Trifluoroacetic acid enhances sulfuric acid–ammonia nucleation in the cold atmosphere: Molecular mechanism and atmospheric implications

Abstract. New particle formation (NPF) is a major source of atmospheric particulates and cloud condensation nuclei (CCN), yet conventional sulfuric acid (SA)–ammonia (NH₃) nucleation cannot fully explain the observed NPF and CCN generation. While recent CLOUD studies have shown that nitric acid can enhance SA–NH₃ nucleation in the cold upper troposphere [Nature, 605, 483-489, 2022], this mechanism can only explain particle formation under certain cold atmospheric environments. Here, we use trifluoroacetic acid (TFA) as a model perfluorocarboxylic acid (PFCA) to investigate the stabilizing effect of PFCAs on SA–NH₃ clusters, given their atmospheric nucleation relevance, long lifetime, and widespread distribution. Using quantum chemical calculations with Atmospheric Cluster Dynamics Code simulations, we find that TFA forms stable cage-like SA–NH₃–TFA clusters via strong hydrogen bonds and proton transfer. At 220 K (favourable cold conditions), TFA enhances the SA–NH₃ nucleation rate by up to 950-fold and contributes up to 93 % of the main simulated growth flux at low SA. TFA assisted stabilization represents a potentially efficient pathway for SA–NH₃ nucleation in cold atmospheres. This mechanism is most directly relevant to the cold boundary layer environments where TFA has been measured. In the upper troposphere, its role serves as a critical low-temperature mechanistic insight, pending direct observations of gas-phase TFA aloft. This study provides a molecular-level foundation for understanding general PFCA-enhanced nucleation mechanisms in cold atmospheric environments.