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

Nitryl chloride (ClNO) is a key precursor of chlorine radicals, influencing atmospheric oxidation and secondary pollutants formation. Few studies have examined the ClNO chemistry from the perspective of the planetary boundary layer. Here, we conducted a vertically resolved investigation of ClNO at six heights (ranging from 5 to 335 m) on a 356 m tower in the Pearl River Delta, China, during winter 2021. Nocturnal ClNO 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, ClNO 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 ClNO chemistry aloft compared with the ground, and may cause the underestimation of ClNO photolysis impacts if solely relying on surface measurements. We highlight that more field studies are needed to elucidate ClNO chemistry across the boundary layer.