Washington, Aug 21 : NASA has discovered large amount of an ozone-depleting compound from an unknown source decades, after the compound was banned worldwide, in the Earth's atmosphere.

The research shows worldwide emissions of carbon tetrachloride (CCl4) average 39 kilotons per year, which was once used in applications such as dry cleaning and as a fire-extinguishing agent.

The compound was regulated in 1987 under the Montreal Protocol along with other chlorofluorocarbons that destroy ozone and contribute to the ozone hole over Antarctica, but approximately 30 percent of peak emissions are prior to the international treaty.

Qing Liang, an atmospheric scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and lead author of the study said that they are not supposed to be seeing this at all and 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.

The scientists said that 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 study was published online in the Geophysical Research Letters. (ANI)