Layers of complexity in ozone hole.

Layers of Complexity in Ozone Hole 

One more mystery has been added tothe seasonal loss of ozone in the stratosphere over Antarctica. It now appears that the "hole' is an uneven one, with 2-to 3-kilometer-thick slices of ozone-poor air sandwiched within layers of only minimal depletion.

In 1985, British researchers reportedthe presence of a sharp ozone drop over Antarctica; previously collected data indicated that the hole made its first appearance in 1975 and has been returning each Antarctic spring. Climate researchers have been struggling mightily to explain why the hole appears (SN: 3/1/86, p.133; 10/11/86, p. 239; 10/25/86, p.261; 11/29/86, p.344), but their theories have been modeled on a generalized ozone depletion.

Some scientists have put forth a chemicalexplanation--that the depletion is caused by chemical events spurred by the presence of chlorofluorocarbons created by industrial processes. This was bolstered this week with the announcement by the head of an ozone-research team that a chlorine-containing molecule related to chlorofluorocarbon use is abundant in the hole.

Others believe the hole is formed bydynamic air movement and mixing. A third group blames it on the sun, suggesting high solar-cycle activity produces ozone-destroying active forms of nitrogen above the stratosphere.

The new data on ozone stratificationwere collected by University of Wyoming researchers who went from Laramie, Wyo., to McMurdo Station in Antarctica last year. They sent up their first ozonesensing balloon Aug. 25, before the seasonal hole began forming, and by Nov. 6 had sent up 32 more. The balloons sampled the atmosphere with sensors as they traveled to about 30 kilometers up, and beamed the results back to earth.

Ozone depletion is confined to a swathof air from 12 to 20 km up, the researchers report in the March 5 NATURE. But while the total ozone loss in that segment is 35 percent, the patch between 14 and 18 km lost more than 70 percent to its ozone from the initial high in August, and the researchers found depletions as great as 90 percent within 1- to 5-km-thick zones.

They also found great differences inadjacent layers--in some cases, a layer that had lost more than 75 percent of its ozone was adjacent to one with a loss of less than 25 percent. Another surprise, says Wyoming researcher David J. Hofmann, was the rapidity with which the depletion occurred--about half the ozone was gone after 25 days.

The findings leave both the chemicaland dynamic theorists to explain the stratification and the speed at which the depletion occurs. The stratification doesn't necessarily hurt the chemical camp, Hofmann says--the layering could occur by air movements, after the ozone has been chemically depleted. But the chemists will have to explain how the depletion can be so quick.

Susan Solomon of the National Oceanicand Atmospheric Administration in Boulder, Colo., favors a chemical explanation. Solomon, who headed the U.S. National Ozone Expedition in Antarctica last year, says the Hofmann study "is a very important observation that's going to have to be explained.'

The data, she says, "pose severe problemsfor all the models.' The September depletion is earlier than predicted by either the chemical or physical model, both of which rely on the sun warming the air and predict an October depletion.

Data gathered recently by her groupsupport the chemical theory, she says. At a congressional subcommittee hearing on ozone loss this week, Solomon said the ozone hole contains 20 to 50 times the expected level of OCIO, a chlorine-containing molecule. Such chlorine molecules have been associated with chlorofluorocarbon use. But it is too early to say that chlorofluorocarbons cause the hole, she says.

The absence of the depletion above 20km makes the solar-cycle theory unlikely, says Hofmann, since that theory predicts the greatest loss at higher altitudes. But one of the formulators of the solar-cycle theory, Linwood B. Callis of NASA Langley Research Center in Hampton, Va., says the date were collected during a period of low solar-cycle activity, when not much solar-related effect was expected; still, he prefers not to comment on what may have caused the hole in 1986, pending further analysis.

Data only alluded to in the NATUREpaper are going to give the dynamicists some problems, Hofmann says. He and his colleagues found that other chemicals in the ozone-poor air were not depleted, making it less likely that the hole is caused by upwellings pulling in aerosoldepleted air. For upwellings to bring in air depleted only of ozone, and not of other trace chemicals, would take "immaculate transport,' says Hofmann. "Our measurements show no upwelling.'

Nonetheless, Mark Schoeberl of theNASA Goddard Space Flight Center in Greenbelt, Md., who is a proponent of dynamics, says the current research does not rule out a physical process. "Ozone is a long-lived tracer,' he says. Only if all aerosols formed and decayed at the same rate and in the same place would air moved by upwellings have uniform concentrations of aerosols, he says.

Hofmann, while suggesting that thepink-and-green stratospheric clouds that form over Antarctica may somehow be a factor, is not taking sides. "I don't push any models,' he says. "I take measurements.'