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

Gagn?? S, Lepp?? J, Pet??j?? T, McGrath M, Vana M, Kerminen V, Laakso L, Kulmala M...(+8 more)

Atmospheric Chemistry and Physics, vol. 12, issue 10 (2012) pp. 4647-4666 Published by Copernicus GmbH

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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.

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