Verlag des Forschungszentrums Jülich
JUEL-4073
Wolters, André
Pesticide Volatilization from Soil and Plant Surfaces: Measurements at Different Scales versus Model Predictions
IX, 127 S., 2003
Simulation of pesticide volatilization from plant and soil surfaces as an integral component
of pesticide fate models is of utmost importance, especially as part of the PEC (Predicted
Environmental Concentrations) models used in the registration procedures for pesticides.
Experimentally determined volatilization rates at different scales were compared to model
predictions to improve recent approaches included in European registration models.
To assess the influence of crucial factors affecting volatilization under well-defined
conditions, a laboratory chamber was set-up and validated. Aerodynamic conditions were
adjusted to fulfill the requirements of the German guideline on assessing pesticide
volatilization for registration purposes. Determination of soil moisture profiles of the upper
soil layer illustrated that a defined water content in the soil up to a depth of 4 cm could be
achieved by water-saturation of the air. Cumulative volatilization of 14 C-parathion-methyl
ranged from 2.4% under dry conditions to 32.9% under moist conditions revealing a clear
dependence of volatilization on the water content in the top layer.
At the semi-field scale, volatilization rates were determined in a wind-tunnel study after
soil surface application of pesticides to gleyic cambisol. The following descending order of
cumulative volatilization was observed: chlorpyrifos > parathion-methyl > terbuthyl-azine
> fenpropimorph.
Parameterization of the models PEARL (Pesticide Emission Assessment at Regional and
Local Scales) and PELMO (Pesticide Leaching Model) was performed to mirror the
experimental boundary conditions. Model predictions deviated markedly from measured
volatilization fluxes and showed limitations of current volatilization models, such as the
uppermost compartment thickness having an enormous influence on predicted volatilization
losses. Moreover, the impact of soil moisture on volatilization from soil was not
reflected by the model calculations. Improvements of PELMO, including the temperature-dependence
of water-air partitioning, the reduction of the compartment size of the top layer
and the introduction of a moisture-dependent sorption coefficient, contributed to a more
realistic reflection of experimental findings, especially at the initial stage of the studies.
Studies on volatilization from plants included a field study and a wind-tunnel study after
simultaneous application of parathion-methyl, fenpropimorph and quinoxyfen to winter
wheat. Parathion-methyl was shown to have the highest volatilization during the wind-tunnel
study of 10 days (29.2%). Volatilization of quinoxyfen was about 15.0%, indicating
a higher volatilization tendency in comparison with fenpropimorph (6.0%), which may be
attributed to enhanced penetration of fenpropimorph counteracting the volatilization
process.
A mechanistic approach using a laminar air-boundary layer concept for the consideration
of volatilization from plant surfaces was adjusted and calibrated on the basis of a series of
wind-tunnel studies. Calibration of the thickness of the air-boundary layer and the rate
coefficients of phototransformation and penetration into the leaves allowed the
implementation of this description in PELMO and enabled the simultaneous estimation of
volatilization from plants and soil.
The need to determine critical factors affecting volatilization, especially phase partitioning
coefficients, resulted in the development and validation of a novel chamber system for
measurements of the temperature dependence of the soil-air partitioning of fenpropimorph.
Additional batch studies allowed for the quantification of the general tendency of
pesticides towards enhanced soil sorption after lowering the temperature.
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