Loss of particulate contaminants from plant canopies under wet and dry conditions

There is a requirement for data describing the loss with time of particulate contamination from plant canopies. Measurements were made of the loss rates of monodispersed silica spheres (three sizes, with Mass Median Aerodynamic Diameters (MMADs) 1.9, 5.3 and 8.4 microm) from wheat (Triticum aestivum) and broad bean (Vicia faba) canopies. The spheres were labelled with tracers detectable by Instrumental Neutron Activation Analysis (INAA). Canopies were contaminated under realistic turbulence conditions in a wind tunnel, then removed to sheltered and exposed field sites or to a glasshouse containing a rain simulator. Samples were taken periodically, and the level of contamination per plant determined 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 order of 1-2 days were obtained for an exposed wheat crop and 3-4 days for a partially sheltered wheat crop, with permanent residues of initial deposit for the exposed crop of 4-8%, and for the partially sheltered crop of 22-52%. A broad bean crop under glasshouse conditions showed loss half-lives of 0.5-1.5 days with residues of 22-26% initial contamination. A double exponential loss model also fitted the data well in some cases, and it is possible that a slow loss of the residual deposit occurs, being masked by noise in the current data set.