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.

Abstract

Herein, the reaction mechanisms and kinetics for the HO2 + SO3 → HOSO2 + 3O2 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⋯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⋯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 for the H2O and (k'WD1/ktot) (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 5-15 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 + 3O2 reaction.