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One of the ozone layer’s most destructive attackers - bromine - is decreasing in the lower atmosphere, according to researchers at the Commerce Department’s National Oceanic and Atmospheric Administration (NOAA). They attribute the decline of total bromine primarily to international restrictions on industrial production of methyl bromide.
In a study to be published in the Aug. 15 issue of Geophysical Research Letters, Stephen A. Montzka and colleagues from NOAA’s Climate Monitoring and Diagnostics Laboratory in Boulder, Colo., say that total bromine in the lower atmosphere has been decreasing since 1998 and is now more than 5 percent below its peak.
“The decrease is driven by a large and rapid decline in methyl bromide, a brominated gas that is regulated internationally by the Montreal Protocol,” said Montzka. The surprisingly large drop in atmospheric methyl bromide about 13 percent since 1998 has more than offset the small increases still observed for bromine from fire-extinguishing agents known as halons. Bromine is about 50 times more efficient at destroying stratospheric ozone than chlorine, another potent ozone destroyer.
“This is welcome news for stratospheric ozone because it means that less bromine and chlorine have been entering the upper atmosphere (stratosphere), where the ozone layer resides, for a number of years now,” said Montzka. Furthermore, while chlorine’s decline in the lower atmosphere had been slowing in recent years, these new data suggest that the overall threat posed to stratospheric ozone from all halogenated gases continues to steadily diminish.”
Methyl bromide is produced industrially for use as a fumigant in agriculture and in the shipment of commercial goods. It is unique among ozone-depleting substances regulated by the Montreal Protocol, however, it also has substantial natural sources including the oceans, wetlands, some plants, and burning of vegetation.
Global industrial production of methyl bromide has declined in recent years in response to restrictions outlined in the amended Montreal Protocol, say the researchers. The Montreal Protocol, which limits production of ozone-damaging compounds, was originally signed by the United States and 22 other nations in 1987, and has been strengthened through revisions and amendments since then.
The NOAA scientists were able to discern this reversal in the long-term upward trend for bromine based on their ground-based measurements of methyl bromide and halons over the past eight years at 10 stations across the globe, including Cape Grim, Tasmania; the South Pole; Mauna Loa, Hawaii; and Barrow, Alaska. Methyl bromide and halons together account for nearly all of the human-released bromine that reaches the stratosphere. These results are an example of the unique benefits a comprehensive coordinated global observing system provide, as envisioned at the recent U.S. sponsored Earth Observation Summit held in Washington, D.C.
Before this study, changes in total atmospheric bromine were quite uncertain because published global trends of methyl bromide were available only through 1991. These new measurements dramatically improve understanding of how bromine levels are changing in the global atmosphere allowing to conclude, for the first time, that levels of bromine are decreasing in the lower atmosphere across the entire globe.
The decrease observed for bromine is largely compared to the declines documented previously for chlorine; with this new result, the authors were able to determine that the overall atmospheric burden of ozone-depleting gases is declining faster than thought previously. International efforts to reduce industrial production of methyl bromide have had a noticeable impact on overall atmospheric levels of ozone-depleting substances.
This good news must be tempered, however, because bromine from halons is still increasing slowly. The surprisingly large decline observed for methyl bromide now dominates the overall trend for bromine.
International restrictions on production of chlorofluorocarbons and related gases have already resulted in atmospheric decreases of the ozone-depleting agent chlorine since 1994. These declines in bromine are a significant additional step forward in the recovery of the stratospheric ozone layer. Full recovery is still expected to take several decades, provided atmospheric levels of both bromine and chlorine continue to drop. Stratospheric ozone forms an invisible shield around the Earth, protecting it from the biologically damaging ultraviolet rays of the sun.
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