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

Area of Science:

• Atmospheric Chemistry
• Computational Chemistry
• Chemical Physics

Background:

• Sulfuric acid (H2SO4) plays a vital role in atmospheric nucleation and the formation of cloud condensation nuclei (CCN).
• Understanding the formation pathways of H2SO4 is essential for atmospheric modeling and predicting climate impacts.

Purpose of the Study:

• To investigate the formation of sulfuric acid (H2SO4) from sulfur dioxide (SO2) using density functional theory (DFT).
• To compute reaction and activation energies for three key reactions involved in H2SO4 formation.
• To evaluate the performance of various DFT functionals, including r2SCAN and DC-r2SCAN, for describing these atmospheric reactions.

Main Methods:

• Density Functional Theory (DFT) calculations were employed.
• Several DFT flavors were utilized: PBE, r2SCAN, DC-r2SCAN, CAM-B3LYP, and PBE0.
• Reaction and activation energies were computed for SO2 oxidation, HOSO2 + O2 reaction, and SO3 hydrolysis.

Main Results:

• Reaction R1 (SO2 oxidation) showed a reaction energy of -23.845 kcal/mol and activation energy of -0.628 kcal/mol with DC-r2SCAN.
• Reaction R2 (HOSO2 + O2) exhibited a small but significant activation barrier of 1.225 kcal/mol at the CAM-B3LYP/6-31G** level, contrary to previous assumptions.
• Reaction R3 (SO3 hydrolysis) yielded a reaction energy of -23.218 kcal/mol and an activation energy of 5.648 kcal/mol using r2SCAN/TZV2P.

Conclusions:

• The study highlights the importance of computational methods like r2SCAN and DC-r2SCAN for accurately describing sulfuric acid formation.
• These DFT functionals offer a computationally efficient alternative to high-level methods for atmospheric chemistry research.
• The findings provide valuable data for improving atmospheric models related to nucleation and cloud formation.