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Stratospheric Ozone Chemistry
Ozone (O3) in the upper atmosphere (the stratosphere) plays a vital role in absorbing harmful solar ultraviolet radiation, particularly in the biologically damaging UV-B part of the spectrum. As a result, depletion of stratospheric ozone following anthropogenic pollution is an environmental concern and model simulations of ozone loss attempt to replicate and predict ozone abundances. Such models also help elucidate the role played by stratospheric composition in affecting the Earth's climate, and it is essential that models therefore contain an accurate description of stratospheric gas phase chemistry under appropriate conditions. In our work, we carry out laboratory studies of gas phase halogen chemistry of relevance to stratospheric ozone loss. Halogens, principally chlorine, are released into the atmosphere in long lived anthropogenic pollutants such as the CFC's (chlorofluorocarbons) and the degradation chemistry following CFC release leads to the polar ozone 'holes' observed routinely in Springtime. We have recently studied the principal gas phase reaction resulting in Springtime polar ozone depletion events, the ClO association reaction. This reaction produces a ClO dimer species, which may then be photolysed by sunlight to regenerate chlorine atoms which destroy further O3: Our study of the ClO radical association reaction monitored ClO radicals using an ensemble of vibronic bands rather than at a single wavelength, wherein other absorbing species could interefere. This work characterized the reaction kinetics of ClO association as a function of temperature and pressure relevant to the upper atmosphere. Our results show that the reaction is more rapid than some previously reported work indicates, with potentially more efficient ozone loss expected to result from the reaction sequence illustrated above. More recently, we have studied the thermal stability of the ClO dimer species through analysis of the equilibrium between ClO radicals and Cl2O2 at near ambient temperatures. In related work, we have been investigating the self and cross- reactions of other halogen monoxide free radicals which play roles in a variety of regions in the atmosphere. Our research also encompasses the absorption spectroscopy of laser generated weakly - bound gaseous molecules which may act as active species 'reservoirs' in the atmosphere, and ab initio studies of the mechanisms of selected atmospherically important reactions.
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