The Kinetics of the ClOOCl Catalytic Cycle

We use simultaneous in situ observations of [ClO] and [ClOOCl] obtained in the Arctic polar vortex to evaluate the kinetics of the ClOOCl catalytic cycle. Available laboratory measurements of the ClOOCl absorption cross sections, the ClO + ClO + M reaction rate constant, and the ClO/ClOOCl equilibrium constant are considered, along with compendium evaluations of these kinetic parameters. We show that the most recent (year 2015) recommendations for the kinetics that govern the partitioning of ClO and ClOOCl put forth by the Jet Propulsion Laboratory (JPL) panel are in extremely good agreement with the atmospheric observations of [ClO] and [ClOOCl]. Hence, we suggest that studies of polar ozone loss adopt these most recent recommendations. The most important difference with respect to calculations that rely on older recommendations is the temperature at which loss of O3 by the ClOOCl catalytic cycle terminates. The latest JPL recommendation for the equilibrium constant suggests that ClOOCl is less stable than previously assumed, resulting in an approximate 2°C downward 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 ClOOCl cycle, will be best addressed by future laboratory determinations of the absolute cross section of ClOOCl at the peak (i.e., close to a wavelength of 245 nm) as well as reduced uncertainty in the rate constant of the ClO + ClO + M reaction.