Polar Ozone Depletion (Nobel Lecture)

摘要

The ozone layer acts as an atmospheric shield that protects life on earth against harmful ultraviolet radiation from the sun. This shield is fragile: in the past two decades it has become very clear that it can be affected by human activities. Roughly90% of the earth's ozone resides in the stratosphere, which is the atmospheric layer characterized by an inverted -that is, increasing--temperature profile that rises typically from about 210 K at its base at 10-15 km altitude, to roughly 275 K at 50 kmaltitude. The maximum concentration of ozone is several parts per million; it is continuously being produced in the upper stratosphere by the action of solar radiation on molecular oxygen, and continuously being destroyed by chemical processes involvingfree radicals. Work by Paul Crutzen in the early 1970s established that nitrogen oxides of natural origin, present at parts per billion levels in the stratosphere, are responsible for most of this chemical destruction, which occurs by means of catalyticcycles. Harold Johnston pointed out in 1972 that the large fleets of supersonic aircraft, which were being considered at that time, could have seriously affected the ozone layer through their emissions of nitrogen oxides.