Banned by the Montreal Protocol in 1987, CFC-11 was seen to be declining as expected but that fall has slowed down by 50% since 2012.
Officially, production of CFC-11 is supposed to be at or near zero - at least, that is what countries have been telling the United Nations body that monitors and enforces the Protocol. A smaller amount of CFC-11 also exists today in chillers.
But in the last few years, it looks like someone has started cheating.
"Emissions today are about the same as it was almost 20 years ago", he said.
Another key question is whether there could be another explanation for a slower decline in CFC-11 post-2012, such as a change in the rate of chemical processes such as UV photolysis that break down CFC-11 in the stratosphere, or an increase in emissions from CFC "banks" - reservoirs that persist in old equipment and products that are still in use. But the data just didn't match up.
Study: banned ozone-destroying gas may still be in production
"In the end, we concluded that it's most likely that someone may be producing the CFC-11 that's escaping to the atmosphere". "We don't know why they might be doing that and if it is being made for some specific goal, or inadvertently as a side product of some other chemical process". Together, this analysis suggested the emissions are coming from east Asia.
If the issue is tackled now, the damage will be minor, Montzka says. This, in turn, will delay the ozone layer's recovery, and in the meantime leave it more vulnerable to other threats. However, in 1987, an worldwide team of scientists proved that the emissions of such chemicals were actually harming the environment, particularly the ozone layer.
As of now, the study points to the chemical originating from Eastern Asia, but that was the maximum information researchers could gather from the samples collected. They found that the difference in CFC-11 concentrations between the northern and southern hemispheres has been increasing, which points to a northern hemisphere source. When the researchers examined measurements from atop Mauna Loa in Hawaii, they found that other industrial emissions are also increasing.
But "continued increase in global CFC-11 emissions will put that progress at risk". Under the treaty's requirements, nations have reported less than 500 tons of new CFC-11 production per year since 2010.
The study, Atmospheric Chemistry and Physics, identified two possible culprits: industrial chemicals not covered by the Montreal Protocol called "very short-lived substances" (VSLSs), or climate change, which would be far more hard to resolve.
"It's disappointing, I would not have expected it to happen", said Dr Michaela Hegglin from Reading University, UK, who was not involved in the study.
However, according to Durwood Zaelke, an expert on the Montreal Protocol, the massive quantities of CFC-11 indicates that someone is acting in defiance of the ban.More news: India looks to revive ties with North Korea