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

<p>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 (<span class="inline-formula">NO₂</span>) level-2 (L2) product (OFFL v1.03.01; 3.5 <span class="inline-formula">km</span> <span class="inline-formula">×</span> 7 <span class="inline-formula">km</span> at nadir observations) has been validated over strongly polluted urban regions by comparison with coincident high-resolution Airborne Prism EXperiment (APEX) remote sensing observations (<span class="inline-formula">∼</span> 75 <span class="inline-formula">m</span> <span class="inline-formula">×</span> 120 <span class="inline-formula">m</span>). 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 spatiotemporal mismatches. In the framework of the S-5P validation campaign over Belgium (S5PVAL-BE), 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 <span class="inline-formula">NO₂</span>. For each flight, 10 to 20 TROPOMI pixels were fully covered by approximately 2700 to 4000 APEX measurements within each TROPOMI pixel. The TROPOMI and APEX <span class="inline-formula">NO₂</span> vertical column density (VCD) retrieval schemes are similar in concept. Overall, for the ensemble of the four flights, the standard TROPOMI <span class="inline-formula">NO₂</span> VCD product is well correlated (<span class="inline-formula"><i>R</i></span> <span class="inline-formula">=</span> 0.92) but biased negatively by <span class="inline-formula">−</span>1.2 <span class="inline-formula">±</span> 1.2 <span class="inline-formula">×</span> 10<span class="inline-formula">¹⁵</span> <span class="inline-formula">molec cm⁻²</span> or <span class="inline-formula">−</span>14 <span class="inline-formula">±</span> 12 %, on average, with respect to coincident APEX <span class="inline-formula">NO₂</span> retrievals. When replacing the coarse 1<span class="inline-formula">^∘</span> <span class="inline-formula">×</span> 1<span class="inline-formula">^∘</span> the massively parallel (MP) version of the Tracer Model version 5 (TM5) a priori <span class="inline-formula">NO₂</span> profiles by <span class="inline-formula">NO₂</span> profile shapes from the Copernicus Atmospheric Monitoring Service (CAMS) regional chemistry transport model (CTM) ensemble at 0.1<span class="inline-formula">^∘</span> <span class="inline-formula">×</span> 0.1<span class="inline-formula">^∘</span>, <span class="inline-formula"><i>R</i></span> is 0.94 and the slope increases from 0.82 to 0.93. The bias is reduced to <span class="inline-formula">−</span>0.1 <span class="inline-formula">±</span> 1.0 <span class="inline-formula">×</span> 10<span class="inline-formula">¹⁵</span> <span class="inline-formula">molec cm⁻²</span> or <span class="inline-formula">−</span>1.0 <span class="inline-formula">±</span> 12 %. The absolute difference is on average 1.3 <span class="inline-formula">×</span> 10<span class="inline-formula">¹⁵</span> <span class="inline-formula">molec cm⁻²</span> (16 %) and 0.7 <span class="inline-formula">×</span> 10<span class="inline-formula">¹⁵</span> <span class="inline-formula">molec cm⁻²</span> (9 %), when comparing APEX <span class="inline-formula">NO₂</span> VCDs with TM5-MP-based and CAMS-based <span class="inline-formula">NO₂</span> VCDs, respectively. Both sets of retrievals are well within the mission accuracy requirement of a maximum bias of 25 %–50 % for the TROPOMI tropospheric <span class="inline-formula">NO₂</span> 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 <span class="inline-formula">NO₂</span> field. For a case study in the Antwerp region, the current TROPOMI data underestimate localized enhancements and overestimate background values by approximately 1–2 <span class="inline-formula">×</span> 10<span class="inline-formula">¹⁵</span> <span class="inline-formula">molec cm⁻²</span> (10 %–20 %).</p>