Large daytime molecular chlorine missing source at a suburban site in East China

Abstract

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.