UV Absorption Cross Sections of ClOOCl Are Consistent with Ozone Degradation Models

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Science  08 May 2009:
Vol. 324, Issue 5928, pp. 781-784
DOI: 10.1126/science.1171305

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Cl in the Sunlight

A major assumption in the long-standing mechanistic model for halogen-induced degradation of stratospheric ozone is the steady generation of Cl atoms by photolysis of molecules such as ClOOCl by sunlight. Recently, however, laboratory data raised uncertainties in the ClOOCl absorption cross sections underlying this assumption. Chen et al. (p. 781) undertook precise mass-sensitive cross-section measurements to explore the discrepancy. Molecules were directly quantified before and after ultraviolet photolysis, avoiding interference from impurity or by-product absorptions that can plague more traditional detection schemes that measure attenuation of a light beam. The measured cross sections at two different ultraviolet wavelengths proved well within the margin necessary to support standard models of ozone degradation.


Recently, discrepancies in laboratory measurements of chlorine peroxide (ClOOCl) absorption cross sections have cast doubt on the validity of current photochemical models for stratospheric ozone degradation. Whereas previous ClOOCl absorption measurements all suffered from uncertainties due to absorption by impurities, we demonstrate here a method that uses mass-selected detection to circumvent such interference. The cross sections of ClOOCl were determined at two critical wavelengths (351 and 308 nanometers). Our results are sufficient to resolve the controversial issue originating from the ClOOCl laboratory cross sections and suggest that the highest laboratory estimates for atmospheric photolysis rates of ClOOCl, which best explain the field measurements via current chemical models, are reasonable.

  • * These authors contributed equally to this work.

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