To protect Earth’s ozone layer, most uses of carbon tetrachloride were banned years ago. According to reports filed by countries around the world, emissions of this ozone-depleting substance were virtually nil since 2007. But researchers now estimate that some 39,000 metric tons of the chemical were released each year from 2000 to 2012.
“We are not supposed to be seeing this,” says Qing Liang, a NASA atmospheric scientist who led an international team that made the finding (Geophys. Res. Lett. 2014, DOI: 10.1002/2014gl060754). “There are either unidentified industrial leakages, large emissions from contaminated sites, or unknown CCl4 sources.”
All countries that are members of the United Nations agreed through the Montreal Protocol on Substances That Deplete the Ozone Layer to phase out uses of CCl4 that lead to emissions, such as in fire extinguishers. Under that treaty, the compound is still legally manufactured, mainly as a feedstock for producing hydrofluorocarbons, but in lower volumes than in decades past. Small amounts of CCl4 can be used for pharmaceutical and solvent applications, but emissions from these are reported under the Montreal protocol.
CCl4 is destroyed by photolysis in the stratosphere and degrades in oceans and soils. Relying on data about the rate of these processes, scientists expected atmospheric concentration of the compound to fall by 4% per year once all but the legally allowed CCl4 releases ended. Yet observations show that level has declined by only about 1% annually.
Liang’s team used atmospheric measurements and computer modeling to conclude that the observed atmospheric levels of CCl4 were mainly from ongoing emissions.