Effect of ammonia, ammonia‐water, and sulfuric acid on the HO2 + HO2 → H2O2 + 3O2 reaction in troposphere: Competition between stepwise and one‐step mechanisms

Abstract A detailed theoretical study on the reaction mechanisms for the formations of H 2 O 2 + 3 O 2 from the self‐reaction of HO 2 radicals under the effect of NH 3 , H 3 N···H 2 O, and H 2 SO 4 catalysts was performed using the CCSD(T)/CBS//M06‐2X/aug‐cc‐pVTZ method. The rate constant was computed using canonical variational transition state theory (CVT) with small curvature tunneling (SCT). Our results indicate that NH 3 ‐, H 3 N···H 2 O‐, and H 2 SO 4 ‐catalyzed reactions could proceed through both one‐step and stepwise routes. Calculated rate constants show that the catalyzed routes in the presence of the three catalysts all prefer stepwise pathways. Compared to the catalytic efficiency of H 2 O, the efficiencies of NH 3 , H 3 N···H 2 O, and H 2 SO 4 are much lower due to their smaller relative concentrations. The present results have provided a definitive example of how basic and acidic catalysts influence the atmospheric reaction of HO 2 + HO 2 → H 2 O 2 + 3 O 2 . These results further encourage one to consider the effects of basic and acidic catalysts on the related atmospheric reactions. Thus, the present investigation should have broad implications in the gas‐phase reactions of the atmosphere.