Volatilization agricultural pesticides

Volatilization of pesticides is an important pathway for their loss from treated agricultural lands. The importance of volatilization in the forest environment has not been established by direct measurement, but can be inferred from volatilization rates of the same pesticides under agricultural conditions and from other data on their behavior in the forest environment. In recent years, several studies of actual volatilization rates of pesticides under field conditions have provided an assessment of the rate of input to the air under typical conditions of use (1). 

Unfortunately, research studies that address environmentally relevant atmospheric fate processes of pesticides are relatively few in comparison to studies that measure transformations on land surfaces and in water. This scarcity of fate information is related to the difficulty in attaining relevant tropospheric photochemical and oxidative information under both environment and controlled laboratory conditions. Only a limited number of studies exist that have measured airborne pesticide reactivity under actual sunlight conditions (d, 7,8), These studies enq)loyed photochemically stable tracer confounds of similar volatility and atmospheric mobility to con5)ensate for physical dilution. The examined airborne sunlight-exposed pesticides in these limited studies had to react quickly to provide environmentally measurable reaction rate constants. The field examination of tropospheric reaction rates for the vast majority of agricultural pesticides is impractical since reaction rates for many of these compounds are probably too slow to yield reliable rate constant information. 

Modem agriculture uses worldwide about 2.5 million tons of pesticides annually (Wijnands 1997), and out of such quantity only about 0.4% reaches the targeted pests, according to Pimentel (Pimentel 1995), while losses through volatilization are on the order of 80-90% (Taylor and Spencer 1990). 

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). 

Sparks DL (ed) (1986) Soil physical chemistry. CRC Press, Boca Raton, Florida Sparks DL (1989) Kinetics of soil processes. Academic Press, San Diego Sparks DL, Huang PM (1985) Physical chemistry of soil potassium. In Munson RE (ed) Potassium in agriculture, ASA, Madison, Wisconsin, pp 201-276 Sparks DL, Jardine PM (1984) Comparison of kinetic equations to describe K-Ca exchange in pure and mixed systems. Soil Sci 138 115-122 Spencer WF, Cliath MM (1969) Vapor densities of dieldrin. Environ Sd Technol 3 670-674 Spencer WF, Chath MM (1973) Pesticide volatilization as related to water loss from soil. J Environ Qual 2 284-289... 

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... 

Extrapolating to the forest environment from field measurements of pesticide volatilization in agricultural environments, along with output from the screening model using benchmark properties, we conclude that volatilization from the canopy foliage will be relatively high for the more commonly used forest pesticides. 

The process of pesticide volatilization from a leaf surface is considered first in terms of the component physical processes of sublimation and molecular diffusion through a saturated boundry layer. Predicted volatilization rates based solely on pesticide vapour pressures often bear little relation to field observations due to myriad interactions of the pesticide with the leaf and the surrounding microenvironment. Observed pesticide fluxes above sprayed agricultural fields together with microclimatological characteristics of coniferous forests are then used to predict general patterns of pesticide volatilization from a treated coniferous stand. 

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... 

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