Arctic ozone springs a leak as winter draws to a close

日期:2019-03-04 01:01:01 作者:郁黾 阅读:

By JOHN GRIBBIN OZONE is definitely being depleted over the Arctic in late winter, according to research published last week in Nature. Measurements by instruments lifted high above the Arctic on balloons show that the same mechanisms that cause the ozone ‘hole’ above Antarctica to grow in the spring were at work in the northern hemisphere in January 1989. Atmospheric scientists attribute the depletion of ozone in the Antarctic winter to the extreme cold and stillness of the air. The conditions permit clouds of icy particles to form in the stratosphere. The particles’ surfaces support chemical reactions which, with the help of energy from sunlight, trigger destruction of ozone in the region as soon as sunlight returns to Antarctic skies. A key element in the destructive reactions is chlorine, which originates from the breakdown of chlorofluorocarbons, industrial gases used to blow foam and for aerosols and refrigerators. Scientists observed the chemistry of ozone destruction over Antarctica almost two years ago, when NASA flew an aircraft into the ozone hole. Last winter, NASA flew into the Arctic stratosphere and reported that ice clouds had formed and that the ozone was ‘primed for destruction’ (‘Is there an ozone hole over the North Pole?’, New Scientist, 25 February). But because the flights took place before the Sun returned, scientists aboard the aircraft failed to record the actual destruction of ozone. Now, another team has succeeded. The balloons were launched from Kiruna, in northern Sweden. Kiruna is an ideal site from which to observe the destruction of ozone, because the region of cold air over the North Pole (the polar vortex) breaks up earlier in spring than its southern counterpart. As a result, only the outer region of the cold air is exposed to enough sunlight to trigger the mechanism that destroys ozone before it warms. Last winter also proved ideal for finding evidence for the destruction of ozone. It was the coldest January in the stratosphere above the Arctic for at least 25 years. On 23 January, the team measured a reduction of ozone of 25 per cent at altitudes of between 22 kilometres and 26 kilometres. The pattern of this reduction was, the researchers say, strikingly similar to the pattern seen at slightly lower altitudes above Antarctica in spring. The loss of ozone is restricted to the altitudes where the icy clouds form,