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

Molecular chlorine (Cl-2) 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 Cl-2 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 Cl-2 formation mechanisms. The standard model run with traditional Cl-2 formation mechanisms underestimates the observed Cl-2 by almost one order of magnitude around noon. The daytime Cl-2 missing source was estimated, accounting for on average (69 +/- 5)% of daytime Cl-2 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 Cl-2 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 Cl-2 formation mechanisms is critical while assessing the impacts of chlorine chemistry on air quality and climate.