Copernicus Sentinel-4: a new era in European air quality monitoring

Air pollution is one of Europe’s most pressing environmental and public health challenges. To ensure the continued growth of Copernicus Earth Observation (EO) data and services, the European Commission is expanding the Copernicus constellation with new satellite missions in 2024 and 2025. Among them, Copernicus Sentinel-4, set to launch in summer 2025, will be Europe’s first geostationary air quality monitoring mission, providing hourly, high-resolution data on pollutants such as ozone (O₃), nitrogen dioxide (NO₂), formaldehyde (HCHO), and sulphur dioxide (SO₂).

Unlike previous Sentinel missions, Sentinel-4 is not relying on its own spacecraft but is a hosted payload aboard EUMETSAT’s MTG-Sounder satellite, operating alongside the Infrared Sounder (IRS) instrument. This innovative and efficient, ‘one satellite, two missions’ approach allows for continuous air quality monitoring alongside meteorological observations, delivering crucial atmospheric composition data to improve pollution forecasting, support policy initiatives, and contribute to efforts to mitigate pollution-related health risks.

Ahead of its launch, a clean room visits in Bremen offered key stakeholders a final opportunity to see Sentinel-4 and the MTG-S satellite before they embark on their mission. 

The growing challenge of air quality

Polluted air carries an acrid taste, thick with exhaust and dust. It lingers in the throat, a silent aggressor penetrating deep into the lungs, tightening chests, stinging eyes, and leaving millions across the globe struggling to breathe. In some cities, pollution isn’t just an inconvenience—it’s a public health crisis. In Europe alone, poor air quality is said to be responsible for nearly 600,000 premature deaths each year.

In urban areas residents are often exposed to pollutant levels which reduce the quality of their lives and make spending time outside hazardous to their health. Northern Italy, for example, frequently experiences stagnant air trapped by the surrounding mountains fencing the Po Valley to the north, allowing pollutants from heavy road traffic, agriculture and industry to accumulate. In February 2024, data from Copernicus Atmosphere Monitoring Service (CAMS) showed PM10 levels in Milan far exceeding healthy limits, with many residents resorting to wearing face masks to reduce the risks of exposure. Across Eastern Europe, air pollution is also a persistent challenge, with some cities regularly breaching safe thresholds. In North Macedonia, unhealthy levels of air pollution recently sparked protests, with citizens demanding urgent action against coal emissions, vehicle exhaust, and industrial pollutants.

Poor air quality stems from multiple factors, including the burning of fossil fuels for power generation, heavy road traffic, and industrial emissions. However, pollution is not only caused by direct emissions. Temperature inversions, which trap pollutants close to the ground, often worsen smog, particularly during winter months during which cold air is unable to rise and disperse. Natural and climate change-driven factors also play a role—Saharan dust storms transport fine particulate matter across Western and Central Europe, temporarily increasing pollution levels, while seasonal wildfires in Southern and Central Europe release smoke and airborne pollutants which degrade air quality both locally and in downwind regions. These complex interactions highlight the challenge of tracking and managing air pollution effectively.

Despite efforts to reduce emissions, tracking and predicting pollution events remains difficult. Weather patterns shift pollutants rapidly, and ground-based monitoring stations, though essential, are not evenly distributed across Europe. Without consistent, high-frequency measurements, it is difficult to understand how pollution levels fluctuate throughout the day or how emissions from different sources interact. Sentinel-4 will fill this gap by providing hourly, high-resolution air quality data, allowing for continuous and standardised assessment of atmospheric pollutants.

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