Large Daytime Molecular Chlorine Missing Source at a Suburban Site in East China

Molecular chlorine (Cl₂) affects atmospheric oxidative capacity by generating chlorine radicals upon photolysis, but it is poorly simulated in atmospheric chemistry models. In this study, we observed up to 40 ppt Cl₂ around noon at a suburban site in East China, and used a box model with up-to-date chlorine chemistry and comprehensive observational constraints to investigate Cl₂ formation mechanisms. The standard model run with traditional Cl₂ formation mechanisms underestimates the observed Cl₂ by almost one order of magnitude around noon. The daytime Cl₂ missing source was estimated, accounting for on average (69 ± 5)% of daytime Cl₂ production for the 1-week study period. It is likely caused by photochemistry within the aerosols, based on its correlation with observed environmental factors, such as sunlight intensity and aerosol abundances. With the daytime Cl₂ missing source implemented into the model, the chlorine radical abundance increases by a factor of 4 in the afternoon, enhancing the oxidation of volatile organic compounds. A good understanding of daytime Cl₂ formation mechanisms is critical while assessing the impacts of chlorine chemistry on air quality and climate.