Altitude-Dependent Role of Nitric Acid in Iodic Acid-Iodous Acid Nucleation: From Marine Boundary Layer Catalyst to Upper Troposphere Core Component

Abstract. With global sulfur emissions declining and concurrent marine iodine emissions rising, new particle formation (NPF) driven by iodic acid (HIO₃) and iodous acid (HIO₂) has become critical to global aerosol and cloud condensation nuclei (CCN) budget. However, the role of ubiquitous nitric acid (HNO₃) in this iodine-driven nucleation across altitudes from the marine boundary layer (MBL) to the upper troposphere (UT) remains poorly understood. Herein, we integrated quantum chemical calculations with Atmospheric Cluster Dynamics Code (ACDC) simulations to unravel the altitude-dependent enhancement mechanism by which HNO₃ enhances HIO₃-HIO₂ nucleation. Under MBL conditions, HNO₃ acts as a catalyst to promote nucleation via collision and re-evaporation processes, yielding a modest 2–3-fold enhancement in nucleation. In contrast, as altitude increases to the UT, where cluster evaporation is effectively suppressed by low temperature, HNO₃ becomes a core component of nucleation clusters, driving a 200-fold enhancement in nucleation. Consequently, the cluster formation rates of the HNO₃–HIO₃–HIO₂ mechanism reach 10³–10⁴ cm^-3 s^-1, exceeding those of the well-documented H₂SO₄–NH₃–HNO₃ mechanism under comparable UT conditions. Our findings establish HNO₃ as a critical atmospheric agent that amplifies iodine oxoacid nucleation across altitudes, providing a critical chemical explanation for intense NPF events in both polluted coastal regions and the UT. This altitude‑dependent role of HNO₃ links marine iodine emissions to troposphere‑wide particle formation, with important implications for global CCN budgets and the refinement of climate models.