Emissions of one of the chemicals most responsible for the Antarctic ozone hole are on the rise, despite an international treaty that required an end to its production in 2010, a new NOAA study shows.

Trichlorofluoromethane, or CFC-11, is the second-most abundant ozone-depleting gas in the atmosphere and a member of the family of chemicals most responsible for the giant hole in the ozone layer that forms over Antarctica each September. Once widely used as a foaming agent, production of CFC-11 was phased out by the Montreal Protocol in 2010.

The new study, published today in Nature, documents an unexpected increase in emissions of this gas, likely from new, unreported production.

"We're raising a flag to the global community to say, 'This is what's going on, and it is taking us away from timely recovery from ozone depletion,'" said NOAA scientist Stephen Montzka, lead author of the paper, which has co-authors from CIRES, the UK, and the Netherlands. "Further work is needed to figure out exactly why emissions of CFC-11 are increasing and if something can be done about it soon."

CFCs were once widely used in the manufacture of aerosol sprays, as blowing agents for foams and packing materials, as solvents, and as refrigerants. Though production of CFCs was phased out by the Montreal Protocol, a large reservoir of CFC-11 exists today primarily contained in foam insulation in buildings, and appliances manufactured before the mid-1990s. A smaller amount of CFC-11 also exists today in chillers.

Because CFC-11 still accounts for one-quarter of all chlorine present in today's stratosphere, expectations for the ozone hole to heal by mid-century depend on an accelerating decline of CFC-11 in the atmosphere as its emissions diminish -- which should happen with no new CFC-11 production.

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Despite the increase in CFC-11 emissions, its concentration in the atmosphere continues to decrease, but only about half as fast as the decline observed a few years ago, and at a substantially slower rate than expected. This means that the total concentration of ozone-depleting chemicals, overall, is still decreasing in the atmosphere. However, that decrease is significantly slower than it would be without the new CFC emissions.

Precise measurements of global atmospheric concentrations of CFC-11 made by NOAA and CIRES scientists at 12 remote sites around the globe show that CFC-11 concentrations declined at an accelerating rate prior to 2002 as expected. Then, surprisingly, the rate of decline hardly changed over the decade that followed. Even more unexpected was that the rate of decline slowed by 50 percent after 2012. After considering a number of possible causes, Montzka and his colleagues concluded that CFC emissions must have increased after 2012. This conclusion was confirmed by other changes recorded in NOAA's measurements during the same period, such as a widening difference between CFC-11 concentrations in the northern and southern hemispheres -- evidence that the new source was somewhere north of the equator.

Measurements from Hawaii indicate the sources of the increasing emissions are likely in eastern Asia. More work will be needed to narrow down the locations of these new emissions, Montzka said.

The Montreal Protocol has been effective in reducing ozone-depleting gases in the atmosphere because all countries in the world agreed to legally binding controls on the production of most human-produced gases known to destroy ozone. Under the treaty's requirements, nations have reported less than 500 tons of new CFC-11 production per year since 2010. CFC-11 concentrations have declined by 15 percent from peak levels measured in 1993 as a result.

That has led scientists to predict that by mid- to late-century, the abundance of ozone-depleting gases would to fall to levels last seen before the Antarctic ozone hole began to appear in the early 1980s.

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However, results from the new analysis of NOAA atmospheric measurements show that from 2014 to 2016, emissions of CFC-11 increased by more than 14,000 tons per year to about 65,000 tons per year, or 25 percent above average emissions during 2002 to 2012.

To put that in perspective, production of CFC-11, marketed under the trade name Freon, peaked at about 430,000 tons per year in the 1980s. Emissions of this CFC to the atmosphere reached about 386,000 tons per year at their peak later in the decade.

These findings represent the first time emissions of one of the three most abundant, long-lived CFCs have increased for a sustained period since production controls took effect in the late 1980s.

If the source of these emissions can be identified and mitigated soon, the damage to the ozone layer should be minor. If not remedied soon, however, substantial delays in ozone layer recovery could be expected, Montzka said.

David Fahey, director of NOAA"s Chemical Science Division and co-chair of the United Nations Environment Programme's Ozone Secretariat 's Science Advisory Panel, said ongoing monitoring of the atmosphere will be key to ensuring that the goal of restoring the ozone layer is achieved.

"The analysis of these extremely precise and accurate atmospheric measurements is an excellent example of the vigilance needed to ensure continued compliance with provisions of the Montreal Protocol and protection of the Earth's ozone layer," Fahey said.