Investigating small ion number size distributions: insight into cluster formation and growth

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. 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.