Ground-based validation of the Copernicus Sentinel-5P TROPOMI NO2 measurements with the NDACC ZSL-DOAS, MAX-DOAS and Pandonia global networks

Verhoelst, Tijl ; Compernolle, Steven ; Pinardi, Gaia ; Lambert, Jean-Christopher ; Eskes, Henk J. ; Eichmann, Kai-Uwe ; Fjæraa, Ann Mari ; Granville, José ; Niemeijer, Sander ; Cede, Alexander ; Tiefengraber, Martin ; Hendrick, François ; Pazmiño, Andrea ; Bais, Alkiviadis ; Bazureau, Ariane ; Boersma, K. Folkert ; Bognar, Kristof ; Dehn, Angelika ; Donner, Sebastian ; Elokhov, Aleksandr ; Gebetsberger, Manuel ; Goutail, Florence ; Grutter de la Mora, Michel ; Gruzdev, Aleksandr ; Gratsea, Myrto ; Hansen, Georg H. ; Irie, Hitoshi ; Jepsen, Nis ; Kanaya, Yugo ; Karagkiozidis, Dimitris ; Kivi, Rigel ; Kreher, Karin ; Levelt, Pieternel F. ; Liu, Cheng ; Müller, Moritz ; Piters, Ankie J. M. ; Pommereau, Jean-Pierre ; Portafaix, Thierry ; Prados-Roman, Cristina ; Puentedura, Olga ; Querel, Richard ; Remmers, Julia ; Richter, Andreas ; Rimmer, John ; Rivera Cárdenas, Claudia ; Saavedra de Miguel, Lidia ; Sinyakov, Valery P. ; Stremme, Wolfgang ; Strong, Kimberly ; Van Roozendael, Michel ; Veefkind, J. Pepijn ; Wagner, Thomas ; Wittrock, Folkard ; Yela González, Margarita ; Zehner, Claus

Année de publication
2021
Résumé
<p align=justify><strong class="journal-contentHeaderColor">Abstract.</strong> This paper reports on consolidated ground-based validation results of the atmospheric <span class="inline-formula">NO<sub>2</sub></span> data produced operationally since April 2018 by the TROPOspheric Monitoring Instrument (TROPOMI) on board of the ESA/EU Copernicus Sentinel-5 Precursor (S5P) satellite. Tropospheric, stratospheric, and total <span class="inline-formula">NO<sub>2</sub></span> column data from S5P are compared to correlative measurements collected from, respectively, 19 Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS), 26 Network for the Detection of Atmospheric Composition Change (NDACC) Zenith-Scattered-Light DOAS (ZSL-DOAS), and 25 Pandonia Global Network (PGN)/Pandora instruments distributed globally. The validation methodology gives special care to minimizing mismatch errors due to imperfect spatio-temporal co-location of the satellite and correlative data, e.g. by using tailored observation operators to account for differences in smoothing and in sampling of atmospheric structures and variability and photochemical modelling to reduce diurnal cycle effects. Compared to the ground-based measurements, S5P data show, on average, (i) a negative bias for the tropospheric column data, of typically <span class="inline-formula">-23</span> % to <span class="inline-formula">-37</span> % in clean to slightly polluted conditions but reaching values as high as <span class="inline-formula">-51</span> % over highly polluted areas; (ii) a slight negative median difference for the stratospheric column data, of about <span class="inline-formula">-0.2</span> Pmolec cm<span class="inline-formula"><sup>-2</sup></span>, i.e. approx. <span class="inline-formula">-2</span> % in summer to <span class="inline-formula">-15</span> % in winter; and (iii) a bias ranging from zero to <span class="inline-formula">-50</span> % for the total column data, found to depend on the amplitude of the total <span class="inline-formula">NO<sub>2</sub></span> column, with small to slightly positive bias values for columns below 6 Pmolec cm<span class="inline-formula"><sup>-2</sup></span> and negative values above. The dispersion between S5P and correlative measurements contains mostly random components, which remain within mission requirements for the stratospheric column data (0.5 Pmolec cm<span class="inline-formula"><sup>-2</sup></span>) but exceed those for the tropospheric column data (0.7 Pmolec cm<span class="inline-formula"><sup>-2</sup></span>). While a part of the biases and dispersion may be due to representativeness differences such as different area averaging and measurement times, it is known that errors in the S5P tropospheric columns exist due to shortcomings in the (horizontally coarse) a priori profile representation in the TM5-MP chemical transport model used in the S5P retrieval and, to a lesser extent, to the treatment of cloud effects and aerosols. Although considerable differences (up to 2 Pmolec cm<span class="inline-formula"><sup>-2</sup></span> and more) are observed at single ground-pixel level, the near-real-time (NRTI) and offline (OFFL) versions of the S5P <span class="inline-formula">NO<sub>2</sub></span> operational data processor provide similar <span class="inline-formula">NO<sub>2</sub></span> column values and validation results when globally averaged, with the NRTI values being on average 0.79 % larger than the OFFL values.</p>
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