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 , 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 
Published: January 2021

Abstract. This paper reports on consolidated ground-based validation results of the atmospheric NO2 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 NO2 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 −23 % to −37 % in clean to slightly polluted conditions but reaching values as high as −51 % over highly polluted areas; (ii) a slight negative median difference for the stratospheric column data, of about −0.2 Pmolec cm−2, 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 cm−2 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−2) but exceed those for the tropospheric column data (0.7 Pmolec cm−2). 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−2 and more) are observed at single ground-pixel level, the near-real-time (NRTI) and offline (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.