Stratospheric Halogen Chemistry
Halogens are among the most reactive chemical elements. In the atmosphere, chlorine and bromine in particular are among the most efficient catalysts in stratospheric ozone removal mechanisms, and the global thinning of the Earth's ozone shield, including the regular occurrence of an “ozone hole” over Antarctica in winter, observed since the 1980s is almost entirely caused by the increase of stratospheric chlorine and bromine levels due to anthropogenic emissions of CFCs and halons. While in the upper stratosphere, rather simple catalytic cycles involving atomic oxygen prevail, somewhat more complex mechanisms are responsible for ozone removal in the lower stratosphere. Two specific cycles, the ClO dimer cycle and the ClO-BrO-cycle, are responsible for the pronounced ozone loss in both hemispheres in polar winter. The reactivity and partitioning within the inorganic halogen families determines their relative ozone depletion efficiency. Unfortunately, the kinetics of some reactions involved are still poorly understood. In particular, the photolysis rate of the ClO dimer—under most conditions rate-limiting for ozone loss—has not been satisfactorily quantified, but some progress towards a better understanding has been made in recent studies. Bromine released from VSLS and chlorine activated on high altitude cirrus clouds also destroy ozone in the UTLS region, where ozone trends can have a significant impact on radiative forcing and thus climate change.