The Mechanism of Ion Growth in the Atmosphere

Malcolm J. Campbell

doi:10.7273/000007806

Recent developments in the theory of formation and growth of neutral particles in the atmosphere can be applied to ions. It appears that neutral particles are produced by one of several processes which resemble kinetically the homogeneous condensation of a supersaturated vapor. Such a condensation process will take place preferentially on small air ions, because of the lower energy barrier involved. A condensational process of this type will cause sudden growth of the ion to sizes of about 3 nm, corresponding the size of the smallest intermediate ions, and slower growth of larger ions; this explains the deficit of ions with sizes between l and 3 nm. It is suggested that condensational growth, while always competitive with growth by coagulation of ions with neutral particles, is frequently an important mechanism for smaller ions. Summary It has been generally assumed that small ions progress to inter mediate ions by capture of relatively large (radius of 3 nm or larger) neutral particles. There is, however, an alternative process, responsible for the growth of small neutral particles, which is likely to be important in the growth of ions; this is the condensation of vapors on to the ions, which act as nuclei of condensation. Several mechanisms have been suggested for the formation of small neutral particles in the atmosphere (Mohnen and Walther, 1974), but of these the three most likely are homogeneous condensation of a supersaturated vapor (for example, an organic compound) produced in the atmosphere, heteromolecular as binary homogeneous nucleation of two subsaturated vapors conjointly (Kiang et al., 1973; Mirabel and Katz, 1974), and chemical nucleation, for example via a polymerization reaction at the particle. All of these mechanisms have similar kinetics in practice, and all share the important characteristic that the energy barrier to particle formation will be lowered substantially if the con densation takes place not homogeneously, but rather on a small ion acting in the role of heteronucleus.

The Mechanism of Ion Growth in the Atmosphere

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