Atmospheric chemistry of the self-reaction of HO2 radicals: stepwise mechanism versus one-step process in the presence of (H2O)n (n = 1–3) clusters

In the incorporation of the catalyst (H₂O)_n (n = 1–3) into the HO₂ + HO₂ → H₂O₂ + ³O₂ reaction, the catalytic effect of water, water dimers and water trimers is mainly derived from the contribution of a single water vapor molecule by a stepwise route.