Rate Constant of the Reaction of OH Radicals with HBr over the Temperature Range 235-960 K.

Abstract

The kinetics of the reaction of hydroxyl radicals with HBr, important in atmospheric and combustion chemistry, has been studied in a discharge flow reactor combined with an electron impact ionization quadrupole mass spectrometer in the temperature range 235-960 K. The rate constant of the reaction OH + HBr → H2O + Br (1) was determined using both a relative rate method (using the reaction of OH with Br2 as a reference) and absolute measurements, monitoring the kinetics of OH consumption under pseudo-first-order conditions in excess of HBr. The observed U-shaped temperature dependence of k1 is well represented by the sum of two exponential functions: k1 = 2.53 × 10-11 exp(-364/T) + 2.79 × 10-13 exp(784/T) cm3 molecule-1 s-1 (with an estimated conservative uncertainty of 15% at all temperatures). This expression for k1, recommended for T = 240-960 K, combined with that from previous low temperature studies, k1 = 1.06 × 10-11 (T/298)-0.9 cm3 molecule-1 s-1 at T = 23-240 K, allows to describe the temperature behavior of the rate constant over an extended temperature range 23-960 K. The current direct measurements of k1 at temperatures above 460 K, the only ones to date, provide an experimental dataset for use in combustion and volcanic plume modeling and an experimental basis to test theoretical calculations.