OZONE LAYER CHECKED; SCIENTISTS FOCUS ON ARCTIC CLOUDS.

Byline: Usha Sutliff Staff Writer

PASADENA - JPL scientists are tracking a killer in the clouds high above the Earth in the Arctic stratosphere.

The victim? The ozone layer. The main suspect is the chlorine gas that comes from factories and other sources. But what remains to be determined is exactly how ozone erosion occurs in the Arctic and whether it can be stopped.

Those are the puzzles that brought an international team of scientists, including several from the California Institute of Technology and the Jet Propulsion Laboratory, to a tiny, remote town in Sweden from last November to March. Stationed roughly 124 miles from the Arctic Circle, the team set to work, conducting the most thorough examination of the Arctic ozone layer to date.

What they found, during one of the coldest winters on record, was an ozone loss of more than 60 percent in the Arctic stratosphere at 60,000 feet above the Earth.

``It got everyone's attention. Was it a surprise? It's hard to say,'' said Ross Salawitch, a JPL radio scientist who was part of the team.

The research was focused on clouds that form in the Arctic stratosphere, which, under certain conditions, release a deadly chemical cocktail that erodes the ozone layer.

There is a recipe for this cocktail, Salawitch said. First, you must have a significant amount of chlorine, which is always present despite tighter restrictions on the use of chlorofluorocarbons.

``The more chlorine we have, the more ozone depletion we get,'' he said.

But chlorine does not work alone. It must be combined with sunlight - present during late winter at the North Pole - and cold temperatures during that same time period, a combination which triggers the chemical's reaction.

``If you have all three ingredients, you get very rapid ozone loss and we saw that this year,'' Salawitch said.

Using planes and high-altitude balloons, the scientists' goal was to analyze the internal life of the polar stratospheric clouds that exist at the center of a vortex, a sort of cold giant tornado whose furious winds seal off warm air.

To start with, the clouds can only be formed when the air in the stratosphere is cold enough, say 110 degrees Fahrenheit below zero, Salawitch said.

The stratosphere is the region of the atmosphere that begins at an altitude of roughly seven to 10 miles and extends to about 30 miles. The troposphere, on the other hand, is where we live and where most of our weather and clouds are formed. It extends from ground level to an altitude of seven to 10 miles.

``If (the clouds) are not present, then the chlorine in the atmosphere resides in forms that are very unreactive toward ozone. If the clouds are present, however, then certain chemical reactions occur on the surface of the clouds that lead to a pretty amazing transformation,'' Salawitch said.

The clouds are both ``unbelievably beautiful'' and, under the right conditions, potentially deadly to the ozone layer, he said.

``The kicker is that the Arctic appears like it may be getting colder with time,'' he added. ``But it's very hard to say if that's what's really happening because of the extraordinary year-to-year variations in temperature.''

Among the instruments JPL provided was one developed by scientist Chris Webster that measures, among other things, the chlorine compounds that, when combined with the clouds, fuel the ozone loss.

Another was provided by Paul Wennberg, associate professor of atmospheric chemistry at Caltech, who developed an instrument that measured the chemical composition of stratospheric cloud particles.

``These are strange clouds. In the lower atmosphere, the clouds are made of water. In the stratosphere, a major component is nitric acid,'' Wennberg said. ``It's on the surface of these clouds that the chemistry occurs that leads to the large ozone loss that we've been seeing. The chemistry on the clouds liberates chlorine atoms that are able then to destroy the massive amounts of ozone. You need the chlorine, but you (also) need the clouds themselves.''

The Arctic stratospheric clouds and the conditions that lead to their formation have led scientists to what may be a crucial link in the process - greenhouse warming.

``This idea has been in development for many years. I think it's the first time it's being taken seriously,'' Salawitch said.

The greenhouse effect describes a phenomenon where gases like CO-2 build up, virtually acting as a blanket that traps heat in the troposphere, preventing warm air from heating the stratosphere above. The result is that the stratosphere gets colder, creating prime conditions for the formation of clouds.

``The real measure of cold is the amount of clouds that we had in this circulation system. We set a new record this year for the amount of clouds that we saw,'' he said.

What worries scientists, he added, is that the three coldest years on record for the Arctic have occurred within the last five years.

``It's not only that we have cold temperatures, it's that they persist to the end of winter,'' Salawitch said. ``At this stage, we have heightened concern that the greenhouse gases might be playing a role in the tendency toward colder temperatures (during the late winter months).''

There is another way the greenhouse effect may be playing a role in the destruction of the ozone layer over the Arctic.

Theoretical studies done by computer calculations have shown that as the greenhouse gases rise, weather systems that may bring warm air are diverted away from the vortex, thus lowering the temperature inside and making cloud formation more likely, Salawitch said.

The data collected during the Arctic expedition will put those theories to the test, he added.