The effect of redistribution on aerosol removal rates

Abstract

A simplified model is constructed for an aerosol interacting with a vapour-gas mixture in a well-mixed cavity. An accurate form is deduced for the initial equilibration timescale between the aerosol and the surrounding mixture, and the corresponding supersaturation is obtained. Analytic estimates are also given for the timescales of gravitational settling, Ostwald ripening from the Kelvin effect, and redistribution of water mass in a hygroscopic salt aerosol. Calculations are performed for a water aerosol at 20°C and 100°C, and the effect on the gravitational fallout is investigated of redistribution due to an aerosol solute content and of radiative redistribution. The solute effect maintains a disperse aerosol size distribution and enhances settling to a moderate but significant extent. For radiative cooling alone, the redistribution hardly affects the settling at 100°C, although it also maintains a wide size distribution. However, when coupled with heating to maintain a constant temperature, the radiative redistribution is shown to be capable of causing a rapid reduction in the mass of the airborne aerosol.