Volatilization of pesticides:: Measurements under simulated field conditions

Volatilization of pesticides is either measured under artificial, constant conditions in the laboratory or using micrometeorological measurements in the field, both having their specific limitations. A wind tunnel erected above a lysimeter combines the advantages of both methods: simulated field conditions, use of radiolabeled compounds, detection of metabolites and mass balances. The volatilization behavior of four pesticides was studied under simulated field conditions in the wind tunnel. In two experiments, simultaneous, parallel measurements were carried out in the field. High volatilization rates from plants were detected for fenpropimorph and parathionmethyl (up to 50 % d(-1)) whereas clopyralid showed only a very low volatilization. Generally, volatilization started immediately after application at relatively high rates, rapidly decreasing to a considerably lower level, where a diurnal rhythm was observed. Volatilization rates corresponded well with data from the simultaneous field measurements. Plant uptake was a major process counteracting pesticide volatilization. Volatilization from bare soil was only slight for parathion-methyl (2 %) and terbuthylazine (0.5 %) during a 3-week wind-tunnel experiment. The volatilization rates were strongly dependent on soil moisture. A diurnal rhythm was observed right from the beginning. Simultaneously measured volatilization rates in the field were considerably higher due to differences in soil moisture, application volume of water, and irrigation intensity. The occurrence of non-identified metabolites in air samples suggests that for many pesticides metabolic processes, possibly due to photolysis on plant or soil surfaces plays an important role in their dissipation.