The Geostationary Environment Monitoring Spectrometer (GEMS), the first ultraviolet-visible hyperspectral imager in geostationary orbit, has been operating since its launch in February 2020. This paper evaluates and discusses the on-orbit performance of GEMS using 4.5 years of irradiance measurements (300-500 nm), with particular emphasis on the long-term changes in its radiometric accuracy and spectral stability. As part of this assessment, we examine various super-Gaussian forms used for deriving on-orbit slit functions and conclude that applying symmetry constraints to both shape and width parameters enhances fitting stability and accuracy. During the initial phase of operation, the full width at half maximum (FWHM) of the derived slit functions ranged from 0.56 to 0.58 nm across spatial and spectral dimensions. The wavelength shift remains spatially uniform and varies smoothly within ± 0.05 nm, showing a consistent, repeating pattern in 300-400 nm and 400-500 nm. However, over time, instrument degradation has led to pronounced spectral and spatial variations, particularly at shorter wavelengths. Slit widths (FWHM) at the center pixels fluctuate between 0.54 and 0.60 nm, and a “frown-shaped” spatial pattern emerges in the wavelength shift. Radiometric offsets exhibit a steady increase—approximately 5 % per year at shorter wavelengths below 330 nm and 0.2 % above 460 nm. By 2024, irradiance values are up to 35 % lower near 325 nm and about 10 % lower above 460 nm compared to the solar reference. To isolate fine-scale uncertainties, we further evaluate the fitting residuals between the measured irradiance and the radiometrically adjusted solar reference. As the degradation progressed, residuals in 310-330 nm increased sharply from 0.4 % in 2020 to 4 % in 2024, while fitting uncertainties nearly doubled in 330-360 nm. This degradation feature indicates the emergence of instrumental artifacts in spectral structures that are unrelated to physical solar absorption features. Such artifacts are most pronounced at the central pixels and have gradually extended toward the southern pixels and adjacent spectral regions throughout the mission. Beyond 400 nm, radiometric offsets occur on a broadband scale, with no noticeable degradation in the fine spectral structures.

Aerosol size information is important to the understanding of aerosol dynamics, which change rapidly during wildfire, dust transport, and volcanic eruption events over Asia. In this study, a deep neural network model was trained using Advanced Meteorological Imager (AMI) level 1b observations, AMI Yonsei aerosol retrieval aerosol products, and observation geometries to retrieve the aerosol optical depth (AOD), Ångström exponent (AE), and spectral derivatives of AE (AE′). The fine-mode fraction (FMF) was calculated with a spectral deconvolution algorithm using retrieved AE and AE′ when AOD > 0.2. The retrieved aerosol products were validated using AERONET (AOD at 550 nm: R = 0.837, RMSE = 0.219, MBE = –0.066; AE: R = 0.726; RMSE = 0.231; MBE = –0.007; FMF: R = 0.875; RMSE = 0.072; MBE = 0.007). Case studies of dust transport, wildfire, and haze events in Asia revealed that the retrieved aerosol size products may be used for analysis of sudden pollution events. Results of this study indicate the potential for comprehensive analysis of aerosol properties in Asia using continuous aerosol size data from geostationary Earth orbit satellite observations.

The Geostationary Environment Monitoring Spectrometer (GEMS), launched in February 2020, performs hourly measurements of earthshine radiances to retrieve column amounts of air pollutants over Asia. However, the charge-coupled device detector of GEMS has bad pixels that exhibit abnormal radiometric responses, which translates to a decrease in the quality of radiance measurements. Permanent bad pixels result in an information gap in the aerosol product at ~14.4–16.1°N latitudes (e.g., in Manila, the Philippines, and Mainland Southeast Asia), which cannot be filled even with long-term observations owing to the structure of the East-West scanning mechanism of GEMS. Here, we propose a robust method to reconstruct radiances measured inaccurately by the bad pixels, based on spectral correlation induced mainly by the Fraunhofer line structures. The reconstruction aims at the bad pixels in the wavelength range of ~485–491 nm, which affects aerosol retrieval. We estimate that uncertainties in the reconstructed optical depths are ~2 orders of magnitude smaller than typical aerosol optical depths. Our results convincingly demonstrate that the reconstructed radiances effectively restore the physical distributions of visible aerosol indices, improving the determination of aerosol types. Furthermore, the reconstructed radiances enhance retrievals of aerosol layer height (ALH), holding particular significance for the long-term accumulation of ALH data over Southeast Asia using GEMS.

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.

Aerosol layer height (ALH) has been retrieved using multi-angle observations or the O2–O2 and O2–A/B absorption bands. This study attempted to retrieve ALH using the Advanced Himawari Imager (AHI), a single passive imager onboard Himawari-8 and -9. ALH retrieval using geostationary Earth orbit (GEO) satellites is advantageous for monitoring diurnal changes in ALH and understanding long-range transport. Before retrieving the ALH, the aerosol optical properties (AOPs) are retrieved using the green-near infrared (NIR) band, which is relatively insensitive to aerosol height. The retrieved AOPs are used as input to the radiative transfer calculation to compute the top-of-atmosphere (TOA) reflectance of a highly sensitive band (the blue band in this study). Then, the ALH is retrieved using the observed and calculated TOA reflectances. Since the retrieval accuracy of the aerosol optical depth (AOD) is better over the ocean, the retrieval was performed only over the ocean during the Korea–United States Air Quality Study (KORUS-AQ) campaign period. The retrieved ALH was validated using the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) and high-spectral-resolution Lidar (HSRL).

The Airborne and Satellite Investigation of Air Quality (ASIA-AQ) is an international field study concept in the active planning stage. The study will implement air quality observations from multiple perspectives (satellite, ground-based, and airborne) in a consistent strategy across selected Asian countries to improve understanding of both specific air quality issues for each location and common challenges in the interpretation of satellite observations and model predictions of air quality. With initial flights planned for early 2024, this paper outlines the basic plans for the study. Ongoing efforts include the development of specific plans for each country that consider the unique conditions and emissions driving local air quality.

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.

We retrieved aerosol effective height (AEH) values from TROPOspheric monitoring instrument (TROPOMI) measurements based on the O4absorption at 477 nm. As the first step in the AEH retrieval algorithm, TROPOMI carbon monoxide and aerosol index products were used for the first time to determine aerosol type. An online radiative transfer calculation method was introduced to the AEH retrieval process. The AEHs were retrieved over northeast Asia (various aerosol types), South Africa (biomass-burning smoke), and the Sahara Desert (dust). The retrieved AEHs were compared with AEHs derived using aerosol extinction profiles from ground and satellite-based lidar measurements. A preliminary evaluation based on comparison with lidar suggests reasonable performance, but shows that cloud masking is the main source of uncertainty. The errors associated with the online-based AEH retrieval algorithm were calculated using synthetic radiance. The total AEH retrieval errors were 0.91 km (0.4 ≤ AOD4absorption band at 477 nm is applicable to aerosol height retrieval from TROPOMI with a high spatial resolution.

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 O3, NO2, SO2and 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.

With the launch of Geostationary Ocean Color Imager(GOCI) and Meteorological Imager(MI) onboard the Communication, Oceanography, and Meteorology Satellite(COMS) in 2010, hourly monitoring of various aerosol properties has been realized. Together with the plan to launch Geostationary Environment Monitoring Spectrometer(GEMS) in 2019, monitoring of trace gas concentration will be possible in high temporal and spatial resolution. In this study, results and plan to monitor atmospheric composition from geostationary earth orbit(GEO) are presented.