The Influence of Atmospheric Stability on Wind Drift from Ultra-Low-volume Aerial Forest Spray Applications

Abstract Measurements of drift cloud mass from 11 cases selected from a study of wind-borne droplet drift from ultra low-volume aerial spray applications over northern Ontario forests are presented as a function of atmospheric stability. Six swaths were overlaid onto a flight line in 30 min to obtain ensemble-averaged data from rotary atomizer emissions from an agricultural spray plane flying at about 21 and 26 m above ground level. The estimated volume median diameters of the spray were 100 µm for the 21-m height and 70 µm for the 26-m height. The mass of spray material in the drift cloud was measured at 400, 1200, and 2200 m downwind of the (crosswind) flight line using Rotorods™ suspended from tethered blimps. Wind speed at aircraft height varied from 2 to 5 m s−1 and meteorological conditions varied from moderately stable to moderately unstable. Analysis of the data revealed that 35% more drift occurred in stable than in unstable conditions. The lowest drift was measured when the aircraft was flown in...