Enhancing SO₃ Hydrolysis and Nucleation: The Role of Formic Sulfuric Anhydride

Abstract. Although the nucleation route driven by sulfuric acid (H₂SO₄) and ammonia (NH₃) primarily dominates new particle formation (NPF) in the atmosphere, exploring the role of other trace species on H₂SO₄-NH₃ system is crucial for a more comprehensive insight into NPF processes. Formic sulfuric anhydride (FSA) has been observed in atmospheric environment and is found in abundance in atmospheric fine particles. Nevertheless, its effect on SO₃ hydrolysis and NPF remain poorly understood. Here, we studied the enhancing effect of FSA on gaseous and interfacial SO₃ hydrolysis as well as its impact on H₂SO₄-NH₃-driven NPF occurring through quantum chemical calculations, atmospheric clusters dynamics code (ACDC) kinetics combined with Born-Oppenheimer molecular dynamics (BOMD). Gaseous-phase findings indicate that FSA-catalyzed SO₃ hydrolysis is nearly barrierless. At an [FSA] = 10⁷ molecules•cm^-3, this reaction competes effectively with SO₃ hydrolysis in the presence of HNO₃ (10⁹ molecules•cm^-3), HCOOH (10⁸ molecules•cm^-3) and H₂SO₄ (10⁶ molecules•cm^-3) in the range of 280.0–320.0 K. At the gas-liquid nanodroplet interface, BOMD simulations reveal that FSA-mediated SO₃ hydrolysis follows a stepwise mechanism, completing within a few picoseconds. Notably, FSA enhances the formation rate of H₂SO₄-NH₃ clusters by over 10⁷ times in regions with relatively high [FSA] at elevated temperatures. Additionally, interfacial FSA^- ion has the ability to appeal precursor species for particle formation from the gaseous phase to the water nanodroplet interface, thereby facilitating particle growth. These results present new comprehensions into both the pathways of H₂SO₄ formation and aerosol particle growth in polluted boundary layer.