HARVARD GAZETTE ARCHIVES

Results also showed that the chemical chain reaction involving the molecule likely runs 20 percent to 30 percent faster than scientists had expected, a

Ozone measurements over the Northern Hemisphere, from the Total Ozone Mapping Spectrometer, or TOMS, satellite, launched in 1996. The NASA satellite conducts daily mapping of the Earth's atmospheric ozone.

finding that will allow them to adjust critical computer models to better approximate real conditions in the atmosphere.

The molecule, made up of two chlorine atoms and two oxygen atoms, is called a chlorine monoxide dimer or chlorine peroxide, Cl-O-O-Cl. It has a crucial role in the process by which chlorine destroys atmospheric ozone.

Though a variety of chemicals are implicated in ozone loss in the polar winter stratosphere, chlorine is thought to dominate, with a large contribution from bromine radicals.

Scientists have been concerned about the impact of man-made processes on the Earth's ozone layer for decades. The ozone layer, a thin band high in the stratosphere, is responsible for shielding the Earth from harmful ultraviolet rays.

Ozone loss is thought to be a byproduct of the release of chemicals into the atmosphere through various man-made processes, including everything from air-conditioning to agricultural fumigants.

During 11 flights of a NASA ER-2 airplane - essentially a converted U-2 spy plane - the automated instrument was mounted in a wing pod, taking measurements as high as 65,000 feet in the atmosphere.

The implications of ozone loss were so serious that nations across the globe banded together to reduce the release of harmful substances into the atmosphere in the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer.

Though release of many ozone-killing substances has been greatly reduced, ozone depletion continues because compounds released over several decades persist in the atmosphere.

Huge holes in the ozone layer have been detected in recent years during the coldest months over both the Antarctic and Arctic regions.

Rick Stimpfle, a senior project scientist with the Division of Engineering and Applied Sciences, was the lead author in a paper published this month in the Journal of Geophysical Research-Atmospheres that outlined the findings. Stimpfle conducted the research along with David Wilmouth, a postdoctoral fellow in atmospheric chemistry, Philip S. Weld Professor of Atmospheric Chemistry James Anderson, and Ross Salawitch, a researcher at NASA's Jet Propulsion Laboratory

Stimpfle (Staff photo Lindsay Pierce/Harvard News Office)

Stimpfle said the existence of the chlorine monoxide dimer has been theorized for some time. It exists only in the extremely cold environment of the stratosphere over the Earth's coldest regions, and only during the coldest months of the year.

In those conditions, however, the molecule is thought to be plentiful and to drive a reaction that destroys atmospheric ozone, a molecule made up of three oxygen atoms.

Though ozone is made up of oxygen, it has different chemical properties from the oxygen we breathe, which is contained in molecules made up of two oxygen atoms.

That difference may seem minor, but ozone has chemical properties that make it a pollutant when found at ground level and which make it an invaluable shield against ultraviolet rays high in the atmosphere.

The ozone-destroying reaction is thought to start with the chlorine monoxide dimer and consist of three steps:

When sunlight strikes the dimer, it breaks up into two free chlorine atoms and an oxygen molecule.

Each chlorine atom reacts easily with an ozone molecule, combining with one of ozone's three oxygen atoms to form chlorine monoxide and releasing an oxygen molecule.

The two chlorine monoxide molecules formed by the reaction then join up to re-form the chlorine monoxide dimer, starting the process again.

The results took several years to produce. The measurements were taken between 1999 and 2000 during a joint U.S.-European science mission in Kiruna, Sweden, called SOLVE/THESEO-2000.

During 11 flights of a NASA ER-2 airplane - essentially a converted U-2 spy plane - the automated instrument was mounted in a wing pod, taking measurements as high as 65,000 feet in the atmosphere. The instrument, which uses a process called "vacuum ultraviolet resonance fluorescence," was able to measure the amount of the chlorine monoxide dimer in the air flowing through it.

These concrete measurements, Stimpfle said, will be invaluable in adjusting computer models of ozone loss and in understanding the complex chemistry high in the atmosphere.

Stimpfle said he is optimistic that the problem of ozone loss will ultimately be solved. Adoption of the Montreal Protocol, he said, indicates that nations of the world understand the potentially serious consequences of ozone destruction.

In addition, he said, levels of chlorine in the atmosphere have begun to decline, albeit slowly. His research will help scientists understand fully the mechanisms through which chlorine destroys ozone, allowing them to turn their efforts to other significant ozone destroyers, such as bromine.

The ongoing problem of global warming, however, could reverse any hopeful trends, Stimpfle said, because as the lower reaches of the atmosphere warm, the upper reaches cool. And a cooler stratosphere will only accelerate the chlorine-ozone reaction.

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