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

Complex formation between HO2 and H2O and kinetics of the HO2‐H2O complex were investigated using near‐infrared two‐tone frequency modulation spectroscopy at 250–350 K and 50 torr with N2 diluent. From the depletion of the HO2 signal in the presence of water, the equilibrium constant of the HO2‐H2O complex formation was estimated. The stabilized energy of the HO2‐H2O 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 HO2 + HO2‐H2O was estimated at 297–350 K and 50 torr with N2 diluent. Weak dependence of the rate constant on temperature indicated that the temperature dependence of water effects on HO2 self‐reaction kinetics was mainly controlled by that of the equilibrium constant of the HO2‐H2O complex formation. Altitude profiles of HO2‐H2O complexes and HO2 self‐reaction rate were also estimated in the northwest Pacific Ocean. The importance of HO2‐H2O complexes in the lower troposphere was suggested.