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

by S. L. Sihto, J. Mikkilä, J. Vanhanen, M. Ehn, L. Liao, K. Lehtipalo, P. P. Aalto, J. Duplissy, T. Petäjä, V. M. Kerminen, M. Boy, M. Kulmala show all authors
Atmospheric Chemistry and Physics ()
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As a part of EUCAARI activities, the annual cycle of cloud condensation\nnuclei (CCN) concentrations and critical diameter for cloud droplet\nactivation as a function of supersaturation were measured using a CCN\ncounter and a HTDMA (hygroscopicity tandem differential mobility\nanalyzer) at SMEAR II station, Hyytiala, Finland. The critical diameters\nfor CCN activation were estimated from (i) the measured CCN\nconcentration and particle size distribution data, and (ii) the\nhygroscopic growth factors by applying kappa - Kohler theory, in both\ncases assuming an internally mixed aerosol. The critical diameters\nderived by these two methods were in good agreement with each other. The\neffect of new particle formation on the diurnal variation of CCN\nconcentration and critical diameters was studied. New particle formation\nwas observed to increase the CCN concentrations by 70-110 %, depending\non the supersaturation level. The average value for the kappa -\nparameter determined from hygroscopicity measurements was kappa = 0.18\nand it predicted well the CCN activation in boreal forest conditions in\nHyytiala. The derived critical diameters and kappa - parameter confirm\nearlier findings with other methods, that aerosol particles at CCN sizes\nin Hyytiala are mostly organic, but contain also more hygrosopic,\nprobably inorganic salts like ammonium sulphate, making the particles\nmore CCN active than pure secondary organic aerosol.

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