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 che mistry. Potentially, HOBr could act as a temporary reservoir for strat ospheric BrO(y) compounds. Recently, the first direct measurement of t he rate coefficient for the reaction BrO + HO2 --> HOBr + O2, which is thought to be the main production channel for HOBr in the stratospher e, 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 N O(2PI) using the discharge-flow kinetic technique coupled to mass spec trometric detection. The HOBr was formed in situ by the reaction seque nce F + H2O --> OH + HF, OH + Br2 --> HOBr + Br. Using an excess of th e 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 re action between Br + HOBr could be observed, but it was not possible to quantify this because of the large amount of interference from backgr ound Br2 on the HOBr signal. This study represents the first determina tion of the rate constants for these reactions. Attention is paid to p ossible 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 wi th 0 is ca. 0.03 h and with respect to reaction with Cl is ca. 9.6 h a t z = 35 km (calculated maxima in [HOBr]).