“When it comes to definitive answers, I think we have to first emphasize that this mystery has yet to be solved,” said Keith Weller, a spokesman for the United Nations Environment Program, which helps organize the ozone layer talks.

The CFC-11 mystery has wide implications. The ozone layer has been healing, but the return of a banned substance is an alarming breach in one of the world’s most effective environmental pacts and could slow the layer’s recovery.

CFC-11 is also a potent greenhouse gas. If it is leaking directly into the atmosphere from factories, even more gas may be held in the products made in those factories — for example, in insulation foam — and may enter the atmosphere when those products are eventually destroyed.

Scientists discovered decades ago that CFC-11 and other manufactured chemicals used as refrigerants and aerosols and in the production of insulating foams were destroying the ozone layer, which shields humans, crops and animals from the most damaging solar rays.

In 1987, countries agreed on the Montreal Protocol to phase out such gases, steadily replacing them with ever safer substitutes. The protocol has been praised as a model environmental initiative.

The Chinese government has said it will investigate and stamp out any illicit production off CFC-11, and Chinese industrial associations have vowed that their businesses will not use the chemical.

Officials announced last month that the police had broken up an illegal CFC-11 plant in Henan, a rural heartland province, and found more 30 metric tons of the chemical on the site, according to an official report.