Water effect on the formation of 3O2 from the self-reaction of two HO2 radicals in tropospheric conditions

Abstract

Second-order perturbation theory and transition state theory rate constant calculations have been performed to gain insight into the effect of one and two water molecules on the process of 3O2 formation from the HO2+HO2 reaction that is proposed to be important in atmospheric chemistry. Based on the reaction of H2O⋯HO2+HO2 investigated by Zhu and Lin (2002), a comprehensive mechanism for a single water-catalyzed the title reaction was suggested in which the additional reactions of HO2⋯H2O+HO2 were also not neglected, with barrier heights between 1.10 and 1.79kcalmol−1, and the estimated reaction rate constants 1–2 orders-of-magnitude larger than the naked reaction estimates. At 298K the total enhancement factor of the reactions of H2O⋯HO2+HO2 and HO2⋯H2O+HO2 is up to ∼5.70%. The question whether two water molecules will affect 3O2 formation in the HO2+HO2 reaction was investigated by studying the reactions of H2O⋯HO2+H2O⋯HO2, H2O⋯HO2+HO2⋯H2O and HO2⋯(H2O)2+HO2. The results show that the reaction occurring through the H2O⋯HO2+HO2⋯H2O reactants is dominant. However, its effective rate constant within the temperature range of 216.7–298.2K is much smaller than that with a water molecule, showing that the positive water effect for the title reaction mainly comes from one water molecule.