A modelling study on the activation of small Aitken‐mode aerosol particles during CIME 97

During February 1997, one of the 2 observational periods of CIME ( c loud i ce m ountain e xperiment), a joint field experiment funded by the European Commission, took place on the summit of the Puy de Dōme in the centre of France. During this experiment the droplet spectra were measured with an FSSP and the aerosol particles in the drops and in the interstitial particle phase were measured with a counterflow virtual impactor and a round jet impactor inside a windtunnel. Very low aerosol particle and drop concentrations were observed and particles as small as 25 nm in diameter were found to activate. Two datasets obtained on 15 February and 17 February were used to study the activation of the small Aitken‐mode particles and the spectral form of the droplet spectrum and the scavenging fraction. Numerous sensitivity studies were performed investigating the rôle of the number density and chemical composition of the aerosol particles. The rôle of mixing inside the orographic cloud was studied by using a new technique. It considers the fact that the air arriving on the summit of the Puy de Dôme is a mixture of air of different origins. Thus, it weighs the results of a spectral scavenging model (DESCAM or EXMIX) calculated along a number of individual trajectories. The weighing function is derived from tracer and trajectory studies with a 3‐dimensional mesoscale model. The model was able to reproduce the activation of aerosol particles as small as 25 nm. It was caused by the low aerosol particle number concentrations. In general, we can conclude that the variability found in the sensitivity tests of the dynamical and chemical factors allows to reproduce the shape of the observed results. As too many free parameters exit at the moment we cannot quantify the contribution of each factor studied to the observed scavenging fraction, however, it seems that dynamics dominates.