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United States Department of Agriculture

Agricultural Research Service

Methyl Bromide
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1 - Background
2 - Chemical and Physical Properties
3 - Reactions with Stratospheric Ozone
4 - Solubility
5 - Henry's Law Constant
6 - Vapor Pressure
7 - Adsorption
8 - Diffusion Coefficient
9 - Air Sampling
10 - Field Experiments
11 - Transformation of MeBr in Water
12 - Transformation of MeBr in Soil
13 - Transport Model
14 - Simulating MeBr Volatilization
15 - Fumigation
16 - Post-Fumigation
17 - Further Reading
Simulating MeBr Volatilization
 
In typical applications, MeBr is applied to soil using metal shanks that cut into the soil. A nozzle is located at the rear of each shank that emits MeBr into the soil. The injection depth may vary from approximately 20 cm to 70 cm. As the tractor moves along the field, a layer of 0.025 mm (thick) by 3.5 m (wide) sheet high-density polyethylene (HDPE) plastic film is placed over the soil surface. For each sheet of plastic film, one side is buried in soil and the other is glued to the previous plastic sheet. This creates a series of panels down the field and a continuous plastic cover over the field. MeBr application rates range from 200 kg/ha to 400 kg/ha and large fractions (24% to 74%) of the applied chemical are lost to the atmosphere (Yagi et al., 1993, 1995; Majewski et al., 1995; Yates et al., 1996bc; Williams et al., 1999).
 
A field experiment was conducted to investigate the environmental fate and transport of MeBr after soil fumigation. The experiment was designed to determine the dynamics of MeBr movement through soil, degradation and cumulative emissions to the atmosphere. MeBr was applied to a 3.5 ha field, at a depth of 0.25 m and a rate of 240 kg/ha. Meteorological measurements of incoming solar radiation, net solar radiation, air temperature gradient, wind speed gradient, wind direction, relative humidity, and barometric pressure were obtained, on site, at 10 min intervals during the experiment. Soil heat flux measurements were taken at 0.05 and 0.1 m below the soil surface and soil temperature was measured at 0.05-, 0.10-, 0.15-, 0.20-, 0.30-, 0.40-, 0.50-, 0.80- and 1-m depths. Periodically during the experiment, MeBr soil-gas samples were taken to a depth of 150 cm.
 
Several methods were used to estimate the MeBr volatilization rate: the flux chamber (Denmead, 1979), aerodynamic (Parmele et al., 1972), theoretical profile shape (Wilson et al., 1982) and integrated horizontal flux methods (Denmead et al., 1977). For the micrometeorological methods, an air sampling mast was used to hold charcoal sampling tubes at heights of 0.1, 0.2, 0.5, 0.8, 1.2, and 1.6 m above the field surface. These measurements provide the air concentrations and concentration gradients required to estimate the volatilization rate. Estimate of the cumulative emission were obtained by integrating the flux measurements and by calculating the difference between the applied mass and the MeBr mass degraded (e.g., Br‾ ion), several months after application. The cumulative emissions obtained from these methods varied from 58% to 70% of the applied MeBr mass. A complete description of the experimental procedures is given in Yates et al. (1996abc).
 
A one-dimensional numerical model was developed to simulate MeBr degradation and movement in soil, and volatilization into the atmosphere. Since shank-injected soil fumigation results in a planar source of MeBr at the target depth, a one-dimensional model is appropriate for simulating the fate and transport process. Three volatilization boundary conditions, with increasing complexity, have been explored: (1) volatilization under isothermal conditions, (2) volatilization in response to solar-driven temperature changes at the soil surface, and (3) volatilization from soil coupled to atmospheric processes.
 
The results are shown in the following figures. Note that the simulated volatilization are predictions, therefore, none of the parameters in the model where adjusted to improve the correspondence between simulated and measured volatilzation.
 
References:
Yates, S.R., Wang, D., Papiernik, S.K. and Gan, J. 2002. Predicting pesticide volatilization from soils, Environmetrics 13:569–578.
 
Stagnant Boundary Layer Theory, Isothermal
Stagnant Boundary Layer Theory, Isothermal
Figure 1. Methyl bromide volatilization as a function of time after injection (i.e., t=0.6 days). Open circles are measured volatilization rates and solid lines are simulated values for a constant mass transfer coefficient. The inset shows cumulative emissions.
 
Stagnant Boundary Layer Theory, Nonisothermal (i.e.,h (T))
Stagnant Boundary Layer Theory, Nonisothermal
Figure 2. Methyl bromide volatilization as a function of time after injection (i.e., t=0.6 days). Open circles are measured volatilization rates and solid lines are simulated values for a mass transfer coefficient that varies with ambient temperature. The inset shows cumulative emissions.
 
Coupling Volatilization to Atmospheric Processes (i.e., Baker et al., 1996)
Coupling Volatilization to Atmospheric Processes
Figure 3. Methyl bromide volatilization as a function of time after injection (i.e., t=0.6 days). Open circles are measured volatilization rates and solid lines are simulated values for a mass transfer coefficient that depends on atmospheric processes (i.e., Baker et al., 1996). The inset shows cumulative emissions.
 
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Last Modified: 10/20/2005
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