Seasonal variation of CCN concentrations and aerosol activation properties in boreal forest

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 (hygroscopicity tandem differential mobility analyzer) at SMEAR II station, Hyytiala, Finland. The critical diameters for CCN activation were estimated from (i) the measured CCN concentration and particle size distribution data, and (ii) the hygroscopic growth factors by applying kappa - Kohler theory, in both cases assuming an internally mixed aerosol. 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 kappa - parameter determined from hygroscopicity measurements was kappa = 0.18 and it predicted well the CCN activation in boreal forest conditions in Hyytiala. The derived critical diameters and kappa - parameter confirm earlier findings with other methods, that aerosol particles at CCN sizes in Hyytiala are mostly organic, but contain also more hygrosopic, probably inorganic salts like ammonium sulphate, making the particles more CCN active than pure secondary organic aerosol.