Chlorine Destroys Ozone Over U.S.

A Harvard researcher has found that the depletion of the ozone layer by a chlorine compound is occurring above densely populated midlatitude regions, not just over the poles.

Darin W. Toohey, a post-doctoral fellow in chemical physics, and his research team, wrote in the January edition of the Geophysical Research Letters that measurements of chlorine monoxide taken February 1989 over the United States were five to 10 times higher than their levels the previous October.

Chlorine monoxide is the main compound responsible for breaking apart the ozone molecules over the poles. Most of the chlorine in the ozone layer comes from chlorofluorocarbons and various forms of pollution.

A recent article in Science News describes how chlorine monoxide levels normally increase during the winter because the icy cloud particles formed allow inactive chlorine to become ozone-destroying chlorine monoxide.

But a wind stream called the polar vortex keeps the polar air from mixing with warmer air over the midlatitudes, the article says.

That is why Toohey's finding is so unusual. Using observations from NASA's high-altitude ER-2 research plane, he found that while normal chlorine levels of 20 to 30 parts per billion were measured in October and early December of 1988, twice that concentration was detected in late December and five to 10 times the October amount was observed on a flight from Virginia to California on February 21, 1989.

If these levels of chlorine monoxide--the highest ever measured at midlatitudes--persist for a month, they could destroy as much as two percent of the ozone in a given region, according to Toohey.

The evidence for the ozone destruction is inconclusive, however. Because the ER-2 only collected data on a few days, it is unclear for how long the chlorine monoxide elevations last. Furthermore, the plane could not make measurements higher up in the main part of the ozone layer, the stratosphere.