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

Abstract

Sulfuric acid (H 2 SO 4 ) 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 H 2 SO 4 from sulfur dioxide (SO 2 ) via three key reactions: SO 2 oxidation by hydroxyl radicals (R1), reaction of HOSO 2 with O 2 (R2), and hydrolysis of SO 3 with water (R3). Reaction and activation energies were computed using the PBE, r 2 SCAN, DC-r 2 SCAN, CAM-B3LYP, and PBE0 flavors of density functional theory. The key findings reveal that reaction R1 has a calculated reaction energy (Δ G ) of −23.845 kcal/mol and an activation energy (Δ G *) of −0.628 kcal/mol by using the DC-r 2 SCAN 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 r 2 SCAN/TZV2P. This study demonstrates that r 2 SCAN and DC-r 2 SCAN provide a computationally efficient alternative to high-level methods, achieving comparable accuracy in the description of sulfuric acid formation.