Vapor , pesticide measurement

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. 

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. 

The good correlation of the results of vapor diffusion and leaching experiments for butylate, alachlor, and metolachlor with their physical properties has given support to the value of physical property measurements to predict pesticide movement in the soil. 

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. 

Air Samples. The atmospheres in the vicinity of the pits were sampled for pesticide vapors using XAD-2 resin and a vacuum pump ( 6). Collection efficiencies for this method were measured in the laboratory using simulated atmospheres and found to be 99.5% average for 10 pesticides spiked at 10 ng/L of air. 

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. 

There are a few data that suggest that pesticides can undergo reactions indoors. For example, Wallace et al. (1996) observed that the aldrin levels inside a home decreased with time, whereas those of dieldrin did not. Dieldrin had been applied with aldrin but is also an oxidation product of aldrin. One of the reasons for the lack of change in dieldrin may be that it was being formed as the aldrin decayed however, this could not be differentiated from the effects of a lower vapor pressure of dieldrin, which could lead to lower overall removal rates. In the same study, pentachloroanisole was also measured inside the home and attributed to formation by degradation of pentachlorophenol, which is used as a wood preservative and termiticide. 

LCS0 Concentration of an active ingredient in the air which, when inhaled, kills half of the test animals exposed to it expression of a compound s toxicity when present in the air as a gas, vapor, dust, or mist generally expressed in ppm when a gas or vapor, and in micrograms per liter when a dust or mist often used as the measure of acute inhalation toxicity. The lower the LC50 number value the more poisonous die pesticide. 

Diazinon air concentrations related to vapors released from pest control strips were measured by Jackson and Lewis (1981). Diazinon levels in indoor air increased from 0.32 pg/m3 at 6 hours after application of the pest strips to 1.34 pg/m3 on day 15, and then declined to 1.21 pg/m3 on day 30. Air sampling in a retail garden store where pesticide containers with diazinon were displayed showed an average diazinon concentration of 3.4 pg/m3 (Wachs et al. 1983). 

No direct measurements of volatilization losses of any pesticide has been made following applications to forests. However, Grover et al. (7) recently measured the volatilization of 2,4-D after application as the isooctyl ester to a wheat field. This same low-volatile ester is used in forest vegetation control. The total vapor loss within 3 days after application of the isooctyl ester of 2,4-D was 20% of the amount applied. The applicability of these findings to volatilization of like pesticides in the forest environment will be discussed. We will indicate how volatilization in forests may differ from that reported from agricultural applications to open fields. The paper also will discuss the transfer of pesticides into the atmosphere from the standpoint of mechanisms involved, factors influencing rates of... 

Measurements of pesticide volatilization in the field have been made by several researchers using microclimate techniques. Vapor... 

Knowledge of the relative role of vaporization in pesticide field dissipation has been markedly advanced by development of methods for measuring the vertical flux of vapors from a soil or soil-crop field plot. The basis and techniques for this method have been presented by Caro et al.(74) and Parmele et al. (7), and summarized recently by Taylor (6). The equation... 

Parmele, L.H. Lemon, E.R. Taylor, A.W. Micrometeorolog-ical measurement of pesticide vapor flux from bare soil and corn under field conditions. Water, Air, Soil Pollut., 1972, 1, 433. 

Vapor pressures at ambient temperatures of a number of pesticides can be determined by using a du Pont 900 differential thermal analyzer to measure boiling points for a series of pressures down to 10 mm., or by using an effusion method for compounds having vapor pressures from 10 3 to 10 7 mm. Less than 100 mg. of the sample are required in either case. Accuracy can be determined by comparison with direct measurements available in the literature. Vapor pressures for phenoxy herbicide esters are lower than values reported in the literature. 

Therefore, any program for measuring vapor pressures must be able to handle a wide range of values—many mm. Hg to 10"5 mm. Hg and less. At present, no one method is capable of covering such a range, and several must be used. Published work in this area shows that virtually all available methods have been used at one time or another for measuring vapor pressures of pesticides. The reader is referred to standard works on this subject (4, 10, 17), but the following brief survey is offered for orientation. 

Besides the magnitude of the vapor pressure value, other considerations will influence the choice of method. For example, if the material is a solid at normal use temperatures, boiling point measurements are obviously out of the question. Of frequent importance to pesticide re-... 

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. 

Personal protection measures to prevent mosquito bites are actively used by householders. In many developed and medium-income countries in the Region, the use of household insecticide products constituted a major component of the total public health use of pesticides. The mosquito coil is used widely, followed by mats, aerosols and liquid vaporizers. The use of mosquito coils has increased every year from 1995 to 1997 China is the major user. Data indicate that the biggest market for household insecticide products is China, where in 1998 more than 10000 million units of household insecticide products were used. 

Air sampling for occupational exposure to pesticides normally consists of measurement of pesticide concentrations in the worker s breathing zone, with a portable air-sampling pump and a sampling train which includes some type of collection device. The latter device, or sampling media, selected are based on the physical and chemical properties of the compound to be measured. Field workers may be exposed to chemical vapors, solid particulates or water-based aerosols. Examples of sampling media include membrane filters, sorbent tubes, polyurethane foam and charcoal. A discussion of pesticide exposure provides a useful review of methods for respiratory exposure measurement (Nigg etal, 1990). 

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. 

The typical end-use product and application method chosen as representative of the extreme-case exposure scenario must be used to attain the highest permissible rate allowed by label directions. Sampling for indoor residues should consider all potential sites where appreciable residues are expected and are accessible. For instance, dermal contact may come from exposure to the pesticide as a residue on carpets, vinyl tile, upholstery and counter tops, while airborne residue (vapor- or particle-phase) may provide the source of inhalation exposure. The measurements taken are linked specifically to the method of application. 

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