Method for the assessment of airborne off-target pesticide spray concentrations due to aircraft wing-tip vortex

Abstract

The purpose of this thesis is to develop a method for assessing airborne concentrations caused by off-target pesticide drift. Concentrations are bounded by the worst credible circumstances within a normal aircraft pesticide spraying. It is assumed that spherical spray droplets are transported by a two dimensional parametric representation of a spray-engulfing aircraft wing-tip vortex pair. Two distinctive flow regions were considered for the spray-vortex dynamics. The first occurs in the vicinity of the vortex pair center where pesticide evaporation/ condensation, advection/convection were the predominant mass transfer processes. The second occurs away from the vortex center where anisotropic dispersion, droplet gravitational settling and decaying vortex-induced motion effects are represented by a physically consistent atmospheric transport model. The solution parameters were determined in a heuristic manner by association to physical entities known to influence the spray drift. Particular cases of the equations closely resembled previously published expressions of the specialized literature. For validation purposes, canopy and ground deposit measurements from an aircraft spraying field experiment were relatively compared to predicted values of airborne concentrations. The correlation with field trials pesticide drift measurements improved as the distance from the direct aircraft flight line increased. The proposed method explained from 24% to 69% of the canopy deposit data variability and from 58% to 79% of the trials ground deposit data. However, fictitious values were always obtained near the direct aircraft swath, which were removed by means of a filter function. Even though the computed estimates matched field data with varying success, the originality of the method and its apparent advantages constitute a persuasive appeal for further study.