Nitrogen oxides are some of the main ingredients in air pollution, smog, acid rain, and greenhouse gas–driven warming. Quantifying large-scale sources of nitrogen oxide pollution has long proved challenging, making regulation difficult, but now a new high-resolution satellite monitoring system, combined with wind modeling, is providing the tools needed to remotely monitor nitrogen oxide emissions anywhere in the world from space.
The Tropospheric Monitoring Instrument (TROPOMI) on board the European Space Agency’s Copernicus Sentinel-5 Precursor satellite, launched in October 2017, offers “unparalleled spatial resolution” of greenhouse gases and pollutants, including nitrogen oxides, carbon dioxide, carbon monoxide, and methane, over industrial complexes and major cities, said Steffen Beirle, a geochemist at the Max Planck Institute for Chemistry in Germany and lead author of the new study published in Science Advances.
But it’s not enough to simply image the gas plumes, as they tend to be smeared horizontally by wind currents. To quantify the amount of gas being emitted, the satellite data must be processed to take wind patterns into account, Beirle said. “If you just look at the map of the satellite measurements, you see polluted spots over the east coast of the U.S. and China, for example. The difficulty comes when you try to quantify the emissions coming from those hot spots.”
The majority of stationary emissions (as opposed to mobile emissions from vehicles) of nitrogen oxides (NO and NO2, commonly combined as NOx) come from power plants. To quantify emissions from individual power plants, Beirle and colleagues combined TROPOMI data with three-dimensional models of wind spatial patterns. “Previous approaches have taken wind data into account, but not in this kind of systematic way,” he said.
The team first focused their efforts on Riyadh, the capital of Saudi Arabia. Riyadh is fairly remote from other cities, industrial areas, and other sources that could complicate the emission signal. Initially, the satellite data showed a strong NOx signal centered over Riyadh, smeared to the south and east by prevailing winds. Further analysis using the wind models revealed five localized point sources within the smear that corresponded to four power plants and a cement plant.
In total they found that the city produces 6.6 kilograms of NOx per second, with the four power plants accounting for about half of those emissions. Individually, emissions from Riyadh’s crude oil– and natural gas–powered plants were comparable to emissions from coal-fired power plants in the United States.
The team also tested their techniques in South Africa and Germany, where cloud cover can make collecting satellite data difficult. They found the method worked well in both places, but with higher uncertainties in quantifying emissions.
The study represents an important step in being able to monitor greenhouse gas emissions from space, said Andreas Richter, an atmospheric chemist at the University of Bremen in Germany who was not involved in the new study.
“In Germany, industrial facilities are required to track and report their emissions. Where it’s not required, being able to monitor emissions remotely using satellites will be very valuable,” Richter said. The method also has the “potential to validate or check emission inventories that are reported by different countries using different methods, using a consistent methodology globally,” Beirle says. In Germany, the emissions calculated using the new satellite and wind model method “matched up well to the inventory provided by the facilities,” he said.
Power plants are the primary concern for point source emissions, with large industrial facilities like steel factories and cement plants also contributing significant amounts of nitrogen oxides. Diffused emissions from moving sources such as vehicles are harder to pin down. “The total emission from cities may be as large as from a big power plant, but because it’s not as localized, this particular method doesn’t work as well,” Richter said.
Beirle and colleagues also hope to apply their methods to other pollutants, such as sulfur dioxide. “We hope to do something similar for sulfur dioxide, but the background noise levels are higher,” he said. “This satellite is opening up a whole new line of inquiry: What other emissions can we track from space? It will be exciting to see what happens in the next few years.”
—Mary Caperton Morton (@theblondecoyote), Science Writer