# Monitoring air quality with satellite data Satellites measure nitrogen dioxide, ozone and other pollutants across whole regions, and the Copernicus Atmosphere Monitoring Service combines models with observations into daily forecasts for Europe. A city sees where pollution builds up and how it changes over time. Satellites do not replace ground stations: they observe the whole column of air, not the air at street level. ## How satellites observe air pollution Spectrometers on satellites measure sunlight reflected by the Earth's surface and atmosphere. Trace gases and aerosols leave spectral fingerprints in this light, and processing turns them into maps of pollutants such as nitrogen dioxide, sulphur dioxide and ozone [3]. The satellite observes the whole column of air above a point. To estimate air quality near the ground, the Copernicus Atmosphere Monitoring Service (CAMS) combines observations from monitoring networks and satellites with atmospheric models [6]. Its European analysis merges model data with observations provided by the European Environment Agency [5]. For cities, EO-based services provide information on air quality in urban areas, including particles that can affect health [8]. **Figure:** A satellite layer of nitrogen dioxide shows where pollution builds up across a region, in pixels of about 5.5 by 3.5 km. It describes the whole column of air, and clouds leave gaps. Ground stations measure the air at ground level and remain the basis. Illustrative map, not real data. ## Data and satellites - **Sentinel-5P:** Launched in October 2017; daily global coverage; resolution as high as 5.5 by 3.5 km since August 2019 - **Sentinel-4:** Hourly scans of Europe and northern Africa; first images in October 2025, operational service expected in 2027 - **CAMS European forecasts:** Daily analysis plus four-day forecast on a 10 km grid - **Ground stations:** Assimilated into the CAMS analysis and used to validate the main pollutants Sentinel-5P provides information on trace gases, aerosols and clouds that affect air quality and climate, and covers the globe once a day [1]. Since August 2019 its pixels measure about 5.5 by 3.5 km at nadir [2]. Sentinel-4 adds an hourly view from geostationary orbit. Its first images, published in October 2025, showed nitrogen dioxide, sulphur dioxide and ozone over Europe. Once operational, it will also map formaldehyde, glyoxal and aerosols, and its data will feed into CAMS [3]. EUMETSAT, which has operated the mission since September 2026, expects operational service to begin in 2027 [4]. The CAMS European forecasts cover nitrogen dioxide, ozone, sulphur dioxide, carbon monoxide, PM2.5, PM10 and pollen, among others. They are the median of an ensemble of regional models, which on average performs better than any single model. Only some species, among them nitrogen dioxide, ozone, PM2.5 and PM10, are regularly validated against ground measurements; the others are experimental [5]. ## What a city gets - **Maps of nitrogen dioxide over a region**, at a resolution of a few kilometres, that show where pollution builds up and how it changes over time [1]. - **Forecasts**: hourly values for the next four days for nitrogen dioxide, ozone, particulate matter and other species [5]. - **Assessments**: CAMS publishes annual air quality assessment reports, and online tools that show which sources contribute to particulate matter and ozone in more than 70 major European cities, including the share from outside the city [6]. The revised EU Ambient Air Quality Directive encourages Member States to use CAMS products for monitoring [6]. For a city, they are a recognised supplement to its own measurements, not a replacement for them. ## Limits **Limits** - **Not street level:** A CAMS grid cell is about 10 by 10 km \[5], and a Sentinel-5P pixel covers about 19 square kilometres \[2]. Differences between one street and the next need local stations, sensors or city-scale models. - **No data under clouds:** Satellites that measure nitrogen dioxide cannot see through clouds, so cloudy days leave gaps \[7]. - **Ground-level values need careful processing:** The quantity a satellite observes is not the same as the concentration at ground level, and the weather changes it from day to day. Only careful processing makes it a proxy for surface levels \[7]. ## How we help Copernicus air quality data is free of charge [9], so a city can start without buying data. The work lies in deciding which question the data should answer, which department uses the result, and whether a commercial service with finer detail is worth its price. In [strategy and use cases](https://leto.space/consulting/strategy/) we test the question against these points; in [requirements and procurement](https://leto.space/consulting/procurement/) we turn it into requirements that providers can answer. To see which providers offer air quality services, [Spacegraph](https://leto.space/spacegraph/), our discovery platform, has a use-case category "Emissions & air quality". ## Related use cases - [Monitoring a city's emissions](https://leto.space/use-cases/city-emissions-monitoring/): What satellite data adds to greenhouse-gas monitoring, and where its limits are. - [Mapping urban heat islands](https://leto.space/use-cases/urban-heat-islands/): Where a city overheats, seen from land surface temperature. - [Monitoring habitats and water quality](https://leto.space/use-cases/habitat-water-quality/): What satellite data shows about habitats, lakes and rivers. - [Satellite data for cities and regions](https://leto.space/cities-and-regions/): All use cases for administrations, and how we work with them. ## Frequently asked questions ### Which air quality data from space is available, and how often is it updated? Sentinel-5P covers the whole globe once a day, around 13:30 local solar time [1]. Sentinel-4 can scan Europe and northern Africa every hour [3]. EUMETSAT took over its operation in September 2026 and expects operational service to begin in 2027 [4]. The Copernicus Atmosphere Monitoring Service publishes a daily analysis and forecasts for the next four days on a grid of about 10 km for Europe [5]. Copernicus services and data are free of charge [9]. ### Can satellite data replace our air quality monitoring stations? No. What a satellite observes is not the same as the concentration at ground level, and satellites that measure nitrogen dioxide cannot see through clouds [7]. Ground stations remain the basis. Models and satellite data add the picture between the stations: the revised EU Ambient Air Quality Directive asks for modelling, where applicable, so that point measurements can be read as a geographical distribution of concentrations [6]. ### How do we start a pilot? Start with one question that a department owns, for example where to place a new sensor or how a traffic measure changes nitrogen dioxide over a district. We check which free Copernicus products already answer it, which commercial services add finer detail, and how to compare the results with your stations. Our [consulting overview](https://leto.space/consulting/) shows the path from the problem to the pilot. ## Sources 1. [Sentinel-5P: Facts and figures, European Space Agency (ESA), accessed 2026-10-06](https://www.esa.int/Applications/Observing_the_Earth/Copernicus/Sentinel-5P/Facts_and_figures) 2. [Sentinel-5P TROPOMI Tropospheric NO2 1-Orbit L2 5.5km x 3.5km V2 (S5P_L2__NO2____HiR), NASA Goddard Earth Sciences Data and Information Services Center (GES DISC), accessed 2026-10-08](https://disc.gsfc.nasa.gov/datasets/S5P_L2__NO2____HiR_2/summary) 3. [Sentinel-4 offers first glimpses of air pollutants, European Space Agency (ESA), 2025-10-21](https://www.esa.int/Applications/Observing_the_Earth/Copernicus/Sentinel-4/Sentinel-4_offers_first_glimpses_of_air_pollutants) 4. [EUMETSAT takes charge of Copernicus Sentinel-4, EUMETSAT, 2026-09-28](https://www.eumetsat.int/eumetsat-takes-charge-copernicus-sentinel-4) 5. [CAMS European air quality forecasts (Atmosphere Data Store, dataset overview), Copernicus Atmosphere Monitoring Service (CAMS), accessed 2026-10-06](https://ads.atmosphere.copernicus.eu/datasets/cams-europe-air-quality-forecasts?tab=overview) 6. [CAMS role highlighted in the revised European Ambient Air Quality Directive, Copernicus Atmosphere Monitoring Service (CAMS), 2024-05-08](https://atmosphere.copernicus.eu/cams-role-highlighted-revised-european-ambient-air-quality-directive) 7. [Caution advised on use and interpretation of satellite-derived nitrogen dioxide tropospheric columns, NASA Goddard Space Flight Center, Air Quality, 2026-09-09](https://airquality.gsfc.nasa.gov/caution-advised-use-and-interpretation-satellite-derived-nitrogen-dioxide-tropospheric-columns) 8. [EU Space Market Report 2026, Issue 1, EU Agency for the Space Programme (EUSPA), 2026-05-26](https://www.euspa.europa.eu/sites/default/files/documents/EU_Space_Market_Report_2026.pdf) 9. [Regulation (EU) 2021/696 of 28 April 2021 establishing the Union Space Programme and the European Union Agency for the Space Programme, European Union, EUR-Lex, 2021-05-12](https://eur-lex.europa.eu/eli/reg/2021/696/oj/eng) Page: