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 on Sunday.
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-to-spring.
Part of the source comes from manmade chlorine-based compounds, once widely used in refrigerants and consumer aerosols, that are being phased out under the UN’s Montreal Protocol.
However, the loss itself is driven by deep cold, which causes water vapor 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 Arctic winter-to-spring from last year until this year found ozone badly depleted at a height of between 15km and 23km.
The biggest loss — of more than 80 percent — occurred between 18km and 20km.
“For the first time, sufficient loss occurred to be reasonably described as an Arctic ozone hole,” says the study, appearing 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, Gloria Manney, of the Jet Propulsion Laboratory in California, said in an e-mail.
“Especially low total column ozone values [below 250 Dobson Units] were observed for about 27 days in March and early April,” Manney said. “The maximum area with values below 250 Dobson Units was about 2 million km2, 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, Manney said.
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.
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