Helena Joyce
The use of satellites to track air pollution and climate change indicators is becoming increasingly common. Satellites can identify large areas of pollution caused by fires, dust or sand storms, volcanic eruptions, large industrial sources, or the transport of man-made pollution from other regions. They can also monitor air quality in near real-time, providing hourly images and observations of air pollutants. This data can be used to improve air quality and mitigate the health impacts of severe pollution. For example, NASA's Tropospheric Emissions: Monitoring of Pollution (TEMPO) instrument will monitor air pollution across North America, providing hourly images and tracking ozone, nitrogen dioxide, and other pollutants. In addition, satellites can also be used to detect climate-warming methane pollution from the oil and gas industry, helping to reduce methane emissions and meet climate targets.
| Characteristics | Values |
|---|---|
| Frequency of monitoring | Hourly |
| Area covered | North America, Europe, East Asia, most of Asia |
| Pollutants tracked | NO2, SO2, CO, CH4, PM2.5, O3, nitrogen dioxide, sulfur dioxide, ozone, methane |
| Sources of pollution tracked | Large industrial sources, transport of man-made pollution, fires, dust or sand storms, volcanic eruptions, transportation, power generation, industry, oil and gas extraction, wildfires, secondary pollutants |
| Use cases | Tracking air quality, climate change, environmental surveillance, public health surveillance, asthma burdens, tropospheric ozone concentrations, urban particulate matter mortality, health impacts of pollution |
| Limitations | Cannot track smaller sources of pollution such as small industries or local roads, cannot directly measure PM2.5, ground measurements are still needed for ground truthing |
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What You'll Learn
How satellites identify pollution sources
Satellites are increasingly being used to monitor air quality and the movement of pollution. They can identify pollution sources by capturing data that is invisible to the human eye, showing where pollution is the worst. This data can be used to inform policies to reduce overall pollution levels and their health impacts. For example, Sentinel 5P, a satellite technology equipped with the latest technologies for atmospheric measurements, was used to analyse the main air pollutants: ozone (O3), nitrogen dioxide (NO2), sulphur dioxide (SO2), and particulate matter (PM10 and PM2.5).
Satellite images can also help identify large areas of pollution caused by fires, dust or sand storms, volcanic eruptions, large industrial sources, or the transport of man-made pollution from other regions. For example, the Moderate Resolution Imaging Spectroradiometer (MODIS) records the location of fires on the ground by observing unusual hot spots. It can also see the transport of smoke from wildfires across regions.
Satellites can also measure the concentration of particles (aerosols) in the atmosphere by observing how much light reaches the surface of the Earth and how much is reflected off the aerosols. This measurement is called aerosol optical depth or aerosol optical thickness.
In addition, satellites can be used to fill in spatial gaps in ground monitoring resources and provide data on air quality in areas without a ground monitor. For example, NASA's Tropospheric Emissions: Monitoring of Pollution (TEMPO) satellite instrument will monitor air quality during daylight hours in geostationary orbit, at a vantage point about 22,000 miles above Earth's equator. TEMPO's sensors detect tiny differences in the light reflected when sunlight strikes molecules in the atmosphere and gets absorbed at specific wavelengths. It can track ozone, nitrogen dioxide, sulfur dioxide, bromine, formaldehyde, and aerosols.
Overall, satellites are a valuable tool for identifying pollution sources and monitoring air quality, particularly in areas where ground monitoring is lacking.
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How satellites measure aerosol optical depth
Aerosol optical depth (AOD) is a measure of the extinction of the sun's rays by particles in the atmosphere, such as dust, smoke, and pollution. These particles can block sunlight by absorbing or scattering it, and AOD tells us how much direct sunlight is prevented from reaching the ground. AOD is a critical tool for safeguarding the environment.
AOD is calculated by measuring the direct solar radiation that passes through these particles in the atmosphere. This is done using sun photometers CIMEL, which take measurements every 15 minutes in the range from 340 to 1020 nm. The optical depth is then calculated based on the spectral attenuation of the ray at each wavelength, using Beer-Lambert-Bouguer Law.
AOD is dependent on the wavelength of light; a common reference wavelength reported by satellite data products is 550 nm. The Ångström Exponent (AE) is a measure of how the AOD changes relative to the wavelength of light, and this is related to the size of the particles.
Satellite instruments such as Moderate Resolution Imaging Spectroradiometer (MODIS) and Multi-angle Imaging Spectroradiometer (MISR) are used to monitor aerosols and provide long-term and continuous coverage of the territory under study. These satellites use a "look-up table" strategy, matching actual observations to values calculated for the range of observable values. This allows for complex calculations to be performed once and saved for easy matching to subsequent observations.
Deep Blue is an algorithm used to calculate AOD over land using data from satellite instruments. By taking measurements at different wavelengths, with different contrasts between surface and atmospheric features, Deep Blue can estimate AOD. This process is known as a 'retrieval'.
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How satellites monitor air pollution in real-time
Satellites have become essential tools for tracking weather events and improving daily weather forecasts. They are also increasingly being used to monitor air quality and the movement of pollution in the air.
The Tropospheric Emissions: Monitoring of Pollution (TEMPO) satellite instrument, for example, will monitor air quality during daylight hours in geostationary orbit at a vantage point about 22,000 miles above Earth's equator. TEMPO will join South Korea's Geostationary Environmental Monitoring Spectrometer (GEMS), which launched in 2020, and the European Space Agency's Sentinel-4, expected to launch in 2023, to form a global air-quality satellite constellation. Together, these satellites will provide coordinated observations of air pollution across all continents in the Northern Hemisphere.
TEMPO's sensors detect tiny differences in the light reflected when sunlight strikes molecules in the atmosphere and gets absorbed at specific wavelengths. It can track ozone, nitrogen dioxide, sulfur dioxide, bromine, and organic molecules such as formaldehyde, as well as tiny airborne particles called aerosols. TEMPO will be able to observe these gases at a much higher temporal and spatial resolution than weather satellites.
The European Space Agency's Sentinel-4, expected to launch in 2023, will focus on Europe. The European Space Agency is also expected to launch Sentinel-5 and CO2M, a small constellation of three satellites that will provide valuable information about the concentration and geographical origin of carbon dioxide in the atmosphere.
The Meteosat-9, -10, -11, and -12 satellites from EUMETSAT observe some aerosols in the atmosphere, particularly dust storms, ash, and volcanic emissions, which can have a severe impact on the health of populations in Africa and Europe.
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How satellites track climate-warming pollution
Satellite images are essential in tracking the long-range transport of pollutants from other regions, although they do not reveal the pollution levels on the ground. They can identify large areas of pollution caused by fires, dust or sand storms, volcanic eruptions, large industrial sources, or the transport of man-made pollution from other regions. Smaller sources, such as small industries or local roads, are not visible in satellite images.
NASA's Health and Air Quality Applied Science Team (HAQAST) has developed satellite-based air quality and climate indicators. These indicators are used to track wildfires, dust storms, pollen counts, urban green space, nitrogen dioxide concentrations, asthma burdens, tropospheric ozone concentrations, and urban particulate matter mortality.
The Tropospheric Emissions Monitoring of Pollution (TEMPO) instrument, mounted on a commercial communications satellite, will provide hourly images for most of North America, from Mexico City to northern Canada. TEMPO's sensors detect tiny differences in the light reflected when sunlight strikes molecules in the atmosphere and gets absorbed at specific wavelengths. It can track ozone and nitrogen compounds, which are critical for understanding air quality. TEMPO will join South Korea's Geostationary Environmental Monitoring Spectrometer (GEMS), which has been delivering daily rhythms of nitrogen dioxide pollution across Asia. GEMS has tracked air-fouling dust storms from northern China and climate-cooling sulfur dioxide from volcanic eruptions.
In 2024, the European Space Agency is expected to launch Sentinel-4, an air pollution tracker focused on Europe. Three satellites in geosynchronous orbits are planned to observe NO2, SO2, CO, and CH4 over East Asia (GEMS), North America (TEMPO), and Europe (Sentinel-4).
A new satellite, MethaneSAT, was recently launched with the specific purpose of detecting methane, a potent climate-warming gas. It will focus on methane from the oil and gas industry, which leaks at various parts of the fossil fuel production process.
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How satellites help improve air quality
Satellite technology has been used to track air pollution for some time, and it is an important tool in the effort to improve air quality.
Satellites can provide a global view of pollution, which is especially useful for monitoring large-scale pollution events such as those caused by fires, dust or sand storms, volcanic eruptions, large industrial sources, or the long-range transport of pollution from other regions. For example, in 2007, satellite images showed smoke from wildfires in Idaho and Montana spreading across the United States and over the Atlantic Ocean. Similarly, South Korea's GEMS satellite has tracked dust storms from northern China and sulfur dioxide from volcanic eruptions.
Satellite images can also be used to observe the daily rhythms of nitrogen dioxide pollution in cities. For instance, GEMS data shows that nitrogen dioxide peaks in the morning in Beijing and in the afternoon in Shanghai. This information can be used to better understand the sources of pollution and inform strategies to improve air quality.
In addition to providing a broad view of pollution, satellites can also deliver detailed, high-resolution data on specific pollutants. For example, the NASA Multi-Angle Imager for Aerosols (MAIA) mission collects high-resolution data on the properties of aerosols, which can be used to infer surface-level PM2.5 concentrations in cities. PM2.5, or fine soot, is one of the most harmful air pollutants, and while TEMPO cannot directly measure it, scientists are working to convert its broader measurements of aerosols into PM2.5 estimates.
The launch of new satellites and instruments is improving the ability to track pollution and air quality. For example, the Tropospheric Emissions: Monitoring Pollution (TEMPO) instrument will provide hourly images of North America, allowing scientists to better understand the origins and behaviour of smog and other airborne contaminants. The Atmospheric Composition Instrument (ACX) will provide hourly observations of air pollutants from transportation, power generation, industry, oil and gas extraction, and volcanoes, as well as secondary pollutants generated from these emissions. Furthermore, the recent launch of MethaneSAT will help to detect methane, a potent planet-warming gas, with a targeted focus on the oil and gas industry.
While satellites provide valuable data on air pollution, they should be used in conjunction with ground measurements to ensure accurate and actionable information for improving air quality.
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Frequently asked questions
Satellites can identify pollution caused by fires, dust or sand storms, volcanic eruptions, large industrial sources, or the transport of man-made pollution from other regions.
Satellites such as MethaneSAT are designed to detect methane, a potent climate-warming gas, and help understand where methane emissions occur in the oil industry.
Satellites such as TEMPO can track air pollution across large regions, such as North America, and provide hourly images to help scientists better understand the situation on the ground.
Satellites can help scientists establish links between air pollution and health. For example, research has correlated air pollution in Mexico City with death rates.
