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| Titel |
Ice nucleation efficiency of clay minerals in the immersion mode |
| VerfasserIn |
V. Pinti, C. Marcolli, B. Zobrist, C. R. Hoyle, T. Peter |
| 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. 13 ; Nr. 12, no. 13 (2012-07-10), S.5859-5878 |
| Datensatznummer |
250011303
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| Publikation (Nr.) |
 copernicus.org/acp-12-5859-2012.pdf |
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| Zusammenfassung |
| Emulsion and bulk freezing experiments were performed to investigate
immersion ice nucleation on clay minerals in pure water, using various
kaolinites, montmorillonites, illites as well as natural dust from the
Hoggar Mountains in the Saharan region. Differential scanning calorimeter
measurements were performed on three different kaolinites (KGa-1b, KGa-2 and
K-SA), two illites (Illite NX and Illite SE) and four natural and
acid-treated montmorillonites (SWy-2, STx-1b, KSF and K-10). The emulsion
experiments provide information on the average freezing behaviour
characterized by the average nucleation sites. These experiments revealed
one to sometimes two distinct heterogeneous freezing peaks, which suggest
the presence of a low number of qualitatively distinct average nucleation
site classes. We refer to the peak at the lowest temperature as "standard
peak" and to the one occurring in only some clay mineral types at higher
temperatures as "special peak". Conversely, freezing in bulk samples is
not initiated by the average nucleation sites, but by a very low number of
"best sites". The kaolinites and montmorillonites showed quite narrow
standard peaks with onset temperatures 238 K<Tonstd<242 K and
best sites with averaged median freezing temperature Tmedbest=257 K,
but only some featuring a special peak (i.e. KSF, K-10, K-SA and SWy-2)
with freezing onsets in the range 240–248 K. The illites showed broad
standard peaks with freezing onsets at 244 K Tonstd<246 K
and best sites with averaged median freezing temperature
Tmedbest=262 K. The large difference between freezing
temperatures of standard and best sites shows that characterizing ice
nucleation efficiencies of dust particles on the basis of freezing onset
temperatures from bulk experiments, as has been done in some atmospheric
studies, is not appropriate. Our investigations demonstrate that immersion
freezing temperatures of clay minerals strongly depend on the amount of clay
mineral present per droplet and on the exact type (location of collection
and pre-treatment) of the clay mineral. We suggest that apparently
contradictory results obtained by different groups with different setups are
indeed in good agreement when only clay minerals of the same type and amount
per droplet are compared. The natural sample from the Hoggar Mountains, a
region whose dusts have been shown to be composed mainly of illite, showed
very similar freezing characteristics (standard and best) to the illites.
Relating the concentration of best IN to the dust concentration in the
atmosphere suggested that the best IN in the Hoggar sample would be common
enough downwind of their source region to account for ambient IN number
densities in the temperature range of 250–260 K at least during dust
events. |
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