1Department of Physical Sciences, P.O. Box 64, 00014 University of Helsinki, Finland
2Finnish Meteorological Institute, P.O Box 503, 00101 Helsinki, Finland
3Department of Forest Ecology, P.O. Box 27, 00014 University of Helsinki, Finland
4Dep. of Experimental Physics and Environmental Change Institute, National Univ. of Ireland, Galway, University Road Galway, Ireland
Abstract. During the calendar years 1998-2002, 147 clear 8nm diameter particle formation events have been identified at the SMEAR I station in Värriö, northern Finland. The events have been classified in detail according to the particle formation rate, growth rate, event starting time, different trace gas concentrations and pre-existing particle concentrations as well as various meteorological conditions. The frequency of particle formation and growth events was highest during the spring months between March and May, suggesting that increasing biological activity might produce the precursor gases for particle formation. The apparent 8nm particle formation rates were around 0.1 /cm3s, and they were uncorrelated with growth rates that varied between 0.5 and 10nm/h. The air masses with clearly elevated sulphur dioxide concentrations (above 1.6ppb) came, as expected, from the direction of the Nikel and Monschegorsk smelters. Only 15 formation events can be explained by the pollution plume from these sources.