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In this study vapour pressure osmometry was used to determine water activityin the solutions of organic acids. The surface tension of the solutions wasalso monitored in parallel and then Köhler curves were calculated fornine organic acids (oxalic, malonic, succinic, glutaric, adipic, maleic,malic, citric and cis-pinonic). Surface tension depression is negligible formost of the organic acids in dilute (≤1 w/w%) solutions. Therefore,these compounds affect equilibrium vapour pressure only in the beginningphase of droplet formation when the droplet solution is more concentratedbut not necessarily at the critical size. An exception is cis-pinonic acidwhich remarkably depress surface tension also in dilute (0.1 w/w%)solution and hence at the critical point. The surface tension of organicacid solutions is influenced by the solubility of the compound, the lengthof the carbon chain and also by the polar functional groups present in themolecule. Similarly to surface tension solubility plays an important rolealso in water activity: compounds with higher solubility (e.g. malonic,maleic and glutaric acid) reduce water activity significantly in the earlyphase of droplet formation while less soluble acids (e.g. succinic andadipic acid) are saturated in small droplets and the solution startsdiluting only in bigger droplets. As a consequence, compounds with lowersolubility have a minor effect on water activity in the early phase ofdroplet formation. To deduce the total effect Köhler curves werecalculated and critical supersaturations (Sc) were determined for theorganic acids using measured surface tension and water activity. It wasfound that critical supersaturation grew with growing carbon number. Oxalicacid had the lowest critical supersaturation in the size range studied andit was comparable to the activation of ammonium sulphate. The Sc valuesobtained in this study were compared to data from CCNC experiments. In mostcases good agreement was found. For modelling purposes Sc vs. ddryplots are given and the dependence of water activity and surface tension onconcentration are also formulated

Modelling the cloud condensation nucleus activity of organic acids on the basis of surface tension and osmolality measurements