Abstract
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. Here we report the daytime Cl2 concentrations of up to 1 ppbv observed in a coastal area of Hong Kong, revealing a large daytime source of Cl2 (2.7 pptv s−1 at noon). Field and laboratory experiments indicate that photodissociation of particulate nitrate by sunlight under acidic conditions (pH < 3.0) can activate chloride and account for the observed daytime Cl2 production. The high Cl2 concentrations significantly increased atmospheric oxidation. Given the ubiquitous existence of chloride, nitrate, and acidic aerosols, we propose that nitrate photolysis is a significant daytime chlorine source globally. This so far unaccounted for source of chlorine can have substantial impacts on atmospheric chemistry.
Highlights
Chlorine atoms (Cl) are highly reactive and can strongly influence the abundances of climate and air quality-relevant trace gases
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
Summary
Chlorine atoms (Cl) are highly reactive and can strongly influence the abundances of climate and air quality-relevant trace gases.
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