Water Dependence of the HO2 Self Reaction:  Kinetics of the HO2−H2O Complex

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 109; no. 14; pp. 3153 - 3158
• Main Authors: Kanno, Nozomu, Tonokura, Kenichi, Tezaki, Atsumu, Koshi, Mitsuo
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 14.04.2005
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp044592+

Transient absorption spectra and decay profiles of HO2 have been measured using cw near-IR two-tone frequency modulation absorption spectroscopy at 297 K and 50 Torr in diluent of N2 in the presence of water. From the depletion of the HO2 absorption peak area following the addition of water, the equilibrium constant of the reaction HO2 + H2O ↔ HO2−H2O was determined to be K 2 = (5.2 ± 3.2) × 10-19 cm3 molecule-1 at 297 K. Substituting K 2 into the water dependence of the HO2 decay rate, the rate coefficient of the reaction HO2 + HO2−H2O was estimated to be (1.5 ± 0.1) × 10-11 cm3 molecule-1 s-1 at 297 K and 50 Torr with N2 as the diluent. This reaction is much faster than the HO2 self-reaction without water. It is suggested that the apparent rate of the HO2 self-reaction is enhanced by the formation of the HO2−H2O complex and its subsequent reaction. Results are discussed with respect to the kinetics and atmospheric chemistry of the HO2−H2O complex. At 297 K and 50% humidity, the concentration ratio of [HO2−H2O]/[HO2] was estimated from the value of K 2 to be 0.19 ± 0.11.