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. In this work, we investigate the NOx emissions inventory inSeoul, South Korea, using a regional ozone monitoring instrument (OMI) NO2 product derived from the standard NASA product. We firstdevelop a regional OMI NO2 product by recalculating the air massfactors using a high-resolution (4 km  ×  4 km) WRF-Chem modelsimulation, which better captures the NO2 profile shapes in urbanregions. We then apply a model-derived spatial averaging kernel to furtherdownscale the retrieval and account for the subpixel variability. These twomodifications yield OMI NO2 values in the regional product that are1.37 times larger in the Seoul metropolitan region and >2 times largernear substantial point sources. These two modifications also yield an OMI NO2 product that is in better agreement with the Pandora NO2 spectrometer measurements acquired during the SouthKorea–United States Air Quality (KORUS-AQ) field campaign. NOx emissions are then derived for the Seoul metropolitanarea during the KORUS-AQ field campaign using a top-down approach with thestandard and regional NASA OMI NO2 products. We first apply thetop-down approach to a model simulation to ensure that the method isappropriate: the WRF-Chem simulation utilizing the bottom-up emissionsinventory yields a NOx emissions rate of 227±94  kt yr −1 , while the bottom-up inventory itself within a 40 kmradius of Seoul yields a NOx emissions rate of198 kt yr −1 . Using the top-down approach on the regional OMI NO2 product, we derive the NOx emissions rate fromSeoul to be 484±201  kt yr −1 , and a 353±146  kt yr −1 NOx emissions rate using the standard NASA OMI NO2 product. This suggests an underestimate of 53 % and 36 % in thebottom-up inventory using the regional and standard NASA OMI NO2 products respectively. To supplement this finding, we compare the NO2 and NOy simulated by WRF-Chem to observationsof the same quantity acquired by aircraft and find a model underestimate.When NOx emissions in the WRF-Chem model are increased by afactor of 2.13 in the Seoul metropolitan area, there is better agreement withKORUS-AQ aircraft observations and the recalculated OMI NO2 tropospheric columns. Finally, we show that by using a WRF-Chem simulationwith an updated emissions inventory to recalculate the air mass factor (AMF),there are small differences ( ∼8  %) in OMI NO2 compared tousing the original WRF-Chem simulation to derive the AMF. This suggests thatchanges in model resolution have a larger effect on the AMF calculation thanmodifications to the South Korean emissions inventory. Although the currentwork is focused on South Korea using OMI, the methodology developed in thiswork can be applied to other world regions using TROPOMI and future satellitedatasets (e.g., GEMS and TEMPO) to produce high-quality region-specifictop-down NOx emissions estimates.

atmospheric chemistry and physics

A top-down assessment using OMI NO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; suggests an underestimate in the NO&amp;lt;sub&amp;gt;&amp;lt;i&amp;gt;x&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt; emissions inventory in Seoul, South Korea, during KORUS-AQ