Understanding the kinetics of the ClO dimer cycle

[摘要] Among the major factors controlling ozone loss in the polar vortices inwinter/spring is the kinetics of the ClO dimer catalytic cycle. Here, wepropose a strategy to test and improve our understanding of these kinetics bycomparing and combining information on the thermal equilibrium between ClOand Cl₂O₂, the rate of Cl₂O₂ formation, and theCl₂O₂ photolysis rate from laboratory experiments, theoreticalstudies and field observations. Concordant with a number of earlier studies,we find considerable inconsistencies of some recent laboratory results withrate theory calculations and stratospheric observations of ClO andCl₂O₂. The set of parameters for which we find the best overallconsistency – namely the ClO/Cl₂O₂ equilibrium constant suggestedby Plenge et al. (2005), the Cl₂O₂ recombination rate constantreported by Nickolaisen et al. (1994) and Cl₂O₂ photolysis ratesbased on absorption cross sections in the range between the JPL 2006assessment and the laboratory study by Burkholder et al. (1990) – is notcongruent with the latest recommendations given by the JPL and IUPAC panelsand does not represent the laboratory studies currently regarded as the mostreliable experimental values. We show that the incorporation of new Pope etal. (2007) Cl₂O₂ absorption cross sections into several models,combined with best estimates for other key parameters (based on either JPLand IUPAC evaluations or on our study), results in severe modelunderestimates of observed ClO and observed ozone loss rates. This findingsuggests either the existence of an unknown process that drives thepartitioning of ClO and Cl₂O₂, or else some unidentified problemwith either the laboratory study or numerous measurements of atmospheric ClO.Our mechanistic understanding of the ClO/Cl₂O₂ system is grosslylacking, with severe implications for our ability to simulate both presentand future polar ozone depletion.

[学科分类] 大气科学