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BBC Science correspondent Julian Siddle
"As well as ice particles the clouds contained frozen crystals"
 real 28k

Thursday, 30 November, 2000, 19:38 GMT
Clouds speed ozone loss
Arctic BBC
The Arctic has also experienced extensive thinning
By BBC News Online's Helen Briggs

An international research team has confirmed that cloud particles speed up the destruction of the ozone layer.

The natural particles activate manmade chlorine-containing compounds that attack ozone.

The observations were made by European scientists who flew a balloon over the Arctic in January this year, taking samples from different layers of cloud.

The presence of the particles could explain why ozone depletion is most marked in the spring. And it could lead to better models for predicting the thinning of the ozone layer over the poles.

Natural shield

A large hole has been appearing in the ozone layer over Antarctica every year since the early 1980s. In recent years, extensive thinning has also occurred over the Arctic.

Instruments Max Planck Institute
The instrument used to detect the particles
Man-made chlorine and bromine compounds cause the depletion of this protective layer of gas, which shields the Earth from harmful rays.

Now scientists have confirmed that certain cloud particles play a central role in hastening the destruction of ozone.

In winter, clouds gather over the South Pole and to lesser extent the North, when air in the Earth's lower atmosphere is trapped and cooled.

Critical behaviour

But ozone-destroying chemicals that accumulate in these clouds are relatively innocuous until activated by frozen particles and then sunlight.

One of the co-researchers of the study, Konrad Mauersberger of the Max Planck Institute, Heidelberg, Germany, said the cloud particles were an important factor.

"The importance of the clouds are that they can activate chlorine molecules which otherwise would not be dangerous to the ozone," he told BBC News Online.

"Because of the activation on the cloud particles, chlorine becomes very critical in the ozone destruction."

'Better prediction'

The particles are called NATs (for nitric acid trihydrate, a mixture of water and nitric acid). They exist in a solid form in polar clouds.

They are thought to speed ozone loss more than plain water crystals, because they stay solid at higher temperatures.

This could explain why the most dramatic ozone losses are seen in spring, when ice crystals thaw out, exposing NAT particles and chlorine-containing molecules to energising sunlight.

The scientists believe that their work could lead to better ways of predicting the size and extent of ozone depletion.

"We now have data on how the particles are composed, how the particles appear, their diameter, and their size," said Professor Mauersberger.

"We can use that to make a better prediction for future ozone developments."

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See also:

26 Oct 00 | Sci/Tech
Arctic ozone damage 'likely by 2020'
05 Apr 00 | Sci/Tech
Severe loss to Arctic ozone
24 Jan 99 | Sci/Tech
Arctic ozone worries grow
01 Oct 98 | Sci/Tech
Ozone hole reaches record size
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