First-time comparison between NO₂ vertical columns from Geostationary Environmental Monitoring Spectrometer (GEMS) and Pandora measurements

The Geostationary Environmental Monitoring Spectrometer (GEMS) is a UV-visible (UV-Vis) spectrometer on board the GEO-KOMPSAT-2B (Geostationary Korea Multi-Purpose Satellite 2B) satellite launched into a geostationary orbit in February 2020. To evaluate the GEMS NO2 total column data, a comparison was carried out using the NO2 vertical column density (VCD) that measured direct sunlight using the Pandora spectrometer system at four sites in Seosan, South Korea, from November 2020 to January 2021. Correlation coefficients between GEMS and Pandora NO2 data at four sites ranged from 0.35 to 0.48, with root mean square errors (RMSEs) from 4 :7 x 10(15) to 5 :5 x 10(15) molec. cm(-2) for a cloud fraction (CF) < 0 :7. Higher correlation coefficients of 0.62-0.78 with lower RMSEs from 3 :3 x 10(15) to 5 :0 x 10(15) molec. cm(-2) were found with CF < 0 :3, indicating the higher sensitivity of GEMS to atmospheric NO2 in less cloudy conditions. Overall, the GEMS NO2 total column data tended to be lower than the Pandora data, owing to differences in the representative spatial coverage, with a large negative bias under high CF conditions. With a correction for horizontal representativeness in the Pandora measurement coverage, correlation coefficients ranging from 0.69 to 0.81, with RMSEs from 3 :2 x 10(15) to 4 :9 x 10(15) molec. cm(-2), were achieved for CF < 0 :3, showing a better correlation with the correction than without the correction.