Banned substance still depleting ozone layer: NASA
Carbon tetrachloride (CCl4), which was once used in applications such as dry cleaning and as a fire-extinguishing agent, was regulated in 1987 under the Montreal Protocol along with other chlorofluorocarbons that destroy ozone.
Parties to the Montreal Protocol reported zero new CCl4 emissions between 2007-12.
However, new research suggests 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. It is now apparent there are either unidentified industrial leakages, large emissions from contaminated sites or unknown CCl4 sources," said Qing Liang, an atmospheric scientist at NASA's Goddard Space Flight Centre in Greenbelt, Maryland.
"Is there a physical CCl4 loss process we do not understand or are there emission sources that go unreported or are not identified?" Liang asked.
With zero CCl4 emissions reported between 2007-12, 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 one percent per year," Liang added.
To investigate the discrepancy, Liang and colleagues used NASA's 3-D GEOS Chemistry Climate Model and data from global networks of ground-based observations.
In addition to unexplained sources of CCl4, the model results showed the chemical stays in the atmosphere 40 percent longer than previously thought.
"People believe the emissions of ozone-depleting substances have stopped because of the Montreal Protocol. Unfortunately, there is still a major source of CCl4 out in the world," said Paul Newman, a chief scientist for atmospheres at NASA's Goddard Space Flight Centre.
The research was published in the journal Geophysical Research Letters.