Key environmental processes affecting the fate of the insecticide chloropyrifos applied to leaves

Chlorpyrifos (CP) is still a commonly employed organophosphorus insecticide worldwide. In semi-arid and Mediterranean climates, applied CP is expected to remain on leaves surfaces for relatively long time due to the lack of summer rains and common use of drip irrigation. The present work examines the loss rate of CP from leaves via different surface processes: evaporation, direct photolysis and reactions with ozone and [rad]OH radicals. Laboratory experiments showed that evaporation rate constant of CP increased from 0.109 to 0.492 h⁻¹with the increase in wind speed up to 4 m/s. First-order rate constant of direct photolysis, measured using a solar simulator, was k’_UV = 1.15 (±0.01) x 10⁻²⁰ cm²photon⁻¹. Second-order rate constants for the reaction of CP with ozone and [rad]OH were measured as 6 × 10⁻²⁰and 6 × 10⁻¹² cm³ molecule⁻¹ s⁻¹, respectively. The above rate constants were applied successfully in an outdoor experiment to predict the disappearance of chloropyrifos under specific environmental conditions. Further modeling showed that the insecticide half-life time on exposed surfaces under typical Mediterranean environment will be in the range of 0.9–6.9 h. Evaporation is expected to be the dominant removal path under most environmental conditions, followed by direct photolysis and reaction with [rad]OH.