Chemistry ties CFCs firmly to ozone hole - chlorofluorocarbons

Science News, Dec 10, 1988 by Janet Raloff

Chemistry ties CFCs firmly to ozone hole

A new study for the first time "convincingly" identifies the dominant chemical process by which the Antarctic ozone hole forms, its authors say. In so doing, the study also appears to indict chlorofluorocarbons (CFCs) for much of the ozone loss.

Since discovering it several years ago, atmospheric scientists have puzzled over the destruction of stratospheric ozone above Antarctica. While many researchers strongly suspected emissions of manufactured CFCs played a role, the chemistry by which CFCs' chlorine breaks down ozone in the upper atmosphere couldn't operate at the lower altitudes where an ozone "hole" now forms. That led some to question chlorine's overall role in the hole.

About 18 months ago, Mario Molina, an atmospheric chemist at NASA's Jet Propulsion Laboratory in Pasadena, Calif., offered a hypothesis to explain chlorine's role in the hole. Unlike chlorine's destruction of ozone in the upper stratosphere, this pathway required the linkage of two chlorine monoxide (CIO) molecules into a fragile dimer (Cl.sub.2.O.sub.2.). Upon exposure to the sun's ultraviolet light, the dimer would ultimately decompose into two chlorine atoms and a molecule of oxygen (O.sub.2.). The recycled free chlorines were then available to destroy more ozone.

The potential hitch in this hypothesis was the general rarity of atmospheric ClO molecules. At levels normally found in the stratosphere, there was little likelihood that two would collide to form a dimer.

Using millimeter-wave spectroscopy, physicists at the State University of New York in Stony Brook studied stratospheric levels of this ClO during September and October of 1986 and 1987. And consistent with Molina's hypothesis, their ground-based measurements of the skies above Antarctica's McMurdo Station found "a huge excess" of ClO -- levels on the order of at least 1 part per billion -- notes Robert de Zafra, one of the scientists. In fact, data published by his group in the Dec. 1 NATURE show this ClO excess occurred only between 17 and 23 kilometers -- the precise altitudes where simultaneous, direct balloon measurements by others showed ozone destruction was occurring. Moreover, the Stony Brook researchers found that ClO excesses disappeared at night and quickly returned with the morning sun.

Such variations in ClO concentrations--by altitude and between day and night--"corroborate one of the important predictions [of the dimer hypothesis]," Molina told SCIENCE NEWS. And while data collected by others from airplanes at about the same time last year also showed an excess of ClO in the Antarctic ozone hole, Molina says the "high quality" of the Stony Brook data provides stronger -- and the first formally published--measurements pointing to dimer formation as the most likely route to ozone destruction over Antarctica.

These high ClO measurements also confirm for the first time that CFCs -- contributing about two-thirds of the atmospheric chlorine--are largely responsible for the Antarctic ozone hole, notes Mark Schoeberl, an atmospheric scientist at NASA's Goddard Space Flight Center in Greenbelt, Md.