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| Titel |
Comparison of measured and calculated collision efficiencies at low temperatures |
| VerfasserIn |
B. Nagare, C. Marcolli, O. Stetzer, U. Lohmann |
| 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 ; 15, no. 23 ; Nr. 15, no. 23 (2015-12-15), S.13759-13776 |
| Datensatznummer |
250120223
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| Publikation (Nr.) |
 copernicus.org/acp-15-13759-2015.pdf |
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| Zusammenfassung |
| Interactions of atmospheric aerosols with clouds influence cloud properties
and modify the aerosol life cycle. Aerosol particles act as cloud
condensation nuclei and ice nucleating particles or become incorporated into
cloud droplets by scavenging. For an accurate description of aerosol
scavenging and ice nucleation in contact mode, collision efficiency between
droplets and aerosol particles needs to be known. This study derives the
collision rate from experimental contact freezing data obtained with the ETH
CoLlision Ice Nucleation CHamber (CLINCH). Freely falling 80 μm diameter water droplets are exposed to an
aerosol consisting of 200 and 400 nm diameter silver iodide particles of
concentrations from 500 to 5000 and 500 to 2000 cm−3, respectively,
which act as ice nucleating particles in contact mode. The experimental data
used to derive collision efficiency are in a temperature range of
238–245 K, where each collision of silver iodide particles with droplets
can be assumed to result in the freezing of the droplet. An upper and lower
limit of collision efficiency is also estimated for 800 nm diameter
kaolinite particles. The chamber is kept at ice saturation at a temperature
range of 236 to 261 K, leading to the slow evaporation of water droplets
giving rise to thermophoresis and diffusiophoresis. Droplets and particles
bear charges inducing electrophoresis. The experimentally derived collision
efficiency values of 0.13, 0.07 and 0.047–0.11 for 200, 400 and 800 nm
particles are around 1 order of magnitude higher than theoretical
formulations which include Brownian diffusion, impaction, interception,
thermophoretic, diffusiophoretic and electric forces. This discrepancy is
most probably due to uncertainties and inaccuracies in the description of
thermophoretic and diffusiophoretic processes acting together. This is, to
the authors' knowledge, the first data set of collision efficiencies acquired
below 273 K. More such experiments with different droplet and particle
diameters are needed to improve our understanding of collision processes
acting together. |
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