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

Abstract. This paper reports on consolidated ground-based validation results of the atmospheric NO2 data produced operationally since April 2018 by the TROPOspheric Monitoring Instrument (TROPOMI) 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 Differential Optical Absorption Spectroscopy (MAX-DOAS), 26 Network for the Detection of Atmospheric Composition Change (NDACC) Zenith-Scattered-Light DOAS (ZSL-DOAS), and 25 Pandonia Global Network (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 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 median difference for the stratospheric column data, of about −0.2 Pmolec cm−2, 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 cm−2 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 cm−2) but exceed those for the tropospheric column data (0.7 Pmolec cm−2). While a part of the biases and dispersion may be due to representativeness differences such as different area averaging and measurement times, 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 chemical transport model used in the S5P retrieval and, to a lesser extent, to the treatment of cloud effects and aerosols. Although considerable differences (up to 2 Pmolec cm−2 and more) are observed at single ground-pixel level, the near-real-time (NRTI) and offline (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.

Variability of nitrogen oxide emission fluxes and lifetimes estimated from Sentinel-5P TROPOMI observations

“…Another possible explanation is the known low bias of current TROPOMI tropospheric NO 2 columns, as compared to ground based or aircraft measurements of the tropospheric NO 2 column. This underestimation is more pronounced for regions with larger NO 2 columns (Verhoelst et al, 2021), which is in agreement with our finding that differences to EDGAR are largest for the source regions with high emissions such as Riyadh, Singapore, Seoul, New York and Tokyo. This underestimation of TROPOMI is discussed in more detail in section 4.6.…”

“…Recent studies comparing TROPOMI NO 2 column with co-located ground based or aircraft measurements reported a low bias for TROPOMI NO 2 columns, which is most likely caused by a-priori information such as the surface albedo, cloud-top-height, cloud fraction and the NO 2 vertical profile, used for tropospheric AMF calculations. This bias differs for different regions and is more pronounced for regions with larger NO 2 columns (Griffin et al, 2019;Ialongo et al, 2020;Judd et al, 2020;Dimitropoulou et al, 2020;Verhoelst et al, 2021). Some studies scaled up the measured NO 2 columns with a factor of 1.33 for Paris (Lorente et al, 2019) up to a factor of 1.98 for Germany (Beirle et al, 2019).…”

Assessment of the TROPOMI tropospheric NO<sub>2</sub> product based on airborne APEX observations

“…On this basis, a S-5P validation campaign over Belgium (S5PVAL-BE), focusing on nitrogen dioxide (NO 2 ) column airborne observations, was identified as having much potential and high priority for TROPOMI validation due to (1) the strong gradients in the NO 2 field over key Belgian cities, (2) the expertise built during the precursor BUMBA (Belgian urban NO 2 monitoring based on Airborne Prism Experiment (APEX) remote sensing) campaigns over Belgium (Tack et al, 2017), and (3) the availability of APEX hyperspectral imager and complementary ground-based infrastructure, such as mobile differential optical absorption spectroscopy (DOAS), multi-axis (MAX)-DOAS, and CIMEL stations. Aircraft remote sensing instruments, such as iDOAS (Heue et al, 2008), ACAM (Kowalewski and Janz, 2009), Geostationary Trace gas and Aerosol Sensor Optimization (GeoTASO) (Nowlan et al, 2016), AirMAP (Meier et al, 2017), Spectrolite (Vlemmix et al, 2017), SWING (Merlaud et al, 2018), GEOstationary Coastal and Air Pollution Events (GEO-CAPE) Airborne Simulator (GCAS) (Nowlan et al, 2018), and APEX (Tack et al, 2017) are considered to be very valuable for satellite validation (van Geffen et al, 2018). The suitability of APEX to serve as independent reference for S-5P validation was assessed as part of the AROMAPEX project (Tack et al, 2019), a preparatory campaign activity focusing on the intercomparison of airborne atmospheric imaging systems (including APEX) and their mutual consistency, and the development of satellite validation strategies.…”

“…), different instrument sensitivities, and the presence of small signals close to the detection limit. In preparation for the S-5P atmospheric mission and the forthcoming Sentinel-5 and Sentinel-4 missions , ESA has supported several projects to test newly developed airborne instruments and to develop satellite validation strategies, such as the AROMAT (Airborne ROmanian Measurements of Aerosols and Trace gases; Meier et al, 2017;Merlaud et al, 2018Merlaud et al, , 2020 and AROMAPEX (Vlemmix et al, 2017;Tack et al, 2019) campaigns. The S-5P validation campaign over Belgium (S5PVAL-BE) builds on the experience and lessons learned from these campaigns.…”

Meteorological Drivers of Permian Basin Methane Anomalies Derived from TROPOMI

“…TROPOMI satellite retrievals of NO 2 and CH 4 have been extensively validated over the past several years. TROPOMI tropospheric and total NO 2 column data have been compared against NO 2 spectrometers, with a general negative bias (−23% to −51%) for tropospheric column data attributed to errors in chemical transport models, cloud effects and aerosols [40]. Other studies, comparing aircraft-and ground-based spectrometer measurements in urban and non-urban areas.…”