More clues to the mysterious ozone hole.
Satellite measurements indicate that ozone has once again started to disappear from the stratosphere above Antarctica, repeating a phenomenon that has intrigued and concerned scientists since it first came to light in 1985. While more than 100 researchers from around the world are currently staging two separate experiments to study this event (SN: 8/8/87, p.95), results from last year's experiments are pointing an ever-more-accusing finger at industrially produced chlorine chemicals as the cause of the ozone hole.
The newest information, reported in the Sept. 10 NATURE, comes from a group at the Jet Propulsion Laboratory (JPL) in Pasadena, Calif., which participated in the first National Ozone Expedition (NOZE I) last September and October at McMurdo Station in Antarctica. Using an interferometer, this group measured the atmospheric concentrations of certain gases thought to be critical to the stratospheric chemistry that might be at the root of the ozone disappearance.
The results from the JPL experiments confirm earlier reports that the chlorine chemistry of the Antarctic stratosphere is "perturbed' during the Antarctic spring, but the JPL report also contains measurements of some previously unstudied chemicals. According to group leader C. Bernard Farmer, the new measurements add to a growing pile of evidence that chlorine causes the depletions.
"The results don't constitute scientific proof yet, certainly not proof in any court,' Farmer told SCIENCE NEWS. "But the evidence is strongly circumstantial, and indeed what's more important is that the results are entirely consistent with the [proposed] chemical hypotheses.'
The ozone hole is actually a large patch of ozone-depleted stratosphere that has appeared since the late 1970s over latitudes south of 45| during each Antarctic spring. Since they learned of its existence two years ago, atmospheric scientists have strived to identify the underlying causes and to determine if the global ozone layer is in jeopardy. They are currently debating whether the hole results from chemical reactions or a "dynamic' reorganization of winds over the Southern Hemisphere. Many scientists, although still unsure about the relative importances of chemistry and dynamics, believe both mechanisms are active.
The proposed chemical mechanisms have thus far focused on chlorine, which primarily reaches the stratosphere in chlorofluorocarbons (CFCs)--a class of chemicals used in refrigeration, aerosol sprays and foam production. When present in the stratosphere, chlorine and certain chlorine chemicals convert ozone (O3) into molecular oxygen (O2).
Farmer's group measured the concentrations of the two "reservoir' species of chlorine: hydrochloric acid (HCl) and chlorine nitrate (CLONO2). In the cold and darkness of the Antarctic winter, most of the chlorine readily assumes one of these two forms, both of which are innocuous and normally unreactive.
The researchers found that the chlorine level in the reservoir species was low throughout the period of greatest ozone loss in September, then returned to more normal levels in October when ozone depletion began to taper off. Where, then, was the missing chlorine during September?
One possible explanation is that some of the normally gaseous HCl and ClONO2 had condensed onto cloud particles, called aerosols. When these reservoir chemicals are in this nongaseous form, the interferometer cannot detect them.
This scenario would seem to confirm the suspicions of scientists who believe chemistry is behind the ozone disappearance. Condensation must be occurring, they reason, because--as lab tests have shown--only in their condensed form on the surface of an aerosol do HCl and ClONO2 react quickly enough to form ozone-destroying chemicals. According to N. Dak Sze, who works with computer models of stratospheric chemistry at Atmospheric and Environmental Research, Inc., in Cambridge, Mass., the JPL data are "definitely indicative of the possibility' of such a scenario.
However, says Farmer, there is another explanation for the missing chlorine. Some of the chlorine could already be present in another, gaseous form, which actively destroys ozone. This explanation would match other results from NOZE I, which found abnormally high September levels of one such form, chlorine monoxide (ClO), a molecule active in ozone destruction (SN: 5/23/87, p.326).
While both explanations are possible, says Farmer, he is certain that ClO cannot account for all of the missing chlorine. "If it had all been in the form of ClO, we would definitely have seen it very clearly, and we didn't,' he says. "So either it is frozen or there's some other chlorine species around.'
Meanwhile, in the Antarctic this month, NOZE II scientists are again collecting ground-based measurements while others are studying the ozone hole from planes. And in Montreal this week, diplomats from more than 40 countries approved a protocol that establishes international limits on the production and consumption of CFCs.
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|Date:||Sep 19, 1987|
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