Freezing of water droplets seeded with atmospheric aerosols and ice nucleation activity of the aerosols

Abstract

A method is proposed for analysing the kinetics of droplet freezing, which makes it possible to determine the nucleation rate from the temperature derivative of the logarithm of the relative number of unfrozen droplets. Experiments were carried out by freezing droplets of distilled water either unseeded or seeded with aerosol dust, and by employing this method the ice nucleation rate was determined. It was found that while in the unseeded droplets ice nucleation is controlled by only one type of nucleation-active centre, in the seeded droplets it is stimulated by the presence of three new types of active centres introduced by the aerosol seeds. Each type of active centre is characterized quantitatively by an activity factor and a corresponding “wetting” angle. The theoretically expected linear dependence of the nucleation rate on the concentration of aerosol seeds is experimentally confirmed and the relative number of the aerosol-introduced active centres is calculated.