Aerosol charging state at an urban site: new analytical approach and implications for ion-induced nucleation
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
*now at: Dept. of Physics and Atmospheric Science, Dalhousie University, Halifax, B3H 3J5, Canada and at Environment Canada, Toronto, M3H 5T4, Canada
**now at: Dept. of Biophysics, Graduate School of Science, Kyoto University, Sakyo Kyoto 606-8502, Japan
Abstract. The charging state of aerosol populations was determined using an Ion-DMPS in Helsinki, Finland between December 2008 and February 2010. We extrapolated the charging state and calculated the ion-induced nucleation fraction to be around 1.3 % ± 0.4 % at 2 nm and 1.3 % ± 0.5 % at 1.5 nm, on average. We present a new method to retrieve the average charging state for a new particle formation event, at a given size and polarity. We improve the uncertainty assessment and fitting technique used previously with an Ion-DMPS. We also use a new theoretical framework that allows for different concentrations of small 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 behaviour 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.