Measurements show elevated levels of molecular chlorine at the coast of Hong Kong. These high levels of reactive chlorine are explained by the photodissociation of particulate nitrate and subsequent oxidation of chloride.

Chlorine atoms (Cl) are highly reactive and can strongly influence the abundances of climate and air quality-relevant trace gases. Despite extensive research on molecular chlorine (Cl2), a Cl precursor, in the polar atmosphere, its sources in other regions are still poorly understood. In this work, an international team including IQFR scientists reports on atmospheric observations of Cl2 and other chemicals obtained at a polluted coastal site in southern China during autumn 2018. The Cl2 concentrations are much higher than those previously measured outside polar regions. We show that previously proposed Cl2 production mechanisms cannot account for the large Cl2 daytime source at this site and this source positively correlates with solar radiation, particulate nitrate, and particulate surface area. Laboratory experiments show that illuminating solution of sodium chloride and nitrate under acidic condition and ambient particulates can produce a large amount of Cl2, which can explain a large fraction of the observed Cl2 at our site. We propose that nitrate photolysis at high aerosol acidity is an important pathway for activating inert chloride to produce photoliable Cl2 during daytime in the polluted troposphere. Model calculations demonstrate significant enhancement of conventional radical levels, hydrocarbon oxidation, and ozone production by the high levels of Cl2 at the study site. We suggest that the same Cl2 production pathway may exist in other places of the world and call for more attention to the role of Cl2 in tropospheric chemistry and air quality of polluted regions. The results are published in Nature Communications: