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| Titel |
Time dependence of immersion freezing: an experimental study on size selected kaolinite particles |
| VerfasserIn |
A. Welti, F. Lüönd, Z. A. Kanji, O. Stetzer, U. Lohmann |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 12, no. 20 ; Nr. 12, no. 20 (2012-10-29), S.9893-9907 |
| Datensatznummer |
250011542
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| Publikation (Nr.) |
 copernicus.org/acp-12-9893-2012.pdf |
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| Zusammenfassung |
| The time dependence of immersion freezing was studied for temperatures
between 236 K and 243 K. Droplets with single immersed, size-selected
400 nm and 800 nm kaolinite particles were produced at 300 K, cooled down
to supercooled temperatures, and the fraction of frozen droplets with
increasing residence time was detected. To simulate the conditions of
immersion freezing in mixed-phase clouds we used the Zurich Ice Nucleation
Chamber (ZINC) and its vertical extension, the Immersion Mode Cooling chAmber
(IMCA). We observed that the frozen fraction of droplets increased with
increasing residence time in the chamber. This suggests that there is a time
dependence of immersion freezing and supports the importance of a stochastic
component in the ice nucleation process. The rate at which droplets freeze
was observed to decrease towards higher temperatures and smaller particle
sizes. Comparison of the laboratory data with four different ice nucleation
models, three based on classical nucleation theory with different
representations of the particle surface properties and one singular, suggest
that the classical, stochastic approach combined with a distribution of
contact angles is able to reproduce the ice nucleation observed in these
experiments most accurately. Using the models to calculate the increase in
frozen fraction at typical mixed-phase cloud temperatures over an extended
period of time, yields an equivalent effect of −1 K temperature shift for
an increase in times scale by one order of magnitude. This suggests that
temperature is more important than time. |
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