Atmospheric Oxidation of Propanesulfinic Acid Initiated by OH Radicals: Reaction Mechanism, Energetics, Rate Coefficients, and Atmospheric Implications

The atmospheric oxidation mechanism and energetics of propanesulfinic acid (CH3CH2CH2S(O)OH, PSIA) initiated by OH radicals have been investigated at the CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ level of theory. The PSIA + •OH reaction proceeds through (i) H-atom abstraction and (ii) •OH addition pathways. The calculated energies indicate that the barrier height for the abstraction of H-atom from the -OH moiety of PSIA leading to the formation of CH3CH2CH2S(O)2 + H2O is estimated to be -4.7 kcal mol-1 relative to that of the separated reactants. The rate coefficients were determined for all possible reaction paths by RRKM-ME calculations using Master equation solver for multienergy well reactions (Mesmer) code in the atmospherically relevant temperatures between 200 and 320 K and bath gas pressures between 0.1 and 10 atm. The calculated bimolecular rate coefficients suggest that the formation of CH3CH2CH2S(O)2 + H2O is predominant compared to the other possible reaction paths in the studied temperature range. The total rate coefficient for the PSIA + •OH reaction was found to be ∼8.40 × 10-11 cm3 molecule-1 s-1 at T = 298 K and P = 1 atm. In addition, branching ratios, thermochemical parameters, atmospheric lifetime, and global warming potentials were determined. Overall, the results indicate that the atmospheric removal of PSIA with •OH results in the formation of sulfur dioxide (SO2) from C-S single bond fission in the CH3-CH2-CH2-S(O)2 radical, which is formed by H-atom abstraction from the OH group of PSIA. Thus, the SO2 product does not originate from the direct elimination of SO2 from unimolecular dissociation of PSIA. The formed SO2, propylene (C3H6), sulfurous acid (H2SO3), and hydroperoxyl (HO2) radical are major products that may contribute to global warming and aerosol formation.