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| Titel |
Immersion mode heterogeneous ice nucleation by an illite rich powder representative of atmospheric mineral dust |
| VerfasserIn |
S. L. Broadley, B. J. Murray, R. J. Herbert, J. D. Atkinson, S. Dobbie, T. L. Malkin, E. Condliffe, L. Neve |
| 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. 1 ; Nr. 12, no. 1 (2012-01-05), S.287-307 |
| Datensatznummer |
250010435
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| Publikation (Nr.) |
 copernicus.org/acp-12-287-2012.pdf |
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| Zusammenfassung |
Atmospheric dust rich in illite is transported globally from arid regions
and impacts cloud properties through the nucleation of ice. We present
measurements of ice nucleation in water droplets containing known quantities
of an illite rich powder under atmospherically relevant conditions. The
illite rich powder used here, NX illite, has a similar mineralogical
composition to atmospheric mineral dust sampled in remote locations, i.e.
dust which has been subject to long range transport, cloud processing and
sedimentation. Arizona Test Dust, which is used in other ice nucleation
studies as a model atmospheric dust, has a significantly different
mineralogical composition and we suggest that NX illite is a better
surrogate of natural atmospheric dust.
Using optical microscopy, heterogeneous nucleation in the immersion mode by
NX illite was observed to occur dominantly between 246 K and the homogeneous
freezing limit. In general, higher freezing temperatures were observed when
larger surface areas of NX illite were present within the drops. Homogenous
nucleation was observed to occur in droplets containing low surface areas of
NX illite. We show that NX illite exhibits strong particle to particle
variability in terms of ice nucleating ability, with ~1 in 105
particles dominating ice nucleation when high surface areas were present. In
fact, this work suggests that the bulk of atmospheric mineral dust particles
may be less efficient at nucleating ice than assumed in current model
parameterisations.
For droplets containing ≤2 × 10−6 cm2 of NX illite, freezing
temperatures did not noticeably change when the cooling rate was varied by
an order of magnitude. The data obtained during cooling experiments (surface
area ≤2 × 10−6 cm2) is shown to be inconsistent with the
single component stochastic model, but is well described by the singular
model (ns(236.2 K ≤ T ≤ 247.5 K) = exp(6.53043 × 104− 8.2153088 × 102T + 3.446885376T2 − 4.822268 × 10−3T3). However,
droplets continued to freeze when the temperature was held constant, which
is inconsistent with the time independent singular model. We show that this
apparent discrepancy can be resolved using a multiple component stochastic
model in which it is assumed that there are many types of nucleation sites,
each with a unique temperature dependent nucleation coefficient. Cooling
rate independence can be achieved with this time dependent model if the
nucleation rate coefficients increase very rapidly with decreasing
temperature, thus reconciling our measurement of nucleation at constant
temperature with the cooling rate independence. |
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