Detecting charging state of ultra-fine particles: instrumental development and ambient measurements L. Laakso1, S. Gagné1, T. Petäjä1, A. Hirsikko1, P. P. Aalto1, M. Kulmala1, and V.-M. Kerminen2 1Department of Physical Sciences, P.O. Box 64, 00014 University of Helsinki, Finland 2Finnish Meteorological Institute, Research and Development, P.O. Box 503, 00101 Helsinki, Finland
Abstract. The importance of ion-induced nucleation in the lower atmosphere has
been discussed for a long time. In this article we describe a new
instrumental setup – Ion-DMPS – which can be used to detect
contribution of ion-induced nucleation on atmospheric new particle
formation events. The device measures positively and negatively charged
particles with and without a bipolar charger. The ratio between
"charger off" to "charger on" describes the charging state of aerosol
particle population with respect to equilibrium. Values above one
represent more charges than in an equilibrium (overcharged state), and
values below unity stand for undercharged situation, when there is less
charges in the particles than in the equilibrium.
We performed several laboratory experiments to test the operation of
the instrument. After the laboratory tests, we used the device to
observe particle size distributions during atmospheric new particle
formation in a boreal forest. We found that some of the events were
clearly dominated by neutral nucleation but in some cases also
ion-induced nucleation contributed to the new particle formation. We
also found that negative and positive ions (charged particles) behaved
in a different manner, days with negative overcharging were more
frequent than days with positive overcharging.
Citation: Laakso, L., Gagné, S., Petäjä, T., Hirsikko, A., Aalto, P. P., Kulmala, M., and Kerminen, V.-M.: Detecting charging state of ultra-fine particles: instrumental development and ambient measurements, Atmos. Chem. Phys., 7, 1333-1345, doi:10.5194/acp-7-1333-2007, 2007.