1Department of Physical Sciences, P.O. Box 64, 00014 University of Helsinki, Finland
2Center for high-performance computing and networking CSC, P.O. BOX 405, 02101 Espoo, Finland
3CSIRO Marine and Atmospheric Research, GPO Box 1666, Canberra ACT 2601, Australia
4Institute of Physics, University of Tartu, Ülikooli Str. 18, EE2400 Tartu, Estonia
5Institute for Environmental Research, ANSTO, PMB 1, Menai NSW 2234, Australia
Abstract. Biogenic aerosol formation is likely to contribute significantly to the global aerosol load. In recent years, new-particle formation (NPF) has been observed in various ecosystems around the world but hardly any measurements have taken place in the terrestrial Southern Hemisphere. Here, we report the first results of atmospheric ion and charged particle concentrations as well as of NPF in a Eucalypt forest in Tumbarumba, South-East Australia, from July 2005 to October 2006. The measurements were carried out with an Air Ion Spectrometer (AIS) with a size range from 0.34 to 40 nm. Daytime aerosol formation took place on 52% of days with acceptable data. Median growth rates (GR) for negative/positive 1.3–3 nm particles were 2.29/2.02 nmh−1; for 3–7 nm particles 3.04/2.94 nmh−1; and for 7–20 nm particles 7.13/5.62 nmh−1, respectively. Intermediate ion growth rates were highest when the wind was blowing from the direction of the native Eucalypt forest, suggesting that the Eucalypts were the strongest source of condensable vapours. Average cluster ion (0.34 to 1.8 nm) concentrations were very high, 2400/1700 cm−3 for negative/positive ions compared to other measurements around the world. These high concentrations are probably the result of the strong radon efflux from the soils around the Tumbarumba field site. Furthermore, comparison between nighttime and daytime concentrations supported the view that cluster ions are produced close to the surface within the boundary layer also at night but that large ions are mostly produced in daytime. Finally, a previously unreported phenomenon, nocturnal aerosol formation, appeared on 32% of the analysed nights but was clustered almost entirely within six months from summer to autumn in 2006. From January to May, nocturnal formation was 2.5 times as frequent as daytime formation. Therefore, it appears that in summer and autumn, nocturnal production was the major mechanism for aerosol formation in Tumbarumba.