Estimating the contribution of ion–ion recombination to sub-2 nm cluster concentrations from atmospheric measurements

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. Our method takes into account the production of recombination products in the collisions between oppositely charged ions and the loss due to coagulation. Furthermore, unlike previous studies, we also consider the effect of condensational growth on the size distribution of recombination products. We applied our 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 6 and 69 cm⁻³ in different size classes, which resulted in a small proportion of all neutral clusters, varying between 0.2 and 13%. 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%. We also investigated how the results change if the effect of condensational growth is neglected. It seems that with that assumption the fragmentation of newly formed recombination products has to be taken into account, or else the concentration of recombination products is overestimated. Overall, we concluded that our method provides reasonable results, which 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 atmospheric clusters would be needed.