1School of Earth Atmospheric and Environmental Sciences, University of Manchester, Manchester, M13 9PL, UK
2National Centre for Atmospheric Sciences, University of Manchester, Manchester, M13 9PL, UK
3Laboratory of Atmospheric Chemistry, Paul Scherrer Institut, 5232 Villigen, Switzerland
4ILAQH, Queensland University of Technology, P.O. Box 4233, Brisbane QLD, 4001, Australia
*now at: Laboratory for Energy Systems Analysis, Paul Scherrer Institut, 5232 Villigen, Switzerland
**now at: Department of Physics, Centre Européen de la Recherche Nucléaire, 1211 Geneva, Switzerland
***now at: Experimental Solid State Physics Group, Blackett Laboratory, Imperial College London, SW7 2BW, UK
Abstract. The link between measured sub-saturated hygroscopicity and cloud activation potential of secondary organic aerosol particles produced by the chamber photo-oxidation of α-pinene in the presence or absence of ammonium sulphate seed aerosol was investigated using two models of varying complexity. A simple single hygroscopicity parameter model and a more complex model (incorporating surface effects) were used to assess the detail required to predict the cloud condensation nucleus (CCN) activity from the sub-saturated water uptake. Sub-saturated water uptake measured by three hygroscopicity tandem differential mobility analyser (HTDMA) instruments was used to determine the water activity for use in the models. The predicted CCN activity was compared to the measured CCN activation potential using a continuous flow CCN counter.
Reconciliation using the more complex model formulation with measured cloud activation required widely different assumed surface tension behavior of the growing droplet; this was entirely determined by the instrument used as the source of water activity data. This unreliable derivation of the water activity as a function of solute concentration from sub-saturated hygroscopicity data indicates a limitation in the use of such data in predicting cloud condensation nucleus behavior of particles with a significant organic fraction. Similarly, the ability of the simpler single parameter model to predict cloud activation behaviour was dependent on the instrument used to measure sub-saturated hygroscopicity and the relative humidity used to provide the model input. However, agreement was observed for inorganic salt solution particles, which were measured by all instruments in agreement with theory.
The difference in HTDMA data from proven instruments means that it cannot be stated with certainty the detail required to predict the CCN activity from sub-saturated hygroscopicity. In order to narrow the gap between measurements of hygroscopic growth and CCN activity the processes involved must be understood. It is impossible to say from the results presented here whether: i) Surface tension suppression occurs ii) Bulk to surface partitioning is important iii) The water activity coefficient changes significantly as a function of the solute concentration.