Leaf-air partition coefficient (Kleaf-air) values are needed to understand and predict pesticide volatilization and persistence in agroecosystems. The objectives of this work were to measure Kleaf-air values and foliar penetration for the insecticide chlorpyrifos (as an active ingredient alone and in a pesticide formulation) on alfalfa (lucerne) leaves at a range of temperatures and relative humidities and when using leaves collected in different summer months. Kleaf-air values were measured using a solid-phase fugacity meter. A portion of the leaves were also used for foliar penetration experiments. Kleaf-air values for chlorpyrifos as an active ingredient alone decreased with temperature, while the effects of temperature on chlorpyrifos in the formulation were negligible. No correlations between Kleaf-air values and relative humidity were observed. Foliar penetration increased with temperature for chlorpyrifos both as an active ingredient and in the formulation. Increasing foliar penetration with temperature is attributed to increasing diffusion into inner leaf layers. Both volatilization and foliar penetration affect the measured Kleaf-air values, so understanding the link between these processes is necessary to predict Kleaf-air values. The leaf collection date had a substantial effect on the measured Kleaf-air values, highlighting the need for a better understanding of the role of leaf properties on Kleaf-air.
Keywords: alfalfa; fugacity meter; organophosphate insecticides; pesticide adjuvants; pesticide fate; semivolatile pesticides.