The Great Dun Fell Experiment 1995: an overview

During March and April of 1995 a major international field project was cond ucted at the UMIST field station site on Great Dun Fell in Cumbria, Norther n England. The hill cap cloud which frequently envelopes this site was used as a natural flow through reactor to examine the sensitivity of the cloud microphysics to the aerosol entering the cloud and also to investigate the effects of the cloud in changing the aerosol size distribution, chemical co mposition and associated optical properties. To investigate these processes , detailed measurements of the cloud water chemistry (including the: chemis try of sulphur compounds, organic and inorganic oxidised nitrogen and ammon ia), cloud microphysics and properties of the aerosol and trace gas concent rations upwind and downwind of the cap cloud were undertaken. It was found that the cloud droplet number was generally strongly correlated to aerosol number concentration, with up to 2000 activated droplets cm(-3) being obser ved in the most polluted conditions. In such conditions it was inferred tha t hygroscopic organic compounds were important in the activation process. O ften, the size distribution of the aerosol was substantially modified by th e cloud processing, largely due to the aqueous phase oxidation of S(IV) to sulphate by hydrogen peroxide, but also through the uptake and fixing of ga s phase nitric acid as nitrate, increasing the calculated optical scatterin g of the aerosol substantially (by up to 24%). New particle formation was a lso observed in the ultrafine aerosol mode (at about 5 nm) downwind of the cap cloud, particularly in conditions of low total aerosol surface area and in the presence of ammonia and HCl gases. This was seen to occur at night as well as during the day via a mechanism which is not yet understood. The implications of these results for parameterising aerosol growth in Global C limate Models are explored. (C) 1999 Elsevier Science B.V. All rights reser ved.