The haze hangs over Tiananmen Square, Beijing. Credit: axz65

Observations of atmospheric composition from geostationary satellites

6 & 10 March 2023

The haze hangs over Tiananmen Square, Beijing. Credit: axz65
The haze hangs over Tiananmen Square, Beijing. Credit: axz65

Geostationary satellite instruments observing trace gases and aerosols have started a new era in space-borne air quality monitoring. These satellites have the capability to provide observations over the monitoring area several times per day, which significantly increases the information on spatial and temporal variation of atmospheric pollutants.

Last Updated

02 June 2023

Published on

01 June 2023

By Anu-Maija Sundström (FMI), Julia Wageman (MEEO), Federico Fierli

Satellites are increasingly used to monitor air quality and transport of atmospheric pollution. The advantages of satellite data are that they provide data from regional to global scale, and they can help to fill the gaps between the ground-based measurement stations. Satellites also provide observations over areas where no other measurements are available.

The majority of current operational satellite instruments observing air quality-related parameters (ie trace gases and aerosols) are on a sun-synchronous Low Earth Orbit (LEO). These satellites circle the globe from pole to pole at about 600-800km altitude, and each time they pass over a location roughly at the same local time (and sun angle). For example, the Metop-B and -C satellites carrying GOME-2 and IASI instruments are on a LEO orbit, with an overpass time corresponding to local 'morning' (at the Equator 09.30 local time). The advantage of these polar orbiting satellites is that they provide global coverage, but observations are obtained only roughly once per day.

In 2020, a new era of satellite air quality monitoring started when the first satellite of the new Air Quality Satellite Constellation was launched. The constellation consists of three satellites that are on a GEostationary Orbit (GEO). These satellites fly at very high altitude (36, 000km) over the Earth’s equator, and their orbit speed is matched with the Earth’s rotation. Therefore, satellites at GEO orbit 'stay' over the same location monitoring a specific area. The special feature of GEO satellites is that they have the capability to provide observations several times per day.

The first instrument of the Air Quality Constellation, launched in 2020, was the South Korean Geostationary Environment Monitoring Spectrometer (GEMS) that monitors atmospheric composition over Asia. The next instrument, NASA's Tropospheric Emissions: Monitoring of Pollution (TEMPO) was successfully launched on 7 April 2023. Once in full operation it will provide observations over North America. The third mission, Copernicus Sentinel-4, onboard the Meteosat Third Generation (MTG) satellite will be monitoring Europe and North Africa. Each of these missions provide/will provide hourly observations during daylight on key air quality parameters. This opens a whole new view on how pollutant concentrations vary at regional scale over a day. The Air Quality Constellation is a significant international effort to improve air quality monitoring and forecasting.

Why is it important for air quality monitoring to get observations many times per day?

Many of the key air quality parameters, for example Nitrogen Dioxide (NO2), have diurnal variation. A typical feature of daytime NO2 in cities is that clear peaks in concentrations are visible in the morning and afternoon rush hours. By providing observations at hourly pace, the new GEO satellites can bring new insights on the daytime variation of air pollutants at a wider regional scale. This can also help to assess the temporal representativeness of the LEO satellites, as they provide observations about once per day approximately at the same local time. Multiple observations per day can also provide more detailed information on the evolution of sudden air quality events, processes involved, and dispersion of pollutants (eg fires). In addition, as cloudiness often varies during the day, multiple GEO observations also increase the chances of getting observations at least for part of the day.

Examples from operational geostationary Tropospheric NO2 observations over Asia

As the GEMS is the first GEO instrument that has been launched from the Air Quality Satellite Constellation, it is currently the only GEO satellite that provides data operationally.

Figure 1: Overview of one day GEMS data coverage. The acquisition time of each scene is indicated in the image

Figure 1 shows the overall coverage of the GEMS for one day. As the measurements at UV and visible wavelengths require a certain amount of solar radiation, observations are obtained first at the eastern part of the GEMS monitoring area (scenes taken at 23:45 and 00:45 UTC). Throughout the day, the observation area extends more towards the west. For each day there are eight separate files provided from GEMS, covering once per hour observations from 00:45 to 06:45 UTC, and 23:45 UTC.

Figure 2:  Daily Tropospheric NO2 observations by GEMS on 10 March 2023. In this example the NO2 observations have been filtered according to solar zenith and viewing zenith angles (<70 deg) as well as cloud fraction (< 30%). Source: GEMS

Figure 2 shows GEMS Tropospheric NO2 concentrations on 10 March 2023. The first scene represents observations close to midnight, and, therefore ,the time stamp is from the previous day. The animation shows how several mega cities in Eastern Asia are covered by six to seven observations per day, including Beijing and Shanghai.

.  GOME-2 (LEO) Tropospheric NO2 over Eastern Asia 6.3.2023
Figure 3: GOME-2 (LEO) Tropospheric NO2 over Eastern Asia, 6 March 2023. GOME-2 data from Metop-B and -C are averaged into a 0.1 degree grid consisting of orbits at 02:11 UTC (C) and 02:59 UTC (B). The nominal resolution for both instruments (forward scan) is 80km x 40km. Source: AC SAF
GEMS instrument (GEO) Tropospheric NO2 over Eastern Asia on 6.3.2023
Figure 4: GEMS instrument (GEO) Tropospheric NO2 over Eastern Asia on 6 March 2023. The GEMS observations are from 02:45 UTC observation time that  is closest to GOME-2 instruments’ overpass times. The spatial resolution of the GEMS NO2 measurements is 3.5 × 8km. Source: NESC

Figures 3 and 4 show Tropospheric NO2 observations on 6 March 2023 from the GEO (GEMS) and LEO (Metop-B/C GOME-2) instruments. The GEMS NO2 map consists of measurements from the 02:45 observation time that is temporally the closest to GOME-2 instruments’ overpass times. The GOME-2 observations are merged from Metop-B (02:59 UTC orbit) and -C (02:11 UTC orbit) observations, and gridded into a 0.1 deg grid. As the original spatial resolution of GEMS is higher (3.5km-8km) compared to GOME-2 (40km-80km, forward scan), the GEMS NO2 map shows finer spatial scale variation.

The GEMS data over Asia illustrates well the new possibilities that GEO satellites open for space-borne air quality monitoring. With the launch of Sentinel-4, hourly data on important trace gases and aerosols will also be available over Europe and Northern Africa, provided by the EUMETSAT Central Facility and the Atmospheric Composition Satellite Application Facility (AC SAF).

Full details on Sentinel-4