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The Korea–United States Air Quality (KORUS-AQ) field study was conducted during May–June 2016. The effort was jointly sponsored by the National Institute of Environmental Research of South Korea and the National Aeronautics and Space Administration of the United States. KORUS-AQ offered an unprecedented, multi-perspective view of air quality conditions in South Korea by employing observations from three aircraft, an extensive ground-based network, and three ships along with an array of air quality forecast models. Information gathered during the study is contributing to an improved understanding of the factors controlling air quality in South Korea. The study also provided a valuable test bed for future air quality–observing strategies involving geostationary satellite instruments being launched by both countries to examine air quality throughout the day over Asia and North America. This article presents details on the KORUS-AQ observational assets, study execution, data products, and air quality conditions observed during the study. High-level findings from companion papers in this special issue are also summarized and discussed in relation to the factors controlling fine particle and ozone pollution, current emissions and source apportionment, and expectations for the role of satellite observations in the future. Resulting policy recommendations and advice regarding plans going forward are summarized. These results provide an important update to early feedback previously provided in a Rapid Science Synthesis Report produced for South Korean policy makers in 2017 and form the basis for the Final Science Synthesis Report delivered in 2020.

Abstract. In this study, NO 2 columns from the US EPA Models-3/CMAQ model simulations carried out using the 2001 ACE-ASIA (Asia Pacific Regional Aerosol Characterization Experiment) emission inventory over East Asia were compared with the GOME-derived NO 2 columns. There were large discrepancies between the CMAQ-predicted and GOME-derived NO 2 columns in the fall and winter seasons. In particular, while the CMAQ-predicted NO 2 columns produced larger values than the GOME-derived NO 2 columns over South Korea for all four seasons, the CMAQ-predicted NO 2 columns produced smaller values than the GOMEderived NO 2 columns over North China for all seasons with the exception of summer (summer anomaly). It is believed that there might be some error in the NO x emission estimates as well as uncertainty in the NO x chemical loss rates over North China and South Korea. Regarding the latter, this study further focused on the biogenic VOC (BVOC) emissions that were strongly coupled with NO x chemistry during summer in East Asia. This study also investigated whether the CMAQ-modeled NO 2 /NO x ratios with the possibly overestimated isoprene emissions were higher than those with reduced isoprene emissions. Although changes in both the NO x chemical loss rates and NO 2 /NO x ratios from CMAQmodeling with the different isoprene emissions affected the CMAQ-modeled NO 2 levels, the effects were found to be limited, mainly due to the low absolute levels of NO 2 in Correspondence to: C. H. Song (chsong@gist.ac.kr) summer. Seasonal variations of the NO x emission fluxes over East Asia were further investigated by a set of sensitivity runs of the CMAQ model. Although the results still exhibited the summer anomaly possibly due to the uncertainties in both NO x -related chemistry in the CMAQ model and the GOME measurements, it is believed that consideration of both the seasonal variations in NO x emissions and the correct BVOC emissions in East Asia are critical. Overall, it is estimated that the NO x emissions are underestimated by ∼57.3% in North China and overestimated by ∼46.1% in South Korea over an entire year. In order to confirm the uncertainty in NO x emissions, the NO x emissions over South Korea and China were further investigated using the ACE-ASIA, REAS (Regional Emission inventory in ASia), and CAPSS (Clean Air Policy Support System) emission inventories. The comparison between the CMAQ-calculated and GOME-derived NO 2 columns indicated that both the ACE-ASIA and REAS inventories have some uncertainty in NO x emissions over North China and South Korea, which can also lead to some errors in modeling the formation of ozone and secondary aerosols in South Korea and North China.

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