The gas-phase degradation of chlorpyrifos and chlorpyrifos-oxon towards OH radical under atmospheric conditions

[Display omitted] •The gas phase atmospheric degradation of chlorpyrifos was investigated at EUPHORE.•The gas phase atmospheric degradation of chlorpyrifos-oxon was also investigated.•First experimental value for the rate constant of chlorpyrifos-oxon with respect to OH.•The atmospheric lifetime with respect to OH of chlorpyrifos is 2h.•The atmospheric lifetime with respect to OH of chlorpyrifos-oxon is 11h. The OH initiated oxidation of chlorpyrifos (a widely used insecticide) and its photooxidation product chlorpyrifos-oxon were investigated at the large outdoor European Photoreactor (EUPHORE). The rate constants for reaction of chlorpyrifos and chlorpyrifos oxon with OH radicals were measured using a conventional relative rate method. The value of the OH reaction rate constants with chlorpyrifos and chlorpyrifos-oxon were determined to be k=(9.1±2.1)×10−11cm3molecule−1s−1 and (1.7±0.9)×10−11cm3molecule−1s−1 at 303±5K and atmospheric pressure. They gave an atmospheric lifetime in relation to the reaction with OH of approximately 2h and 11h for chlorpyrifos and chlorpyrifos-oxon, respectively. Photolysis was found to be unimportant relative to reaction with OH. The main products detected in the gas phase from the reaction of OH with chlorpyrifos were SO2, chlorpyrifos-oxon, 3,5,6-trichloro-2-pyridinol and diethylphosphate with molar yields of 17±5%, ∼10%, 8±4% and 30±9%, respectively.