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October 1, 2007
All Sorts of Holes
One of the things I like most about science is how thoroughly you can be taken by surprise. A good check on a field’s vigor is whether or not its practitioners are being ambushed by new data. By that standard, what at first looks like an embarrassment for the ozone-hole chemists actually makes them look pretty good.
The chemistry of ozone depletion over the Antarctic is well understood. Or is it? One of the key molecules in the process is chlorine peroxide (also known as chlorine monoxide dimer). It’s understood to be split by sunlight into reactive free chlorine radicals, which go on to catalyze the conversion of ozone into plain oxygen. In the process, the peroxide forms again, and the whole cycle starts over. While this is by no means the only means by which chlorine depletes ozone, it’s long been thought to be the main one.
But chlorine peroxide is a difficult molecule to work with. Extremely unstable by sea-level laboratory standards, it’s been hard to isolate in pure form for study. And despite the generally accepted cascade of ozone depletion reactions, it hadn’t even been detected in the Antarctic until 2004, which difficulty had been largely chalked up to its short lifetime. Now, though, a team at JPL has produced the best synthetic samples of chlorine peroxide to date, and they’ve checked how quickly it decomposes in the presence of ultraviolet light. And, well. . .the problem is, the stuff falls apart much more slowly than anyone had predicted – many, many times more slowly. If they’re right, it’s hard to see how the accepted chemistry of chlorine peroxide-driven ozone depletion can be correct.
This has produced all sorts of surprised reactions in the atmospheric chemistry world, summed up here at Nature News. Everyone is taking this report seriously, as well they should, and a number of explanations are already being tentatively advanced. All of them are going to require a lot of revision of what we thought we knew, though. (I should note that the depletion of ozone itself isn’t in question; that’s an experimental fact. Just how it’s being lost is the problem). Nature quotes researcher Marcus Rex:
"Overwhelming evidence still suggests that anthropogenic emissions of CFCs and halons are the reason for the ozone loss. But we would be on much firmer ground if we could write down the correct chemical reactions."
I have little doubt that this will get figured out eventually. The reason I’m optimistic is that this area of research is going along the way it’s supposed to. People are spending the time and effort to check assumptions, and when something turns up unexpectedly, the results are published in a good journal for everyone to see and argue over. That will lead to another round of theorizing, then more rounds of experimentation as people try to prove the latest ideas right or wrong. And thus we close in on the truth. That’s exactly, exactly how it should work.
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