1Centre of Atmospheric Science, School of Earth Atmospheric and Environmental Sciences, University of Manchester, Manchester, UK
2National Centre for Atmospheric Sciences, University of Manchester, Manchester, UK
Abstract. Results from a measurement study performed in the Tropical Atlantic on board the RHaMBLe Discovery Cruise D319 are presented. Measurements of aerosol composition, hygroscopicity and CCN activity were used to test the ability of a single parameter model to describe water uptake in sub- and supersaturated conditions.
It was found that the magnitude and variability of the sub-saturated water uptake could be well represented using the non-refractory composition to derive the model input when most of the aerosol mass is non-refractory. As may be expected, when a significant fraction of the aerosol mass is refractory the sub-saturated water uptake is not well predicted by the non-refractory composition. When predicting the cloud activation potential from the composition and the hygroscopicity there is a consistent under-prediction of the CCN activity. The prediction of CCN activity from the sub-saturated water uptake gives a better prediction of the CCN activity than the composition when the non-refractory components are not fully representative of the aerosol composition.
Based on these observations it appears that a single parameter cannot always capture the behavior fully across the sub- and supersaturated regimes when the surface tension is assumed to be that of water. The magnitude of the discrepancy varies according to the air mass sampled and the reasons for the difference may vary too. It is postulated that this is a result of a lack of characterisation of the behaviour of components at the particle surface or that the water activity dependence on the solution concentration is not sufficiently constrained by the single parameter approach. The water activity appears satisfactorily represented by a single parameter derived at 90% relative humidity (RH) for RHs less than 94%. To reconcile the CCN activity with the aerosol composition, surface tensions in the range of 52 to 60 mN/m are required at the point of activation.