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

Abstract

Abstract. We investigate the role of background ionization, associated mainly with galactic cosmic radiation, in the generation and evolution of ultrafine particles in the marine boundary layer. We follow the entire course of aerosol evolution, from the initial buildup of molecular clusters (charged and uncharged) through their growth into stable nanoparticles. The model used for this purpose is based on a unified collisional (kinetic) mechanism that treats the interactions between vapors, neutral and charged clusters, and particles at all sizes. We show that air ions are likely to play a central role in the formation of new ultrafine particles. The nucleation of aerosols under atmospheric conditions involves a series of competing processes, including molecular aggregation, evaporation, and scavenging by preexisting particles. In this highly sensitive nonlinear system, electrically charged embryos have a competitive advantage over similar neutral embryos. The charged clusters experience enhanced growth and stability as a consequence of electrostatic interactions. Simulations of a major nucleation event observed during the Pacific Exploratory Mission (PEM) Tropics-A can explain most of the observed features in the ultrafine particle behavior. The key parameters controlling this behavior are the concentrations of precursor vapors and the surface area of preexisting particles, as well as the background ionization rate. We find that systematic variations in ionization levels due to the modulation of galactic cosmic radiation by the solar cycle are sufficient to cause a notable variation in aerosol production. This effect is greatest when 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 population of cloud condensation nuclei and hence cloud optical properties, the magnitude of the effect cannot be directly inferred from the present analysis, and requires additiorial analysis based on specific aerosol-cloud interactions.

Seasonal and spatial trends in particle number concentrations and size distributions at the children's health study sites in Southern California

“…The actual formation mechanism of nanoparticles in the range of 1-3 nm remains largely unknown and has recently become the subject of intensive research in the field of atmospheric science. Current hypotheses on the composition of these fresh nuclei include the binary nucleation of water and sulfuric acid (Kulmala, 2002), ternary nucleation of ammonia-sulfuric acid-water (Weber et al, 1997) and ion-induced nucleation (Yu and Turco, 2001). There is also general consensus that the species responsible for further growth of these nanoparticles to the 410 nm range are different than the nucleating species (Stanier et al, 2004).…”

Investigation of Diesel Nanoparticle Nucleation Mechanisms

“…We assumed D p0 as 1.8 nm, which is a typical critical embryo size for ionic nucleation (Yu and Turco 2001).…”

“…However, ionic nucleation could not be completely ruled out because experimental uncertainty made it impossible to ascertain that there was no charge on the nucleation mode particles. Further, Yu et al (2001Yu et al ( , 2002Yu et al ( , 2004 have argued strongly that ionic nucleation is an 336 H. MA ET AL. important mechanism in the formation of the nucleation mode in diluted diesel exhaust.…”

Clusters: A bridge across the disciplines of environment, materials science, and biology

“…One such proposed mechanism involves ion-induced nucleation, which has recently become a topic of significant interest because of a number of computational and experimental findings that implicate direct ion nucleation as well as ion-ion recombination as potentially significant sources of aerosol particles (14)(15)(16)(17)(18). Interestingly, the energy barriers to ion-induced nucleation involve thermodynamic considerations similar to ones mentioned in the section on solvation effects on reactions.…”