Ozone holes and cosmic rays

ozone hole sept 2007Browsing through the blogosphere recently, I came across an interesting little story about the scientific method, scientific progress, and un-scientific spin (h/t Hank Roberts). The subject concerns the polar ozone hole in Antarctica and a possible role for cosmic rays in its variability on solar cycle timescales. The proponents of this link are a small research group at the University of Sherbrooke in Canada, who find themselves up against the mainstream stratospheric chemistry community and whose ideas are twisted out of all recognition by the more foolish of the usual suspects.

The story hit the ‘tubes earlier this year when researcher Q.B. Lu predicted that this years Antarctic ozone hole would be the biggest ever due to the actions of increased galactic cosmic rays (GCR) (because we are at solar minimum and GCR are inversely correlated to solar activity). This years peak ozone hole has now come and gone, and the prediction can therefore be evaluated. Unfortunately for Dr. Lu, this year’s hole was merely about average for the decade – a result that wasn’t too supportive for his theory.

This story made me a little curious about this though. Firstly, I didn’t initially understand why cosmic rays should be playing a role in ozone depletion – most of the cosmic ray effects that are usually discussed revolve around cloud-aerosol connections, but there are not many clouds in the stratosphere where the ozone holes form, and the ones there are (Polar Stratospheric Clouds – PSCs) are much more sensitive to temperature and water vapour than they are likely to be to background aerosols. On further investigation, it turns out that this idea has been out there for a few years (and was reported on then) and has subsequently been discussed in the ozone literature.

So let’s start with the background theory. Standard (Nobel-prize winning) stratospheric chemistry has tied ozone depletion to the increasing chlorine (Cl) load in the stratosphere which catalytically destroys ozone and comes from the photolytic dissolution of human-sourced chloro-fluoro-carbons (CFCs) high in the stratosphere. In the polar night, the presence of PSCs allows for a specific class of heterogeneous Cl reactions to occur on the surface of the cloud particles which turn out to be very efficient at destroying ozone. Hence the presence of an ozone hole in the very cold Antarctic polar vortex. Since PSCs are very sensitive to temperature, cold winter vortex conditions often presage a large ozone depletion the following spring (note that polar ozone depletion only occurs in sunlight and so is a spring time phenomena in both hemispheres). This is pretty much undisputed at this point (well, at least by serious scientists). We here at RealClimate even used this relationship to predict (successfully) a particularly large Arctic ozone depletion event in 2005.

Dr. Lu’s theory though posits an additional mechanism to release the Cl from the CFCs – and that is through GCR effects. Specifically, Lu suggests that the action of the GCR on CFCs attached to PSCs causes more Cl to be released, thus potentially delivering more Cl exactly where it could enhance polar depletion most effectively. The evidence for this comes from correlations of ozone loss with GCR (over a couple of solar cycles) and some suggestive lab experiments. Note that this does not call into question the anthropogenic source of the Cl which is still from CFCs.

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