used to study the semi-volatile organic species with vapor pressures up to 1E-4 mmHg. Using the SEDB, the hygroscopicity, the crystallization relative humidity (CRH) and the deliquescence relative humidity (DRH) of aqueous solutions of maleic acid and glutaric acid were measured. The effects of glycerol, succinic acid, malonic acid, citric acid and glutaric acid on the water cycles (water activities during particle evaporation and growth), CRH and DRH of sodium chloride and ammonium sulfate were also examined. Among the hygroscopic characteristics of atmospheric aerosols, efflorescence of supersaturated droplets to form solid (or partially solid) particles is least understood. At high supersaturation, the interaction of water with solute is expected to be very important. Hence, the knowledge of the state of water molecules, particularly the amounts of solvated water and free water in aqueous droplets, is valuable in understanding the hydration properties of atmospheric aerosols. A novel technique combining the use of an electrodynamic balance (EDB) and a fluorescence dye, 8- hydroxy1-1,3,6-pyrenetrisulfonate (pyranine), was used to study the state of the water molecules in single levitated aqueous droplets from subsaturation to supersaturation concentrations. The steady-state fluorescence spectra of sucrose, glucose, NaC1, MgSO4 and Mg(NO3)2 solutions doped with 100 ppm pyranine were measured. The fluorescence emission of pyranine is sensitive to the proton transfer capacity of its microenvironment. When excited by radiation at around 345 nm, pyranine fluoresces and the spectrum consists of two peaks, one at about 440 nm and the other at about 510 nm, which correspond to the presence of solvated water (or the absence of free water) and free water, respectively. The fluorescence peak intensity ratios of the 440 nm peak to the 510 nm peak and the hygroscopic measurements were used to calculate the amounts of solvated and free water in the droplets as a function of relative humidity. This study has demonstrated that fluorescence spectroscopy is a unique tool in understanding the hydration properties, hydration structure and efflorescence of supersaturated droplets.