Dr. Montzka and colleagues reported in 2018 that worldwide CFC-11 emissions were rising, following years of steady decline as new production was outlawed and most emissions came from existing foams as they aged. If the emissions increase continued, they said, that would slow progress in restoring the ozone layer, which protects living organisms, including humans, from harmful solar radiation.

The 2018 study suggested the source of most of the rogue emissions was East Asia. Additional research this year pinpointed the source more closely, suggesting that at least 40 to 60 percent of the increased emissions were coming from eastern China. Several investigations, including one by The New York Times, found evidence that factories in that part of China were making the gas or using it to make insulating foams.

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The Montreal Protocol, which is generally considered the most successful environmental pact in history, was drawn up in response to research showing how chlorofluorocarbons, once widely used as refrigerants and propellants, were harming the ozone layer. (Because they are greenhouse gases, they also contribute to global warming.)

Through requirements for monitoring and phasing out production of the chemicals, the pact has led to an increase in stratospheric ozone, with a full recovery of the ozone layer expected by the middle of the century.

Mr. Zaelke said the new findings showed that “the treaty regime is continuing to do its job and we’re seeing a response.”