Sentinel-5p TROPOMI HCHO tropospheric column data is validated through comparisons to the following correlative ground-based measurements: 

• Fourier Transform Infrared measurements (FTIR) of the NDACC network.  The sensitivity of this instrument increases with altitude. The procedure of Rodgers and Connor (2003) is followed before the comparison is executed. Details on the methodology and results are described in Vigouroux et al., AMT 2020. The comparison method follows Vigouroux et al., ACP 2009.
• Multi-axis DOAS UV-visible measurements (MAX-DOAS).  This measurement method has the highest sensitivity close to the surface, and is therefore complementary to the FTIR measurement. Details on the methodology and results are described in De Smedt et al., ACP 2021. The comparison method follows Vigouroux et al., ACP 2009.
• Direct Sun DOAS UV-Visible Pandora measurements from Pandonia Global Network, which measure the total column.

The figure below demonstrates the vertical sensitivity (by means of the total column averaging kernel) of TROPOMI (red), FTIR (blue) and MAX-DOAS (green). Both FTIR and TROPOMI have a low sensitivity near the surface and an increasing sensitivity at higher altitude, while MAX-DOAS has its highest sensitivity near the surface, which then decreases with altitude.

Sentinel-5p TROPOMI HCHO data are also compared to alternative retrievals, and to corresponding satellite data from the MetOp-B GOME-2 and Aura OMI instruments processed with the QA4ECV community method (De Smedt et al., 2018, EC FP7 QA4ECV project). Methodology and results on TROPOMI-OMI comparisons are described in De Smedt et al., ACP 2021.

References

De Smedt, I. et al. Comparative assessment of TROPOMI and OMI formaldehyde observations against MAX-DOAS network column measurements, Atmos. Chem. Phys., 10.5194/acp-21-12561-2021, 2021. 

Vigouroux, C. et al., TROPOMI-Sentinel-5 Precursor formaldehyde validation using an extensive network of ground-based Fourier-transform infrared stations, Atmos. Meas. Tech., 7, 3751-3767, (2020), doi:10.5194/amt-13-3751-2020

Vigouroux, C. et al. Ground-based FTIR and MAX-DOAS observations of formaldehyde at Réunion Island and comparisons with satellite and model data, Atmos. Chem. Phys., 9, 9523-9544, (2009), doi:10.5194/acp-9-9523-2009

De Smedt, I. et al. Algorithm theoretical baseline for formaldehyde retrievals from S5P TROPOMI and from the QA4ECV project, Atmos. Meas. Tech., 11, 2395-2426 (2018) doi:10.5194/amt-11-2395-2018

Rodgers, C. D. and Connor, B. J. Intercomparison of remote sounding instruments, J. Geophys. Res., D3 ,108, 4116, (2003) doi:10.1029/2002JD002299