A generalised scaling correlation for airborne drift potential of agricultural nozzles

Agrochemical spraying presents a major environmental challenge, as off-target drift can contaminate nearby soil and water bodies, posing risks to ecosystems, human health, and agricultural sustainability. This study introduces a generalised, physics-based scaling metric for classifying and evaluating the airborne drift risk of agricultural hydraulic nozzles under controlled crosswind conditions. Wind tunnel experiments were conducted using four commercially representative nozzles: Air Induction (AI), Drift Guard (DG), Extended Range (XR), and hollow cone (TX), spanning the ASABE droplet classification spectrum from Very Fine to Very Coarse. Tests were performed across three wind velocities (1, 3, and 5 m s⁻¹) and three nozzle release heights (0.8, 1.0, and 1.2 m) at a constant injection pressure of 300 kPa. Droplet size distributions were measured using Malvern Spraytec laser diffraction, and airborne drift fractions were quantified by volumetric mass balance. Lagrangian droplet trajectory simulations and independent Agricultural Dispersal (AGDISP) numerical modelling provided complementary validation datasets. Three-way ANOVA and Sobol sensitivity analysis identified nozzle type, wind speed, and release height as significant ADP drivers, with D₄₃ as the dominant parameter. The proposed generalised scaling law collapses all four nozzle types onto a single self-similar curve (R² = 0.92), independently validated against AGDISP simulations (R² = 0.98). These results demonstrate that the ADP metric provides a promising, standardised tool for regulatory nozzle classification and drift risk assessment at the nozzle-selection stage, without requiring field trials or a reference nozzle, though generalisation beyond the tested conditions warrants further investigation.