Reduction in the atmosphere of a single chemical, methyl chloroform, is primarily responsible for the overall decline of ozone-depleting substances observed in the atmosphere during the past five years. Because methyl chloroform will soon become a much smaller part of the remaining ozone-depleting substances due to its rapid natural disintegration, further reductions in the threat to the ozone layer would have to come from reductions in other ozone-depleting gases, say researchers at the Commerce Department's National Oceanic and Atmospheric Administration.

In a paper appearing in the current issue of Nature, scientists at NOAA's Climate Monitoring and Diagnostics Laboratory in Boulder, Colo., say that ozone-depleting chemicals have declined about 3 percent overall since 1994, mainly due to the loss of methyl chloroform from the atmosphere. Methyl chloroform, a chemical previously used as a cleaning solvent, is destroyed much more rapidly in the atmosphere by natural processes than other ozone-depleting substances. Lead author Stephen A. Montzka says that "based on our understanding of atmospheric removal processes, the level of methyl chloroform will diminish to such an extent over the next 5-10 years that it will no longer influence the overall amount of ozone-depleting substances in the atmosphere."

Restrictions on the production of ozone-depleting chemicals are outlined in the Montreal Protocol, an international agreement that limits the production of ozone-damaging chemicals. These restrictions are designed to ensure that the threat posed by ozone-depleting substances diminishes over the next 50 years and will allow a return to "pre-ozone hole" levels sometime in the middle of the next century.

While the concentrations of some ozone-depleting substances (CFC-11, CFC-113, CCL4) are decreasing slightly in response to the Montreal Protocol, the authors say that the global amount of a few chemicals continues to increase in the atmosphere despite international regulation. Levels of the most abundant and persistent CFC (CFC-12), a bromine compound (halon-1211), and the chemicals used as replacements for CFCs (the hydrochlorofluorocarbons or HCFCs) continue to increase in the atmosphere.

Despite the very low concentrations of halon-1211, the researchers have determined that continued increases of this gas are slowing the collective decline of ozone-depleting chemicals in the atmosphere more than any other persistent man-made gas. Halons have a significant influence on stratospheric ozone because they contain bromine, which is about 50 times more efficient at destroying ozone than the chlorine released by CFCs. Considering this enhanced efficiency, all halons account for about 10-15% of the ozone-depleting potential of today's atmosphere.

To discover the root causes of these trends, the authors computed emissions of ozone-depleting substances based on observations from NOAA's network of air-sampling sites around the globe. The results suggest that during the past 10 years, emissions of most chemicals have diminished substantially as a result of the Montreal Protocol. Emissions of the two most common CFCs, CFC-11 and CFC-12, have diminished to between one-half and one-third of the amount released during the late 1980s, which were the peak years. Despite these reductions, concentration increases continue for CFC-12 because it is extremely persistent in the atmosphere.

"Emissions of halon-1211 have not decreased despite the ban on production in developed countries since 1994," Montzka says. Some of the continued release of this chemical results because a substantial reservoir or "bank" exists in fire extinguishers that are still in use today in many developed countries. Production figures from the United Nations Environment Programme suggest that accelerated production of this halon in developing countries also accounts for a large portion of the continued increase.

Hydrochlorofluorocarbons (HCFCs) are also increasing because they are used as interim replacements for ozone-depleting substances. But, the authors say, these chemicals are less efficient at destroying stratospheric ozone than CFC-12 and the halons, and at present, only account for about 2 percent of the ozone-depleting substances in the atmosphere.

"Progress towards reducing the threat to stratospheric ozone posed by CFCs and related gases is evident in measurements of air throughout the globe, and continuation of this trend would ensure the eventual recovery of the ozone layer," Montzka says. "However, a sustained reduction of ozone-depleting agents will be realized only if use and release of these substances continues to decline in future years."

According to David Hofmann, director of NOAA's Climate Monitoring and Diagnostics Laboratory, "continued monitoring of atmospheric levels of ozone-depleting gases will be necessary to assess our progress towards recovery of the ozone layer as mandated by the Protocol."