Abstract 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 cycle, 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 Ox production by 28%. Our study reveals a feedback loop that enhances atmospheric oxidation at this coastal site and potentially other areas.