Satellite remote sensing has played a key role in understanding distribution and changes of atmospheric composition including aerosols, ozone, air pollutants, and greenhouse gases. These contributions have been achieved extensively with Low Earth Orbit (LEO) satellite instruments providing one to two observations per day, including but not limited to MODIS, VIIRS, OMI, TROPOMI, GAOFEN, and so on. Geostationary Environment Monitoring Spectrometer (GEMS) was launched in February, 2020 as the first component of GEO-Ring for atmospheric composition observation from geostationary Earth orbit. GEMS observation is complemented by AMI and GOCI-2 on the same spacecraft for aerosols, and hyperspectral instruments such as Chinese GIIRS. NASA’s TEMPO was launched in 2023 over North America and ESA’s Sentinel 4 UVN was launched in 2025 over Europe, to establish the GEO ring of Air Quality observation. GEMS has provided hourly observation of key air quality components, including aerosol, ozone, and their precursors such as NO2, HCHO, SO2 etc. In this talk, achievements of GEMS observations to monitor atmospheric composition of aerosol and gases from GEMS are presented with algorithm updates and validation results. Achievements and related issues with GEMS observations are discussed for further improvements and harmonization of dataset for the GEO-RING.

Satellite measurements of nitrogen dioxide have been used to infer nitrogen oxide emissions, a critical component in tropospheric chemistry and pollution. New observations from the Geostationary Environmental Monitoring Spectrometer offer a breakthrough by providing a daytime record of nitrogen dioxide over Asia. Here we present the summertime diurnal patterns of nitrogen dioxide at major cities, power plant regions, and the Strait of Malacca. The Geostationary Environmental Monitoring Spectrometer data across various regions show high nitrogen dioxide in the morning which decrease in the afternoon, with varying hourly peaks, troughs, and amplitudes reflecting diurnal characteristics of local emissions and chemistry. Nitrogen oxide emissions inferred from Geostationary Environmental Monitoring Spectrometer and the Weather Research and Forecasting model coupled with Chemistry also show distinct patterns among regions: early morning peaks occur over Hanoi, Guangzhou, and Bangkok; mid-to-late morning peaks appear over Seoul and Beijing; and late afternoon peaks are noted in the Yangtze River Delta region. Top-down emissions incorporating temporal changes in the Geostationary Environmental Monitoring Spectrometer nitrogen dioxide yield the most accurate nitrogen dioxide simulations.

The Geostationary Environment Monitoring Spectrometer (GEMS) is the first geostationary earth orbit (GEO) environmental instrument, onboard the Geostationary Korea Multi-Purpose Satellite–2B (GEO-KOMPSAT-2B) launched on 19 February 2020, and is measuring reflected radiance from the earth's surface and atmosphere system in the range of 300–500 nm in the ultraviolet–visible (UV–Vis) region. The radiometric response of a satellite sensor that measures the UV–Vis wavelength region can depend on the polarization states of the incoming light. To reduce the sensitivity due to polarization, many current low earth orbit (LEO) satellites are equipped with a scrambler to depolarize the signals or a polarization measurement device (PMD) that simultaneously measures the polarization state of the atmosphere, then utilizes it for a polarization correction. However, a novel polarization correction algorithm is required since GEMS does not have a scrambler or a PMD. Therefore, this study aims to improve the radiometric accuracy of GEMS by developing a polarization correction algorithm optimized for GEMS that simultaneously considers the atmosphere's polarization state and the instrument's polarization sensitivity characteristics. The polarization factor and axis were derived by the preflight test on the ground as a function of wavelengths, showing a polarization sensitivity of more than 2 % at some specific wavelengths. The polarization states of the atmosphere are configured as a look-up table (LUT) using the Vector Linearized Discrete Ordinate Radiative-Transfer model (VLIDORT). Depending on the observation geometry and atmospheric conditions, the observed radiance spectrum can include a polarization error of 2 %. The performance of the proposed GEMS polarization algorithm was assessed using synthetic data, and the errors due to polarization were found to be larger in clear regions than in cloudy regions. After the polarization correction, polarization errors were reduced close to zero for almost all wavelengths, including the wavelength regions with high peaks and curvatures in the GEMS polarization factor, which sufficiently demonstrates the effectiveness of the proposed polarization correction algorithm. From the actual observation data after the launch of GEMS, the diurnal variation for the spatial distribution of polarization error was confirmed to be minimum at noon and maximum at sunrise/sunset. This can be used to improve the quality of GEMS measurements, the first geostationary environmental satellite, and then contribute to the retrieved accuracy of various Level-2 products, such as trace gases and aerosols in the atmosphere.

Geostationary Environment Monitoring Spectrometer (GEMS), the first UV-Vis hyperspectral imaging spectrometer onboard a geostationary satellite launched in February 2020, is working with overall performances as well as characteristics aligned with ground-based characterizations. However, there are noticeable issues, especially in the solar irradiances which show a significant discrepancy compared to reference datasets, the focus of current study. The key discrepancy is the variation of measured solar irradiance along the time as well as space of which the root causes are traced back to the angular dependence of the diffuser transmittance and its degradation, both of which critically impact the accuracy of the GEMS Level-2 data products. To mitigate the discrepancy, the current study introduces an empirical correction approach that uses the correlation between the azimuth angle and the measured daily irradiance using 3.5 years of data. With the correction, the spatial and seasonal discrepancies in both irradiance and Earth reflectance disappeared almost completely. Furthermore, the mean bias and root-mean-square deviation (RMSD) against the solar reference spectrum decreased by 12% and 5%, respectively. However, the corrected irradiance values are still lower than those from reference data and other satellites, indicating the potential need for future updates to the radiometric calibration coefficients.

The Geostationary Environment Monitoring Spectrometer (GEMS) onboard the Geostationary Korea Multi-Purpose Satellite-2B (GEO-KOMPSAT-2B) satellite was launched in February 2020 and observes the hourly volcanic SO2 in geostationary orbit. We For the first time show the hourly changes in volcanic SO2 distributions emitted and transported from several volcanoes over Asia. The various physical characteristics of volcanic plumes have been investigated based on hourly volcanic SO2 measurements. We estimated transport direction, path and speed, and altitude of volcanic SO2 plume emitted from Nishinoshima in Japan, Etna in Italy, Taal volcano in the Philippines and Dukono located in Halmahera, Indonesia. Before the eruption, Taal volcanic SO2 plumes, which were found to present within PBL, were transported mostly less than 100 km in various azimuth directions. Gradual increase in SO2 column densities was observed for about two months before a volcanic eruption from Taal. It implies that it might be possible to warn a volcanic eruption in advance which is subject to further investigation. GEMS can be further utilized for an improvement in prediction accuracy of SO2 plume transport using chemical transport model due to the availability of hourly volcanic SO2 height information.

Nitrogen dioxide (NO2) is generally emitted from the anthropogenic source such as fossil fuel combustion and natural sources such as lightning, forest fires, and soil emission. These NO2 have adverse effects on human health and are known to affect regional climate as a short lived climate forcer. In addition, it is a precursor of aerosol nitrate and plays a key role the photochemistry of tropospheric Ozone. Up to date, NO2 observation has been possible only once a day using low earth orbit satellite sensors such as GOME, SCIAMACHY, GEMS-2, OMI, OMPS, and TROPOMI. However, hourly NO2 monitoring is expected to provide better understanding of atmospheric chemistries and climate effects related with NOx in regional and global scales. From February, 2020, it is possible, for the first time, to observe the diurnal NO2 variations using Geostationary Environment Monitoring Spectrometer (GEMS). Here, we present first results of diurnal changes in total and tropospheric NO2 columns observed over Asia with high temporal and spatial resolutions using the GEMS operational NO2 algorithm. NIER of Ministry of Environment in South Korea plans to release the GEMS NO2 data in real-time. The GEMS operational NO2 algorithm based on DOAS technique and LUT based NO2 AMF to retrieve the total NO2 columns. We, in addition, retrieve the GEMS tropospheric NO2 columns by subtracting stratospheric NO2 columns from the total NO2 columns. The stratospheric NO2 columns are calculated from scaling stratospheric NO2 from SLIMCAT model using the real GEMS observation data over Pacific ocean. In this present study, we introduce diurnal characteristics at various major cities including, ports, and industrial regions. We also evaluate the performance of the GEMS NO2 retrieval algorithm by comparing GEMS NO2 columns and those observed from ground based Pandora at Seosan in South Korea and MAX-DOAS at Xianghe in China. The comparisons also are made between the total and tropospheric GEMS NO2 data and that of TROPOMI. The validation results show good agreements of GEMS data against those from others.

The Geostationary Environment Monitoring Spectrometer (GEMS), an ultraviolet and visible imaging spectrometer, provides air-quality information over a large area of the Asia Pacific region with a high spatiotemporal resolution. To assure the reliability of trace gas retrieval, accurate knowledge of the spectral response function (SRF) is critical for spectral calibration as well as retrieval algorithms. Here, we characterize the GEMS SRF using prelaunch SRFs obtained with the monochromatic laser measurements during the ground test and inflight SRFs retrieved using the solar irradiance measurements after the launch. The prelaunch SRFs are analyzed in terms of shape (skewness and kurtosis), width, and under-sampling and show that the full-width at half-maximum is smaller than 0.6 nm with a maximum of 0.589 nm. The variations along both the spectral and spatial directions are smooth and within 3.65%, indicating a highly homogenous and stable optical system of GEMS. To characterize the prelaunch SRFs and monitor the behavior of inflight SRFs, we applied several analytical functions including asymmetric super Gaussian (ASG) and hybrid Gaussians to the prelaunch SRFs. The spectral fitting of the measured GEMS irradiance with a reference spectrum shows that the ASG to be the best representative of the GEMS SRFs. The inflight SRFs, retrieved with the GEMS irradiances and the ASG, agree well with the prelaunch SRFs, suggesting that the inflight spectral performance and characteristics of GEMS are similar to those investigated from the on-ground characterization.

The Geostationary Korean Multi-Purpose Satellite (GK-2) program consisting of GK-2A and GK-2B provides consistent monitoring information in the Asia Pacific region, including the Korean peninsula. The Geostationary Environment Monitoring Spectrometer (GEMS) onboard GK-2B in particular provides information on the atmospheric composition and aerosol properties, retrieved from the calibrated radiance (Level 1B) with high spectral resolution in 300-500 nm. GEMS started its extended validation measurement after the in-orbit test (IOT) in October following the launch of the satellite in February 2020. One of issues found during the IOT is that GEMS shows a spatial dependence in the measured solar irradiance along the north-south direction, albeit the solar irradiance does not have such a dependency. Thus, such a dependence should be from the optical system or the solar diffuser which is placed in front of the scan mirror. To clarify the root cause of the dependence, we utilize inter-comparison of the Earth measurement between GEMS and the Advanced Meteorological Imager (AMI), a multi-channel imager onboard GK-2A for meteorological monitoring. As the spectral range of GEMS fully covers the spectral response function (SRF) of the AMI visible channel having a central wavelength of 470 nm, spectral matching is properly done by convolving the SRF with the hyperspectral data of GEMS. By taking advantage of the fact that the position of GK-2A and GK-2B is maintained within a 0.5 degree square box centered at 128.2°E, match-up data set for the inter-comparison is prepared by temporal and spatial collocation. To reduce spatio-temporal mis-match and increase the signal to noise, zonal mean is applied to the collocated data. Results show that the north-south dependence occurs in the comparison of reflectance, the ratio between the earth radiance and solar irradiance, while not in the comparison of radiance. This indicates the dependence occurs due to the characteristics of the solar diffuser, not because of optical system. It is further deduced that dependence of diffuser transmittance on the solar azimuth angle is the main cause of the north-south dependency which was not characterized during the pre-flight ground test.

The successful launch of Geostationary Environment Monitoring Spectrometer (GEMS) onboard the Geostationary Korea Multipurpose Satellite 2B (GK-2B) opens up a new possibility to provide daily air quality information for trace gases and aerosols over East Asia with high spatiotemporal resolution. As a part of major efforts to calibrate and validate the performance of the GEMS, accurate characterization of the spectral response functions (SRFs) is critical. The characteristics of preflight SRFs examined in terms of shape, width, skewness, and kurtosis vary smoothly along both the spectral and spatial direction thanks to highly symmetrical optic system of GEMS. While the preflight SRFs are determined with high accuracy, there is possibility of changes of in-flight SRFs during the harsh launch processes and/or operations over the mission lifetime. Thus, it is important to verify the in-flight SRFs after launch and to continue monitoring of their variability over time to assure the reliable trace gases retrievals. Here, we retrieve the in-flight SRFs for all spectral and spatial domain of the GEMS using spectral fitting of observed daily solar measurement and high-resolution solar reference spectrum. A variety of analytic model functions including hybrid form of Gaussian and flat-topped function, asymmetric super Gaussian, Voigt function are tested to determine the best representative function for GEMS SRF. The SRFs retrieved from early solar irradiances measured during the in-orbit tests agree well with the preflight SRFs indicating that no significant change occurred during the launch process. Continuous monitoring of the in-flight SRF is planned, using daily solar irradiances to investigate the temporal variation along with spectral and spatial directions. The detailed results of the in-flight SRF retrieval are to be presented.

The Geostationary Environment Monitoring Spectrometer (GEMS) is scheduled for launch in February 2020 to monitor air quality (AQ) at an unprecedented spatial and temporal resolution from a geostationary Earth orbit (GEO) for the first time. With the development of UV–visible spectrometers at sub-nm spectral resolution and sophisticated retrieval algorithms, estimates of the column amounts of atmospheric pollutants (O3, NO2, SO2, HCHO, CHOCHO, and aerosols) can be obtained. To date, all the UV–visible satellite missions monitoring air quality have been in low Earth orbit (LEO), allowing one to two observations per day. With UV–visible instruments on GEO platforms, the diurnal variations of these pollutants can now be determined. Details of the GEMS mission are presented, including instrumentation, scientific algorithms, predicted performance, and applications for air quality forecasts through data assimilation. GEMS will be on board the Geostationary Korea Multi-Purpose Satellite 2 (GEO-KOMPSAT-2) satellite series, which also hosts the Advanced Meteorological Imager (AMI) and Geostationary Ocean Color Imager 2 (GOCI-2). These three instruments will provide synergistic science products to better understand air quality, meteorology, the long-range transport of air pollutants, emission source distributions, and chemical processes. Faster sampling rates at higher spatial resolution will increase the probability of finding cloud-free pixels, leading to more observations of aerosols and trace gases than is possible from LEO. GEMS will be joined by NASA’s Tropospheric Emissions: Monitoring of Pollution (TEMPO) and ESA’s Sentinel-4 to form a GEO AQ satellite constellation in early 2020s, coordinated by the Committee on Earth Observation Satellites (CEOS).

To consistently observe deteriorating air quality over East Asia, the National Institute of Environmental Research, Republic of Korea, is planning to launch an environmental observation sensor, the Geostationary Environment Monitoring Spectrometer (GEMS), onboard the GK-2B satellite (a successor to the GeoKOMPSAT-1) in late 2019. GEMS is a hyperspectral spectrometer that covers the ultraviolet-visible range (300 to 500 nm) with full-width at half-maximum of 0.6 nm. It has been designed for the observation of air pollutants and short-lived climate pollutants. GEMS captures images at hourly intervals in the daytime, alternating with the Geostationary Ocean Color Imager-II every 30 min. Over the Seoul Special Metropolitan area, South Korea, the spatial sampling resolution of GEMS is 3.5 x 8 km (north-south and east-west, respectively). There are 16 baseline products, including aerosol optical depth and the vertical column density of trace gases such as nitrogen dioxide, sulfur dioxide, formaldehyde, and ozone. Research continues into additional applications (e.g., ground-level concentrations and emissions). (C) The Authors.

Monitoring of aerosol optical properties in high temporal and spatial resolution has been realized with the launch of Geostationary Ocean Color Imager (GOCI) and Meteorological Imager (MI) onboard the Communication, Oceanography, and Meteorology Satellite (COMS), also known as GEO-KOMPSAT (GK)-1 in 2010. In 2018, Advanced MI (AMI) will be launched with further enhanced capability onboard the GK-2A. GOCI-2 onboard GK-2B is planned to be launched in late 2019-early 2020 with Geostationary Environment Monitoring Spectrometer (GEMS). GEMS is a UV-visible spectrometer to monitor column concentration of trace gas including O 3 , NO 2 , SO 2 and HCHO, for the first time in high temporal and spatial resolution. In this study, results and plan to monitor atmospheric composition from geostationary earth orbit(GEO) are presented.