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Application of Osmolality for the Determination of Water Activity and the Modelling of Cloud Formation : Volume 4, Issue 6 (24/11/2004)

By Kiss, G.

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Book Id: WPLBN0003997662
Format Type: PDF Article :
File Size: Pages 23
Reproduction Date: 2015

Title: Application of Osmolality for the Determination of Water Activity and the Modelling of Cloud Formation : Volume 4, Issue 6 (24/11/2004)  
Author: Kiss, G.
Volume: Vol. 4, Issue 6
Language: English
Subject: Science, Atmospheric, Chemistry
Collections: Periodicals: Journal and Magazine Collection (Contemporary), Copernicus GmbH
Publication Date:
Publisher: Copernicus Gmbh, Göttingen, Germany
Member Page: Copernicus Publications


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Hansson, H., & Kiss, G. (2004). Application of Osmolality for the Determination of Water Activity and the Modelling of Cloud Formation : Volume 4, Issue 6 (24/11/2004). Retrieved from

Description: Air Chemistry Group of the Hungarian Academy of Sciences, University of Veszprém, P.O. Box 158, Veszprém, 8201, Hungary. A simple approach is suggested here to give reliable estimates on the Raoult term of the Köhler equation when calculating critical supersaturation (Sc) for real atmospheric samples. Water activity is calculated from osmolality and thus the original Köhler equation can be applied avoiding the difficulties with unknown molecular weights, solubilities, van't Hoff factors of aerosol constituents and also the interactions in the growing droplet. First, water activity calculated from osmolality data was compared to literature values both for electrolytes and a non-electrolyte. Then the applicability of the approach was demonstrated by generating Köhler curves from osmolality derived and literature activity data as well as by using the simplified Köhler equation. Sc values calculated with the osmolality approach fitted those obtained by using literature water activity data within a relative deviation of less than 0.3%, 0.8%, 1.1% and 3.4% for sucrose, CaCl2, NaCl and H2SO4, respectively, while the corresponding errors with the simplified Köhler equation were 11%, 8.5%, 4.5% and 19% in the dry nucleus size range of 20 nm to 100 nm. Finally, the osmolality method was used to show how considerably Sc is underestimated for organic acids if complete dissociation is assumed.

The method described in this paper can be applied to real atmospheric samples (aerosol extracts, fog water or cloud water) thus improving the reliability of estimates on critical supersaturation and critical droplet diameter in atmospheric modelling.

Application of osmolality for the determination of water activity and the modelling of cloud formation


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