S5P/TROPOMI NO_2 slant column retrieval: method, stability, uncertainties and comparisons with OMI

Van Geffen, J.H.G.M., Boersma, K.F., Eskes, H.J., Sneep, M., ter Linden, M., Zara, M. and Veefkind, J.P.: 2020,
Atmos. Meas. Tech. 13, 1315-1335.


The Tropospheric Monitoring Instrument (TROPOMI), aboard the Sentinel-5 Precursor (S5P) satellite, launched on 13 October 2017, provides measurements of atmospheric trace gases and of cloud and aerosol properties at an unprecedented spatial resolution of approximately 7x3.5 km^2 (approx. 5.5x3.5 km^2 as of 6 August 2019), achieving near-global coverage in 1 d. The retrieval of nitrogen dioxide (NO_2) concentrations is a three-step procedure: slant column density (SCD) retrieval, separation of the SCD in its stratospheric and tropospheric components, and conversion of these into vertical column densities. This study focusses on the TROPOMI NO_2 SCD retrieval: the retrieval method used, the stability of the SCDs and the SCD uncertainties, and a comparison with the Ozone Monitoring Instrument (OMI) NO_2 SCDs.

The statistical uncertainty, based on the spatial variability of the SCDs over a remote Pacific Ocean sector, is 8.63 µmol m^-2 for all pixels (9.45 µmol m^-2 for clear-sky pixels), which is very stable over time and some 30 % less than the long-term average over OMI–QA4ECV data (since the pixel size reduction TROPOMI uncertainties are ~8 % larger). The SCD uncertainty reported by the differential optical absorption spectroscopy (DOAS) fit is about 10 % larger than the statistical uncertainty, while for OMI–QA4ECV the DOAS uncertainty is some 20 % larger than its statistical uncertainty. Comparison of the SCDs themselves over the Pacific Ocean, averaged over 1 month, shows that TROPOMI is about 5 % higher than OMI–QA4ECV, which seems to be due mainly to the use of the so-called intensity offset correction in OMI–QA4ECV but not in TROPOMI: turning that correction off means about 5 % higher SCDs. The row-to-row variation in the SCDs of TROPOMI, the "stripe amplitude", is 2.15 µmol m^-2, while for OMI–QA4ECV it is a factor of ~2 (~5) larger in 2005 (2018); still, a so-called stripe correction of this non-physical across-track variation is useful for TROPOMI data. In short, TROPOMI shows a superior performance compared with OMI–QA4ECV and operates as anticipated from instrument specifications.

The TROPOMI data used in this study cover 30 April 2018 up to 31 January 2020.


   1. Introduction
   2. Satellite data sources and data selection
      2.1  TROPOMI aboard Sentinel-5 Precursor
        2.1.1  TROPOMI instrument
        2.1.2  TROPOMI observations used in this study
      2.2  OMI aboard EOS-Aura
        2.2.1  OMI instrument
        2.2.2  OMI observations used in this study
      2.3 Latitudinal range for uncertainty studies
   3. NO_2 slant column retrieval
      3.1  DOAS technique
      3.2  TROPOMI intensity fit retrieval
        3.2.1  TROPOMI wavelength calibration
      3.3  OMI–QA4ECV optical density fit retrieval
        3.3.1  OMI–QA4ECV wavelength calibration
      3.4  OMI–OMNO_2A intensity fit retrieval
   4. NO_2 slant column retrieval evaluation
      4.1  GCD and SCD error comparison for one orbit
        4.1.1  Geometric column density
        4.1.2  Slant column density error
        4.1.3  Impact of NO_2 processor updates to v2.1.0
      4.2  TROPOMI NO_2 SCD: different QDOAS options
      4.3  De-striping: correcting across-track features
      4.4  Quantitative TROPOMI-OMI GCD comparison
      4.5  Impact of time difference between radiance and 
           irradiance measurements
      4.6  Time dependence of the slant column uncertainty
   5. Discussion
      5.1  Intensity offset correction
      5.2  Validation of stratospheric NO_2
      5.3  NO_2 retrieval over strongly polluted areas
   6. Concluding remarks
      A. Implementation of the Ring correction in the 
         intensity and optical density fit models
      B. Relationship between the rms error in the 
         intensity and optical density fit models
      C. TROPOMI spike removal

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created: 18 February 2020
last modified: 19 August 2020