Preprints
https://doi.org/10.5194/amt-2021-129
https://doi.org/10.5194/amt-2021-129

  02 Jul 2021

02 Jul 2021

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

Impact of 3D radiative transfer on airborne NO2 imaging remote sensing over cities with buildings

Marc Schwaerzel1,2, Dominik Brunner1, Fabian Jakub3, Claudia Emde3,4, Brigitte Buchmann1, Alexis Berne2, and Gerrit Kuhlmann1 Marc Schwaerzel et al.
  • 1Empa, Swiss Federal Laboratories for Materials Science and Technology, Dübendorf, Switzerland
  • 2Environmental Remote Sensing Laboratory, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland
  • 3Meteorological Institute, Ludwig Maximilian University of Munich, Munich, Germany
  • 4German Aerospace Center (DLR), Oberpfaffenhofen, Germany

Abstract. Airborne imaging remote sensing is increasingly used to map the spatial distribution of nitrogen dioxide (NO2) in cities. Despite the small ground-pixel size of the sensors, the measured NO2 distributions are much smoother than one would expect from high-resolution model simulations of NO2 over cities. This could be caused by 3D radiative transfer effects due to observation geometry, adjacency effects and effects of buildings. Here, we present a case study of imaging a synthetic NO2 distribution for a district of Zurich using the 3D MYSTIC solver of the libRadtran radiative transfer library. We compute NO2 slant column densities (SCD) using the recently implemented 3D-box air mass factors (3D-box AMF) and a new urban canopy module to account for the effects of buildings. We found that for a single ground pixel (50 m × 50 m) more than 50 % of the sensitivity is located outside of the pixel, primarily in the direction of the main optical path between sun, ground pixel, and instrument. Consequently, NO2 SCDs are spatially smoothed, which results in an increase over roads when they are parallel to the optical path and a decrease otherwise. When buildings are included, NO2 SCDs are reduced on average by 5 % due to the reduced sensitivity to NO2 in the shadows of the buildings. The effects of buildings also introduce a complex pattern of variability in SCDs that would show up in airborne observations as an additional noise component (about 12 μmol m−2) similar to typical measurement uncertainties. The smearing of the SCDs cannot be corrected using 1D-layer AMFs that assume horizontal homogeneity and thus remains in the final NO2 map. 3D radiative transfer effects by including buildings need to be considered to compute more accurate AMFs and to reduce biases in NO2 vertical columns obtained from high-resolution city-scale NO2 remote sensing.

Marc Schwaerzel et al.

Status: open (until 30 Aug 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on amt-2021-129', Frederik Tack, 14 Jul 2021 reply
  • RC2: 'Comment on amt-2021-129', Anonymous Referee #2, 22 Jul 2021 reply

Marc Schwaerzel et al.

Marc Schwaerzel et al.

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Short summary
NO2 maps from airborne imaging remote sensing often appear much smoother than one would expect from high-resolution model simulations of NO2 over cities, despite the small ground-pixel size of the sensors. Our case study over Zurich, using the newly implemented building module of the MYSTIC radiative transfer solver, shows that 3D effect can explain the smearing and that building shadows cause a noticeable underestimation of and noise in the measured NO2 columns.