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Dr. James Butler 5/22/97New, possibly biological, oceanic mechanisms are involved in removing methyl bromide -- an ozone-depleting chemical -- from the atmosphere, suggests two recent research papers published by scientists at the National Oceanic and Atmospheric Administration.
Based on this research, these scientists also believe that the atmospheric lifetime of methyl bromide is shorter than was previously estimated. Bromine, a chemical derived from methyl bromide, is estimated to be about 50 times more effective in depleting stratospheric ozone than the principal ozone-destroying chemical, chlorine.
The most recent study, published in the May 15, 1997 issue of Geophysical Research Letters by Shari Yvon-Lewis and James Butler of NOAA's Climate Monitoring and Diagnostics Laboratory in Boulder, Colo., incorporates previously reported data on presumably biological removal mechanisms into a global model of gas exchange between the ocean and atmosphere. The study suggests that the atmospheric lifetime of methyl bromide is even shorter than previously predicted. Butler, principal investigator of these studies, stated, "It now appears that loss of atmospheric methyl bromide to the ocean is just about as fast as oxidation in the atmosphere, which historically has been considered the predominant removal mechanism for methyl bromide."
In the earlier paper, scientists conducted a study of the high latitude waters near Antarctica and the atmosphere above them, determining that the ocean there removes methyl bromide from the atmosphere. It had previously been suggested that these waters, unlike most of the open ocean, could be a large source of methyl bromide. These results, published in the Jan. 15, 1997 issue of Geophysical Research Letters by Juergen Lobert from NOAA's CMDL, and co-authors, indicated that some mechanisms other than the known chemical removal processes were also responsible for degrading methyl bromide in seawater.
Yvon-Lewis and Butler said that according to their calculations, the lifetime of methyl bromide in the atmosphere is about 0.7 years. The calculated lifetime of methyl bromide has gradually declined from a high of 2.0 years in 1992 to the present figure as research in this topic has progressed. The researchers looked at 40 years of wind speed and ocean temperature measurements across the globe, as well as recent measurements of methyl bromide concentrations from research cruises and laboratory studies, to reach this conclusion.
Unlike the chlorofluorocarbons (CFCs) and halons, which are entirely man-made, methyl bromide has both natural and man-made sources. Major human-related sources of this gas include biomass burning and fumigation of soils, produce and buildings. It appears that fumigation emissions may constitute 20-30% of all methyl bromide produced, but further research is being conducted to refine this estimate. Although there is some uncertainty, studies suggest that man-made methyl bromide is responsible for 3-10% of global stratospheric ozone destruction. It has been identified as a Class I ozone-depleting substance and its production is to be phased out in the near future according to current international agreements.