A Missing Source of New Particles in the Cold Arctic Atmosphere: The Synergistic Nucleation of Halogen Oxyacids

High concentrations of chloric acid (HClO3) have been detected in the sea-ice-covered Arctic atmosphere. While iodic acid (HIO3) is a confirmed key driver of new particle formation (NPF) in the Arctic, and its analogue iodous acid (HIO2) significantly enhances initial nucleation, NPF events are frequently observed even when HIO3 concentrations are insufficient to explain them. Owing to its strong acidity, we propose that HClO3 participates in aerosol nucleation, bridging this observational gap. Here, we systematically investigate the HIO3-HIO2-HClO3 ternary nucleation mechanism. Molecular-level analysis reveals that within the ternary clusters, HIO2 exhibits distinct basicity and preferentially accepts protons. Concurrently, the strong acidity and halogen-bond-inducing effects of HClO3 enable HIO3 to accept protons transferred from HClO3. Thermodynamic and kinetic stability analyses demonstrate that at typical Arctic temperatures (≤278.15 K), HClO3 significantly enhances HIO3-HIO2 nucleation. Specifically, under relevant atmospheric conditions ([HIO3] ≤ 107 cm-3 and [HClO3] ≥ 106 cm-3), the incorporation of HClO3 enhances cluster formation rates by up to an order of magnitude. This synergistic enhancement effectively explains the NPF events observed at Arctic sites (e.g., Ny-Ålesund) under iodine-poor conditions. These results indicate that HIO3-HIO2-HClO3 ternary nucleation is a crucial NPF pathway in marine environments with active halogen chemistry, providing a fundamental theoretical basis for understanding the sources, sinks, and climate impacts of marine chlorine-containing components.