Kinetics and branching ratio studies of the reaction of C2H5O2 + HO2 using chemical ionisation mass spectrometry

Abstract

The overall rate coefficient for the reaction of C₂H₅O₂ with HO₂ was determined using a turbulent flow chemical ionization mass spectrometer (TF-CIMS) system over the pressure range of 75 to 200 Torr and temperatures between 195 and 298 K. The temperature dependence of the overall rate coefficient for the reaction between C₂H₅O₂ and HO₂ was fitted using the following Arrhenius expression: k(T) = (2.08^+0.87_−0.62) × 10⁻¹³ exp [(864 ± 79)/T] cm⁻³ molecule⁻¹ s⁻¹. The upper limits for the branching ratios for reactive channels leading to O₃ and OH production were quantified for the first time. A tropospheric model 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 very small and will not in itself be a major cause of uncertainty in modelled concentrations. However, at low temperatures, where there is a wide discrepancy between existing kinetic studies, modelling using the range of kinetic data in the literature shows a small but significant variation for [C₂H₅O₂], [C₂H₅OOH], [NO_x] and the HO₂ : OH ratio. Furthermore, a structure–activity relationship (SAR) was developed to rationalise the reactivity of the reaction between RO₂ and HO₂.