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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 29 Jun 2020

Submitted as: research article | 29 Jun 2020

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This preprint is currently under review for the journal AMT.

Assessment of the TROPOMI tropospheric NO2 product based on airborne APEX observations

Frederik Tack1, Alexis Merlaud1, Marian-Daniel Iordache2, Gaia Pinardi1, Ermioni Dimitropoulou1, Henk Eskes3, Bart Bomans2, Pepijn Veefkind3, and Michel Van Roozendael1 Frederik Tack et al.
  • 1BIRA-IASB, Royal Belgian Institute for Space Aeronomy, Brussels, 1180, Belgium
  • 2VITO, Flemish Institute for Technological Research, Mol, 2400, Belgium
  • 3KNMI, Royal Netherlands Meteorological Institute, De Bilt, 3731, The Netherlands

Abstract. Sentinel-5 Precursor (S-5P), launched in October 2017, carrying the TROPOspheric Monitoring Instrument (TROPOMI) nadir-viewing spectrometer, is the first mission of the Copernicus Programme dedicated to the monitoring of air quality, climate, and ozone. In the presented study, the TROPOMI tropospheric nitrogen dioxide (NO2) L2 product (OFFL v1.03.01; 3.5 km × 7 km at nadir observations) has been validated over strongly polluted urban regions by comparison with coincident high-resolution Airborne Prism EXperiment (APEX) remote sensing observations (~75 m × 120 m). Satellite products can be optimally assessed based on (APEX) airborne remote sensing observations as a large amount of satellite pixels can be fully mapped at high accuracy and in a relatively short time interval, reducing the impact of spatio-temporal mismatches. In the framework of the S5PVAL-BE campaign, the APEX imaging spectrometer has been deployed during four mapping flights (26–29 June 2019) over the two largest urban regions in Belgium, i.e. Brussels and Antwerp, in order to map the horizontal distribution of tropospheric NO2. For each flight, 10 to 20 TROPOMI pixels were fully covered by approximately 2800 to 4000 APEX measurements within each TROPOMI pixel. The TROPOMI and APEX NO2 vertical column density (VCD) retrieval schemes are similar in concept. Overall for the ensemble of the four flights, the standard TROPOMI NO2 VCD product is well correlated (R = 0.92) but biased negatively by −1.2 ± 1.2 × 1015 molec cm−2 or −14 % ± 12 %, on average, with respect to coincident APEX NO2 retrievals. When replacing the coarse 1° × 1° TM5-MP a priori NO2 profiles by NO2 profile shapes from the CAMS regional CTM ensemble at 0.1° × 0.1°, the slope increases by 11 % to 0.93, and the bias is reduced to −0.1 ± 1.0 × 1015 molec cm−2 or −1.0 % ± 12 %. When the absolute value of the difference is taken, the bias is 1.3 × 1015 molec cm−2 or 16 %, and 0.7 × 1015 molec cm−2 or 9 % on average, when comparing APEX NO2 VCDs with TM5-MP-based and CAMS-based NO2 VCDs, respectively. Both sets of retrievals are well within the accuracy requirement of a maximum bias of 25–50 % for the TROPOMI tropospheric NO2 product for all individual compared pixels. Additionally, the APEX data set allows the study of TROPOMI subpixel variability and impact of signal smoothing due to its finite satellite pixel size, typically coarser than fine-scale gradients in the urban NO2 field. The amount of underestimation of peak plume values and overestimation of urban background values in the TROPOMI data is in the order of 1–2 × 1015 molec cm−2 on average, or 10 %–20 %, in case of an urban scene.

Frederik Tack et al.

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Frederik Tack et al.

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Latest update: 10 Jul 2020
Publications Copernicus
Short summary
We assess the TROPOMI tropospheric NO2 product based on coinciding airborne APEX observations (~75 m × 120 m), acquired over Brussels and Antwerp, Belgium. The TROPOMI NO2 product meets the requirements in terms of precision and accuracy. However, we show that TROPOMI is biased low over polluted areas due to a combination of 1) its limited spatial resolution with respect to the strong urban NO2 gradients, and 2) the limited spatial resolution of a priori input for the AMF computation.
We assess the TROPOMI tropospheric NO2 product based on coinciding airborne APEX observations...