Sky-High Smoke

Some wildfires are so intense, they create their own weather – thunderstorms driven by heat that hurtle smoke as high as 10 miles into the sky like giant chimneys. 

When these smoke plumes reach the thin, calm air of the upper troposphere and lower stratosphere, they can persist for weeks or even months – yet their exact effects on the Earth’s climate aren’t well known because they’re difficult to capture and measure.

 An atmospheric science team in the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) took an audacious swing at directly observing fresh wildfire smoke that found its way into the uppermost troposphere, about nine miles above Earth’s surface.

In a study published in Science Advances, the researchers report that unusually large particles they observed inside a wildfire-induced, high-altitude smoke plume had a significant cooling effect on that region, a phenomenon with potential consequences for the Earth’s climate yet aren’t incorporated into current climate models.

“We’re seeing more and more wildfires in Canada, the western U.S., all over the world,” said senior author Frank Keutsch, Stonington Professor of Engineering and Atmospheric Science at SEAS. “We are particularly interested in what the impact is on climate, and extending from that, what is the impact on atmospheric composition – the things that we care about, for example, the stratospheric ozone layer that protects us from UV radiation.”

Absorbed or scattered sunlight

Wildfire smoke, as well as other aerosols like industrial air pollution, can change the amount of radiation that gets to the ground by absorbing sunlight or scattering it back toward space. Better understanding the behavior of high-altitude smoke clouds could lead to new insights into the balance between incoming and outgoing radiation, and thus, how Earth processes like the hydrological cycle might be responding to fires, Keutsch explained.

For example, said study co-author and project scientist John Dykema, local heating caused by the smoke absorbing sunlight could cause atmospheric circulation to change, which could in turn shift positions of jet streams and may have implications for weather. “I think all of these things are possible, and we don’t currently have enough information to say which way they could go,” he said.

The research team used the NASA ER-2 high-altitude aircraft outfitted with specialized equipment to make unprecedented observations of a smoke plume that climbed into the uppermost troposphere shortly after the eruption of a New Mexico wildfire in June 2022. 

Aboard the aircraft that was shared with other research groups, they deployed a portable optical spectrometer that measures the concentration and size of particles, as well as an instrument that measures plume composition — complemented by an instrument from a group at Purdue University to identify smoke particles. 

With the help of geostationary satellite technology, the researchers were able to pinpoint the plume, fly into it, and capture detailed information from within it just five days post-fire. This compares with previous observers who’d managed to measure stratospheric smoke that was several weeks old.

Coagulated aerosols

Within the young plume, the Harvard team observed concentrations of surprisingly large aerosols, about 500 nanometers in diameter, or double the size of typical smoke aerosols at lower altitudes. With the help of modeling experts at Colorado State University, they showed that efficient particle coagulation could explain their large observed size. 

“Particles can coagulate at any place in the atmosphere,” said lead author and former Ph.D. student Yaowei Li. “But in that specific region, the air mixes very slowly. That allows wildfire smoke particles to remain concentrated and collide more often, making coagulation much more efficient.”

These larger-sized particles, they continued, had a much stronger cooling effect, increasing outgoing radiation by 30-36% compared with smaller smoke particles typically found at lower altitudes. This effect has not been included in current climate models. The results could have important implications for how scientists understand the Earth’s future climate.

“Our study provides new insights to better constrain how particles from these specific phenomena of wildfire-driven thunderstorms affect the Earth’s energy budget,” Li said.

The study was co-authored by Harvard researchers Xu Feng, Jasna Pittman, Bruce Daube, Steven Wofsy, and Loretta Mickley. Other co-authors were David Peterson, Xiaoli Shen, Nicole June, Michael Fromm, Theodore McHardy, Justin Jacquot, Jonathan Dean-Day, Anita Rapp, Kenneth Bowman, Daniel Cziczo, and Jeffrey Pierce. 

The research was supported by NASA under the Earth Venture Suborbital-3 program awards for the Dynamics and Chemistry of the Summer Stratosphere (DCOTSS) mission, for which Keutsch is the Deputy-PI. Additional support came from the Naval Research Laboratory, the National Science Foundation, and the Salata Institute for Climate and Sustainability at Harvard.