Abstract
Nucleation of liquid water in the Earth's atmosphere occurs via heterogeneous nucleation on aerosol particles. We consider nucleation on both water-insoluble and water-soluble aerosol particles. We find that, for particles of the same radius, nucleation on soluble particles dominates. Soluble particles dissolve in the liquid phase and form a droplet even at coexistence. The radius of this droplet essentially determines the supersaturation at which nucleation occurs: the larger the droplet the smaller the supersaturation required before it nucleates to form the bulk liquid. We find that the supersaturation is best measured by the Kelvin radius, which is the radius of a droplet of pure liquid that coexists with vapour of a given supersaturation. We show that nucleation occurs at a universal value of the ratio between the radius of the droplet at coexistence and the Kelvin radius.