A Review of Aerosol Nanoparticle Formation from Ions

Ion-induced nucleation (IIN) mechanism has been widely accepted as an efficient source of new particle formation in the middle and upper troposphere. However, there have been debates on its contribution to the nucleation events observed in the boundary layer. To reveal the current understanding of IINs, we here summarise investigations on microphysical mechanisms of aerosol formation from ions, i.e., ion induced homogeneous nucleation (HN). The basic formation steps of ion-induced HN include primary ion production, charged cluster formation from ions, and stable nanoparticle formation from clusters. Two essential controlling processes (ion generation and formation of condensable species) are emphasized in the viewpoint of plasma performance for three case studies, i.e., galactic cosmic ray travelling in the troposphere, lightning discharges, and charging processes by aerosol chargers. The first two cases result in IINs due to the simultaneous generation of ions and condensable species (such as H2SO4 and oxidized organic vapours) from oxidizing trace atmospheric molecules. The third case leads to ion-induced clustering or nucleation in aerosol chargers. Due to a relatively short residence time in chargers (normally less than 1 minute), big charged clusters often exist in the outgoing aerosol flows.