In a groundbreaking study, researchers have leveraged Landsat and Sentinel-2 satellites to pinpoint and quantify nitrogen oxide (NOx) emissions from power plants with unprecedented precision. These satellites, originally designed for land surveying, now show promise for environmental monitoring, offering a new tool in the fight against air pollution.

The Sentinel-2 satellite, part of the European Space Agency’s Copernicus program, is designed for Earth observation and environmental monitoring. It carries a high-resolution multispectral imager with several spectral bands, which allows it to capture detailed images of the Earth’s surface.

Sentinel-2 can monitor vegetation health, detect changes in land cover, and assess the impacts of natural disasters like floods and wildfires. It provides frequent and high-quality data, and is already playing a vital role in supporting sustainable land management and environmental protection efforts worldwide.

But it can do more.

A team of researchers led by Daniel Varon from Harvard used it to map NOx — nitrogen oxides, significant pollutants that contribute to poor air quality and climate change.

Seeing emissions around plants

The study focused on several power plants in Saudi Arabia and the United States, including a comprehensive 13-year analysis of a power plant, Riyadh Power Plant 9. This plant, which uses a combination of crude oil, natural gas, and diesel, showed significant NOx emissions that were detectable by both Landsat and Sentinel-2 satellites.

A single-pass observation demonstrated the satellites’ ability to detect pollution, even in this type of complex urban environment.

 “This unexpected capability means that Landsat and Sentinel-2 can be used to detect nitrogen dioxide emissions with fine spatial resolution, which is particularly useful in urban areas where pollution sources are numerous and close together.

“Our findings add to the utility of the Sentinel-2 satellites, allowing them to contribute to air quality monitoring by pinpointing pollution sources, which has been challenging for traditional nitrogen dioxide-sensing satellite instruments.”

The research not only quantified emissions but also revealed seasonal and long-term trends. For example, the data showed a strong summer peak in emissions from Riyadh Power Plant 9, likely due to increased air conditioning use during hotter months. Over the 13-year period, a slight decrease in emissions was observed following the introduction of Saudi Arabia’s Ambient Air Standard in 2012.

Not meant for this

The researchers used a clever method involving specific parts of the satellites’ sensors called the blue and ultrablue bands, which are particularly good at detecting nitrogen dioxide (NO₂) in the atmosphere. When the satellite takes pictures, it measures how much light is reflected from the Earth’s surface. By focusing on the light in the blue and ultrablue bands, they can see how much NO₂ is present because NO₂ absorbs light in these bands.

To figure out how much NO₂ is in the air, the scientists compared the light measurements from areas with NO₂ plumes to similar areas without these plumes (called reference scenes). This comparison helps them see the difference caused by the NO₂. From this difference, they can calculate the concentration of NO₂ in the atmosphere and determine the emission rates from sources like power plants.

The idea of tracking emissions with satellites is crucial. Measuring emissions from satellites provides a comprehensive and accurate way to monitor air pollution on a global scale. The problem is that launching satellites for monitoring pollution and emissions is expensive. But this study shows that some existing satellites, not originally meant for this application, can also provide useful information for atmospheric pollution and emissions.

To make matters even better, Sentinel-2C, the follow-up mission Sentinel-2A and -2B, will be launched soon. From an altitude of 786 km, the ‘C’ satellite will provide continuous imaging in 13 spectral bands, per the European Space Agency.

Journal Reference: Daniel J. Varon et al, Quantifying NO_x point sources with Landsat and Sentinel-2 satellite observations of NO₂ plumes, Proceedings of the National Academy of Sciences (2024). DOI: 10.1073/pnas.2317077121