The Cl 2 photosensitized decomposition of O 3: the reactions of ClO and OClO with O 3

Abstract

Cl 2 was photolyzed in the temperature range 254 – 296 K with 366 nm radiation in the presence of O 3· O 3 was removed with quantum yields of 5.8 ± 0.5, 4.0 ± 0.3, 2.9 ± 0.3 and 1.9 ± 0.2 at 24, 10,0 and—21 °C respectively, independent of the initial O 3 or Cl 2 removal quantum yields were 0.11 ± 0.2 at 24 °C for Cl 2 conversions of about 30%, much higher than expected from mass balance considerations based on the initial quantum yield of 0.089 ± 0.013 for OClO formation at 24 °C. The final chlorine-containing product was Cl 2O 7 which was observed. It was produced at least in part through the formation of OClO as an intermediate which was also observed with an initial quantum yield of Φ i[;OClO]; = 2.5 × 10 3 exp [;—(3025 ± 625)/ T]; independent of [O 3] or I a. The results are consistent with the reaction sequence ▪ The relative importance of the channels for reaction (2) at 296 K are the following: k 2a/ k 2 = 0.63; k 2b/ k 2 = 0.34; k 2c/ k 2 = 0.0032. Also, k 2c/ k 2b = 2.5 × 10 3 exp [;—(3025 ± 625)/ T];. The upper limit for the rate coefficient was found to be less than 1 × 10 −18 cm 3 s −1 for channels (24a) and (4b): ▪ The addition of nitrogen had no effect, but oxygen reduced —Φ[;O 3]; for unknown reasons and several possibilities are discussed. At temperatures below 296 K the equilibrium ▪ becomes apparent. The reaction of OClO with O 3 was also studied by direct mixing of OClO and O 3 in a quartz vessel in the temperature range 253 – 296 K, and in the Cl 2O 3 system by monitoring OClO decay in the dark in the temperature range 264 – 296 K: ▪ The Arrhenius rate coefficient recommended for reaction (5) is ▪ The low values of k 4 and k 5 obtained in this study indicate that reactions (4) and (5) are probably not important in atmospheric chemistry.