Atmospheric Chemistry and Physics, vol. 7, issue 17 (2007) pp. 4601-4611
In this study vapour pressure osmometry was used to determine water activity in the solutions of organic acids. The surface tension of the solutions was also moni- tored in parallel and then K ohler curves were calculated for nine organic acids (oxalic, malonic, succinic, glutaric, adipic, maleic, malic, citric and cis-pinonic). Surface tension de- pression is negligible for most of the organic acids in di- lute (1w/w%) solutions. Therefore, these compounds af- fect equilibrium vapour pressure only in the beginning phase of droplet formation when the droplet solution is more con- centrated but not necessarily at the critical size. An exception is cis-pinonic acid which remarkably depress surface tension also in dilute (0.1w/w%) solution and hence at the critical point. The surface tension of organic acid solutions is influ- enced by the solubility of the compound, the length of the carbon chain and also by the polar functional groups present in the molecule. Similarly to surface tension solubility plays an important role also in water activity: compounds with higher solubility (e.g. malonic, maleic and glutaric acid) re- duce water activity significantly in the early phase of droplet formation while less soluble acids (e.g. succinic and adipic acid) are saturated in small droplets and the solution starts diluting only in bigger droplets. As a consequence, com- pounds with lower solubility have a minor effect on water activity in the early phase of droplet formation. To deduce the total effect K that critical supersaturation grew with growing carbon num- ber. Oxalic acid had the lowest critical supersaturation in the size range studied and it was comparable to the activation of ammonium sulphate. The Sc values obtained in this study were compared to data from CCNC experiments. In most cases good agreement was found. For modelling purposes Sc vs. ddry plots are given and the dependence of water activity and surface tension on concentration are also formulated.
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