Measurements of aerosol charging states in Helsinki, Finland
1Department of Physics, University of Helsinki, P.O. Box 64, 00014 Helsinki, Finland
2Finnish Meteorological Institute, Climate Change, P.O. Box 503, 00101 Helsinki, Finland
3Institute of Physics, University of Tartu, Ülikooli 18, 50090 Tartu, Estonia
4School of Physical and Chemical Sciences, North-West University, Private Bag x6001, Potchefstroom 2520, South Africa
Abstract. The charging state of aerosol populations was measured with an Ion-DMPS in Helsinki, Finland between December 2008 and February 2010. Based on the charging states, we calculated the ion-induced nucleation fraction to be around 0.8 % ± 0.9 %. We review the role of ion-induced nucleation and propose different explanations for a low ion-induced nucleation participation in urban areas. We present a new method to retrieve the average charging state for an event, and a given size. We also use a new theoretical framework that allows for different concentrations of small cluster ions for different polarities (polarity asymmetry). We extrapolate the ion-induced fraction using polarity symmetry and asymmetry. Finally, a method to calculate the growth rates from the variation of the charging state as a function of the particle diameter using polarity symmetry and asymmetry is presented and used on a selection of new particle formation events.