Kinetics and Mechanism of the BrO Self-Reaction:  Temperature- and Pressure-Dependent Studies

Abstract

The flash photolysis/UV absorption technique has been used to study the self-reaction of BrO radicals over the temperature range 222−298 K and the pressure range 100−760 Torr of N 2 or O 2 . Two chemical sources of BrO radicals were used: photolysis of Br 2 in the presence of excess ozone and photolysis of O 2 in the presence of excess Br 2 . The overall rate constant, k 1, for the BrO self-reaction (defined by −d[BrO]/d t = 2 k 1 [BrO] 2 ) was found to be temperature and pressure independent at T ≥ 250 K, with k 1 = (2.88 ± 0.20) × 10 -12 cm 3 molecule -1 s -1 . At temperatures below 250 K, k 1 was found to be pressure dependent, due to the emergence of a new termolecular channel of the BrO self-reaction 1c, −1c forming the BrO dimer, Br 2 O 2 (BrO + BrO + M ⇌ Br 2 O 2 + M). Channel-specific rate constants were determined for the two bimolecular channels of the BrO self-reaction above 250 K, giving for (1a) (BrO + BrO → 2Br + O 2 ) k 1a = (5.31 ± 1.17) × 10 -12 exp{(−211 ± 59)/ T } cm 3 molecule -1 s -1 and for (1b) (BrO + BrO → Br 2 + O 2 ) k 1b = (1.13 ± 0.47) × 10 -14 exp{(983 ± 111)/ T } cm 3 molecule -1 s -1 . Below 250 K, the overall rate coefficient of the two bimolecular channels is reduced as the dimer forming channel emerges. At 235 and 222 K, rate constants for the formation ( k 1c ) and decomposition ( k - 1c ) of Br 2 O 2 were recorded. Using the values for K 1c, Δ H r for reaction 1c was estimated as −58.6 ± 0.1 kJ mol -1 . A UV absorption spectrum attributed to Br 2 O 2 was also recorded over the wavelength range 300−390 nm. The cross section of the smooth Br 2 O 2 spectrum was found to be 1.2 × 10 -17 cm 2 molecule -1 at 320 nm. These results are rationalized in terms of a mechanism of the BrO self-reaction that shows competition, at low temperatures, between collisional quenching and unimolecular dissociation of an excited BrOOBr* intermediate. The rate constant for the reaction of oxygen atoms with molecular bromine was also determined in the course of these experiments [O + Br 2 → BrO + Br (5)], giving k 5 = (5.12 ± 1.86) × 10 -13 exp{(989 ± 91)/ T } cm 3 molecule -1 s -1 . All errors are 1σ. The atmospheric implications of these results are discussed.