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The link between measured sub-saturated hygroscopicity and cloud activationpotential of secondary organic aerosol particles produced by the chamberphoto-oxidation of α-pinene in the presence or absence of ammoniumsulphate seed aerosol was investigated using two models of varyingcomplexity. A simple single hygroscopicity parameter model and a more complexmodel (incorporating surface effects) were used to assess the detail requiredto predict the cloud condensation nucleus (CCN) activity from thesub-saturated water uptake. Sub-saturated water uptake measured by threehygroscopicity tandem differential mobility analyser (HTDMA) instruments wasused to determine the water activity for use in the models. The predicted CCNactivity was compared to the measured CCN activation potential using acontinuous flow CCN counter.  Reconciliation using the more complex model formulation with measured cloudactivation could be achieved widely different assumed surface tensionbehavior of the growing droplet; this was entirely determined by theinstrument used as the source of water activity data. This unreliablederivation of the water activity as a function of solute concentration fromsub-saturated hygroscopicity data indicates a limitation in the use of suchdata in predicting cloud condensation nucleus behavior of particles with asignificant organic fraction. Similarly, the ability of the simpler singleparameter model to predict cloud activation behaviour was dependent on theinstrument used to measure sub-saturated hygroscopicity and the relativehumidity used to provide the model input. However, agreement was observed forinorganic salt solution particles, which were measured by all instruments inagreement with theory.  The difference in HTDMA data from validated and extensively used instrumentsmeans that it cannot be stated with certainty the detail required to predictthe CCN activity from sub-saturated hygroscopicity. In order to narrow thegap between measurements of hygroscopic growth and CCN activity the processesinvolved must be understood and the instrumentation extensively qualityassured. It is impossible to say from the results presented here due to thedifferences in HTDMA data whether: i) Surface tension suppression occursii) Bulk to surface partitioning is important iii) The water activitycoefficient changes significantly as a function of the solute concentration

Widening the gap between measurement and modelling of secondary organic aerosol properties?