Assessment of Fine Droplets (<10 Μm) in Primary Airborne Spray Drift: A New Methodological Approach

Passive samplers are especially inefficient when collecting and measuring the finest particles in primary airborne spray drift far from the spray source. We describe a method to improve the measurement of <10 mu m-sized droplets that become airborne during crop spray applications. An airblast sprayer was combined with Conventional (VMD 90 mu m) and Low Drift (VMD 276 mu m) nozzles. Downwind droplet concentrations were measured at six sampling distances (5, 10, 15, 20, 25, and 30 m) from the sprayer pathway using an Optical Particle Counter (OPC). The instrument simultaneously measures particle mass (mu g m(-3)) and particle number (n). Three particle size ranges were considered (<1.0, 1.0-2.5, and 2.5-10.0 mu m). Weather conditions were monitored to ensure optimal spray drift generation as dictated by the ISO22866:2005 which allowed comparison of the two nozzle technologies. Significant downwind particle mass and number increases were observed using proposed methodology compared to the background level with either Conventional or Low Drift nozzles. As expected, more massive fine droplets (+55%) resulted with Conventional nozzles. Interestingly, the Low Drift nozzles highlighted their potential to generate more fine particles (<1.0 mu m) at 25 and 30 m from the sprayer. This study verified the potential to use OPCs to assess drift and understand particle size variation and dynamics during drift events.