The mystery of the ozone-depleting chemical leak

Decades after ozone-destroying chlorofluorocarbons were banned, NASA scientists have found one of the compounds is still leaking into the atmosphere from an unknown source.

"We are not supposed to be seeing this at all," said Qing Liang, an atmospheric scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and lead author of a study into the problem. "It is now apparent there are either unidentified industrial leakages, large emissions from contaminated sites, or unknown carbon tetrachloride sources."

Carbon tetrachloride, or CCl4, was once used as a dry cleaning fluid and fire extinguisher, but was banned in 1987 under the Montreal Protocol when it was found it and other chlorofluorocarbons were contributing to the ozone hole over Antarctica.

There were no new CCl4 emissions were reported between 2007-2012, but the new research shows that now an average 39 kilotons is leaking into the atmosphere a year – 30% of peak emissions prior to the ban.

Scientists have been puzzled why CCl4 levels in the atmosphere have declined so slowly.

"People believe the emissions of ozone-depleting substances have stopped because of the Montreal Protocol," said Paul Newman, chief scientist for atmospheres at NASA's Goddard centre, and a co-author of the study. "Unfortunately, there is still a major source of CCl4 out in the world."

NASA has an ozone watch website here.

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