 |
| Titel |
Thermodynamic derivation of the activation energy for ice nucleation |
| VerfasserIn |
D. Barahona |
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| ISSN |
1680-7316
|
| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 15, no. 24 ; Nr. 15, no. 24 (2015-12-16), S.13819-13831 |
| Datensatznummer |
250120227
|
| Publikation (Nr.) |
 copernicus.org/acp-15-13819-2015.pdf |
|
|
|
|
|
| Zusammenfassung |
| Cirrus 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. |
| |
|
| Teil von |
|
|
|
|
|
|
|
|