Abstract: Experimental studies demonstrate that sulfur trioxide (SO₃) and formic acid (HCOOH, FA) rapidly react to form formic sulfuric anhydride (HCOOSO₃H, FSA). Compared with sulfuric acid (H₂SO₄, SA), a well-established nucleation precursor, FSA exhibits lower saturation vapor pressure and a greater number of intermolecular interaction sites, suggesting its potential contribution to atmospheric new particle formation (NPF). However, its nucleation capability remains unclear. This study employs density functional theory to evaluate the nucleation potential of FSA with 62 common atmospheric species, and compares it with that of its parent compound formic acid and the typical nucleation precursor sulfuric acid, thereby comprehensively assessing FSA’s potential role in NPF and its atmospheric implications. Results indicate that FSA can spontaneously form dimers with common atmospheric monomers, and proton transfer occurs within dimer clusters formed with 18 amine-containing compounds. Among these, the FSA–monoethanolamine (MEA) dimer exhibits the most negative ΔG value, indicating that MEA possesses the strongest ability to promote initial nucleation of FSA. Furthermore, based on the most stable clusters (FSA–MEA, FA–MEA, and SA–MEA), the hydration behavior and hygroscopicity of these dimers were investigated. It was found that cluster stability increases with the number of water molecules (n=0–6). Under varying humidity conditions, the sensitivity of dimer hydrates to humidity follows the order: SA–MEA > FSA–MEA > FA–MEA. As cluster size increases, both the isotropic mean polarizability and Rayleigh scattering intensity increase linearly, in the order: FSA–MEA > SA–MEA > FA–MEA. This suggests that FSA–MEA has a stronger capacity to enhance the light extinction properties of atmospheric aerosols than FA–MEA and SA–MEA, thereby exerting a more adverse impact on atmospheric visibility.

Key words: Formic acid thionic anhydride, Dimer, Atmospheric aerosol, Hydrated clusters, Nucleation capacity