Ground-based validation of the Copernicus Sentinel-5p TROPOMI NO<sub>2</sub> measurements with the NDACC ZSL-DOAS, MAX-DOAS and Pandonia global networks

Abstract

<jats:p>Abstract. This paper reports on consolidated ground-based validation results of the atmospheric NO2 data produced operationally since April 2018 by the TROPOMI instrument 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 DOAS (MAX-DOAS), 26 NDACC Zenith-Scattered-Light DOAS (ZSL-DOAS), and 25 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 an 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 bias for the stratospheric column data, of about −0.2 Pmolec/cm2, 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/cm2 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/cm2), but exceed those for the tropospheric column data (0.7 Pmolec/cm2). While a part of the biases and dispersion may be due to representativeness differences, 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 chemistry transport model used in the S5p retrieval, and to a lesser extent, to the treatment of cloud effects. Although considerable differences (up to 2 Pmolec/cm2 and more) are observed at single ground-pixel level, the near-real-time (NRTI) and off-line (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. </jats:p>

NOx emissions reduction and rebound in China due to the COVID-19 crisis

“…Validation studies (Judd et al, 2020;Tack et al, 2020;Verhoelst et al, 2020) show that the currently available TROPOMI product (versions 1.2.2 and 1.3.0) has tropospheric columns with effectively a typical systematic bias of about −15% (see Supporting Information), and we expect the derived emissions from these observations to be low by such an amount on average. In the relative comparisons discussed in this paper for both columns and emissions we expect a large part of such a multiplicative bias to cancel out.…”

Nitrogen dioxide reductions from satellite and surface observations during COVID-19 mitigation in Rome (Italy)

“…The tropospheric NO 2 VCD is the TROPOMI product used to evaluate the NO 2 reduction from space for the high variability of the pollutant occurring in the lower atmospheric layers, especially over urban sites taking advantage of the high spatial resolution (ground pixel at nadir 7 × 3.5 km 2 before August 6, 2019, and then 5.5 × 3.5 km 2 , as reported in Verhoelst et al 2020). The TROPOMI products are chosen in the central Italian region comprehensive of the urban and the surrounding rural areas (Fig.…”

“…The description of the spatial-temporal variation of the NO 2 concentration includes the tropospheric NO 2 columnar content provided by the TROPOMI (TROPOspheric Monitoring Instrument) sensor on board of Sentinel 5 Precursor (S5P) satellite (Veefkind et al 2012). In the framework of Copernicus program, the S5P was launched in October 2017 by European Space Agency (ESA) to monitor the density of several compounds (e.g., NO 2 , CO, O 3 , CH 4 , CH 2 O) and cloud distributions affecting air quality and climate with higher spatial resolution (ground pixel at nadir 7 × 3.5 km 2 before August 6, 2019, and then 5.5 × 3.5 km 2 , as reported in Verhoelst et al 2020) than the other remote sensing sensors such as Ozone Monitoring Instrument (OMI) with a spatial resolution of 13 × 24 km 2 (Krotkov et al 2016;Levelt et al 2006). Previous studies considered TROPOMI for its high spatial resolution to monitor air pollutants at urban level (Ialongo et al 2020) and in remote areas with different types of industrial sources (Griffin et al 2019).…”

Evaluating Sentinel-5P TROPOMI tropospheric NO&lt;sub&gt;2&lt;/sub&gt; column densities with airborne and Pandora spectrometers near New York City and Long Island Sound

“…Fitted trace gas absorption cross sections in the slant column spectral fit include NO 2 (Vandaele et al, 1998), O 4 (Thalman and Volkamer, 2013), water vapor (Rothman et al, 2009), CHOCHO (Volkamer et al, 2005 ± 0.1 × 10 15 molecules cm −2 . The differences in uncertainty between spectral fits are likely due to a minor amount of undersampling of the GeoTASO slit function, which has a slightly flattened top hat shape compared to the more purely Gaussian shape exhibited by GCAS.…”