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

Abstract

Abstract. 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 4.7×1015 to 5.5×1015 molec. cm−2 for a cloud fraction (CF) <0.7. Higher correlation coefficients of 0.62–0.78 with lower RMSEs from 3.3×1015 to 5.0×1015 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 3.2×1015 to 4.9×1015 molec. cm−2, were achieved for CF <0.3, showing a better correlation with the correction than without the correction.

Estimating Hourly Nitrogen Oxide Emissions over East Asia from Geostationary Satellite Measurements

“…The correlation coefficients (R) between the NO 2 vertical column densities by the GEMS instrument and the ground-based Pandora spectrometer measurements ranged from 0.62 to 0.78 for a cloud fraction of <0.3. 37 In this study, the GEMS NO 2 v1.0 products from May to September in 2021 and 2022 were used and were filtered with an effective cloud fraction of less than 0.3 and a solar zenith angle of less than 65°. The Level 2 data were then gridded to a resolution of 0.02°× 0.02°using an errorweighted conservative interpolation.…”

“…Recent ground validation results demonstrated the high accuracy of GEMS NO 2 retrievals. The correlation coefficients ( R ) between the NO 2 vertical column densities by the GEMS instrument and the ground-based Pandora spectrometer measurements ranged from 0.62 to 0.78 for a cloud fraction of <0.3 . In this study, the GEMS NO 2 v1.0 products from May to September in 2021 and 2022 were used and were filtered with an effective cloud fraction of less than 0.3 and a solar zenith angle of less than 65°.…”

First Top-Down Diurnal Updates to NOx Emissions Inventory in Asia Informed by the Geostationary Environment Monitoring Spectrometer (GEMS) Tropospheric NO2 Columns

“…To ensure the quality of the observation references, we used pixels satisfying algorithm quality ags of 0 bit (good sample) and cloud fractions smaller than 0.3. We also considered the inherent bias in GEMS tropospheric NO 2 columns, observing an average discrepancy of -0.255 × 10 15 molecules/cm 2 compared to Pandora spectrometer measurements in highly polluted grid cells (≥ 0.5×10 16 molecules/cm 2 ) 49 , by selectively adjusting the observed values upwards.…”

“…However, a gap remains in studies concerning gaseous air pollutants, particularly in terms of better inventorying the quantity of their emissions, owing to the historical absence of instruments capable of monitoring gas-phase species in a geostationary manner. In ful lling such a need, the launch of the Geostationary Environment Monitoring Spectrometer (GEMS) in February 2020, the rst instrument of its kind to measure the loadings of both gas-phase air pollutants and aerosols 45 , set the stage for new research by providing enhanced monitoring capability over the Asia-Paci c region 36, [49][50][51] . Deployed aboard the Geostationary Korean Multi-Purpose Satellite 2 (GEO-KOMPSAT-2), GEMS offers hourly measurements of trace gases, such as nitrogen dioxide (NO 2 ), sulfur dioxide, formaldehyde, and ozone, as well as aerosol properties during daylight hours in a consecutive manner 45 -capability that was previously unattainable with instruments on LEO platforms.…”

Validation of GEMS tropospheric NO2 columns and their diurnal variation with ground-based DOAS measurements

“…A study by Kim et al (2023) evaluated GEMS L2 v1.0 total NO 2 column data from November 2020 to January 2021 with four ground-based Pandora instruments, all located in Seosan, South Korea. They found correlation coefficients of 0.62-0.78 and an underestimation of the ground-based NO 2 measurements by the GEMS data set.…”