References

ACP

Atmospheric Chemistry and Physics

ACP

1680-7324

Copernicus GmbH

Göttingen, Germany

10.5194/acp-11-13269-2011

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

Sihto

S.-L.

¹ Mikkilä

¹ ² Vanhanen

¹ ² Ehn

¹ Liao

¹ Lehtipalo

¹ ² Aalto

P. P.

¹ Duplissy

¹ ³ Petäjä

¹ Kerminen

V.-M.

¹ ⁴ Boy

¹ Kulmala

Department of Physics, University of Helsinki, P.O. Box 48, 00014 University of Helsinki, Finland

AirModus Oy, Gustaf Hällströmin katu 2 A, 00560 Helsinki, Finland

Physics Department, CERN, CH1211, Geneva, Switzerland

Finnish Meteorological Institute, Climate and Global Change, P.O. Box 503, 00101 Helsinki, Finland

21 12 2011

11 24 13269 13285

This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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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.

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