1Department of Physics, University of Helsinki, P.O. Box 64, 00014 University of Helsinki, Finland
2Helsinki Institute of Physics and University of Helsinki, Department of Physics, P.O. Box 64, 00014 University of Helsinki, Finland
3Institute of Physics, University of Tartu, 18 Ülikooli Str., 50090 Tartu, Estonia
4Finnish Meteorological Institute, Research and Development, P.O. Box 503, 00101, Finland
5School of Physical and Chemical Sciences, North-West University, Potchestroom, Republic of South Africa
6Particle Technology Laboratory, University of Minnesota, Minneapolis, Minnesota, USA
Abstract. 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 (<1.6 nm in diameter) in the atmosphere, chemical composition, or formation and growth rates of sub-3 nm 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-3 nm 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 ion-mediated 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.