ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-11-767-2011 Atmospheric ions and nucleation: a review of observations Hirsikko A. 1 Nieminen T. 1 Gagné S. 1 2 Lehtipalo K. 1 Manninen H. E. 1 Ehn M. 1 Hõrrak U. 3 Kerminen V.-M. 1 4 Laakso L. 1 4 5 McMurry P. H. 6 Mirme A. 3 Mirme S. 3 Petäjä T. 1 Tammet H. 3 Vakkari V. 1 Vana M. 1 3 Kulmala M. 1 Department of Physics, P.O. Box 64, 00014 University of Helsinki, Finland Helsinki Institute of Physics and University of Helsinki, Department of Physics, P.O. Box 64, 00014 University of Helsinki, Finland Institute of Physics, University of Tartu, 18 Ülikooli Str., 50090 Tartu, Estonia Finnish Meteorological Institute, Research and Development, P.O. Box 503, 00101 Helsinki, Finland School of Physical and Chemical Sciences, North-West University, Potchestroom, Republic of South Africa Particle Technology Laboratory, University of Minnesota, Minneapolis, Minnesota, USA 26 01 2011 11 2 767 798 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/11/767/2011/acp-11-767-2011.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/11/767/2011/acp-11-767-2011.pdf

This review is based on ca. 260 publications, 93 of which included data on the temporal and spatial variation of the concentration of small ions (<1.6 nm in diameter) especially in the lower troposphere, chemical composition, or formation and growth rates of sub-3 nm ions. This information was collected on tables and figures. 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<sup>−3</sup>. However, concentrations up to 5000 cm<sup>−3</sup> have been observed. The results are in agreement with observations of ion production rates in the atmosphere. 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 (<i>J</i><sub>2</sub>[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 <i>J</i><sub>2</sub>[ion] and GR[ion]. Based on the publications, the formation rates of 2-nm ions were 0–1.1 cm<sup>−3</sup> s<sup>−1</sup>, while the total 2-nm particle formation rates varied between 0.001 and 60 cm<sup>−3</sup> s<sup>−1</sup>. Due to small changes in <i>J</i><sub>2</sub>[ion], the relative importance of ions in 2-nm particle formation was determined by the large changes in <i>J</i><sub>2</sub>[tot], and, accordingly the contribution of ions increased with decreasing <i>J</i><sub>2</sub>[tot]. Furthermore, small ions were observed to activate for growth earlier than neutral nanometer-sized particles and at lower saturation ratio of condensing vapours.

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