Chlorine atoms (Cl) are highly reactive and can strongly influence atmospheric abundances of climate and air quality-relevant trace gases such as methane and ozone (O3). Despite extensive research on molecular chlorine (Cl2), an important Cl atom precursor, in the polar atmosphere, its sources in other regions are still poorly understood. Here we report unprecedented levels of daytime Cl2 (up to 1 ppbv, the highest atmospheric concentration ever reported, with an average of ~400 pptv) in a coastal area in Hong Kong, indicating the presence of a large daytime source (Cl2 ~2.7 pptv/s at noon) as the lifetime of Cl2 is only ~7 minutes at noon. Field and laboratory results reveal that photolysis of particulate nitrate under acidic conditions (pH <3.0) can activate chloride and account for the observed daytime Cl2 production. The high Cl2 concentrations observed at the site increased the atmospheric oxidation rate of volatile organic compounds (VOCs) by 24 to 132%, ROx radical by 4-27%, and daytime O3 integrated production by 17%. 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 a new source of chlorine can have substantial impacts on global atmospheric chemistry.