| Popis: |
Spray transects through an 11-m-tall pecan orchard with an air-blast orchard sprayer were replicated 10 times over a wide range of atmospheric stability conditions. Drift was measured by collecting tracer (malathion) using ground plates (f), high volume air samplers (hv), and string (s) collectors distributed downwind in the adjacent field out to a maximum distance of 18 canopy heights (198 m). Atmospheric conditions were measured above the orchard canopy with fast-response, three-dimensional wind, temperature, and humidity sensors. Atmospheric stability, characterized by the surface layer stability parameter ( ae), was the primary factor affecting drift amounts measured on and above the adjacent field. On average, the total amount of drift caught by the samplers in very stable conditions was 5.9, 3.6, and 2.1 times the amount of drift in unstable conditions for the plates, the hvs, and the strings, respectively. The transition from one condition to the other took place over a very small stability range where the air was dynamically stable (0.0 < ae< 0.3). In this range, very small changes in atmospheric conditions resulted in very large changes in the atmosphere’s aerosol transport characteristics. The effect of stability on the amount of drift collected in the adjacent field is described by a symmetric hyperbolic tangent function over the entire range of stability encountered. |
