KINETICS AND MECHANISM OF THE BRO SELF-REACTION - TEMPERATURE-DEPENDENT AND PRESSURE-DEPENDENT STUDIES

The flash photolysis/UV absorption technique has been used to study th e self-reaction of BrO radicals over the temperature range 222-298 K a nd 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 exce ss ozone and photolysis of O-2 in the presence of excess Br-2. The ove rall 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 independe nt at T greater than or equal to 250 K, with k(1) = (2.88 +/- 0.20) x 10(-12) cm(3) molecule(-1) s(-1). At temperatures below 250 K, k(1) wa s found to be pressure dependent, due to the emergence of a new termol ecular channel of the BrO self-reaction 1c, -1c forming the BrO dimer, Br2O2 (BrO + BrO + M reversible arrow Br2O2 + M) Channel-specific rat e constants were determined for the two bimolecular channels of the Br O self-reaction above 250 K, giving for (1a) (BrO + BrO --> 2Br + O-2) k(1a) = (5.31 +/- 1.17) x 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) x 10(-14) exp{(983 +/- 111)/T} cm(3) molecule(-1) s(-1). Below 25 0 K, the overall rate coefficient of the two bimolecular channels is r educed as the dimer forming channel emerges. At 235 and 222 K, rate co nstants for the formation (k(1c)) and decomposition (k(-1c)) of Br2O2 Were recorded. Using the values for K-1c, Delta H-r for reaction Ic wa s-estimated as -58.6 +/- 0.1 kJ mol(-1). A UV absorption spectrum attr ibuted 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 x 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 competitio n, at low temperatures, between collisional quenching and unimolecular ,dissociation of an excited BrOOBr intermediate. The rate constant fo r the reaction of oxygen atoms with molecular bromine was also determi ned in the course of these experiments [O + Br-2 --> BrO + Br (5)], gi ving k(5) = (5.12 +/- 1.86) x 10(-13) exp{(989 +/- 91)/T} cm(3) molecu le(-1) s(-1). All errors are 1 sigma. The atmospheric implications of these results are discussed.