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| Titel |
Heterogeneous freezing of water droplets containing kaolinite particles |
| VerfasserIn |
B. J. Murray, S. L. Broadley, T. W. Wilson, J. D. Atkinson, R. H. Wills |
| 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 ; 11, no. 9 ; Nr. 11, no. 9 (2011-05-06), S.4191-4207 |
| Datensatznummer |
250009698
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| Publikation (Nr.) |
 copernicus.org/acp-11-4191-2011.pdf |
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| Zusammenfassung |
| Clouds composed of both ice particles and supercooled liquid water droplets
exist at temperatures above ~236 K. These mixed phase clouds, which
strongly impact climate, are very sensitive to the presence of solid
particles that can catalyse freezing. In this paper we describe experiments
to determine the conditions at which the clay mineral kaolinite nucleates
ice when immersed within water droplets. These are the first immersion mode
experiments in which the ice nucleating ability of kaolinite has been
determined as a function of clay surface area, cooling rate and also at
constant temperatures. Water droplets containing a known amount of clay
mineral were supported on a hydrophobic surface and cooled at rates of
between 0.8 and 10 K min−1 or held at constant sub-zero temperatures.
The time and temperature at which individual 10–50 μm diameter droplets
froze were determined by optical microscopy. For a cooling rate of 10 K min−1, the
median nucleation temperature of 10–40 μm diameter
droplets increased from close to the homogeneous nucleation limit (236 K) to
240.8 ± 0.6 K as the concentration of kaolinite in the droplets was
increased from 0.005 wt% to 1 wt%. This data shows that the
probability of freezing scales with surface area of the kaolinite
inclusions. We also show that at a constant temperature the number of liquid
droplets decreases exponentially as they freeze over time. The constant
cooling rate experiments are consistent with the stochastic, singular and
modified singular descriptions of heterogeneous nucleation; however,
freezing during cooling and at constant temperature can be reconciled best
with the stochastic approach. We report temperature dependent nucleation
rate coefficients (nucleation events per unit time per unit area) for
kaolinite and present a general parameterisation for immersion nucleation
which may be suitable for cloud modelling once nucleation by other important
ice nucleating species is quantified in the future. |
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