A Laboratory Study of the Photolysis of the ClO Dimer

The stratospheric ozone layer, located between altitudes of approximately 15 and 40 km, performs a number of critical roles in the Earth's atmosphere: It shields the biosphere from harmful UV radiation, determines the temperature structure and hence affects the circulation of the stratosphere, and is a radiatively active gas, that is, it acts as a greenhouse gas in our atmosphere. Following discovery of the 'ozone hole' over Antarctica in the early 1980s, considerable scientific effort has focussed upon understanding the causes of ozone depletion. Anthropogenic emissions have increased the stratospheric halogen loading, while the meteorological conditions of the polar stratosphere following the polar night favour a specific chemical reaction cycle: ClO radicals undergo self-reaction to form a dimer, Cl2O2, which photolyses releasing the constituent Cl atoms, which in turn react with ozone reforming ClO. The rate of this cycle, which is the major route for polar stratospheric ozone destruction, depends upon the photolysis rate (absorption cross sections) of Cl2O2. A number of laboratory studies of the absorption cross sections of Cl2O2 have been performed previously, with some disagreement between studies, particularly at wavelengths above 300 nm, where the signal is small and hence hard to measure, and interference effects from laboratory precursors may be significant. Unfortunately this is also the key region for the atmosphere / due to the spectral distribution of actinic flux, only wavelengths above 300 nm contribute significantly to the atmospheric photolysis of Cl2O2. Recently, measurements of ClOx species in the atmosphere from various remote sensing and in situ techniques have been used to constrain the photochemistry of Cl2O2, with results suggesting the cross sections should be *higher* than the evaluations (NASA-JPL, IUPAC) suggest. However, in March 2007 a new study of the Cl2O2 cross sections was published, from a highly respected laboratory kinetics group, which found the Cl2O2 photolysis rate to be a factor of 6 *lower* than earlier measurements indicated. This result implies that we do not have a quantitative understanding of polar stratospheric ozone loss, a finding of great scientific and societal importance. The aim of this project is to apply a new approach to the study of the photochemistry of Cl2O2, using a range of novel instrumentation to unequivocally constrain the various species present. In essence, we will generate Cl2O2 in a laboratory system under conditions representative of the polar stratosphere, photolyse the Cl2O2 at selected wavelengths using a laser, and measure the Cl atoms produced. We will use a resonance fluorescence technique to detect the Cl atoms, affording orders of magnitude greater sensitivity than the absorption approach employed previously, and will use Chemical Ionisation Mass Spectrometry (CIMS) to quantify both the Cl2O2, and interferant species such as Cl2 and Cl2O / the presence of which is likely to be responsible for discrepancies between previous studies. Again the detection limits for the CIMS system are orders of magnitude better than for the absorption approaches used previously. Our focus will be on the 300-350 nm region critical to the stratosphere. Experiments will be conducted at Birmingham, led by Dr William Bloss, using a new CIMS system developed for atmospheric field measurements by Dr Carl Percival from the University of Manchester. Our results will determine the photolysis rate for Cl2O2, and hence the rate of ozone destruction through the ClO + ClO cycle, with much greater accuracy and precision than has been achieved previously, and will address the discrepancies between previous measurements. Through our Project Partner, Prof. Martyn Chipperfield at the University of Leeds, our results will be incorporated in models of stratospheric chemistry and transport, to determine revised ozone loss rates for comparison with observations.