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| Titel |
Ice nucleation efficiency of clay minerals in the immersion mode |
| VerfasserIn |
Valeria Pinti, Claudia Marcolli, Bernhard Zobrist, Christopher Hoyle, Thomas Peter |
| 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 |
250052320
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| Zusammenfassung |
| Up to 5700 Tg dust from deserts are emitted every year into the atmosphere (IPCC, 2001).
These aerosols may reach high altitudes and can be transported over long distances. They
further may act as ice nuclei (IN) and thus can modify the optical properties of clouds and
their lifetime (indirect effect). Ice nucleation in the atmosphere can either occur by
homogeneous or heterogeneous ice nucleation. While the first process occurs typically at
temperatures below ~238 K, the latter one takes place at higher temperatures (De Mott et al.,
1997).
Desert dust aerosols are mainly composed of illite (Illite NX; Illite SE), kaolinite
(KGa-2; KGa-1b; Kaol) and montmorillonite (Mont SWy-2; Mont K-10; Mont KSF;
Mont STx-1b). These three clay minerals from different suppliers (Fluka, Clay
Mineral Society and Arginotec) were investigated in order to study their behaviour as
ice nuclei in immersion mode. Additionally a Saharan dust sample (Hoggar) was
investigated.
A commercial Differential Scanning Calorimeter (DSC, TA Q10) was used. The
emulsified samples were prepared with 80 wt% of a mineral oil/lanoline mixture and 20 wt%
of aqueous clay suspensions. This led to aqueous droplets of roughly 1.5 μm diameter with
immersed clay particles. For every clay mineral we investigated aqueous suspensions with six
different clay concentrations (from 0.01 to 10 wt%).
The studied samples showed three different types of behavior (case a, b and c).
Characteristics of each case are listed below, samples which belong to the case are in
brackets:
Case a: shifting of the onset of freezing toward higher temperatures with
increasing amount of dust in the suspension (KGa-2, Clay Mineral Society).
Case b: constant freezing onsets with increasing amount of dust in the suspension
(KGa-1b, Mont STx-1b, both from Clay Mineral Society; Illite NX, from
Arginotec; Hoggar).
Case c: second heterogeneous peak appears for higher wt% suspensions (Kaol.,
Mont Mont K-10, Mont KSF, all from Fluka; SWy-2, Clay Mineral Society; Illite
SE, from Arginotec).
The heterogeneous freezing temperatures depend on the type but also on the source and
pretreatment of the different clay minerals. Thus, Kaol. showed freezing onsets in the range
from 238 K to 248 K while KGa-1b and KGa-2 only started to freeze in the range from 238 K
to 240 K.
This shows how the same type of clay can behave in a highly different way and highlights
the need to better understand which property is important for the heterogeneous ice
nucleation.
For comparison with literature studies where heterogeneous onset temperatures were
much higher (Hoffer et al. (1961) and Pitter and Pruppacher (1973) for kaolinite), we
performed a series of freezing cycles with the pan filled with the Kaol. suspension leading to
a similar sample volume (~20 mm3) as in the second literature study. This data agrees nicely
with literature suggesting that large sample volumes contain some highly potent IN. This
indicates that some of the high freezing temperatures reported in the literature result mainly
from the large volume of the samples, and might lead to an overrating of IN efficiencies when
applied to the atmosphere.
References
De Mott, P. J., Rogers, D. C. & Kreidenweis (1997), J. Geophys. Res. 102,
19575–19584.
Hoffer, T. E., Journal of Meterology, 18, 766–778, 1961.
Pitter, R. L., and H. R. Pruppacher (1973), Q. J. R. Meteorol. Soc., 99, 540–550,
doi:10.1002/qj.49709942111. |
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