Chlorine-Catalyzed Ozone Destruction: Cl Atom Production from ClOOCl Photolysis

Recent laboratory measurements of the absorption cross sections of the ClO dimer, ClOOCl, have called into question the validity of the mechanism that describes the catalytic removal of ozone by chlorine. Here we describe direct measurements of the rate-determining step of that mechanism, the production of Cl atoms from the photolysis of ClOOCl, under laboratory conditions similar to those in the stratosphere. ClOOCl is formed in a cold-temperature flowing system, with production initiated by a microwave discharge of Cl2 or photolysis of CF2Cl2. Excimer lasers operating at 248, 308, and 352 nm photodissociate ClOOCl, and the Cl atoms produced are detected with time-resolved atomic resonance fluorescence. Cl2, the primary contaminant, is measured directly for the first time in a ClOOCl cross section experiment. We find the product of the quantum yield of the Cl atom production channel of ClOOCl photolysis and the ClOOCl absorption cross section, (φσ)ClOOCl ) 660 ( 100 at 248 nm, 39.3 ( 4.9 at 308 nm, and 8.6 ( 1.2 at 352 nm (units of 10-20 cm2 molecule-1). The data set includes 468 total cross section measurements over a wide range of experimental conditions, significantly reducing the possibility of a systematic error impacting the results. These new measurements demonstrate that long-wavelength photons (λ ) 352 nm) are absorbed by ClOOCl directly, producing Cl atoms with a probability commensurate with the observed rate of ozone destruction in the atmosphere.