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

Atmospheric Chemistry and Physics (Impact Factor: 4.88). 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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    ABSTRACT: Many atmospheric aerosols contain both organic compounds and inorganic material, such as sulfate salts. In this work, we show that these sulfates could trigger some chemical transformations of the organic compounds by producing sulfate radicals, SO4−, when exposed to UV light (280–320 nm). In particular, we show by mass spectrometry (LC/ESI-MSMS) that isoprene, methyl vinyl ketone, methacrolein, and α-pinene in irradiated sulfate solutions (ammonium and sodium sulfate) produce the same organosulfates as previously identified in aerosols, and even some that had remained unidentified until now. With a typical time constant of 9 h instead of 4600 days for esterifications, these radical reactions would be a plausible origin for the atmospheric organosulfates. These reactions also produced efficient surfactants, possibly resembling the long-chain organosulfates found in the experiments. Thus, photochemistry in mixed sulfate/organic aerosols could increase cloud condensation nuclei (CCN) numbers, which would be supported by previous atmospheric observations.
    Geophysical Research Letters 03/2010; 37(5). DOI:10.1029/2009GL041683 · 4.20 Impact Factor
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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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    ABSTRACT: A significant fraction of the organic material in aerosols is made of highly soluble compounds such as sugars (mono- and polysaccharides) and polyols such as the 2-methyltetrols, methylerythritol and methyltreitol. Because of their high solubility these compounds are considered as potentially efficient CCN material. For the 2-methyltetrols, this would have important implications for cloud formation at global scale because they are thought to be produced by the atmospheric oxidation of isoprene. To investigate this question, the complete Köhler curves for C3-C6 polyols and the 2-methyltetrols have been determined experimentally from osmolality and surface tension measurements. Contrary to what was expected, none of these compounds displayed a higher CCN efficiency than organic acids. Their Raoult terms show that this limited CCN efficiency is due to their absence of dissociation in water, this in spite of slight surface-tension effects for the 2-methyltetrols. Thus, compounds such as saccharides and polyols would not contribute more to cloud formation than other organic compounds studied so far. In particular, the presence of 2-methyltetrols in aerosols would not particularly enhance cloud formation in the atmosphere, in contrary to recently suggested.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 02/2009; 9(3). DOI:10.5194/acp-9-973-2009 · 5.05 Impact Factor
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    ABSTRACT: Critical supersaturations for internally mixed particles of adipic acid, succinic acid and sodium chloride were determined experimentally for dry particles sizes in the range 40-130nm. Surface tensions of aqueous solutions of the dicarboxylic acids and sodium chloride corresponding to concentrations at activation were measured and parameterized as a function of carbon content. The activation of solid particles as well as solution droplets were studied and particle phase was found to be important for the critical supersaturation. Experimental data were modelled using Köhler theory modified to account for limited solubility and surface tension lowering.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 02/2005; 4(6). DOI:10.5194/acp-5-575-2005 · 5.05 Impact Factor
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