Chlorine (Cl) and bromine (Br) atoms are highly reactive and can strongly influence atmospheric abundances of climate and air quality-relevant trace gases such as methane and ozone (O₃). We observed high concentrations of Cl₂ and Br₂ during daytime in a coastal area in Hong Kong, indicating the presence of a large daytime source as the photolytic lifetime of Cl₂ and Br₂ is only ~7 and ~4 minutes at noon, respectively. This source was positively correlated with the product of solar actinic flux, nitrate and aerosol surface areas, suggesting a photochemical source involving nitrate and occurring at air-particle interface. Illuminating acid doped nitrate-NaCl solution with a xenon lamp in laboratory revealed that photolysis of particulate nitrate under acidic conditions could activate chloride and bromide. Chemical box model constrained by the observation data shows that the Cl₂ and Br₂, through photodissociation to produce Cl and Br, atoms, significantly increases the atmospheric oxidation rate of volatile organic compounds (VOCs), conventional RO_x radical, O₃ production, and Hg oxidation. Chemical transport model also demonstrates considerable impact on ozone and PM_2.5 in the coastal areas. Our study suggests the need for considering this new pollution-induced activation of sea salt halogens and its impact on atmospheric chemistry and air quality in coastal areas.

Halogens,oxidation,ozone,PM2.5,Hg