LEGO-4-AQ: Monitoring Belgian air quality from space

2023-2024
The LEGO-4-AQ system integrates Sentinel-5P satellite observations with in-situ reference measurements to track how NO2 concentrations near the surface respond to local, regional, and (inter)national air quality policies.

For the past 5 years LEGO-4-AQ reports marked decreases of NO2 over Belgium, from 5% per year in rural areas to 10% per year and more in polluted urban areas. In the Low Emission Zones enforced in Brussels, Antwerp, and Ghent, LEGO-4-AQ does not see stronger NO2 reductions than in the suburban parts of central Belgium.

LEGO-4-AQ: Synergistic space+ground data analysis for Air Quality monitoring

To make optimal use of the emerging constellation of satellites dedicated to Air Quality (AQ) monitoring, BIRA-IASB developed the LEGO-4-AQ system. It combines spatially oversampled and seasonally aggregated satellite data with near-surface in-situ network measurements into a 1-km resolution synergistic dataset.

Applied to the Sentinel-5P TROPOMI nitrogen dioxide (NO2) observations and the IRCEL-CELINE network measurements, LEGO-4-AQ is able to resolve local features such as residential areas, harbours, airports, and industrial sites, while at the same time providing global spatial coverage.

Its systematic and random data uncertainties of respectively 5% and 12% are one order of magnitude below the amplitude of atmospheric NO2 patterns and variability typically seen in Western Europe.   

NO2 trends over Belgium

The data produced by LEGO-4-AQ is used to evaluate NO2 trends over Belgium since the beginning of Sentinel-5P data provision (May 2018) through 2024. This assessment reports decreases of 5% per year (rural areas) to 10% per year and more (polluted urban areas) in the midday column data measured by the satellite and even somewhat stronger reductions in the synergistic near-surface concentration data (Figure 1).

While the decreases in (sub)urban areas are in general larger than in rural areas, the large-scale Low Emission Zones (LEZ) enforced in Brussels, Antwerp, and Ghent yield weaker reductions than the suburban parts of central Belgium. This peculiar behaviour results likely from a combination of several factors:

  1. the strength of LEZ enforcement may have not yet had a substantial impact on the NO2 levels,
  2. the impact on transportation means may be more widespread, and
  3. the impact of regional/federal policy may be a stronger driver (e.g., fiscal policy to move from diesel to hybrid/EV). 

In any case, a decrease in midday NO2 levels from often well above the WHO AQ thresholds of 10 𝜇g/m3 to mostly below is observed over many urban areas. The eagerly awaited Sentinel-4 geostationary satellite will tell if this noticeable improvement in midday NO2 also happens during the emission peaks observed in weekday mornings and evenings.

Operational integration in TERRASCOPE

As a next step towards operational integration in support to AQ stakeholders, the new LEGO-4-AQ data sets will be offered to the community through CAELOSCOPE as an element of the Belgian Copernicus Collaborative Ground Segment TERRASCOPE.

 

Reference

Verhoelst, Tijl; Compernolle Steven; Lambert, Jean-Christopher; Fierens, Frans; Vanpoucke, Charlotte, LEGO-BEL-AQ: Low-Earth and Geostationary Observations of Belgian Air Quality, Final Report. Brussels: Belgian Science Policy Office 2024 – 49 p. (BRAIN-be 2.0).

  

NO2 Belgium
Near-surface NO2 concentration Belgium
Average annual reduction in near-surface NO2 concentration (in %/year) over the period 2019-2023 as derived by LEGO-4-AQ from the synergistic use of Sentinel-5P TROPOMI satellite and IRCEL-CELINE in-situ network data. © 2024 – Contains modified Copernicus data processed at BIRA-IASB.