Emissions of a banned gas from China, which is damaging the earth’s ozone layer, have plummeted after several years, two teams of scientists said Wednesday, a sign that the Beijing government had honored its vows to combat the illegal production of the industrial chemical.

The results alleviate concerns that increased emissions of the CFC-11 gas would slow progress in the decade-long environmental struggle to repair the ozone layer, which filters out ultraviolet rays from the sun that can cause skin cancer and damage plants.

“We’re seeing a huge drop in both global emissions rates and emissions from east China,” said Stephen A. Montzka, research chemist with the National Oceanic and Atmospheric Administration and lead author of one of the studies. Works by Dr. Montzka and others first exposed illegal emissions three years ago.

“It looks like there has been a significant response, possibly because we hoisted a flag and said, ‘Hey, something is not happening the way it should,” said Dr. Montzka.

Matthew Rigby, an atmospheric chemist at the University of Bristol in England and author of the second study, said if emissions hadn’t gone down, we could see years of ozone recovery delay. A full recovery is expected by the middle of the century.

Chinese government officials did not immediately respond to requests for comment.

Chemical traders in Shandong, a highly industrialized province in east China where CFC-11 was widely used to make insulation foams, said trade in the banned gas had largely dried up. “It hasn’t completely gone, but it’s much rarer than before,” said Gao Shang, a chemical distributor in Shandong, in a telephone interview.

CFC-11 was banned a decade ago under the Montreal Protocol, the treaty that was signed in the 1980s, when research revealed its effects on atmospheric ozone, as well as the effects of similarly widespread chemicals.

The revelation in a 2018 study of rogue emissions from China that began five years earlier came as a shock to scientists, policymakers, environmentalists and others overseeing the protocol, which is largely viewed as the most effective environmental deal in history.

Meg Seki, acting executive secretary of the ozone secretariat, the United Nations agency that administers the treaty, said the organization was pleased to see emissions have decreased and the impact on the ozone layer likely to be limited. “However, it is important to prevent such unexpected emissions in the future through continuous high-level monitoring by the scientific community,” she said in a statement.

The 2018 study did not identify the source of most emissions other than that they came from East Asia. However, research by the Environmental Investigation Agency, an independent advocacy group based in Washington, DC, and the New York Times earlier this year revealed evidence that the gas is still being produced and used in eastern China, particularly Shandong.

One of Dr. Atmospheric analysis conducted by Rigby in 2019 found that Shandong, as well as a neighboring province, Hebei, were important sources.

When first confronted with the evidence, Chinese environmental agencies backed the results and expressed doubts, suggesting that there might be other, undetected sources of the chemical or that foam insulation manufacturers might not be using as much CFC-11.

At the same time, the Chinese Ministry of Ecology and Environmental Protection promised “zero tolerance” for companies that illegally manufacture or use CFC-11.

Political announcements, industry reports, and court rulings suggest that the Chinese government cracked down on the illegal trade, despite repeated denial that there was ever a serious problem. Last year, the government published the conviction of a businessman, Qi Erming, as the first case in China for criminal prosecution for illegal trafficking in ozone-depleting chemicals.

In addition to law enforcement, the government tightened regulations and oversight of the chemical and foam industries, promising to put in place a comprehensive data system to track the movement of chemicals that could be used to make CFC-11.

There are legal gases that can replace CFC-11 in foam production. Mr. Gao, the chemical dealer in Shandong, said his company specializes in one of them.

The availability of substitutes may have helped China’s efforts to reduce CFC-11 emissions. Zhu Xiuli, sales manager at another company in Shandong that sells foam concentrate, said customers had previously asked if they had CFC-11. But “there have been fewer and fewer requests in recent years,” she said.

CFC-11 has also been used in refrigerators. As the gear ages and as foams containing CFC-11 degrade, the gas is slowly released. Although the exact size of this “bank” of CFC-11 is not known, it is recorded in the protocol and is one of the reasons full ozone recovery will take decades.

The new publications, published in Nature magazine, also do not take into account the total global increase in CFC-11 emissions since 2013. The gas may still be produced or used in other countries or other parts of China, but the researchers said that there aren’t enough air sampling stations around the world to know for sure.

“This is a useful lesson that we really need to expand our monitoring capabilities,” said Dr. Rigby.

Avipsa Mahapatra, a climate campaign leader for the Environmental Investigation Agency, said of the new findings that it was “exciting to see atmospheric studies that confirm that on-site intelligence and subsequent enforcement resulted in a spectacular climate victory”. But she said her group has evidence that enforcement may have been more successful in some parts of China than others. “This is not the time for complacency,” she said.

Susan Solomon, an atmospheric chemist at the Massachusetts Institute of Technology who was not involved in the research, said the work was “a real triumph for science”.

But the problem isn’t over yet, said Dr. Solomon, because in addition to CFC-11, other, similar chemicals are also emitted. “There’s a whole zoo of molecules,” she said, and while the amounts are smaller, they add up.

They’re also powerful greenhouse gases, she said, though their contribution to warming is much less than that of the widely used heat scavenger gases like carbon dioxide and methane. “The chemical industry around the world is still not being monitored closely enough to be really sure how much greenhouse gases they are producing and how much ozone-depleting gases they are producing,” she said.

Liu Yi contributed to the research.