Kinetics and mechanisms of halogen reactions involved in atmospheric ozone depletion
The purpose of this thesis is to address some of the uncertainties associated with the key gas phase reactions implicated in atmospheric ozone depletion. Laboratory studies, employing flash photolysis coupled with time resolved UV/visible absorption spectroscopy have been used to investigate the kinetics of several free radical reactions. The spectroscopy incorporated charge coupled device (CCD) technology which facilitated rapid time scale broad band data acquisition giving rise to unequivocal monitoring of multiple absorbing species. Classical or numerical models were used to simulate the experimental data by optimising kinetic parameters. The improvements in both the accuracy and the precision associated with the CCD studies have led to a profound improvement in our understanding of the roles of the free radical reactions studied in the Earth's atmosphere. The focus of the studies in this thesis was the reactions of halogenated species. These species are known to destroy atmospheric ozone through the reaction schemes such as: (1) XO + XO → 2X + O2 (2) X + O3 → XO + O2 Here, species X represents a halogen atom: Cl, Br or I, and this halogen atom is continously regenerated following flux through reactions (1) and (2) promoting further ozone destruction. The relatively high atmospheric abundance of ozone (typically parts per million in the lower stratosphere) compared to halogen atoms and free radicals (parts per trillion) ensures that reaction (1) is the rate determining step in the ozone depletion cycle. Consequently, a knowledge of the rate of reactions of type (1) under appropriate atmospheric conditions is fundamental in assessing the extent of halogen induced atmospheric ozone depletion. This thesis reports results from extensive investigations into three reactions of type (1) (i) BrO + BrO A study at 298 K and 760 Torr (ii) CIO + CIO A study over the ranges 206-320 K and 25-760 Torr, incorporating measurement of the differential CIO absorption cross section over this temperature range. (iii) BrO + CIO A study over the ranges 210-320 K and 100-760 Torr.