Photolytic oxidation of ammonium chloride as a source of Cl₂ in the atmosphere

Chlorine atoms (·Cl) exhibit high reactivity and exert a substantial influence on the atmospheric oxidation capacity and chlorine cycle in the troposphere. The photolysis of molecular chlorine (Cl₂) is a crucial source of ·Cl. However, the daytime peaks of Cl₂ concentration cannot be entirely accounted for by the currently known pathways. Here we show that the photolytic oxidation of ammonium chloride (NH₄Cl), a ubiquitous component of atmospheric aerosol, serves as an important daytime source of Cl₂. Laboratory experiments demonstrate that oxygen, water vapour, and acidity are indispensable for Cl₂ generation and release, and that Cl₂ production is enhanced in the presence of black carbon aerosol. Field observation combined with model simulation demonstrates that the mechanism explains 12-55% of daytime Cl₂ concentration. These results reveal a photoactivation pathway for chlorine production that depends only on chlorine salts and solar radiation, with significant implications in regions with abundant chloride salts.