Spray Drift and its Mitigation

When forced under pressure through sprayer nozzles liquids emerge as thin elongated sheets with edge instabilities that break up into small aerosols or particles having nearly a thousand fold range in spherical diameters. Owing to gravitational forces and the viscosity of air, the rate of fall to ground can be predicted by Stokes Law and is proportional to the radius of the particles. The rate of fall before a particle hits the ground (or conversely how long a particle remains in air before it falls a given distance) is modified by entrainment in a mobile air mass. Rate of fall of a spray particle will also be influenced by the rate of evaporation of the liquid constituting the aerosol. The longer the aerosol remains in air before falling to ground (or alternatively striking an object above ground) the greater the opportunity to be carried away from its intended target (e.g., crop canopy). In general, all size classes of spray particles are capable of movement off-target, but the smallest particles would move the farthest before depositing on the ground. Drift has been historically considered to be the movement of pesticide residues via air masses during and after application. Post application movement of pesticide residues (i.e., after deposition on plants or soil) via volatilization can be classified as secondary or indirect drift. A comprehensive review of spray drift and its mitigation was published in 2005 (Felsot 2005)¹ and is currently the subject of an IUPAC funded project². A publication based on this project should be available in 2010.

Although drift has a negative connotation because of its usual association with off-target (or out of field) impacts, spray drift within, for example, the canopy during application can increase the potentially bioavailable material on foliage. On the other hand, off-target or out-of-field drift during application may produce a high concentration of residues that potentially have an immediate or acute effect on nontarget organisms.