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 the GEOS-Chem global model tends to underestimate Cl2 concentrations due to incomplete representations of its formation pathways. Here, we adapt an iron (Fe)-mediated Cl2 formation mechanism into the GEOS-Chem model, explicitly representing the dynamic solubility of iron and Cl2 production. This implementation enables the GEOS-Chem model to better reproduce observed Cl2 concentrations, increasing correlation coefficient 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, causing 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 Cl-driven O3 removal 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. Consequently, the strengthened AOC intensifies regional secondary aerosol formation with wintertime PM2.5 in eastern China surges by up to 6 %, driven primarily by accelerated nitrate production. Conversely, a discernible decline in PM2.5 occurs in the downwind regions with enhanced AOC and also remote marine regions. These findings underscore the importance to consider iron-chlorine coupling chemistry in the GEOS-Chem global model for accurately representing atmospheric oxidation processes and enhancing its reliability of air quality assessments.