Vertical Gradient of Nitryl Chloride and Implications for Atmospheric Photochemistry in Pearl River Delta, China, during Wintertime

Nitryl chloride (ClNO₂) is a key precursor of chlorine radicals, influencing atmospheric oxidation and secondary pollutants formation. Few studies have examined the ClNO₂ chemistry from the perspective of the planetary boundary layer. Here, we conducted a vertically resolved investigation of ClNO₂ at six heights (ranging from 5 to 335 m) on a 356 m tower in the Pearl River Delta, China, during winter 2021. Nocturnal ClNO₂ is notably lower at the surface than the upper layers, with the nocturnal median concentration at 220 m (51 parts per trillion by volume, pptv) being approximately three times higher than that recorded in the surface layer (16 pptv). Observation-constrained box model simulations show that the NO gradients primarily account for the vertical disparities. Compared to the hydroxyl radical (OH) production via the nitrous acid and ozone photolysis, ClNO₂ photolysis contributes to radical formation by 3.8% (1.8%) at 220 m (5 m) in the morning (07:00-08:00), indicates the enhanced significance of ClNO₂ chemistry aloft compared with the ground, and may cause the underestimation of ClNO₂ photolysis impacts if solely relying on surface measurements. We highlight that more field studies are needed to elucidate ClNO₂ chemistry across the boundary layer.