| Abstract
| - The knowledge of the state of water molecules, particularly the amounts of solvated water and free water inaqueous droplets, is valuable in understanding the hydration properties of atmospheric aerosols. A noveltechnique combining the use of an electrodynamic balance and a fluorescence dye, 8-hydroxyl-1,3,6-pyrenetrisulfonate (pyranine), was used to study the state of the water molecules in single levitated aqueousdroplets from subsaturation to supersaturation concentrations. The steady-state fluorescence spectra of sucrose,glucose, and NaCl solutions doped with 100 ppm pyranine were measured. The fluorescence emission ofpyranine is sensitive to the proton-transfer capacity of its microenvironment. When excited by radiation ataround 345 nm, pyranine fluoresces and the spectrum consists of two peaks, one at about 440 nm and theother at about 510 nm, which correspond to the presence of solvated and free water, respectively. Thefluorescence peak intensity ratios of the 440-nm peak to the 510-nm peak and the hygroscopic measurementswere used to calculate the amounts of solvated and free water in the droplets as a function of relative humidity.The amount of free water equals the amount of solvated water when crystallization or saturation (fornoncrystallizating chemicals) occurs. Imaging analysis has revealed that the solvated to free water ratio oscillateswithin the droplets, which indicates the spatial heterogeneity of aqueous droplets. This study demonstratesthat fluorescence spectroscopy is a unique tool in understanding the hydration properties, the efflorescence,and the structural heterogeneity of aqueous droplets.
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