Pollution-derived Cl2 and Br2 boost oxidation power of the coastal atmosphere

The chlorine (Cl·) and bromine atom (Br·) are known to destroy ozone, and Br· accelerates the deposition of toxic mercury (Hg). However, their abundances, sources and impacts in polluted regions are not well understood. In this talk, we give an overview of recent measurements of significant levels of Cl₂ and Br₂— a producer of Cl· and Br·, respectively —in the coastal atmosphere of Hong Kong. We present field and laboratory evidence to show photodissociation of particulate nitrate being a major production pathway for daytime Cl₂ and Br₂ and reactive uptake of N₂O₅ on aerosols as an important nighttime source. Model-calculated and Cl· and Br· from these di-halogens have significant impacts on VOC oxidation, OH radical, ozone production, and Hg oxidation. The findings suggest that reactive halogens may play a larger role in the atmospheric chemistry and air quality of polluted coastal regions than previously thought and call for more research on this issue.