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

  • Sihto S
  • Mikkilä J
  • Vanhanen J
 et al. 
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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, 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.

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  • S. L. Sihto

  • J. Mikkilä

  • J. Vanhanen

  • M. Ehn

  • L. Liao

  • K. Lehtipalo

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