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 N2 or O2. Two chemical sources of BrO radicals were used: photolysis of Br2 in the presence of excess ozone and photolysis of O2 in the presence of excess Br2. The overall rate constant, k1, for the BrO self-reaction (defined by −d[BrO]/dt = 2k1[BrO]2) was found to be temperature and pressure independent at T ≥ 250 K, with k1 = (2.88 ± 0.20) × 10-12 cm3 molecule-1 s-1. At temperatures below 250 K, k1 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, Br2O2 (BrO + BrO + M ⇌ Br2O2 + 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 + O2) k1a = (5.31 ± 1.17) × 10-12 exp{(−211 ± 59)/T} cm3 molecule-1 s-1 and for (1b) (BrO + BrO → Br2 + O2) k1b = (1.13 ± 0.47) × 10-14 exp{(983 ± 111)/T} cm3 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 (k1c) and decomposition (k-1c) of Br2O2 were recorded. Using the values for K1c, ΔHr for reaction 1c was estimated as −58.6 ± 0.1 kJ mol-1. A UV absorption spectrum attributed to Br2O2 was also recorded over the wavelength range 300−390 nm. The cross section of the smooth Br2O2 spectrum was found to be 1.2 × 10-17cm2 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 + Br2 → BrO + Br (5)], giving k5 = (5.12 ± 1.86) × 10-13 exp{(989 ± 91)/T} cm3 molecule-1 s-1. All errors are 1σ. The atmospheric implications of these results are discussed.