The value of the diffusion coefficient of a chemical in the vapor phase is generally 10^{4} times larger than that in the liquid phase (Jury et al., 1983). The diffusion coefficient can be estimated using a variety of methods (Reid et al., 1987), including the Fuller correlation

where D_{ab} is the binary diffusion coefficient (cm^{2}/s), T is the absolute temperature (K), M_{ab} = 2/(M_{a}^{-1} + M_{b}^{-1}), M_{a} and M_{b} are the molecular weights of air and MeBr, respectively, P is the pressure (bars) and E_{L} is obtained using the atomic diffusion volumes (Reid et al., 1987). Using this Equation yields an estimated diffusion coefficient for MeBr of 0.114 cm^{2} s^{-1} at 20°C and 1 atmosphere ambient pressure. The temperature dependence of the diffusion coefficient is shown in Figure 3 and appears to be nearly a straight line over the temperature range 0-60°C. The temperature dependence of the binary diffusion coefficient can be described using the Equation above or using activation energy and the Arrhenius Equation as shown in Figure 3.

Using a screening model, Jury et al., (1991) found that the movement of a chemical is dominated by vapor-phase diffusion if the air-to-water partition coefficient, or the Henry’s Law coefficient (K_{H}) is »10^{-4}. Since the K_{H} for MeBr is approximately 0.25, transport in the vapor phase is important in describing the fate and transport in soil.