Mechanistic Insights into Sulfuric Acid Formation in the Atmosphere via First-Principles Simulations.

Sulfuric acid (H2SO4) is essential in the formation of atmospheric nucleation and cloudy condensation nuclei (CCN). This study uses density functional theory (DFT) to investigate the formation of H2SO4 from sulfur dioxide (SO2) via three key reactions: SO2 oxidation by hydroxyl radicals (R1), reaction of HOSO2 with O2 (R2), and hydrolysis of SO3 with water (R3). Reaction and activation energies were computed using the PBE, r2SCAN, DC-r2SCAN, CAM-B3LYP, and PBE0 flavors of density functional theory. The key findings reveal that reaction R1 has a calculated reaction energy (ΔG) of -23.845 kcal/mol and an activation energy (ΔG*) of -0.628 kcal/mol by using the DC-r2SCAN functional. Reaction R2, which was previously assumed to be barrierless, showed a small but significant activation barrier of 1.225 kcal/mol at the CAM-B3LYP/6-31G** level. Reaction R3 led to a calculated chemical reaction energy of -23.218 kcal/mol, with an activation energy of 5.648 kcal/mol using r2SCAN/TZV2P. This study demonstrates that r2SCAN and DC-r2SCAN provide a computationally efficient alternative to high-level methods, achieving comparable accuracy in the description of sulfuric acid formation.