Taking ozone measurements in Tibet. Credit: John Semple
Reviewing satellite imagery of the atmosphere over the Tibetan Plateau, G. W. Kent Moore, a researcher at the University of Toronto, found himself looking at a halo that seemed to trace the border of Tibet.
"It is uncanny how closely it demarks the boundary," Moore said regarding the unexpected filament of atmospheric gas ringing the region. The plateau is one of the planet’s most prominent features, with an average elevation of over 4,000 m above sea level. It is the home to most of the world’s 8,000-meter peaks, including Everest and K2.
The halo is a band, several hundred kilometers wide, made up of high levels of ozone. In searching for an explanation for the phenomenon, Moore turned to a fluid-dynamic occurrence called a Taylor Cap; until now it had only been observed in the ocean. Taylor Caps form when rotating fluid (of the sort that occurs when an ocean sits on the surface of a rotating planet) hits a solid column, such as the large mountains protruding from the ocean floor. Under these conditions, the fluid will predictably be deflected around the column itself. But even above the column, the fluid flows around a phantom column, leaving a shaft of still fluid in the center.
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Moore found that, for much of the year, the atmosphere (another rotating fluid) does the same thing. As it hits the Tibetan Plateau, the atmosphere creates a ring of concentrated ozone, marking the border of a "phantom column." As far as Moore is aware, this occurrence hasn't been observed elsewhere on land; in the low-density fluid of the atmosphere, there is no feature comparable to the size and height of the Tibetan Plateau.
Moore has collaborated for a number of years with John Semple, a University of Toronto surgeon interested in high altitude physiology. The recent ozone finding was published in an article they coauthored in the November edition of Geophysical Research Letters. Semple tried to figure out where the Tibetan ozone was coming from by taking an ozone reader on a trip to the mountains of Bhutan, which lie on the edge of the Plateau. His measurements showed that the ozone concentrations increased with elevation, suggesting that the source was not industrialized areas in the lowlands, but was the stratosphere.
At Tibet's latitude, the tropopause—the boundary layer of the atmosphere between the stratosphere and the troposphere—is found at elevations of 12 km (and can descend to heights as low as 9 km). While this wouldn't matter to people in most areas of the world, a number of Tibetan Plateau's mountains are exposed to a high level of ozone from the stratosphere. In addition, elevated concentrations of ozone can be found at altitudes as low as 5,000 m, just over half the height of Everest. For Semple, this is significant because mountaineers working hard in thin air may be taking in quantities of ozone that can impact respiratory function.
Though there is a significant body of research on high altitude physiology, there has only been limited consideration of the role of ozone. Semple has drawn from the work of physiologists who study the impact of concentrated ozone on exercisers in industrialized cities, as well as aviation-industry studies on safeguarding crews and passengers from ozone exposure at cruising altitudes. He says people generally think that there is bad ozone at ground level, due to pollution, and good ozone in the atmosphere, which protects us from the sun's ultraviolet rays. Stratospheric ozone does provide UV protection, but no matter where it comes from, ozone is an irritant and direct exposure to it is never good.
"It can trigger asthmatic attacks, shortness of breath, or chest pains," Semple said, "and in very severe cases, pulmonary inflammation or bronchitis."