Observations and modelling of the processing of aerosol by a hill cap cloud

Observations are presented of the aerosol size distribution both upwind and downwind of the Great Dun Fell cap cloud. Simultaneous measurements of the cloud microphysics and cloud chemistry, and of the chemical composition of the aerosol both upwind and downwind of the hill were made along with measurements of sulphur dioxide, hydrogen peroxide and ozone. These observations are used for initialisation of, and for comparison with the predictions of a model of the air flow, cloud microphysics and cloud chemistry of the system. A broad droplet size distribution is often observed near to the hill summit, seemingly produced as a result of a complex supersaturation profile and by mixing between parcels with different ascent trajectories. The model generates several supersaturation peaks as the airstream ascends over the complex terrain, activating increasing numbers of droplets. In conditions where sulphate production in-cloud (due to the oxidation of S(IV) by hydrogen peroxide and ozone) is observed, there is a marked effect on the chemical evolution of the aerosol particles on which the droplets form. When sulphate production occurs, a significant modification of the aerosol size distribution and hygroscopic properties is both predicted and observed. The addition of sulphate mass to those aerosol particles nucleation scavenged by the cloud generally increases the ease with which they are subsequently able to act as cloud condensation nuclei (CCN). Often, this will lead to an increase in the number of CCN available for subsequent cloud formation, although this latter effect is shown to be strongly dependent upon the activation history of the droplets and the concentration of pollutant gases present in the interstitial air. Situations are also identified where cloud processing could lead to a reduction in the capacity of smaller aerosol to act as CCN.