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September 1998

ACD studies a newfound ozone disappearance in the Arctic

The Canadian military base that hosted PSE 98 sits next to the Arctic Ocean near latitude 82degreesN.

Within the last ten years, a new ozone mystery has been uncovered that Brian Ridley (Atmospheric Chemistry Division) calls "one of the most interesting tropospheric discoveries of the decade."

About 500 miles from the North Pole, on the northern tip of Ellesmere Island, is a Canadian military base called Alert. Established in the 1950s, it's the Northern Hemisphere's farthest-north permanent settlement. About ten years ago, the Canadian Atmospheric Environment Service (AES) started measuring surface ozone at Alert. In springtime, the usual measurement is 30 to 50 parts per billion by volume (ppbv). During some periods, however, the ozone concentration drops to zero.

"The exact mechanism for the depletion is not known," explains Brian, "but from the previous campaigns that the AES has organized, by measuring a bunch of hydrocarbons and other chemicals, all the evidence points to bromine and chlorine atoms and radicals destroying the ozone"--the same reactions as in stratospheric ozone depletion. The bromine and chlorine radicals are somehow liberated from more stable compounds in the Arctic Ocean's sea salt or perhaps from biological sources in the ocean itself, but again, the process by which they are released is unknown.

It may not be Winter Park, but it's definitely winter: Brian Ridley uses skis to get around the base camp on Ellesmere Island during PSE 98.

Brian and three other ACD scientists spent last March and April at Alert, hoping to sample some ozoneless air. He describes the program, called the Polar Sunrise Experiment (PSE) 98, as "a mopping-up experiment," an attempt to pin down some of the details of the depletion process.

"It was an adventure," Brian says. "You are absolutely in the middle of nowhere, and the scenery is just spectacular." During early March, temperatures ranged from -30 to -40 degrees Celsius (-22 to -40 degrees Fahrenheit) in near-darkness; in April, it was -15 to -30 degrees C (+5 to -22 degrees F) with 24 hours of sunlight. "I spent a lot of time skiing," Brian says. "I also did some science, but I had a good time." He particularly enjoyed the Arctic foxes that live around the base, where they formerly found a gourmet selection of garbage. "Now [the base personnel] are much more careful," says Brian, so the foxes must make do with natural prey.

Arctic foxes like this one kept the NCAR crew company at PSE 98.

The NCAR team also included Jim Walega, Deedee Montzka, and Frank Grahek. Their mission was to record levels of nitric oxide (NO), one of the most reactive of the atmosphere's nitrogen oxides, along with nitrogen dioxide. The two chemicals are jointly called NOx. "For bromine and chlorine to affect ozone, there can't be much NOx and formaldehyde [in the atmosphere]," Brian says. Thus, "It's important [to know] whether NOx is 30 parts per trillion or 3 parts per trillion. It can make all the difference between bromine being able to be responsible for what's happening or not."

Earlier PSEs had attempted to measure NOx levels, but the instrumentation wasn't stable enough to record very small concentrations. Ridley's group can measure concentrations of NO as low as one-half part per trillion using a chemiluminescence instrument. The cold didn't affect the trailer-housed sensor. "We had more problems keeping the trailer cool, because there were so many instruments and people inside," Ridley says. "A lot of instruments don't like running at 80 and 90 degrees [F; 26-32degreesC]." Although the inlets on the trailer sometimes froze up, on the whole, the system worked well.

The ACD crew hoped to see three or four ozone-loss episodes over their two-month stay. As it happened, there was only one, lasting 36 hours. "It's hard to base a lot of science on one event," Brian says, "but the indications are that certainly formaldehyde was low, and in general NOx was pretty low. One of the problems we had was that the base uses 500,000 gallons of diesel fuel a year, and that creates a lot of NOx. We received a lot of pollution from the base, even during the ozone-depletion episode. But we should be able to filter that data out." •Carol Rasmussen

For more on this project and other related ACD work, see the upcoming issue of the UCAR Quarterly (fall 1998).

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Edited by Bob Henson, bhenson@ucar.edu

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