HOBR KINETICS - REACTIONS OF HALOGEN ATOMS, OXYGEN-ATOMS, NITROGEN-ATOMS, AND NITRIC-OXIDE WITH HOBR

Relatively little is known about the role of HOBr in stratospheric chemistry. Potentially, HOBr could act as a temporary reservoir for stratospheric BrO(y) compounds. Recently, the first direct measurement of the rate coefficient for the reaction BrO + HO2 --> HOBr + O2, which is thought to be the main production channel for HOBr in the stratosphere, was found to be 6 times larger than previously suggested at (3.3 +/- 0.5) x 10(-11) cm3 molecule-1 s-1. In this work we have investigated the reactions of HOBr with F(2P), Cl(2P), O(3P), N(4S), Br(2P), and NO(2PI) using the discharge-flow kinetic technique coupled to mass spectrometric detection. The HOBr was formed in situ by the reaction sequence F + H2O --> OH + HF, OH + Br2 --> HOBr + Br. Using an excess of the atomic species or NO, the rate coefficients at T 298 K for the HOBr reactions were determined to be k(F) = (2.0 +/- 0.7) X 10(-10), k(Cl) = (1.1 +/- 0.3) x 10(-10), k(O) = (2.5 +/- 0.4) x 10(-11), k(N) < 2 X 10(-13), and k(NO) < 5 X 10(-14) (all units CM3 molecule-1 s-1). No reaction between Br + HOBr could be observed, but it was not possible to quantify this because of the large amount of interference from background Br2 on the HOBr signal. This study represents the first determination of the rate constants for these reactions. Attention is paid to possible interferences from reaction of HOBr precursors. The reactions of HOBr with Cl and 0 are markedly faster than the analogous reactions of HOCl. The atmospheric lifetime of HOBr with respect to reaction with 0 is ca. 0.03 h and with respect to reaction with Cl is ca. 9.6 h at z = 35 km (calculated maxima in [HOBr]).