Chlorine radicals in the environment: A critical review of reactivity, formation, and reaction across aqueous and atmospheric compartments

Chlorine radicals (Cl • ), as short-lived yet highly reactive oxidants, critically govern redox equilibrium and pollutant fate in both aqueous and atmospheric environments. Despite their significance, no prior review has systematically integrated their formation pathways, identification techniques, and reaction mechanisms across these diverse compartments. Here, we critically review Cl • sources (e.g., photolysis of chlorinated compounds, photocatalytic/electrochemical activation of chloride anions (Cl - ), and sea-salt-derived processes in the atmosphere) and evaluate current approaches for their identification and quantification, including electron spin resonance (ESR) for direct detection, quenching experiments, and probe compounds targeting specific reaction pathways. The dominant reaction mechanisms of Cl • with organic contaminants (addition to unsaturated bonds, H-abstraction, and single electron transfer (SET)) are elucidated, highlighting the controlling effects of molecular structure and environmental conditions. Furthermore, we systematically examine the roles of pH, oxidant dosage, and co-existing species (e.g., inorganic anions and NOM) in regulating Cl • formation and reactivity. Unlike existing reviews, this work uniquely synthesizes cross-media data (aqueous vs. atmospheric) and identifies knowledge gaps, such as the limited availability of in situ identification and quantification methods for transient Cl • intermediates in complex matrices. This review provides a scientific basis for optimizing Cl • -based advanced oxidation processes for aqueous-phase remediation and for improving atmospheric models that assess Cl • impacts on ozone and methane chemistry.