Monitoring Belgian air quality with LEO and GEO atmospheric composition data
Authors: Verhoelst Tijl , Compernolle Steven , Lambert Jean-Christopher , Fierens Frans , Vanpoucke Charlotte 
Published: February 2023

Air Quality (AQ) monitoring in Belgium has hitherto been relying mostly on in-situ measurements of surface concentration, with geographical gaps between observations filled in with numerical modelling ingesting (proxies for) bottom-up emission estimates.&#160; &#160;However, a new generation of satellite sounders on sun-synchronous Low Earth Orbits (LEO) &#8211; like the Copernicus Sentinel-5(p) series &#8211; performs now daily global mapping of atmospheric composition down to the 3-km scale. Soon this daily global mapping will be complemented with geostationary instruments (GEO, e.g. Sentinel-4) observing the diurnal cycle in trace gas concentrations, although over the limited geographical area accessible from a geostationary viewpoint. This new constellation of satellite sounders is built to support detailed monitoring of AQ on the different relevant scales: from point-like emissions to intercontinental transport, and from city-level to international regulations established by public authorities to manage AQ in their area of responsibility. Nevertheless, uptake of these new satellite AQ data by the various Belgian stakeholders is not guaranteed.&#160; Indeed, to realize the full complementary impact of this constellation of LEO and GEO satellites, i.e. to make their observations fit-for-purpose for air quality applications at the different scales, several challenges need to be addressed. These include (1) the need to enhance to sub-city scales the resolution of satellite data to make them better suited for the monitoring of e.g. the impact of the Low Emission Zones enforced in several European cities, (2) to characterize the non-trivial relation between the column amount of the pollutant measured by a satellite and the near-surface concentrations measured by in-situ networks, and (3) to determine how the different LEO and GEO vantage points lead to a different perception of atmospheric and surface details and how we can benefit from - or correct for - these differences.Work on these challenges is taking place in the dedicated Belgian federal research project LEGO-BEL-AQ (2020-2023, https://lego-bel-aq.aeronomie.be/index.php) funded by BELSPO, with a particular focus on AQ in Belgium.&#160; In this contribution, we demonstrate that a combination of temporal aggregation, careful data selection, and horizontal oversampling can produce a meaningful increase in horizontal resolution in S5P tropospheric NO2 column maps, revealing policy-relevant features in the NO2 distribution over Belgium&#8217;s major cities. Comparisons between our high-resolution S5P NO2 maps and the near-surface in-situ observations as procured by the Belgian authorities, reveal high correlation when considering longer time scales (seasonal and annual), allowing a pragmatic conversion from tropospheric columns to near-surface concentrations over the complete Belgian domain, and consequently also a confrontation to WHO annual thresholds at the level of individual Belgian municipalities.Acknowledgements This work has been supported by the BELSPO BRAIN-be 2.0 project LEGO-BEL-AQ (https://lego-bel-aq.aeronomie.be)