Large Quantities of Carbon tetrachloride Sill Lingering in the Atmosphere: NASA

NASA said Wednesday that Carbon tetrachloride (CCl4) - a chemical phased out in recent years - is still lingering in large amounts in the atmosphere.

The chemical was earlier used in dry cleaning as well as a fire extinguisher agent. In 1987, CCl4 along with other chlorofluorocarbons were regulated under the Montreal Protocol. Countries that signed the Protocol reported zero new CCl4 emissions between 2007 and 2012.

CCl4 and chlorofluorocarbons are known to play an important role in the ozone hole over Antarctica.

NASA researchers recently found that CCl4 emissions worldwide have reached an average of 39 kilotons per year. The amount is about 30 percent of the peak emissions before the Protocol was signed.

"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, according to a news release. "It is now apparent there are either unidentified industrial leakages, large emissions from contaminated sites, or unknown CCl4 sources."

Scientists at 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 worked on the current research.

For the study, the researchers used 3-D GEOS Chemistry Climate Model and data from ground-based observations.

Several ozone depleting chemicals have been phased out. The researchers say that the CCl4 in the atmosphere isn't enough to alter the rate at which ozone hole is growing. Still, the team wants to know why CCl4 is lingering in the environment. The recent study challenges some common assumptions about the chemical. The researchers had previously believed that solar radiation and other natural processes break down this chemical.

"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 in a news release.

The study is published in the journal Geophysical Research Letters.