Reactivity of certain organophosphorus insecticides toward hydroxyl radicals at elevated air temperatures

Methods were developed to determine OH reaction rates for medium-weight organophosphorus pesticides in the gas phase. A 57-L chamber was constructed that utilized xenon arc irradiation (>290 nm) to photolyze the OH precursor, methyl nitrite. Experiments were performed at elevated temperatures ranging from 60 to 80 degrees C to minimize wall sorption. Solid-phase microextraction (SPME) sampling of the gas phase was employed to assess the simultaneous rate of loss of the pesticides in relation to the rate of loss of two reference substances with known OH rate constants. An internal gas-phase standard (hexachlorobenzene), relatively stable to hydroxyl radicals, was used to assess other processes, which included dilution and wall sorption. The relative reaction rates of the organophosphorus insecticides, diazinon and chlorpyrifos, when compared to reference compounds, were unaffected by various air temperatures between 60 and 80 degrees C. Although both insecticides were expected to react at similar rates on the basis of structural activity model predictions, diazinon reacted 3 times more rapidly than chlorpyrifos and gave estimated environmental OH half-lives of 0.5 and 1.4 h, respectively. The degree of sorption onto the chamber walls was minimal and similar for each of the compounds examined. Experimental gas-phase determinations at elevated temperatures may provide important information that can be used when in the assessment of the potential of airborne pesticide risks to nontarget and ecologically sensitive areas.