GEMS (GEO-KOMPSAT-2B)¶

Geostationary Environment Monitoring Spectrometer (GEMS) on GEO-KOMPSAT-2B — hourly air-pollutant and aerosol observations over Asia

Available

At a glance¶

Note on period: GEO-KOMPSAT-2B was launched on 19 February 2020 and reached its target orbit on 6 March (Cho et al., 2024; Yonsei University GEMS algorithm team). The Environmental Satellite Center website does not state when the data service began or the period available for each product. Published studies mostly use L2 data from November 2021 onward, so check that the period you need exists with a web-download search or the Open-API observation-time query (getFileDateList) before planning an analysis.

Overview¶

GEMS, on the GEO-KOMPSAT-2B (Chollian-2B) satellite launched in February 2020, is the first geostationary instrument dedicated to air-quality monitoring. Its UV–visible imaging spectrometer (300–500 nm, 0.6 nm resolution) observes East and Southeast Asia every daylight hour. The NIER Environmental Satellite Center distributes Level 2 products for NO2, SO2, formaldehyde (HCHO), glyoxal (CHOCHO), ozone (total column and profile), aerosols (optical depth, single-scattering albedo, effective height), clouds and UV indices, plus daily and monthly NO2 means (L3) and estimated surface PM10 and PM2.5 (L4). Unlike low-Earth-orbit sensors such as TROPOMI and OMI, which pass once a day, GEMS captures the diurnal cycle over the same region, which makes it useful for tracking emission sources, long-range transport and daily variation.

Available variables¶

The product list and versions follow the center's SFTP page and web-download menu. Variable names are the in-file names given in Annex Table 2 of the center's GEMSAT quick user guide (April 2025), and units follow Annex Table 1 of the same guide. The guide gives no group paths, scale factors or quality flags, so check the attributes of the files you receive. HCHO and CHOCHO share one name (ColumnAmount), as do O3T and O3P (ColumnAmountO3); tell them apart by the product file (the product code in the file name).

instantaneous mean accumulated maximum minimum static — instantaneous: value at the time stamp · mean/accumulated/maximum/minimum: over the indicated interval · static: does not change with time

L2 trace gases (hourly scans)¶

L2 aerosol, cloud, surface and UV (hourly scans)¶

L3 NO2 means¶

L4 surface concentration estimates¶

The full variable list is in the official documentation.

How to get the data¶

• NIER ESC Web Download (data service cart) — Web search and cart request → download with the issued request key
  • There is no user account or login. Select satellite, format (image or data) and product, search, add files to the cart, enter the purpose of use and a notification e-mail, agree to the personal-data notice and submit; the page then issues a request key. Copy the key or save it as a key file, and once the completion e-mail arrives, enter the key (or upload the key file) under 'Data request status' to look up and download the files. An existing key can be reused with later requests.
• GEMS Open-API — REST API (JSON, XML or TEXT responses) — build request URLs on the 'Create Open-API URL' page after a key is issued
  • Apply on the key-issuance page with an e-mail address and purpose of use; after administrator approval the key is sent by e-mail. Approval can take about 7 days, keys are valid for one year, and the e-mail may land in a spam folder. Query types are observation-time listing (getFileDateList), file-name listing (getFileList) and download (getFileItem); dates use yyyyMMddHHmm (date, sDate, eDate) and the product version is set with ver (e.g. v2.1.0). A package of shell-script samples (api_sample_download.zip) is on the same page.
• NIER ESC SFTP service (bulk data) — SFTP (application form → public IP registered with the center's web firewall → account issued)
  • Use this for long periods or many products. Overseas users e-mail the 'Application for GEMS Data' form (.docx) to Kim Goo (kimgoo9@korea.kr) or Seo Jeonghyeon (savetheearth@korea.kr); Korean users submit an official letter. An account is issued only after the public IP of your machine is allowed through the center's firewall, and existing users must reapply for each new product. Mailing an external hard drive to the center is an alternative. Algorithm questions go to Hong Hyunkee (wanju77@korea.kr).
• GEMSAT environmental satellite toolkit — Desktop software (Lite, Standard, Expert v120; Linux v110)
  • Display, editing and comparison tool for GEMS imagery; the researcher (Expert) edition supports Python. Manuals are in Korean.
• Satellite image viewer and GIS viewer — Web map viewer
  • Useful for browsing hourly fields before downloading.

Example code¶

Prints the group and variable structure of a downloaded L2 file (assumed to be netCDF4/HDF5-based). GEMS variable names differ between products and versions, so inspect the structure and attributes (units, _FillValue, scale_factor) first and then pick the variables you need. The file name is a placeholder.

# pip install h5py
import h5py

path = "GEMS_L2_NO2_example.nc"   # replace with a file from web download or SFTP

with h5py.File(path, "r") as f:
    def show(name, obj):
        if isinstance(obj, h5py.Dataset):
            units = obj.attrs.get("units", b"")
            print(f"{name:60s} {obj.shape} {units}")
    f.visititems(show)          # group/variable names, array shapes, units
    print(dict(f.attrs))        # global attributes (product version, scan time, etc.)

Practical tips¶

• Always record the product version. Each product is versioned separately (as of October 2026, NO2 v4.0.1, AERAOD v2.1.0 and so on), and 2024 papers used NO2 v2.0. Do not join different versions into one time series.
• Apply the quality flags and cloud screening. For example, Yang et al. (2024) used NO2 v2.0 retrievals passing the quality and final-algorithm flags, with solar and viewing zenith angles below 70° and cloud fraction below 0.3. Flag definitions can change between versions, so check the ATBD.
• Korea is viewed nearly head-on from geostationary orbit, but viewing zenith angles and pixel sizes grow toward the edges of the field of regard (India, Indonesia). The 7 km × 8 km pixel size refers to Seoul.
• Morning and evening scans have large solar zenith angles, lower quality and often cover only half the area. When analysing diurnal cycles, report the number of samples and the area for each hour.
• Open-API keys take about 7 days to approve and expire after one year. For long projects note the expiry date, and start the SFTP application (which needs a formal request) early for bulk data.
• The center asks for an acknowledgement in publications. The standard reference is Kim et al. (2020, BAMS); product-specific validation papers (e.g. Cho et al., 2024, for aerosols) are usually cited as well.
• GEMS data are not registered in NASA CMR/Earthdata (checked 2026-10-13). When comparing with the other geostationary air-quality missions, TEMPO (US) and Sentinel-4 (Europe), note that distributors and versioning differ.

Notices and change history¶

No notices yet.

References for your paper¶

Reference papers¶

GEMS mission overview (BAMS) — DOI 10.1175/BAMS-D-18-0013.1

@article{kim2020gems,
  author  = {Kim, Jhoon and Jeong, Ukkyo and Ahn, Myoung-Hwan and Kim, Jae H. and Park, Rokjin J. and Lee, Hanlim and others},
  title   = {New Era of Air Quality Monitoring from Space: Geostationary Environment Monitoring Spectrometer ({GEMS})},
  journal = {Bulletin of the American Meteorological Society},
  volume  = {101},
  number  = {1},
  pages   = {E1--E22},
  year    = {2020},
  doi     = {10.1175/BAMS-D-18-0013.1}
}

First GEMS aerosol results (AMT) — DOI 10.5194/amt-17-4369-2024

@article{cho2024gemsaerosol,
  author  = {Cho, Yeseul and Kim, Jhoon and Go, Sujung and Kim, Mijin and Lee, Seoyoung and Kim, Minseok and Chong, Heesung and Lee, Won-Jin and Lee, Dong-Won and Torres, Omar and Park, Sang Seo},
  title   = {First atmospheric aerosol-monitoring results from the Geostationary Environment Monitoring Spectrometer ({GEMS}) over Asia},
  journal = {Atmospheric Measurement Techniques},
  volume  = {17},
  pages   = {4369--4390},
  year    = {2024},
  doi     = {10.5194/amt-17-4369-2024}
}

Diurnal NO2 from GEMS (ACP) — DOI 10.5194/acp-24-7027-2024

@article{yang2024gemsno2,
  author  = {Yang, Laura Hyesung and Jacob, Daniel J. and Dang, Ruijun and Oak, Yujin J. and Lin, Haipeng and Kim, Jhoon and others},
  title   = {Interpreting Geostationary Environment Monitoring Spectrometer ({GEMS}) geostationary satellite observations of the diurnal variation in nitrogen dioxide ({NO2}) over East Asia},
  journal = {Atmospheric Chemistry and Physics},
  volume  = {24},
  pages   = {7027--7039},
  year    = {2024},
  doi     = {10.5194/acp-24-7027-2024}
}

Acknowledgements¶

GEMS data have no dataset DOI. The center recommends that publications include: "This study was conducted using the GEMS (Geostationary Environment Monitoring Spectrometer) data provided by the Environmental Satellite Center of the National Institute of Environmental Research (NIER)." Also state the product names and versions used (e.g. NO2 v4.0.1).

Information on the datasets used in this study was compiled with the help of the Korean Climate Data Navigator (KCDN) (Sim, 2026; https://doi.org/10.5281/zenodo.23300923).

Links¶

• NIER Environmental Satellite Center (English)
• GEMS operation (scan schedule and areas)
• GEMS Open-API guide
• SFTP service (current product versions)
• Algorithm documents (ATBD, Korean site)
• GEMS International Workshop
• NIER ESC copyright policy (Korean)
• Yonsei University GEMS algorithm team