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 PM_2.5 (10-16%) concentrations in high-NO_x areas and reducing O₃ (3%) concentration in low-NO_x areas. Accounting for chlorine chemistry shifts the O₃ - precursor relationships from VOC limited to mixed or NO_x -limited regimes, enhancing the benefits of NO_x emission reduction in mitigating O₃ pollution. Our findings suggest that tightening emission control for two acidic pollutants, NO_x and SO₂, would alleviate reactive Cl^· production and its adverse impact on air quality.