How Aerosols Helped Untangle Carbon Monoxide Trends

How Aerosols Helped Untangle Carbon Monoxide Trends

NASA’s Terra satellite has been taking measurements of Earth’s atmosphere for more than two decades. One of the five sensors on board, Measurement of Pollution in the Troposphere (MOPITT), makes daily measurements of the air pollutant carbon monoxide.

Over the past two decades, MOPITT’s science team has observed a significant decline in global concentrations of carbon monoxide. “We wanted to know what’s driving that trend,” said Rebecca Buchholz, an atmospheric scientist at the U.S. National Center for Atmospheric Research (NCAR). “But that’s challenging to untangle because there are several sources of carbon monoxide, including fires, transportation, industry, and vegetation.”

Carbon monoxide also stays in the atmosphere for a few months, meaning it has plenty of time to mix and spread out. This makes it harder for researchers to pinpoint the sources or understand what factors are contributing to changes. But they do have ways of getting around this problem. By comparing MOPITT observations of carbon monoxide with measurements of shorter-lived pollutants that were released from some of the same sources, Buchholz and her colleagues were able to better understand where the carbon monoxide came from and why the concentration of the gas has fallen. They reported their results in Remote Sensing of Environment.

They relied partly on data from the Moderate Resolution Imaging Spectroradiometer (MODIS), a sensor on the Terra satellite. MODIS measures short-lived airborne particles called aerosols—things like dust, soot, ash, and sea salt. Aerosols typically remain in the atmosphere for about a week before chemical reactions, gravity, or precipitation remove them.

“Of particular use to us were black carbon and organic carbon aerosols,” explained Buchholz. “Since they are both directly emitted along with carbon monoxide during the burning of fossil fuels and biomass, they can help trace carbon monoxide from these sources.”

In addition to analyzing carbon monoxide and aerosol trends from MOPITT and MODIS, the researchers also considered observations from other satellite sensors and incorporated insights about regional aerosol emissions from a range of other research.

The map above shows trends in aerosols since 2002 as measured by Terra’s MODIS. In several parts of the Northern Hemisphere, aerosols, like carbon monoxide, have declined in recent decades. The trend was especially notable in eastern Asia, the eastern United States, and western Europe—areas where environmental regulations and cleaner-burning technologies have limited emissions from transport and industrial sectors.

Another key factor behind the decline was a reduction in fires and burned area. In semi-arid parts of South America, Central Africa, and Asia, the expansion and intensification of agriculture into grasslands and savannas has led to more fire suppression as agricultural interests move in and protect high-value crops, livestock, homes, and infrastructure. In many places, there has been a trend toward using machinery rather than periodic burning to maintain land in rural areas.

Parts of India, the U.S. Pacific Northwest, and the Caspian Sea region have experienced increases in aerosol particles for other reasons. Drying of the Caspian Sea has probably added to aerosols in that area by making skies dustier, according to the researchers. In India, loose environmental controls, combined with growing populations and lots of agricultural burning, has likely fueled the increases in aerosols there. In the western U.S., the aerosol load has increased as drought and climate change have fueled more frequent and intense fires.

The second map shows the strong positive aerosol trend signal in the Pacific Northwest for the month of August. “The changes in the U.S. Pacific Northwest stand out in both carbon monoxide and aerosol data,” said Buchholz. “The seasonal pattern has shifted, and now we’re seeing an intensification of the summer smoke season creating a new carbon monoxide peak in August.”

The extra smoke has potentially exposed as many as 130 million people to toxic substances in recent years and threatens to reverse years of air quality gains. In April 2022, Buchholz co-authored a Nature Communications study that detailed the problem. The data indicates a similar process may be playing out in Siberia—another region that has seen increased fire activity in recent years.

“Research like this really underscores how powerful it can be to fold data from multiple sensors into one analysis,” said Buchholz. “In this case, the carbon monoxide and aerosol data reinforce each other and tell a deeper story about what’s going on in our atmosphere than either could alone.”

Terra was launched in 1999 with an expected design life of six years. “Nobody expected that MODIS or MOPITT or other sensors on Terra would still be operating more than two decades after launch,” said Helen Worden, the U.S. principal investigator for MOPITT. “Their longevity has opened all sorts of new doors regarding the science we can do.”

Editor’s Note: Read the first part of this story in yesterday’s Image of the Day.


NASA Earth Observatory images by Joshua Stevens, using data courtesy of Buchholz, R., et al. (2021). Story by Adam Voiland.

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