Cooperative Institute for Research in Environmental Sciences

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Laboratory study of the UV absorption spectrum of the ClO dimer (Cl2O2)

Dimitrios K. Papanastasiou (1,2), Vassilis C. Papadimitriou (1,2) and James B. Burkholder (1)

(1) Earth System Research Laboratory, Chemical Sciences Division, NOAA, 325 Broadway, Boulder Colorado 80305 3328, USA (2) Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder 80309, USA

Catalytic ozone destruction cycles involving chlorine species play an important role in polar stratospheric ozone depletion. The UV photolysis of Cl2O2 (the ClO dimer) is a critical step in the ClO dimer catalytic cycle, which accounts for the majority of the observed polar ozone depletion. Consequently the determination of the ClO dimer UV spectrum has been the focus of a number of studies since the late 1980’s. However, to this day, discrepancies exist among the published laboratory studies of the Cl2O2 spectrum. Here, we report a study of the UV absorption cross sections of the ClO dimer, in the gas-phase, in the wavelength region 200 – 450 nm at low temperatures (200 – 228 K) that was designed to resolve these discrepancies. Experiments were carried out in a thermostated Pyrex (or Quartz) reactor where Cl2O2 was formed from the self-reaction of the ClO radical, which was produced following the pulsed laser photolysis of various precursor mixtures (e.g. 351 nm photolysis of Cl2 and Cl2O mixtures), under static or flow conditions. UV spectra of the gas-phase species were acquired using diode array spectroscopy. The findings of the present study are compared with previously published results. The impact of the present work on calculated polar ozone depletion will also be discussed.