Atmospheric ions and nucleation: A review of observations

This review is based on ca. 250 publications, from which 92 published data on the temporal and spatial variation of the concentration of small ions (<1.6nm in diameter) in
the atmosphere, chemical composition, or formation and growth rates of sub-3nm ions.
The small ions exist all the time in the atmosphere, and the average concentrations of
positive and negative small ions are typically 200–2500 cm−3. However, concentrations
up to 5000 cm−3 have been observed. The results are in agreement with observations
of ion production rates in the atmosphere. Concentrations of small ions increased in
the early morning hours due to night time inversion, which leads to accumulation of
radon. We also summarised observations on the conversion of small ions to intermediate ions, which can act as embryos for new atmospheric aerosol particles. Those observations include the formation rates (J2[ion]) of 2-nm intermediate ions, growth rates (GR[ion]) of sub-3nm ions, and information on the chemical composition of the ions. Unfortunately, there were only a few studies which presented J2[ion] and GR[ion]. Based on the publications, the formation rates of 2-nm ions were 0–1.1 cm−3 s−1, while the total 2-nm particle formation rates varied between 0.001 and 60 cm−3 s−1. The ionmediated processes were observed to dominate when the total particle formation rates were small, and, accordingly the importance of ion-induced mechanisms decreased with increasing total 2-nm particle formation rates. Furthermore, small ions were observed to activate for growth earlier than neutral nanometer-sized particles and at lower saturation ratio of condensing vapours.