Volatilization estimation, pesticides

Smit AAMFR, van den Berg F, Leistra M (1998) Estimation method for the volatilization of pesticides from plants. Environmental Planning Bureau series 4, Wageningen, Netherlands, DLO Winand Staring Centre... 

Woodrow J. E., Seiber J. N., and Baker L. W. (1997) Correlation techniques for estimating pesticide volatilization flux and downwind concentrations. Environ. Sci. Technol. 31(2), 523-529. 

Results of volatilization and leaching estimations are reported for six pesticides that span a wide range of the physical/chemical properties that affect fate at the soil/air interface. The pesticides are Mirex, toxaphene, methoxychlor, lindane, malathion, and dibromochloropropane (DBCP). These particular pesticides were chosen for discussion here because they illustrate the methods for assessing the fate of organics at the... 

Sufficient samples at the proper locations were not taken to allow for accurate estimates of the vaporization losses for those pesticides which could be proven to be emanating from the disposal pit. However the low concentrations measured above the pit suggest that losses by volatilization are not a source of significant contamination of surrounding air. The pit therefore is a source of air contamination due to volatilization but the amount is negligible given the normal dilution effect for point sources of air pollution. 

A simple environmental chamber is quite useful for obtaining volatilization data for model soil and water disposal systems. It was found that volatilization of low solubility pesticides occurred to a greater extent from water than from soil, and could be a major route of loss of some pesticides from evaporation ponds. Henry s law constants in the range studied gave good estimations of relative volatilization rates from water. Absolute volatilization rates from water could be predicted from measured water loss rates or from simple wind speed measurements. The EXAMS computer code was able to estimate volatilization from water, water-soil, and wet soil systems. Because of its ability to calculate volatilization from wind speed measurements, it has the potential of being applied to full-scale evaporation ponds and soil pits. 

Inhalation Route - Estimation of Vapour Exposure. In a study of drift exposure following aerial application of an organo-phosphorus pesticide, Crabbe t al (16) found that the vapour concentration in areas remote from the spray line increased gradually up to 10 hours after the spraying. Increasing temperature was undoubtedly the major explanation for this. Other factors such as volatility of the pesticide, windspeed and sorption properties of the target would also influence the actual vapour concentration on the target. 

Samples for YOC (EPA Method 8260) and pesticide analysis (EPA Method 8081) arrived to the laboratory at 12°C and were analyzed. Because of the target analyte volatility, during data evaluation the chemist qualifies the VOC results as estimated values. Unlike YOCs, non-volatile pesticides are not affected by the elevated temperature, and the chemist chooses not to qualify the results of pesticide analysis. 

A key parameter used to estimate or model volatilization processes is the pesticide vapour pressure a fundamental property of the chemical agent which is uniquely defined by the temperature. This parameter is readily and reproducibly measured in the laboratory. Two... 

In the absence of direct field measurements of pesticide fluxes eminating from a sprayed forest a series of suppositions may be drawn from similar observations of losses from treated agricultural crops. The volatilization of dieldrin and heptachlor from a grass pasture was characterized by rather marked diurnal variations in vertical flux intensities of both insecticides during the initial days post application (12). The authors concluded that the volatilization ceased or was greatly reduced with decreased solar radiance. Estimated relative vapour concentrations of dieldrin rapidly declined from saturation 2 hours post application to 10% by evening. This parameter reached a maximum of 30 - 40% on day 2 and 20 - 25% on day 3. Although the saturated vapour concentration of heptachlor is approximately fifty... 

Another type of experiment has been used to assess the chemical reactivity of pesticides in the air. This principally employs downwind sampling from a treatment site during application (for measuring conversion in the spray drift) and for several days following application (for conversions involving volatilized residues) (24). The principal data are in the form of product(s)/parent ratios with increasing downwind distance, from which estimates of the rate of conversion can be made knowing the air residence time calculated from windspeed measurements. 

The two methods described for determining vapor pressures appear to give reliable vapor pressure data for volatile pesticides and fumigants. The precision is better than 10 to 20% in most cases. Care must be taken, particularly with the determination of low vapor pressures which can be difficult. Estimation of vapor pressures by Clausius-Clapeyron and other related functions is dependable for interpolation and limited extrapolation. Extensive extrapolation is, as always, dangerous, and a direct measurement should be made as closely as possible to the desired temperature. The literature contains a number of examples which violate this principle. 

Measurement of pesticide metabolites in nrine holds the potential for developing a more accurate estimate of internal dose, and is particnlarly nsefnl when exposnre is from multiple routes, oral, as well as respiratory and dermal, as is almost always the case for pesticide-exposed workers. If the total nrinary ontput is collected, until either there are no detectable residues or background levels are reached (usually 48-96 h), the levels can be used to estimate the internal dose. Stndies carried out in animals and humans for several pesticides have shown a good correlation between the amount of pesticide applied to the skin and the urinary output (Franklin et al., 1983, 1986 Popendorf and Eranklin, 1987). However, there are limitations to using this approach. The pharmacokinetics of the pesticide must be known in humans, while those pesticides that are highly volatile are extensively metabolized to numerous minor metabolites or seqnestered and are... 

Whatever their design, passive samplers will collect only pesticide vapors. Therefore, air measurements made with them will not always be appropriate for estimating total respiratory exposure, especially in the case of low-volatility pesticides and pesticides tracked indoors on soil particles. In addition, some pesticides may undergo chemical degradation on sorbents or in solvents over such long sampling periods. Therefore, internal standards or other means will need to be used to assure acceptable analyte recoveries. 

Numerical validation for pesticide movement addresses the question of whether the results generated from the model predict actual experimental values. A few models have been validated by correlating the estimated airborne pesticides and/or the amount on room materials with actual measurements in certain specific cases, van Veen et al. (1999) reported an experiment to validate a painting model of CONSEXPO which describes concentrations of a volatile solvent in room air both during and after the application. The concentrations depended on evaporation, initial concentration of solvent in two layers of paint, volume of paint and removal of solvent by ventilation from the room. Model parameters were either measured from the room before the experiment (ventilation rate, room size, physico-chemical parameters, etc.) from the act of painting (surface painted and amount of paint used), or fixed in advanced (relative size of the two layers of paint, transfer rate between the layers, etc.). The model predicted room concentrations that were within 80 % of the actual measured concentrations (Figure 6.1). Important with respect to the evaporation term is that peak concentrations could be predicted very well, so indicating that the source term is appropriate. 

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