Published online 7 May 2009 | Nature | doi:10.1038/news.2009.456

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Ozone data conflict resolved

Disturbing laboratory measurements put down to impurities

Ozone hole over AntarcticThe Antarctic ozone hole shows no signs of recovery

Two years after puzzling experiments threatened to shatter established models of ozone depletion in the atmosphere, Taiwanese chemists have published data that support the currently accepted theory.

The standard model for how chlorofluorocarbons (CFCs) destroy ozone in the Arctic and Antarctic stratospheres was called into question when experiments challenged the rate at which chlorine peroxide (Cl2O2), generated when CFCs decompose, is split apart by light (photolysed).

This reaction produces aggressive chlorine radicals that help to deplete ozone. But in 2007, Francis Pope, an atmospheric chemist then at NASA's Jet Propulsion Laboratory in Pasadena, California, and his colleagues, found that Cl2O2's photolysis rate in stratospheric conditions was almost an order of magnitude lower than that needed to explain the rate of late-winter polar ozone depletion seen above Antarctica1.

Their results challenged the validity of the Nobel prize-winning model of ozone destruction in the polar stratosphere, and sent shock waves through the atmospheric research community. Unsure whether they had the Cl2O2 photolysis rate correctly figured out, scientists could not confidently include ozone depletion processes in climate models.

Impure, impure

Pope's team reached their conclusion by measuring the amount of light that Cl2O2 absorbs at various wavelengths - giving a spectrum called the absorption cross section: a standard way of working out how rapidly it would photolyse under the wavelengths of light available in the stratosphere.

Some scientists suspected that Pope's results might have been an artefact. "It immediately occurred to me that the problem must be sample impurity," says Jim Lin, a chemist at the Institute of Atomic and Molecular Sciences in Taipei, Taiwan, who became interested in the problem when he came across Nature 's account of this worrying hole in ozone theory (see Nature 449, 382-383; 2007).

“If their numbers are correct, ours were wrong.”

Francis Pope
Centre for Atmospheric Science, University of Cambridge

In order to accurately measure the absorption spectrum of Cl2O2, samples of the molecule must be extremely pure. Other light-absorbing molecules - which can be formed by side reactions - may easily lurk unsuspected, causing misinterpretation of experimental results. Pope's team, however, had carefully used a new method to prepare their Cl2O2, which they had hoped would exclude impurities and secondary chemical reactions.

Thinking that Pope's preparations might have been at fault, Lin and his colleagues introduced a different experimental approach, which works whether or not the sample can be prepared free of all impurities.

Lin had his team fire a beam of Cl2O2 molecules — together with impurities — into a mass spectrometer, which was selectively set to detect only the Cl2O2 that came through. The researchers then irradiated the beam with laser light, which depleted the Cl2O2 molecules with a probability proportional to their absorption cross section. With the help of reference molecules, the scientists could then determine the precise absorption values for two specific wavelengths available in the stratosphere. The researchers report their results in Science2.

The absorption values they obtained are much larger than those reported by Pope, and agree well with previously calculated values. Reassuringly, they point to a photolysis rate that is large enough to support established models of ozone depletion and suggest that chlorine-catalysed ozone loss works even more efficiently in the polar stratosphere than thought.

Open question

"Impurity does pose a problem, and their method seems like a rigorous way of getting round it," says Pope, now at the Centre for Atmospheric Science at the University of Cambridge, UK. "If their numbers are correct, ours were wrong."

It is important to understand where measurements methods get stuck, and where they do well, he adds. Lin's group has proposed another set of values, but its results may not be the last word on the issue, he says.

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Several groups around the world, including Pope's, are conducting further experiments. If Lin's results hold up, scientists will have come close to a full quantitative understanding of the processes of ozone loss at the poles.

"Lin's study is a key step towards resolving previous concerns that our understanding of the polar ozone loss mechanisms may be incomplete," says Markus Rex, an atmospheric scientist at the Alfred Wegener Institute for Polar and Marine Research in Potsdam, Germany.

Even though Pope's results now seem to be wrong, his study did prompt other groups to think about alternative approaches, Rex adds. 

  • References

    1. Pope, F. D., Hansen, J. C., Bayes, K. D., Friedl, R. R. & Sander, S. P. J. Phys. Chem. A 111, 4322-4332 (2007). | Article | PubMed | ISI | ChemPort |
    2. Chen, H.-Y., Lien, C.-Y., Lin, W.-L., Lee, T. T. & Lin, J. J. Science 324, 781-784 (2009). | Article | PubMed | ChemPort |
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