High above Earth, more than 20 miles above sea level, a diaphanous layer of ozone surrounds our planet, absorbing energetic UV rays from the sun. It is, essentially, sunscreen for planet Earth. Without the ozone layer, we would be bathed in dangerous radiation on a daily basis, with side effects ranging from cataracts to cancer.
People were understandably alarmed, then, in the 1980s when scientists noticed that manmade chemicals in the atmosphere were destroying this layer. Governments quickly enacted an international treaty, called the Montreal Protocol, to ban ozone-destroying gases such as CFCs then found in aerosol cans and air conditioners. On September 16, 1987, the first 24 nations signed the treaty; 173 more have signed on in the years since.
Fast forward 27 years. Ozone-depleting chemicals have declined and the ozone hole appears to be on the mend. The United Nations has called the Montreal Protocol “the most successful treaty in UN history.” Yet, despite Montreal’s success, something is not … quite … right.
A new study by NASA researchers shows that a key ozone-depleting compound named carbon tetrachloride (CCl4) is surprisingly abundant in the ozone layer.
“We are not supposed to be seeing this at all,” says NASA atmospheric scientist Qing Liang.
Between 2007 and 2012, countries around the world reported zero emissions of CCl4, yet measurements by satellites, weather balloons, aircraft, and surface-based sensors tell a different story. A study led by Liang shows worldwide emissions of CCl4 average 39 kilotons per year, approximately 30 percent of peak emissions prior to the international treaty going into effect.
In the 1980s, chlorofluorocarbons became well-known to the general public. As the ozone hole widened, “CFC” became a household word. Fewer people, however, have heard of CCl4, once used in applications such as dry cleaning and fire-extinguishers.
“Nevertheless,” says Liang, “CCl4 is a major ozone-depleting substance. It is the 3rd most important anthropogenic ozone-depleting compound behind CFC-11 and CFC-12.”
Levels of CCl4 have been declining since the Montreal Protocol was signed, just not as rapidly as expected. With zero emissions, abundances should have dropped by 4% per year. Instead, the decline has been closer to 1% per year.
To investigate the discrepancy, Liang and colleagues took CCl4 data gathered by NOAA and NASA and plugged it into a NASA computer program, the 3-D GEOS Chemistry Climate Model. This sophisticated program takes into account the way CCl4 is broken apart by solar radiation in the stratosphere as well as how the compound can be absorbed and degraded by contact with soil and ocean waters. Model simulations pointed to an unidentified ongoing current source of CCl4.
“It is now apparent there are either unidentified industrial leakages, large emissions from contaminated sites, or unknown CCl4 sources,” says Liang.
Another possibility is that the chemistry of CCl4 might not be fully understood. Tellingly, the model showed that CCl4 is lingering in the atmosphere 40% longer than previously thought. “Is there something about the physical CCl4 loss process that we don’t understand?” she wonders.
It all adds up to a mystery in the ozone layer.