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.