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]/dt ) 2k 1 [BrO] 2 ) was found to be temperature and pressure independent at T g 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 h 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 f 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 f 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 , ∆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 f 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.

OIO and the atmospheric cycle of iodine

“…The laboratory measurements were performed using the technique of laser flash photolysis with kinetic UV-visible absorption spectroscopy employing a charged coupled device detection system [Rowley et al, 1996;Harwood et al, 1998]. …”

Kinetics of the BrO + HO2 reaction over the temperature range T = 246–314 K

“…This enabled unequivocal determination of BrO concentrations at each time point, despite the presence of other absorbers, as has been demonstrated in our previous work. 33,34,35 The BrO reference spectra were taken from the work of Wilmouth et al 36 These crosssections are reported at higher spectral resolution (0.4 nm FWHM) than spectra recorded in the present study (1.1 nm FWHM), so a smoothing function using a…”

“…Signal (photocharge) was transferred row by row down the array, out of the illuminated region, to record sequential wavelength and time resolved transmission spectra of the photolysed mixture on kinetic timescales, as shown, specifically for BrO, in our previous studies. 33,34,35 Spectra were recorded over the wavelength range λ = 274 -338 nm, at a spectral resolution of 1.1. nm full width half maximum. Typically, 1000…”

“…34,34,35 The experimental conditions chosen for the present study were similar, the only difference being the use of Cl 2 in the present work as an extra source of Br atoms via the reaction of Cl with Br 2 (8). Therefore the results of the current work are compared to the more comprehensive and extensive study of Ferracci et al 33 Figure 7(a) shows that the overall rate constant at T = 298 K obtained for reaction (3) However, the overall effect on the values of k 1 arising from these differences in approach appears to be small as indicated by their general agreement and the sensitivity parameters discussed above.…”

Trace measurement of BrO at the ppt level by a transportable mode-locked frequency-doubled cavity-enhanced spectrometer

“…A photolysis yield of 4% for reaction (I ) was estimated by CRDS measurements for relatively highly concentrated samples (up to 5 × 10 12 molecule cm −3 ) of It should be noted that the concentration of BrO produced is proportional to that of Br 2 only at low concentrations of both Br and BrO. Indeed, for more concentrated samples, the self-reaction of BrO, together with its reaction with bromine atoms, becomes relevant to the kinetics of reaction, leading to BrO depletion [25,26]. Ring-down measurements were taken with the photolysis LEDs off, before and after each series of measurements in order to acquire a reference spectrum, free of BrO.…”