Pollution Transfer by Occult Deposition.

Available from UMI in association with The British Library. Requires signed TDF. Occult deposition is the transfer of dissolved and suspended pollutants in fog droplets to the ground by turbulent deposition and sedimentation. The flux density of fog droplets to a moorland site in central England was measured by the flux-profile technique, and was usually less than 15 mg m^{-2} s^{-1}. On average, turbulent deposition accounted for a third of the total flux. The strongest determinants of the droplet flux were properties of the fog rather than of the surface. The annual deposits of acidity, nitrogen (as nitrate) and sulphur were estimated to be 7, 50 and 80 mg m^{-2}, respectively. For sulphur, occult deposition represented about 10% of the wet deposition and only 2% of the total (dry plus wet) deposition at the moorland site. Wind-tunnel studies were done to determine the isokinetic correction for the impactors used to measure liquid water concentration in the field. This correction factor varied with the ratio of inlet velocity to windspeed. Moreover, an increase in the intensity of turbulence from 0.5 to 5% reduced the factor by about 30%. Field measurements of liquid water concentration were corrected using published formulae, although these do not account for this dependence on turbulence. The ionic composition of fog was highly variable, depending in part on synoptic-scale weather. Concentrations of the major ions dissolved in fog droplets were largest in anticyclonic weather when subsidence inversions had formed. The composition of fog in the E. Midlands was investigated by analysing rime samples from different sites. Acidity, sulphate and lead concentrations were significantly larger in rime than in snow collected at the same time. The largest sulphate concentration in rime was 2500 mu eq 1^{-1} and concentrations were similar at urban and rural sites. In contrast, the largest lead concentration was only 7 mu eq 1^{-1} and found in urban/roadside samples of rime. There was areal variation in the composition of fog in the Pennines. The concentrations of all ions measured decreased immediately after rain and were largest near cloud base.