Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 5 2 2005 10.5194/acp-5-461-2005 http://www.atmos-chem-phys.net/5/461/2005/ http://www.atmos-chem-phys.net/5/461/2005/acp-5-461-2005.html http://www.atmos-chem-phys.net/5/461/2005/acp-5-461-2005.pdf 461 464 2005-02-14 Commentary 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 commentary we note that in cloud modelling, the primary quantity of interest is the droplet growth rate rather than the mass accommodation coefficient, and that experimental investigations of droplet growth rates provide more direct verification of cloud models than do measurements of the mass accommodation coefficient. Furthermore, we argue that the droplet growth rates calculated in cloud model studies are consistent with experimental results obtained so far only if a mass accommodation coefficient of unity is applied.