In a feat of atmospheric detective work, researchers have uncovered evidence that thousands of metric tons of an ozone-destroying chemical are still being released into the air every year, despite a global ban on production (Nature 2018, DOI: 10.1038/s41586-018-0106-2).
Their investigation traced trichlorofluoromethane (a chlorofluorocarbon, CFC-11) back to east Asia, and concluded that it was probably emitted by new, unreported production facilities. “If it continues at this rate, it will delay ozone recovery by about a decade,” says Stephen A. Montzka of the National Oceanic and Atmospheric Administration, who led the research.
CFC-11 is one of a suite of halocarbons that are controlled under the Montreal Protocol on Substances That Deplete the Ozone Layer, widely hailed as being the most effective environmental treaty in history. When the protocol came into force in 1989, it set a timetable to phase out various industrial halocarbons used as refrigerants, solvents, and propellants. In Earth’s stratosphere, these compounds release halogen atoms that catalyze the breakdown of ozone, thinning a vital atmospheric shield that filters out most of the sun’s harmful ultraviolet radiation. Current estimates suggest that the protocol’s restrictions should help the concentration of stratospheric ozone to return to 1980 levels by the middle of this century.
CFC-11 was widely used to help expand foams during manufacturing, and emissions peaked in the late 1980s at about 350,000 metric tons per year. The protocol stipulated that developed countries should stop producing CFC-11 by 1996, while developing countries had until 2010. Global reported production has been close to zero since 2006.
But with an average lifetime of 57 years in the atmosphere, CFC-11 remains the second most abundant ozone-depleting substance, after dichlorodifluoromethane (CFC-12). Meanwhile, foams already in place still contain a hefty reservoir of CFC-11, estimated at about 1.4 million metric tons in 2008, which is gradually leaking into the air.
Montzka’s team used gas chromatography and mass spectrometry to measure the concentration of CFC-11 in air samples taken from 12 remote sites around the globe including Antarctica, the northern tip of Canada, Ireland,and Australia. Then they poured these data into computational simulations that included climate and atmospheric chemistry models.
The team calculated that the atmospheric concentration of CFC-11 declined at a steady 2 parts per trillion every year from 2002 to 2012. That in itself is unexpected—the decline should have accelerated over that time, given that the foam reservoir is shrinking and production was supposedly being scaled back. The reason, the team concludes, is that CFC-11 emissions remained fairly constant over that time, at around 54,000 metric tons per year.
The real shock came during the period from 2014 to 2016, when they found that the decline had slowed to just 1 part per trillion each year. The implication is that emissions must have risen to about 67,000 metric tons per year. “It’s a quite astounding finding,” says Michaela I. Hegglin of the University of Reading. “It’s extremely important to have these measurements so that we can go and check that people are adhering to the regulations.”
Air samples from Hawaii offered the researchers other important clues. They found that in 2013, plumes of air containing higher levels of CFC-11 than before began to arrive from east Asia. The CFC-11 concentrations changed in lockstep with other industrial chemicals, including chlorodifluoromethane (HCFC-22) and dichloromethane, suggesting that the chemicals were all emitted from production facilities rather than the foam reservoir. The researchers also found significantly higher concentrations of CFC-11 in the northern hemisphere, where more industrial sites are located, compared with the southern hemisphere.
Montzka’s team did consider other factors that could affect CFC-11 concentrations, such as circulation changes in the atmosphere, but these could not explain the scale of the apparent rise in emissions.
So why would anyone restart CFC-11 production? Montzka notes that developing countries were due to start reducing hydrochlorofluorocarbon (HCFCs) production in 2013, so it’s possible that CFC-11 could be making up a shortfall. “But it’s pretty much speculation at this point,” he says.
Montzka has been talking with researchers in Japan, China, and South Korea about his findings for the past couple of years, and says they are making local air measurements that could narrow down the origin of the emission to particular cities—and, if there is only one factory in that city that produces halocarbons, finger a potential culprit.
Sunyoung Park of Kyungpook National University has preliminary results from an air sampling station on Jeju Island, off the southern tip of the Korean Peninsula, which show a similar spike in CFC-11 concentrations from late 2012. Based on air flow modelling, she believes that the emissions are coming largely from mainland China, particularly along the Yangtze River.
That sort of evidence could help Montreal Protocol signatory states to apply pressure on Chinese authorities to shut down unauthorized production. “Let’s narrow in and see if we can pinpoint these sources,” says Montzka.
UPDATE: On May 18, 2018, the story was updated with preliminary results from Sunyoung Park.