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| Titel |
Can we define an asymptotic value for the ice active surface site density for heterogeneous ice nucleation? |
| VerfasserIn |
Dennis Niedermeier, Stefanie Augustin-Bauditz, Susan Hartmann, Heike Wex, Karoliina Ignatius, Frank Stratmann |
| 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 |
250105726
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| Publikation (Nr.) |
 EGU/EGU2015-5282.pdf |
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| Zusammenfassung |
| The formation of ice in atmospheric clouds has a substantial influence on the radiative
properties of clouds as well as on the formation of precipitation. Therefore much effort has
been made to understand and quantify the major ice formation processes in clouds.
Immersion freezing has been suggested to be a dominant primary ice formation process in
low and mid-level clouds (mixed-phase cloud conditions). It also has been shown that mineral
dust particles are the most abundant ice nucleating particles in the atmosphere and thus
may play an important role for atmospheric ice nucleation (Murray et al., 2012).
Additionally, biological particles like bacteria and pollen are suggested to be potentially
involved in atmospheric ice formation, at least on a regional scale (Murray et al.,
2012).
In recent studies for biological particles (SNOMAX and birch pollen), it has been
demonstrated that freezing is induced by ice nucleating macromolecules and that an
asymptotic value for the mass density of these ice nucleating macromolecules can be
determined (Hartmann et al., 2013; Augustin et al., 2013, Wex et al., 2014). The question
arises whether such an asymptotic value can also be determined for the ice active surface site
density ns, a parameter which is commonly used to describe the ice nucleation activity of
e.g., mineral dust. Such an asymptotic value for ns could be an important input parameter for
atmospheric modeling applications.
In the presented study, we therefore investigated the immersion freezing behavior of droplets
containing size-segregated, monodisperse feldspar particles utilizing the Leipzig
Aerosol Cloud Interaction Simulator (LACIS). For all particle sizes considered in the
experiments, we observed a leveling off of the frozen droplet fraction reaching a
plateau within the heterogeneous freezing temperature regime (T > -38°C) which was
proportional to the particle surface area. Based on these findings, we could determine an
asymptotic value for the ice active surface site density, which we named ns/, for the
investigated feldspar sample. The comparison of these results with those of other studies
elucidates the general feasibility of determining such an asymptotic value and also show
that the value of ns/ strongly depends on the method of the particle surface area
determination.
Acknowledgement
This work is partly funded by the Federal Ministry of Education and Research (BMBF -
project CLOUD 12) and by the German Research Foundation (DFG project WE 4722/1-1,
part of the research unit INUIT, FOR 1525). D. Niedermeier acknowledges financial support
from the Alexander von Humboldt-foundation.
References
Augustin et al.: Immersion freezing of birch pollen washing water, Atmos. Chem. Phys.,
13, 10989–11003, doi:10.5194/acp-13-10989-2013, 2013.
Hartmann et al.: Immersion freezing of ice nucleation active protein complexes, Atmos.
Chem. Phys., 13, 5751–5766, doi:10.5194/acp-13-5751-2013, 2013.
Murray et al.: Ice nucleation by particles immersed in supercooled cloud droplets, Chem.
Soc. Rev., 41, 6519–6554, 2012.
Wex et al.: Intercomparing different devices for the investigation of ice nucleating
particles using Snomax® as test substance, Atmos. Chem. Phys. Discuss., 14, 22321-22384,
doi:10.5194/acpd-14-22321-2014, 2014. |
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