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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.

Abstract. Organic aerosol (OA) is an important fraction of submicron aerosols. However,it is challenging to predict and attribute the specific organic compounds andsources that lead to observed OA loadings, largely due to contributions fromsecondary production. This is especially true for megacities surrounded bynumerous regional sources that create an OA background. Here, we utilizein situ gas and aerosol observations collected on board the NASA DC-8 duringthe NASA–NIER KORUS-AQ (Korea–United States Air Quality) campaign toinvestigate the sources and hydrocarbon precursors that led to the secondaryOA (SOA) production observed over Seoul. First, we investigate thecontribution of transported OA to total loadings observed over Seoul byusing observations over the Yellow Sea coupled to FLEXPART Lagrangiansimulations. During KORUS-AQ, the average OA loading advected into Seoul was∼1–3 µg sm−3. Second, taking this background intoaccount, the dilution-corrected SOA concentration observed over Seoul was∼140 µgsm-3ppmv-1 at 0.5 equivalent photochemicaldays. This value is at the high end of what has been observed in othermegacities around the world (20–70 µgsm-3ppmv-1 at 0.5equivalent days). For the average OA concentration observed over Seoul(13 µg sm−3), it is clear that production of SOA from locallyemitted precursors is the major source in the region. The importanceof local SOA production was supported by the following observations.(1) FLEXPART source contribution calculations indicate anyhydrocarbons with a lifetime of less than 1 day, which are shown to dominate theobserved SOA production, mainly originate from South Korea. (2) SOAcorrelated strongly with other secondary photochemical species, includingshort-lived species (formaldehyde, peroxy acetyl nitrate, sum of acyl peroxynitrates, dihydroxytoluene, and nitrate aerosol). (3) Results froman airborne oxidation flow reactor (OFR), flown for the first time, show afactor of 4.5 increase in potential SOA concentrations over Seoul versus overthe Yellow Sea, a region where background air masses that are advected intoSeoul can be measured. (4) Box model simulations reproduce SOAobserved over Seoul within 11 % on average and suggest that short-livedhydrocarbons (i.e., xylenes, trimethylbenzenes, and semi-volatile and intermediate-volatility compounds) were the main SOA precursors over Seoul. Toluenealone contributes 9 % of the modeled SOA over Seoul. Finally, along withthese results, we use the metric ΔOA/ΔCO2 toexamine the amount of OA produced per fuel consumed in a megacity, whichshows less variability across the world than ΔOA∕ΔCO.

Organic aerosol (OA) is an important fraction of submicron aerosols. However, it is challenging to predict and attribute the specific organic compounds and sources that lead to observed OA loadings, largely due to contributions from secondary production. This is especially true for megacities surrounded by numerous regional sources that create an OA background. Here, we utilize in situ gas and aerosol observations collected on board the NASA DC-8 during the NASA-NIER KORUS-AQ (Korea-United States Air Quality) campaign to investigate the sources and hydrocarbon precursors that led to the secondary OA (SOA) production observed over Seoul. First, we investigate the contribution of transported OA to total loadings observed over Seoul by using observations over the Yellow Sea coupled to FLEXPART Lagrangian simulations. During KORUS-AQ, the average OA loading advected into Seoul was ∼ 1-3 μgsm -3 . Second, taking this background into account, the dilutioncorrected SOA concentration observed over Seoul was ∼ 140 μgsm -3 ppmv -1 at 0.5 equivalent photochemical days. This value is at the high end of what has been observed in other megacities around the world (20-70 μgsm -3 ppmv -1 at 0.5 equivalent days). For the average OA concentration observed over Seoul (13 μgsm -3 ), it is clear that production of SOA from locally emitted precursors is the major source in the region. The importance of local SOA production was supported by the following observations. (1) FLEXPART source contribution calculations indicate any hydrocarbons with a lifetime of less than 1 day, which are shown to dominate the observed SOA production, mainly originate from South Korea. (2) SOA correlated strongly with other secondary photochemical species, including short-lived species (formaldehyde, peroxy acetyl nitrate, sum of acyl peroxy nitrates, dihydroxytoluene, and nitrate aerosol). (3) Results from an airborne oxidation flow reactor (OFR), flown for the first time, show a factor of 4.5 increase in potential SOA concentrations over Seoul versus over the Yellow Sea, a region where background air masses that are advected into Seoul can be measured. (4) Box model simulations reproduce SOA observed over Seoul within 11% on average and suggest that shortlived hydrocarbons (i.e., xylenes, trimethylbenzenes, and semi-volatile and intermediate-volatility compounds) were the main SOA precursors over Seoul. Toluene alone contributes 9% of the modeled SOA over Seoul. Finally, along with these results, we use the metric ΔOA/ΔCO2 to examine the amount of OA produced per fuel consumed in a megacity, which shows less variability across the world than ΔOA=ΔCO.