Methyl bromide (CH3Br) is an ozone-depleting trace gas in the atmosphere that for the past decade has been the focus of considerable scientific and political controversy. This controversy stems from the role of CH3Br in stratospheric ozone depletion and its toxicity to humans, contrasted with its value as an agricultural fumigant. Although CH3Br has been employed in a variety of applications, including fumigation of buildings, preservation of grains in silos, and treatment of fresh fruits, vegetables, or timber for export, its main use continues to be fumigation of soils (see article by Batchelor, this issue). Methyl bromide is particularly effective because it attacks a broad spectrum of pests, including weeds, nematodes, insects, fungi, bacteria, and some parasitic plants [Klein, 1996]. Because it penetrates the soil quickly, requires only a short period of exposure, and dissipates rapidly from the soil, it is ideal for pre-planting applications. Crop yield is considerably enhanced following such treatment and, unlike treatment with persistent pesticides, no organic residue is left in the soil to be concentrated through the food web. Much of the use of CH3Br is for annual crops such as strawberries, melons, tobacco, flowers, and a variety of vegetables, but it is also effective in treating soils before planting fruit trees and grape vines.

Methyl bromide at high concentrations is toxic to humans. This has given rise to objections from farm workers and communities located near areas of its application. Efforts to tarp the soils and apply CH3Br only during specific meteorological conditions have been made to reduce human exposure, but this is not always completely successful, nor is it always met with enthusiasm from nearby residents. Recent research suggests that employment of virtually impermeable film (VIF) tarps during application can reduce the emission of CH3Br to the atmosphere to a few percent of that applied ([Yates et al., 1998]; see article by Yates et al., this issue). Deeper application into wetter soils can also reduce the flux of CH3Br from the soil.

Despite this immediate health controversy, it is methyl bromide's role in depleting stratospheric ozone that has led to the planned, global phase-out of its anthropogenic production and use. Methyl bromide is responsible for over half of the bromine reaching the stratosphere [Schauffler et al., 1999], although not all of this is anthropogenic. Because stratospheric bromine is so efficient in depleting ozone (about 50 or 60 times that of chlorine on a per-atom basis), 10 pmol mol-1 (parts-per-trillion) of CH3Br represents a chlorine equivalent on par with any one of the major CFC's at current atmospheric levels. However, unlike the CFCs and all other compounds being phased out by the Montreal Protocol and its amendments (e.g., UNEP, 1997), most CH3Br in the atmosphere is emitted by natural processes and will continue to be emitted by these processes after fumigant emissions cease. The questions being addressed by the scientific community at this time are: (1) How much of the CH3Br in the atmosphere is man-made? (2) What is the anticipated change in atmospheric mole fraction with change in emissions? (3) Where is CH3Br in the atmosphere coming from? (4) Where is it going? and (5) How will natural fluxes be altered in the face of global change?

The most recent estimates of the global budget for CH3Br cannot account for almost half of its sources ([Kurylo et al., 1999; Yvon-Lewis and Butler, 1997]; see article by Yvon-Lewis, this issue). Uncertainties on at least one of the budget terms, the soil sink, are large, but it is difficult to devise scenarios to balance the budget that do not require including additional, unidentified sources. Reaction of CH3Br with tropospheric OH [Mellouki et al., 1992] and its irreversible loss to the ocean [Butler, 1994; Yvon-Lewis and Butler, 1997], two of the major loss terms, are reasonably well constrained. Recent studies have narrowed the calculated gap between sources and sinks, but a significant deficit of sources remains, amounting to about 30% of the identified sinks.

One of the difficulties in assessing natural, predominantly biological influences upon an atmospheric gas is that organisms or ecosystems can simultaneously consume or produce the gas (see article by Crill, this issue). They can do this much faster than the gas is exchanged with the atmosphere. This is clearly evident for CH3Br, which is produced and degraded at fast rates in the oceans, produced and degraded by terrestrial plants, and perhaps produced and degraded in soils and sediments. Thus a measured net flux of the gas only tells part of the story. Although the net flux of a gas can be small, it may result from the near balance of two large, but opposite, gross fluxes, as evidenced by the exchange of CH3Br (or CO2, for that matter) between the atmosphere and ocean. The net flux also can be large and dominant, and thus approximate a gross flux to or from the atmosphere. The relative roles of these production and loss processes are at the core of the last question posed above: Just how might these production and loss processes be affected by some significant global change? The most obvious of possible global changes is, of course, temperature, but it also could be the amount, type and distribution of precipitation, or the dramatic reformation, replacement or loss of ecosystems. It is unlikely that any change, however widespread, will affect the rates of production and loss equally.

Methyl bromide's role in ozone depletion depends not only upon the amount of bromine it delivers to the stratosphere, but also upon the amount of chlorine present, as the removal of ozone by bromine involves the reaction of BrO with ClO. With plenty of chlorine available in the stratosphere in the future [Montzka et al., 1996; Montzka et al., 1999], increases or decreases in the total natural flux of CH3Br could still have a significant effect on stratospheric ozone or the timing of the recovery of the ozone hole. What is important now is to try to understand the cycling of CH3Br in nature, how its fluxes might change in the future, and what effect this might have upon the atmosphere.

In this issue of IGACtivities, articles are presented addressing the current status of our understanding of the behavior of CH3Br in nature and the potential for its use or replacement as a fumigant in the future. Shari Yvon-Lewis of NOAA's Atlantic Oceanographic and Meteorological Laboratory summarizes the state of our knowledge on CH3Br in the atmosphere and oceans. Patrick Crill of the University of New Hampshire discusses the effects of natural terrestrial systems upon atmospheric CH3Br and the implications of some of the most recent findings in this field. Scott Yates of the USDA and the University of California at Riverside provides a synopsis of the fate of CH3Br in soils following application and the factors controlling its emission. And Tom Batchelor, Co-Chair of the UNEP Methyl Bromide Technical Options Committee, gives us some insight into potential replacements for CH3Br, their uses and limitations.

1. Butler, J.H., The potential role of the ocean in regulating atmospheric CH3Br, Geophys. Res. Letts., 21, 185-188, 1994.
2. Klein, L., Methyl bromide as a soil fumigant, in: The Methyl Bromide Issue, edited by C.H. Bell, N. Price, and B. Chakrabarti, pp. 191-235, John Wiley and Sons Ltd., West Sussex, UK, 1996.
3. Kurylo, M.J., J.M. Rodriguez, M.O. Andreae, E.L. Atlas, D.R. Blake, J.H. Butler, S. Lal, D.J. Lary, P.M. Midgley, S.A. Montzka, P.C. Novelli, C.E. Reeves, P.G. Simmonds, L.P. Steele, S. W.T., R.F. Weiss, and Y. Yokouchi, Short-lived ozone-related compounds, in Scientific Assessment of Ozone Depletion: 1998, edited by C.A. Ennis, World Meteorological Organization, Geneva, Switzerland, 1999.
4. Mellouki, A., R.K. Talukdar, A. Schmoltner, T. Gierczak, M.J. Mills, S. Solomon, and A.R. Ravishankara, Atmospheric lifetimes and ozone depletion potentials of methyl bromide (CH3Br) and dibromomethane (CH2Br2), Geophys. Res. Letts., 19, 2059-2062, 1992.
5. Montzka, S.A., J.H. Butler, J.W. Elkins, T.M. Thompson, A.D. Clarke, and L.T. Lock, Present and future trends in the atmospheric burden of ozone-depleting halogens, Nature, 398, 690-694, 1999.
6. Montzka, S.A., J.H. Butler, R.C. Myers, T.M. Thompson, T.H. Swanson, A.D. Clarke, L.T. Lock, and J.W. Elkins, Decline in the tropospheric abundance of halogen from halocarbons: Implications for stratospheric ozone depletion, Science, 272, 1318-1322, 1996.
7. Schauffler, S.M., E.L. Atlas, D.R. Blake, F. Flocke, R.A. Lueb, J.M. Lee-Taylor, V. Stroud, and W. Travnicek, Distributions of brominated organic compounds in the troposphere and lower stratosphere, J. Geophys. Res., 104, 21,513-21,535, 1999.
8. UNEP, Report of the Ninth Meeting of the Parties to the Montreal Protocol on Substances that Deplete the Ozone Layer (Montreal), United Nations Environmental Programme, New York, 1997.
9. Yates, S.R., D. Wang, J. Gan, F.F. Ernst, and W.A. Jury, Minimizing methyl bromide emissions from soil fumigation, Geophys. Res. Letts., 25, 1633-1636, 1998.
10. Yvon-Lewis, S.A., and J.H. Butler, The potential effect of oceanic biological degradation on the lifetime of atmospheric CH3Br, Geophys. Res. Letts., 24, 1227-1230, 1997.