Rapid HCl Production From Chlorine Oxidation Sustains Coastal Atmospheric Chlorine Cycling

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 (Cl 2 ), exceeding acid displacement of Cl − (∼36%). Rapid HCl formation from chlorine radical oxidation, coupled with concurrent Cl − activation to Cl 2 , 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, HO 2 , and RO 2 concentrations by 11%, 19%, and 30%, respectively, and increases O x production by 28%. Our study reveals a feedback loop that enhances atmospheric oxidation at this coastal site and potentially other areas.