Ozone-depleting compound persists, new research shows

WASHINGTON, D.C. – New research shows Earth’s atmosphere contains an unexpectedly large amount of an ozone-depleting compound from an unknown source decades after the compound was banned worldwide.

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 and contribute to the ozone hole over Antarctica. Parties to the Montreal Protocol reported zero new CCl₄ emissions between 2007-2012.

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. New research shows Earth’s atmosphere contains an unexpectedly large amount of an ozone-depleting compound, carbon tetrachloride, from an unknown source decades after the compound was banned worldwide.
Credit: NASA

However, the new research shows worldwide emissions of CCl₄ average 39 kilotons (about 43,000 U.S. tons) 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 published online in the Aug. 18 issue of Geophysical Research Letters, a journal of the American Geophysical Union. “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.

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. The CCl₄ measurements used in the study were made by scientists at the National Oceanic and Atmospheric Administration’s (NOAA’s) Earth System Research Laboratory and NOAA’s Cooperative Institute for Research in Environmental Sciences at the University of Colorado, Boulder.

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. The results produced the first quantitative estimate of average global CCl₄ emissions from 2000-2012.

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,” 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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For more information about NASA’s Earth science activities in 2014, visit: http://www.nasa.gov/earthrightnow

For information on the Antarctic ozone hole, visit: http://ozonewatch.gsfc.nasa.gov

Nanci Bompey
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NASA Headquarters Contact:
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NASA Goddard Space Flight Center Contact:
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