A computational study of the HO2 + SO3 → HOSO2 + 3O2 reaction catalyzed by a water monomer, a water dimer and small clusters of sulfuric acid: kinetics and atmospheric implications

Herein, the reaction mechanisms and kinetics for the HO2 + SO3 -> HOSO2 + O-3(2) reaction catalyzed by a water monomer, a water dimer and small clusters of sulfuric acid have been studied theoretically by quantum chemical methods and the Master Equation/Rice-Ramsperger-Kassel-Marcus (ME/RRKM) rate calculations. The calculated results show that when H2O is introduced into the HO2 + SO3 reaction, it not only enhances the stability of the reactant complexes by 9.0 kcal mol(-1) but also reduces the energy of the transition state by 8.7 kcal mol(-1). As compared with H2O, catalysts (H2O)(2), H2SO4, H2SO4 center dot center dot center dot H2O and (H2SO4)(2) are more effective energetically, which not only results from a higher binding energy of 21.3-26.0 kcal mol(-1) for the reactant complexes but also from a reduction of the energy of the transition states by 8.6-17.2 kcal mol(-1). Effective rate constant calculations show that, as compared with H2O, catalysts (H2O)(2), H2SO4, H2SO4 center dot center dot center dot H2O and (H2SO4)(2) can never become more efficient catalysts within the altitude range of 0-15 km due to their relatively lower concentrations. Besides, at 0 km altitude, the enhancement factor k' (WM1)/k(tot)) for the H2O and (k'(WD1)/k(tot)) (H2O)(2)-assisted HO2 + SO3 reaction within the temperature range of 280-320 K was respectively calculated to be 0.31%-0.34% and 0.16%-0.27%, while the corresponding enhancement factor of H2O and (H2O)(2) at higher altitudes of 515 km was respectively found only 0.002%-0.12% and 0.00001%-0.022%, indicating that the contributions of H2O and (H2O)(2) are not apparent in the gas-phase reaction of HO2 with SO3 especially at higher altitude. Overall, the present work will give a new insight into how a water monomer, a water dimer and small clusters of sulfuric acid catalyze the HO2 + SO3 -> HOSO2 + O-3(2) reaction.