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| Titel |
Ice cloud processing of ultra-viscous/glassy aerosol particles leads to enhanced ice nucleation ability |
| VerfasserIn |
R. Wagner, O. Möhler, H. Saathoff, M. Schnaiter, J. Skrotzki, T. Leisner, T. W. Wilson, T. L. Malkin, B. J. Murray |
| 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. 18 ; Nr. 12, no. 18 (2012-09-25), S.8589-8610 |
| Datensatznummer |
250011466
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| Publikation (Nr.) |
 copernicus.org/acp-12-8589-2012.pdf |
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| Zusammenfassung |
| The ice nucleation potential of airborne glassy aqueous aerosol particles
has been investigated by controlled expansion cooling cycles in the AIDA
aerosol and cloud chamber of the Karlsruhe Institute of Technology at
temperatures between 247 and 216 K. Four different solutes were used as
proxies for oxygenated organic matter found in the atmosphere: raffinose,
4-hydroxy-3-methoxy-DL-mandelic acid (HMMA), levoglucosan, and a
multi-component mixture of raffinose with five dicarboxylic acids and
ammonium sulphate. Similar to previous experiments with citric acid
aerosols, all particles were found to nucleate ice heterogeneously before
reaching the homogeneous freezing threshold provided that the freezing
cycles were started well below the respective glass transition temperatures
of the compounds; this is discussed in detail in a separate article. In this
contribution, we identify a further mechanism by which glassy aerosols can
promote ice nucleation below the homogeneous freezing limit. If the glassy
aerosol particles are probed in freezing cycles started only a few degrees
below their respective glass transition temperatures, they enter the liquid
regime of the state diagram upon increasing relative humidity
(moisture-induced glass-to-liquid transition) before being able to act as
heterogeneous ice nuclei. Ice formation then only occurs by homogeneous
freezing at elevated supersaturation levels. When ice forms the remaining
solution freeze concentrates and re-vitrifies. If these ice cloud processed
glassy aerosol particles are then probed in a second freezing cycle at the
same temperature, they catalyse ice formation at a supersaturation threshold
between 5 and 30% with respect to ice. By analogy with the enhanced ice
nucleation ability of insoluble ice nuclei like mineral dusts after they
nucleate ice once, we refer to this phenomenon as pre-activation. We propose
a number of possible explanations for why glassy aerosol particles that have
re-vitrified in contact with the ice crystals during the preceding
homogeneous freezing cycle exhibit pre-activation: they may retain small ice
embryos in pores, have footprints on their surface which match the ice
lattice, or simply have a much greater surface area or different surface
microstructure compared to the unprocessed glassy aerosol particles.
Pre-activation must be considered for the correct interpretation of
experimental results on the heterogeneous ice nucleation ability of glassy
aerosol particles and may provide a mechanism of producing a population of
extremely efficient ice nuclei in the upper troposphere. |
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