The currently hopeful progress in removing ozone-damaging gases from the atmosphere may stall early in the new millennium, according to new research. Writing in the 22 April issue of
Much of the decline in atmospheric halocarbon concentration can be put down to the removal of just one halocarbon, trichloroethane, once widely used as a cleaning solvent. The bad news is that beneath the marked drop in trichloroethane, the concentrations of other important halocarbons have remained static or are even rising. In a few years, when trichloroethane has been all but completely removed from the atmosphere, upward trends in other halocarbons will dampen the overall rate of decline of halocarbons in the atmosphere.
Halocarbon gases have been used for many years in refrigeration, as aerosol propellants, in the plastics industry, in dry-cleaning and in fire extinguishers. Because they are rather reluctant to get involved in chemical reactions, they were thought to be perfectly safe. But this inertness carried a sting. Once in the atmosphere, halocarbons tend to hang around, and drift up to the stratosphere, where they catalyze reactions with the chemically highly reactive ozone layer, approximately 22 kilometres above our heads. This ozone abosrbs harmful ultraviolet light from the Sun: some people have suspected that there may be a connection between increased incidences of skin cancer, and increased UV flux as a result of damage to the ozone layer.
These ozone-damaging reactions are especially marked in springtime in the polar regions. During the long, sunless winter, halocarbon derivatives accumulate to high concentrations in high, icy stratospheric clouds. The first rays of the spring sunshine provide energy to fuel reactions in which ozone is depleted almost to extinction by reacting with a massive halocarbon dose. This explains why ozone depletion is most marked in the Arctic and Antarctic.
The realization that halocarbons were causing serious damage to the ozone layer led to the Montréal Protocols. Signed in 1987, and amended several times since, this pledge to phase out the production and use of ozone-damaging halocarbons represented perhaps the single most hopeful evidence for international collaboration to solve global environmental problems. Montzka and colleagues report the good news that by 1997, ten years after Montréal, the overall burden of halocarbons in the atmosphere had dropped by 3% from its peak in 1993–1994. But they then lay out the bad news, that most of this decline relates to trichloroethane, global emissions of which are now almost nil.
The researchers single out several halocarbons for concerted international effort. One of these is halon-1211, widely used in fire-fighting equipment. Like all halons, this substance contains bromine as well as the chlorine and fluorine found in CFCs. Bromine is 50 times more effective than chlorine in ozone-depleting reactions, making halons more of a worry than many CFCs.
Halon-1211 is used increasingly in countries such as Korea and China, and now accounts for more than 95% the world production of halons. Emissions of halon-1211 were thought to have peaked in the 1980s, but they still seem to be going on, and may even be rising. Indeed, the most recent estimates suggest that emissions are 50–60% greater than predicted from global production data. This suggests either that more halon-1211 is being produced than pople thought, or, once produced, it is leaking out existing fire-fighting equipment, despite stringent regulations on its use.
Faced with the new challenges posed by halons, the world is not standing by and wringing its hands – the Uneited Nations Environment Program is looking into an accelerated phasing-out of halon-1211 production in China, with production ending in 2006, four years ahead of the Montréal schedule. Montzka and colleagues emphasize the urgency: if emissions of halon-1211 and other halocarbons apart from trichloroethane continue at 1997 rates, the halocarbon burden would stop decreasing in 2010. This scenario is not expected if all the signatories of the Montréal Protocols perservere, but the job is far from over.
Overall, though, researchers are cautiously optimistic. The long residence times of halocarbons in the atmosphere mean that the threat to the ozone layer will not disappear overnight, but the problems have been identified and are, to an extent, being addressed.