Articles | Volume 12, issue 10
Research article
25 May 2012
Research article |  | 25 May 2012

Aerosol charging state at an urban site: new analytical approach and implications for ion-induced nucleation

S. Gagné, J. Leppä, T. Petäjä, M. J. McGrath, M. Vana, V.-M. Kerminen, L. Laakso, and M. Kulmala

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.

Final-revised paper