Kinetic studies of the ClO + ClO association reaction as a function of temperature and pressure

Abstract

The kinetics of the association reaction of ClO radicals: ClO + ClO + M → Cl₂O₂ + M (1), have been investigated as a function of temperature T between 206.0–298.0 K and pressure p between 25–760 Torr using flash photolysis with time-resolved UV absorption spectroscopy. ClO radicals were generated following the photolysis of Br₂/Cl₂O mixtures in nitrogen diluent gas. Charge coupled device (CCD) detection of time resolved absorptions was used to monitor ClO radicals over a broad wavelength window covering the ClO (A ²Π ← X ²Π) vibronic absorption bands. The high pass filtered ClO absorption cross sections were calibrated as a function of temperature between T = 206.0–320 K, and exhibit a negative temperature dependence. The ClO association kinetics were found to be more rapid than those reported in previous studies, with limiting low and high pressure rate coefficients, in nitrogen bath gas, k₀ = (2.78 ± 0.82) × 10⁻³² × (T/300)^{−3.99 ± 0.94} molecule⁻² cm⁶ s⁻¹ and k_∞ = (3.37 ± 1.67) × 10⁻¹² × (T/300)^{−1.49 ± 1.81} molecule⁻¹ cm³ s⁻¹, respectively, (obtained with the broadening factor F_c fixed at 0.6). Errors are 2σ. The pressure dependent ClO association rate coefficients (falloff curves) exhibited some discrepancies at low pressures, with higher than expected rate coefficients on the basis of extrapolation from high pressures (p > 100 Torr). Reanalysis of data excluding kinetic data recorded below p = 100 Torr gave k₀ = (2.79 ± 0.85) × 10⁻³² × (T/300)^{−3.78 ± 0.98} molecule⁻² cm⁶ s⁻¹ and k_∞ = (3.44 ± 1.83) × 10⁻¹² × (T/300)^{−1.73 ± 1.91} molecule⁻¹ cm³ s⁻¹. Potential sources of the low pressure discrepancies are discussed. The expression for k₀ in air bath gas is k₀ = (2.62 ± 0.80) × 10⁻³² × (T/300)^{−3.78 ± 0.98} molecule⁻² cm⁶ s⁻¹. These results support upward revision of the ClO association rate coefficient recommended for use in stratospheric models, and the stratospheric implications of the results reported here are briefly discussed.