Evaporation of pesticides

Under field conditions water evaporation normally occurs from the same surfaces as evaporation of pesticide. Any temperature effect caused by the former will therefore affect the latter, and in this repect the state of affairs is simpler. On the other hand, leaves can restrict water evaporation, and the soil surface is very complex hence, other complications appear under field conditions. All these factors, however, will make the pesticide evaporation rate (if the pesticide is fully exposed on an outer surface) faster than that calculated from the water rate. 

Where a solution of a moderately volatile organic substance in water is distilled, the distillate is often richer in organic substance than the residue and may form two separate phases. The evaporation of pesticide along with water from wet soil is often referred to as steam distillation or codistillation. The implication that the pesticide evaporates more rapidly because of the presence of the water is entirely correct, but to imply that steam distillation is the explanation directs attention to the wrong part or the total process. 

There is abundant evidence that water can accelerate the evaporation of pesticides from the soil. Two mechanisms operate one which depends on the evaporation of water, the other requiring only its presence. The first is best called wick evaporation and is the phenomenon which should be referred to by the agronomist when he speaks of steam distillation. 

Because rate of loss is roughly proportional to p / M, but the capacity of a given volume of air is proportional to p X Af, a smaller depth of air is necessary to accommodate the predicted evaporation of pesticide, around 50 meters instead of 200 meters. This is still substantial, but two qualifications are necessary. 

INTRODUCTION 210 STATUS OF RESIDENTIAL MODELS 211 EXPOSURE PHASES IN RESIDENTIAL EXPOSURE 212 Mixing and Loading Phase 212 Application Phase 212 Post-Apphcation Phase 213 MODEL CONCEPTS FRAMEWORKS 214 Mass-Balanced Air Qnality Model 214 Fngacity Model 215 Flnid Dynamics Model 216 MODEL CONCEPTS SOURCES AND SINKS 216 Sonrce Evaporation of Pesticides 216 Vapor-Pressnre-Driven Evaporation 216 Chinn Evaporation 217... 

Kerosene may enter the water or soil environment as a result of regular use (e.g., evaporation of pesticide solvent), from spills during use or transportation, or from leaking storage facilities. The relatively low vapor pressure of kerosene makes inhalation exposure unlikely under ordinary occupational conditions unless conditions of poor ventilation exist. The combustion product of burned kerosene, carbon monoxide, is of real concern when kerosene heaters are not vented. Exposure to kerosene mist can occur as kerosene is often applied in the form of a spray. Eye and skin contact with kerosene and kerosene mists and vapors can occur. The exposure pathway usually of... 

G. S. Hartley, Evaporation of Pesticides , in Pesticidal Formulation Research— Physical and colloidal Aspects, Advances in Chemistry Series, 86, American Chemical Society, Washington, D.C., 1969, pp. 115-134. 

Solvents spilled onto the surface of soil may volatilize into the atmosphere. The Dow Method " was used in this section to estimate half-life values of each solvent if spilled on the surface of a dry soil. The Dow Method is a simple relationship that was derived for the evaporation of pesticides fi om bare soil ... 

All of the 23 plasticizers in Table 18.1 occur as viscous or oily liquids that range from colorless to an amber color. If these liquids were spilled on soil or sediments, a portion of the liquid could volatilize into the air, depending on the specific compound, but most of the 23 plasticizers have vapor pressures that are less than 10 mm Hg at 25°C (Table 18.13). The vapor pressures of nine of the compounds have not been measured. For these plasticizers, vapor pressures were estimated using the Fragment Constant Method. As noted earlier, most of these chemicals will also be adsorbed by soil and sediments which would reduce the extent of volatiUzation. The rate of volatilization of plasticizers from soil has not been measured. For the purpose of illustration, the Dow Method was applied to estimate the half-life of each plasticizer if it was spilled on the surface of a dry soil. The Dow Method is a simple relationship that was derived for the evaporation of pesticides from bare soil ... 

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