Soot from massive 2017 fire clouds persisted in stratosphere for months

Analysis of fire cloud’s impact will help calibrate climate models

Thunderstorms generated by a group of giant wildfires in summer 2017 injected a small volcano’s worth of particles into the stratosphere, creating a smoke plume that lasted for almost nine months. CIRES and NOAA researchers studying the plume found that black carbon or soot in the smoke was key to the plume’s rapid rise: the soot absorbed solar radiation, heating the surrounding air and allowing the plume to quickly rise.

The billowing smoke clouds provided researchers with an ideal opportunity to test climate models that estimate how long the particulate cloud would persist. After achieving a maximum altitude of 14 miles (23 kilometers), it remained in the stratosphere for many months.

“We compared observations with  model calculations of the smoke plume. That helped us understand why the smoke plume rose so high and persisted so long, which can be applied to other stratospheric aerosol injections, such as from volcanoes or nuclear explosions,” said NOAA scientist Karen Rosenlof, a member of the author team that also included scientists from University of Colorado Boulder, Office of Naval Research, Rutgers and other institutions.  The findings were published today in the journal Science.

During the summer of 2017, wildfires raged across the Pacific Northwest. On August 12 in British Columbia, a group of fires and weather conditions produced five near-simultaneous towering clouds of smoke, called pyrocumulonimbus clouds, that lofted smoke high into the stratosphere. Within two months, the plume rose from its initial height of about seven to 14 miles (12 km) up to 14 miles (23 km) and persisted in the atmosphere for much longer—satellites could spot it even after eight months. 

“The forest fire smoke was an ideal case study for us because it was so well observed by satellites,” said lead author Pengfei Yu, a former CIRES scientist at NOAA, now at the Institute for Environment and Climate Research at Jinan University in Guangzhou, China. 

Instruments on two satellites—the International Space Station and NASA’s CALIPSO—and on NOAA’s balloon-borne Printed Optical Particle Spectrometer, or POPS provided the aerosol measurements the researchers needed. 

Yu and his colleagues compared those observations with results from a global climate and chemistry model to get a match for how high up the smoke rose and how long it lasted in the atmosphere. With measurements of the rise rate and evolution of the smoke plume, the researchers could estimate the amount of black carbon in the smoke and how quickly the organic particulate material was destroyed in the stratosphere. The team included scientists from Rutgers, CU, NCAR, Naval Research Laboratory, among others.

They found that the plume’s rapid rise could only be explained by the presence of black carbon or soot, which comprised about two percent of the total mass of the smoke. The soot absorbed solar radiation, heated the surrounding air and forced the plume high into the atmosphere. 

Next, the team modeled the degradation of the smoke plume in the atmosphere. They found that to mimic the smoke’s observed rate of decay over the multi-month plume, there had to be a relatively slow loss of organic carbon through photochemical processes. The scientists said what they learned about modeling the lifetime of soot and other aerosols in the stratosphere could be applied to pyrocumulonimbus clouds triggered by nuclear detonations or stratospheric geoengineering concepts. 

“We have a better understanding of how our models represent smoke,” said co-author Ru-Shan Gao, a NOAA scientist who is participating in FIREX-AQ, a massive NOAA- and NASA- led mission to investigate the chemistry of wildfire smoke. “And because we can model this process, we know we can model other aerosol-related processes in the atmosphere.” 

The work was funded in part by NOAA, the National Science Foundation, the Department of Energy, and the Open Philanthropy Project.

For more information, contact Karin Vergoth, CIRES Communications, at karin.vergoth@colorado.edu.