A Comparative Analysis of Wintertime ClNO ₂ Chemistry Across Three Megacities in China

Nitryl chloride (ClNO 2 ), a key precursor to chlorine radicals that accelerates atmospheric oxidation, has chemistry and spatial distribution, particularly its vertical distribution, that remain insufficiently constrained. Here we presented wintertime observations of ClNO 2 across three Chinese megacities: Beijing (2024), Shanghai (2020), and Guangzhou (2025). Diurnal average peak concentrations reached 68.8, 112.6, and 77.6 ppt, respectively, with a maximum of 931.0 ppt recorded at 03:26 on November 10 in Beijing. Median nocturnal NO 3 ‐to‐ClNO 2 production efficiency ( ϵ ) were 0.3%, 0.5%, and 0.2% in Beijing, Shanghai, and Guangzhou, but variability was much larger in Beijing (SD 10.0%), reflecting perturbation from changing air masses. The inter‐city contrast in ϵ is driven by temperature, particulate chloride,and N 2 O 5 partitioning: the warmer coastal cities operate in a chloride‐supply‐limited regime in which ϵ increases with Cl − , whereas cold, chloride‐rich Beijing is chloride‐saturated. In Beijing, post‐sunrise ClNO 2 increases occurred on 13 of 34 days. For the 8 November case, the BL‐RL (boundary layer‐residual layer) model revealed that the surge resulted from entrainment of ClNO 2 stored in the RL with an inferred 720.2 ppt that subsequently decayed by photolysis as the boundary layer deepened. This morning BL‐RL interaction, observed in Beijing but not detected in our Shanghai and Guangzhou records, indicates that vertical exchange of reactive chlorine is a critical factor in the wintertime chlorine chemistry of North China Plain, amplified by regional residential coal combustion and elevated particulate chloride.