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| Titel |
Mixed-phased particles in polar stratospheric ice clouds |
| VerfasserIn |
Anatoli Bogdan, Mario J. Molina, Thomas Loerting |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250037943
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| Zusammenfassung |
| Keywords: polar stratospheric clouds (PSCs), ozone depletion, differential scanning
calorimeter.
The rate of chlorine activation reactions, which lead to ozone depletion in the
winter/spring polar stratosphere (Molina, 1994), depends on the phase state of the surface of
polar stratospheric cloud (PSC) ice crystals (McNeil et al., 2006). PSCs are thought to consist
of solid ice and NAT (nitric acid trihydrate, HNO3-
3H2O) particles and supercooled
HNO3/H2SO4/H2O droplets. The corresponding PSCs are called Type II, Ia, and Ib PSCs,
respectively (Zondlo et al., 1998). Type II PSCs are formed in the Antarctic region below the
ice frost point of  189 K by homogeneous freezing of HNO3/H2SO4/H2O droplets
(Chang et al., 1999) with the excess of HNO3. The PSC ice crystals are thought to
be solid. However, the fate of H+, NO3-, SO42- ions during freezing was not
investigated.
Our differential scanning calorimetry (DSC) studies of freezing emulsified
HNO3/H2SO4/H2O droplets of sizes and compositions representative of the polar
stratosphere demonstrate that during the freezing of the droplets, H+, NO3-, SO42- are
expelled from the ice lattice. The expelled ions form a residual solution around the formed ice
crystals. The residual solution does not freeze but transforms to glassy state at ~150 K
(Bogdan et al., 2010). By contrast to glass-formation in these nitric-acid rich ternary mixtures
the residual solution freezes in the case of sulphuric-acid rich ternary mixtures (Bogdan and
Molina, 2009).
For example, we can consider the phase separation into ice and a residual solution during
the freezing of 23/3 wt% HNO3/H2SO4/H2O droplets. On cooling, ice is formed at ~189
K. This is inferred from the fact that the corresponding melting peak at ~248 K
exactly matches the melting point of ice in the phase diagram of HNO3/H2SO4/H2O
containing 3 wt % H2SO4. After the ice has formed, the glass transition occurs at
Tg - 150 K. The appearance of the glass transition indicates that the droplets do
not freeze completely. After freezing, a fraction of each droplet remains liquid
until its transformation to glass. The liquid, which remains unfrozen, is a residual
solution formed by the expulsion of H+, NO3-, SO42- ions from the ice lattice
during freezing. The residual solution undergoes the glass transition even if the
cooling rate as small as 0.05 K/min (3 K/h) is applied. This cooling rate is similar to
synoptic temperature change. Thus our results indicate that Type II PSC ice crystals
cannot be completely solid, as is usually thought, but are enveloped by a supercooled
HNO3/H2SO4/H2O coating. These results also suggest that chlorine-activation reactions
are better studied on supercooled HNO3/H2SO4/H2O solutions than on a pure ice
surface.
Molina, M. J. (1994), The chemistry of the atmosphere: The impact of global
change. Ed. Calvet, J. G. (Blackwell, Boston) pp. 27-38.
McNeil, V. F., Loerting, T., Geiger, F. M., Trout, B. L. & Molina, M. J. (2006),
Proc. Nat. Acad. Sci., 103, 9422. |
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