Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 4 6 2004 10.5194/acpd-4-7281-2004 http://www.atmos-chem-phys-discuss.net/4/7281/2004/ http://www.atmos-chem-phys-discuss.net/4/7281/2004/acpd-4-7281-2004.html http://www.atmos-chem-phys-discuss.net/4/7281/2004/acpd-4-7281-2004.pdf 7281 7290 2004-11-05 On cloud modelling and the mass accommodation coefficient of water A. Laaksonen T. Vesala M. Kulmala P. M. Winkler P. E. Wagner Department of Applied Physics, University of Kuopio, P.O. Box 1627, FIN-70211 Kuopio, Finland Department of Physical Sciences, University of Helsinki, P.O. Box 64, FIN-00014 Helsinki, Finland Institut für Experimentalphysik, Universität Wien, Boltzmanngasse 5, A-1090 Wien, Austria The mass accommodation coefficient of water is a quantity for which different experimental techniques have yielded conflicting values in the range 0.04–1. From the viewpoint of cloud modelling, this is an unfortunate situation, since the value of the mass accommodation coefficient affects the model results, e.g. the number concentration of activated cloud droplets. In this paper we argue that a mass accommodation coefficient of unity should be used in cloud modelling, since this value has been obtained in experimental studies of water droplet growth rates, a quantity which is explicitly described in cloud models. In contrast, mass accommodation coefficient values below unity have been derived from experimental results which are analyzed with different theoretical expressions than those included in cloud models.