A combined soil-atmosphere model for evaluating the fate of surface-applied pesticides. 2. The effect of varying environmental conditions

Simulations using the one-dimensional nonisothermal model (Reichman et al. Environ Sci Technol. 2000, 34,1313-1320). this issue) for pesticides that h ave been applied to the soil surface revealed that diurnal variation of cli matic factors, soil type (silty clay and sand), initial soil-moisture distr ibution, and pesticide type (parathion, dieldrin, and trifluralin) potentia lly have major influences on pesticide volatilization rates and persistence . The diurnal variation of water evaporation, which is closely related to t he diurnal variation in solar radiation, has potentially significant effect on the diurnal variation of parathion volatility and mobility (low volatil ity and high mobility). However, the volatilization rates of low-water-solu ble pesticides (e.g., trifluralin and dieldrin) are only slightly affected by water evaporation rate. Their migration downward via vapor-phase diffusi on reduces their concentration in the soil surface layer and decreases thei r actual volatilization rates. The limiting factor for these pesticides is their movement downward in the soil. The volatilized masses of parathion an d dieldrin from a silty clay soil are higher than from sandy soil. However, trifluralin's volatilization rate, volatilized mass, and penetration depth are lower for the silty clay than for the sandy soil due to adsorption. Hi gher volatilization rates were obtained from initially dry soils for triflu ralin and from initially wet soils for parathion and dieldrin. Comparison b etween isothermal and nonisothermal models revealed that use of the former might lead to erroneous conclusions with respect to the relative volatility of pesticides especially those with a low Henry constant and high water so lubility.