A computational study of the HO + SO → HOSO + O 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 HO + SO → HOSO + O 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 HO is introduced into the HO + SO reaction, it not only enhances the stability of the reactant complexes by 9.0 kcal mol but also reduces the energy of the transition state by 8.7 kcal mol. As compared with HO, catalysts (HO), HSO, HSO⋯HO and (HSO) are more effective energetically, which not only results from a higher binding energy of 21.3-26.0 kcal mol for the reactant complexes but also from a reduction of the energy of the transition states by 8.6-17.2 kcal mol. Effective rate constant calculations show that, as compared with HO, catalysts (HO), HSO, HSO⋯HO and (HSO) 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 HO and ('/) (HO)-assisted HO + SO 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 HO and (HO) 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 HO and (HO) are not apparent in the gas-phase reaction of HO with SO 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 HO + SO → HOSO + O reaction.