Hydrogen chloride (HCl) is a major tropospheric reservoir of reactive chlorine. In coastal environments, it is thought to primarily originate from acid displacement of particulate chloride (Cl⁻). Here we demonstrate that chlorine radical oxidation of volatile organic compounds dominates HCl formation (~64%) at a polluted coastal site with elevated molecular chlorine (Cl₂), exceeding acid displacement of Cl⁻ (~36%). Rapid HCl formation from chlorine radical oxidation, coupled with concurrent Cl⁻ activation to Cl₂, shifts the HCl–Cl⁻ partitioning equilibrium and drives efficient aerosol uptake of gaseous HCl. The dynamic balance between Cl⁻ depletion and replenishment reveals a previously overlooked self-sustaining multiphase chlorine cycling, where HCl acts as a central hub. This cycle (Cl-HCl-Cl⁻-Cl) amplifies daytime OH, HO₂, and RO₂ concentrations by 11%, 19%, and 30%, respectively, and increases O_x production by 28%. Our study reveals a feedback loop that enhances atmospheric oxidation in this coastal site and potentially other areas.
 

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