Ion-Induced Nucleation
Nucleation is the formation of aerosol particles from precursor vapours like sulphuric acid. The process depends on both thermodynamic and kinetic properties of the system. Gas particles form clusters through kinetic collisions, which will often evaporate because a single-particle state is more energetically favourable than a multi-particle cluster.
When clusters of particles reach a certain critical size, there is an energy cost rather than an energy gain associated with evaporation. The inclusion of an ion within the cluster can reduce the size of this critical cluster significantly, making new particle formation much more stable.
Because atmospheric ion concentrations are largely determined by the intensity of galactic cosmic rays, ion-induced nucleation is a mechanism which could explain the observed cosmic ray-climate correlation. This is especially true in a clean, pre-industrial atmosphere, where nucleation would contribute a much large proportion of atmospheric aerosol.