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| Methyl Bromide |
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Background
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Environmental Concerns |
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In 1991, MeBr was identified as a potential ozone depleting compound (Chakrabarti and Bell, 1993). In 1992, on the Fourth Conference of the Parties to the Montreal Protocol, MeBr was officially added to the list of ozone depleting chemicals, with its production suggested to be frozen at the 1991 level, effective from 1995. |
| The inclusion of MeBr in the ozone-depleting chemicals list naturally brought this fumigant within the scope of the U.S. Environmental Protection Agency's (EPA) Clean Air Act, which has an amendment that mandates discontinuation of any chemical with an ozone depletion potential (ODP) greater than 0.2 at the beginning of 2001. The ODP index for MeBr was determined to be 0.60-70 in 1992 (UNEP, 1995); the ODP estimate was reduced to 0.4 in 1998. In March 1993, EPA announced that MeBr was scheduled for a phase out in the United States by the year 2001 (USEPA, 1993). The phase out date was later changed to the year 2005 (USEPA, 2000). |
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| Over the last decade, there has been an increased research effort devoted to understanding of the effects of halogenated gases emitted into the atmosphere on the depletion of the stratospheric ozone layer. According to the Ozone Assessment Synthesis Panel of the United Nations Environmental Programme (UNEP), the hole in the Antarctic ozone layer is due primarily to increases in chlorine- and bromine-containing chemicals in the atmosphere. Even though most of the ozone loss is due to chlorinated compounds (90-95%, Watson et al., 1992), attention has been focused more recently on MeBr because stratospheric bromine is believed to be 40 times more efficient than chlorine in breaking down ozone on a per atom basis (Wofsy et al., 1975). Although the largest effects from ozone-depleting gases have been observed in the southern hemisphere, there are indications that atmospheric ozone is also decreasing in the northern hemisphere. |
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| There is a great deal of uncertainty in estimates of the global MeBr budget. In the early 1990s, the ocean was viewed as a net source of MeBr. More recent global balances account for larger sinks than sources (Yvon-Lewis and Butler, 1997), with the ocean acting as a net sink of MeBr, the magnitude of which is being refined (Lobert et al., 1997; King et al., 2000). Soil fumigation is thought to contribute 32 Gg y-1 of MeBr to the atmosphere (~20% of the total MeBr source) (Yvon-Lewis and Butler, 1997). The oceans represent the largest known source of atmospheric MeBr, followed by fumigation (Butler, 2000). Other natural sources of atmospheric MeBr include biomass burning and production by plants, salt marshes, and fungi (Butler, 2000). In recent global budgets, only 60% of the MeBr sinks were accounted for by the quantified source terms and the "missing source" outweighed all other sources in the budget (Butler, 2000). Agricultural use of MeBr, including soil fumigation, may be responsible for 3-10% of stratospheric ozone depletion (USDA, 2001). The relative significance of each global source of MeBr, including that from agricultural uses, needs to be better quantified to assist in developing rational national and worldwide policy. |
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Economic Concerns |
| An economic assessment conducted by the U.S. Department of Agriculture indicated that the phase out of MeBr as a fumigant will have a substantial impact on many commodities because currently alternative control practices are either less effective or more expensive than MeBr (NAPIAP, 1993; Ferguson and Padula, 1994). A recent estimate of the annual economic loss to U.S. producers and consumers resulting from a ban of agricultural uses of MeBr is $500 million (Carpenter et al., 2000). In addition, at present there does not exist a single chemical alternative which can completely replace MeBr (Anderson and Lee-Bapty, 1992; Duafala, 1996). Under these circumstances, MeBr has become the topic of many heated discussions, and the 'methyl bromide issue' has received widespread attention (Anonymous, 1994; Noling and Becker, 1994; Taylor, 1994; Sauvegrain, 1995; Butler, 1995, 1996; Duafala, 1996; Thomas, 1996). The many unanswered questions have also stimulated extensive research on the environmental fate of MeBr, particularly on estimating the relative contribution from each source, obtaining accurate estimates for volatilization losses from fumigated fields, understanding the processes and factors that affect the volatilization, and identifying and developing emission-reduction techniques. |
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| The USDA National Agricultural Pesticide Impact Assessment Program conducted one of the first assessments of the economic impact of eliminating MeBr (NAPIAP, 1993). This assessment determined that there would be a substantial adverse impact on the agricultural community and would be most strongly felt in two states, California and Florida, the primary users of MeBr. It was estimated that a MeBr phase-out for pre-plant soil fumigation would cause $1.5 billion dollars in annual lost production in the United States. This estimate ignored post-harvest, non-quarantine uses and quarantine treatments of imports and other future economic aspects such as lost jobs, markets, etc. The report predicted that the major crop losses would occur with tomatoes ($350 M), ornamentals ($170M), tobacco ($130M), peppers ($130M), strawberries ($110M) and forest seedlings ($35M). |
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| More recently, the National Center for Food and Agricultural Policy (NCFAP) conducted an assessment of the economic implications of the methyl bromide ban (Carpenter et al., 2000). The NCFAP estimates a smaller economic loss of $479M to producers and consumers with the ban of preplant uses of methyl bromide. These losses are due to decreases in yield with use of alternative pest control strategies, increased production costs, changes in the marketing window in response to supply and demand, etc. The NCFAP estimates that losses of $235M may occur in annual crops (tomatoes, strawberries, peppers, etc.), $143M in perennials (orchards and grapes), and $101M in nurseries and ornamentals. As research on methyl bromide alternatives continues to progress and regulatory issues surrounding soil fumigation take effect, the economic impact of the methyl bromide ban will become more clearly defined. |
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Plant Production of Methyl Bromide |
| Production of MeBr by plants has been demonstrated in laboratory studies (Saini et al., 1995; Gan et al., 1998a). Floating plant leaf disks on solutions containing high concentrations of halide ions resulted in evolution of MeBr and other methyl halides (MeI, MeCl) (Saini et al., 1995). The capability of plant leaves to methylate halide ions and release methyl halides appears to be common among higher plants. A high proportion of species tested demonstrated the capability to methylate halide ions, with the family Brassicaceae being particularly efficient (Saini et al., 1995). Plants may contain a variety of methyl transferase enzymes that effect this reaction. Intact plants grown in soil containing BrG were also observed to produce MeBr. In a study by Gan et al. (1998a), several species from the Brassicaceae family were grown in soil containing 0.4 to 100 mg/kg BrG. Production of MeBr per gram of plant mass was correlated to the soil BrG concentration. No MeBr production was observed in BrG-treated soil without plants. No MeBr was observed in microcosms from which the aboveground plant mass was removed, indicating that MeBr production and/or release could not be caused by plant roots only. Because low concentrations of BrG are ubiquitous in soils, terrestrial plants may be an important component of the MeBr cycle. |
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| Flux of MeBr and other methyl halides from rice paddies was measured by Redeker et al. (2000). Results indicated production of MeBr by rice paddies, although the study did not include a plant-free control, so it was not verified that plants were responsible for MeBr production. The flux of MeBr depended on the rice growth stage, and maximum MeBr flux occurred during heading and flowering. Plots in which the rice straw was incorporated into the soil had increased soil organic matter content and increased MeBr production. At one site, MeBr production was proportional to soil BrG concentrations. Muramatsu and Yoshida (1995) also observed production of methyl halide (methyl iodide) by rice plants, with seasonal variation in MeI production. Unplanted soil controls also produced measurable MeI, but both flooded and unflooded soils produced less MeI than the treatments containing rice plants. |
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| Gan et al. (1998a) estimated a global production level of MeBr by rapeseed and cabbage plants to be 6.6 ± 1.6 Gg/yr and 0.4 ± 0.2 Gg/yr, respectively. Rice paddies may contribute approximately 1.3 Gg/yr (Redeker et al., 2000). These values are of the same order of magnitude as some anthropogenic sources of MeBr (Yates et al., 1998). It seems likely that the contribution of plants is important in the MeBr cycle, although it is often not accounted for in current MeBr budgets. |
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