Kinetics of HO2 + HO2 → H2O2 + O2: Implications for Stratospheric H2O2

[1] The reaction HO₂ + HO₂ → H₂O₂ + O₂(1) has been studied at 100 Torr and 222 K to 295 K. Experiments employing photolysis of Cl₂/CH₃OH/O₂/N₂ and F₂/H₂/O₂/N₂ gas mixtures to produce HO₂ confirmed that methanol enhanced the observed reaction rate. At 100 Torr, zero methanol, k₁ = (8.8 ± 0.9) 10⁻¹³ × exp[(210 ± 26)/T] cm³ molecule⁻¹ s⁻¹ (2σ uncertainties), which agrees with current recommendations at 295 K but is nearly 2 times slower at 231 K. The general expression for k₁, which includes the dependence on bath gas density, is k₁ = (1.5 ± 0.2) × 10⁻¹² × exp[(19 ± 31)/T] + 1.7 × 10⁻³³ × [M] × exp[1000/T], where the second term is taken from the JPL00-3 recommendation. The revised rate largely accounts for a discrepancy between modeled and measured [H₂O₂] in the lower to middle stratosphere.