Kinetics of the HO2 + NO reaction: A temperature and pressure dependence study using chemical ionisation mass spectrometry

Abstract

The overall rate coefficient (k₂) for the reaction HO₂ + NO (2) has been measured using the turbulent flow technique with chemical ionisation mass spectrometry (CIMS) for the detection of reactants and products. The temperature dependence of the rate coefficient was investigated between 183 and 300 K. Across the temperature range the experimentally determined rate coefficients showed good agreement with previous studies and were fitted using an Arrhenius type analysis to yield the expression k₂ = (3.98^+0.29_−0.27) × 10⁻¹² exp [(223 ± 16.5)/T] cm³ molecules⁻¹ s⁻¹. Experiments were carried out in the pressure range of 75 to 220 Torr within the stated temperature range, where the rate coefficients were shown to be invariant with pressure. Such invariance with pressure is in accord with recent theoretical calculations. This work represents an extension to the range of temperature and pressure over which the rate coefficient has been studied. A model of the troposphere has been used to assess the impact of the experimental error of the rate coefficients determined in this study on predicted concentrations of a number of key species, including O₃, OH, HO₂, NO and NO₂. In all cases it is found that the propagated error is rather small and will not in itself be a major cause of uncertainty in modelled concentrations.