1Dept. of Physics, University of Helsinki, P.O. Box 48, 00014 Helsinki, Finland
2Airmodus Oy, Gustaf Hällströmin katu 2 a, 00560 Helsinki, Finland
3Physics Department, CERN, 1211, Geneva, Switzerland
4Finnish Meteorological Institute, Climate and Global Change, P.O. Box 503, 00101 Helsinki, Finland
Abstract. As a part of EUCAARI activities, the annual cycle of cloud condensation nuclei (CCN) concentrations and critical diameter for cloud droplet activation as a function of supersaturation were measured using a CCN counter and a HTDMA (hygroscopic tandem DMA) at SMEAR II station, Hyytiälä, Finland. The critical diameters for cloud droplet activation were estimated from (i) the CCN concentration and particle size distribution data, and (ii) the hygroscopic growth factors by applying κ-Köhler theory. The critical diameters derived by these two methods were in good agreement with each other. The effect of new particle formation on the diurnal variation of CCN concentration and critical diameters was studied. New particle formation was observed to increase the CCN concentrations by 70–110%, depending on the supersaturation level. The average value for the κ-parameter determined from hygroscopicity measurements was κ = 0.18 and it predicted well the CCN activation in Hyytiälä boreal forest conditions. The derived critical diameters and κ-parameter indicate that aerosol particles at CCN sizes in Hyytiälä are mostly organic, but contain also more soluble, probably inorganic salts like ammonium sulphate, making the particles more CCN active than pure secondary organic aerosol.