Ground-based validation of the Copernicus Sentinel-5p TROPOMI
NO2 measurements with the NDACC ZSL-DOAS, MAX-DOAS and
Pandonia global networks
Authors: 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 , Navarro Comas Monica , Piters Ankie J. M. , Pommereau Jean-Pierre , Portafaix Thierry , Puentedura Olga , Querel Richard , Remmers Julia , Richter Andreas , Rimmer John , Rivera Cárdenas Claudia , Saavedra de Miguel Lidia , Sinyakov Valery P. , Strong Kimberley , Van Roozendael Michel , Veefkind J. Pepijn , Wagner Thomas , Wittrock Folkard , Yela González Margarita , Zehner Claus 
Published: May 2020

Abstract. This paper reports on consolidated ground-based validation results of the atmospheric NO2 data produced operationally since April 2018 by the TROPOMI instrument on board of the ESA/EU Copernicus Sentinel-5 Precursor (S5p) satellite. Tropospheric, stratospheric, and total NO2 column data from S5p are compared to correlative measurements collected from, respectively, 19 Multi-Axis DOAS (MAX-DOAS), 26 NDACC Zenith-Scattered-Light DOAS (ZSL-DOAS), and 25 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 an average: (i) a negative bias for the tropospheric column data, of typically −23 to −37 % in clean to slightly polluted conditions, but reaching values as high as −51 % over highly polluted areas; (ii) a slight negative bias for the stratospheric column data, of about −0.2 Pmolec/cm2, i.e. approx. −2 % in summer to −15 % in winter; and (iii) a bias ranging from zero to −50 % for the total column data, found to depend on the amplitude of the total NO2 column, with small to slightly positive bias values for columns below 6 Pmolec/cm2 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/cm2), but exceed those for the tropospheric column data (0.7 Pmolec/cm2). While a part of the biases and dispersion may be due to representativeness differences, 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 chemistry transport model used in the S5p retrieval, and to a lesser extent, to the treatment of cloud effects. Although considerable differences (up to 2 Pmolec/cm2 and more) are observed at single ground-pixel level, the near-real-time (NRTI) and off-line (OFFL) versions of the S5p NO2 operational data processor provide similar NO2 column values and validation results when globally averaged, with the NRTI values being on average 0.79 % larger than the OFFL values.