Falling CFC-11 emissions offer a reprieve for the ozone layer

Global emissions of a key contributor to the depletion of stratospheric ozone fell suddenly between 2018 and 2019, largely driven by reductions from within China, according to new analysis (Nature 2021, DOI: 10.1038/s41586-021-03260-5; Nature 2021, DOI: 10.1038/s41586-021-03277-w). These findings mark the reversal of a previously observed rise in the atmospheric concentrations of trichlorofluoromethane, known as CFC-11, a member of the chlorofluorocarbon (CFC) family of potent ozone-depleting molecules.

CFC-11 was commonly used as a propellant gas in aerosol cans, a refrigerant in air conditioners, and as an insulating agent in the construction industry. But after researchers realized, in the 1980s, that the growing hole in the stratospheric ozone layer was caused by CFCs, 197 countries ratified the Montreal Protocol on Substances That Deplete the Ozone Layer—an international agreement that outlawed CFC production after 2010. Many hailed the treaty as a success that would heal the ozone layer, restoring its ability to absorb the sun’s harmful ultraviolet rays.

That’s why it came as a surprise when Stephen A. Montzka of the National Oceanic and Atmospheric Administration and colleagues reported in 2018 that annual emissions of CFC-11 actually increased after 2012. A 2019 study estimated that 40–60% of this global rise in CFC-11 emissions came from China.

“If the rise in emissions had been sustained, it would have delayed the recovery of the ozone layer,” Montzka says. In the two new studies, Montzka and colleagues looked at global and Chinese CFC-11 emissions and now report that this outcome appears to have been avoided.

“These studies are a good news story in relation to the 2018 papers when it was first flagged that CFCs were on the rise,” says Martyn Chipperfield, an atmospheric scientist at the University of Leeds who was part of the new global emissions study. “I’m surprised how quickly things have changed again.”

The researchers took ambient air measurements of CFCs from 13 remote field stations dotted around the globe. These measurements allowed them to calculate that average global CFC-11 emissions between 2018 and 2019 fell by 26%, returning to pre-2012 levels.

The results are important, says Martin Vollmer, of the Laboratory for Air Pollution/Environmental Technology at the Swiss Federal Laboratories for Materials Science and Technology (Empa). “They show the world that through atmospheric measurements, it’s possible to validate the emissions that countries are self-reporting and point fingers at any discrepancies,” he says.

Related work focused specifically on data collected from two field stations—one in South Korea and the other in Japan. Computer models used this data to simulate the expected atmospheric-travel patterns of CFC-11 molecules in the region, revealing that around 60% of the observed global decrease in CFC-11 seems to have come from China. Where the rest of the decrease is coming from is hard to know, says study author Luke M. Western, a chemist at the University of Bristol.

“You have to be reasonably close to the source to pinpoint the location. There are large areas of the planet where we don’t know what’s going on, but there are efforts to expand these networks,” Western says.

“It’s a positive outcome and our research has hopefully been a catalyst in bringing this about,” Western says, pointing to the team’s earlier work that first identified the rising CFC-11 emissions in China.

CFC-11 may not be the only ozone-damaging chemical to worry about, Vollmer cautions. In a separate study published last month, he and colleagues detected new and unexpected emissions of three other, less ozone-depleting hydrochlorofluorocarbon (HCFC) chemicals. One, called HCFC-132b, hadn’t been picked up the atmosphere before. But after analyzing archived canisters of air, the team determined that the chemical had in fact first appeared in northern hemisphere during the late 1990s (Proc. Natl. Acad. Sci. 2021, DOI: 10.1073/pnas.2010914118). The researchers concluded that approximately 95% of the world’s HCFC-132b emissions are coming from China, Japan, North Korea, Taiwan, and South Korea. China alone accounts for 50% of global emissions. Bizarrely, no one knows what these chemicals are used for, which makes eliminating them from the atmosphere difficult to tackle.

“It’s a fascinating find but on the other hand, you kind of despair that here’s another thing that humans produce,” Vollmer says. Fortunately, these chemicals are less damaging to the ozone layer than CFC-11 and for now their atmospheric concentrations are also far lower, he says.

These studies underline the need to keep making these types of atmospheric measurements, says Montzka. “The ozone layer is far from fully recovered,” he says. “Vigilance is warranted.”