Two models of long-range drift of forest pesticide aerial spray

Abstract Long-range drift and dry deposition of an aerial insecticide spray onto a forest are calculated from gradient transfer and Markov chain models to 80 km downwind of the spray line. Horizontal homogeneity and neutral stratification with a capping inversion are assumed. The spray droplets are seven-eighths water which evaporates and one-eighth non-volatile Fenitrothion; the initial drop-size distribution is highly polydispersed with mass mean diameter ~ 82 μm. Results show that at 80 km the models agree within a factor of 2.3 on the fraction of Fenitrothion still airborne (~0.2%), to within a few per cent on the mode of the spectrum of surviving droplets, ~ 22 μm, and substantially on the largest surviving droplet, ~ 40 μm. Results are shown to be sensitive to the assumed source configuration, the initial droplet spectrum and the droplet evaporative behaviour.