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.