Diurnal variations of NO₂ tropospheric vertical column density over the Seoul Metropolitan Area from the Geostationary Environment Monitoring Spectrometer (GEMS): seasonal differences and impacts of varying a priori NO₂ profile data

Abstract. Geostationary Environment Monitoring Spectrometer (GEMS), launched in 2020, provides both temporally and spatially continuous air quality data from geostationary Earth orbit (GEO). In this study, we analyzed seasonal characteristics of GEMS tropospheric NO2 vertical column density (NO2 TropVCD) diurnal patterns and impacts of a priori data from diverse chemical transport model (CTM) simulations over the Seoul Metropolitan Area (SMA), using the GEMS products retrieved by the IUP algorithm. We found that both the amounts of NO2 TropVCD and the peak time vary according to the season – the maximum value occurs earlier in July (10 KST) compared to other months (12 KST), with relatively lower value (8.53 – 9.81 × 1015 molec. cm-2). In wintertime, the decrease in NO2 TropVCD over time was relatively slower than in summertime. Also, we examined the impact of changes in a priori data on the GEMS NO2 TropVCD. When we compare GEMS NO2 data retrieved with default NOx emissions and uniformly 20 %-reduced NOx emissions, there are no notable discrepancies as simulated NO2 profiles from CTM are nearly identical over the SMA. However, when the vertical profile at 06:45 UTC (13:45 KST) was applied for retrievals at all times, there are 11.9 – 16.1 % lower values before 13:45 KST and up to 4.9 % higher values after 13:45 KST compared to the control run case. Our study highlighted two key findings: (1) GEMS NO2 products describe distinct seasonal features, including the absolute values (highest in January and lowest in July) and diurnal patterns (persisting longer in January and declining rapidly in July), (2) changes of a priori data have the impacts of up to 19.2 % on the GEMS NO2 TropVCD.