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 (Cl2), 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 Cl2 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 Cl2 level measured at a coastal site, revealing a discernible presence of Cl2 and released Cl· in coastal and adjacent areas. Abundant Cl2 alters the oxidative capacity of the atmosphere, consequently increasing O3 (6–12%) and PM2.5 (10–16%) concentrations in high-NOx areas and reducing O3 (3%) concentration in low-NOx areas. Accounting for chlorine chemistry shifts the O3 – precursor relationships from VOC limited to mixed or NOx -limited regimes, enhancing the benefits of NOx emission reduction in mitigating O3 pollution. Our findings suggest that tightening emission control for two acidic pollutants, NOx and SO2, would alleviate reactive Cl· production and its adverse impact on air quality.