Paul Fraser, an honorary fellow at Australia's CSIRO Climate Science Centre and co-author of the paper said while eastern China accounts for about half of the rise in CFC-11, global scientists don't have the technology in place to monitor large parts of the rest of the world.
"CFCs are the main culprit in depletion of the stratospheric ozone layer, which protects us from the Sun's ultra-violet radiation", said lead author Matt Rigby, an atmospheric chemist at the University of Bristol.
Before it was phased out CFC-11, or Chlorofluorocarbon-11, was widely used in the 1970s and 1980s as a refrigerant and to make foam insulation. However, past year scientists discovered the pace of that slowdown slowed by half between 2013 and 2017. It is assumed that CFCs have been used illegally in the manufacture of new products.
China ratified the treaty in 1991.
The concentration of CFC-11 gas in the atmosphere has declined significantly since the mid-1990s, though researchers have noticed a slowdown in that decline since then.
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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 past year 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.
China launched a special campaign to inspect 3,000 foam manufacturers across the country a year ago and promised to punish any violations of the Montreal treaty.
"We did not find evidence of increased emissions from Japan, the Korean peninsula or any other country", added Luke Western, a post-doctoral researcher at the University of Bristol.
The findings also has implications for the fight against climate change. They also showed that this increase comes mainly from the north-eastern provinces of Shandong and Hebei, and concluded that they are probably the result of new CFC-11 production. The source of the emissions remained unknown, however, sparking concerns that it could hamper years of worldwide effort to fix the protective ozone layer.