Over the Seoul metropolitan area, tropospheric NO₂ vertical column densities from the Geostationary Environment Monitoring Spectrometer show distinct seasonal features. Also, varying a priori data have substantial impacts on the observed NO₂ columns. The a priori data from different chemical transport models resulted in differences of up to −18.3 %. Notably, diurnal patterns of observed NO₂ columns are similar for all datasets, although their a priori data exhibit contrasting diurnal patterns.