Environmental, resident and bystander exposure from spraying applications with unmanned aerial vehicles equipped with flat-fan nozzles or rotary atomizers

Adoption of agricultural unmanned aerial vehicles (UAVs) for spraying plant protection products has proceeded at a rapid pace. A trend towards usage of UAV sprayers fitted with rotary atomizers has been noted. Regarding the risks associated with UAV-based plant protection treatments, the existing body of literature on environmental and resident/bystander exposure primarily relates, however, to UAVs equipped with hydraulic nozzles. Rotary atomizers and hydraulic nozzles are different in how they produce spray. The influence of the delivery system on the risks posed by UAV sprayers was therefore assessed in several laboratory and field experiments relating to a reference UAV sprayer fitted with hydraulic flat-fan nozzles as well as two UAV sprayers equipped with rotary atomizers. Droplet measurements revealed similar volumes of drift-prone droplets for a given D V0.5 independent of the delivery system. Considering the results of spray pattern experiments and other relevant aspects, swath widths of 3.0–4.5 m were chosen in drift and exposure trials. Drift and resident/bystander exposure generally decreased at increasing distance from the edge of the target area and at increasing height above the ground. Crosswind and droplet size rather than the delivery system were identified as primary drivers of off-target droplet movement.