The potential influence of ClO·O2 on stratospheric ozone depletion chemistry
The inability to explain the observed oxygen suppression of chlorine photosensitized ozone loss remains a gap in our understanding of the photochemistry responsible for depletion of the stratospheric ozone layer. It has been suggested that the presence of a weakly bound ClO·O2 complex could explain this effect. The existence of this complex would alter the chlorine budget of the stratosphere, perhaps reducing the chlorine available for catalytic ozone destruction. On the other hand, the chemistry of ClO·O2 provides two new pathways for ClO dimer formation, which could increase the rate of catalytic ozone loss. In this paper we constrain the kinetic rate system of ClO·O2 to match the measured Cl(y) budget. It is shown that ClO·O2 cannot be both fairly stable and rapidly form the ClO dimer, or the resulting partitioning of chlorine becomes incompatible with observations of both CLO and total available chlorine. These constraints allow that either: (1) the ClO·O2 is fairly stable, but does not significantly enhance ClO dimer formation and therefore has a negligible effect on ozone loss rates, or (2) the ClO·O2 complex is only very weakly stable, but does rapidly form the ClO dimer, and therefore can influence stratospheric ozone depletion. Even at the ClO·O2 mixing ratios allowed under the assumption of weak stability, 0.1 to 0.2 ppbv, significant ozone loss rate enhancements were calculated. Of course, the chlorine budget constraint also allows for a third possibility; that ClO·O2 is neither very stable nor forms Cl2O2 very rapidly. Measured limits on the reaction rates for ClO·O2 to form the ClO dimer would greatly aid the resolution of this issue. Since the uncertainties about ClO·O2 chemistry are so large, a potential role for ClO·O2 in stratospheric ozone loss cannot be ruled out at this time.
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