Formation of Chlorine in the Atmosphere by Reaction of Hypochlorous Acid with Seawater

The highly reactive dihalogens play a significant role in the oxidative chemistry of the troposphere. One of the main reservoirs of these halogens is hypohalous acids, HOX, which produce dihalogens in the presence of halides (Y^-), where X, Y = Cl, Br, I. These reactions occur in and on aerosol particles and seawater surfaces and have been studied experimentally and by field observations. However, the mechanisms of these atmospheric reactions are still unknown. Here, we establish the atomistic mechanism of HOCl + Cl^- → Cl₂ + OH^- at the surface of the water slab by performing ab initio molecular dynamics (AIMD) simulations. Main findings are (1) This reaction proceeds by halogen-bonded complexes of (HOCl)···(Cl^-)_aq surrounded with the neighboring water molecules. (2) The halogen bonded (HOCl)···(Cl^-)_aq complexes undergo charge transfer from Cl^- to OH^- to form transient Cl₂ at neutral pH. (3) The addition of a proton to one proximal water greatly facilitates the Cl₂ formation, which explains the enhanced rate at low pH.