Integration of Airborne and Ground Observations of NitrylChloride in the Seoul Metropolitan Area and the Implications onRegional Oxidation Capacity During KORUS-AQ 2016

Abstract. Nitryl chloride (ClNO 2 ) is a radical reservoir species that releases chlorine radicals upon photolysis. An integrated analysis of the impact of ClNO 2 on regional photochemistry in the Seoul Metropolitan Area (SMA) during the Korean-United States-Air Quality (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 region downwind, respectively. The overall diurnal variations of ClNO 2 in both sites appeared similar but the night time variation were systematically different. For about half of the observation days at the OP site the level of ClNO 2 increased at sunset but rapidly decreased at around midnight. On the other hand, high levels were sustained throughout the night at the TRF site. Significant levels of ClNO 2 were sustained at both sites for 4–5 hours after sunrise. Airborne observations, box model calculations, and back trajectory analysis consistently show that this high levels of ClNO 2 in the morning is likely due to the transport of air masses within the boundary layer. Box model results show that chlorine radical initiated chemistry can impact the regional photochemistry by elevating net ozone production rate up to ~ 25 % in the morning.