An international team of 27 scientists published their findings about the new class of particles in this week's issue of Science magazine. The Earth's ozone layer protects life below from harmful ultraviolet radiation coming from the Sun that can lead to the formation of skin cancers.

"The size of these particles was surprisingly big, and they are part of a process that is removing nitric acid from the stratosphere, eventually leading to ozone loss," said Hansjurg Jost, co-author of the Science article and a scientist at NASA Ames Research Center, in California's Silicon Valley. Lead author of the paper is David W. Fahey of the National Oceanic and Atmospheric Administration (NOAA), Boulder, CO.

"Ames teams contributed to these findings with crucial high-precision measurements of trace gases and temperature," said co-author Thaopaul Bui of Ames.

The newly discovered class of particles has given scientists a better understanding of the processes that "set the stage" for chlorine-caused ozone depletion in the Arctic stratosphere. Scientists have observed unusually low levels of ozone over the Arctic during recent winters.

Icy, nitric acid-containing "polar stratospheric cloud" (PSC) particles are formed in the polar regions during winter, where they enhance the destruction of ozone caused by human emissions of chlorine and bromine.

Fahey, a scientist at NOAA's Aeronomy Laboratory, and colleagues, reported the discovery of a new population of very large PSCs. They have diameters of 10-20 microns (millionths of a meter), which is about 10-20 times larger in diameter than typically observed in PSCs.

-more- -2- These particles have eluded detection to date because of their large size and very small abundance in the atmosphere, scientists said. The PSCs are laden with nitric acid and serve as reservoirs for nitrogen in the polar stratosphere. As the particles sediment, or fall out of the atmosphere, the stratosphere becomes "denitrified."

The loss of nitrogen has consequences for ozone, because ozone-destroying forms of chlorine and bromine are longer-lived in a denitrified stratosphere.

The discovery of this new class of large PSC particles helps to explain a long-standing mystery that the extent of denitrification observed in the polar stratosphere could not be accounted for by the smaller (and slower-to-sediment) PSCs.

Scientists made their observations in the Arctic stratosphere from January-March 2000. Instruments onboard a NASA ER-2 high-altitude research aircraft measured reactive nitrogen species as the aircraft traveled toward the North Pole and deep into the region of highest ozone loss.

The large-sized PSCs observed in some of the air samples contained 15-20 percent of the available reactive nitrogen in the Arctic stratosphere, and were falling at a rate of 1-2 kilometers per day.

These values demonstrate the potential for significant denitrification by the large particles. The 2000 Arctic winter stratosphere was extensively denitrified, which set the stage for significant chlorine- and bromine-caused ozone loss in the winter and spring.

Cold temperatures promote the growth of large PSCs, enhancing the loss of ozone by chlorine and bromine. Unusually cold winters, or climate shifts that reduce stratospheric temperatures or alter the amount of water vapor in the stratosphere, could prolong chemical ozone loss in the Arctic even as chlorine levels fall in response to international curtailments in the use of ozone-depleting chemicals.

"Our findings suggest that we need to learn more about how these large PSCs are formed, so that we'll have a better understanding of how the ozone layer will recover in the future," Fahey said.

Cooling of the stratosphere will likely increase ozone loss during Arctic winters in coming decades, even as chlorine and bromine levels decrease as a result of the Montreal Protocol, according to scientists.

The buildup of greenhouse gases, such as carbon dioxide, tends to trap more heat near the Earth's surface, while at the same time colder than normal temperatures are experienced above, in the stratosphere, where ozone breakdown occurs, researchers said.

The ER-2 science flights took place as part of two international field experiments: NASA's SAGE III Ozone Loss and Validation Experiment (SOLVE) and the European Commission-sponsored Third European Stratospheric Experiment on Ozone.

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