Photodissociation of particulate nitrate as a source of daytime tropospheric Cl2

10 pags., 3 figs.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.This research is supported by the Hong Kong Research Grants Council (T24-504/17-N and A-PolyU502/16 to T.W.), the Agence Nationale de la Recherche (ANR-16-CE01-0013 to C.G.), the Swedish Research Council (2013-6917 to M.H. and C.M.S), and the European Research Council Executive Agency under the European Union s Horizon 2020 Research and Innovation programme (Project ‘ERC2016-COG726349 CLIMAHAL’ to Q.L. and A.S-L.).Peer reviewe