Size dependence of volume and surface nucleation rates for homogeneous freezing of supercooled water droplets
The relative roles of volume and surface nucleation were investigated\nfor the homogeneous freezing of pure water droplets. Experiments were\ncarried out in a cryogenic laminar aerosol flow tube using supercooled\nwater aerosols with maximum volume densities at radii between 1 and 3\nmu m. Temperature- and size-dependent values of volume- and\nsurface-based homogeneous nucleation rates between 234.8 and 236.2 K\nwere derived using a microphysical model and aerosol phase compositions\nand size distributions determined from infrared extinction measurements\nin the flow tube. The results show that the contribution from\nnucleation at the droplet surface increases with decreasing droplet\nradius and dominates over nucleation in the bulk droplet volume for\ndroplets with radii smaller than approximately 5 mu m. This is\ninterpreted in terms of a lowered free energy of ice germ formation in\nthe surface-based process. The implications of surface nucleation for\nthe parameterization of homogeneous ice nucleation in numerical models\nare considered.