Gaseous results indicated that SO₃ hydrolysis with formic sulfuric anhydride (FSA) has a Gibbs free energy barrier as low as 1.5 kcal mol^-1 and can effectively compete with other SO₃ hydrolysis. Interfacial BOMD (Born–Oppenheimer molecular dynamics) simulations illustrated that FSA-mediated SO₃ hydrolysis at the gas–liquid interface occurs through a stepwise mechanism and can be completed within a few picoseconds. ACDC (Atmospheric Clusters Dynamics Code) kinetic simulations indicated that FSA significantly enhances cluster formation rates in the H₂SO₄–NH₃ system.