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| Titel |
Ice nucleation from aqueous NaCl droplets with and without marine diatoms |
| VerfasserIn |
P. A. Alpert, J. Y. Aller, D. A. Knopf |
| 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. 12 ; Nr. 11, no. 12 (2011-06-16), S.5539-5555 |
| Datensatznummer |
250009844
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| Publikation (Nr.) |
 copernicus.org/acp-11-5539-2011.pdf |
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| Zusammenfassung |
| Ice formation in the atmosphere by homogeneous and heterogeneous
nucleation is one of the least understood processes in cloud
microphysics and climate. Here we describe our investigation of the
marine environment as a potential source of atmospheric IN by
experimentally observing homogeneous ice nucleation from aqueous
NaCl droplets and comparing against heterogeneous ice
nucleation from aqueous NaCl droplets containing intact and
fragmented diatoms. Homogeneous and heterogeneous ice nucleation are
studied as a function of temperature and water activity, aw.
Additional analyses are presented on the dependence of diatom
surface area and aqueous volume on heterogeneous freezing
temperatures, ice nucleation rates, ωhet, ice nucleation
rate coefficients, Jhet, and differential and cumulative ice
nuclei spectra, k(T) and K(T), respectively. Homogeneous freezing
temperatures and corresponding nucleation rate coefficients are in
agreement with the water activity based homogeneous ice nucleation
theory within experimental and predictive uncertainties. Our results
confirm, as predicted by classical nucleation theory, that a stochastic
interpretation can be used to describe the homogeneous ice nucleation
process. Heterogeneous ice nucleation initiated by intact and
fragmented diatoms can be adequately represented by a modified water
activity based ice nucleation theory. A horizontal shift in water
activity, Δaw, het = 0.2303, of the ice melting curve
can describe median heterogeneous freezing temperatures. Individual
freezing temperatures showed no dependence on available diatom surface
area and aqueous volume. Determined at median diatom freezing
temperatures for aw from 0.8 to 0.99, ωhet~0.11+0.06−0.05 s−1, Jhet~1.0+1.16−0.61×104
cm−2 s−1, and K~6.2+3.5−4.1
×104 cm−2. The
experimentally derived ice nucleation rates and nuclei spectra allow
us to estimate ice particle production which we subsequently use for
a comparison with observed ice crystal concentrations typically found in
cirrus and polar marine mixed-phase clouds. Differences in application
of time-dependent and time-independent analyses to predict ice
particle production are discussed. |
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