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