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Temperature Effect on Methyl Bromide Volatilization in Soil Fumigation

Soil Physics & Pesticides Res. Unit, U.S. Salinity Laboratory, 450 West Big Springs Road, Riverside, CA 92507-4617.

^* Corresponding author (dwang{at}ussl.ars.usda.gov).

ABSTRACT

Recent interest in characterizing methyl bromide emission has focused on field and laboratory measurements, which are expensive to conduct and very time consuming. Predicting methyl bromide volatilization with numerical or analytical models has been limited to idealized situations without considering environmental conditions such as diurnal temperature change. It has been found that temperature can strongly affect methyl bromide volatilization under field conditions. To quantitatively characterize temperature effect, we adopted a two-dimensional numerical model that can solve simultaneous equations of water, heat, and solute transport (including both liquid and vapor phases). Functional relationships were established between temperature and methyl bromide liquid-gas phase partition coefficient or the Henry's constant, diffusion coefficient in soil air space, and the permeability of polyethylene tarp. To test the model, soil properties and boundary conditions from Yates et al. (1996a,b,c) were used. The model prediction was completely independent of the field measurement. The model simulation by considering diurnal variations of soil temperature predicted the cumulative emission that agreed well with the measured flux density. Prediction without considering temperature missed the diurnal nature in emission flux density. Comparable results were also obtained for methyl bromide concentration in the soil profile. The key advantage of this model is its ability of describing diurnal variations in methyl bromide emission flux. Based on the temperature effect on temporal variations of methyl bromide emission, we believe that small sampling intervals are needed to determine the dynamic nature of methylbromide emission under field conditions, especially during the first 24 h after application.

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