- An experimental protocol to determine the effects of temperature and light on the dissipation of xenobiotic substances in pesticide deposits is presented. A factorial design laboratory experiment is used to develop isothermal models of the dissipation of compounds over time in authentic pesticide deposits on excised leaves arid glass microscope slides. The laboratory study provides a means of evaluating the redistribution of a substance from pesticide deposits on foliage and glass slides in darkness or light at various temperatures, and generates data suitable for modeling the behavior of a compound under field conditions. The protocol is particularly suited for vegetation that would be difficult to study in a microcosm. Models developed from the laboratory study are tested with data collected from outdoor exposures. TCDD in 2,4,5-T herbicide deposits on Pacific madrone and giant chinkapin leaves, and glass microscope slides was exposed to 5, 20, and 145°C treatments in artificial sunlight or the dark for 0, 120, 240, 480, 960, and 1920 mm. The laboratory experiment showed that losses in the light were about two to twenty times greater than losses in the dark at the same temperature. Results from glass slides in the dark suggest that TCDD volatilizes from surface deposits. Losses of TCDD from deposits on madrone and chinkapin in the dark suggest that the behavior of TCDD on foliar surfaces depends on the plant species. The butoxyethyl ester of triclopyr in authentic herbicide deposits was also studied in this system using Pacific madrone and giant chinkapin leaves, and glass microscope slides. Treatment conditions were 10, 25, and 4O°C in artificial sunlight or the dark for 0, 831, 1662, 21493, 332k, and 14155 mm. Results from the dark showed that triclopyr is lost at a rate that changes exponentially with temperature. Foliar penetration of triclopyr was greater in giant chinkapin leaves than in madrone leaves. After about 27 h in the dark on foliar surfaces, the redistribution of triclopyr from surface deposits appeared to reach an equilibrium. Losses were greater in the light than in the dark, suggesting that photodegradation of triclopyr was occurring in the surface deposits. Losses at 10°C in the light were 10 times greater than in the dark, however, this difference was not apparent at 40°C. Assuming that photodegradation was occurring, then the quantum yield of triclopyr degradation is 1.71 X l0[superscript -4] moles/einstein in herbicide deposits on glass microscope slides at 100C.