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Sentinel-5p TROPOMI validation campaigns

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The new sensor technology and retrieval approaches of S-5p/TROPOMI, launched in October 2017, introduce many new opportunities and challenges. This requires to carefully assess the quality and validity of the generated data products by comparison with independent airborne and ground-based reference observations. The UV-Visible DOAS group is coordinating a series of international S-5p validation campaigns (2020-2022), including the S5PVAL-BE airborne campaign over Brussels and Antwerp, Belgium.
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S-5p validation campaigns relevant to air quality and climate

Somewhere out there in space, there is a sentinel with the task to keep a close eye on the Earth’s climate and air quality. It is a European satellite named Sentinel-5 Precursor (S-5p), with the ability to get highly detailed data on the most important gases around the entire globe, and this every 24 hours.

Scientists are working together over international borders to ensure that the raw data is processed into useful information, but also to ensure that S-5p measures everything correctly, by comparing with ground-based and airborne reference data.

The Royal Belgian Institute for Space Aeronomy is currently coordinating a series of these international validation campaigns (2020-2022), in close collaboration with the European Space Agency. The campaign goals are to assess the quality (uncertainty and error characterisation) of a wide range of S-5p products that are relevant to air quality and climate:

  • nitrogen dioxide (NO2)
  • carbon monoxide (CO)
  • methane (CH4)
  • formaldehyde (HCHO)
  • sulphur dioxide (SO2)
  • the aerosol layer (dust particles)

The validation process provides the assurance of reliable data, so that scientists can make correct assessments of local air quality and predictions of the future of Earth’s climate, and inform governments and policy makers.

NO2 emissions in the S5PVAL-BE campaign

The TROPOMI tropospheric NO2 L2 product (OFFL v1.03.01; 3.5 km × 5.5 km at nadir observations) is one of the most valuable assets of the mission, as NO2 is a trace gas with documented harmful effects on human health as well as a proxy for air pollution. The principal sources of NO2 are fuel combustion due to traffic, domestic heating and industrial activities.

In the context of the S-5p validation campaigns, the S5PVAL-BE campaign took place in Belgium in June 2019 to monitor NO2 emissions and to validate the TROPOMI NO2 product over strongly polluted urban regions, such as Brussels and Antwerp, by comparison with coincident high-resolution Airborne Prism EXperiment (APEX) remote sensing observations (~75 m × 120 m).

Overall for the ensemble of four flights, the TROPOMI NO2 VCD product was well correlated (R = 0.92) but biased negatively by  −1.2 ± 1.2 × 1015 molec cm−2 or −14 % ± 12 %, on average, with respect to coincident APEX NO2 reference data. The validation results, including potential explanations for the observed bias, have been a key input for the improvement of the NO2 retrieval algorithm (planned release of v2.0 by early 2021).



  • Tack, F., Merlaud, A., Iordache, M.-D., Pinardi, G., Dimitropoulou, E., Eskes, H., Bomans, B., Veefkind, P., and Van Roozendael, M. (2021). Assessment of the TROPOMI tropospheric NO₂ product based on airborne APEX observations. Atmospheric Measurement Techniques, 14(1), 615-646. Open Access Logo

  • Royal Belgian Institute for Space Aeronomy. (2021, February). Sentinel-5 Precursor Campaigns.

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Figure 2 caption (legend)
Tropospheric NO2 VCD grid retrieved from APEX over Brussels on 28 June 2019. White dots indicate the point sources, emitting more than 10 kg of NOx per hour, according to the emission inventory (2017) of the Belgian Interregional Environment Agency. Coinciding TROPOMI tropospheric NO2 VCD retrievals are overlayed as color-coded polygons.
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