Temperature and Supersaturation Dependent Nucleation Rates of Water by Molecular Cluster Model Calculations

Abstract

The homogeneous nucleation process for water has been the subject of great interest, despite the fact that water droplet formation in the atmosphere occurs by heterogeneous nucleation. This is because the homogeneous nucleation process is the simplest of the nucleation processes to treat, both theoretically and experimentally, and because it is strongly dependent on, and hence can be used to illuminate, the molecular interactions and processes that are important for heterogeneous nucleation and aerosol hydration. Using a microphysical approach to nucleation, we present an extensive study of water nucleation rates for wide ranges of both temperature and supersaturation ratio. Based on the fundamental molecular properties of clusters instead of bulk properties, the microphysical approach is demonstrated to predict good agreement with measured nucleation rates over this broad range of conditions. Predicted critical sizes for nucleation are found to be relatively small, and are in the molecular cluster size regime rather than in a size regime that should be characterized by bulk values. Estimated sticking coefficient values cover the range of ?0.9 to ?0.2 for the temperature range considered, whereas sticking coefficient values corresponding to Becker-Doring theory suffer an unreasonably large three-orders of magnitude decrease for temperature increase from 220 K to 285 K.