Kinetics of the ClO + HO2 reaction over the temperature range T = 210–298 K

Abstract

The rate coefficient for the atmospherically important radical reaction: ClO + HO₂ → Productswhich leads to ozone depletion, has been studied over the temperature range T = 210–298 K and at ambient pressure p = 760 ± 20 Torr. The reaction was studied using laser flash photolysis radical generation coupled with broadband charge coupled device absorption spectroscopy employing a two-dimensional charge-coupled-device (CCD) detection system. ClO radicals were generated following the photolysis of Cl₂ and Cl₂O gas mixtures diluted in nitrogen and oxygen. ClO radicals were monitored using broadband fingerprinting of their characteristic vibronic (A²Π ← X²Π) spectral structure, representing a definitive monitoring of this radical. Addition of hydroperoxy radical precursors to the gas mixture (methanol and oxygen) subsequently led to a competition for photolytically generated Cl atoms and a simultaneous prompt formation of both ClO and HO₂ radicals. Detailed analysis and modelling of the radical production routes provided a degree of constraint into numerical integration simulations which were then used to interrogate and fit to ClO temporal profiles to extract the rate coefficient k₁. The ambient temperature (T = 298 K) rate coefficient reported is k₁ = (8.5 ± 1.5) × 10⁻¹² cm³ molecule⁻¹ s⁻¹. The rate coefficient, k₁, is described by the Arrhenius expression:where errors are 1σ statistical only. This significant rate coefficient is greater than previously reported, with a stronger negative temperature dependence than previously observed. Consequently this suggests that the contribution of reaction (1) to ozone loss, in particular at mid-latitudes might be currently underestimated in models. This work reports atmospheric pressure kinetic parameters for this reaction which are greater than those reported from low pressure studies, perhaps supporting ClO and HO₂ association as predicted by previous theoretical studies of this process and highlighting the need for further pressure dependent experimental studies of the title reaction, which has been demonstrated here to be effective as an ozone loss process over a wide temperature range.