A new way to measure atmospheric gases could track down sources of CFCs thought to be slowing the recovery of Earth's ozone layer, European researchers say.

CFCs, chlorofluorocarbons, were used in refrigerants and aerosol propellants until restricted by a global treaty in 1987, but they have stayed in the air longer than many expected, the BBC reported Friday.

A team of British and German researchers says it is now possible to chemically "fingerprint" CFCs to potentially trace their origin.

The scientists worked on samples of atmosphere retrieved from 115,000 feet in the stratosphere by French space agency balloons.

Using mass spectrometers, they detailed the ratios of different isotopes of chlorine atoms present in small concentrations of chlorofluorocarbon-12.

The sharp falls in global emissions of CFCs seen in the early years following the signing of the treaty have leveled off, suggesting some chlorofluorocarbons, which should have been exhausted in developed countries by now, are still in use.

"Even though the production and use of CFC-12 is forbidden by the Montreal Protocol, we still find it in the atmosphere," Jan Kaiser of the University of East Anglia said.

The ability to make fine measurements opens the door to chemical fingerprinting -- of being able to tie a particular sample to a known origin.

Such information could help authorities identify continuing sources, the BBC said.