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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
Reactions with Stratospheric Ozone
It is known that bromine can catalytically destroy stratospheric ozone (Wofsy et al., 1975; Yung et al., 1980; McElroy et al., 1986; Salawitch et al., 1988; Anderson et al., 1989; Prather et al., 1990). Reactions involving bromine are believed to be responsible for 20-25% of the Antarctic 'ozone hole' that develops each austral spring (Anderson et al., 1989), which implies that a bromine atom is approximately 40 times more efficient than a chlorine atom in destroying ozone (Wofsy et al., 1975; Salawitch et al., 1988; Solomon et al., 1992). Methyl bromide is unique because it is a significant source of bromine to the stratosphere (Wofsy et al., 1975; Yung et al., 1980; Penkett et al., 1985; Cicerone et al., 1988; Schauffler et al., 1993). However, the case for restricting the use of MeBr is not clear-cut. Unlike the CFCs, atmospheric MeBr is not entirely contributed by human activities. Atmospheric MeBr has abundant natural and anthropogenic sources. Also, its sinks result not only from reactions with the atmosphere, but also from interaction with the oceans and land. Thus, estimating the contribution of MeBr fumigation (currently ~80% of the entire anthropogenic source) to the depletion of stratospheric ozone is much more complex than it is for other regulated halogenated compounds.
To justify the pending suspension of MeBr use in agriculture, it should be established that the known sources of atmospheric MeBr surpass the sinks, and the surplus is contributed by anthropogenic emissions. However, current estimates of global MeBr are out of balance, with sinks exceeding sources by a wide margin (Yvon-Lewis and Butler, 1997). The total atmospheric burden of MeBr is believed to be around 145 Gg y-1 (100-194 Gg y-1), and the concentration about 10 pptv, increasing at 0.1-0.3 pptv y-1 (Khalil et al., 1993; Singh and Kanakidou, 1993). The sinks currently thought to remove MeBr from the atmosphere include reactions with OH radicals in the atmosphere (accounting for ~86 Gg y-1 MeBr), removal by oceans (~77 Gg y-1), degradation in soil (42 Gg y-1 ) and uptake and degradation by plants. The relative strength of each of these sinks is not well quantified. The estimated lifetime of atmospheric MeBr is 0.7 y (range of 0.4 to 0.9 y), with a calculated ozone depletion potential (ODP) of 0.4 (range 0.2 to 0.5) according to the World Meteorological Organization's 1998 Scientific Assessment of Ozone Depletion (WMO, 1999).
The known sources of atmospheric MeBr include oceanic emissions, biomass burning, automobile emissions from leaded gasoline, and fumigation. Together, these emissions combine to produce 122 Gg y-1 of MeBr (range of 43 to 244 Gg y-1) (WMO, 1999). The 1998 Scientific Assessment of Ozone Depletion estimated oceanic MeBr emissions to be 60 Gg y-1, with the ocean acting as a net MeBr sink of -21 Gg y-1 (WMO, 1999). Recent research has indicated that the magnitude of the oceanic sink may be -11 to -20 Gg y-1 (King et al., 2000). Biomass burning (Manö and Andreae, 1994) is another significant, natural source of atmospheric MeBr, and its contribution is poorly quantified. Global emission of MeBr from biomass burning is estimated to be 20 Gg y-1 (range of 10 to 40 Gg y-1) (WMO, 1999). It has also been demonstrated that automobile exhaust from the combustion of leaded gasoline, which contains bromine compounds, can include measurable amounts of MeBr (Harsch and Rasmussen, 1977). Emissions from this source could range from 0 to 5 Gg y-1 (WMO, 1999). Additional potential MeBr sources which have recently been identified include production by plants (Gan et al., 1998a), salt marshes (Rhew et al., 2000), and fungi (Butler, 2000). Salt marshes may be a globally important source of MeBr (contributing 7-29 Gg y-1) (Rhew et al., 2000) and production of MeBr has been observed for a variety of plants (Gan et al., 1998a); therefore, plant sources may account for a large proportion of the "missing source" in current MeBr budgets.
Some anthropogenic emissions, such as fumigation of structures, perishables, and durables, are relatively well quantified, since nearly 100% of the applied MeBr is vented into the air during these fumigation processes. The use of MeBr for these fumigations accounts for about 15% of the total production. Trapping and/or decomposing MeBr during structural fumigation can drastically decrease atmospheric emissions of MeBr during these operations. Approximately 85% of the industrially-produced MeBr is used as a soil fumigant, equivalent to ~65 Gg y-1 in 1996. The actual discharge of MeBr from fumigated fields into the air is largely determined by the proportion of the applied MeBr that is emitted from the treated soil, which can be reduced through management practices (for example, Wang et al., 1997a; Yates et al., 1998; Gan et al., 1998d).
New measurements of the sources and sinks of MeBr are still being actively obtained, as evidenced by many recent reported studies. It is a fact that the relative contribution of MeBr used in fumigation practices is far from well quantified. Despite this, being a significant controllable source, the agricultural use of MeBr becomes a natural target for elimination. It is also assumed that due to a short atmospheric lifetime of less than 1 year, the effect of cessation of anthropogenic MeBr emissions on the restoration of stratospheric ozone will be nearly immediate. In comparison, all the chlorofluorocarbons (CFCs) have extremely long lifetimes, and with a complete elimination of emissions of these compounds, it may take many years, or even centuries, to reduce the atmospheric burden of the CFCs to an insignificant level (Butler, 1995).
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Last Modified: 10/20/2005
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