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| Titel |
Ambient in-situ immersion freezing measurements – findings from the ZAMBIS 2014 field campaign for three ice nucleation techniques |
| VerfasserIn |
Monika Kohn, James D. Atkinson, Ulrike Lohmann, Zamin A. Kanji |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250103288
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| Publikation (Nr.) |
 EGU/EGU2015-2698.pdf |
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| Zusammenfassung |
| To estimate the influence of clouds on the Earth’s radiation budget, it is crucial to understand
cloud formation processes in the atmosphere. A key process, which significantly affects cloud
microphysical properties and the initiation of precipitation thus contributing to the
hydrological cycle, is the prevailing type of ice nucleation mechanism. In mixed-phase clouds
immersion freezing is the dominant ice crystal forming mechanism, whereby ice nucleating
particles (INP) first act as cloud condensation nuclei (CCN) and are activated to cloud
droplets followed by freezing upon supercooling.
There are a number of experimental methods and techniques to investigate the ice
nucleating ability in the immersion mode, however most techniques are offline for field
sampling or only suitable for laboratory measurements. In-situ atmospheric studies are
needed to understand the ice formation processes of ‘real world’ particles. Laboratory
experiments simulate conditions of atmospheric processes like ageing or coating but are still
idealized. Our method is able to measure ambient in-situ immersion freezing on
single immersed aerosol particles. The instrumental setup consists of the recently
developed portable immersion mode cooling chamber (PIMCA) as a vertical extension
to the portable ice nucleation chamber (PINC, [1]), where the frozen fraction of
activated aerosol particles are detected by the ice optical depolarization detector
(IODE, [2]). Two additional immersion freezing techniques based on a droplet
freezing array [3,4] are used to sample ambient aerosol particles either in a suspension
(fraction larger ~0.6 μm) or on PM10-filters to compare different ice nucleation
techniques.
Here, we present ambient in-situ measurements at an urban forest site in Zurich,
Switzerland held during the Zurich ambient immersion freezing study (ZAMBIS) in spring
2014. We investigated the ice nucleating ability of natural atmospheric aerosol with
the PIMCA/PINC immersion freezing setup as well as a droplet freezing method
on aerosol particles either collected in a suspension or on PM10-filters to obtain
atmospheric IN concentrations based on the measured ambient aerosol. Investigation of
physical properties (number and size distribution) and chemical composition as
well as the meteorological conditions provide supplementary information that help
to understand the nature of particles and air masses that contribute to immersion
freezing.
Acknowledgements
We thank Hannes Wydler and Hansjörg Frei from ETH Zurich for their technical support.
Furthermore, the authors want thank Franz Conen from the University of Basel for sharing
equipment and training in the drop freezing experiment.
References
[1] Chou et al. (2011), Atmos. Chem. Phys., 11, 4725-4738.
[2] Nicolet et al. (2010), Atmos. Chem. Phys., 10, 313-325.
[3] Conen et al. (2012), Atmos. Meas. Tech., 5, 321-327.
[4] Stopelli et al. (2014), Atmos. Meas. Tech., 7, 129-134. |
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