Ozone-depleting compound still being released

NASA researchers have found that Earth’s atmosphere contains an unexpectedly large amount of an ozone-depleting compound, despite the fact that the substance was banned worldwide decades ago. Their work has been published in the journal Geophysical Research Letters.

Carbon tetrachloride (CCl₄), which was once used in applications such as dry cleaning and as a fire-extinguishing agent, is described by the researchers as “a major anthropogenic ozone-depleting substance and greenhouse gas”. It was regulated in 1987 under the Montreal Protocol along with other chlorofluorocarbons that destroy ozone and contribute to the ozone hole over Antarctica.

Parties to the Montreal Protocol reported zero new CCl₄ emissions between 2007 and 2012, which meant atmospheric concentrations of the compound should have declined at a rate of 4% per year. But observations from the ground showed atmospheric concentrations were only declining by 1% per year.

A team led by Dr Qing Liang, an atmospheric scientist at NASA’s Goddard Space Flight Center, used NASA’s 3D GEOS Chemistry Climate Model and data from global networks of ground-based observations to investigate the discrepancy. Model simulations of global atmospheric chemistry and the losses of CCl₄ due to interactions with soil and the oceans pointed to an unidentified ongoing current source of CCl₄, with emissions averaging 39 kilotons per year - approximately 30% of peak emissions prior to the international treaty going into effect.

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Satellites observed the largest ozone hole over Antarctica in 2006. Purple and blue represent areas of low ozone concentrations in the atmosphere; yellow and red are areas of higher concentrations. Image credit: NASA.

“We are not supposed to be seeing this at all,” said Dr Liang. “It is now apparent there are either unidentified industrial leakages, large emissions from contaminated sites or unknown CCl₄ sources.”

As of 2008, CCl₄ accounted for about 11% of chlorine available for ozone depletion - “a pretty major compound”, said Dr Liang. And while this is not enough to alter the decreasing trend of ozone-depleting substances, the model results show that CCl₄ stays in the atmosphere 40% longer than previously thought.

“A longer lifetime means the amount of carbon tet accumulating in the atmosphere in the past decades will be removed from the atmosphere slower than our original expected rate,” said Dr Liang.

“Is there a physical CCl₄ loss process we don’t understand, or are there emission sources that go unreported or are not identified?”

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