Photodegradation and volatility of pesticides - Chamber experiments

Background and Objectives. Among the factors affecting the environmental fate of surface-applied pesticides several biological as well as abiotic factors, such as volatilization and photochemical transformations are of particular interest. Whereas reliable measurement methods and models for estimating direct photodegradation are already available for the compartments of water and atmosphere and individual subprocesses have already been described in detail, there is still a need for further elucidation concerning the key processes of heterogeneous photodegradation of environmental chemicals on surfaces. Methods. In order to systematically examine the direct and indirect photoclegradation of C-14-labeled pesticides on various surfaces and their volatilization behavior, a new laboratory device ('photovolatility chamber') was designed according to US EPA Guideline 161-3. Model experiments under controlled conditions were conducted investigating the impact of different surfaces, i.e. glass, soil dust and radish plants, and environmental factors, i.e. irradiation and atmospheric ozone (O-3), on the photodegradation and volatilization of surface-deposited [phenyl-UL-C-14]parathion-methyl (PM). Results and Discussion. 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 photodegradation of PM (experiments without O-3), the major products were polar compounds and (CO2)-C-14, due to the rapid photochemical mineralization of 4-nitrophenol to (CO2)-C-14. Paraoxon-methyl and 4-nitrophenol formation was mainly mediated by the combination of light, O-3, and (OH)-O-. radicals. In radish experiments PM photoclegradation was presumably located in the cuticle compartment, which exhibited a sensitized photodegradation, as more unknown products were yielded compared to the glass and soil dust experiments. This could be explained by intensifying the inherent PM degradation in the dark with the same product spectrum. Due to photochemical product formation, which is an antagonistic process to the volatilization of parent compound, the volatilization of unaltered parathion-methyl from each surface generally decreased in the presence of light, particularly in combination with increasing O-3 concentrations and (OH)-O-. radical production rates. Conclusion. First results demonstrated that the photovolatility chamber provides a special tool for the systematic evaluation of (a) photodegradation of surface-located pesticide residues, i.e. measuring qualitative aspects of direct and indirect photodegradation together with relative photodegradation rates, and (b) volatilization of pesticides on surfaces by including and optionally varying relevant parameters such as light, atmospheric O-3 concentration, surface temperature, air temperature, air flow rate. Outlook. The experimental facility represents an important complement to lysimeter and field studies, in particular for experiments on the volatilization of pesticides using the wind tunnel system. With the photovolatility chamber special experiments on photodegradation, volatilization and plant uptake can be conducted to study key processes in more detail and this will lead to a better understanding of the effects of certain environmental processes on the fate of released agrochemicals contributing to an improved risk assessment.