Geoscientific Model Development Discussions www.geosci-model-dev-discuss.net 1991-9611 1991-962X 3 3 2010 10.5194/gmdd-3-1139-2010 http://www.geosci-model-dev-discuss.net/3/1139/2010/ http://www.geosci-model-dev-discuss.net/3/1139/2010/gmdd-3-1139-2010.html http://www.geosci-model-dev-discuss.net/3/1139/2010/gmdd-3-1139-2010.pdf 1139 1159 2010-07-30 A simplified treatment of surfactant effects on cloud drop activation T. Raatikainen tomi.raatikainen@fmi.fi A. Laaksonen Finnish Meteorological Institute, P.O. Box 503, 00101 Helsinki, Finland Dissolved surface active species, or surfactants, have a tendency to partition to solution surface and thereby decrease solution surface tension. Activating cloud droplets have large surface-to-volume ratios, and the amount of surfactant molecules in them is limited. Therefore, unlike with macroscopic solutions, partitioning to the surface can effectively deplete the droplet interior of surfactant molecules. <br><br> Surfactant partitioning equilibrium for activating cloud droplets can be solved numerically from a group of equations. This can be a problem when surfactant effects are examined by using large-scale cloud models. Namely, computing time increases significantly due to the partitioning calculations done in the lowest levels of nested iterations. <br><br> The purpose of this paper is to present analytical equations for surfactant partitioning equilibrium. Some simplifications are needed in deriving the equations, but the numerical errors caused by the simplifications are shown to be very minor. In addition, computing time is decreased roughly by an order of magnitude. 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