Thanks to the Montreal Protocol, which brought the major causes of ozone depletion such as chlorofluorocarbons (CFCs) under control, the ozone hole over Antarctica is healing, and global ozone levels are expected to return to 1980 levels by about 2050. But recent studies have found that a lesser known ozone depleter, carbon tetrachloride, isn’t going away as fast as it should be. The substance persists, scientists now suggest, because an unidentified source is still emitting it into the atmosphere.

“Most of the Montreal Protocol–controlled gases are decreasing in the atmosphere in exactly the way we had anticipated that they would decrease,” says atmospheric chemist John Pyle of the University of Cambridge in the United Kingdom, who was not involved in the new study. “[There may be] leaks into the atmosphere from old refrigerators but essentially they’re all behaving exactly as we would have expected … with the exception of carbon tetrachloride.”

The Montreal Protocol, the 1989 treaty intended to phase out emissions of ozone-destroying chemicals, is widely considered one of the most successful international agreements, with near-global participation. Every year, signatory countries are required to report to the U.N. Environment Programme their imports, exports, and production of eight groups of controlled ozone-depleting chemicals. To keep an eye on how well the controls are working, the World Meteorological Organization (WMO) measures atmospheric levels of these substances every 4 years, as mandated by Article 6 of the Protocol.

But scientists preparing the 2010 WMO assessment noticed a curious thing. The primary ozone depleters—CFCs (previously used as refrigerants, propellants, and solvents) and halons (used in portable fire extinguishers)—are decreasing in the atmosphere just as expected based on countries’ production reports. But another substance, carbon tetrachloride, is lingering in the atmosphere longer than expected from the reported production numbers.

“It’s a nasty little compound,” says Paul Newman, an atmospheric chemist at NASA Goddard Space Flight Center in Greenbelt, Maryland. “It’s toxic, carcinogenic, ozone-depleting, and a greenhouse gas.” It’s also completely humanmade and has been used as a solvent, a cleaning agent, and a chemical feedstock to help synthesize other chemicals such as hydrofluorocarbons (which are potent greenhouse gases but not ozone depleters).

Although developing countries were allowed to delay their phaseout of “carbon tet,” it was banned in 2010. And expected emissions based on reported production and feedstock usage after about 2007 were zero. Models based on those expected emissions numbers and on the lifetime of the compound in the atmosphere estimate that its level should be decreasing by about 4% each year. But the WMO data showed that it was decreasing by only 1%.

One possible explanation for the discrepancy, Newman says, is that scientists have been underestimating how long the molecule sticks around in the atmosphere. The other possibility is that there’s another source sending the compound into the air.

Since the 2010 report, Newman says, “there’s been a big effort to look at the lifetime in the atmosphere of carbon tetrachloride.” It’s known to be at least 25 years—long enough for the gas to distribute evenly around the globe. In that case, measurements of carbon tetrachloride in North America and Australia ought to be equal, assuming there are no new emissions. But they aren’t. The WMO report noted that the amount of the compound was significantly higher in the Northern Hemisphere than in the Southern Hemisphere.

“It turns out we can use the gradient to determine what global emissions ought to be,” Newman says. He and his colleagues ran a number of climate simulations to quantify those emissions. They varied how much of the gas would be absorbed by the ocean or soils and how much might be emitted in each hemisphere, and they considered different estimated lifetimes of the gas. In the end, they simply couldn’t reconcile the observed decline with the reports of zero emissions. Instead, about 31,000 tonnes per year of carbon tetrachloride must still have entered the atmosphere from 2007 to 2012, they report online this week in Geophysical Research Letters.

“If you take a train with 100 tanker cars of carbon tetrachloride derailing once a month, that’s how much is being emitted,” Newman says. “That’s a lot.” The question is “Where’s this stuff coming from? We really don’t know.”

Illegal production of carbon tetrachloride is one possibility—but not the only one, he notes. Another potential source is brownfields, old chemical disposal sites from which the gas is still leaking into the air. Yet another alternative is the ongoing use of carbon tetrachloride as a feedstock to produce other compounds; the process should destroy it, but some could be leaking out of production facilities. “But if there’s a lot of leakage, that’s money [the companies] are losing. We don’t have a good handle on that leakage, but we think it’s small,” Newman says. Even washing machines might be a source. Trace amounts of carbon tetrachloride can be produced by combining sodium hypochlorite and soap. “That would be a very, very tiny amount. But times all the clothes washers in the world, that could be a source also,” Newman says.

Identifying potential sources is tricky because carbon tetrachloride “is kind of an unusual molecule,” says A. R. Ravishankara, an atmospheric chemist at the University of Colorado, Boulder, who was not involved in the new study but who was a co-chair of group that produced the 2010 WMO report (with Newman and Pyle). “It may be being produced in ways that we don’t understand.” The lifetime remains another question mark, Ravishankara says. The new paper removes some uncertainty and estimates an atmospheric lifetime of 35 years rather than 25 years. But Ravishankara says the molecule is different from CFCs in that it “has some removal processes that are kind of weird. It gets a little more difficult.” And, he adds, “it would also help to be more confident about the emissions numbers reported from countries participating in the Montreal Protocol. It would not be such a bad idea to ask for better accounting.”

Newman stresses that his findings do not imply that the Montreal Protocol has failed. Levels of carbon tetrachloride are falling—albeit more slowly than they should—and that’s because of the treaty, he says. And even the fact that scientists are focusing on these additional sources is a victory. “It’s had two successes here: [Carbon tetrachloride] is going down, and we’re looking into what could be an additional source.”

Ravishankara agrees, noting that “the key point is that carbon tetrachloride contributes about 10% to the ozone depletion rate right now, so it’s not like other things are not working.” And, Pyle notes, “the overall story of the Montreal Protocol is still that it’s massively successful. What this paper is doing is saying we now understand [the discrepancy] discussed 4 years ago.”

The next WMO assessment of ozone-depleting gases in the atmosphere is slated to come out in about a month, Ravishankara says, although the study was published too late to be included.