A Model Analysis of Halogen Kinetics: The ClOOCl Catalytic Cycle Revisited

We revisit prior analyses of simultaneous in situ observations of [ClO] and [ClOOCl] obtained in the Arctic polar vortex to evaluate the recommended updates that govern the kinetics of the ClOOCl catalytic cycle.  Available laboratory measurements of the ClOOCl absorption cross sections, the ClO+ClO reaction rate constant, and the ClO/ClOOCl equilibrium constant are considered, along with compendium evaluations of these kinetic parameters.  We show that the latest recommendations for the kinetics that govern the partitioning of ClO and ClOOCl put forth by the JPL panel in Spring 2016 (JPL 15-10) are in good agreement with atmospheric observations of [ClO] and [ClOOCl].  Hence, we suggest that studies of polar ozone loss adopt these most recent recommendations.  The latest JPL recommendation for the equilibrium constant suggests that ClOOCl is less stable than previously assumed, resulting in a shift in the termination temperature of polar ozone loss due to the ClOOCl catalytic cycle.  Remaining uncertainties in our knowledge of the kinetics that govern the partitioning of ClO and ClOOCl within the activated vortex, and hence the efficiency of O₃ loss by the ClO+ClO cycle, will be best addressed by future laboratory determinations of the absolute cross section of ClOOCl as well as measurements designed to reduce the uncertainty in the rate constant of the ClO+ClO reaction at cold temperatures characteristic of the polar, lower stratosphere.