October 22, 2024
Research Highlight

Light Absorption by Black Carbon in Wildfire-Driven Storms

Some wildfires produce pyrocumulonimbus clouds, which can inject smoke into the upper troposphere and lower stratosphere

wildfire in a mountainous area

Thick coatings on black carbon (BC) inside a pyrocumulonimbus cloud cause higher absorption enhancement when compared to BC from other sources. Absorption enhancement plateaus at ∼2.0, providing an upper limit for absorption enhancement by atmospheric BC.

The Science    

Pyrocumulonimbus (pyroCb) clouds form from wildfire-driven convection. PyroCb clouds contain large amounts of black carbon (BC) mixed with water and organics. BC particles have a large effect on our climate because they can absorb sunlight, but BC absorption depends on how it is mixed with other materials. Previously, the extent of BC mixing was assumed, and absorption properties were calculated based on these assumptions. This study used measurements of BC mixing to improve our understanding of BC light absorption in a pyroCb cloud and found that BC absorption reaches a maximum value—beyond which additional mixing does not increase absorption.

The Impact

PyroCb clouds are increasing in frequency as large wildfires become more prevalent in a warming climate. PyroCb clouds can in some cases inject smoke particles into the upper troposphere and lower stratosphere, where they can remain suspended for several months. To understand how these particles influence Earth’s climate, how much light they absorb must be understood. Measurements of particles inside a pyroCb cloud were used in this study to quantify their absorption properties. This research uses some of the first observations of particles inside a pyroCb cloud to improve our understanding of how they absorb sunlight.

Summary

PyroCb clouds are a subset of cumulonimbus clouds that form via heat-driven convection driven by wildfires. Previous observations of pyroCb firestorm systems have shown that pyroCbs can inject significant amounts of BC-containing smoke to the stratosphere with a residence time of several months. A distinguishing feature of BC particles in pyroCb clouds is their thick surface coatings compared to BC from other sources. When coated with non-refractory materials, absorption by BC is enhanced, which is usually quantified by an enhancement factor (Eabs). Previous studies of absorption by BC in pyroCb clouds have relied on models that make simplifying assumptions about the thickness of coatings on BC particles.

In this study, direct measurements of coating thickness on BC particles in a pyroCb cloud were used to quantify Eabs. Eabs in a pyroCb cloud was higher than Eabs by BC from urban and wildfire sources. The results indicate that Eabs has an upper limit of 2—beyond which the addition of coatings does not increase absorption.

Contacts

  • Payton Beeler, Pacific Northwest National Laboratory, payton.beeler@pnnl.gov, primary author
  • Rajan K. Chakrabarty, Washington University - St. Louis, chakrabarty@wustl.edu, corresponding author
  • Jerome Fast, Pacific Northwest National Laboratory, jerome.fast@pnnl.gov, Integrated Cloud, Land-Surface, and Aerosol System primary investigator

Funding

This research has been supported by the National Aeronautics and Space Administration; the National Oceanic and Atmospheric Administration; the National Science Foundation; the Department of Energy (DOE); the Simons Foundation’s Mathematics and Physical Sciences division; the DOE Atmospheric System Research program via the Integrated Cloud, Land-Surface, and Aerosol System Study Science Focus Area; and the Laboratory Directed Research and Development program (Linus Pauling Distinguished Postdoctoral Fellowship Program) at Pacific Northwest National Laboratory.

Published: October 22, 2024

Beeler, Payton, Joshin Kumar, Joshua P. Schwarz, Kouji Adachi, Laura Fierce, Anne E. Perring, J. M. Katich, and Rajan K. Chakrabarty. 2024. “Light Absorption Enhancement of Black Carbon in a Pyrocumulonimbus Cloud.” Nature Communications 15 (1). [DOI:10.1038/s41467-024-50070-0]