Validation Summary

The Sentinel-5P (S5P) L2_NO₂ products—covering tropospheric, stratospheric, and total columns from versions 02.04.00 (RPRO) and 02.04.00–02.09.01 (NRTI, OFFL)—show overall good agreement with independent reference measurements. Validation is based on ground-based networks, including MAX-DOAS (tropospheric), NDACC ZSL-DOAS/SAOZ (stratospheric), and the Pandora Global Network (total columns), as well as comparisons with OMI satellite data. Across products, a systematic negative bias is observed that generally increases with higher NO₂ column amounts in the tropospheric and total columns. Similar bias characteristics and uncertainty estimates are found for both NRTI and OFFL/RPRO datasets.

For tropospheric NO₂ columns, comparisons with MAX-DOAS data from 34 stations yield a median bias of −27.7% (−1.5 Pmolec/cm²), which meets the mission requirement of 50%. The bias is strongly dependent on pollution levels: it is positive (+12%) in clean conditions (< 2 Pmolec/cm²) and negative (−42%) in highly polluted environments (> 15 Pmolec/cm²). Applying TROPOMI averaging kernels to vertically smooth MAX-DOAS profiles reduces the absolute bias by 20%. The median dispersion is approximately 2.9 Pmolec/cm², exceeding the mission precision requirement (0.7 Pmolec/cm²), though it remains within acceptable limits at clean sites.

For stratospheric NO₂ columns, validation against ZSL-DOAS UV-visible measurements from 25 NDACC stations (globally distributed) accounts for horizontal smoothing effects and the NO₂ diurnal cycle. The data show a small negative offset of about −0.1 Pmolec/cm², corresponding to a median bias of −2.5%, well within the 10% mission requirement. The dispersion (0.3 Pmolec/cm²) also meets the requirement of 0.5 Pmolec/cm² when considering combined random errors and co-location mismatches. Comparisons with FTIR measurements at 26 NDACC stations indicate a positive median bias of +5.2% and a similar dispersion (0.3 Pmolec/cm²). Regional deviations are larger, with biases of 10–12% at Northern Hemisphere high latitudes and 13–16% in the tropics.

For total NO₂ columns, comparisons with Pandora observations from 81 stations show a median bias of −10.8% (−0.7 Pmolec/cm²) and a dispersion of 2.0 Pmolec/cm². As with tropospheric columns, the bias depends on pollution levels: it is slightly positive (+3.3%) at low column amounts (< 6 Pmolec/cm²), typically at clean or high-altitude sites, and becomes negative (−16.6%) in more polluted conditions.

Overall, the S5P NO₂ products meet mission accuracy requirements for most applications, with performance varying systematically as a function of NO₂ abundance and environmental conditions.

The figure below visualizes, as a box-whisher graph, the bias and dispersion between TROPOMI and MAX-DOAS tropospheric NO2 VCDs per ground-based station, ordered by mean MAX-DOAS tropospheric NO2 VCD (clean stations at the bottom, polluted stations at the top). The pink shaded area represents the mission requirement of at most 50% bias. The light and dark green shaded areas represent the 1- and 2-sigma uncertainty on the differences (combined uncertainty of satellite and ground-based measurements). 

A detailed description of the method and a comprehensive discussion of validation results can be found in the Sentinel-5p Quarterly Validation Report #30: April 2018 - February 2026

The graph below shows up-to-date time series of the difference between TROPOMI and NDACC ZSL-DOAS stratospheric NO₂ column. (© Update of Verhoelst et al., AMT 2021). Stations are ordered by latitude.

The graph below shows up-to-date time series of the difference between TROPOMI and NDACC MAX-DOAS tropospheric NO₂ column. (© Update of Verhoelst et al., AMT 2021). Stations are ordered by median tropospheric column value.

The graph below show up-to-date time series of the difference between TROPOMI and PGN/Pandora total NO₂ column data. (© Update of Verhoelst et al., AMT 2021). Stations are ordered by median total column.

The retrieval of Sentinel-5p TROPOMI NO₂ data involves the successive determination of three geophysical quantities: the stratospheric, tropospheric and total column amount of NO₂.  These three quantities are validated as follows: 

• The stratospheric NO₂ column is compared to ground-based zenith-scattered twilight DOAS measurements (ZSL-DOAS) of the NDACC network.  The zenith-sky twilight geometry increases the ground-based sensitivity to the stratosphere.  To account for the often large difference in solar local time between the satellite (afternoon) and ground-based (twilight) observations, a diurnal cycle correction is applied based on the PSCBOX 1D stacked-box photochemical model initiated by SLIMCAT CTM fields (Errera and Fonteyn, 2001,Hendrick et al., 2004).
• The tropospheric NO₂ column is compared to ground-based multi-axis DOAS measurements (MAX-DOAS).  The multi-axis geometry can increase the ground-based sensitivity to the troposphere.  
• The total NO₂ column is compared to ground-based direct-sun measurements from Pandora spectrometers of the PGN network.  Direct-sun measurements are sensitive to the entire column.

Details on the methodology and results are described in Verhoelst, et al. (2021).

Sentinel-5p TROPOMI NO₂ data are also compared to alternative retrievals, and to corresponding satellite data from the Aura OMI instrument processed with the QA4ECV community method (Boersma et al., 2018, EC FP7 QA4ECV project).

References

Verhoelst, T. et al., Ground-based validation of the Copernicus Sentinel-5p TROPOMI NO₂ measurements with the NDACC ZSL-DOAS, MAX-DOAS and Pandonia global networks, Atmos. Meas. Tech., v14, 2021, p481-510 doi.org/10.5194/amt-14-481-2021.

Errera, Q. and Fonteyn, D. Four-dimensional variational chemical assimilation of CRISTA stratospheric measurements
J. Geophys. Res., 2001 , 106, 12253-12265, doi:10.1029/2001JD900010

Hendrick, F. et al., Retrieval of nitrogen dioxide stratospheric profiles from ground-based zenith-sky UV-visible observations: validation of the technique through correlative comparisons
Atmos. Chem. Phys., 8, 2004, 2091-2106, 10.5194/acp-4-2091-2004

Correlative measurements used as reference for satellite validation are collected from several ground-based monitoring networks contributing to WMO's Global Atmosphere Watch:

• Zenith-Scattered-Light Differential Optical Absorption Spectroscopy UV-Visible spectrometers (ZSL-DOAS): About 30 ZSL-DOAS instruments perform network operation from the Arctic to the Antarctic, in the framework of the Network for the Detection of Atmospheric COmposition Change (NDACC).
• Multi-Axis Differential Optical Absorption Spectroscopy UV-Visible spectrometers (MAX-DOAS): Several of those instruments perform network operation in the framework of the Network for the Detection of Atmospheric COmpoisition Change (NDACC).
• Fourier Transform Infra Red (FTIR) spectrometers: stratospheric NO2 column retrievals are performed on measurements by the FTIR instruments that contribute to the Network for the Detection of Atmospheric COmpoisition Change (NDACC).
• Direct-sun Pandora spectrometers, also based on Differential Optical Absorption Spectroscopy (DOAS), perform network operation in the framework of the Pandonia Global Network (PGN).

Geographical distribution of the UV-visible DOAS and FTIR spectrometers contributing NO₂ correlative measurements to the validation of Sentinel-5p. (© Update of Verhoelst et al., 2021.)

Correlative data is obtained via the following channels:

• ESAs FRM programme and LuftBlick/U. Innsbruck (A. Cede, M. Gebetsberger and M. Tiefengraber) for rapid data delivery from the Pandonia Global Network,
• the SAOZ_RT processing facility at IPSL/UVSQ/UPMC/CNRS LATMOS (A. Pazmino, A. Bazureau, F. Goutail, and J.-P. Pommereau) for rapid data delivery from the NDACC UV-Vis/SAOZ network,
• S5PVT AO projects CESAR (ID #28596, PI A. Apituley, KNMI) and NIDFORVAL (ID #28607, PI G. Pinardi, BIRA-IASB) for rapid data delivery from NDACC MAX-DOAS and ZSL-DOAS stations,
• the NDACC Data Host Facility, mirrored at EVDC and in BIRA-IASB's CORR-2 database for validation purposes,
• the PGN data archive, mirrored at EVDC and in BIRA-IASB's CORR-2 database for satellite validation purposes,

The Sentinel-5p Mission Performance Centre provides an operational validation service relying on the VDAF Automated Validation Server (VDAF-AVS).  This fully automated data analysis system collects correlative measurements from quality-controlled monitoring networks and compares them to Sentinel-5p data using community endorsed protocols.  It outputs a suite of traceable quality indicators enabling users to judge the fitness-for-purpose of the Sentinel-5p data.

The illustration given hereafter shows the automated comparison of Sentinel-5p TROPOMI total NO₂ column data with respect to correlative measurements aquired by a Pandora UV/Visible spectrometer of the Pandonia Global Network.  More TROPOMI NO₂ validation results are available at http://mpc-vdaf-server.tropomi.eu/no2 .

The VDAF Automated Validation Server in brief:

• hosted at BIRA-IASB and operated with s[&]t,
• provides routine validation for the ESA/Copernicus Sentinel-5p Mission Performance Centre,
• ingests correlative measurements from the ESA Atmospheric Validation Data Centre (EVDC) hosted at NILU, collected from ESA's Fiducial Reference Measurements (FRM) programme and from ground-based monitoring networks contributing to WMO's Global Atmosphere Watch,
• builds on heritage from the Multi-TASTE satellite validation system, the QA4ECV validation server (EC FP7 QA4ECV project) and the CAMS NDACC validation server (EC FP7 NORS project),
• is co-funded by ESA and the Belgian Science Policy Office (BELSPO) as a contribution to the Copernicus Space Component.

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Quarterly Validation Report of the Copernicus Sentinel-5 Precursor Operational Data Products #30: April 2018 - February 2026

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