An aqueous emulsion of permethrin containing a water-soluble red dye was applied at a dosage rate of 70 g of the active ingredient in 4.0 L per ha, using a Cessna 188B aircraft (equipped with 4 × AU3000 Micronair® atomizers), over a 400 m × 400 m spray block in Thessalon in northern Ontario, Canada. Spray mass recovery was assessed on glass plates at ground level and droplets were collected on Kromekote® cards. The stain sizes were grouped into different categories. The area containing the stains were excised and permethrin was extracted. The mass of permethrin present in the extract was determined by gas-liquid chromatography (GLC). The average mass per droplet in each size category was calculated. The mass (of permethrin) median diameter (DM.5) and volume median diameter (DV.5) at the spray release height were computed. The present method showed, for the first time in literature, the advantage of not having to use the conventional tedious procedures to determine spread factors to arrive at the DM.5 and DV.5 values.
The DM.5 was 113 μm both at spray release height and at ground level, and was identical to the volume median diameter (DV.5) at spray release height. The present study provided a method to determine the DV.5 at spray release height for that fraction of the spray cloud that deposited on the Kromekote cards at ground level.
The present study also provided information, for the first time, on the percentage decrease in droplet volume and droplet diameter due to evaporation in-flight. When the emitted droplet sizes increased gradually from 33 to 185 μm, evaporation in-flight decreased progressively resulting in droplet sizes of 16 to 181 μm at ground level. The percent decrease in droplet volume was 88.4 for the smallest droplet and 5% for the largest one. Correspondingly, the droplet diameter decreased from 51 to 1.7%, indicating that the small droplets evaporated to a greater extent than the large ones because of their large surface area to mass ratio.