We have shown that water adsorbed to the (001) plane of muscovite mica has an infrared spectrum consistent with a bonding network that is more structured than that found in bulk water. Isotherms taken at temperatures ranging from near the ice melting point to room temperature suggest that water wets mica incompletely. Additionally, we find that the enthalpy and entropy of the adsorbed water molecules imply a strongly bound first layer. Both enthalpy and entropy approach the bulk values as the thickness of the adsorbed layer increases.

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Abstract. Sodium chloride droplets with a median diameter of --•0.4/•m were generated in the laboratory by atomizing an aqueous solution of NaC1 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 H20 associated with the particulates, it was necessary to subtract from the extinction spectra the absorption by H20 vapor and the scattering by the particulates. H20 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 NaC1 particles contain an anomalously large amount of H20. Several different particle models are considered to explain the H20 content. The model in which the NaC1 particles contain pockets of aqueous NaC1 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.

Water adsorbed on the (001) face of NaCl under ambient conditions has been studied by infrared spectroscopy. From these measurements, combined with recent Monte Carlo calculations, we find evidence of two structures for the adsorbed water. At low coverages, the water molecules aggregate into islands on the surface. When a critical concentration is reached, multilayer growth becomes favorable, creating a thin film on the surface with properties similar to liquid water.

Charged water droplets generated by electrospray, sonic spray, and a vibrating orifice aerosol generator (VOAG) have been studied by digital macrophotography and image charge detection mass spectrometry. Image charge detection mass spectrometry provides information on the droplet size, charge, and velocity after transmission through a capillary interface. The digital images provide the droplet size distribution before they enter the capillary. Droplets with 10-100 microm radii generated by sonic spray and VOAG are reduced to 2-3 microm radii by transmission through the capillary interface. The droplets from sonic spray and VOAG are much more highly charged than expected for random charging, and positive droplets are much more prevalent than negative. For positive mode electrospray, >99% of the detected droplets carry a positive charge, whereas for negative mode electrospray, <30% of the detected droplets carry a negative charge (i.e., >70% carry a positive charge). These observation can all be accounted for by the aerodynamic breakup of the droplets in the capillary interface. This breakup reduces the droplets to a terminal size at which point further breakup does not occur. Charge separation during droplet breakup is responsible for the relatively high charges on the sonic spray and VOAG droplets and for the preference for positively charged droplets. The charge separation can be explained using the bag mechanism for droplet breakup and the electrical bilayer at the surface of water.