Peering into the ozone hole

Concentrations of ozone-destroying gases are down, but the Antarctic ozone hole is bigger than ever. It turns out there's more to ozone destruction than just CFCs.

SCIENTISTS HAVE some good news and some bad news for ozone watchers. Concentrations of ozone-depleting chlorofluorocarbons (CFCs) have levelled off in the stratosphere and actually declined in the lower atmosphere, raising hopes for a recovery of the ozone layer. That's the good news. The bad news is that NASA satellites spotted the largest Antarctic ozone hole ever recorded on September 9, 2000, and the effects of global climate change may exacerbate the problem.

Why are we seeing the worst-ever ozone hole when 13 years of regulation are finally bringing CFC levels under control? "The first point is that these processes are really slow," said Dr. Richard McPeters, principal investigator for NASA's Total Ozone Mapping Spectrometer (TOMS) at the NASA Goddard Space Flight Center (GSFC). "It takes a long time for the CFCs to get up into the stratosphere in the first place, so it's going to take a long time for them to come back out," McPeters said.

CFCs released at the ground diffuse upward through the lowest layer of the atmosphere, called the troposphere. The vertical air currents of tropospheric weather help push CFCs up to the next layer, the stratosphere. Once there, CFCs rise more slowly because stratospheric air has less vertical air movement.

In fact, it can take a CFC molecule about 2 years after being released at the ground to make it to the stratosphere where the ozone is. And it can take decades for it to be converted by sunlight into a form that is harmful to ozone, according to Dr. Charles Jackman, an atmospheric modeler at GSFC.

Once a CFC molecule is converted to its destructive form, it can linger in the stratosphere for a few years before it drifts back down into the troposphere in the form of hydrogen chloride (HCl) and is washed out of the atmosphere by rain, Jackman said. In 1994, NOAA scientists first measured a decrease in the amount of CFCs in the lowest layer of the atmosphere. Since these CFCs would eventually work their way up to the stratosphere - where the ozone is - this finding gave hope that CFC concentrations in the stratosphere would also soon begin to decline."It'll be a number of years before you start to see real reductions in the CFCs in the stratosphere," McPeters said. Model calculations suggest that ozone recovery to pre-1980 levels could take 20 to 40 years, he explained. "So it's not something where you'd expect to see a big change this year."

Although the concentration of CFCs in the stratosphere appears to have levelled off, the size of the ozone hole won't necessarily level off with it. "What's happening right now is you have the CFCs at a very high level, and this gives you a background of low ozone," McPeters explained. "And then from one year to the next, whether you have a particularly deep hole or not sort of depends on the stratospheric `weather' that you have in the Southern Hemisphere."

This year's record ozone hole occurred largely as a result of the particularly cold winter in Antarctica, McPeters said. During the Antarctic winter, the total or partial lack of sunlight causes air to drop to very low temperatures. Clouds of ice crystals called "polar stratospheric clouds" form in the upper atmosphere. These ice crystals are bad news for ozone. The crystals provide a surface for a chemical reaction that changes chlorine in molecules that do not affect ozone (such as hydrogen chloride) into more active forms that do destroy ozone."That's the accelerator," McPeters said. "If you didn't have the ice crystals, you would not be seeing the kind of ozone destruction that you see every year." A colder winter will result in more extensive polar stratospheric clouds, greater destruction of ozone, and a larger ozone hole.

Increases in the amount of carbon dioxide in the atmosphere can also create this same effect, noted Dr. Mike Newchurch, senior research scientist at the Atmospheric Science Department of The University of Alabama in Huntsville and a member of NASA's Global Hydrology and Climate Center. While higher carbon dioxide concentrations are thought to cause a warming of the atmosphere's lowest layer (the troposphere), scientists know that this same carbon dioxide actually causes the stratosphere to cool down. This cooling can exacerbate ozone destruction just as a particularly cold winter does.

``Although the magnitude and direction of the lower atmosphere's temperature change has been very hotly debated for several years, the cooling of the stratosphere is very clear and not a matter of question," Newchurch said.

The Antarctic vortex

The cold air over Antarctica in winter creates a huge "whirlpool" of fast-moving air circling Antarctica called the "Antarctic vortex."This vortex effectively insulates Antarctica from the rest of the atmosphere."It forms up almost as a whirlpool that sits there and is very stable. It locks in that body of air and it keeps the outside high-ozone air from coming in," McPeters said.

Most stratospheric ozone is created in the tropics, because the intensity of the solar radiation that causes formation of ozone is higher nearer the equator. The ozone is then transported by stratospheric air currents to the Arctic and to Antarctica.The strong and stable vortex prevents the migration of ozone into the stratosphere over Antarctica, exacerbating the low levels caused by the ice crystal-catalyzed destruction of ozone.

By virtually sealing Antarctica off from the warmer air surrounding it, the vortex causes temperatures in Antarctica to drop even lower. Lower temperatures cause the formation of more ice-crystal clouds and the destruction of even more ozone. "The (Antarctic) vortex sets up in the (Southern Hemisphere's) fall and runs all winter," Newchurch said. "The ozone depletion occurs in the springtime when sunlight becomes available. Then toward the end of the spring the vortex breaks down, and over the summer there really isn't a vortex. And then it sets up again the next fall."

A similar vortex forms around the Arctic, but "atmospheric waves" caused by landmasses with high mountain ranges in the Northern Hemisphere frequently push the vortex off the pole, allowing warmer air into the Arctic. The relative warmth of the Arctic is the main reason why a similar ozone hole doesn't form over the North Pole. "The weather systems (in the Northern Hemisphere) are a lot less stable than they are in the South," McPeters said.