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2/8/01-Scientists have discovered a new class of particles present in the Arctic's winter stratosphere that likely play an important role in ozone loss in that region, the Commerce Department's National Oceanic and Atmospheric Administration announced today. These findings give scientists a better understanding of the processes that set the stage for chlorine-caused ozone depletion in the stratosphere above the Arctic.

An international team of scientists published their findings in the Feb. 9 issue of Science magazine. Lead author David W. Fahey, a scientist at NOAA's Aeronomy Laboratory in Boulder, Colo., and colleagues, discovered a new class of polar stratospheric cloud (PSC) particles, very large in size, that had previously been undetected.

Scientists have known for some time that during the cold Arctic winter, icy, nitric acid-containing polar stratospheric cloud particles are formed, which enhance the destruction of ozone by human emissions of chlorine and bromine.

The newly discovered particles have diameters of 10-20 microns (millionths of a meter), which is about 10-20 times larger in diameter than those typically observed in PSCs. These particles have eluded detection to date because of their unexpectedly large size and their very small abundance in the atmosphere. The PSCs are laden with nitric acid (HNO3), and thus serve as reservoirs for nitrogen in the polar stratosphere. As the particles "sediment" or fall out of the atmosphere, the stratosphere becomes "denitrified". The discovery of this new class of large PSCs helps to explain a longstanding mystery, namely, that the extent of denitrification observed in the polar stratosphere could not be accounted for by the smaller, more slowly-falling PSCs.

The researchers say that the loss of nitrogen has important consequences for the ozone because ozone-destroying forms of chlorine and bromine are longer-lived in a denitrified stratosphere.

Fahey and colleagues made the observations during the period of January-March, 2000, using instruments placed onboard a NASA ER-2 high-altitude research aircraft. Those instruments measured reactive nitrogen species as the aircraft traveled toward the Pole and deep into the region of highest ozone loss.

The large particles 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. This indicates that these particles may play an important role in denitrifying the stratosphere. The 2000 Arctic winter stratosphere was extensively denitrified. Large ozone losses of up to 70 percent occurred in the lower stratosphere throughout the Arctic vortex, according to Fahey.

Cold temperatures promote the growth of large PSCs and enhance the loss of ozone by chlorine and bromine. Unusually cold winters, or climate shifts that reduce stratospheric temperatures or increase 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 still 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.

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