Gas-phase photolysis of phorate, a phosphorothioate insecticide

Novel methods are described for determining atmospheric photolysis rates for the moderately volatile pesticide, phorate. The gas-phase sunlight photolysis of this substance was determined in three test systems which include: (1) 3-L borosilicate flasks exposed to sunlight and laboratory solar simulation, (2) 100-L Tedlar sunlight exposed air sample bags, and (3) 12,800-L Tedlar sunlight exposed chamber studies using a photochemically stable tracer compound. The kinetic results from all systems indicate rapid gas-phase photolysis under midsummer sunlight conditions with observed half-lives being less than 30 minutes. A pronounced increase in transformation rates with increasing volume to surface area suggests photoreactions occur at more rapid rates in the gas-phase. 14C Radiolabeled phorate irradiations resulted in one conversion product, phorate sulfoxide, and this photoproduct accounted for essentially all of the phorate lost.