Evaluation and Mitigation of Spray 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 o