Associate Professor Darin Toohey of the Program in Atmospheric and Oceanic Sciences said scientists have known for several decades that chlorofluorocarbon-derived compounds can deplete stratospheric ozone.

More recently, some have proposed that adverse chemical reactions caused by man-made compounds occurring just seven to 10 miles in altitude could lead to additional ozone losses.

While such chemical reactions at lower altitudes may be occurring, "What we see is that ozone-depleting reactions of chlorine and bromine compounds are occurring rapidly at high latitudes in winter," he said.

The winter chemical reactions are occurring in an atmospheric region where the air can readily mix with air at mid-latitudes like the skies over Reno, Denver and Philadelphia.

PAOS researchers and students have used balloons and aircraft to show that ozone-gobbling chlorine "free radicals" produced by the breakdown of CFCs are more concentrated at high latitudes than previously believed. During winter and spring, the reactions appear to be accelerated from about 50 degrees to 60 degrees in latitude -- roughly from Vancouver, B.C., north to Great Slave Lake in the Northwest Territories -- all the way to the North Pole.

These chemical reactions occur in regions where there are ice clouds, based on measurements of CU-Boulder Professor Linnea Avallone of PAOS and the Laboratory for Atmospheric and Space Physics, said Toohey.

Toohey presented his research at the Spring American Geophysical Union Meeting last week in Washington, D.C.

The invisible ozone layer, which shields Earth from harmful ultraviolet radiation, is produced naturally in the stratosphere -- 10 miles to 30 miles above Earth's surface. Until several decades ago, the equilibrium of the layer has been maintained by several competing chemical reactions among naturally occurring oxygen and hydrogen molecules.

"This ozone-depleting chemistry is important because it occurs outside the region where large amounts of ozone depletions have previously been reported, such as the poles and at higher altitudes," said Toohey.

While ozone losses at higher altitudes inside the polar vortex are thought to be confined to the high latitudes, it is possible that transport and mixing of lower-altitude air is having a wider influence on ozone abundance at lower latitudes.

In addition, new studies from Boulder scientist Stephen Reid of the National Oceanic and Atmospheric Administration suggest climate change could alter the motion of air in the lower stratosphere.

"These results raise the possibility that halogen chemistry occurring at high latitudes is more important for ozone trends at mid-latitudes than was previously believed," said Toohey.

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