Sentinel-5p TROPOMI SO2 total column data is validated through comparisons to the following correlative ground-based measurements.

• Direct-sun measurements from Pandora spectrometers of the PGN network. Note that these are still in demonstration phase. Pandora direct-sun measurements (Knepp et al., 2015; Fioletov et al., 2016; McLinden et al., 2021) are sensitive to the entire column.
• Multi-axis DOAS UV-visible measurements (MAX-DOAS). MAX-DOAS measurements (e.g., Wang et al., 2017; Theys et al., 2015) are mainly sensitive to the lower tropospheric column.

References

Knepp, T. et al., Estimating surface NO2 and SO2 mixing ratios from fast-response total column observations and potential application to geostationary missions, J. Atmos. Chem., 2015, 72, 261-286, doi.org/10.1007/s10874-013-9257-6

Fioletov, V.E. et al., Sulfur dioxide (SO2) vertical column density measurements by Pandora spectrometer over the Canadian oil sands, Atmos. Meas. Tech., 2016, 9, 2961-2976, doi.org/10.5194/amt-9-2961-2016

McLinden, C.A. et al., Inconsistencies in sulfur dioxide emissions from the Canadian oil sands and potential implications, Environmental Research Letters, 2021, 16, 014012, doi.org/10.1088/1748-9326/abcbbb

Wang, Y. et al., Validation of OMI, GOME-2A and GOME-2B tropospheric NO2, SO2 and HCHO products using MAX-DOAS observations from 2011 to 2014 in Wuxi, China: investigation of the effects of priori profiles and aerosols on the satellite products, Atmos. Chem. Phys., 2017, 17, 5007-5033, doi.org/10.5194/acp-17-5007-2017

Theys, N. et al. Sulfur dioxide vertical column DOAS retrievals from the Ozone Monitoring Instrument: Global observations and comparison to ground-based and satellite data,
J. Geophys. Res. Atmos., 2015, 120, 1-22, doi.org/10.1002/2014jd022657