From its vantage point outside Earth’s atmosphere, more than 36,000 km above Earth’s surface, the Copernicus Sentinel-4 mission will detect major air pollutants over Europe in unprecedented detail. It will observe how they vary on an hourly basis—a real breakthrough for air quality forecasting.

Copernicus Sentinel-4 marks a significant step up for monitoring the air we breathe. It measures levels of major air pollutants, such as the toxic trace gases nitrogen dioxide, which is formed when fossil fuels are burned, and ozone, which is typically produced downwind of the pollution sources. Sentinel-4 data will help to produce a clearer picture of pollution patterns, to protect public health and ultimately help save lives.

Many aspects of the mission are new and even groundbreaking. Here are just a few ways in which the Sentinel-4 mission has been innovatively designed to bring us unique data on air quality over Europe and part of northern Africa.

1. Why orbit matters for air quality monitoring

Copernicus Sentinel-4 is the first mission to monitor European air quality from geostationary orbit. It is implemented as an ultraviolet, visible, and near-infrared (UVN) imaging spectrometer mounted on the Meteosat Third Generation Sounder (MTG-S), a geostationary satellite that is positioned about 50 times farther away than low-Earth orbit missions.

So what is so special about geostationary orbit? It means that the Sentinel-4 instrument remains at a fixed position over the equator well above our atmosphere, much like a hawk hovering high above a field. The instrument keeps Europe and northern Africa constantly in its field of view and scans this area at hourly intervals. With a continuous data connection between the satellites and the ground, a near-real-time data distribution within less than one hour is achieved.

Ben Veihelmann, Sentinel-4 and Sentinel-5 Mission Scientist at ESA, stresses that “Sentinel-4 is a complete game changer for air quality forecasting.”

Pollution sources associated with traffic, domestic fuel burning and industry vary rapidly throughout the day. Air pollution levels evolve further due to transport by wind and due to a variety of chemical processes. Therefore, air quality can change quickly; drastic changes on hourly time scales are not uncommon. The short revisit and data distribution times place Sentinel-4 in a unique position to capture the variability of air pollution levels, much better than sensors on low-Earth orbiting satellites, which provide a snapshot at a fixed time each day.

“For the first time, we’ll be able to provide hourly information about air quality across Europe,” says Giorgio Bagnasco, ESA’s Sentinel-4 Project Manager. “It’s like switching from a weather report every day to an update every hour. This is crucial when you’re trying to track something as dynamic as air pollution, which can shift with wind and traffic in a matter of hours,” adds Bagnasco. “This kind of temporal resolution will transform how we predict pollution events and understand air quality trends.”

2. Joining forces

Whereas previous Copernicus Sentinel missions relied on their own spacecraft, Sentinel-4’s instrument is mounted on board the MTG-S satellite. The two-missions-one-satellite approach is both innovative and efficient, uniting missions for meteorology and air pollution.

“We have embedded a dedicated air quality instrument into a broader operational weather satellite, making the most of shared infrastructure while expanding the mission’s reach. It’s a great example of collaborative, multi-purpose space technology,” observes Bagnasco.

ESA and its sister organization, Eumetsat, have joined forces to make Sentinel-4 happen: ESA has taken care of the design and the building of the satellite instrument and a set of data processors. Eumetsat has developed the ground segment for receiving, processing and distributing the mission data.

3. Part of a global air quality monitoring constellation

Copernicus Sentinel-4 is the European contribution to the global constellation of geostationary air quality sensors, which includes the Korean sensor GEMS for Asia, and the NASA sensor TEMPO for North America. Together, the three geostationary sensors provide hourly observations covering a large part of the densely populated areas in the northern hemisphere.

“Sentinel-4 is part of a broader constellation alongside American and Korean instruments, providing complementary coverage. Together, they form a global system that helps track air quality across continents. Pollution doesn’t respect borders, so this international cooperation gives us the best chance to monitor and respond on a global scale,” adds Veihelmann, who is also one of the chairpersons of the Atmospheric Composition Virtual Constellation, where coordination of this geostationary air quality constellation is pursued.

4. Using data to make a difference

Copernicus Sentinel-4 data products are free and openly accessible. They will be used by a wide range of end users, from air quality service providers to the scientific community. The data will be systematically exploited by the Copernicus Atmosphere Monitoring Services (CAMS) and the Copernicus Climate Change Service. Sentinel-4 observations are expected to boost the air quality monitoring and forecast services of CAMS.

Data will be used for monitoring and forecasting air pollution and issuing health warnings, as well as for verifying compliance with global and EU guidelines on ambient air quality and emissions reductions. Sentinel-4 observations will also help to support the solar power industry, to provide UV warning services, as well as for detecting volcanic emissions for air traffic controllers.

“Sentinel-4 isn’t just about monitoring the atmosphere—it’s about improving people’s lives,” notes Veihelmann. “Its data will feed into air quality forecasts, pollution alerts, and health warnings for vulnerable groups such as asthma sufferers. It helps model how pollution moves across Europe, from industrial zones to cities, allowing authorities to act. This mission supports everything from environmental policy to public health, and even climate research through Copernicus services.”

5. How it works

The Copernicus Sentinel-4/UVN spectrometer measures light arriving both directly from the sun, as well as sunlight reflected by Earth’s surface and atmosphere. When light passes through the atmosphere, trace gases leave a signature, or “footprint,” on the light arriving at the satellite. These signatures are resolved by the UVN spectrometer and are exploited to estimate the amount of the trace gases nitrogen dioxide, ozone, sulfur dioxide, formaldehyde, and glyoxal, as well as aerosols, present in the atmosphere. For ozone, sulfur dioxide and aerosol, the signatures also give information on the vertical distribution.

Bagnasco adds, “It’s a scientific marvel—measuring trace gases at parts per billion levels, from 36,000 km away. The orbit is 50 times farther from Earth than low-Earth orbit missions, which makes the engineering challenge immense. That’s like spotting a needle in a haystack—from space. But we will be able to detect pollution patterns in near real-time, supporting decision-makers and protecting public health.”