Water content and morphology of sodium chloride aerosol particles

Sodium chloride droplets with a median diameter of ∼0.4 μm were generated in the laboratory by atomizing an aqueous solution of NaCl under ambient conditions. Infrared extinction spectra of the aerosols under controlled relative humidity (RH) ranging from 15 to 95% were obtained. The extinction spectra contained both scattering and absorption components. In order to obtain an absorption spectrum of the condensed phase H₂O associated with the particulates, it was necessary to subtract from the extinction spectra the absorption by H₂O vapor and the scattering by the particulates. H₂O vapor subtraction was accomplished by a standard technique. A procedure using Mie theory to subtract the scattering component of the extinction spectrum is described. The absorption spectra were used to determine the water content and structure of the particulates. Above ∼50% RH the aerosols contain aqueous droplets that have not reached equilibrium with the water vapor during the timescale of the experiments (∼10 s). There is a sharp transition in water content at around 50% RH which is consistent with other measures of the recrystallization point. Below 50% RH the NaCl particles contain an anomalously large amount of H₂O. Several different particle models are considered to explain the H₂O content. The model in which the NaCl particles contain pockets of aqueous NaCl solution was found to be most consistent with the spectroscopic observations. The relevance of salt particle morphology and water content to atmospheric aerosol chemistry is discussed.