Significant Impact of a Daytime Halogen Oxidant on Coastal Air Quality

Chlorine radicals (Cl · ) are highly reactive and affect the fate of air pollutants. Several field studies in China have revealed elevated levels of daytime molecular chlorine (Cl 2 ), which, upon photolysis, release substantial amounts of Cl · but are poorly represented in current chemical transport models. Here, we implemented a parametrization for the formation of daytime Cl 2 through the photodissociation of particulate nitrate in acidic environments into a regional model and assessed its impact on coastal air quality during autumn in South China. The model could reproduce over 70% of the high Cl 2 level measured at a coastal site, revealing a discernible presence of Cl 2 and released Cl · in coastal and adjacent areas. Abundant Cl 2 alters the oxidative capacity of the atmosphere, consequently increasing O 3 (6–12%) and PM 2.5 (10–16%) concentrations in high-NO x areas and reducing O 3 (3%) concentration in low-NO x areas. Accounting for chlorine chemistry shifts the O 3 – precursor relationships from VOC limited to mixed or NO x -limited regimes, enhancing the benefits of NO x emission reduction in mitigating O 3 pollution. Our findings suggest that tightening emission control for two acidic pollutants, NO x and SO 2, would alleviate reactive Cl · production and its adverse impact on air quality.