Water activity as the determinant for homogeneous ice nucleation in aqueous solutions

The unique properties of water in the supercooled (metastable) state are no t fully understood(1). In particular, the effects of solutes and mechanical pressure on the kinetics of the liquid-to-solid phase transition of superc ooled water and aqueous solutions to ice have remained unresolved. Here we show from experimental data that the homogeneous nucleation of ice from sup ercooled aqueous solutions is independent of the nature of the solute, but depends only on the water activity of the solution-that is, the ratio betwe en the water vapour pressures of the solution and of pure water under the s ame conditions. In addition, we show that the presence of solutes and the a pplication of pressure have a very similar effect on ice nucleation. We pre sent a thermodynamic theory for homogeneous ice nucleation, which expresses the nucleation rate coefficient as a function of water activity and pressu re. Recent observations from clouds containing ice are in good agreement wi th our theory and our results should help to overcome one of the main weakn esses of numerical models of the atmosphere, the formulation of cloud proce sses.