Experimental studies of the evaporation and growth of individual aqueous droplets

The condensational growth rates of haze and cloud droplets are limited by both the gas-phase diffusion of water vapor and the accommodation of water molecules by the liquid (surface kinetics). The effect of surface kinetics is large when particles are small and/or the ambient pressure is low. When nitric acid, generated by photochemical reactions, is present in the air, it enhances the growth of haze and cloud droplets due to its high solubility and dissociation into ions in water. Therefore, the mass accommodation coefficients of both H2O and HNO3 on the surfaces of haze and cloud droplets become important in the process of cloud formation and need to be resolved.;A series of experiments was conducted using an electrodynamic levitation system. In each experimental run, a single droplet with radius in the range of 15--25 mum and of known composition was introduced into a gaseous environment that was intentionally out of equilibrium with the droplet properties in order to study the kinetics of droplet growth and evaporation. Binary HNO 3/H2O and ternary H2SO4/HNO3/H 2O solutions, as well as pure water were studied at various pressures and temperatures. Droplet sizes during the evaporation and growth were measured to high precision (+/-0.03 mum) through a Mie scattering technique.;Three different models for the transition-regime mass flux and heat flux were used to calculate the time-dependent droplet size, mass, composition and surface temperature. For solution droplets containing HNO3 and/or H2SO4, a thermodynamic model for the individual saturation vapor pressures over the droplet surface was coupled with each of the three transition-regime transport models. The thermal accommodation coefficient was set to be 0.7, and the mass accommodation coefficients of H2O and HNO3 were varied in the model to minimize the standard deviation (close to measurement precision) between the calculated and measured radii. It was found that the mass accommodation coefficient of H2O is 0.05 and that of HNO3 is 0.06 at room temperature and atmospheric pressure. The determination of the mass accommodation coefficients is not sensitive to which transport: model was used. The results also showed that the kinetics of droplet evaporation/growth is strongly limited/enhanced by the presence of HNO3 and occurs in two stages, one dominated by H2O mass transfer and the other by HNO3 mass transfer. This effect is driven by the different time scales for H2O and HNO3 to approach gas-droplet equilibrium. (Abstract shortened by UMI.)...