Revisiting atmospheric sulfur chemistry: Acid-catalyzed pathways do not control H 2 SO 4 formation

Sulfuric acid (H₂SO 4 ) drives new particle formation and radiatively important aerosol processes in the atmosphere, but its formation pathways remain poorly resolved because of limited kinetics data. To clarify these pathways, we combined coupled-cluster calculations, reaction rate theory, and a global chemistry transport model to quantify acid-catalyzed SO 3 hydration and its atmospheric significance. We find that the overall kinetics is not governed by a tight transition state but rather is controlled by a preceding loose transition state associated with the formation of the precursor complex of the acid-catalyzed SO 3 hydration. Our results reverse a recent view that acid catalysis is the dominant contributor to atmospheric SO 3 hydration, instead supporting an earlier view that water catalysis dominates SO 3 hydration. Incorporating the revised kinetics into the GEOS-Chem atmospheric model reveals no significant impact of acid-catalyzed SO 3 hydration on the simulated concentrations of H 2 SO 4 . The present findings highlight the critical roles of variational transition states in kinetic modeling.