Kinetics and Products of the IO + BrO Reaction

Abstract

The kinetics and products of reaction 1 between gas-phase BrO and IO radicals have been studied using the technique of laser photolysis with time-resolved UV−vis absorption spectroscopy. The O + IBr reaction 8, used as one source of IO and BrO radicals, was found to produce predominantly IO radicals at 295 K. The rate coefficient for reaction 8 is correlated with the branching ratio for IO production (channel 8a). Using a value of 0.7 for the branching ratio for IO production ( k 8a / k 8 ), k 8 was found to be (3.6 ± 2.4) × 10 -11 cm 3 molecule -1 s -1 at 295 K, with no significant temperature dependence between 210 and 333 K. Sensitivity tests using a kinetic model showed that, in addition to (1), decay traces were also sensitive to the rate coefficient for reaction 12: I + BrO → Br + IO. This rate coefficient was found to be (1.3 ± 1.2) × 10 -11 molecule -1 cm 3 s -1 at 295 K, with no significant temperature dependence between 235 and 333 K. The title reaction (1), IO + BrO → products was found to have a rate coefficient (8.5 ± 1.4) × 10 -11 cm 3 molecule -1 s -1 at 295 K. Reaction 1 exhibited a negative temperature dependence between 210 and 333 K, adequately described by k 1 = (6.7 ± 0.8) × 10 -12 exp ((760 ± 30)/ T ) molecules -1 cm 3 s -1 . No pressure dependence to k 1 was found between 100 and 760 Torr. All errors are 2σ. Five potential products exist for the IO + BrO reaction: IO + BrO → I + Br + O 2 (1a), IO + BrO → IBr + O 2 (1b), IO + BrO → OIO + Br (1c), IO + BrO → OBrO + I (1d), and IO + BrO → IBrO 2 (1e). No direct measurement of I or Br formation was perfomed. IBr was observed as a minor product ( k 1b / k 1 < 0.2). OIO formation was observed and shown to result from the IO + BrO reaction for the first time in this work. No evidence for OBrO formation was observed ( k 1d / k 1 < 0.15). No evidence for the formation or existence of IBrO 2 was observed. The results obtained here, in conjunction with other published work, were used to constrain the branching ratio, α, for OIO production in the IO + IO reaction, giving {0.30 ± 0.05 ≤ α ≤ 0.46 ± 0.08}. This constraint allowed the absorption cross section of OIO to be constrained giving (1.29 ± 0.22) ≥ σ OIO × 10 17 ≥ 0.87 ± 0.15) molecules -1 cm 2 at the (5,1,0) peak at 549 nm, 295 K, and 760 Torr. Results are compared with previous studies of the IO + BrO reaction, and the atmospheric implications are briefly discussed.