Assessment of the TROPOMI tropospheric NO2 product based on recurrent airborne campaigns

<p>Sentinel-5 precursor (S-5p), launched on 13 October 2017, is the first mission of the Copernicus Programme dedicated to the monitoring of air quality, climate, ozone and UV radiation. The S-5p characteristics, such as the fine spatial resolution, introduce many new opportunities and challenges, requiring to carefully assess the quality and validity of the generated data products by comparison with independent measurements and analyses.</p> <p>While routine validation is performed within the ESA Mission Performance Center (MPC) based on a limited number of Fiducial Reference Measurements (FRM), additional validation activities including airborne and ground-based campaigns are conducted in research mode as part of the S-5p Validation Team (S5PVT). A series of decentralized campaign activities take place since 2021, which have been identified to address key priorities for S5-p validation as well as key S5-p products (see s5pcampaigns.aeronomie.be for an overview of all campaigns performed in the context of the ESA QA4EO and SVANTE projects).</p> <p>Here, we will focus on recurrent observations with the airborne hyperspectral imager SWING, developed by BIRA-IASB. A SWING instrument is regularly installed in the Cessna T207A from FUB, while a second SWING instrument is permanently installed in a BN-2 from INCAS, alongside other instruments such as in-situ samplers.  Recurrent validation activities are performed over the cities of Berlin, Germany and Bucharest, Romania to map the horizontal distribution of tropospheric NO<sub>2 </sub>and its urban/industrial sources, in close coincidence with the overpass of the TROPOMI sensor. During each flight, approximately 5 to 15 TROPOMI pixels can be fully covered by airborne measurements. The recurrent nature of this type of campaigns, meaning that a flight can take place whenever weather conditions allow, results in a large statistical set of reference data covering various meteorological and geo-physical conditions (including autumn and winter conditions), as well as different satellite overpass configurations.</p> <p>In this context, a general airborne data format, following the Climate and Forecast (CF) conventions, has been developed at BIRA, as well as harmonized tools for the airborne L2 data processing and the TROPOMI validation in order to obtain consistent results over the different reference data sets. The developed validation strategies and developed tools are suitable for other airborne imagers than SWING and will be suitable as well for validation of future atmospheric missions such as Sentinel-5 and Sentinel-4. First results of the assessment of the TROPOMI tropospheric NO<sub>2</sub> L2 product based on observations from recurrent airborne SWING campaigns will be discussed.</p>