A simple model for the formation of ice clouds

Atmospheric ice clouds strongly affect both the chemistry and the radiant properties of the Earth. However, the formation of ice particles in the atmosphere through homogeneous ice nucleation is not fully understood. This is due to the fact that ice forms in aqueous aerosol droplets which can be composed of a great variety of constituents. Scientists from the Federal Institute of Technology Zurich (Switzerland) and the Max Planck Institute for Chemistry in Mainz (Germany) show that the formation of atmospheric ice particles can be described by a general thermodynamic model (Nature, 10 August 2000).

Using laboratory data on a large number of solutes, they show that ice nucleation is independent of the nature of the solute. The only important parameters required to describe ice particle formation are temperature and relative humidity. They further show that their model is in good agreement with recent observation of ice clouds.

These results should help to overcome one of the main weaknesses of numerical models of the atmosphere, the formulation of cloud processes. Important applications range from issues like denitrification and future ozone depletion in the Arctic polar stratosphere to the aerosol indirect effect due to homogeneously formed ice clouds in climate studies. The simplicity of the formulation makes it suitable not only for small scale process studies of individual clouds but also for global scale three-dimensional models.

The results of this study also have important implications in other areas of research. For example, the scientists show that solutes and applied pressure have a very similar effect on ice nucleation which might be important for the understanding of the physics of supercooled water and aqueous solutions. The results also challenge classical theories of nucleation of crystals from liquid solutions. Finally, the new ice nucleation model may also have implications in the field of cryobiology, because it sets a inferior limit to the freeze resistance of cells in plants and animals at low temperatures.