First-time comparison between NO₂ vertical columns from GEMS and Pandora measurements

Abstract. The Geostationary Environmental Monitoring Spectrometer (GEMS) is a UV–visible spectrometer onboard the GEO-KOMPSAT-2B satellite launched into geostationary orbit in February 2020. To evaluate GEMS NO₂ column data, comparison was carried out using NO₂ vertical column density (VCD) measured using direct-sunlight observations by the Pandora spectrometer system at four sites in Seosan, South Korea, during 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 × 10¹⁵ molec. cm^-2 to 5.5 × 10¹⁵ molec. cm^-2 for cloud fraction (CF) < 0.7. Higher correlation coefficients of 0.62–0.78 with lower RMSEs from 3.3 × 10¹⁵ molec. cm^-2 to 4.3 × 10¹⁵ molec. cm^-2 were found with CF < 0.3, indicating the higher sensitivity of GEMS to atmospheric NO₂ in less-cloudy conditions. Overall, GEMS NO₂ column data tend to be lower than those of Pandora due to differences in representative spatial coverage, with a large negative bias under high-CF conditions. With correction for horizontal representativeness in Pandora measurement coverage, the correlation coefficients range from 0.69 to 0.81 with RMSEs from 3.2 × 10¹⁵ molec. cm^-2 to 4.9 × 10¹⁵ molec. cm^-2 were achieved for CF < 0.3, showing the better correlation with the correction than that without the correction.