Seasonal variation of CCN concentrations and aerosol activation properties in boreal forest S.-L. Sihto1, J. Mikkilä1,2, J. Vanhanen1,2, M. Ehn1, L. Liao1, K. Lehtipalo1,2, P. P. Aalto1, J. Duplissy1,3, T. Petäjä1, V.-M. Kerminen1,4, M. Boy1, and M. Kulmala1 1Department of Physics, University of Helsinki, P.O. Box 48, 00014 University of Helsinki, Finland 2AirModus Oy, Gustaf Hällströmin katu 2 A, 00560 Helsinki, Finland 3Physics Department, CERN, CH1211, 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 (hygroscopicity tandem differential mobility analyzer)
at SMEAR II station, Hyytiälä, 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 κ-Köhler 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 κ-parameter determined from hygroscopicity measurements was
κ = 0.18 and it predicted well the CCN activation in boreal forest
conditions in Hyytiälä. The derived critical diameters and κ-parameter confirm earlier findings with other methods, that aerosol
particles at CCN sizes in Hyytiälä 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.
Citation: Sihto, S.-L., Mikkilä, J., Vanhanen, J., Ehn, M., Liao, L., Lehtipalo, K., Aalto, P. P., Duplissy, J., Petäjä, T., Kerminen, V.-M., Boy, M., and Kulmala, M.: Seasonal variation of CCN concentrations and aerosol activation properties in boreal forest, Atmos. Chem. Phys., 11, 13269-13285, doi:10.5194/acp-11-13269-2011, 2011.