Validation of Sentinel-5P TROPOMI tropospheric NO₂ products by comparison with NO₂ measurements from airborne imaging DOAS, ground-based stationary DOAS, and mobile car DOAS measurements during the S5P-VAL-DE-Ruhr campaign

<p>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 NO<span class="inline-formula">₂</span> tropospheric vertical column density (VCD) data products from the Sentinel-5 Precursor (S5P) TROPOspheric Monitoring Instrument (TROPOMI).</p> <p>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 NO<span class="inline-formula">₂</span> in the layer below the aircraft. These flights cover many S5P ground pixels within an area of 30 km <span class="inline-formula">×</span> 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 NO<span class="inline-formula">₂</span> VCDs and show high Pearson correlation coefficients of 0.88 and 0.89 and slopes of 0.90 <span class="inline-formula">±</span> 0.09 and 0.89 <span class="inline-formula">±</span> 0.02 for the stationary and car DOAS, respectively.</p> <p>Having a spatial resolution of about 100 m <span class="inline-formula">×</span> 30 m, the AirMAP tropospheric NO<span class="inline-formula">₂</span> VCD data create a link between the ground-based and the TROPOMI measurements with a nadir resolution of 3.5 km <span class="inline-formula">×</span> 5.5 km and are therefore well suited to validate the TROPOMI tropospheric NO<span class="inline-formula">₂</span> VCD. The observations on the 7 flight days show strong NO<span class="inline-formula">₂</span> variability, which is dependent on the three target areas, the day of the week, and the meteorological conditions.</p> <p><span id="page1358"/>The AirMAP campaign data set is compared to the TROPOMI NO<span class="inline-formula">₂</span> operational offline (OFFL) V01.03.02 data product, the reprocessed NO<span class="inline-formula">₂</span> data using the V02.03.01 of the official level-2 processor provided by the Product Algorithm Laboratory (PAL), and several scientific TROPOMI NO<span class="inline-formula">₂</span> data products. The AirMAP and TROPOMI OFFL V01.03.02 data are highly correlated (<span class="inline-formula"><i>r</i>=0.87</span>) but show an underestimation of the TROPOMI data with a slope of 0.38 <span class="inline-formula">±</span> 0.02 and a median relative difference of <span class="inline-formula">−</span>9 %. With the modifications in the NO<span class="inline-formula">₂</span> retrieval implemented in the PAL V02.03.01 product, the slope and median relative difference increased to 0.83 <span class="inline-formula">±</span> 0.06 and <span class="inline-formula">+</span>20 %. However, the modifications resulted in larger scatter and the correlation decreased significantly to <span class="inline-formula"><i>r</i>=0.72</span>. 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 NO<span class="inline-formula">₂</span> vertical profiles and surface reflectivity are investigated using scientific TROPOMI tropospheric NO<span class="inline-formula">₂</span> 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 NO<span class="inline-formula">₂</span> VCD retrieval in the campaign region and season. In comparison, the replacement of the a priori NO<span class="inline-formula">₂</span> profile in combination with the improvements in the retrieval of the PAL V02.03.01 product regarding cloud heights can further increase the tropospheric NO<span class="inline-formula">₂</span> VCDs. This study demonstrates that the underestimation of the TROPOMI tropospheric NO<span class="inline-formula">₂</span> VCD product with respect to the validation data set has been and can be further significantly improved.</p>