A computational study of the HO 2 + SO 3 → HOSO 2 + 3 O 2 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 2 + SO 3 → HOSO 2 + 3 O 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 H 2 O is introduced into the HO 2 + SO 3 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 H 2 O, catalysts (H 2 O) 2 , H 2 SO 4 , H 2 SO 4 ⋯H 2 O and (H 2 SO 4 ) 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 H 2 O, catalysts (H 2 O) 2 , H 2 SO 4 , H 2 SO 4 ⋯H 2 O and (H 2 SO 4 ) 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 H 2 O and ( k ' WD1 / k tot ) (H 2 O) 2 -assisted HO 2 + SO 3 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 H 2 O and (H 2 O) 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 H 2 O and (H 2 O) 2 are not apparent in the gas-phase reaction of HO 2 with SO 3 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 2 + SO 3 → HOSO 2 + 3 O 2 reaction.