By Amy Gunia
May 23, 2019

A new study has determined that the mysterious source of ozone-destroying gases that have increased in the atmosphere recently is eastern China.

Last year, scientists discovered that chlorofluorocarbons, more commonly known as CFCs, were increasing despite an international ban on their use, according to CNN. The findings of a team of scientists from across the world who used computer models to determine where the gases were being emitted, will be published in Nature on Thursday, reports CNN.

“We used our models to show that emissions of CFC-11 from north eastern China had increased by around 7,000 tons per year after 2013, particularly in or around the provinces of Shandong and Hebei,” said Luke Western, a University of Bristol atmospheric modeler, according to a statement on the Scripps Institution of Oceanography, where several of the researchers hailed from.

CFCs, which were invented in 1928 and commercially used as refrigerants and in aerosol cans, are highly damaging to the earth’s ozone layer over Antarctica. In 1987, the U.S. and about two-dozen other countries signed the Montreal Protocol, which agreed to phase out the use of CFCs. China ratified the treaty in 1991. According to CNN, a global ban on the use of CFCs has been in place since 2010.

China’s Ministry of Foreign Affairs has not yet commented on the study, according to CNN.

The scientists say that their work is not done.

“It is now vital that we find out which industries are responsible for the new emissions,” Matt Rigby, a lead author of the study, said in the Sripps Institution statement.

The ozone layer is a layer in the earth’s atmosphere that protects the earth from ultraviolet radiation. In recent years, scientists have said that the ozone layer was starting to show signs of healing.

Write to Amy Gunia at amy.gunia@time.com.

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