LOSS OF PARTICULATE CONTAMINANTS FROM PLANT CANOPIES UNDER WET AND DRY CONDITIONS

There is a requirement for data describing the loss with time of parti culate contamination from plant canopies. Measurements were mane of th e loss rates of monodispersed silica spheres (three sizes, with Mass M edian Aerodynamic Diameters (MMADs) 1.9, 5.3 and 8.4 mu m) from wheat (Triticum aestivum) and broad bean (Vicia faba) canopies. The spheres were labelled with tracers detectable by Instrumental Neutron Activati on Analysis (INAA). Canopies were contaminated under realistic turbule nce conditions in a wind tunnel, then removed to sheltered and exposed field sites or to a glasshouse containing a rain simulator. Samples w ere taken periodically, and the level of contamination per plant deter mined by INAA. Statistical analysis of the resulting data suggested an offset exponential loss model, with a residue of deposit that is not lost over time. Loss half-lives in the older of 1-2 days were obtained for an exposed wheat crop and 3-4 days for a partially shelter ed whe at crop, with permanent residues of initial deposit for the exposed cr op of 4-8%, and for the partially sheltered crop of 22-52%. A broad be an crop under glasshouse conditions showed loss half-lives of 0.5-1.5 days with residues of 22-26% initial contamination. A double exponenti al loss model also fitted the data well in some cases, and it is possi ble that a slow loss of the residual deposit occurs, being masked by n oise in the current data set.