Abstract

Photolysis of the ClO dimer is recognized as the rate determining step in seasonal polar stratospheric ozone depletion. As the photolysis rate of any species depends on the absorption cross section of that molecule the absorption cross-section of ClOOCl must be known to high precision if atmospheric models are to accurately predict the rate of ozone depletion in polar regions. Currently the published literature suggests discrepancies exist that become noticeably more significant at the longer wavelength region (>300nm). A study of Pope et al. showed substantially smaller values for the absorption cross section of the ClO dimer than previously suggested. This is important in the polar lower stratosphere where the intensity of light at longer wavelengths is higher, causing greater effects to ozone concentrations. One possible explanation for the wide range of results is due to the difficulties in producing ClOOCl under laboratory conditions. Molecular chlorine is present as an impurity in the laboratory synthesis of the ClO dimer. Therefore, it is vital to be able to subtract the Cl2 spectral interference from the ClO dimer spectrum. The use of broadband cavity enhanced absorption spectroscopy (BBCEAS) will allow the detection of absorption spectra over the 520-570nm wavelength range where Cl2 is believed to exhibit vibrational structure in this region. This data, together with UV spectra detected simultaneously, will allow the ClO dimer spectrum to be determined unambiguously by subtracting the influence of the Cl2 spectrum.