Photodegradation and volatilization of parathion-methyl on glass and soil dust under laboratory conditions

A new device ("photovolatility chamber") was designed to simultaneously exa mine the direct and indirect photolysis of C-14-labelled pesticides on surf aces and their volatilization behaviour, excluding photolysis in air. Both continuous air sampling, which quantities volatile organic compounds and (C O2)-C-14 separately, and the detection of surface-located residues guarante e a complete radioactivity and mass balance. In model experiments using a B orofloat glass surface and soil dust, the volatilization and photodegradati on of [phenyl-UL-C-14]parathion-methyl were measured under constant climati c conditions lair temperature and humidity) determining the influence of si mulated sunlight and O-3. Depending on the experimental conditions, parathion-methyl was converted to paraoxon-methyl 4-nitrophenol, unknown polar products and (CO2)-C-14. With respect to the direct photolysis of parathion-methyl (experiments without O-3), polar compounds and (CO2)-C-14 were the major metabolites, due to the rapid photochemical mineralization of 4-nitrophenol to (CO2)-C-14, which w as confirmed in separate experiments with [C-14]4-nitrophenol. Paraoxon-met hyl and 4-nitrophenol formation was mainly mediated by the combination of O -3 and light, suggesting an indirect photochemical reaction with OH radical s, since neither light alone nor O-3 alone showed comparable effects. Due to photochemical product formation, which is an antagonistic process, t he volatilization of unaltered parathion-methyl from both surfaces generall y decreased in the presence of light, particularly in combination with incr easing O-3 concentrations.