Consistency between parameterisations of aerosol hygroscopicity and CCN activity during the RHaMBLe discovery cruise 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
*now at: Laboratoire de Météorologie Physique, Blaise Pascal University, Clermont Ferrand, France
Received: 16 September 2009 – Published in Atmos. Chem. Phys. Discuss.: 26 October 2009 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
Revised: 15 March 2010 – Accepted: 23 March 2010 – Published: 01 April 2010
It was found that the magnitude and temporal variability of the sub-saturated
water uptake could be well represented using the non-refractory composition
to derive the model input for 2 periods when the large majority of the
aerosol mass was non-refractory. As may be expected, when a significant
fraction of the aerosol volume 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.
Measurements made at relative humidities (RHs) up to 94% showed that the
water activity appears satisfactorily represented by a single parameter
derived at 90% RH. It therefore appears that the change in the observed
hygroscopicity take place between 94% RH and the point of activation. This
change may be due in part to a change solution non-ideality, surface tension
effects or the presence of sparingly soluble compounds for example, but
cannot be reconciled without measurements at higher RHs.
Citation: Good, N., Topping, D. O., Allan, J. D., Flynn, M., Fuentes, E., Irwin, M., Williams, P. I., Coe, H., and McFiggans, G.: Consistency between parameterisations of aerosol hygroscopicity and CCN activity during the RHaMBLe discovery cruise, Atmos. Chem. Phys., 10, 3189-3203, doi:10.5194/acp-10-3189-2010, 2010.