Impact of recent Laboratory Measurements of the Absorption Cross Section of ClOOCl on our Understanding of polar Ozone Chemistry: Part I, Theory

• Main Authors: Rex, Markus, Schofield, Robyn, Canty, T., Salawitch, R., Bayes, K.
• Format: Conference Object
• Language: unknown
• Published: 2007
• Subjects: Arctic
• Online Access: https://epic.awi.de/id/eprint/17255/; https://hdl.handle.net/10013/epic.27101

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 between 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 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 the impact of these new results on our understanding of polar ozone chemistry.For typical Arctic conditions calculated 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 observations. Also the calculated levels of ClO become much smaller than those observed in the stratosphere. These results suggest that a major fraction of the observed ozone loss in the polar stratosphere is due to a currently unknown mechanism - a major challenge of our fundamental understanding of the polar stratospheric ozone loss process.We will discuss potential new chemistry that would lead to improved agreement between calculations of ozone loss and ClO diurnal variations with in-situ observations in the stratosphere.