1Department of Physics, University of Helsinki, Helsinki, Finland
2Department of Applied Physics, University of Eastern Finland and Finnish Meteorological Institute, Kuopio, Finland
3Helsinki Institute of Physics, Helsinki, Finland
4Airmodus Oy, Helsinki, Finland
Abstract. The significance of ion–ion recombination for atmospheric new particle formation is not well quantified. Here we present and evaluate a method for determining the size distribution of recombination products from the size distributions of neutral and charged clusters. We applied this method to the data measured in Hyytiälä, Finland, to estimate the contribution of ion–ion recombination to the concentrations of atmospheric clusters in the size range of 0.9–2.1 nm. We observed that the concentration of recombination products was highest in the size classes between 1.5 and 1.9 nm. The median concentrations of recombination products were between 1 and 79 cm−3 in different size classes, which resulted in a small proportion of all neutral clusters, varying between 0.05% and 15%. When examining the whole size range between 0.9 and 2.1 nm, the median fraction of recombination products of all neutral clusters was only 1.5%. Overall, the applied method was concluded to be reasonable, and the results are consistent with the earlier estimates on the contribution of recombination products to atmospheric cluster population in Hyytiälä. Still, in order to determine the size distribution of recombination products more accurately in the future, more precise measurements of the size distribution of sub-2 nm clusters would be needed.