Abstract. Small ions, consisting mostly of charged molecular clusters with mobility diameters below 2 nm, exist continuously in the atmosphere. Here, we studied small ion number size distributions measured with neutral cluster and air ion spectrometer measurements in Hyytiälä, Finland, and Beijing, China. We found that in Hyytiälä, there is a strong positive relationship between the concentration and diameter of small ions of both polarities and highly oxidized organic molecule (HOM) and sulfuric acid concentrations, and that the relationship with the former is especially strong. The relationship between the negative sulfuric acid cluster ions and the small ion number size distribution in Hyytiälä was found to be more complex but overall positive. In contrast to Hyytiälä, we found that in Beijing, the small ion number size distribution does not have a clear relationship with sulfuric acid or oxidized organic molecule (OOM) concentration. We found that the small ion size distribution in Hyytiälä behaved as expected with respect to varying coagulation sink (CoagS), with concentrations of the smallest ions decreasing most with increasing CoagS. Surprisingly, the small ion size distribution in Beijing did not vary significantly with varying CoagS. However, in both locations, the impact of growth on the small ion number size distribution during periods of intense cluster formation and new particle formation is clearly seen. Our results show that although in Hyytiälä the growth of small ions to larger diameters is limited by the concentrations of sulfuric acid and OOMs, in Beijing there are additional factors required for the small ions to grow.
Tuovinen et al. (2026) studied this question.