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| Titel |
The role of surfactants in Köhler theory reconsidered |
| VerfasserIn |
R. Sorjamaa, B. Svenningsson, T. Raatikainen, S. Henning, M. Bilde, A. Laaksonen |
| 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 ; 4, no. 8 ; Nr. 4, no. 8 (2004-10-26), S.2107-2117 |
| Datensatznummer |
250002064
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| Publikation (Nr.) |
 copernicus.org/acp-4-2107-2004.pdf |
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| Zusammenfassung |
| Atmospheric aerosol particles typically consist of inorganic salts
and organic material. The inorganic compounds as well as their
hygroscopic properties are well defined, but the effect of organic
compounds on cloud droplet activation is still poorly
characterized. The focus of the present study is the organic
compounds that are surface active i.e. tend to concentrate on
droplet surface and decrease the surface tension. Gibbsian
surface thermodynamics was used to find out how partitioning
between droplet surface and the bulk of the droplet affects the surface tension
and the surfactant bulk concentration in droplets large enough to act as cloud
condensation nuclei. Sodium dodecyl sulfate (SDS) was used
together with sodium chloride to investigate the effect
of surfactant partitioning on the Raoult effect (solute effect).
While accounting for the surface to bulk partitioning is known to lead to
lowered bulk surfactant concentration and thereby to increased surface tension
compared to a case in which the partitioning is neglected, the present
results show that the partitioning also alters the Raoult effect,
and that the change is large enough to further increase the critical
supersaturation and hence decrease cloud droplet activation. The fraction
of surfactant partitioned to droplet surface increases with decreasing droplet
size, which suggests that surfactants might enhance the activation
of larger particles relatively more thus leading to less dense
clouds. Cis-pinonic acid-ammonium sulfate aqueous solutions were
studied in order to study the partitioning with compounds found in the
atmosphere
and to find out the combined effects of
dissolution and partitioning behavior. The results show that
the partitioning consideration presented in this paper alters the shape of the
Köhler curve when compared to calculations in which the
partitioning is neglected either completely or in the Raoult
effect. In addition, critical supersaturation was measured for SDS
particles with dry radii of 25-60nm using a static parallel
plate Cloud Condensation Nucleus Counter. The experimentally
determined critical supersaturations agree very well with
theoretical calculations taking the surface to bulk partitioning
fully into account and are much higher than those calculated neglecting
the partitioning. |
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