Pesticides vapor pressure

The deviations observed between extrapolated estimates from GLC data, and direct measurements with the effusion measurements appear to be too large to be accounted for by extrapolation uncertainties. The best estimate can probably be obtained by fitting the combined data to the Clausius-Clapeyron equation (footnote b of Table IV). The obvious implication is that where possible, extrapolation of pesticide vapor pressures obtained at elevated temperatures be converted to interpolation by including a direct measurement at room temperature. In terms of the work described here, vapor pressure measurements requiring the DTA should be supplemented with Knudsen cell measurements. This would require a temperature at which the vapor pressure was 10 3 mm. or less. 

An extensive pesticide properties database was compiled, which includes six physical properties, ie, solubiUty, half-life, soil sorption, vapor pressure, acid pR and base pR for about 240 compounds (4). Because not all of the properties have been measured for all pesticides, some values had to be estimated. By early 1995, the Agricultural Research Service (ARS) had developed a computerized pesticide property database containing 17 physical properties for 330 pesticide compounds. The primary user of these data has been the USDA s Natural Resources Conservation Service (formerly the Soil Conservation Service) for leaching models to advise farmers on any combination of soil and pesticide properties that could potentially lead to substantial groundwater contamination. 

Many factors affect the mechanisms and kinetics of sorption and transport processes. For instance, differences in the chemical stmcture and properties, ie, ionizahility, solubiUty in water, vapor pressure, and polarity, between pesticides affect their behavior in the environment through effects on sorption and transport processes. Differences in soil properties, ie, pH and percentage of organic carbon and clay contents, and soil conditions, ie, moisture content and landscape position climatic conditions, ie, temperature, precipitation, and radiation and cultural practices, ie, crop and tillage, can all modify the behavior of the pesticide in soils. Persistence of a pesticide in soil is a consequence of a complex interaction of processes. Because the persistence of a pesticide can govern its availabiUty and efficacy for pest control, as weU as its potential for adverse environmental impacts, knowledge of the basic processes is necessary if the benefits of the pesticide ate to be maximized. 

The environmental fate and behavior of compounds depends on their physical, chemical, and biochemical properties. Individual OPs differ considerably from one another in their properties and, consequently, in their environmental behavior and the way they are used as pesticides. Pesticide chemists and formulators have been able to exploit the properties of individual OPs in order to achieve more effective and more environment-friendly pest control, for example, in the development of compounds like chlorfenviphos, which has enough stability and a sufficiently low vapor pressure to be effective as an insecticidal seed dressing, but, like other OPs, is readily biodegradable thus, it was introduced as a more environment-friendly alternative to persistent OCs as a seed dressing. 

The vaporous collar contains a relatively high-vapor-pressure liquid pesticide mixed throughout the collar. The pesticide is slowly released and fills the atmosphere adjacent to the animal s surface with a vapor of pesticide that kills the pest but is innocuous to the animal. 

At the fundamental level of equilibrium modeling the advantages are many. The model can combine a number of compartments through simple relationship to describe a realistic environment within which chemicals can be ranked and compared. Primary compartments that chemicals will tend to migrate toward or accumulate in can be identified. The arrangement of compartments and their volumes can be selected to address specific environmental scenarios. Data requirements are minimal, if the water solubility and vapor pressure of a chemical are known, other properties can be estimated, and a reasonable estimate of partitioning characteristics can be made. This is an invaluable tool in the early evaluation of chemical, whether the model be applied to projected environmental hazard or evaluation of the behavior of a chemical in an environmental application, as with pesticides. Finally, the approach is mathematically very simple and can be handled on simple computing devices. 

The difference between the concentration in the ultra-filtered water and the concentration inside the ultrafiltration cell is therefore a measure of the bound concentration. Griffin and Chian7-7-, Hassett 7-, and Diachenko have used volatilization measurements to determine the extent of binding of pesticides and pollutants to dissolved humic materials. In these experiments either the rate of gas stripping of a compound or its equilibrium vapor pressure is measured in the presence and absence of humic materials. The results obtained can be manipulated in such a way to determine the percentage of the pollutant bound. 

As described earlier, Henry s law constants can be calculated from the ratio of vapor pressure and aqueous solubility. Henry s law constants do not show a simple linear pattern as solubility, Kqw or vapor pressure when plotted against simple molecular descriptors, such as numbers of chlorine or Le Bas molar volume, e.g., PCBs (Burkhard et al. 1985b), pesticides (Suntio et al. 1988), and chlorinated dioxins (Shiu et al. 1988). Henry s law constants can be estimated from ... 

Dobbs, A. J., Grant, C. (1980) Pesticide volatilization rate a new measurement of the vapor pressure of pentachlorophenol at room temperature. Pestic. Sci. 11, 29-32. 

Kim, Y.-H., Woodrow, J. E., Seiber, J. N. (1984) Evaluation of a gas chromatographic method for calculating vapor pressures with organophosphorus pesticides. J. Chromatogr. 314, 37-53. 

Despite endrin s low vapor pressure of 2.0xlCl7 mm Hg (EPA 198la), initial volatilization of 20-30% after agricultural application to soil has been reported to be rapid (Nash 1983). Within 11 days, however, further volatilization was no longer detected (Nash 1983). Unlike some other chlorinated pesticides, endrin volatilization was not enhanced after a rainfall. Small amounts of endrin in soil may also be transported to the air by dust particles. 

Endrin is relatively nonvolatile with a vapor pressure of 2.0xl0 7 mm Hg (EPA 198 la Worthing and Walker 1983). Despite its low volatility, initial loss of agriculturally applied endrin through volatilization was found to be comparable to more volatile pesticides (Nash 1983). No recent data on atmospheric concentrations of endrin could be found in the available literature. Endrin was detected in air samples collected at 4 of the 102 NPL sites where endrin has been detected in some environmental medium however, concentrations were not available (HazDat 1996). 

Sites suitable for conventional SVE have certain typical characteristics. The contaminating chemicals are volatile or semivolatile (vapor pressure of 0.5 mm Hg or greater). Removal of metals, most pesticides, and PCBs by vacuum is not possible because their vapor pressures are too low. The chemicals must be slightly soluble in water, or the soil moisture content must be relatively low. Soluble chemicals such as acetone or alcohols are not readily strippable because their vapor pressure in moist soils is too low. Chemicals to be removed must be sorbed on the soils above the water table or floating on it (LNAPL). Volatile dense nonaqueous liquids (DNAPLs) trapped between the soil grains can also be readily removed. The soil must also have sufficiendy high effective porosity (permeability) to allow free flow of air through the impacted zone. 

Becanse the vapor pressnre of chemicals is a key factor in controlling their dissipation within the snbsnrface, and from the snbsnrface to the atmosphere, accurate estimation of this valne is reqnired. Comprehensive reviews on this subject are given by Plimmer (1976) and Glotfelty and Schombnrg (1989). For contaminants with low vapor pressnre that reach the snbsnrface as a result of a nonpoint disposal (e.g., pesticides nsed in agricnltnral practices), their vapor pressure is sufficiently low to be below detection limits, which may explain some discrepancies in the reported results. 

Pesticides are characterized by a range of (saturation) vapor pressure and densities (Table 8.2) they therefore evaporate from the land surface in different patterns. Peck and Hombuckle (2005) studied atmospheric concentrations of currently used pesticides in Iowa (United States) during the years 2000-2002. The average detected concentrations of five heavily used herbicides were 0.52 ng/m ... 

Pesticide Molecular weight (g) Temperature ( O Vapor pressure (mPa) Vapor density (lig/L)... 

Due to the movement of the pesticides to the bed surface, air samples were taken to determine any volatilization and subsequent concentration In the air along the berm on the downwind side of the bed. In most Instances, the top of the berm was only about 12 vertical Inches above the bed surface. Spencer and Farmer ( ) have reviewed the literature on the transfer of pesticides Into the atmosphere. Even though pesticide volatility Is related to vapor pressure of the chemical, there are many factors Influencing the effective vapor pressure from soil and water surfaces. 

Table XIV shows the levels of pesticides In air samples In the five field stations that had detectable levels. In general, the pesticide air levels found did compare favorably with the vapor pressure and residue levels of chemicals In the top 0-1 Inch surface of soil. These same conclusions have been made from previous studies (. However, levels were not always detected In the quantities that might be predicted ( ), perhaps due to other variables such as Inconsistent wetting of the beds, oil film In some of the beds and unidentified foreign matter on the surface of some beds Although the beds were all made up of sandy loam, the degree of sand, silt and clay varied appreciably.

The percent pesticide volatilized in one day from wet soil correlated positively with the factor [vapor pressure/(water solubility X binding constant)]. This factor has been reported to be linearly related to the volatilization rate of chemicals from soil surfaces (27). For pesticides with Henry s law constants and soil binding constants within the range studied, the factor is also approximately proportional to the fraction of chemical in soil air at equilibrium (28). In the present study, it was found that four of the pesticides had low factors, and less than 1% volatilized in 1 day (Table III). Diazinon, on the other hand, had a higher factor, and 2% of it volatilized. The use of this factor therefore does seem to have some merit for qualitative prediction. 

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