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In order to predict the physical properties of aerosol particles, it isnecessary to adequately capture the behaviour of the ubiquitous complexorganic components. One of the key properties which may affect thisbehaviour is the contribution of the organic components to the surfacetension of aqueous particles in the moist atmosphere. Whilst the qualitativeeffect of organic compounds on solution surface tensions has been widelyreported, our quantitative understanding on mixed organic and mixedinorganic/organic systems is limited. Furthermore, it is unclear whethermodels that exist in the literature can reproduce the surface tensionvariability for binary and higher order multi-component organic and mixedinorganic/organic systems of atmospheric significance. The current studyaims to resolve both issues to some extent. Surface tensions of single andmultiple solute aqueous solutions were measured and compared withpredictions from a number of model treatments. On comparison with binaryorganic systems, two predictive models found in the literature provided arange of values resulting from sensitivity to calculations of pure componentsurface tensions. Results indicate that a fitted model can capture thevariability of the measured data very well, producing the lowest averagepercentage deviation for all compounds studied. The performance of theother models varies with compound and choice of model parameters. Thebehaviour of ternary mixed inorganic/organic systems was unreliably capturedby using a predictive scheme and this was dependent on the composition ofthe solutes present. For more atmospherically representative higher ordersystems, entirely predictive schemes performed poorly. It was found that useof the binary data in a relatively simple mixing rule, or modification of anexisting thermodynamic model with parameters derived from binary data, wasable to accurately capture the surface tension variation with concentration.Thus, it would appear that in order to model multi-component surfacetensions involving compounds used in this study one requires the use ofappropriate binary data. However, results indicate that the use oftheoretical frameworks which contain parameters derived from binary data maypredict unphysical behaviour when taken beyond the concentration ranges usedto fit such parameters. The effect of deviations between predicted andmeasured surface tensions on predicted critical saturation ratios wasquantified, by incorporating the surface tension models into an existingthermodynamic framework whilst firstly neglecting bulk to surfacepartitioning. Critical saturation ratios as a function of dry size for allof the multi-component systems were computed and it was found thatdeviations between predictions increased with decreasing particle dry size.As expected, use of the surface tension of pure water, rather than calculatethe influence of the solutes explicitly, led to a consistently higher valueof the critical saturation ratio indicating that neglect of thecompositional effects will lead to significant differences in predictedactivation behaviour even at large particle dry sizes. Following this twocase studies were used to study the possible effect of bulk to surfacepartitioning on critical saturation ratios. By employing various assumptionsit was possible to perform calculations not only for a binary system butalso for a mixed organic system. In both cases this effect lead to asignificant increase in the predicted critical supersaturation ratiocompared to the above treatment. Further analysis of this effect will formthe focus of future work

atmospheric chemistry and physics

Surface tensions of multi-component mixed inorganic/organic aqueous systems of atmospheric significance: measurements, model predictions and importance for cloud activation predictions