臭氧层空洞Ozone depletion

氯氟烃化学性质稳定，可上升到平流层，在紫外线作用下分解出 Cl 原子：CCl₂F₂ → CClF₂ + Cl。Cl 与 O₃ 反应生成 ClO 和 O₂，ClO 再与 O 原子反应重新释放 Cl，一个氯原子可破坏数万个臭氧分子。南极冬季形成的极地平流层云表面使 HCl、ClONO₂ 转化为活性氯，春季阳光重现时臭氧迅速被消耗，这解释了臭氧洞为何集中在南极。

CFCs are so stable that they reach the stratosphere, where ultraviolet light splits off chlorine atoms: CCl₂F₂ → CClF₂ + Cl. Cl reacts with O₃ to give ClO and O₂, and ClO reacts with O atoms to release Cl again, so one chlorine atom can destroy tens of thousands of ozone molecules. Polar stratospheric clouds that form in the Antarctic winter convert HCl and ClONO₂ into active chlorine on their surfaces, so ozone is destroyed rapidly when sunlight returns in spring, explaining why the hole is centred on Antarctica.

1974 年莫利纳和罗兰发表论文，预言氯氟烃会破坏平流层臭氧；克鲁岑此前（1970 年）已揭示氮氧化物的催化作用。三人共获 1995 年诺贝尔化学奖。1985 年英国南极调查局的法曼、加德纳和尚克林报道了南极上空臭氧的大幅下降，随后卫星数据证实了“臭氧洞”。

In 1974 Molina and Rowland predicted that CFCs would destroy stratospheric ozone; Crutzen had shown in 1970 that nitrogen oxides catalyse ozone loss. The three shared the 1995 Nobel Prize in Chemistry. In 1985 Farman, Gardiner and Shanklin of the British Antarctic Survey reported a dramatic fall in ozone over Antarctica, and satellite data confirmed the 'ozone hole'.

1987 年签订的《蒙特利尔议定书》逐步淘汰了氯氟烃、哈龙、四氯化碳等消耗臭氧层物质，是迄今最成功的国际环境条约之一。替代品先是氢氯氟烃（HCFC），后是氢氟烃（HFC）和氢氟烯烃（HFO）。科学评估预计南极臭氧层将在 2060 年代前后恢复到 1980 年水平。

The Montreal Protocol of 1987 phased out CFCs, halons, carbon tetrachloride and other ozone-depleting substances and is regarded as one of the most successful international environmental treaties. Replacements were first HCFCs, then HFCs and HFOs. Scientific assessments expect the Antarctic ozone layer to return to 1980 levels around the 2060s.