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Tailoring Sentinel data to inform Belgian air quality policies

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LEGO-BEL-AQ, a new activity initiated by BIRA-IASB in partnership with the Interregional Environment Agency IRCEL-CELINE, aims at tailoring atmospheric observations from the Copernicus constellation of Sentinel satellites to support air quality policies in Belgium. LEGO-BEL-AQ integrates daily global data from the polar orbiting Sentinel-5(p) series with hourly data over Europe from the upcoming geostationary Sentinel-4, to generate maps of nitrogen dioxide (NO2) at sub-city resolution and to elaborate synergies with regional surface pollution analyses.
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Monitoring air quality and the impact of environmental policies at the needed scales

Processes controlling Air Quality (AQ) cover a wide range of scales, from point-like emissions to intercontinental transport, and so do the regulations enacted by public authorities to manage air quality over their domain of legal competence. AQ monitoring has been hitherto relying mostly on in-situ measurements of surface concentration, with geographical gaps between observations filled in with numerical modelling.

New generation satellite sounders on sun-synchronous Low Earth Orbits (LEO) – like the Copernicus Sentinel-5(p) series started in 2017 – perform now daily global mapping of air pollution down to the 3-km scale.

Soon this daily global mapping will be complemented with geostationary instruments (GEO) observing the diurnal cycle of pollutants, although over the limited geographical area accessible from a geostationary viewpoint: European AQ will be observed hourly by the Sentinel-4 satellites, while other GEO sounders will point to North-America and East-Asia.

Key challenges to realize the full potential of the Air Quality satellite constellation

To realize the full potential of the constellation of LEO and GEO satellites being assembled, and to make their observations fit-for-purpose for air quality applications at the different scales, several challenges need to be addressed, among others:

  1. Enhance to sub-city scales the resolution of satellite data, typically from 3-4 km down to 1 km, to make them better suited for the monitoring of e.g. the Low Emission Zones enforced in several Belgian cities,
  2. Determine the non-trivial relation between the column amount of pollutant measured by a satellite and the surface concentration measured by in-situ networks,
  3. 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.

LEGO-BEL-AQ: Pioneering an integrated LEO+GEO AQ monitoring system for Belgium

Funded by the Belgian Federal Science Policy Office under the BRAIN-Be 2.0 programme, and carried out in collaboration with IRCEL-CELINE, the LEGO-BEL-AQ project (for Low-Earth and Geostationary Observations of BELgian Air Quality) pioneers the synergistic use of the Copernicus LEO+GEO AQ Sentinels in support of Belgian air quality policies.

Figure 1 illustrates over the Brussels area the sub-city scale capacity of the system by showing contrasts in nitrogen dioxide (NO2) between the summers of 2019 and 2020. Further particulars and illustrations are available on the LEGO-BEL-AQ project website at

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Figure 1: Tropospheric column of nitrogen dioxide (NO2) at 1-km spatial resolution over Brussels and surroundings: (i) during the normal emission case of summer 2019 (left-hand panel), and (ii) during the SARS-CoV-2 related low emission case of summer 2020 (right-hand panel). © Generated by LEGO-BEL-AQ using modified Copernicus Sentinel-5p TROPOMI data processed at BIRA-IASB (2020).