Aerosol charging state at an urban site: New analytical approach and implications for ion-induced nucleation
The charging state of aerosol populations was determined using an\nIon-DMPS in Helsinki, Finland between December 2008 and February 2010.\nWe extrapolated the charging state and calculated the ion-induced\nnucleation fraction to be around 1.3 % +/- 0.4 % at 2 nm and 1.3 %\n+/- 0.5 % at 1.5 nm, on average. We present a new method to retrieve\nthe average charging state for a new particle formation event, at a\ngiven size and polarity. We improve the uncertainty assessment and\nfitting technique used previously with an Ion-DMPS. We also use a new\ntheoretical framework that allows for different concentrations of small\nions for different polarities (polarity asymmetry). We extrapolate the\nion-induced fraction using polarity symmetry and asymmetry. Finally, a\nmethod to calculate the growth rates from the behaviour of the charging\nstate as a function of the particle diameter using polarity symmetry and\nasymmetry is presented and used on a selection of new particle formation\nevents.