Earth's atmosphere still contains large amounts of ozone depleting chemical CC14
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The Earth's atmosphere contains an unexpectedly large amount of an ozone-depleting compound banned over 25 years ago.
Nasa scientists have admitted they are baffled by the discovery of Carbon tetrachloride (CCl4), which was once used in applications such as dry cleaning and as a fire-extinguishing agent.
It was regulated in 1987 under the Montreal Protocol along with other chlorofluorocarbons that destroy ozone and contribute to the ozone hole over Antarctica.
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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.
Parties to the Montreal Protocol reported zero new CCl4 emissions between 2007-2012.
However, the new research shows worldwide emissions of CCl4 average 39 kilotons per year, approximately 30 percent of peak emissions prior to the international treaty going into effect.
'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 the study.
'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 percent of chlorine available for ozone depletion, which is not enough to alter the decreasing trend of ozone-depleting substances.
Still, scientists and regulators want to know the source of the unexplained emissions.
For almost a decade, scientists have debated why the observed levels of CCl4 in the atmosphere have declined slower than expectations, which are based on what is known about how the compound is destroyed by solar radiation and other natural processes.
'Is there a physical CCl4 loss process we don't understand, or are there emission sources that go unreported or are not identified?' Liang said.
With zero CCl4 emissions reported between 2007-2012, atmospheric concentrations of the compound should have declined at an expected rate of 4 percent per year.
Observations from the ground showed atmospheric concentrations were only declining by 1 percent per year.
In addition to unexplained sources of CCl4, the model results showed the chemical stays in the atmosphere 40 percent longer than previously thought.
The research was published online in the Aug. 18 issue of Geophysical Research Letters.
'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 Space Flight Center, and a co-author of the study.
'Unfortunately, there is still a major source of CCl4 out in the world.'
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