Impact of recent laboratory measurements of the absorption cross section of ClOOCl on our understanding of polar ozone chemistry

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Rex, M. , Schofield, R. , Canty, T. and Salawitch, R. (2008): Impact of recent laboratory measurements of the absorption cross section of ClOOCl on our understanding of polar ozone chemistry , SPARC 4th General Assembly, 31 August - 5 September 2008, Bologna, Italy. .
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Uncertainties of the photolysis cross sections of ClOOCl have long been a limiting factor in our theoretical understanding of the rate of polar stratospheric ozone losses. Previous work suggested that values slightly larger than current recommendations, which are based on laboratory measurements, result in improved agreement be- tween model calculations of polar stratospheric ozone loss rates and observations while at the same time also leading to improved agreement between observations of the diurnal variation of ClO and model calculations of this species. But new laboratory work (Pope et al, 2007) on the cross sections of ClOOCl suggest that its photolysis under polar stratospheric winter/spring conditions is nearly an order of magnitude slower than what would be required to explain the observations of ozone loss and ClO in the atmosphere and a factor of six slower than a value based on the current recommendations. We show what the impact of these new results on our understand- ing of polar ozone chemistry is. In model calculations that are based on the new cross sections and for typical Arctic conditions ratios of ClO/ClOx decrease by about a factor of two. The ozone loss rate by the ClO-dimer cycle, so far believed to be the most efficient ozone loss cycle, drops by about a factor of four and the loss rate by the coupled ClO-BrO cycle by nearly a factor of two. Overall ozone loss rates calculated based on the known ozone loss mechanisms drop by a factor of two to three and become much smaller than ob- servations. Also the calculated levels of ClO become much smaller than those ob- served in the stratosphere. These results demonstrate the tremendous uncertainty of current ozone loss calculations that comes from the broad range of the published cross sections for ClOOCl. In particular they suggest that, if the most recent publica- tion of the cross sections (Pope et al., 2007) is correct, a major fraction of observed polar ozone loss is due to a currently unknown mechanism - clearly are a major chal- lenge of our fundamental understanding of the polar stratospheric ozone loss proc- ess. We will discuss potential chemical mechanisms that would lead to improved agree- ment between calculations of ozone loss based on the new cross sections with in-situ observations of ClO and ozone loss rates in the stratosphere.

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