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| Titel |
Deposition nucleation viewed as homogeneous or immersion freezing in pores and cavities |
| VerfasserIn |
C. Marcolli |
| 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 ; 14, no. 4 ; Nr. 14, no. 4 (2014-02-21), S.2071-2104 |
| Datensatznummer |
250118423
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| Publikation (Nr.) |
 copernicus.org/acp-14-2071-2014.pdf |
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| Zusammenfassung |
| Heterogeneous ice nucleation is an important mechanism for the glaciation of
mixed phase clouds and may also be relevant for cloud formation and
dehydration at the cirrus cloud level. It is thought to proceed through
different mechanisms, namely contact, condensation, immersion and deposition
nucleation. Conceptually, deposition nucleation is the only pathway that does
not involve liquid water, but occurs by direct water vapor deposition onto a
surface. This study challenges this classical view by putting forward the
hypothesis that what is called deposition nucleation is in fact pore
condensation and freezing (PCF) occurring in voids and cavities that may form
between aggregated primary particles and host water at relative humidity
RHw < 100% because of the inverse Kelvin effect.
Homogeneous ice nucleation is expected to occur below 235 K when at least
one pore is filled with water. Ice nucleation in pores may also happen in
immersion mode but with a lower probability because it requires at least one
active site in a water filled pore. Therefore a significant enhancement in
ice nucleation efficiency is expected when temperature falls below 235 K.
For a deposition nucleation process from water vapor no discontinuous change
in ice nucleation efficiency should occur at T = 235 K because no
liquid water is involved in this process. Studies on freezing in confinement
carried out on mesoporous silica materials such as SBA-15, SBA-16, MCM-41,
zeolites and KIT have shown that homogeneous ice nucleation occurs abruptly
at T = 230–235 K in pores with diameters (D) of 3.5–4 nm or
larger but only gradually at T = 210–230 K in pores with
D = 2.5–3.5 nm. Pore analysis of clay minerals shows that kaolinites
exhibit pore structures with pore diameters (Dp) of 20–50 nm.
The mesoporosity of illites and montmorillonites is characterized by pores
with Dp = 2–5 nm. The number and size of pores is
distinctly increased in acid treated montmorillonites like K10. Water
adsorption isotherms of MCM-41 show that pores with
Dp = 3.5–4 nm fill with water at RHw
= 56–60% in accordance with an inverse Kelvin effect. Water in such
pores should freeze homogeneously for T < 235 K even before
relative humidity with respect to ice (RHi) reaches ice
saturation. Ice crystal growth by water vapor deposition from the gas phase
is therefore expected to set in as soon as
RHi > 100%. Pores with
D > 7.5 nm fill with water at
RHi > 100% for T < 235 K and are
likely to freeze homogeneously as soon as they are filled with water. Given
the pore structure of clay minerals, PCF should be highly efficient for T < 235 K and may occur at T > 235 K in
particles that exhibit active sites for immersion freezing within pores. Most
ice nucleation studies on clay minerals and mineral dusts indeed
show a strong increase in ice
nucleation efficiency when temperature is decreased below 235 K in
accordance with PCF and are not explicable by the classical view of
deposition nucleation. PCF is probably also the prevailing ice nucleation
mechanism below water saturation for glassy, soot, and volcanic ash aerosols.
No case could be identified that gives clear evidence of ice nucleation by
water vapor deposition onto a solid surface. |
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