ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-6-879-2006 Cn to ccn relationships and cloud microphysical properties in different air masses at a free tropospheric site Dupuy R. 1 2 Laj P. 1 Sellegri K. 1 Laboratoire de Météorologie Physique, Observatoire de Physique du Globe de Clermont-Ferrand, CNRS UMR 6016, Université Blaise Pascal, Clermont-Ferrand, France now at: The Physics Department, NUIG, Galway, Ireland 01 02 2006 6 1 879 898 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. This article is available from http://www.atmos-chem-phys-discuss.net/6/879/2006/acpd-6-879-2006.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/6/879/2006/acpd-6-879-2006.pdf

The fraction of aerosol particles activated to droplets (CCN) is often derived from semi-empirical relationships that commonly tend to overestimate droplet number concentration leading to major uncertainties in global climate models. One of the difficulties in relating aerosol concentration to cloud microphysics and cloud albedo lies in the necessity of working at a constant liquid water path (LWP), which is very difficult to control. <br><br> In this study we observed the relationships between aerosol number concentration (N<sub>CN</sub>), cloud droplet concentration (N<sub><i>d</i></sub>) and effective radius (R<sub>eff</sub>), at the Puy de D&#244;me (France). A total of 20 cloud events were sampled representing a period of more than 250 h of cloud sampling. Samples are classified first according to air mass origins (Modified Marine, Continental and Polluted) and then according to their liquid water content (Thin, Medium and Thick clouds). <br><br> The CCN fraction of aerosols appears to vary significantly according to the air mass origin. It is maximum for Continental air masses and minimum for Polluted air masses. Surprisingly, the CCN fraction of Modified Marine air masses fraction is lower than the continental air mass and from expected from previous studies. The limited number of activated particles in Modified Marine air masses is most likely the result of the presence of hydrophobic organic compounds. The limited activation effect leads to a 0.5 to 1 &mu;m increase in R<sub>eff</sub> with respect to an ideal Marine case. This is significant and implies that the dR<sub>eff</sub>/dN<sub>CN</sub> of low-continental clouds is higher than expected.