Nitryl chloride (ClNO₂) is a radical reservoir species that releases chlorine radicals upon photolysis. An integrated analysis of the impact of ClNO₂ on regional photochemistry in the Seoul metropolitan area (SMA) during the Korea-United States Air Quality Study (KORUS-AQ) 2016 field campaign is presented. Comprehensive multiplatform observations were conducted aboard the NASA DC-8 and at two ground sites (Olympic Park, OP; Taehwa Research Forest, TRF), representing an urbanized area and a forested suburban region, respectively. Positive correlations between daytime Cl₂ and ClNO₂ were observed at both sites, the slope of which was dependent on O₃ levels. The possible mechanisms are explored through box model simulations constrained with observations. The overall diurnal variations in ClNO₂ at both sites appeared similar but the nighttime variations were systematically different. For about half of the observation days at the OP site the level of ClNO₂ increased at sunset but rapidly decreased at around midnight. On the other hand, high levels were observed throughout the night at the TRF site. Significant levels of ClNO₂ were observed at both sites for 4-5 h after sunrise. Airborne observations, box model calculations, and back-trajectory analysis consistently show that these high levels of ClNO₂ in the morning are likely from vertical or horizontal transport of air masses from the west. Box model results show that chlorine-radical-initiated chemistry can impact the regional photochemistry by elevating net chemical production rates of ozone by 25% in the morning.