The Ministry of Science and ICT of South Korea has spearheaded the development of geostationary satellite missions, starting with Cheollian-1 designed for communication, ocean monitoring, and weather observations. Building on this success, the government launched the Geostationary Korea Multi-Purpose Satellite (GEO-KOMPSAT)-2 program, which encompasses two geostationary satellites: GEO-KOMPSAT-2A (GK-2A) and GEO-KOMPSAT-2B (GK-2B). GK-2A launched on December 5, 2018, carries two payloads including the Advanced Meteorological Imager (AMI) for meteorological monitoring and the Korean Space Environment Monitor for space weather monitoring.

GK-2B, the primary satellite discussed in this paper, also carries two payloads: the Geostationary Ocean Color Imager-II (GOCI-II) and the Geostationary Environment Monitoring Spectrometer (GEMS). These payloads were developed to meet the growing demand for monitoring the ocean environment and air quality across Asia, including South Korea. GEMS (Choi et al., 2018; Kim et al., 2020) was developed by the National Institute of Environmental Research (NIER) under the Ministry of Environment, in collaboration with the Korea Aerospace Research Institute under the Ministry of Science and ICT. This achievement is the result of NIER’s efforts since 2008 to lead the geostationary environmental satellite project aimed at real-time monitoring of air pollutants.

GEMS stands for Geostationary Environment Monitoring Spectrometer and symbolizes “gems”, representing precious jewels shining in the sky. This sophisticated ultraviolet-visible hyperspectral imager, designed to observe atmospheric pollutants from a geostationary orbit, was launched on February 19, 2020, from the Guiana Space Center, marking a significant milestone in environmental monitoring. GEMS is the first geostationary instrument capable of providing hourly daylight observations of air quality over a wide area, with a high spatial resolution of 3.5 × 8 km². It continuously monitors key atmospheric pollutants, including aerosols (Cho et al., 2024; Park et al., 2023b), nitrogen dioxide (NO₂; Kim et al., 2023), sulfur dioxide (SO₂; Park et al., 2021), ozone (O₃; Baek et al., 2023), and formaldehyde (HCHO; Lee et al., 2024), covering more than 20 countries in East and Southeast Asia.

The Environmental Satellite Center (ESC) of NIER was established in 2018 to serve as the ground station for GEMS, responsible for receiving, calibrating, retrieving, and distributing GEMS data. The center produces multi-level data products, ranging from Level 1 (radiometrically and geometrically calibrated data) to Level 4 (value-added products), which play a crucial role in air quality management, supporting scientific research, environmental policy, and public health initiatives across the region.

Simultaneously, similar geostationary missions have been developed globally. The United States successfully launched the Tropospheric Emissions: Monitoring of Pollution (TEMPO) on April 7, 2023, while Europe’s Sentinel-4 is scheduled for launch after 2024. Together, these three geostationary satellites including GEMS, form a global constellation that enhances the spatial and temporal coverage of air pollutants and provides reliable data to support air quality improvement strategies and climate change mitigation.

This review offers a comprehensive overview of GEMS data products, distribution methods, key applications in air quality monitoring, and the national and international collaborations of ESC. It also outlines the strategic plans of ESC to enhance GEMS data services for public use, emphasizing the critical role of GEMS in supporting environmental policy with scientifically robust data, and in developing the next generation of GEMS missions for continued air quality observation. This review aims to provide crucial information on how ongoing advancements in air-quality monitoring research will support the development of environmental policies and further scientific research.

This section provides a detailed overview of the observation characteristics, methods for accessing data, and statistical analysis of data distribution and downloads by users for GEMS.

2.1. Observation Characteristics

GEMS conducts Earth observations to provide hourly air quality measurements eight times daily during daylight hours across four distinct observation modes: Half East (HE), Half Korea (HK), Full Central (FC), and Full West (FW), as depicted in Fig. 1. GEMS starts scanning at the 45th minute of each hour and lasts for approximately 30 minutes, ensuring comprehensive coverage over East and Southeast Asia. The observation area spans from 45°N to 5°S (covering regions from Mongolia to Indonesia) and from 152°E to 80°E (extending from Japan to India), encompassing more than 20 countries within this region.

Figure 1. Observation areas of GEMS: (a) Half East (HE), (b) Half Korea (HK), (c) Full Central (FC), and (d) Full West (FW).

Since February 2024, the observation schedule and coverage areas of GEMS have been adjusted (Fig. 2) to increase the number of observations during winter, a season characterized by severe air pollution in East Asia, while reducing observations during summer, when atmospheric instability is typically higher. These adjustments support the Airborne and Satellite Investigation of Asian Air Quality/Satellite Integrated Joint Monitoring of Air Quality (ASIA-AQ/SIJAQ) campaign, conducted from February to March 2024, by maximizing the availability of GEMS data. Additionally, the observation domain at various times has been expanded to enhance the retrieval accuracy of volatile organic compounds (VOCs), particularly HCHO and glyoxal (CHOCHO). This improvement has been achieved by ensuring better retrieval accuracy based on more reliable background concentrations over the expanded coverage of the Pacific region.

Figure 2. GEMS observation schedule changes: (a) before February 2024 and (b) after February 2024.

For continuous in-orbit calibration, in addition to Earth observations, solar irradiance is measured daily using a working diffuser when the solar incident angle is approximately 30.5 degrees. A reference diffuser is employed every six months to monitor any degradation of the working diffuser. Normal dark images are collected twice daily, immediately before and after Earth observations. Furthermore, onboard Light-Emitting Diode (LED) measurements are conducted weekly to monitor the linearity of the charge-coupled device (CCD), a light-sensitive integrated circuit that captures images by converting photons to electrons.

2.2. GEMS Data Distributions

ESC offers three primary methods for accessing GEMS data. Users can download up to 20 GB or 40 files directly from the ESC website (https://nesc.nier.go.kr/) without logging in. For larger data requests, users can apply for a Secure File Transfer Protocol (SFTP) account, granted upon approval of their public IP address through the Web Application Firewall (WAF) of ESC. Additionally, since June 2023, an OPEN-API service has been available, allowing users to request data by obtaining a key code and generating a URL.

As illustrated in Fig. 3, over the last 18 months from January 2023 to June 2024, more than 5 million GEMS data files have been downloaded by users, with 92.5% of these downloads occurring via the SFTP service. Among approximately 40 countries accessing GEMS data, the United States, China, and Germany represent the largest users outside of South Korea. This reflects the global interest in GEMS data, applicable to both scientific research and air quality policy-making.

Figure 3. Average downloads by (a) method and (b) country from January 2023 to June 2024.

Fig. 4 presents the average search numbers, image file downloads, and NetCDF file downloads by species. Overall, aerosols, NO₂, and O₃ exhibit higher search and download volumes compared to other species. The highest search volume for aerosols is influenced by the default first page on the ESC website, which features aerosol data and automatically contributes to the search count. However, distinguishing users specifically interested in aerosols from those navigating to other species after landing on the default page is challenging. Despite this, aerosols remain the most frequently downloaded species for image files, indicating strong user demand. Similarly, both aerosols and NO₂ dominate NetCDF file downloads, underscoring the critical role of these species in air quality monitoring and environmental research.

Figure 4. Average (a) searches, (b) image file downloads, and (c) NetCDF file downloads by species from January 2023 to June 2024.

GEMS offers a comprehensive range of data products, categorized into Level 2 (21 products), Level 3 (4 products), and Level 4 (7 products), as summarized in Table 1 and shown in Fig. 5. These datasets provide essential information on air quality, cloud characteristics, and surface properties, addressing the needs of diverse users in research, policy-making, and public health. Note that information on algorithm explanations for each product, and precautions for interpretation can be found on the ESC website.

Figure 5. Spatial distributions of GEMS official products: (a) Level 2, (b) Level 3, and (c) Level 4 data produced by the ESC of NIER.

Table 1 . Lists of GEMS products from Level 2 to Level 4 produced by the ESC of NIER.

Level	No.	Product		Unit	Release date
Level 2	1	Aerosol	Aerosol optical depth	-	2021.03
	2		Single scattering albedo	-	2021.10
	3–4		UV-VIS aerosol index	-	2021.10
	5		Aerosol effective height	km	2022.11
	6	O3	Total ozone	DU	2021.03
	7		Stratospheric ozone	DU	2022.11
	8		Tropospheric ozone	DU	2022.11
	9	Surface reflectance	Surface reflectance	-	2022.11
	10	Cloud	Cloud-centered pressure	hPa	2021.10
	11		Effect cloud amount	-	2021.03
	12		Cloud radiation fraction	-	2021.10
	13	VOCs	Formaldehyde	molec/cm2	2022.06
	14		Glyoxyal	molec/cm2	2022.06
	15	NO2	Tropospheric NO2	molec/cm2	2022.11
	16		Total NO2	molec/cm2	2021.03
	17	SO2	Total SO2	molec/cm2	2021.03
	18	UV	UV index	-	2021.10
	19		Plant response rate	-	2021.03
	20		DNA damage rate	-	2021.03
	21		Vitamine D production rate	-	2021.03
Level 3	1	Average concentrations	Daily total NO2	molec/cm2	2023.05
	2		Daily trophosperic NO2
	3		Monthly total NO2
	4		Monthly trophosperic NO2
Level 4	1	Mass flow rates	Aerosol	Mg/hour	2021.11
	2		SO2	Mg/hour	2022.11
	3	Estimated surface concentrations	PM2.5	μg/m3	2021.12
	4		PM10	μg/m3	2021.12
	5		NO2	ppb	2022.12
	6–7	Ratios	Atmospheric emissions characteristic ratio (NO2/CO2)	(1015molec/cm2)/ppm	2023.11

3.1. Level 2 Data (Standard Products)

Since November 2022, all Level 2 products have been publicly available via the ESC website (https://nesc.nier.go.kr/). These products encompass key atmospheric gases, aerosols, clouds, and surface reflectance.

Among the atmospheric gases, GEMS measures O₃, SO₂, NO₂, HCHO, and CHOCHO. The O₃ product provides both total and tropospheric O₃ densities, which are critical for understanding stratospheric depletion and surface-level pollution. SO₂, primarily emitted from volcanic activity and industrial processes, is essential for monitoring emissions and their impacts on air quality. NO₂, mainly associated with fossil fuel combustion, is monitored in both total and tropospheric forms, offering insights into urban air pollution. HCHO and CHOCHO, important precursors to O₃ formation, serve as key markers of pollution in photochemical processes.

Regarding aerosols, Aerosol Optical Depth (AOD) measures the extent to which aerosols scatter or absorb sunlight, serving as a critical parameter for understanding particulate matter concentrations in the atmosphere. In addition to AOD, GEMS provides Single Scattering Albedo (SSA), quantifying the ratio of scattering to total extinction efficiency by aerosols, which offers insights into their radiative properties. Furthermore, the ultraviolet (UV) index and visible (VIS) index are utilized to infer aerosol absorption properties and aerosol sizes, respectively, providing a deeper understanding of aerosol behavior and their effects on radiation.

In addition to aerosols, GEMS monitors cloud properties (Kim et al., 2024a), surface reflectance (Sim et al., 2024), and the UV index. The cloud products provide data on effective cloud fraction, cloud centroid pressure, and cloud radiance fraction, crucial for interpreting satellite measurements of trace gases and aerosols under varying cloud conditions. Surface reflectance products, which quantify the fraction of sunlight reflected by the Earth’s surface, are essential for accurately retrieving atmospheric data, especially in regions with diverse land cover. Furthermore, the UV index, derived from GEMS data, is used to assess potential UV radiation exposure and can play a key role in issuing public health advisories on sun exposure.

Additionally, GEMS employs a hyperspectral sensor capable of measuring wavelengths in the range of 300 to 500 nm at 0.2 nm intervals, allowing for the potential development of new data products beyond the current standard products. As a result, it has been confirmed that GEMS can observe substances such as bromine monoxide (BrO), nitrous acid (HONO), and iodine monoxide (IO). While the accuracy for IO observations remains low, data processing algorithms are being developed for BrO, and HONO. Observational data for these new products are anticipated to be available after 2024.

3.2. Level 3 Data (Averaged Products)

The ESC provides daily and monthly averaged products for both tropospheric and total column NO₂, available since May 2023. Due to variations in the GEMS observation domains by time and month, as discussed in Section 2.1, hourly NO₂ data is spatially collocated into a 5 km resolution grid over the Korean Peninsula and an approximately 10 km resolution across the broader GEMS observation area for averaging. These averaged products are designed to reduce short-term variability and noise, making them particularly valuable for long-term and seasonal trend analysis, as well as for environmental monitoring by users and policymakers.

3.3. Level 4 Data (Value-added Products)

Level 4 GEMS products are generated through the integration of additional datasets and include surface concentrations of particulate matter (PM_2.5 and PM₁₀) and NO₂, mass flow rates for aerosols and SO₂, and atmospheric emission characteristics such as the NO₂ to CO₂ ratio.

The PM_2.5 and PM₁₀, which are derived by combining satellite data with ground-based measurements and meteorological information from the Unified Model (UM) simulations by the Korea Meteorological Administration (KMA) (Park et al., 2022). The satellite-derived PM concentrations are critical for public health assessments, as fine particulate matter poses significant respiratory risks, and help overcome the spatial limitations of ground-based in-situ measurements, which are sparsely distributed across land.

The mass flow rates of aerosol and SO₂ track the movement of these pollutants across regions, providing essential data for understanding transboundary pollution and its impact on downwind areas (Lee et al., 2021). Additionally, the NO₂ to CO₂ ratio offers insights into emission characteristics, aiding in the identification of industrial and vehicular pollution sources.

These Level 4 products are crucial for researchers and policymakers, offering actionable data to support public health initiatives, environmental regulations, and cross-border cooperation in managing air quality. Future expansions of the Level 4 product suite are also planned by ESC, including fused aerosol information utilizing geostationary satellites and estimated emissions for aerosols and NO₂, further enhancing the utility of GEMS data for a broader range of applications.

Section 4 discusses the diverse applications of GEMS data, focusing on the analysis of exceptional events and the ongoing development of advanced application techniques by ESC.

4.1. Analysis of Exceptional Events

This section presents examples of utilizing GEMS data to analyze exceptional events such as wildfires and volcanic eruptions. In March 2022, two major wildfires broke out along the East Sea of South Korea. The first wildfire commenced in Gyeongsangbuk-do on March 4, while the second began in Gangwon-do on March 5. Fig. 6 shows the spatial distribution of the UV aerosol index (UVAI), NO₂, and CHOCHO observed by GEMS, capturing two distinct smoke plumes dispersing into the surrounding areas on March 5, 2022.

Figure 6. Spatial distribution of (a) UV aerosol index (UVAI), (b) total column NO₂, and (c) total column CHOCHO on March 5, 2022.

Additionally, GEMS data can be employed to monitor and analyze volcanic eruptions, providing valuable insights into the dispersion of volcanic gases. For instance, the Ruang Volcano, located on Ruang Island in North Sulawesi, Indonesia, erupted on April 16, 2024, with a subsequent eruption occurring on April 30, 2024. As depicted in Fig. 7, GEMS effectively captured the extensive dispersion of SO₂ across Indonesian seas and neighboring countries.

Figure 7. Total column SO₂ retrieved from GEMS observations between April 29 and May 2, 2024.

4.2. Application Techniques

The ESC is conducting several research projects in collaboration with academic institutions and the industrial sector, aimed at developing novel techniques for utilizing GEMS data across various fields. One ongoing project leverages the capability of geostationary satellites to produce hourly data, focusing on generating fused AOD by integrating data from South Korea’s three geostationary satellites: GEMS, GOCI-II, and AMI (Kim et al., 2024b). Each of these satellites provides aerosol information with different temporal and spatial resolutions, so their data is first collocated into a spatial resolution of 0.5° × 0.5°, aligned with GEMS observation times. The collocated AOD from each satellite is then fused using Maximum Likelihood Estimation (MLE) and deep learning techniques. The fused AOD shows strong agreement with ground-based reference data from the Aerosol Robotic Network.

The fused AOD can be utilized for various additional applications. ESC is also developing techniques to estimate top-down emissions of PM using the fused AOD data through inverse modeling (Park et al., 2023a). Moreover, GEMS NO₂ column data is used to estimate nitrogen oxides (NOx) emissions. These estimated PM and NOx emissions serve as input data, alongside initial and boundary conditions and meteorological fields, for chemical transport models. Compared to bottom-up emissions, which are derived from statistical methods and may take years to be publicly released, the top-down emissions estimated from GEMS data can facilitate more accurate model simulations by reflecting recent changes in emissions.

Furthermore, ESC is developing techniques to estimate AOD and NO₂ data during nighttime. Here, nighttime refers to the period within 1 to 2 hours after the completion of observation. Since GEMS relies on reflected sunlight from the Earth’s surface, it is currently unable to retrieve air pollutant information during nighttime hours. However, nighttime estimates are crucial, particularly during high pollution episodes when pollutants can be transported overnight. The estimated nighttime AOD and NO₂ data would provide valuable insights into the movement of air pollutants during these periods.

In addition to the aforementioned techniques, ESC is collaborating with numerous researchers to develop advanced application techniques using GEMS data. The outputs from these collaborations are planned to be provided by ESC in the near future, offering further insights and enhancements in environmental monitoring.

ESC plays a pivotal role in fostering both national and international collaborations aimed at maximizing the utility of GEMS data as the operator of the GEMS satellite. By partnering with governmental bodies, research institutions, and international organizations, ESC has been instrumental in advancing air quality monitoring and analysis efforts. These collaborations ensure that GEMS data not only meets rigorous scientific and regulatory standards but also addresses regional and global air quality challenges. The cooperative efforts encompass a wide range of activities, from technical calibration and validation to practical applications in air quality policy-making, thereby supporting public health initiatives.

5.1. National Cooperation

The ESC has actively engaged in national collaborations to enhance the accuracy and applicability of GEMS data. These efforts involve partnerships with government agencies, academic institutions, and industry stakeholders to ensure that GEMS data supports a wide range of environmental research and policy-making initiatives.

A pivotal national collaboration is the GK-2A/2B Integrated Applications Consortium, established between the ESC of the NIER, the KMA’s National Meteorological Satellite Center (NMSC), the National Ocean Satellite Center (NOSC) of the Korea Hydrographic and Oceanographic Agency (KHOA), and the Korea Ocean Satellite Center (KOSC) of the Korea Institute of Ocean Science and Technology (KIOST). The consortium was established in October 2020 and expanded through an agreement in March 2022, to facilitate the operation, calibration, and utilization of the GK-2A/2B satellites. It focuses on sharing satellite data, improving calibration accuracy, developing fused products, and enhancing satellite services. Furthermore, the consortium organizes an annual academic conference to share research achievements with the public, with the 6th conference held in Busan on September 5, 2024. On September 6, primary and working group meetings were conducted to discuss further collaborative efforts between consortium member organizations. The ESC plans to continue strengthening cooperation not only with national organizations but also with academic institutions and industry stakeholders to expand the use and application of GEMS data.

5.2. International Cooperation

Since 2020, the ESC has collaborated with more than 20 research institutions across North America, Europe, and Asia to validate the accuracy of GEMS data. This includes partnerships with prominent organizations such as the National Aeronautics and Space Administration (NASA) and the National Oceanic and Atmospheric Administration (NOAA) in North America, the Max Planck Institute (MPI), the European Space Agency (ESA), the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT), and the Royal Belgian Institute for Space Aeronomy (BIRA-IASB) in Europe, and the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) in Asia. Additionally, the ESC actively participates in international collaborations under the United Nations (UN) framework and other global environmental initiatives.

One of the major international collaborations is the Airborne and Satellite Investigation of Asian Air Quality/Satellite Integrated ASIA-AQ/SIJAQ campaign, governed by NASA and NIER, which took place from February to March 2024 (Fig. 8). This campaign, involving multiple Asian countries, focuses on enhancing air quality monitoring and assessment across the region. By leveraging GEMS data, the campaign aims to monitor air pollutants during high-pollution periods, thereby improving the understanding of air quality issues and their impacts on the environment and public health. This collaboration has also strengthened the calibration and validation of satellite data by integrating ground- and airborne-based observations, ensuring the accuracy and reliability of GEMS products across Asia.

Figure 8. Diagram of the Airborne and Satellite Investigation of Asian Air Quality/Satellite Integrated Joint Monitoring of Air Quality (ASIA-AQ/SIJAQ) campaign.

The ESC also leads the development of the Pandora Asia Network (PAN), a ground-based air quality monitoring initiative spanning seven countries in Asia. By 2023, ten monitoring sites were established in Mongolia, Laos, Cambodia, Thailand, and Indonesia, with plans to expand to 20 sites by the end of 2024. Supported by the United Nations Economic and Social Commission for Asia and the Pacific (UN ESCAP) and the Korea International Cooperation Agency (KOICA), this network plays a crucial role in enhancing regional air quality monitoring and capacity-building efforts, contributing to the validation of GEMS data and fostering a more comprehensive understanding of air quality across Asia.

The GEMS mission, as part of the GK-2 satellite program led by the South Korean government, has pioneered air quality monitoring across East and Southeast Asia by providing air quality data with high spatial and temporal resolution, retrieved and distributed by the ESC of NIER. The ability of GEMS to measure aerosols and gases such as NO₂, SO₂, and O₃ is invaluable for understanding the spatial distribution, transport, and variability of air pollution. The satellite has also proven essential for analyzing extraordinary events such as high PM episodes, wildfires, and volcanic eruptions. Furthermore, the ESC is actively engaged in several research projects aimed at developing novel techniques, including satellite data fusion, top-down emissions estimations, nighttime data retrievals, and more.

Looking ahead, ESC is preparing to enhance its environmental monitoring capabilities with the launch of a microsatellite constellation for greenhouse gas monitoring. The first satellite is planned for launch in 2027, followed by four more in 2028. These microsatellites, along with the next generation of GEMS, will strengthen the leadership of South Korea in satellite-based air quality monitoring, contributing to global efforts to improve air quality and address climate change. The continuous evolution of GEMS, along with the strategic planning by ESC for public access and collaboration, will ensure that GEMS data remains critical in environmental research, public health initiatives, and policymaking for clean air and climate mitigation.

This work was supported by a grant from the National Institute of Environment Research (NIER), funded by the Ministry of Environment of the Republic of Korea (NIER-2024-01-01-078). We would also like to express our sincere appreciation to the researchers, including the GEMS science team, for their tremendous efforts in the development, calibration, retrieval, and various applications related to the GEMS project.

No potential conflict of interest relevant to this article was reported.