If you study the Earth from a spy plane, you're bound to turn up a few interesting surprises. A team of Canadian scientists did just that when they put instruments on a high-flying US spy plane modified by NASA for environmental monitoring. Environment Canada scientist Tom McElroy (PhD'85) and York researchers Chris McLinden and Earth and Atmospheric science professor Jack McConnell found unexpectedly high levels of bromine monoxide in the atmosphere several kilometres above the surface.

Scientists are interested in bromine because it destroys ozone. Located higher in the atmosphere (the stratosphere), ozone protects the Earth from the sun's damaging ultraviolet radiation; at lower altitudes, though, it is a constituent of smog and a damaging pollutant. One set of measurements was "much larger than ever expected," said McLinden, a then-York graduate student who's now doing post-doc work at the University of California, Irvine. He said the team was sceptical at first because such high levels "had never been observed before. But they didn't go away after further analysis."

McElroy said there's a correlation between the high levels of bromine and the disappearance of both ozone and mercury in the Arctic atmosphere just above the surface, in what's known as the "boundary layer." Although the ozone destruction is well understood - it's similar to what goes on in the stratosphere - the link between bromine and mercury is less certain and is being actively studied.

Scientists thought that bromine didn't get above the boundary layer because ozone levels there are higher than within the boundary layer. They theorized bromine was trapped by a temperature inversion caused by the fact that Arctic air near the surface is warmer than the ice-covered surface. But new data contradict these assumptions.

McElroy says the key to unravelling the puzzle may be large cracks or "leads" in the Arctic ice that expose water to the air. "The rapidly rising air in these open leads will carry moisture and bromine up to higher levels, up to about five kilometres higher than the boundary layer."

Illustration: Jeff Halmos