Validation of Sentinel-5P TROPOMI tropospheric NO 2 products by comparison with NO 2 measurements from airborne imaging, ground-based stationary, and mobile car DOAS measurements during the S5P-VAL-DE-Ruhr campaign

Abstract. Airborne imaging differential optical absorption spectroscopy (DOAS), ground-based stationary DOAS, and car DOAS measurements were conducted during the S5P-VAL-DE-Ruhr campaign in September 2020. The campaign area is located in the Rhine-Ruhr region of North Rhine-Westphalia, western Germany, which is a pollution hotspot in Europe comprising urban and large industrial sources. The DOAS measurements are used to validate spaceborne NO2 tropospheric vertical column density (VCD) data products from the Sentinel-5 Precursor (S5P) TROPOspheric Monitoring Instrument (TROPOMI). Seven flights were performed with the airborne imaging DOAS instrument for measurements of atmospheric pollution (AirMAP), providing measurements that were used to create continuous maps of NO2 in the layer below the aircraft. These flights cover many S5P ground pixels within an area of 30 km × 35 km and were accompanied by ground-based stationary measurements and three mobile car DOAS instruments. Stationary measurements were conducted by two Pandora, two Zenith-DOAS, and two MAX-DOAS instruments. Ground-based stationary and car DOAS measurements are used to evaluate the AirMAP tropospheric NO2 VCDs and show high Pearson correlation coefficients of 0.88 and 0.89 and slopes of 0.90 ± 0.09 and 0.89 ± 0.02 for the stationary and car DOAS, respectively. Having a spatial resolution of about 100 m × 30 m, the AirMAP tropospheric NO2 VCD data create a link between the ground-based and the TROPOMI measurements with a nadir resolution of 3.5 km × 5.5 km and are therefore well suited to validate the TROPOMI tropospheric NO2 VCD. The observations on the 7 flight days show strong NO2 variability, which is dependent on the three target areas, the day of the week, and the meteorological conditions. The AirMAP campaign data set is compared to the TROPOMI NO2 operational offline (OFFL) V01.03.02 data product, the reprocessed NO2 data using the V02.03.01 of the official level-2 processor provided by the Product Algorithm Laboratory (PAL), and several scientific TROPOMI NO2 data products. The AirMAP and TROPOMI OFFL V01.03.02 data are highly correlated (r=0.87) but show an underestimation of the TROPOMI data with a slope of 0.38 ± 0.02 and a median relative difference of −9 %. With the modifications in the NO2 retrieval implemented in the PAL V02.03.01 product, the slope and median relative difference increased to 0.83 ± 0.06 and +20 %. However, the modifications resulted in larger scatter and the correlation decreased significantly to r=0.72. The results can be improved by not applying a cloud correction for the TROPOMI data in conditions with high aerosol load and when cloud pressures are retrieved close to the surface. The influence of spatially more highly resolved a priori NO2 vertical profiles and surface reflectivity are investigated using scientific TROPOMI tropospheric NO2 VCD data products. The comparison of the AirMAP campaign data set to the scientific data products shows that the choice of surface reflectivity database has a minor impact on the tropospheric NO2 VCD retrieval in the campaign region and season. In comparison, the replacement of the a priori NO2 profile in combination with the improvements in the retrieval of the PAL V02.03.01 product regarding cloud heights can further increase the tropospheric NO2 VCDs. This study demonstrates that the underestimation of the TROPOMI tropospheric NO2 VCD product with respect to the validation data set has been and can be further significantly improved.

[1]  K. F. Boersma,et al.  Comparing Sentinel-5P TROPOMI NO2 column observations with the CAMS regional air quality ensemble , 2023, Geoscientific Model Development.

[2]  K. F. Boersma,et al.  Sentinel-5P TROPOMI NO2 retrieval: impact of version v2.2 improvements and comparisons with OMI and ground-based data , 2022, Atmospheric Measurement Techniques.

[3]  K. F. Boersma,et al.  Sentinel-5P TROPOMI NO2 retrieval: impact of version v2.2 improvements and comparisons with OMI and ground-based data , 2021 .

[4]  E. O'connor,et al.  Validation of the Sentinel-5 Precursor TROPOMI cloud data with Cloudnet, Aura OMI O2–O2, MODIS, and Suomi-NPP VIIRS , 2021, Atmospheric Measurement Techniques.

[5]  H. Eskes,et al.  Assessment of the TROPOMI tropospheric NO2 product based on airborne APEX observations , 2021, Atmospheric Measurement Techniques.

[6]  J. Veefkind,et al.  Evaluating Sentinel-5P TROPOMI tropospheric NO2 column densities with airborne and Pandora spectrometers near New York City and Long Island Sound , 2020, Atmospheric measurement techniques.

[7]  T. Wagner,et al.  Satellite validation strategy assessments based on the AROMAT campaigns , 2020 .

[8]  François-Marie Bréon,et al.  Simulating Multi-Directional Narrowband Reflectance of the Earth's Surface Using ADAM (A Surface Reflectance Database for ESA's Earth Observation Missions) , 2020, Remote. Sens..

[9]  K. Kreher,et al.  Intercomparison of NO2, O4, O3 and HCHO slant column measurements by MAX-DOAS and zenith-sky UV–visible spectrometers during CINDI-2 , 2020, Atmospheric Measurement Techniques.

[10]  M. V. Roozendael,et al.  Intercomparison of four airborne imaging DOAS systems for tropospheric NO2 mapping – the AROMAPEX campaign , 2019, Atmospheric Measurement Techniques.

[11]  Claudia Rivera,et al.  NO2 vertical profiles and column densities from MAX-DOAS measurements in Mexico City , 2018, Atmospheric Measurement Techniques.

[12]  J. Burrows,et al.  Near-surface and path-averaged mixing ratios of NO2 derived from car DOAS zenith-sky and tower DOAS off-axis measurements in Vienna: a case study , 2018, Atmospheric Chemistry and Physics.

[13]  Jan-Peter Muller,et al.  The importance of surface reflectance anisotropy for cloud and NO2 retrievals from GOME-2 and OMI , 2018, Atmospheric Measurement Techniques.

[14]  D. Nicolae,et al.  The Small Whiskbroom Imager for atmospheric compositioN monitorinG (SWING) and its operations from an unmanned aerial vehicle (UAV) during the AROMAT campaign , 2017 .

[15]  M. V. Roozendael,et al.  High-resolution mapping of the NO 2 spatial distribution over Belgian urban areas based on airborne APEX remote sensing , 2017 .

[16]  J. Burrows,et al.  High-resolution airborne imaging DOAS-measurements of NO2 above Bucharest during AROMAT , 2016 .

[17]  Sebastian Donner.  Mobile MAX-DOAS measurements of the tropospheric formaldehyde column in the Rhein-Main region , 2016 .

[18]  J. Burrows,et al.  A wide field-of-view imaging DOAS instrument for two-dimensional trace gas mapping from aircraft , 2015 .

[19]  Clemens Simmer,et al.  JOYCE: Jülich Observatory for cloud evolution , 2015 .

[20]  J. Herman,et al.  The use of NO 2 absorption cross section temperature sensitivity to derive NO 2 profile temperature and stratospheric–tropospheric column partitioning from visible direct-sun DOAS measurements , 2014 .

[21]  A. Kokhanovsky,et al.  Radiative transfer through terrestrial atmosphere and ocean: Software package SCIATRAN , 2014 .

[22]  Pinhua Xie,et al.  Observations of SO 2 and NO 2 by mobile DOAS in the Guangzhou eastern area during the Asian Games 2010 , 2013 .

[23]  J. Burrows,et al.  Improvements to the retrieval of tropospheric NO 2 from satellite – stratospheric correction using SCIAMACHY limb/nadir matching and comparison to Oslo CTM2 simulations , 2013 .

[24]  Michel Van Roozendael,et al.  Measurements of Tropospheric NO2 in Romania Using a Zenith-Sky Mobile DOAS System and Comparisons with Satellite Observations , 2013, Sensors.

[25]  D P Edwards,et al.  Tropospheric emissions: monitoring of pollution (TEMPO) , 2012, Optics & Photonics - Optical Engineering + Applications.

[26]  A. Merlaud.  Development and use of compact instruments for tropospheric investigations based on optical spectroscopy from mobile platforms , 2013 .

[27]  Andreas Hilboll,et al.  Long-term changes of tropospheric NO 2 over megacities derived from multiple satellite instruments , 2012 .

[28]  M. V. Roozendael,et al.  High-resolution NO 2 remote sensing from the Airborne Prism EXperiment (APEX) imaging spectrometer , 2012 .

[29]  Henk Eskes,et al.  TROPOMI on the ESA Sentinel-5 Precursor: A GMES mission for global observations of the atmospheric composition for climate, air quality and ozone layer applications , 2012 .

[30]  Paul Ingmann,et al.  Requirements for the GMES Atmosphere Service and ESA's implementation concept: Sentinels-4/-5 and -5p , 2012 .

[31]  Steffen Beirle,et al.  Estimation of NO x emissions from Delhi using Car MAX-DOAS observations and comparison with OMI satellite data , 2011 .

[32]  Henk Eskes,et al.  An improved tropospheric NO 2 column retrieval algorithm for the Ozone Monitoring Instrument , 2011 .

[33]  Ulrich Platt,et al.  Car MAX-DOAS measurements around entire cities: quantification of NOx emissions from the cities of Mannheim and Ludwigshafen (Germany) , 2010 .

[34]  S. Beirle,et al.  Retrieval of tropospheric column densities of NO2 from combined SCIAMACHY nadir/limb measurements , 2009 .

[35]  Ulrich Platt,et al.  Mobile MAX-DOAS observations of tropospheric trace gases , 2009 .

[36]  Gilles Foret,et al.  Comparison of OMI NO2 tropospheric columns with an ensemble of global and European regional air quality models , 2009 .

[37]  Quintus Kleipool,et al.  Earth surface reflectance climatology from 3 years of OMI data , 2008 .

[38]  Weeks,et al.  A Photochemical Theory of Tropospheric Ozone , 2007 .

[39]  Heikki Saari,et al.  The ozone monitoring instrument , 2006, IEEE Transactions on Geoscience and Remote Sensing.

[40]  V. Grewe.  The origin of ozone , 2005 .

[41]  John P. Burrows,et al.  Validation of SCIAMACHY tropospheric NO2-columns with AMAXDOAS measurements , 2004 .

[42]  John P. Burrows,et al.  TROPOSPHERIC NO2 FROM GOME MEASUREMENTS , 2002 .

[43]  U. Platt,et al.  Differential optical absorption spectroscopy (DOAS) , 1994 .

[44]  Michael Eisinger,et al.  The Global Ozone Monitoring Experiment (GOME): Mission Concept and First Scientific Results , 1999 .

[45]  Ann Carine Vandaele,et al.  Measurements of the NO2 absorption cross-section from 42 000 cm−1 to 10 000 cm−1 (238–1000 nm) at 220 K and 294 K , 1998 .

[46]  D. Jacob,et al.  Origin of ozone and NOx in the tropical troposphere: A photochemical analysis of aircraft observations over the South Atlantic basin , 1996 .

[47]  Liu Xinwu.  This is How the Discussion Started , 1981 .

[48]  Paul J. Crutzen,et al.  The origin of ozone in the troposphere , 1978, Nature.