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/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 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.