Abstract
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.
Highlights
Atmospheric aerosol particles influence clouds, and thereby climate, by altering the albedo, lifetime and precipitation patterns of clouds (e.g. Twomey, 1974; Lohmann and Feichter, 2005; Rosenfeld et al, 2008; Stevens and Feingold, 2009)
We present an analysis of simultaneous measurements of cloud condensation nuclei (CCN) concentrations, aerosol number size distributions and hygroscopic properties at a boreal forest site
We aim to shed light on the following questions: (i) what are the seasonal and diurnal variations of the CCN concentrations, corresponding activated fractions and critical diameters, (ii) how are the CCN concentrations affected by atmospheric new particle formation events, and (iii) can we predict the CCN activity of boreal forest aerosols based on measured aerosol size distributions and hygroscopic properties
Summary
Atmospheric aerosol particles influence clouds, and thereby climate, by altering the albedo, lifetime and precipitation patterns of clouds (e.g. Twomey, 1974; Lohmann and Feichter, 2005; Rosenfeld et al, 2008; Stevens and Feingold, 2009). Snider and Brenguier, 2000; Bigg and Leck, 2001; Hudson and Yum, 2002; Roberts et al, 2006; Hudson and Noble, 2009). Another class of investigations, that has gained a large interest, is so-called CCN closure studies Determining the relative role of humans and nature in aerosol-cloud interaction requires identifying and quantifying different CCN sources. In addition to primary natural and anthropogenic sources, increasing empirical evidence has been obtained on secondary CCN production in the atmosphere via nucleation and growth of new aerosol particles
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