A potent source of ozone-eating gas in Earth's upper atmosphere is showing signs of leveling off. A 20-year study by German scientists has found that the rate of increase of CFC-12, a chlorofluorocarbon compound that spawns ozone-destroying reactions in the stratosphere, has slowed since 1990, although absolute levels are still rising. The study, reported in the 1 September Geophysical Research Letters, credits international treaties designed to protect the ozone layer with turning the tide.
Earth's fragile shield of ozone is under assault by CFCs, substances once used widely as refrigerants, propellants, and foam-blowing agents. CFCs are inert near the surface, but once they drift more than 10 kilometers high into the stratosphere, ultraviolet light splits them apart to release atoms of chlorine. A single chlorine atom then can catalyze the destruction of thousands of ozone molecules. The Montreal Protocol of 1989 and later amendments slashed CFC production worldwide. However, CFCs linger in the air for dozens of years, so researchers knew the ozone layer would take time to heal.
CFC-12 is the longest lived CFC and the source of 30% of the stratosphere's chlorine. Using balloon flights over France and Sweden, Andreas Engel of Johann Wolfgang Goethe University in Frankfurt, Germany, and his colleagues found that CFC-12 increased at an annual rate of 18.5 parts per trillion from 1978 to 1990. Since then, however, the average annual growth rate has slowed to 11.9 parts per trillion. A similar trend has occurred near the ground. If that trend continues, Engel says, "we expect CFC-12 to level off in the stratosphere and then start declining," although that may take several more years. After that, he observes, it will take decades for chlorine to subside to the levels that existed when researchers first detected the Antarctic ozone hole.
"This is the only long-term data set with regular measurements of ozone-destroying compounds in the stratosphere," says atmospheric chemist Darin Toohey of the University of California, Irvine. But he urges continued monitoring of other harmful and long-lived compounds still in use, including halons with bromine that are present in fire extinguishers.