Kinetics of HO 2 + HO 2 ! H 2 O 2 + O 2 : Implications for Stratospheric H

[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] cm molecule 1 s 1 (2s 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.