Chlorine Activation in Marine Air: Insights From Chemical Budgets of Molecular Chlorine and Hypochlorous Acid

Abstract Chlorine chemistry considerably affects air quality and climate in marine environments. Nitrogen oxides (NO x ), emitted by ocean‐going vessels, react with sea salt chloride to generate reactive chlorine species. However, the exact mechanisms and chemical budget of chlorine remain poorly understood. In this study, we explore chlorine activation through field observations in Hong Kong, complemented by box modeling. Over the 2‐week measurement period, the summer monsoon introduced abundant NO x , producing molecular chlorine (Cl 2 , 0.64 ± 0.69 ppt) and hypochlorous acid (HOCl, 8.9 ± 5.1 ppt). Daytime Cl 2 production was attributable to nitrate (NO 3 − ) photolysis and the uptake on hydroxyl radicals (OH·) on chloride‐containing aerosols. A budget analysis using the box model revealed that the production rate of HOCl, primarily driven by chlorine nitrate (ClONO 2 ) hydrolysis, was substantially lower than its loss rate. This discrepancy indicates either uncertainties in known HOCl sources or a missing source of chlorine atoms (Cl·). We examined the potential precursors of Cl· by incorporating emerging reactive chlorine species, such as, trichloramine (NCl 3 ) and iodine chloride (ICl), into the model. However, the inclusion of NCl 3 caused an overestimation of ambient Cl 2 levels, while adding ICl led to excessive ozone (O 3 ) depletion. Incorporating an unknown Cl· source (equivalent to ∼46 ppt Cl 2 ) remarkably enhanced atmospheric oxidation capacity, increasing daytime OH· levels by 12.8% and net ozone production by 35.7% while decreasing the mercury (Hg) lifetime by a factor of 3. These findings highlight the incomplete understanding of chlorine chemistry and suggest the existence of unidentified Cl· sources in coastal environments.