ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-11-2853-2011 Size dependence of volume and surface nucleation rates for homogeneous freezing of supercooled water droplets Kuhn T. 1 2 Earle M. E. 1 3 Khalizov A. F. 1 4 Sloan J. J. 1 Department of Earth and Environmental Sciences, University of Waterloo, Waterloo, ON, Canada now at: Division of Space Technology, Department of Computer Science, Electrical and Space Engineering, Luleå University of Technology, Kiruna, Sweden now at: Cloud Physics and Severe Weather Research Section, Environment Canada, Toronto, ON, Canada now at: Department of Atmospheric Sciences, Texas A&M University, College Station, TX, USA 28 03 2011 11 6 2853 2861 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.net/11/2853/2011/acp-11-2853-2011.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/11/2853/2011/acp-11-2853-2011.pdf

The relative roles of volume and surface nucleation were investigated for the homogeneous freezing of pure water droplets. Experiments were carried out in a cryogenic laminar aerosol flow tube using supercooled water aerosols with maximum volume densities at radii between 1 and 3 μm. Temperature- and size-dependent values of volume- and surface-based homogeneous nucleation rates between 234.8 and 236.2 K were derived using a microphysical model and aerosol phase compositions and size distributions determined from infrared extinction measurements in the flow tube. The results show that the contribution from nucleation at the droplet surface increases with decreasing droplet radius and dominates over nucleation in the bulk droplet volume for droplets with radii smaller than approximately 5 μm. This is interpreted in terms of a lowered free energy of ice germ formation in the surface-based process. The implications of surface nucleation for the parameterization of homogeneous ice nucleation in numerical models are considered.

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