Validation of GEMS operational v2.0 total column NO₂ and HCHO during the GMAP/SIJAQ campaign.

The Geostationary Environmental Monitoring Spectrometer (GEMS), the first geostationary air quality instrument, onboard the GEO-KOMPSAT-2B (GK2B) satellite, produces hourly observations over Asia with 3.5 km<br>8 km spatial resolution. To evaluate the GEMS L2 products, the National Institute of Environmental Research (NIER) organized the GEMS Map of Air Pollutants 2021 (GMAP2021) and the Satellite Integrated Joint monitoring of Air Quality 2022 (SIJAQ2022) campaigns during October 2021 to November 2021 and from June 2022 to July 2022, respectively. While GMAP2021 mainly targeted the SMA (Seoul Metropolitan Area), the SIJAQ2022 campaign extended to the southeastern area of South Korea. In this study, a comparison between Pandora and Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) products and an evaluation of the GEMS operational v2.0 total column NO2 and HCHO products are conducted.<br>A comparative analysis between the Pandora (P189) and the IUP Bremen MAX-DOAS instrument at the Incheon NIER-ESC site was performed to analyze discrepancies between the retrieval processors (Pandora: PGN official processor, MAX-DOAS: MMF in FRM4DOAS framework). Aligning the viewing directions of both Pandora and MAX-DOAS leads to a significant increase in the slope and correlation coefficient from 0.87 to 0.96 and from 0.86 to 0.96, respectively, in the case of NO2 tropospheric columns. Similarly, for HCHO tropospheric columns, slope and correlation coefficient change from 0.94 to 1.09 and from 0.81 to 0.90 when matching the viewing geometries of both instruments. In contrast to tropospheric columns, total HCHO columns derived from Pandora (P189) direct-sun measurements show significantly larger values than the MAX-DOAS ones, with a mean relative difference (MRD) of 126 %. This bias can however be reduced to 33 % after suitable adjustment of the direct-sun retrieval settings.<br>The GEMS v2.0 NO2 total column product, evaluated over 6 official PGN sites in South Korea, shows good agreement with a correlation coefficient of 0.87 and similar seasonal and diurnal NO2 variation. However, GEMS tends to report higher values than Pandora with a mean relative difference of +41 %. The magnitude of the GEMS overestimation is amplified in highly polluted conditions (i.e. during winter and at noontime).<br>Compared to 6 MAX-DOAS stations and 6 Pandora stations, the GEMS HCHO product captures well the seasonal and diurnal variation of HCHO and shows good agreement both with MAX-DOAS and Pandora with slopes of 0.84 and 0.79, respectively, and correlation coefficients of 0.86 for both. Large columns, however, tend to be systematically underestimated.