Published online 28 October 2008 | Nature | doi:10.1038/news.2008.1191

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Expedition records stratospheric distortion and hints of pollution.

The first ozone measurement taken on the summit of Mount Everest has revealed surprisingly high levels of ozone, which scientists involved in the expedition suggest might have originated from urban pollution.

John Semple, holding an ozone meter at Everest base camp. John Semple

The measurement was made by British climber Ian Wade, part of the Karrimor 2005 Everest Expedition, who carried an ozone meter the size of a pack of cards to the mountain's peak, more than 8,800 metres above sea level.

John Semple from the University of Toronto, Canada, who was team doctor for the expedition, says that Wade had to memorise the measurement. "He wasn't about to take out his pen and paper," he says.

Semple and his Toronto colleague, atmospheric physicist Kent Moore, have now analysed this and other ozone data taken during the expedition. The results are published in Geophysical Research Letters1.

On top of the world

Wade's measurement, taken on 30 May 2005, found an ozone level of 50 parts per billion. Using meteorological records of atmospheric circulation patterns in the region, Moore traced the origin of the ozone back to southeast Asia, where urban areas are plagued by ozone-rich photochemical smog. "It is likely that the elevated ozone observed at the summit was associated with this phenomenon," the authors note.

"We were quite surprised," says Wade. "You tend to think of the Himalayas as pristine."

But others are not convinced. "The geographical source of observed 'pollution' ozone is not so unambiguous," says atmospheric scientist Paolo Bonasoni at the Institute of Atmospheric Sciences and Climate in Bologna, Italy, who has helped to establish a permanent high-level meteorological station in the Himalayas. He points out that they didn't measure the levels of any other gases that might have helped to trace pollution, such as carbon monoxide and nitrogen oxides.

Unhealthy levels

Semple took his own ozone measurements on 19 May, at Camp V on the North Ridge of Everest, at a height of 7,500 metres, and found concentrations of 70 parts per billion — close to the 75 parts per billion limit set by the US Environmental Protection Agency on ground-level ozone in polluted cities.

The authors say that this ozone came directly from the stratosphere — the ozone-rich layer of the atmosphere that sits on top of the troposphere and protects us from harmful ultraviolet radiation.

Meteorological records show that a tropopause fold happened near Everest on that day. This event sees the boundary between the troposphere and the stratosphere — the tropopause, which normally lies between 8 and 18 kilometres in height — fold downwards, bringing it closer to Earth's surface. It can often follow a cold front, and in the climbing season at Everest these events can happen up to once a week, Moore says. "[Everest is the] only place in the world where the land touches the stratosphere."

Semple's Camp V measurements also saw regular variation in ozone levels from day to night — evidence that glacier winds, triggered by changing surface temperatures in daylight and darkness, bring ozone down the mountain, says Moore

"When at extreme altitude you're breathing extremely hard, taking in a lot of ozone and it is extremely toxic to the lungs," says Semple. If tropopause folds can be predicted, it could warm climbers that a summit attempt could incur lung damage or breathing problems. "I am a climber," Semple says. "It's important stuff."