THE REACTION O(3P)- TEMPERATURE-DEPENDENCE OF THE RATE-CONSTANT AND IMPORTANCE OF THE REACTION AS AN HOBR STRATOSPHERIC LOSS PROCESS(HOBR )

The absolute rate constant for the reaction O(P-3) + HOBr has been mea sured between T=233K and 423K using the discharge-flow kinetic techniq ue coupled to mass spectrometric detection. The value of the rate coef ficient at room temperature is (2.5+/-0.6) x 10(-11) cm3 molecule-1 s- 1 and the derived Arrhenius expression is (1.4+/-0.5) x 10(-10) exp[(- 430 +/- 260)/T] cm3 molecule-1 s-1. From these rate data the atmospher ic lifetime of HOBr with respect to reaction with O(P-3) is about 0.6h at z = 25 km which is comparable to the photolysis lifetime based on recent measurements of the UV cross section for HOBr. Implications for HOBr loss in the stratosphere have been tested using a 1D photochemic al box model. With the inclusion of the rate parameters and products f or the O + HOBr reaction, calculated concentration profiles of BrO inc rease by up to 33% around z = 35 km. This result indicates that the in clusion of the O + HOBr reaction in global atmospheric chemistry model s may have an an impact on bromine partitioning in the middle atmosphe re.