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| Titel |
The structure of ice crystals at amospheric conditions |
| VerfasserIn |
Christian Sippel, Thomas C. Hansen, Michael M. Koza, Werner F. Kuhs |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250057707
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| Zusammenfassung |
| There is laboratory evidence that the metastable ice phase Ic can form under atmospheric
conditions1)2). The presence of this so-called cubic ice in the upper troposphere and
lower stratosphere would affect important processes occurring at these altitudes by
changing crystal size distributions, dehydrating cold clouds and increasing the vapour
pressure3).
Especially the latter effect makes the question for the crystal structure of atmospheric ice
a crucial one since the discovery of supersaturations of water vapour inside and outside of
cirrus clouds4).
Ice obtained from vapour deposition ("frost") was investigated by neutron powder
diffraction. A structural model5), based on hexagonal and cubic stacking probabilities, allows
us to fit the obtained diffraction patterns. By refining these probabilities and other parameters
related to the crystal structure, we can analyze the stacking behaviour and determine the
anisotropic particle size.
We observed that these properties change as a function of both temperature and time.
While about 50% of the stacking sequences are cubic in the original frost, exposure to
increasing temperatures leads to a monotonic decrease of their fraction, until near extinction
at 220K. This correlates with measurements of supersaturations in this temperature
range6).
The transformation from ice Ic to nearly hexagonal ice is accompanied by an increase
of the particle size, indicating that mass transfer takes place during the annealing
process.
Annealing experiments at constant temperatures showed that isothermal structural
changes are still very slow at 175K, but occur on the timescale of several hours at
180 and 185K, which is in the same order of magnitude as the life time of cirrus
clouds.
At present we make efforts to improve our control over deposition conditions. A specially
developed condensation insert for the neutron diffractometer’s cryostat allows us to
prepare ice in-situ at atmospherically relevant and reproducible temperatures and
pressures.
1) B. J. Murray, D. A. Knopf, A. K. Bertram, Nature 434, 203 (2005)
2) W. F. Kuhs, G. Genov, D. K. Staykova, T. C. Hansen, Phys. Chem. Chem. Phys. 6,
4917-4920 (2004)
3) D. M. Murphy, Geophys.Res.Lett. 30, 018566 (2003)
4) T. Peter, C. Marcolli, P. Spichtinger, T. Corti, M. C. Baker, T. Koop, Science 314, 1399
(2006).
5) T. C. Hansen, M. M. Koza, W. F. Kuhs, Journal of Physics: Condensed Matter, 20,
285104 (2008).
6) R. S. Gao & 19 other authors, Science 303, 516-520 (2004) |
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