Kinetics of HO_2 + HO_2 → H_2O_2 + O_2: Implications for Stratospheric H_2O_2

Abstract

The reaction HO_2 + HO_2 → H_2O_2 + O_2(1) has been studied at 100 Torr and 222 K to 295 K. Experiments employing photolysis of Cl_2/CH_3OH/O_2/N_2 and F_2/H_2/O_2/N_2 gas mixtures to produce HO_2 confirmed that methanol enhanced the observed reaction rate. At 100 Torr, zero methanol, k_1 = (8.8 ± 0.9) 10^(−13) × exp[(210 ± 26)/T] cm^3 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 k_1, which includes the dependence on bath gas density, is k_1 = (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 [H_2O_2] in the lower to middle stratosphere.

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