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Airborne pesticide residues were collected both within and downwind from a parathion-treated plum orchard by high volume sampling through XAD-4 macroreticular resin. Levels of paraoxon in excess of 100 ng/m3 were found in orchard air, along with parathion, during the early days of two 21-day sampling studies. Paraoxon:parathion ratios in the orchard air were relatively constant, averaging ca. 0.5 for days 1 to 21 following treatment. Likely sources of airborne paraoxon include vaporization and dislodgement from soil and leaf surfaces, and chemical conversion of parathion in the air. Support for the latter came from observation of an increased paraoxon:parathion ration in air samples collected downwind from the orchard. Atmospheric conversion of parathion to paraoxon, accelerated by sunlight, was indicated by both field and laboratory studies. Overall dissipation of parathion from the orchard air, soil, and leaf tissue proceeded to a considerable extent through breakdown to paraozon under the dry climatic conditions of these studies. Eventual conversion to the relatively stable breakdown product, p-nitrophenol, was indicated from analysis of air in the orchard vicinity.

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