Detectives have sniffed out a big source of an outlawed, invisible, and odorless gas, currently wafting through Earth's atmosphere. This chemical, CFC-11, is illegal everywhere in every country, because it depletes Earth's ozone layer — which protects life from solar radiation.

The detectives are a team of global scientists who exposed specific provinces in eastern China that are responsible for emitting loads of this ozone-killing gas into the atmosphere since 2013. The researchers published a study on Wednesday in the journal Nature, describing how they used detection instruments to sleuth out major sources of this banned chemical, without stepping foot in China.

"The results point to an emissions increase primarily in two provinces of China," said Steve Montzka, a National Oceanic and Atmospheric Administration (NOAA) scientist who has been following the CFC-11 trail. "A big part of the [CFC-11] global emissions can be accounted for."

But, although about half of global emissions of CFC-11 — used to make foam insulation — have almost certainly been produced in the eastern Chinese provinces of Shandong and Hebei, the rest of globe's perpetrators remain unknown, for now. They could be operating in another part of China, Asia, or the opposite side of the globe. "We don’t know where the rest of it is coming from," explained Montzka, a study coauthor. "It could be coming from other regions around the world."

The thinned-out region of ozone over Antarctica. Image: nasa

The evidence against the rogue CFC emitters has been mounting. Last year, Montzka published research that found an "unexpected and persistent increase" in CFC-11, likely from somewhere in Eastern Asia. Separately, the Washington D.C.-based Environmental Investigation Agency (EIA), which investigates environmental crimes and abuse, engaged in an undercover probe to film and record Chinese factories who were using CFC-11, a relatively cheap chemical, to make their goods. To top it off, The New York Times conducted its own investigation, which pointed to culprits at a "scrappy industrial boomtown in rural China."

This latest research essentially seals the deal — for a good chunk of the outlawed emissions, anyhow.

"The paper corroborates what we found on the ground," said Avipsa Mahapatra, the Climate Campaign Lead at the EIA. "These illegal actors were practiced, fly-by-night operators who were circumnavigating their own laws of government."

"It's not like elephant ivory that you can easily see," added Mahapatra, who had no role in the new study. "They’re odorless, colorless gases."

And they promise to slow down the planet's ozone-recovery. Since global nations agreed to ban CFC chemicals in the late 1980s — a landmark climate and environmental treaty called the Montreal Protocol — the ozone layer began to repair itself, after a substantial and worrisome thinning. But with the new rise in CFC-11, by some 7,000 tons (7 Gigagrams) per year since 2013, the recovery will be slowed.

"It means we should immediately find the [exact] source of the emissions," said Sunyoung Park, a study coauthor and researcher at Kyungpook National University's Department of Oceanography in South Korea.

"It certainly won't help the recovery of the ozone layer. The question is how much [will it delay the recovery]," added Ray Weiss, an atmospheric scientist at the Scripps Institution of Oceanography and also a study coauthor.

NOAA's Montzka has estimated the unexpected uptick in CFC-11 will delay the ozone layer's recovery by yet another decade, which already will take at least a half-century to fully restore.

To better pinpoint the source of CFC-11 emissions, the researchers used two atmosphere detecting stations, one on a Korean island, and another on a Japanese island, which sampled the air every two hours for some 10 years. The scientists then employed sophisticated weather models of how winds have swirled over East Asia in recent years to identify where the CFC-11 must have originated. "You can back-calculate where that air is coming from," said Weiss.

"These illegal actors were practiced, fly-by-night operators who were circumnavigating their own laws of government."

The clues pointed to two provinces in Eastern China, though the two monitoring stations could not pick up what was happening in rest of the burgeoning industrial nation, nor places beyond South Korea, Taiwan, and southern Japan.

"This pinpoints a specific region," said the EIA's Mahapatra. "That’s key to the international effort to understand where the emissions are coming from."

And it's key for assisting the Chinese government in catching the polluters. There is no league of international, law-enforcing guardians of the ozone. Under the Montreal agreement, it's up to each nation to stop their CFC emissions. Fortunately, the Chinese have taken action, likely spurred on by last year's surprise detections and resulting media coverage.

"The Chinese have undertaken massive enforcement efforts," said Mahapatra. They investigated over 1,000 enterprises, she added, and have already shut down CFC-11 producers.

This is no easy task in a booming industrial society with a population of over 1.3 billion. There's industry everywhere. Banned chemicals will inevitably be used, whether in bad faith or not.

"It's the product of the enormous growth of China," said Weiss.

Yet the ability of rogue-operators to use CFC is coming to its end. You can't hide CFC-11 from atmospheric scientists.

This scientific sleuthing is critical to enforcing the Montreal Protocol. And this type of scientific verification, Weiss emphasized, is also a model for how researchers can monitor emissions of another odorless, invisible gas that's now saturating the skies at unprecedented rates: the heat-trapping greenhouse gas carbon dioxide, which has driven 18 of the last 19 years to be the warmest on record.

If global nations agree in 2020 to hold themselves accountable for human-generated carbon emissions that are stoking weather extremes around the globe, scientists will need to watch closely — to see who's following the rules, and who isn't.

"We ought to have some numbers. We can't manage things without having numbers," said Weiss.