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Ozone again

Ozone never ceases to surprise. Scientists report in this issue of Nature that halogen compounds such as bromide and iodine oxide play an unexpectedly large role in chemically breaking down the gas in the troposphere.

Ozone is best known for its presence – or in fact for its seasonal absence – in the upper atmosphere. The stratospheric ozone layer, which protects the earth from harmful ultraviolet radiation, is still being chemically attacked by reactions involving long-lived chlorofluorocarbons (CFCs) banned 20 years ago. But Nature reported last year about a puzzling inconsistency discovered in the established chemical model of stratospheric ozone destruction.

Ozone_Molecule_VdW.png

The troposphere is a very different story, though. Near the ground, ozone can affect human health; in high concentrations it is even toxic. Above the ground layer it acts as a greenhouse gas which, when it chemically breaks up, initiates the removal of methane (an even stronger greenhouse gas) and other hydrocarbons from the atmosphere. So ozone loss in the troposphere is a good thing, really.

The gas is typically destroyed in marine regions by chemical reactions involving water vapour. Tropical oceans regions, where levels of solar radiation are high, are thought to be the most important ozone sink.

So what about halogens? Anthropogenically produced CFCs are stable in the troposphere, and are therefore not a reactive halogen source there. But models studies have suggested that bromine and iodine chemistry does contribute to breaking down ozone in the troposphere, potentially changing the global ozone budget by up to 20 %.

The measurements which Katie Read of the University of York and her team have made in the tropical Atlantic Ocean now provide strong confirmation of substantial ‘halogen-mediated’ ozone destruction. An editor’s summary and the paper are here.

The team analysed the first eight months of measurements from the new Cape Verde Atmospheric Observatory on the remote island of São Vicente in the tropical Atlantic. They found that average daily ozone loss was around 50 % more than predicted by a state-of-the-art chemistry model that excludes halogen chemistry. Aircraft-borne observations in the area confirmed these findings.

But the team also measured ubiquitous daytime concentrations of iodine and bromine oxides in the layer of the troposphere where most ozone loss occurs. These reactive molecules are emitted by marine algae and liberated from salty sea spray.

In a simple model calculation, the observed halogen concentrations induced just about the extra 50 % ozone loss in the region. The omission of halogen sources in atmospheric models “may lead to significant errors in calculations of global ozone budgets,” the authors write.

What’s special about this study, says Roland von Glasow in an accompanying news and views article here (subscription required), is that the measurements were made in the open ocean where they cannot be influenced by local features that produce halogens.

But even so, the results are significant. Ozone is a powerful oxidizing agent and a key constituent of the troposphere; changes in its concentration will feedback on the lifetime of methane and other hydrocarbons in the air.

It is good to know that tropical ocean regions are a larger-than-thought ozone sink. But the composition of the atmosphere is in such fine balance that its cleansing ability cannot be taken for granted as being permanent. In the face of the new measurements it seems essential to include halogen sources and chemistry in global climate models.

The measurements are likely more representative of the troposphere above the global ocean than previous studies near coastlines. But as Cape Verde is surrounded by biologically highly productive waters one must still be cautious when extrapolating the results, says von Glasow.

Note: RealClimate has an interesting discussion on how the media have spun the story.

Quirin Schiermeier

Picture caption: Model of an ozone molecule. Image created by Ben Mills.

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