Just over three months after its successful launch on August 13, 2025, the European mission Copernicus Sentinel-5A has delivered its first impressive views of gases in Earth's atmosphere. The first set of products includes a global map of total ozone, detailed maps of nitrogen dioxide over the Middle East and South Africa, formaldehyde over Central Africa, and a cloud of sulphur dioxide over an active volcano in Kamchatka. With this, Sentinel-5A has demonstrated that it is ready to become a key data source for monitoring air quality, climate, and the state of the ozone layer on a global scale.

A new era of atmospheric monitoring: Sentinel-5A on MetOp-SG-A1

Sentinel-5 is a mission within the European Copernicus program, dedicated to measuring atmospheric composition. Instead of being a standalone satellite, Sentinel-5 is an advanced imaging spectrometer embarked on the second-generation meteorological satellites MetOp-SG type A. The first such satellite, MetOp-SG-A1, entered a polar Sun-synchronous orbit at an altitude of about 830 kilometers on August 13, 2025, and flies over the entire planet from pole to pole every day, recording traces of gases and aerosols worldwide.

On MetOp-SG-A1, Sentinel-5A operates in the company of a range of other instruments – from infrared and microwave sounders to advanced imaging sensors – combining information on temperature, humidity, clouds, and surface properties with highly precise data on air composition. In the coming years, the same type of instrument will be installed on subsequent MetOp-SG type A satellites, ensuring a continuous series of measurements that will extend well into the 2040s.

The mission is managed by the European Space Agency (ESA) and EUMETSAT, while the data is used within Copernicus services for atmospheric monitoring and climate, as well as in national meteorological and environmental services across Europe and the world. Unlike the geostationary mission Sentinel-4, which monitors Europe and North Africa from a fixed point above the equator, Sentinel-5 provides a global image of the entire Earth every day, enabling the monitoring of air pollution and greenhouse gases on a planetary scale.

How Sentinel-5 “sees” invisible gases

The heart of the Sentinel-5A mission is a high-resolution imaging spectrometer operating in seven spectral ranges – from the ultraviolet and visible parts of the spectrum to the near and shortwave infrared regions. The instrument measures how Earth and the atmosphere reflect and scatter sunlight, and from fine differences in the spectrum, scientists reconstruct the amount of various gases along the line of sight.

In this way, Sentinel-5A can daily measure concentrations of a range of key air components: ozone, nitrogen dioxide, sulphur dioxide, formaldehyde, glyoxal, carbon monoxide, and methane, as well as aerosol properties and the UV index. The swath width is large enough to cover almost the entire planet in just one day, while the spatial resolution is adapted to clearly distinguish urban pollution hotspots, large industrial zones, and remote areas where natural gas sources prevail.

This approach builds on the legacy of instruments like GOME, SCIAMACHY, and Sentinel-5P (TROPOMI), but with a greater number of spectral channels, improved stability, and more precise calibration. Thanks to this, Sentinel-5 will become a reference for long-term monitoring of changes in atmospheric composition, including trends in greenhouse gases and pollution traces from cities, energy, industry, and transport.

First global ozone views: a look at Earth's protective shield

The first released image from Sentinel-5A is a global map of total ozone recorded on October 13, 2025. The map shows the total ozone column, meaning the amount of ozone summed through the entire thickness of the atmosphere above each point on Earth. Although some ozone exists in ground layers where it acts as a harmful pollutant, the total column is largely determined by stratospheric ozone – a protective layer at an altitude of 15–35 kilometers that absorbs most of the Sun's ultraviolet radiation.

On the Sentinel-5A map, the ozone hole over Antarctica stands out very clearly, an area where ozone column values fall below 220 Dobson units. This is a threshold often used as the operational definition of the ozone hole. At this time of year, during the Antarctic spring, the ozone hole traditionally expands and deepens due to chemical reactions driven by chlorine and bromine from compounds released into the atmosphere in the past.

The use of ozone-depleting substances, such as chlorofluorocarbons, was banned by the international Montreal Protocol in the late 1980s. However, these compounds persist in the atmosphere for decades, so the recovery of the ozone layer is slow and gradual. Long-term measurement series show that signs of recovery have only appeared in recent years, and Sentinel-5A now takes on part of the responsibility for continuing this global “health record” of the ozone layer.

By combining new Sentinel-5A data with archives from previous missions, scientists can more precisely track how quickly ozone is returning to values before intense pollution, distinguish natural variations from long-term trends, and verify the effectiveness of international agreements.

Nitrogen dioxide: the footprint of human activity over the Middle East

The second image shown displays the vertical column of nitrogen dioxide (NO₂) over the Middle East, also recorded on October 13, 2025. The map clearly distinguishes “hotspots” near major cities, oil and gas refineries, thermal power plants, and metallurgical facilities. Additional hotspots follow major transport corridors, where intense road and maritime traffic directly contributes to nitrogen oxide emissions.

Nitrogen dioxide is one of the most important gases when discussing air pollution in cities. It arises primarily from the combustion of fossil fuels – in car and truck engines, in the chimneys of power plants, industrial facilities, but also in households using gas stoves or fossil fuel heating. In the atmosphere, it participates in the formation of ground-level ozone and secondary particles, worsening smog and affecting respiratory health.

In the Middle East region, Sentinel-5A allows these emissions to be observed uniformly for the first time, day by day, across national borders. This is particularly important in a region where maritime tanker traffic, industrial development, and rapid urbanization lead to a complex mosaic of pollution sources. Data from Sentinel-5 can be directly used in air quality models, as well as for verifying declared emissions from the energy and industrial sectors.

The map also shows parts where the signal is less clear or “cut out” – most often these are areas covered by clouds. Since clouds obstruct the instrument's view of air layers near the surface, NO₂ data cannot be reliably calculated in such cases, and the areas are marked as uncovered or unreliable. Data processing systems therefore deliver information on the quality and reliability of each pixel alongside physical quantities.

High concentrations of nitrogen dioxide over South Africa

The third released image of nitrogen dioxide focuses on South Africa. Elevated NO₂ values stand out particularly over the Highveld region, where the majority of South African coal-fired power plants and a large number of mines and heavy industries are concentrated. In that area, emissions from the energy sector form the backbone of local air pollution, and satellite measurements provide an external, independent view of the level of those emissions.

In combination with surface measurements from networks of national and regional stations, Sentinel-5 data will enable a more detailed assessment of the energy sector's impact on air quality and population health, as well as on the climate commitments of countries relying on coal. High levels of NO₂ are often associated with emissions of other pollutants, such as sulphur dioxide and particulate matter, which jointly contribute to acid rain, atmospheric haze, and respiratory problems.

Formaldehyde over Africa: the footprint of fires and vegetation

The fourth Sentinel-5A image shows the vertical column of formaldehyde (HCHO) over Africa on October 13, 2025. Elevated concentrations along the northwestern coast of Angola are linked to vegetation fires and agricultural stubble burning, while increased levels over the Central African Republic originate from a combination of fires and natural emissions from vegetation in tropical forests.

Formaldehyde is a reactive, potentially carcinogenic gas that also acts in the atmosphere as a proxy indicator for the presence of other exhaust compounds, especially volatile organic compounds (VOCs). Part of the formaldehyde is generated directly in fires, and part through chemical reactions of other organic gases released by plants or human activities. Precisely for this reason, monitoring formaldehyde from space helps scientists better understand how tropospheric chemistry changes over areas with intense fires or lush vegetation.

Since formaldehyde is often concentrated near the surface, clouds have an even greater impact on data coverage than with some other gases. In areas of partial cloud cover, data processing algorithms can introduce systematic errors, so the first formaldehyde products from Sentinel-5A are marked as preliminary. As calibration and processing models are refined, improvements in accuracy and spatial consistency of these maps are expected.

Sulphur dioxide over the Klyuchevskoy volcano

The fifth image highlighted by experts shows a cloud of sulphur dioxide (SO₂) over the Klyuchevskoy volcano on the Kamchatka Peninsula, in the Russian Far East. It is one of the most active volcanoes in the world, having erupted more than fifty times since the beginning of the 18th century. Even outside eruptive peaks, gases and smoke often constantly escape from the crater.

Sentinel-5A very clearly captures a narrow but intense cloud of sulphur dioxide stretching hundreds of kilometers downwind from the source. Sulphur dioxide in the atmosphere can cause respiratory tract irritation, and prolonged presence of high concentrations leads to an increased risk of respiratory and cardiovascular diseases. SO₂ is also a precursor to acid rain, as it oxidizes in the atmosphere and creates sulphate particles.

For volcanologists and civil protection services, such measurements have a dual value. On one hand, they help in issuing warnings for air traffic, as clouds of gas and volcanic ash can damage jet engines. On the other hand, series of SO₂ maps enable an assessment of how much volcanic emissions contribute to the total amount of sulphur in the atmosphere compared to industry and energy.

False color of Earth's radiation: instrument health check

The last highlighted image from the first dataset shows Earth's radiation in the period from October 5 to 6, 2025. It is a false-color composite, in which red, green, and blue components are assigned to different channels, giving a global map where continents, oceans, and clouds are clearly distinguished. Unlike specialized maps of individual gases, this image primarily serves to check the overall “health” of the instrument.

Such checks are crucial in the early phase of the mission, when the instrument is still being calibrated and algorithms for converting raw measurements into geophysical products (e.g., gas concentrations) are being intensively tested. by comparing these images with data from other satellites and models, teams at ESA, EUMETSAT, and partner institutions confirm that Sentinel-5A is correctly measuring the intensity and spectral shape of radiation across the entire field of view.

Only when such “engineering” tests are completed and the long-term stability of the instrument is confirmed, are products like maps of ozone, nitrogen dioxide, or formaldehyde declared operational and ready for use in official forecast and warning systems.

Complementary view from Sentinel-4 and the legacy of Sentinel-5P

Sentinel-5 does not operate in a vacuum – neither literally nor figuratively. In geostationary orbit over Europe, the Sentinel-4 mission has been operating since 2025, measuring key gases over Europe and North Africa every hour. It provides exceptionally good temporal resolution over one part of the world, while Sentinel-5 in polar orbit covers the entire planet daily. Together, they create a system that can simultaneously track short-term pollution episodes and long-term global trends.

Sentinel-5P (Precursor) builds upon this picture, having delivered highly detailed maps of gases like NO₂, SO₂, CH₄, and CO since 2017. Sentinel-5P data remains key for analyzing the period before 2025, while Sentinel-5A and its successors will take on the role of carriers of atmospheric statistics in the decades to follow. Continuity between these missions is of crucial importance for all scientific studies dealing with climate trends, long-term changes in air quality, and the effects of international agreements on emission reductions.

A long-term mission for policy, science, and public health

With the planned operational life of MetOp-SG satellites and a series of Sentinel-5 instruments on multiple successive platforms, the mission is expected to provide key data for at least two decades. Such a time span allows short-term oscillations – like seasonal changes or individual smog episodes – to be clearly separated from slow but important trends linked to climate change and changes in the energy mix.

Sentinel-5 data is already included in air quality forecast and UV index systems, where it complements surface measurements and numerical models. When maps of ozone, nitrogen dioxide, particles, and other air components are included in operational models, meteorological and environmental services can issue timely warnings to citizens about elevated pollution levels, advice for sensitive groups, and recommendations for reducing exposure.

At the same time, detailed information on the spatial distribution and evolution of gases serves as an important tool for policymakers. Countries and regions can compare actual concentrations and estimated emissions with targets defined by European air quality directives and climate plans. In practice, this means that satellite measurements will increasingly serve as an additional “independent judge” showing how much national emission inventories and announced measures align with what is actually happening in the atmosphere.

From first images to operational service

Although Sentinel-5A is still in the commissioning phase, the first views of ozone, nitrogen dioxide, formaldehyde, sulphur dioxide, and Earth's radiation show that the instrument is working according to expectations. Engineers and scientists are now continuing fine calibration, comparing data with ground measurements and other satellites, and refining algorithms used to derive atmospheric quantities from spectra.

As data processing stabilizes, Sentinel-5 will become a standard source of input information for climate studies, air quality forecast models, health warnings on UV radiation, and monitoring of global greenhouse gas emissions. The first released products are just a preview of what the mission will bring during its long operational life – continuous, unprecedented global monitoring of the atmosphere, in the service of science, public health, environmental protection, and the economy.