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| Titel |
New cloud chamber experiments on the heterogeneous ice nucleation ability of oxalic acid in the immersion mode |
| VerfasserIn |
R. Wagner, O. Möhler, H. Saathoff, M. Schnaiter, T. Leisner |
| 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 ; 11, no. 5 ; Nr. 11, no. 5 (2011-03-09), S.2083-2110 |
| Datensatznummer |
250009449
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| Publikation (Nr.) |
 copernicus.org/acp-11-2083-2011.pdf |
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| Zusammenfassung |
| The heterogeneous ice nucleation ability of oxalic acid in the immersion
mode has been investigated by controlled expansion cooling runs with
airborne, ternary solution droplets composed of, (i), sodium chloride,
oxalic acid, and water (NaCl/OA/H2O) and, (ii), sulphuric acid, oxalic
acid, and water (H2SO4/OA/H2O). Polydisperse aerosol
populations with median diameters ranging from 0.5–0.7 μm and
varying solute concentrations were prepared. The expansion experiments were
conducted in the AIDA aerosol and cloud chamber of the Karlsruhe Institute
of Technology at initial temperatures of 244 and 235 K. In the ternary
NaCl/OA/H2O system, solid inclusions of oxalic acid, presumably
nucleated as oxalic acid dihydrate, were formed by temporarily exposing the
ternary solution droplets to a relative humidity below the efflorescence
point of NaCl. The matrix of the crystallised NaCl particulates triggered
the precipitation of the organic crystals which later remained as solid
inclusions in the solution droplets when the relative humidity was
subsequently raised above the deliquescence point of NaCl. The embedded
oxalic acid crystals reduced the critical ice saturation ratio required for
the homogeneous freezing of pure NaCl/H2O solution droplets at a
temperature of around 231 K from 1.38 to about 1.32. Aqueous solution
droplets with OA inclusions larger than about 0.27 μm in diameter
efficiently nucleated ice by condensation freezing when they were activated
to micron-sized cloud droplets at 241 K, i.e., they froze well above the
homogeneous freezing temperature of pure water droplets of about 237 K. Our
results on the immersion freezing potential of oxalic acid corroborate the
findings from a recent study with emulsified aqueous solutions containing
crystalline oxalic acid. In those experiments, the crystallisation of oxalic
acid diyhdrate was triggered by a preceding homogeneous freezing cycle with
the emulsion samples. The expansion cooling cycles with ternary
H2SO4/OA/H2O solution droplets were aimed to analyse whether
those findings can be transferred to ice nucleation experiments with
airborne oxalic acid containing aerosol particles. Under our experimental
conditions, the efficiency by which the surface of homogeneously nucleated
ice crystals triggered the precipitation of oxalic acid dihydrate was very
low, i.e., less than one out of a hundred ice crystals that were formed by
homogeneous freezing in a first expansion cooling cycle left behind an
ice-active organic crystal that acted as immersion freezing nucleus in a
second expansion cooling cycle. |
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