The Korea-United States Air Quality Study (KORUS-AQ) took place in spring 2016 to better understand air pollution in Korea. In support of KORUS-AQ, 2554 whole air samples (WAS) were collected aboard the NASA DC-8 research aircraft and analyzed for 82 C 1 –C 10 volatile organic compounds (VOCs) using multi-column gas chromatography. Together with fast-response measurements from other groups, the air samples were used to characterize the VOC composition in Seoul and surrounding regions, determine which VOCs are major ozone precursors in Seoul, and identify the sources of these reactive VOCs. (1) The WAS VOCs showed distinct signatures depending on their source origins. Air collected over Seoul had abundant ethane, propane, toluene and n -butane while plumes from the Daesan petrochemical complex were rich in ethene, C 2 –C 6 alkanes and benzene. Carbonyl sulfide (COS), CFC-113, CFC-114, carbon tetrachloride (CCl 4 ) and 1,2-dichloroethane were good tracers of air originating from China. CFC-11 was also elevated in air from China but was surprisingly more elevated in air over Seoul. (2) Methanol, isoprene, toluene, xylenes and ethene were strong individual contributors to OH reactivity in Seoul. However methanol contributed less to ozone formation based on photochemical box modeling, which better accounts for radical chemistry. (3) Positive Matrix Factorization (PMF) and other techniques indicated a mix of VOC source influences in Seoul, including solvents, traffic, biogenic, and long-range transport. The solvent and traffic sources were roughly equal using PMF, and the solvents source was stronger in the KORUS-AQ emission inventory. Based on PMF, ethene and propene were primarily associated with traffic, and toluene, ethylbenzene and xylenes with solvents, especially non-paint solvents for toluene and paint solvents for ethylbenzene and xylenes. This suggests that VOC control strategies in Seoul could continue to target vehicle exhaust and paint solvents, with additional regulations to limit the VOC content in a variety of non-paint solvents.

The Seoul Metropolitan Area (SMA) has a population of 24 million and frequently experiences unhealthy levels of ozone (O 3 ). In this work, measurements taken during the Korea-United States Air Quality Study (KORUS-AQ, 2016) are used to explore regional gradients in O 3 and its chemical precursors, and an observationally-constrained 0-D photochemical box model is used to quantify key aspects of O 3 production including its sensitivity to precursor gases. Box model performance was evaluated by comparing modeled concentrations of select secondary species to airborne measurements. These comparisons indicate that the steady state assumption used in 0-D box models cannot describe select intermediate species, highlighting the importance of having a broad suite of trace gases as model constraints. When fully constrained, aggregated statistics of modeled O 3 production rates agreed with observed changes in O 3 , indicating that the box model was able to represent the majority of O 3 chemistry. Comparison of airborne observations between urban Seoul and a downwind receptor site reveal a positive gradient in O 3 coinciding with a negative gradient in NO x , no gradient in CH 2 O, and a slight positive gradient in modeled rates of O 3 production. Together, these observations indicate a radical-limited (VOC-limited) O 3 production environment in the SMA. Zero-out simulations identified C 7+ aromatics as the dominant VOC contributors to O 3 production, with isoprene and anthropogenic alkenes making smaller but appreciable contributions. Simulations of model sensitivity to decreases in NO x produced results that were not spatially uniform, with large increases in O 3 production predicted for urban Seoul and decreases in O 3 production predicted for far-outlying areas. The policy implications of this work are clear: Effective O 3 mitigation strategies in the SMA must focus on reducing local emissions of C 7+ aromatics, while reductions in NO x emissions may increase O 3 in some areas but generally decrease the regional extent of O 3 exposure.

We examine O 3 production and its sensitivity to precursor gases and boundary layer mixing in Korea by using a 3-D global chemistry transport model and extensive observations during the KORea-US cooperative Air Quality field study in Korea, which occurred in May–June 2016. During the campaign, observed aromatic species onboard the NASA DC-8 aircraft, especially toluene, showed high mixing ratios of up to 10 ppbv, emphasizing the importance of aromatic chemistry in O 3 production. To examine the role of VOCs and NO x in O 3 chemistry, we first implement a detailed aromatic chemistry scheme in the model, which reduces the normalized mean bias of simulated O 3 mixing ratios from –26% to –13%. Aromatic chemistry also increases the average net O 3 production in Korea by 37%. Corrections of daytime PBL heights, which are overestimated in the model compared to lidar observations, increase the net O 3 production rate by ~10%. In addition, increasing NO x emissions by 50% in the model shows best performance in reproducing O 3 production characteristics, which implies that NO x emissions are underestimated in the current emissions inventory. Sensitivity tests show that a 30% decrease in anthropogenic NO x emissions in Korea increases the O 3 production efficiency throughout the country, making rural regions ~2 times more efficient in producing O 3 per NO x consumed. Simulated O 3 levels overall decrease in the peninsula except for urban and other industrial areas, with the largest increase (~6 ppbv) in the Seoul Metropolitan Area (SMA). However, with simultaneous reductions in both NO x and VOCs emissions by 30%, O 3 decreases in most of the country, including the SMA. This implies the importance of concurrent emission reductions for both NO x and VOCs in order to effectively reduce O 3 levels in Korea.