Intense wildfires can generate immense energy, wind shear, and turbulence, building towering clouds that shoot smoke high into the stratosphere, where it can linger. Scientists refer to these as pyrocumulonimbus (pyroCb) clouds, and they are full of black carbon, a main component of soot.
In a recent study published in Nature Communications, scientists found that organic matter coating this black carbon can significantly enhance the amount of sunlight the clouds absorb and may affect warming around the globe.
It is important to understand the properties of smoke to determine how pyrocumulonimbus clouds affect local weather and atmospheric heating or cooling, said David Peterson, a meteorologist at the Naval Research Laboratory in Monterey, Calif., who was not involved in the study. “This study is one of the big components to [understand] that,” he added.
Firestorm Chasers
“Ever heard of tornado chasers? We were like firestorm chasers!”
In 2019, Washington University in St. Louis atmospheric chemist Rajan Chakrabarty spent weeks in a small van chasing wildfires as part of a multi-institutional field campaign called Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ), an initiative led jointly by NOAA and NASA. As a part of the initiative, Chakrabarty and his colleagues were mapping fresh smoke plumes on the ground and tracking their spread.
“Ever heard of tornado chasers?” Chakrabarty joked. “We were like firestorm chasers!” By day 45 the team had traveled through Idaho, Arizona, Washington, Oregon, and California.
Meanwhile, NASA’s DC-8 aircraft, retrofitted to collect and analyze air samples at high altitudes, flew about 10 kilometers (6 miles) above Earth’s surface within the stratosphere to gather black carbon particles from the clouds above the fires. The scientists then measured the mass and shape of the particles. In their study, they included data from the Williams Flats Fire in eastern Washington and the Castle and Ikes fires in northern Arizona.
“The initiative was the first and so far the only existing measurement of pyroCb smoke as it was being released through the top of an active thunderstorm.”
“The initiative was the first and so far the only existing measurement of pyroCb smoke as it was being released through the top of an active thunderstorm,” Peterson said.
The group wanted to study how soot residing in these clouds could affect the climate. Typically, black carbon particles in wildfire clouds are short-lived—they are often removed from the stratosphere within around 10 days after a typical wildfire event. Some wildfires, however, produce pyroCb clouds and can eject black carbon into the stratosphere, where it stays for a long time. The Canadian wildfires of 2017, for example, generated a smoke plume that encircled part of the globe, and its effects persisted for about 10 months.
The team observed a key difference between black carbon in typical wildfire clouds and that in pyroCb clouds: As the black carbon in pyroCb clouds reached the cold stratosphere, it served as a surface on which gases within the clouds could condense. As a result, the black carbon particles no longer resembled those typically found in wildfire soot; instead, they acquired an additional layer of organic matter around them.
The group measured the coating thickness and analyzed its effect on individual particles. Black carbon with this extra coating absorbed up to twice as much heat as uncoated particles of the same size.
The finding has tremendous implications for simulating Earth’s future climate. A climate model that calculates heat absorption on the basis of uncoated black carbon particles could underpredict warming. Though these first measurements are exciting, the researchers pointed out that it’s equally important to gather more data for a full picture.
“Imagine you’re an alien researcher studying humans,” said Joshua Schwarz, a coauthor of the study and an atmospheric scientist at NOAA. “Meeting just one person might reveal basic facts, like humans have two arms and two legs, but it wouldn’t capture the full complexity of human interaction.” Similarly, studying a few pyroCb clouds isn’t enough to understand what’s possible from those clouds. But “it’s really valuable that we have these first measurements,” Schwarz added.
—Saugat Bolakhe (@saugat_optimist), Science Writer
