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. Sunlight triggers ammonium chloride in aerosols to produce Cl2 gas, explaining 12-55% of daytime levels. This chlorine source, boosted by black carbon, impacts atmospheric oxidation.