Improvements in satellite monitoring are changing how we measure air quality, widening its potential to include regulation. Rick Gould investigates
In April, Nasa launched TEMPO, short for Tropospheric Emissions: Monitoring of Pollution, on board the satellite Intelsat 40e. TEMPO will be used to monitor air quality and emissions sources from a geostationary orbit above North America.
Equipped with an ultraviolet (UV) spectrometer, TEMPO will measure columns of key air pollutants, such as tropospheric ozone, nitrogen dioxide, formaldehyde, sulphur dioxide and halogen oxides. Additionally, the sensors on the satellite will measure water vapour, aerosols, foliage properties, clouds and UV-B radiation.
The data from TEMPO will help scientists tackle key questions, such as: What are the sources of these pollutants? How do their concentrations vary hourly and seasonally? How is air quality affecting public health? In what ways does monitoring pollution with satellites improve air-quality forecasts, and how do major incidents such as wildfires affect regional and national air quality?
TEMPO will go further than previous satellite monitors by providing a finer resolution and hourly measurements. A UK research group has also shown the potential of satellite monitoring in regulation, rather than purely for research. A team at the University of Leicester, working with the Environment Agency, the University of Birmingham and the National Physical Laboratory (NPL), have demonstrated how measuring pollution with satellites can meld synergistically with conventional, ground-based measurements of air quality and emissions for regulation in the UK.
Satellites are changing how we measure air quality. “Traditionally, if you wanted to understand the air quality in a given region, you’d put down as many air-quality sensors as you could afford and you’d get a detailed time series of air quality, but at a limited number of very specific points,” explains Daniel Potts, who is in the third year of a PhD within the Earth Observation Science group at the University of Leicester.
Air-quality models can fill the gaps between these point measurements, but have their limitations and often miss hotspots of pollution or short-term peaks. “Satellites provide coverage over the entire area, so we can see a snapshot of pollutants as they’re transported across the whole region,” says Potts.
NPL’s Dr Andrew Brown adds: “Satellite air-pollution data is already used in modelling activities. It also complements existing ground-based monitoring techniques by providing a wide-field view, rather than measuring at fixed points.”
For example, air-pollution data from satellites is important for the models used to forecast air quality globally, such as the Copernicus Atmosphere Monitoring Service. “The fact that data from publicly funded satellite missions is freely available to download in near real time makes its use an attractive option to researchers and other end-users,” explains Brown.
Satellites can also tell us about changes to air quality following incidents, or alert us to major emissions sources, and both Brown and Potts are quick to highlight how satellite monitoring informed us about possible air-quality improvements during the Covid-19 lockdowns, giving us an insight into the effects of decarbonising the transport sector.
Potts’ research initially focused on nitrogen dioxide, which is a major pollutant and relatively easy to detect. “It is linked with a variety of health issues and has a myriad of sources – from wildfires to power stations, to the vehicles we drive. In terms of satellite observations of nitrogen dioxide, it is a very mature field of research, and it is relatively short-lived in the atmosphere, which makes it easier for us to find emissions sources,” he explains.
Potts is expanding his research to look at methane and ammonia too. Ammonia emissions are a current concern for environmental researchers and regulators, while a better understanding of methane sources is crucial if we are to successfully reduce greenhouse gas emissions.
The team at Leicester is using data from the European Space Agency’s Tropomi, or Tropospheric Monitoring Instrument, located on the Sentinel-5 precursor satellite that ESA launched in 2017.
Monitoring air quality with satellites began in the 1970s, using satellites with sensors designed for meteorology, yet scientists found that they could also detect particulate matter and sulphate from volcanic reactions.
Monitoring ozone started at this time, and organisations such as Nasa and ESA have since added monitoring capabilities. Over time, satellite-based spectrometers have enabled researchers to measure more pollutants, with an increasingly finer resolution and faster frequency. For example, the earliest satellites had grid-resolutions in tens or even hundreds of kilometres and temporal resolutions from days to weeks. The global coverage was also patchy. All these parameters have been improved.
“Satellite sensors like TROPOMI cover 95% of the globe once every single day, and map the distribution of nitrogen dioxide, sulphur dioxide, methane, carbon monoxide, aerosol and other pollutants at a resolution of up to 3.5km by 5.5km, clouds permitting,” says Potts.
So far, satellite monitoring has been limited to research, especially in the UK. However, the Leicester team and their partners discovered they could use data from TROPOMI to resolve NO2 emissions from three industrial sources in north-east England, including an oil refinery.
Satellite monitoring can also reveal major incidents and emissions sources – for example, data from TROPOMI and GHGSat showed huge methane leaks from oil and gas producers in Turkmenistan. “These satellite observations led to outside pressure for these companies to rapidly plug these leaks. Now we have the technology to detect, quantify and rectify [leaks] in a matter of days,” says Potts.
There are limitations, however. “Satellite monitoring does need further development before it can be used operationally. Uncertainties are high, resolutions are still quite coarse for some applications, and we lose data every time we have cloud cover,” says Potts.
“They also lack standardisation, full metrological traceability and robust estimates of measurement uncertainty. NPL is currently working on projects to address the latter two issues in order to provide more confidence in air-pollutant data from satellites for facility-level contributions,” adds Brown.
That said, the future looks promising. With TEMPO over North America, and its cousins the Geostationary Environment Monitoring Spectrometer (GEMS) over East Asia and Sentinel-4 over Europe, they will form a virtual constellation of geostationary satellites that allows scientists to monitor air quality over large parts of the northern hemisphere, with a high spatial and temporal resolution.
“There are also methane-specific satellites, launched by commercial entities and organisations such as GHGSat and MethaneSAT, providing high-resolution observations of methane from the oil and gas industry, waste management and now even agriculture, helping us reduce emissions of a strong greenhouse gas,” says Potts.
“We also have Sentinel-7 launching soon, giving us 2km-by-2km global observations of CO2 and NO2, allowing us to directly measure the progress towards net zero of some of the most polluting industries,” Potts concludes.
Rick Gould is an air-quality adviser with the Environment Agency. He is writing in a personal capacity.