Charged particle size distributions and analysis of particle formation events at the Finnish Antarctic research station Aboa

We measured the size distributions of positively and negatively charged particles in the size range 0.34-40 nm at the Finnish Antarctic research station Aboa from 14 December 2004 to 30 January 2005 with an Air Ion Spectrometer (AIS). Our focus was to study secondary particle formation in a clean environment. Therefore, we classified the 48 measurement days either as a particle formation event day, an undefined day or a non-event day. Approximately 23% of the days were particle formation event days, a fraction similar to that observed at a boreal forest site. The median diameter growth rates of negatively charged particles in size classes 1.3-3, 3-7 and 7-20 nm were 1.1, 1.5 and 4.3 nm h -1 , respectively, and somewhat lower for positively charged particles. Furthermore, we observed a clear positive correlation between the wind speed and cluster and intermediate ion concentrations, which suggests that ions were produced by friction processes in fast moving snow and ice crystals during the periods with strong winds. Cluster ions (diameter < 1.6 nm) were present during the whole measurement period. The average (± SD) positive and negative cluster ion concentrations were 557 ± 974 and 587 ± 543 cm -3 , respectively.