臭氧洞复习

很高兴向大家介绍英国南极勘测局的节目,这是1980年代中期首次发现臭氧层洞的团队之一:Jonathan ShankinChris -   What is the ozone layer, and where is it?Jonathan -   The ozone layer is high up in the atmosphere, from about 10 to maybe 30 kilometres.  If you brought all the ozone in that layer down to the surface, you would have just 3 mm of pure ozone gas, but that 3mm is spread through maybe 20km of atmosphere, so even in the ozone layer there's not much ozone.Chris -   So it's very thin, isn't it?Jonathan -   It is very thin, and the worry that we have for the Antarctic is that during the Antarctic spring we have a hole in the ozone layer.Chris -   How did you actually discover that the hole was there, what were you actually doing that enabled you to make that finding?Jonathan -   We were actually making routine measurements of ozone.  They started in the international geophysical year 1957-58 and we've been making them ever since.  Really, the work was looking not to show that there's anything wrong with the ozone layer, we were trying to show that it was okay, that spray cans weren't destroying it, Concorde wasn't destroying it.  We spotted that, during the Antarctic spring, ozone levels were dropping.  We followed that up and found that it was a systematic change!年复一年不同 渐渐下降 自然杂志上发布 美国人回溯卫星数据What's the significance of the season?Jonathan -   The significance of spring is really why it's over Antarctica.  It's to do with the temperature high in the atmosphere.  Once you get up to 14-20km, temperatures get to below -80 Celsius.  At that sort of temperature, you get clouds forming in the ozone layer.  Chemical reactions can occur on the surface of those clouds that convert the chlorine and bromine from halons into an active form, chlorine monoxide or bromine monoxide, and then when the sun returns in the arctic spring you get very efficient catalytic cycles that convert ozone back into oxygen.  A molecule of oxygen has two atoms of oxygen, ozone has three atoms, and we get about 1% per day being converted back into oxygen.Chris -   So what's the significance of the spray cans, what do they contribute, and also why just Antarctica, why not the North pole?Jonathan -   The spray cans used to be powered by chlorofluorocarbons (CFCs) as a propellant, it was also found in upholstery foams, in plastic cartons and a whole host of exciting uses.  They were mostly released in the northern hemisphere, but the process of diffusion means they get well mixed throughout the atmosphere.  Roughly speaking, the concentration of CFCs at the South pole and the North pole are exactly the same, but what is different is the temperature.  Over the Antarctic, every winter it's cold enough to form these stratospheric clouds high up in the atmosphere, but for the Arctic, it's only exceptionally cold winters due to the circulation of the atmosphere that enables these clouds to form.  By and large, the Northern Hemisphere circulation is more complex, the atmosphere is better mixed and it's about 10 degrees warmer, so these clouds are quite rare.  But they have been seen over Cambridge.Chris -   So why doesn't the ozone from elsewhere in the atmosphere just flow in to Antarctica and replace the deficit, filling the hole?Jonathan -   Essentially it can't.  During the Antarctic winter you get very strong circumpolar circulation - called a polar vortex.  That effectively acts as a barrier, it stops the air mixing from out of the Antarctic into the Antarctic.  So what happens is during the spring you get a build up of ozone rich air around the Antarctic, in the middle you get very low levels of ozone, and that's the ozone hole.Kat -   I would just like to backtrack a bit and ask why is the ozone layer so important, does it really matter if it's vanished?  Can't we just take the ozone from down at street level and shove it back up there?Jonathan -   It does matter if it disappears, what happens is that more ultra-violet (UV) light from the sun can get through to the surface when there's less ozone.  This UV light can trigger skin cancers, cataracts and genetic damage in micro-organisms.  So it's not very good for us to have too much UV.  For example when we have the ozone hole in the Antarctic, you can get burned in 5 minutes if you don't put on factor 30 sun block.Kat -   Wow.  How is it actually linked in to global warming, which is something that we're becoming increasingly aware of?Jonathan -   It's quite a complex thing.  The ozone hole is actually completely separate in many ways from global warming, but global warming actually makes the ozone hole worse.  That's because greenhouse gases act like a blanket, and though the surface of the Earth is heating up, the ozone layer is actually getting colder.  The fact that it's getting colder means that more of those clouds can form, then more ozone destruction means it gets colder still, and it's a positive feedback cycle.Chris -   Have the mechanisms we've put in place to try to limit the use of the agents, the CFCs that we think are responsible, actually worked?  Is the hole shrinking?Jonathan -   There's a big international treaty called the Montreal protocol, and all bar one of the worlds countries have signed up to the basic thing, so that's really good news.  The amounts of these ozone-damaging things in the atmosphere are actually going down, so were definitely getting there with this one.  But it is a symptom and not a thing in itself, and unless we tackle at a basic level what's causing many of these environmental problems, things can only get worse.Chris -   Just to crystallise this in people's minds!how big is that hole over Antarctica?Jonathan -   The Antarctic is 50 times the size of the UK, the ozone hole is bigger than that.  Not by much but it gives you an idea.Chris -   So at least fifty times the size of out own nation, it's quite a sobering thought.Jonathan -   It is, and something we created entirely off our own bat.Chris -   In actually quite a short space of time.Jonathan -   In as little as 10 years.  It shows you how fragile our atmosphere is, and how much care we need to take in case something similar happens in the future.