Application of fluorescence spectroscopy to study the state of water in aerosols

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 and a fluorescence dye, 8-hydroxyl-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, and NaCl 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 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. The amount of free water equals the amount of solvated water when crystallization or saturation (for noncrystallizating chemicals) occurs. Imaging analysis has revealed that the solvated to free water ratio oscillates within the droplets, which indicates the spatial heterogeneity of aqueous droplets. This study demonstrates that fluorescence spectroscopy is a unique tool in understanding the hydration properties, the efflorescence, and the structural heterogeneity of aqueous droplets.