The Potential Influence of ClO·O2 on Stratospheric Ozone Depletion Chemistry

The inability to explain the observedoxygen suppression of chlorine photosensitized ozoneloss remains a gap in our understanding of thephotochemistry responsible for depletion of thestratospheric ozone layer. It has been suggested thatthe presence of a weakly bound ClO·O₂complexcould explain this effect. The existence of thiscomplex would alter the chlorine budget of thestratosphere, perhaps reducing the chlorine availablefor catalytic ozone destruction. On the other hand,the chemistry of ClO·O₂ provides two newpathwaysfor ClO dimer formation, which could increase the rateof catalytic ozone loss. In this paper, we constrainthe kinetic rate system of ClO·O₂ tomatch themeasured Cl_y budget. It is shown thatClO·O₂cannot be both fairly stable and rapidly form the ClOdimer, or the resulting partitioning of chlorinebecomes incompatible with observations of both ClO andtotal available chlorine. These constraints allow thateither: (1) the ClO·O₂ is fairly stable,but doesnot significantly enhance ClO dimer formation andtherefore has a negligible effect on ozone loss rates,or (2) the ClO·O₂ complex is only veryweaklystable, but does rapidly form the ClO dimer, andtherefore can influence stratospheric ozone depletion.Even at the ClO·O₂ mixing ratios allowedunderthe assumption of weak stability, 0.1 to 0.2 ppbv,significant ozone loss rate enhancements werecalculated. Of course, the chlorine budget constraintalso allows for a thirdpossibility; that ClO·O₂ is neither verystablenor forms Cl₂O₂ very rapidly. Measuredlimits on the reaction rates for ClO·O₂to form the ClO dimer would greatly aid the resolution of thisissue. Since the uncertainties aboutClO·O₂chemistry are so large, a potential role forClO·O₂ in stratospheric ozone loss cannotbe ruled out at this time.