Understanding the kinetics of the ClO dimer cycle

Abstract

Abstract. Among the major factors controlling ozone loss in the polar vortices in winter/spring is the kinetics of the ClO dimer catalytic cycle. Here, we propose a strategy to test and improve our understanding of these kinetics by comparing and combining information on the thermal equilibrium between ClO and Cl 2 O 2 , the rate of Cl 2 O 2 formation, and the Cl 2 O 2 photolysis rate from laboratory experiments, theoretical studies and field observations. Concordant with a number of earlier studies, we find considerable inconsistencies of some recent laboratory results with rate theory calculations and stratospheric observations of ClO and Cl 2 O 2 . The set of parameters for which we find the best overall consistency -namely the ClO/Cl 2 O 2 equilibrium constant suggested by Plenge et al. (2005), the Cl 2 O 2 recombination rate constant reported by Nickolaisen et al. (1994) and Cl 2 O 2 photolysis rates based on absorption cross sections in the range between the JPL 2006 assessment and the laboratory study by Burkholder et al. (1990) -is not congruent with the latest recommendations given by the JPL and IUPAC panels and does not represent the laboratory studies currently regarded as the most reliable experimental values. We show that the incorporation of new Pope et al. (2007) Cl 2 O 2 absorption cross sections into several models, combined with best estimates for other key parameters (based on either JPL and IUPAC evaluations or on our study), results in severe model underestimates of observed ClO and observed ozone loss rates. This finding suggests either the existence of an unknown process that drives the partitioning of ClO and Cl 2 O 2 , or else some unidentified problem with either the laboratory study or numerous measurements of atmospheric ClO. Our mechanistic understanding of the ClO/Cl 2 O 2 system is grossly lacking, with severe implications for our ability to simulate both present and future polar ozone depletion.

Retrievals of chlorine chemistry kinetic parameters from Antarctic ClO microwave radiometer measurements

“…Given these studies and studies performed in the laboratory (e.g., Plenge et al, 2005), it is more likely that K eq scale lies between 0.27 and 0.5 than being far greater than recommended by JPL06. Furthermore, the study by von Hobe et al (2007) found that their stratospheric ClO observations were described well using a similar scaling for K eq . The JPL09 recommendation also provides a smaller K eq value (0.7×K eq JPL06 ) than previously recommended in JPL06.…”

“…Furthermore, field measurements of ClO and ClOOCl by von Hobe et al (2007) and Stimpfle et al (2004) and satellite measurements of ClO by Santee et al (2010) indicate discrepancies in the equilibrium constant K eq for ClOOCl compared to the JPL06 recommendation, and the more recent JPL09 recommendation. Therefore, there is a need to reduce the uncertainties in these reaction kinetic parameters, and thereby reduce uncertainties in modelled polar ozone loss.…”

A Match-based approach to the estimation of polar stratospheric ozone loss using Aura Microwave Limb Sounder observations

“…The approach has also been applied to observations from spaceborne solar occultation sounders (e.g., Sasano et al, 2000;Terao et al, 2002Terao et al, , 2012Hoppel et al, 2005). Similar techniques have been used in a variety of other studies, including quantification of chemical kinetics constants from observations in the polar vortex (von Hobe et al, 2007;Schofield et al, 2008;Sumińska-Ebersoldt et al, 2012); studies of PSC formation, chlorine activation and denitrification (e.g., Danilin et al, 2000;Santee et al, 2002;Rivière et al, 2003); dehydration processes occurring in the tropical tropopause layer (Inai et al, 2013); long-range transport of tropospheric pollutants (e.g., Methven et al, 2006); and validation of satellite observations (e.g., Morris et al, 2002;Danilin et al, 2002).…”

Evaluation of stratospheric chlorine chemistry for the Arctic spring 2005 using modelled and measured OClO column densities

“…Schofield et al (2008) retrieved a possible range for the combinations of the three model parameters from airborne measurements of ClO and they propose using a decreased forward rate constant for the Cl 2 O 2 formation in combination with a smaller equilibrium constant in comparison to the JPL recommendations. Von Hobe et al (2007) also reviewed the available laboratory references as well as rate theory calculations for the equilibrium constant between ClO and Cl 2 O 2 . They concluded that the best overall consistency with atmospheric observations as well as rate theory calculations can be found for the combination of the photolysis rates based on the Cl 2 O 2 absorption cross section in the range of Burkholder et al (1990) and JPL 2006, the equilibrium constant by Plenge et al (2005), and the Cl 2 O 2 formation rate constant by Nickolaisen et al (1994).…”