Development of iron-mediated molecular chlorine chemistry in GEOS-Chem: model description, evaluation and global atmospheric implication

Abstract. Molecular chlorine (Cl₂) plays a significant role in shaping atmospheric oxidative capacity (AOC), yet global models tend to underestimate Cl₂ concentrations due to incomplete representations of its formation pathways. Here, we implement an iron (Fe)-mediated Cl₂ formation mechanism into the GEOS-Chem model, explicitly representing the dynamic solubility of iron driven by acid processing, organic complexation, and mineralogical variability. The updated mechanism substantially improves the model performance for tropospheric Cl₂, increasing correlation coefficient with observations from 0.55 to 0.88 relative to the Base simulation (without Fe-Cl mechanism). Global surface mean Cl₂ concentration increases about fivefold (from 0.4 to 2.2 pptv) which strengthens radical propagation, leading to approximately threefold and fourfold rise in global Cl and ClO radicals, respectively. These radical perturbations further result in pronounced spatial heterogeneity in AOC. While global mean OH decreases by 5.7 % due to removal of O₃ by Cl and conversion of HOx to ClOx, eastern China experiences concurrent increases in O₃ and OH (up to 14 %), as enhanced RO₂ formation from Cl-accelerated VOCs oxidation elevates both OH and O₃ under high‑NOx conditions. The enhanced AOC also intensifies secondary aerosol formation in eastern China, yielding a maximum of 6 % increase in PM_2.5 concentrations during wintertime, driven primarily by accelerated nitrate production. These findings demonstrate that iron-mediated chlorine activation is an important but previously underrepresented driver of global halogen chemistry. Incorporating iron-mediated photochemistry into global models is therefore essential for accurately representing atmospheric oxidation processes and enhancing the reliability of air quality assessments.