Growing up in South Korea, Heesung Chong was acutely aware of air quality. “The aerosol pollution is relatively severe in South Korea. On bad days, you could feel it,” he says. The air on such days was not only fouled with the tiny drops of liquid and solid particles but also contaminated with high concentrations of other pollutants such as NO2, SO2, and ground-level ozone. All are health hazards.
As a child, Chong had not considered pursuing atmospheric science. In fact, he went into college for a degree in chemical engineering which he received in 2015. But when he discovered an atmospheric science laboratory at his university as a senior trying to figure out what to study in graduate school, he thought, “This is it.”
“Working in that field seemed really interesting, scientifically, and also important,” Chong says. His choice was serendipitous.
Chong entered the field at the dawn of a new era of space-based air quality measurement. Through the course of his PhD and post-graduate studies, he has worked on the world’s first two geostationary air quality monitoring satellites. One watches pollution develop over North America and the other monitors air quality in Asia. A third will launch this summer to observe air pollutants in Europe and complete the constellation planned to watch the northern hemisphere.
For decades, scientists and community members relied on ground-based air quality monitors to understand air pollution. Each monitoring station gives a hyperlocal point measurement of pollutant concentrations. But the further away a person is from a station, the less sure they can be about the air around them.
With a network of monitoring stations, people can get a sense of pollution across a larger area. Unfortunately, such networks are uncommon in most parts of the world. Rural areas in particular are often monitoring dark zones.
In the US, Idaho is an example of a place with relatively sparse monitors, says physicist Caroline Nowlan of the Smithsonian Astrophysical Observatory, part of the Harvard & Smithsonian Center for Astrophysics. The state only has one monitor providing the public with near real-time measurements of NO2, “but that’s an area that has a lot of air quality impacts from forest fires,” she says. If a fire’s plume doesn’t drift into the monitored area, people could be exposed to elevated levels of NO2 without realizing it.
Monitoring pollution from space
Satellites equipped with spectrometers can dramatically expand on the information provided by ground-based monitoring networks. They give scientists, and the public, a picture of an entire country or continent that urban-centric ground-based monitoring networks just can’t capture, Nowlan says. These maps put air pollution in context which helps people identify the origin of specific pollutants and determine how far they travel.
Over the years, a handful of satellites carrying spectrometers have been launched into low earth orbit. These flying spectrometers measure air pollution as they zip around Earth at an altitude of about 700 km. Despite the distance, the approach is very similar to light-absorption experiments in a lab, Nowlan says.
The measurement starts with a blank—a background spectrum of the primary light source free of pollutants. In this case, that’s the sun. Then, as the satellites whip around the planet, the instruments they carry take spectral snapshots of light reflected and scattered off the atmosphere from below. In cloud-free regions, researchers can determine the identity and concentration of pollutants within the column of air by comparing these spectra to the blank. The analysis relies on Beer’s law, a classic mathematical relationship between light absorbance and concentration widely used in analytical chemistry. The equations are just tweaked to account for atmospheric scattering and other real-world effects, Nowlan says.
"Some pollutants have a much shorter lifetime. If you take a measurement once per day, you are missing a good part of their movement."
But the very nature of these satellites prevents atmospheric scientists from collecting all the data they might want to model pollutants’ migration through the air. A satellite in low earth orbit travels at a speed different than that of Earth’s spin, so the instruments aboard can only provide a measurement for the same location once a day. “Some pollutants have a much shorter lifetime,” says Jhoon Kim, an atmospheric scientist at Yonsei University. “If you take a measurement once per day, you are missing a good part of their movement.”
Going geostationary
Not all satellites are limited to once-a-day data collection, however. Weather satellites, for example, have long been capable of collecting hourly data thanks to their unique orbit.
Unlike low earth orbit satellites, weather satellites have been launched into orbits tens of thousands of kilometers above the equator. At this distance and location, the spacecraft loops the planet at the speed of Earth’s rotation, essentially locking the satellite in place over a particular area. In other words, the satellite is geostationary. “It's always over our head at the equator, which means we can take snapshots much more frequently,” Kim says.
Kim remembers participating in discussions at a satellite-science conference in 2008 to develop a “geostationary constellation to cover the most polluted regions over the globe.” Twelve years later, the world’s first geostationary air quality monitoring satellite was successfully launched into orbit. It was the first of three.
The Geostationary Environment Monitoring Spectrometer (GEMS) was developed for one purpose: to track hourly changes in air pollution across parts of Asia. By focusing on a small region of the globe, the team increased GEMS’s spatial resolution.
“I devoted a good part of my life to the GEMS mission,” Kim says. He was in the room when GEMS was conceptualized in 2007, became the mission’s principal investigator 5 years later, and saw it launched in 2020. He plans to retire with GEMS, at the end of its lifetime in the 2030s.
Today, the researchers use GEMS to watch “long-range transport of wildfires and Asian dust and even volcanic eruptions,” Kim says. In fact, anyone in the scientific research community can download GEMS data to analyze the hourly daytime concentration of aerosol, ozone, NO2, SO2, and various volatile organic compounds across Asia.
It took about a year for the team to calibrate and validate the data collected with GEMS. “Characterizing an instrument in space is much more difficult, because we need to rely on indirect methods because we cannot do it in the laboratory,” Kim says. Despite the difficulty, characterization is necessary to ensure an instrument is working properly after being blasted into orbit.
As spectral data stream back to Earth, the scientists use ground-based measurements to confirm that their algorithms correctly identify and quantify air pollutants. This is an iterative process (Atmos. Meas. Tech. 2024, DOI: 10.5194/amt-17-145-2024). Over the last 5 years, the GEMS team has updated its algorithms multiple times to ensure the best data product, Kim says: “It's an evolution of data accuracy as time goes on.”
Chong started his career in satellite science as a graduate student working on GEMS with Kim. “My PhD was about GEMS,” he says. “I worked on instrument calibration and also worked on trace gas retrieval”—the process of determining the concentration of a gas from spectral data collected by a satellite. Chong focused on analyzing existing satellite data and producing new retrieval data. One was bromine monoxide, an air pollutant that readily destroys ozone. Understanding the movement of bromine monoxide in the troposphere—the lowest region of the atmosphere—can give scientists a better understanding of the ozone chemistry there, he says.
TEMPO goes up
Chong’s work on GEMS perfectly positioned him to work on the next geostationary air quality satellite launched into space. Tropospheric Emissions: Monitoring of Pollution, or TEMPO, is a NASA-hosted instrument on a commercial geostationary satellite. It’s a sister to GEMS, but TEMPO’s eye is focused on North America. Built by Ball Aerospace & Technologies, both spectrometers can detect similar ultraviolet and visible wavelengths, where air pollutants readily absorb light.
Chong joined TEMPO’s scientific and operations team at the Harvard & Smithsonian Center for Astrophysics, of which Nowlan is a deputy principal investigator, a year before the satellite’s launch in 2023. Since then, Chong has been busy calibrating TEMPO’s “basic input data for retrieving air pollutant concentrations,” he says.
Similarly to the experience of the GEMS scientists, it has taken time for the TEMPO team to calibrate and validate its data. In December 2024, the group announced that the collected data is of provisional quality: the data have been validated by a substantial but not exhaustive number of independent measurements, so researchers can use TEMPO products to answer scientific questions.
The change in status has led to “a big rush on writing papers right now,” Nowlan says. One paper she has seen describes using TEMPO to estimate nitric oxide and NO2 emissions across North America. Another examines the daily cycles of NO2 in urban areas in North America and the Caribbean. And she expects far more science to be published in the next 6 months or so.
Even though the data have been deemed of provisional quality, TEMPO’s data validation is not over. “They'll continue to do validation as long as the instrument’s alive,” says Kevin Daugherty, TEMPO’s project manager at NASA. Flight campaigns and ground-based measurements will provide more calibration data. “All that combined will continue to improve the algorithms for years to come,” Daugherty says.
"The instrument itself is just as healthy today as it was the day of its launch."
TEMPO’s baseline mission ends in June, but the instrument is designed to last longer. “Fortunately for us, TEMPO doesn't have anything that we call a life-limiting part,” Daugherty says. “The instrument itself is just as healthy today as it was the day of its launch.” TEMPO will be a relatively inexpensive mission to extend because the cost is shared between NASA and the commercial owner of the satellite, he says. As long as there’s funding and the parts continue to function, the team plans to “keep getting this revolutionary air quality measurement over Greater North America,” Daugherty adds.
Neither the NASA nor the Harvard-Smithsonian TEMPO team members can speak to how the recently proposed cuts to federally funded science would affect running the instrument in the long term. The team “has already been told that it's being extended at least through the government fiscal year 2026, which goes through September of 2026,” Daugherty says. At that point, the mission will go through a standard review expected of all operating Earth science division missions to ensure that it is meeting its science goals.
“We're confident that the TEMPO instrument will be ready and waiting in the event that in 2026 they decide to extend us,” Daugherty adds.
Completing the constellation
The planned constellation of geostationary air quality monitoring satellites will be completed in July with the launch of Sentinel-4. That satellite is the first of the European Space Agency’s to monitor “the quality of the air we breathe in Europe” on an hourly basis during the day, project manager Giorgio Bagnasco says in an email. “[A]bout 1 million excess deaths are estimated in Europe due to health complications linked to air pollution,” Bagnasco says. Hourly daytime measurements will help improve models that predict the movement of air pollution and ultimately save lives, he adds.
Having worked on the two other satellites in the constellation, Chong says he is excited to possibly add Sentinel-4 to his repertoire: he and others from TEMPO will be part of an early validation team for Sentinel-4 data. They plan to apply their retrieval algorithms—the formulas needed to tease out pollutant concentration from the spectral data—to the first spectra sent down from the new satellite. Chong says he’ll learn more specifics about the validation plan at the first collaborative meeting, which is to take place in June before the satellite’s launch.
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With the successful launch of Sentinel-4, scientists will be able to monitor the air quality above some of the most populated areas of the northern hemisphere every hour of the day with kilometer-scale spatial resolution. But the global picture is incomplete without information from the southern hemisphere. “Leading scientists are working on the initiative to put a similar instrument over the Middle East and Africa,” Kim says. It’s the missing piece: people in these areas are often affected by dust and other air pollutants.
Ultimately, understanding the hourly movement and evolution of pollution across Earth will require an international coalition of scientists. “I'm very honored and happy to be a part of this global effort to monitor air quality over the globe,” Kim says.