Evaluation and Mitigation of Spray Drift

Definition of Drift When liquids are forced under pressure through small orifices (like sprayer nozzles) they are sheared 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 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 has been distinguished as secondary or indirect drift. Whereas drift is specifically the movement of aerosolized chemical during the application period, volatilization post application can occur over prolonged periods and constitutes mass transfer in the gaseous phase. Although drift has a negative connotation because of its usual association with off-target (or out of field) impacts, sprays drift within the canopy itself during an application swath and serve to increase the potentially bioavailable residues on foliage. On the other hand, off-target or out-of-field drift during application will produce a high concentration of residues that potentially has an immediate or acute effect on nontarget receptors. Volatilization over a prolonged period generates a more dilute concentration of residues but a significantly lower probability of an adverse effect. However, both drift and post-application volatilization movement of residues can produce inadvertent contamination of crops for which the pesticide is not registered. Because the physical principles controlling generation of spray drift and secondary drift are different, distinguishing the two modes of off-target movement as related but distinct phenomena may be logical because measures to mitigate either will necessarily be different. This paper will provide a historical overview of primary or direct drift and review current status of activities for its evaluation and mitigation. Regulatory assessment and mitigation of agricultural spray drift is currently the subject of an IUPAC technical review project organized by the Advisory Committee for Crop Protection Chemistry.