Jan. 5 (UPI) -- New satellite data has offered the first direct evidence of a slowdown in ozone depletion.
Using a special instrument designed by scientists at NASA's Jet Propulsion Laboratory in California, the agency's Aura satellite has measured declines in the concentration of ozone-killing chlorine. The reduction in chlorine has led to a decrease in ozone depletion, allowing a slow but steady recovery of the ozone hole.
In 1976, scientists first showed the use of a class of manmade chemicals called chlorofluorocarbons, found in aerosols, refrigerators and other appliances, was leading to heightened levels of ozone-killing chlorine in the atmosphere.
In 1986, an international ban of CFCs was signed by 46 nations. The latest research -- published this week in the journal Geophysical Research Letters -- suggests the ban is working.
"We see very clearly that chlorine from CFCs is going down in the ozone hole, and that less ozone depletion is occurring because of it," lead study author Susan Strahan, an atmospheric scientist at NASA's Goddard Space Flight Center in Maryland, said in a news release.
Chlorofluorocarbon compounds have a lengthy lifespan. When they rise to the stratosphere, they are broken down by ultraviolet solar radiation, releasing chlorine, which eats away at the ozone layer.
The ozone helps protect Earth and its inhabitants from harmful radiation. In the 1980s, researchers measured worryingly low concentrations of ozone above the poles, confirming the predictions of previous CFC studies. The research spawned the idea that the hole was forming in the ozone layer, and inspired international action.
Previous studies have used measurements of seasonal changes in the size of the ozone hole to gauge impact of the CFC ban. The newest NASA study is the first to survey the chemical composition of the ozone hole and show a reduction in ozone depletion as a result of declining levels of CFCs.
Since 2005, NASA scientists have used the Aura satellite to measure ozone depletion rates during the winter months.
"During this period, Antarctic temperatures are always very low, so the rate of ozone destruction depends mostly on how much chlorine there is," Strahan said. "This is when we want to measure ozone loss."
Scientists were also able to use Aura's instruments to link the decline in ozone depletion with reductions in chlorine. Chlorine is difficult to measure, but when it's run out of ozone to eat, chlorine molecules react with methane to form hydrochloric acid, which is easier to measure. By measuring hydrochloric acid levels each fall, scientists were able to gauge the amounts of chlorine in the ozone hole.
Their analysis showed ozone depletion has declined by 20 percent over the last two decades.
"This is very close to what our model predicts we should see for this amount of chlorine decline," Strahan said. "This gives us confidence that the decrease in ozone depletion through mid-September shown by MLS data is due to declining levels of chlorine coming from CFCs. But we're not yet seeing a clear decrease in the size of the ozone hole because that's controlled mainly by temperature after mid-September, which varies a lot from year to year."
And while CFC levels are in decline, some scientists say other ozone-depleting chemicals are increasing, slowing the ozone's recovery. Last year, scientists at Leicester University measured an increase in concentrations of atmospheric dichloromethane, a short-lived, ozone-depleting substance and a popular substitute for CFCs.