Equilibrium constant of the HO₂‐H₂O complex formation and kinetics of HO₂ + HO₂‐H₂O: Implications for tropospheric chemistry

Abstract

Complex formation between HO 2 and H 2 O and kinetics of the HO 2 ‐H 2 O complex were investigated using near‐infrared two‐tone frequency modulation spectroscopy at 250–350 K and 50 torr with N 2 diluent. From the depletion of the HO 2 signal in the presence of water, the equilibrium constant of the HO 2 ‐H 2 O complex formation was estimated. The stabilized energy of the HO 2 ‐H 2 O complex formation was estimated to be 31 ± 4 kJ mol −1 , and it was suggested that the complex has two hydrogen bonds. Using the equilibrium constant obtained from spectral measurements, the rate constant of the reaction HO 2 + HO 2 ‐H 2 O was estimated at 297–350 K and 50 torr with N 2 diluent. Weak dependence of the rate constant on temperature indicated that the temperature dependence of water effects on HO 2 self‐reaction kinetics was mainly controlled by that of the equilibrium constant of the HO 2 ‐H 2 O complex formation. Altitude profiles of HO 2 ‐H 2 O complexes and HO 2 self‐reaction rate were also estimated in the northwest Pacific Ocean. The importance of HO 2 ‐H 2 O complexes in the lower troposphere was suggested.