Earth’s system is a coupled system where the different components (the atmosphere, the hydrosphere, the lithosphere, and the croyosphre) are constantly interacting at different spatial and time scales.
Atmospheric composition and thus air quality (what impacts human health as well as other lifeforms inluding agriculture) is a hot topic and active area of research. When it comes to air quality, measurements of pollutant concentrations like particulate matter and trace gases is not new and continues to develop and expand as technology keeps improving and measurement networks keeps growing.
However, like in atmospheric science and hydrology point based measurements can only do so much. For example, determining the water level or water speed at a point along a river stream requires an understanding of the surface elevation (known bathymetry when under water) and whether the point of interest is upstream or downstream the river. Similarly, how the concentration of a pollutant evolves is governed by the underlying driving forces most notably the wind field, the pollutant lifetime, and the nature of the chemical interactions that arise.
It is therefore critical that the evolution of different chemical elements (pollutants if they affect human life) are tracked and studied in detail. Some of the questions science is trying to answer when it comes to air quality include:
- What are the spatial and temporal variations of the concentrations of pollutants?
- How are local and regional air quality affected by long-range transport?
- How does air quality and climate change drive each other?
- How is air quality affected by metrology and how are pollutants dispersed by weather?
- How can fluxes between different regions be quantified or estimated?
For these and other questions to be answered monitoring is required at the appropriate scale and temporal frequency of the underlying phenomena. Here then the role of satellite-based monitoring comes into play. A number of organisations have teamed up towards the same goal of making this monitoring a reality. A “virtual” constellation of satellites will be composed of three missions that will monitor air quality from space at unprecedented quality.
The Air Quality Virtual Satellite Constellation
The air quality virtual constellation is the result of international cooperation to monitor air quality in the Northern hemisphere towards improving our understanding of air quality and how it changes on local, regional, and global scale.
The constellation, as of writing, is composed of three satellite missions. One is South Korea’s Geostationary Environment Monitoring Spectrometer (GEMS) for monitoring the Asian region. Another is the ESA’s Copernicus Sentinel-4 that will cover the European continent. The third is NASA’s Tropospheric Emissions: Monitoring of Pollution (TEMPO) that will monitor the North American continent. GEMS has been operational for some time while TEMPO that has recently launched will commence data collection in June 2023. Sentinel-4 is yet to be launched.
TEMPO – Tropospheric Emissions: Monitoring Pollution
TEMPO, short for Tropospheric Emissions: Monitoring Pollution, is a satellite mission with an instrument operating in the UV-visible spectrum. The payload is carried onboard the Intellisat IS40e platform and will be monitoring from a geostationary orbit at an elevation of 35-37,000 km at a zero degree inclination above Earth’s surface. The geostationary orbit and high temporal resolution will allow to monitor the high diurnal variability of emissions and atmospheric chemistry. The data is planned to be delivered in near-real time (NRT) with a delay of about 2 hours.
Science questions addressed by TEMPO
- What are the temporal and spatial variations of emissions of gases and aerosols important for air quality and climate?
- How do physical, chemical, and dynamical processes determine tropospheric composition and air quality over scales ranging from urban to continental, diurnally to seasonally?
- How does air pollution drive climate forcing and how does climate change affect air quality on a continental scale?
- How can observations from space improve air quality forecasts and assessments?
- How does intercontinental transport affect air quality?
- How do episodic events, such as wild fires, dust outbreaks, and volcanic eruptions, affect atmospheric composition and air quality?
The main instruments on-board TEMPO are the UV, VIS, and NIR spectrometers. The spatial resolution at the center of the field of regard (FOR) will be about 2 km per pixel in the North-South direction and 4.5 km per pixel in the East-West direction. The temporal frequency of data collection will be hourly and the collection will start at the East coast and scan towards the west coast covering the whole field of regard with 2.5 x 106 spectra.
The Sentinel-4 mission is part of the Copernicus program by the European Space Agency (ESA). The Sentinels included Sentinel-1 (Synthetic Aperture Radar, SAR), Sentinel-2 (multi-spectral), Sentinel-3 (sea surface, temperature, surface color), and Sentinel-5 (see below).
GEMS – Geostationary Environment Monitoring Spectrometer
- Is identical to to TEMPO
- Will help in the understanding of his pollution events affect the global pollution through
- identifying sources and sinks of pollution
- the transport of pollution over large distances known by the term long-range transport
- SLCFs (Short-Lived Climate Forcers)
Mission Characteristics Comparison
|Altitude||37000 km||35786 km|
|Data collect start||May/June 23||TBD||H2 2019|
|Data distributor||SAO||ESA Copernicus|
|Scanning mode||Push broom|
|Scanning direction||East to West||East to West||East to West|
|Spectral range||290-490 nm (UV)|
540-740 nm (VIS)
|305-400 nm (UV)|
400-500 nm (VIS)
750-775 nm (NIR)
|Spectral resolution||UV @ 0.6 nm FWHM|
VIS @ 0.2 nm
|UV-VIS @ 0.5 nm|
NIR @ 0.12 nm
|< 0.6 nm|
|Spectral sampling||0.2 nm||< 0.2 nm|
|Maximum S/N||2700 @ 330-340 nm, EOL|
|Spatial resolution||2.1 × 4.7 km2 @ FOR center||8×8 km2||8×8 km2|
|Field of Regard (FOR) center||36.5N, 100W||17.04 N, 114.1 E|
|Spectra per hour||2000 N/S × 1250 E/W|
Other air quality and atmospheric missions include Sentinel-5 and Sentinel-5P, which are complementary to Sentinel-4, with its TROPOspheric Monitoring Instrument (TROPOMI) onboard Meteosat Third Generation (MTG), and the NASA Aura with its Ozone Monitoring Instrument (OMI).
The main reason these missions are not part of the virtual constellation is simply due to the nature of their (low earth) orbit which affects their monitoring frequency (or orbital cycle) of a specific geographical area. As these missions have sun-synchronous polar orbits they can make revisits daily compared to the hourly frequency of the missions composing the virtual constellation.
Though Sentinel-5P is in a Low Earth Orbit (LEO) and is not part of the air quality virtual constellation it will nevertheless serve a complimentary role and will cover the gap not covered by the virtual constellation helping increase the spatial coverage and continuity of collected data.
Main Characteristics of Sentinel-4 and of its Heritage Instruments
|Instrument||Technical Concept||Spectral Range||Spatial resolution (km x km)||Earth Coverage||Revisit time||Operational|
|GOME||Whisk-broom (scanning)||UV-VIS-NIR (240-790 nm)||320 x 40||Global||1 ½ day||1995-2011|
|GOME-2||Whisk-broom (scanning)||UV-VIS-NIR (240-790 nm)||80 x 40||Global||1 ½ day||2006-present|
|SCIAMACHY||Whisk-broom (scanning)||UV to SWIR (240-2400 nm)||30 x 215||Global||6 days||2002-2012|
|OMI||Push-broom (staring)||UV-VIS-NIR (240-790 nm)||13 x 24||Global||1 day||2004-present|
|TROPOMI||Push-broom (staring)||UV-VIS-NIR-SWIR (270 – 2385 nm)||7 x 7||Global||1 day||Launch scheduled in 2016|
|Sentinel-4/UVN||Push-Broom (scanning)||UV-VIS-NIR (305- 775 nm)||8 x 8||Europe + parts of North Africa and the Atlantic||1 hour||Launch scheduled in 2021|
- SpaceX Falcon 9 Rocket Launches NASA’s TEMPO High-Resolution Air Quality Control Mission, SciTechDaily, April 7 2023, https://scitechdaily.com/spacex-falcon-9-rocket-launches-nasas-tempo-high-resolution-air-quality-control-mission/
- TEMPO, Smithsonian Institution, https://tempo.si.edu/index.html
- TEMPO Mission Overview, Marshall Space Flight Center Earth Science Office, https://weather.ndc.nasa.gov/tempo/
- GEMS, https://www.eoportal.org/satellite-missions/geo-kompsat-2#gems-geostationary-environment-monitoring-spectrometer
- Air Quality Satellite Constellation Begins Taking Shape, NASA, March 6 2020, https://www.nasa.gov/feature/langley/air-quality-satellite-constellation-begins-taking-shape
- Sentinel-4 Heritage Missions, ESA, https://sentinel.esa.int/web/sentinel/missions/sentinel-4/overview/heritage-missions
- Introducing the geostationary environment monitoring spectrometer, Won Jun Choi et al. 2018, https://www.spiedigitallibrary.org/journals/journal-of-applied-remote-sensing/volume-12/issue-04/044005/Introducing-the-geostationary-environment-monitoring-spectrometer/10.1117/1.JRS.12.044005.full?SSO=1
- Description of a formaldehyde retrieval algorithm for the Geostationary Environment Monitoring Spectrometer (GEMS), Hyeong-Ahn Kwon et al. 2019, https://amt.copernicus.org/articles/12/3551/2019/
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