Kinetics of the gas phase HO2 self-reaction: Effects of temperature, pressure, water and methanol vapours

Abstract

The kinetics of the gas phase HO₂ self-reaction have been studied using flash photolysis of Cl₂/CH₃OH/O₂/N₂ mixtures coupled with time-resolved broadband UV absorption spectroscopy. The HO₂ self-reaction rate coefficient (HO₂ + HO₂ → H₂O₂ + O₂ (R1)) has been determined as a function of temperature (236 < T < 309 K, at 760 Torr) and pressure (100 < p < 760 Torr, at 296 K). In addition, the effects of water vapour ((0–6.0) × 10¹⁷ molecules cm⁻³, 254 < T < 309 K at 760 Torr, 400 < p < 760 Torr at 296 K) and methanol vapour ((0.06–4.7) × 10¹⁷ molecules cm⁻³, 254 < T < 309 K, at 760 Torr) on the rate coefficient have been characterised. The observed rate coefficient, k₁, was found to exhibit a negative temperature dependence with both pressure dependent and pressure independent components, in agreement with previous studies. Furthermore, the rate coefficient k₁ was found to be enhanced in the presence of elevated H₂O or CH₃OH concentrations, as reported previously. This study reports the most extensive characterisation of the rate coefficient k₁ as a function of T, p, [H₂O] and [CH₃OH]. The present results indicate that k₁ is higher at low temperatures, and that enhancement of k₁ by H₂O is greater, than has been reported previously. The pressure dependence of k₁ at ambient temperature is in good agreement with previous studies. The rate enhancement by CH₃OH reported here is in good agreement with previous studies at ambient temperatures but is smaller at low temperatures than the most recent previous investigation suggests. The rate coefficient k₁ is adequately parameterised by: k₁(760 Torr) = {(1.8 ± 0.8) × 10⁻¹⁴ exp((1500 ± 120)/T/K)} × {1 + (2.0 ± 4.9) × 10⁻²⁵ [H₂O] exp((4670 ± 690)/T/K)} × {1 + (0.56 ± 1.00) × 10⁻²¹ [CH₃OH] exp((2550 ± 500)/T/K)} cm⁻³ molecule⁻¹ s⁻¹, where [H₂O] and [CH₃OH] are in molecules cm⁻³. Errors are 1σ, and statistical only. The atmospheric implications of these results are briefly discussed.