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

Atmospheric Chemistry and Physics (Impact Factor: 5.05). 11/2004; 4(6). DOI: 10.5194/acpd-4-7667-2004
Source: DOAJ


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.

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    • "To estimate the overall effect of these reactions on the formation of cloud droplets, the Köhler curves were determined for solutions 1/1 of methacrolein in ammonium sulfate, before irradiation and after 2 h of irradiation. The surface tension and osmolality of these two types of solutions and their subsequent dilutions were measured, and their Köhler curves were determined by a method described recently [Kiss and Hansson, 2004; Varga et al., 2007; Ekström et al., 2009] (see also Text S1). The results show that, for a particle of a dry diameter of 80 nm, for instance, the critical supersaturation, Sc, would decrease by about 13% after exposure to light. "
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    • "The Köhler curves were thus built point by point for the different organic compounds and salt solutions . As discussed previously [Kiss and Hansson, 2004], this method is experimentally simple, accurate and has the advantage of eliminating the uncertainties contained in the simplified Köhler equation, in particular in the Van't Hoff factors. Finally, this method can be applied to particles of almost any size, the range accessible to measurements being only limited by the solubility of the compounds in the solutions of interest. "
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