02 Mar 2021

02 Mar 2021

Review status: this preprint is currently under review for the journal AMT.

An improved tropospheric NO2 column retrieval algorithm for TROPOMI over Europe

Song Liu1, Pieter Valks1, Gaia Pinardi2, Jian Xu1, Ka Lok Chan1, Athina Argyrouli1,3, Ronny Lutz1, Steffen Beirle4, Ehsan Khorsandi5, Frank Baier5, Vincent Huijnen6, Alkiviadis Bais7, Sebastian Donner4, Steffen Dörner4, Myrto Gratsea8, François Hendrick2, Dimitris Karagkiozidis7, Kezia Lange9, Ankie J. M. Piters6, Julia Remmers4, Andreas Richter9, Michel Van Roozendael2, Thomas Wagner4, Mark Wenig10, and Diego G. Loyola1 Song Liu et al.
  • 1Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Methodik der Fernerkundung (IMF), Oberpfaffenhofen, Germany
  • 2Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Brussels, Belgium
  • 3Technical University of Munich (TUM), Department of Civil, Geo and Environmental Engineering, Chair of Remote Sensing Technology, Munich, Germany
  • 4Max Planck Institute for Chemistry (MPI-C), Mainz, Germany
  • 5Deutsches Zentrum für Luft- und Raumfahrt (DLR), German Remote Sensing Data Center (DFD), Oberpfaffenhofen, Germany
  • 6Royal Netherlands Meteorological Institute (KNMI), De Bilt, the Netherlands
  • 7Laboratory of Atmospheric Physics, Aristotle University of Thessaloniki (AUTH), Thessaloniki, Greece
  • 8Institute for Environmental Research and Sustainable Development, National Observatory of Athens, Greece
  • 9Institute of Environmental Physics (IUP-UB), University of Bremen, Bremen, Germany
  • 10Meteorological Institute (MIM), Ludwig-Maximilians-Universität München (LMU), Munich, Germany

Abstract. Launched in October 2017, the TROPOspheric Monitoring Instrument (TROPOMI) aboard Sentinel-5 Precursor provides the potential to monitor air quality over point sources across the globe with a spatial resolution as high as 5.5 km × 3.5 km (7 km × 3.5 km before 6 August 2019). The nitrogen dioxide (NO2) retrieval algorithm for the TROPOMI instrument consists of three steps: the spectral fitting of the slant column, the separation of stratospheric and tropospheric contributions, and the conversion of the slant column to a vertical column using an air mass factor (AMF) calculation. In this work, an improved tropospheric NO2 retrieval algorithm from TROPOMI measurements over Europe is presented.

The stratospheric estimation is implemented using the STRatospheric Estimation Algorithm from Mainz (STREAM), which was developed as a verification algorithm for TROPOMI and does not require chemistry transport model data as input. A directionally dependent STREAM (DSTREAM) is developed to correct for the dependency of the stratospheric NO2 on the viewing geometry by up to 2 × 1014 molec/cm2. Applied to synthetic TROPOMI data, the uncertainty in the stratospheric column is 3.5 × 1014 molec/cm2 for polluted conditions. Applied to actual measurements, the smooth variation of stratospheric NO2 at low latitudes is conserved, and stronger stratospheric variation at higher latitudes are captured.

For AMF calculation, the climatological surface albedo data is replaced by geometry-dependent effective Lambertian equivalent reflectivity (GE_LER) obtained directly from TROPOMI measurements with a high spatial resolution. Mesoscale-resolution a priori NO2 profiles are obtained from the regional POLYPHEMUS/DLR chemistry transport model with the TNO-MACC emission inventory. Based on the latest TROPOMI operational cloud parameters, a more realistic cloud treatment is provided by a clouds-as-layers (CAL) model, which treats the clouds as uniform layers of water droplets, instead of the clouds-as-reflecting-boundaries (CRB) model, in which clouds are simplified as Lambertian reflectors.

For the error analysis, the tropospheric AMF uncertainty, which is the largest source of NO2 uncertainty for polluted scenarios, ranges between 20 % and 50 %, leading to a total uncertainty in the tropospheric NO2 column in the 30–60 % range. From a validation performed with ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements, the improved tropospheric NO2 data shows good correlations for nine European urban/suburban stations with an average correlation coefficient of 0.78. The implementation of the algorithm improvements leads to a decrease of the relative difference from −55.3 % to −34.7 % on average.

Song Liu et al.

Status: open (until 20 May 2021)

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Song Liu et al.


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Short summary
In this work, an improved tropospheric NO2 retrieval algorithm from TROPOMI measurements over Europe is presented. The stratospheric estimation is implemented with correction for the dependency of the stratospheric NO2 on the viewing geometry. The AMF calculation is implemented using improved surface albedo, a priori NO2 profiles, and cloud correction. The improved tropospheric NO2 data show good correlations with ground-based MAX-DOAS measurements.