Charging State of Aerosols during Particle Formation Events in an Urban Environment and Its Implications for Ion-Induced Nucleation

ABSTRACT

The aim of this study was to investigate the charging state of atmospheric ions during particle formation (PF) events in an urban environment. We measured small ion (charged cluster) and large ion (charged particle) concentrations over a period of 13 months in Brisbane, Australia, using a neutral cluster and air ion spectrometer (NAIS) and obtained 245 complete days of data. PF events were observed on 110 days which count as 45% of the total. On the average, the positive small ion concentration was 40% higher than the negative. The positive large ion concentration was 20% higher than the negative. Generally, small ion concentrations peaked during the night, while large ion concentrations were a maximum during the day. Next, we classified the results into days on which there was a PF event and when there was not. We showed that PF events have a profound effect on small and large ion concentrations in the environment; the small ion concentrations decreased by about 30% while large ion concentrations increased by up to 100%. We identified two phenomena that have a bearing on ion-induced nucleation. Firstly, the fraction of particles that were charged consistently decreased during PF events and rarely exceeded the equilibrium value, suggesting that the formation process was not significantly influenced by an ion-induced mechanism. Secondly, small ion concentrations were higher in the pre-dawn hours of PF event days than on other days, while the fraction of clusters that were charged often showed an increase immediately prior to a PF event, both observations supporting an ion-induced mechanism of PF. This study serves as basis for future work explaining these observations.