From molecular clusters to nanoparticles: Role of ambient ionization in tropospheric aerosol formation

We investigate the role of background ionization, associated mainly with ga lactic cosmic radiation, in the generation and evolution of ultrafine parti cles in the marine boundary layer. We follow the entire course of aerosol e volution, from the initial buildup of molecular clusters (charged and uncha rged) through their growth into stable nanoparticles. The model used for th is purpose is based on a unified collisional (kinetic) mechanism that treat s the interactions between vapors, neutral and charged clusters, and partic les at all sizes. We show that air ions are likely to play a central role i n the formation of new ultrafine particles. The nucleation of aerosols unde r atmospheric conditions involves a series of competing processes, includin g molecular aggregation, evaporation, and scavenging by preexisting particl es. In this highly sensitive nonlinear system, electrically charged embryos have a competitive advantage over similar neutral embryos. The charged clu sters experience enhanced growth and stability as a consequence of electros tatic interactions. Simulations of a major nucleation event observed during the Pacific Exploratory Mission (PEM) Tropics-A can explain most of the ob served features in the ultrafine particle behavior. The key parameters cont rolling this behavior are the concentrations of precursor vapors and the su rface area of preexisting particles, as well as the background ionization r ate. We find that systematic variations in ionization levels due to the mod ulation of galactic cosmic radiation by the solar cycle are sufficient to c ause a notable variation in aerosol production. This effect is greatest whe n the ambient nucleation rate is limited principally by the availability of ions. Hence we conclude that the greatest influence of such ionization is likely to occur in and above the marine boundary layer. While a systematic change in the ultrafine particle production rate is likely to affect the po pulation of cloud condensation nuclei and hence cloud optical properties, t he magnitude of the effect cannot be directly inferred from the present ana lysis, and requires additional analysis based on specific aerosol-cloud int eractions.