Reactive halogens in polluted atmospheres: Sources, processes, and impacts on secondary pollutants

Halogen radicals have long been recognized for their role in depleting ozone in the stratosphere and in polar and pristine maritime boundary layers. Over the past 2 decades, an expanding body of research has highlighted the significant influence of halogen radicals on atmospheric chemistry and air quality degradation in anthropogenically impacted maritime and continental regions. This article reviews recent advances in the study of reactive halogens in polluted environments, with a particular focus on China, covering atmospheric observations, emission sources, chemical transformations, and the effects on atmospheric oxidation processes and the levels of ozone and secondary aerosol. Key recent findings include: (1) the widespread detection of photolabile halogen gases such as ClNO2 and dihalogens across various locations; (2) the substantial impact of halogen radicals, particularly those derived from elevated daytime Cl2, on VOC oxidation, the HOx budget, and the concentrations of ozone and secondary aerosol; (3) significant progress in identifying anthropogenic sources and chemical activation pathways for chlorine and bromine; and (4) the underrepresentation of daytime halogen activation processes in mainstream chemical transport models, leading to a likely underestimation of their atmospheric impacts. Future research efforts should include (1) conducting comprehensive and updated field measurements of halogens in diverse and changing environments and targeting emerging halogen, (2) improving understanding of sources and chemical cycles of halogen species, and (3) enhancing the capability of air quality models in simulating reactive halogen impact in polluted regions.