Scientists have solved an international environmental mystery by pinpointing the source of a troubling uptick in a dangerous, ozone-destroying chemical: factories in northeastern China.

Why it matters: The Montreal Protocol on Substances that Deplete the Ozone Layer is viewed as the most successful environmental treaty ever enacted. However, its success depends on rigorous monitoring and enforcement, particularly regarding the ban on the production and use of ozone-depleting substances from developing countries since 2010.

According to the new study, published Wednesday in Nature, many factories in northeastern China have flouted the ban when it comes to a compound known as trichlorofluoromethane (CFC-11), which is an efficient destroyer of Earth's protective ozone layer and a far more powerful — albeit less abundant — global warming pollutant than carbon dioxide.

CFC-11 is used to produce foam insulation for refrigerators, air conditioners and buildings, among other uses.

Context: Using data from Mauna Loa Observatory in Hawaii, a 2018 study found that the global concentrations of CFC-11 were not declining at the predicted rate. It also found an uptick in emissions of the synthetic compound and suggested that East Asia was a possible source region.

Subsequent research by the New York Times and the Environmental Investigations Agency (EIA) blamed illicit CFC-11 production and use on the growing Chinese polyurethane market.

However, observational data was too sparse to conclusively show where in East Asia the source was.

What they did: For the new study, researchers utilized high-frequency environmental observations from South Korea, Japan and other areas to investigate CFC-11 emissions patterns.

They also used multiple computer models to trace weather patterns that corresponded to spikes in CFC-11 levels at particular stations, which showed that air blowing from two provinces in northeastern China — Shandong and Hebei —were tied to spikes in CFC-11 concentrations.

What they found: Emissions of CFC-11 from eastern mainland China were about 7 million kilograms per year higher from 2014 to 2017 when compared to 2008 to 2012, the study finds.

This amounts to at least 40%–60% of the global increase in CFC-11 emissions observed from previous research, but this may be an underestimate, the study says.

The study shows the emissions are most likely coming from new production, rather than slow emissions from prior stockpiles.

The big picture: New production and emissions of CFC-11 could delay the healing of the stratospheric ozone layer, which protects us from harmful ultraviolet radiation from the sun, by at least a decade.

Stratospheric ozone loss can lead to increased skin cancer rates and harm wildlife, particularly in high latitudes.

The threat: Each molecule of CFC-11 stays in the atmosphere for 52 years, according to the study. "With no emissions, it still lingers on and on and on," says co-author Ron Prinn of MIT.

What they're saying: Avipsa Mahapatra of the EIA tells Axios the new study is "critical because it provides precise scientific measurements that a large magnitude of the global emissions of CFC-11 that were detected before are indeed coming from China and even pinpoints specific provinces in China." Mahapatra was not involved in the new study.

But, but, but: The lack of observing stations to sift through the air for evidence of banned CFCs in China, parts of Africa and elsewhere blinds scientists to possible emissions sources that could also harm the ozone layer.

What's next: It's now up to China to crack down on producers and users of this banned substance, experts tell Axios. "It's up to the Chinese government to fulfill their requirements under the Montreal Protocol," Prinn says.