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Thermodynamic Derivation of the Activation Energy for Ice NucleationCirrus clouds play a key role in the radiative and hydrological balance of the upper troposphere. Their correct representation in atmospheric models requires an understanding of the microscopic processes leading to ice nucleation. A key parameter in the theoretical description of ice nucleation is the activation energy, which controls the flux of water molecules from the bulk of the liquid to the solid during the early stages of ice formation. In most studies it is estimated by direct association with the bulk properties of water, typically viscosity and self-diffusivity. As the environment in the ice-liquid interface may differ from that of the bulk, this approach may introduce bias in calculated nucleation rates. In this work a theoretical model is proposed to describe the transfer of water molecules across the ice-liquid interface. Within this framework the activation energy naturally emerges from the combination of the energy required to break hydrogen bonds in the liquid, i.e., the bulk diffusion process, and the work dissipated from the molecular rearrangement of water molecules within the ice-liquid interface. The new expression is introduced into a generalized form of classical nucleation theory. Even though no nucleation rate measurements are used to fit any of the parameters of the theory the predicted nucleation rate is in good agreement with experimental results, even at temperature as low as 190 K, where it tends to be underestimated by most models. It is shown that the activation energy has a strong dependency on temperature and a weak dependency on water activity. Such dependencies are masked by thermodynamic effects at temperatures typical of homogeneous freezing of cloud droplets; however, they may affect the formation of ice in haze aerosol particles. The new model provides an independent estimation of the activation energy and the homogeneous ice nucleation rate, and it may help to improve the interpretation of experimental results and the development of parameterizations for cloud formation.
Document ID
20160006706
Document Type
Reprint (Version printed in journal)
Authors
Barahona, D.
(NASA Goddard Space Flight Center Greenbelt, MD United States)
Date Acquired
May 31, 2016
Publication Date
December 16, 2015
Publication Information
Publication: Atmospheric Chemistry and Physics
Volume: 15
Issue: 24
Subject Category
Meteorology And Climatology
Report/Patent Number
GSFC-E-DAA-TN31836
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.
Keywords
Cirrus clouds
Activation Energy
Ice Formation
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