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Scientists have discovered new evidence linking chlorine to the annual depletion of atmospheric ozone over the Antarctic, it was reported today in the British science journal Nature.
Results from Jet Propulsion Laboratory experiment performed in the Antarctic last September and October show direct correlation betwen seasonal temperature changes in the Antarctic and chlorine's transition from non-reactive phase to the reactive state in which it breaks down ozone.
The results are reported in paper by principal investigator Dr. Crofton B. Farmer and co-investigators Geoffrey C. Toon, Peter W. Schaper, Jean-Francois Blavier, and L.L. Lowes, all of JPL. Their findings indicate that chlorine may hibernate in condensed, non-reactive phase during the frigid Antarctic winter, residing in polar stratospheric clouds or in some molecular state not yet identified. With springtime warming, the chlorine emerges from these reservoirs in reactive, ozone-destroying vapor.
The findings bolster the hypothesis that increased amounts of chlorine in the Earth's atmosphere are responsible for the ozone hole, region of severe reduction of the ozone layer. British researchers who discovered the ozone hole two years ago found that as much as 40 percent of the ozone normally present in the stratosphere above Antarctica disappears during the months of September and October. The ozone gradually returns to normal levels by late November.
The new results reported by the JPL researchers show unusually large amounts of chlorine nitrate, one of two molecular forms in which chlorine released from chlorofluorocarbons is stored in the stratosphere. The other reservoir is hydrochloric acid. The relative amounts of hydrochloric acid and chlorine nitrate found in the polar vortex over the McMurdo Station in the Antarctic during September were very different from the amounts normally found at lower latitudes. (The polar vortex is an area of the polar atmosphere isolated from the rest of the circulation of the global atmosphere throughout the Antarctic winter.)
The findings strongly suggest that the reactions -- on the surfaces of ice crystals -- which release reactive forms of chlorine during the first weeks of sunlight in the Antarctic stratosphere, are probable cause of the ozone reduction observed to occur during that season.
The ozone layer in the stratosphere shields the Earth from most of the Sun's ultraviolet radiation. Most of the chlorine in the Earth's stratosphere is from industrially produced chlorofluorocarbons. In the atmosphere, the chlorofluorocarbons are broken down by ultraviolet light into their constituent parts -- one of which, chlorine, destroys ozone through an efficient cyclic reaction involving chlorine oxide.
The JPL results were derived from detailed ground- based infrared measurements of the composition of the Antarctic atmosphere Farmer and his colleagues made as part of his experiment, conducted during the National Ozone Expedition (NOZE-1) to the Antarctic last year.
Farmer is conducting follow-up studies of the ozone hole as part of cooperative investigation sponsored by NASA, the National Oceanic and Atmospheric Administration (NOAA), the National Science Foundation, and the Chemical Manufacturers Association. Researchers from these agencies and other institutions are flying wide range of experiments in and around the ozone hole on two specially fitted NASA aircraft stationed at Punta Arenas, Chile, until the end of September.