Aerosol researchers are assessing how much light is absorbed by black carbon in fire clouds to improve models of the climate effects resulting from severe wildfires.
As global temperatures rise, large wildfires are happening more frequently. These wildfires release black carbon into the atmosphere, which is one of the most effective short-term warming agents due to its capability to absorb sunlight. However, scientists are still trying to understand how much atmospheric warming is caused by black carbon found in pyrocumulonimbus (pyroCb) clouds that form during severe wildfires.
In their most severe instances, these wildfire clouds can spread smoke into the upper troposphere and lower stratosphere, where it can remain for months and affect the temperatures and composition of the stratosphere. Recent research from Washington University’s Center for Aerosol Science & Engineering (CASE) has begun to clarify some of these effects.
The study was spearheaded by Rajan Chakrabarty, a professor at WashU’s McKelvey School of Engineering, and his former student, Payton Beeler, who is now a distinguished post-doctoral fellow at Pacific Northwest National Laboratory. The findings were published in Nature Communications.
“This research tackles a significant issue in measuring the radiative impact of black carbon in the upper atmosphere,” Chakrabarty explained.
The researchers conducted airborne observations within the upper section of a pyroCb thunderstorm in Washington state as part of the 2019 NOAA/NASA Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) field campaign.
“We took into account the intricate and varied characteristics of black carbon particles to accurately estimate their solar absorption. Our findings indicate that black carbon particles from pyroCbs absorb visible sunlight twice as effectively as black carbon particles that come from smaller fires and urban areas,” he noted.
The researchers uniquely merged data on black carbon mass and the thickness of organic coatings around individual particles with a sophisticated single-particle optics model. They utilized an exact numerical model to analyze the optical properties of black carbon and determined how much light these particles absorb (and how much additional heat they contribute to the upper atmosphere).
Moreover, this work emphasizes the different light absorption properties of black carbon found in pyroCbs compared to black carbon from other wildfires and urban sources.
The next phase of the study will involve gathering more data and conducting a thorough examination of black carbon behavior in the stratosphere.
Black carbon released into the lower stratosphere from recent pyroCb events in Canada and Australia has traveled worldwide, lasted for months, and influenced circulation patterns and radiative forcing across vast areas, Chakrabarty pointed out. These thunderstorms account for 10% to 25% of black carbon currently present in the lower stratosphere, affecting both the Northern and Southern Hemispheres. While researchers are increasingly noticing its climate effects, there is still much to discover.
“We require additional direct measurements of black carbon light absorption from pyroCbs to refine our climate model forecasts of stratospheric warming,” Chakrabarty emphasized.
This research has received funding from the National Aeronautics and Space Administration (grant nos. 80NSSC18K1414 and NNH20ZDA001N- ACCDAM), the National Oceanic and Atmospheric Administration (grant no. NA16OAR4310104), the National Science Foundation (grant nos. AGS-1455215 and AGS-1926817), the U.S. Department of Energy (grant no. DE-SC0021011), and the Simons Foundation’s Mathematics and Physical Sciences division. L.F. was supported by the U.S. Department of Energy (DOE) through the Atmospheric System Research (ASR) program, part of the Integrated Cloud, Land-Surface, and Aerosol System Study (ICLASS) Science Focus Area. Extra support was provided by the Laboratory Directed Research and Development program, specifically the Linus Pauling Distinguished Postdoctoral Fellowship Program. The Pacific Northwest National Laboratory operates for the DOE under contract DE-AC05-76RL01830 through Battelle Memorial Institute.
