Why Is a Long-Banned Chemical Still Choking the Ozone Layer?

More than 20km above Earth's surface is a layer of ozone enveloping our planet. It's a key part in what makes this place habitable. That layer of ozone absorbs UV rays from the Sun like a layer of gaseous sunscreen, offering some protection from radiation that can cause health issues ranging from cataracts to cancer.

Since the 1980s, scientists have been tracking growing holes in this protective ozone layer, and while the use of ozone-depleting chemicals have declined in recent years, one damaging chemical is stubbornly sticking around. No one knows why.

That chemical is carbon tetrachloride (CCl4) and, according to a new study from NASA, it remains overly abundant in our ozone layer. "We are not supposed to be seeing this at all," said NASA atmospheric scientist Qing Liang in a statement.

Carbon tetrachloride was once widely produced to make refrigerants and propellants for aerosol cans, and as a solvent for oils, fats, lacquers, varnishes, rubber waxes, and resins. It's been used as a grain fumigant, a dry cleaning agent, and as a combustion inhibitor in fire extinguishers. Also: the stuff in lava lamps. Consumer uses have waned in recent years, though it's still used in industry. CCl4 is pretty bad for humans.

The immediate symptoms of exposure to carbon tetrachloride include headache, weakness, lethargy, nausea, and vomiting. Longer term exposure can lead to acute symptoms affecting the liver, kidneys, and central nervous system. Information on CCl4's human health impacts is somewhat limited, but the Environmental Protection Agency notes that studies on animals have shown its ingestion increases the risk of liver cancer. The EPA considers it a "probable" human carcinogen.

With these effects in mind, countries around the world have been monitoring their outputs of the gas; between 2007 and 2012, NASA notes that most nations reported zero CCl4 emissions. But satellite, weather balloon, aircraft, and surface-based measurements all tell a different story.

Liang and colleagues started with data on CCl4 levels gathered by NOAA and NASA instruments. The team then entered that data into NASA's 3-D GEOS Chemistry Climate Model computer program, a program that takes into account the way CCl4 is broken apart by solar radiation and how much is absorbed by the Earth and oceans.

The study led by Liang was not only unable to find the source of the excess CCl4; the results found that worldwide emissions of CCl4 averaged 39 kilotons each year. That's about 30 percent of peak emissions before the Montreal Protocol, an international agreement to ban ozone-destroying gases like chlorofluorocarbons, went into effect in 1987.

Those results mean that levels of CCl4 aren't going down as rapidly as they should in the wake of the Protocol's signing. With zero emissions internationally, the level of CCl4 should have dropped by about four percent per year, according to a NASA release. Instead, the decline has been closer to one percent per year.

Carbon tetrachloride, unfortunately, is a less well known issue than the more common chlorofluorocarbons, so it's not something we tend to think about, at least compared to the notorious CFCs (chlorofluorocarbons). "Nevertheless," said Liang, "CCl4 is a major ozone-depleting substance. It is the third most important anthropogenic ozone-depleting compound behind CFC-11 and CFC-12."

It's possible there are some unknown industrial leakages causing the persistence of CCl4. Or, perhaps we just don't know enough about the chemistry of CCl4 yet; Liang's model shows that CCl4 lingers in the atmosphere 40 percent longer than expected.

It's certainly a bit of a mystery, and because we happen to need that ozone layer to survive, it's one we should probably try to solve sooner rather than later.