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Geoscientific Model Development An interactive open-access journal of the European Geosciences Union
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Volume 4, issue 1
Geosci. Model Dev., 4, 107–116, 2011
https://doi.org/10.5194/gmd-4-107-2011
© Author(s) 2011. This work is distributed under
the Creative Commons Attribution 3.0 License.
Geosci. Model Dev., 4, 107–116, 2011
https://doi.org/10.5194/gmd-4-107-2011
© Author(s) 2011. This work is distributed under
the Creative Commons Attribution 3.0 License.

Development and technical paper 28 Feb 2011

Development and technical paper | 28 Feb 2011

A simplified treatment of surfactant effects on cloud drop activation

T. Raatikainen1,* and A. Laaksonen1,2 T. Raatikainen and A. Laaksonen
  • 1Finnish Meteorological Institute, P.O. Box 503, 00101 Helsinki, Finland
  • 2Department of Physics and Mathematics, University of Eastern Finland, P.O. Box 1627, 70211 Kuopio, Finland
  • *now at: School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia, USA

Abstract. 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.

Surfactant partitioning equilibrium for activating cloud droplets has so far been solved numerically from a group of non-linear equations containing the Gibbs adsorption equation coupled with a surface tension model and an optional activity coefficient model. 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.

Our purpose is to reduce the group of non-linear equations to simple polynomial equations with well known analytical solutions. In order to do that, we describe surface tension lowering using the Szyskowski equation, and ignore all droplet solution non-idealities. It is assumed that there is only one surfactant exhibiting bulk-surface partitioning, but the number of non-surfactant solutes is unlimited. It is shown that the simplifications cause only minor errors to predicted bulk solution concentrations and cloud droplet activation. In addition, computing time is decreased at least by an order of magnitude when using the analytical solutions.

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