Chlorofluorocarbon-11 (CFC-11) is a powerful ozone-depleting chemical that plays a major role in the appearance, each spring, of the ozone "hole" over Antarctica.
Although atmospheric levels of CFC-11 have fallen since the protocol was signed, United States scientists revealed past year that the rate at which it is declining appears to be slowing down.
"The study represents a particularly policy-relevant milestone in atmospheric scientists' ability to tell which regions are emitting ozone-depleting substances, greenhouse gases, or other chemicals, and in what quantities", said co-author Professor Ray Weiss from the University of California San Diego.
Concentrations in the atmosphere fell steadily from 2002 to 2012, but the decline from 2012 onwards has been 50% slower than expected, suggesting an increase in emissions.
"In recent decades, we've primarily seen declining CFC emissions reflected in these measurements because of the Montreal Protocol", Matt Rigby, the study's lead author, said in a statement. Because the chemical does not occur in nature, the change could only have been produced by new emissions.
There were indications that some region in eastern Asia was still emitting thousands of tonnes of CFC-11, but the exact location was not known.
A report a year ago by the UK-based non-profit Environmental Investigation Agency, said it had collected evidence from 18 companies in 10 Chinese provinces that showed use of CFC-11 is "widespread and pervasive" in factories that make insulation foam for buildings. The new study uses data collected every two hours by the AGAGE global monitoring network, including data from Gosan, South Korea, and from an AGAGE-affiliated station at Hateruma, Japan.
The team also ran computer simulations that confirmed the origin of the CFC-11 molecules.
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China continues to emit an increasing amount of ozone depleting chlorofluorocarbons (CFCs), despite a global initiative to phase out CFCs under the Montreal Protocol, aggravating the climate crisis, scientists have found.
"It is vital that we find out which activities and industries are responsible for the new emissions".
The findings also have implications for the fight against climate change.
"If emissions do not decline, it will delay the recovery of the Antarctic ozone hole, possibly for decades", said Paul Fraser, an honorary fellow at the CSIRO Climate Science Centre in Australia.
CFCs are also potent greenhouse gasses, with one tonne of CFC-11 equivalent to about 5,000 tonnes of CO2.
At its most depleted, around the turn of the 21st century, the ozone layer had declined by about five per cent.
And it's slowing down the rate of recovery for the hole in the crucial ozone layer.
Up to now, CFCs and other molecules have mainly eroded ozone in the upper stratosphere, and over the poles.