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Deep chill tore new hole in ozone shield

South China Morning Post – Oct. 4, 2011

Massive rift opened in protective atmospheric layer over the Arctic for first time last winter, exposing Europe and Asia to higher levels of ultraviolet light

An ozone hole five times the size of California opened over the Arctic this spring, matching ozone loss over Antarctica for the first time on record, scientists said.

Formed by a deep chill over the North Pole, the unprecedented hole at one point shifted over eastern Europe, Russia and Mongolia, exposing populations to higher, but unsustained, levels of ultraviolet light.

Ozone, a molecule of oxygen, forms in the stratosphere, filtering out ultraviolet rays that damage vegetation and can cause skin cancer and cataracts. The shield comes under seasonal attack in both polar regions in the local winter-spring.

Part of the source comes from man-made chlorine-based compounds, once widely used in refrigerants and consumer aerosols, that are being phased out under the UN’s Montreal Protocol.

But the loss itself is driven by deep cold, which causes water vapour and molecules of nitric acid to condense into clouds in the lower stratosphere. These clouds in turn become a “bed” where atmospheric chlorine molecules convert into reactive compounds that gobble up ozone.

Ozone loss over the Antarctic is traditionally much bigger than over the Arctic because of the far colder temperatures there. In the Arctic, records have – until now – suggested that the loss, while variable, is far more limited.

Satellite measurements conducted in the Arcitic last winter and spring found ozone badly depleted at a height of between 15 kilometres and 23 kilometres. The biggest loss – of more than 80 per cent – occurred at a height of between 18 kilometres and 20 kilometres.

“For the first time, sufficient loss occurred to be reasonably described as an Arctic ozone hole,” said the study, which appeared in the British science journal Nature.

The trigger was the polar vortex, a large-scale cyclone that forms every winter in the Arctic stratosphere but which last winter was born in extremely cold conditions, said Dr Gloria Manney, of Nasa’s Jet Propulsion Laboratory in California.

“The ozone destruction began in January, then accelerated in late February and March, so that ozone values in the polar vortex region were much lower than usual from early March through late April, after which the polar vortex dissipated,” the study said.

“Especially low total column ozone values … were observed for about 27 days in March and early April,” The study said. “The maximum area with [such] values … was about two million square kilometres, roughly five times the area of Germany or California.”

This was similar in size to ozone loss in Antarctica in the mid-1980s.

In April, the vortex shifted over more densely populated parts of Russia, Mongolia and eastern Europe for about two weeks.

Measurements on the ground showed “unusually high values” of ultraviolet, although human exposure was not constant as the vortex shifted location daily before eventually fading, said Manney. The study challenges conventional thinking about the Arctic’s susceptibility to ozone holes.

This thinking is based on only a few decades of satellite observations.

Stratospheric temperatures in the Arctic have been extraordinarily varied in the past decade, the paper notes. Four out of the last 10 years have been amongst the warmest in the past 32 years, and two are the coldest.

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