Following the banning of most ozone depleting molecules concentrations have started to drop, allowing the ozone layer to start to heal. However, NASA has announced that one of these, Carbon tetrachloride, is being found in unexpectedly large quantities and no one knows why.

When the Montreal Protocol was signed in 1987 the major concerns were chlorofluorocarbons (CFCs), and the Protocol dictated the production of most of these must be cut back to 25% of 1986 levels by 1994, with production ending two years later. Other molecules, considered less dangerous, were given longer time-lines.

Carbon tetrachloride, CCl 4 , was given its own section. Its use in refrigeration and lava lamps was banned. However, CCl 4 is still used in scientific instruments and to make other, less damaging refrigerants.

Nevertheless, the quantities of CCl 4 in the atmosphere were expected to be dropping, along with all the other chemicals largely banned for more than 20 years. Instead, NASA has found signs worldwide emissions remain at 39,000 tonnes a year, 30% of their pre-treaty level.

"We are not supposed to be seeing this at all," says Dr Qing Liang of the Goddard Space Flight Center. "It is now apparent there are either unidentified industrial leakages, large emissions from contaminated sites, or unknown CCl 4 sources."

The good news is that the Protocol has been so successful with most of the other chemicals it targeted that the overall trend is down, leading to a slow restoration of the ozone layer. Nevertheless, in 2008 11% of the chlorine ripping the ozone layer apart came from CCl 4 , and that percentage will rise unless the source can be found and stopped.

Although persistent in the atmosphere CCl 4 is eventually destroyed by radiation and chemical reactions. For some time atmospheric scientists debated whether it was being removed more slowly than models suggest, or still being emitted. However, in Geophysical Research Letters Liang and her colleagues used the differences in concentrations between the hemispheres to demonstrate that both are the case. CCl 4 stays in the atmosphere 40% longer than was thought, as well as being emitted at rates we cannot account for.

Goddard’s Dr Paul Newman warns against complacency. "People believe the emissions of ozone-depleting substances have stopped because of the Montreal Protocol," he says. "Unfortunately, there is still a major source of CCl 4 out in the world."