OROGRAPHIC ENHANCEMENT OF WET DEPOSITION IN THE UNITED-KINGDOM - CONTINUOUS MONITORING

Continuous monitoring of cloud and rain samples at three mountain site s in the UR has allowed consideration of the long term impact of the e nhancement of the wet deposition of pollutants by orographic effects, specifically the scavenging of cap cloud droplets by rain falling from above (the seeder-feeder effects). The concentration of the major pol lutant ions in the cloud water is related to the relative proximity of each site to marine and anthropogenic sources of aerosol. In general, the concentrations of major ions in precipitation at summit sites exc eed those in precipitation to low ground nearby by 20% to 50%. Concent rations in orographic cloud exceed those in upwind rain by between a f actor of five and ten. The results are consistent with seeder-feeder s cavenging of hill cloud by falling precipitation in which the average concentration of ions in scavenged hill cloud exceed those in precipit ation upwind by a factor of 1.7 to 2.3 for sulphate and nitrate respec tively at Dunslair Heights and 1.5 to 1.8 for sulphate and nitrate at Holme Moss. The results suggest that the parameterisation of this rela tionship with scavenged feeder cloud water concentrations assumed to e xceed those in seeder rain by a factor of two for the production of pr edictive maps of wet deposition in mountainous regions of the U.K. is satisfactory.