CaltechAUTHORS
Published August 19, 2025 | Version Supplemental material
Journal Article Open

Atmospheric Evolution of Brown Carbon from Wildfires in North America

Creators

  • 1. ROR icon Washington University in St. Louis
  • 2. ROR icon National Oceanic and Atmospheric Administration
  • 3. ROR icon Cooperative Institute for Research in Environmental Sciences
  • 4. ROR icon California Institute of Technology
  • 5. ROR icon University of Colorado Boulder
  • 6. ROR icon National Center for Atmospheric Research
  • 7. ROR icon Langley Research Center
  • 8. ROR icon Forschungszentrum Jülich
  • 9. ROR icon University of Montana
  • 10. Tofwerk USA, Boulder, Colorado, 80301, United States
  • 11. ROR icon University of Wyoming
  • 12. ROR icon Colgate University
  • 13. ROR icon Yokohama City University
  • 14. ROR icon University of Michigan–Ann Arbor
  • 15. ROR icon Colorado State University
  • 16. ROR icon University of Washington
  • 17. ROR icon Georgia Institute of Technology

Abstract

Atmospheric brown carbon (BrC) from wildfires is a key component of light-absorbing carbon that significantly contributes to global radiative forcing, but its atmospheric evolution and lifetime remain poorly understood. In this study, we investigate BrC evolution by synthesizing data from one laboratory campaign and four aircraft campaigns spanning diverse spatial scales across North America. To estimate initial conditions for evaluating plume evolution, we develop a method to parametrize the emission ratios of BrC and other species using commonly measured inert tracers, acetonitrile and hydrogen cyanide. The evolution of BrC absorption in the free troposphere is characterized as a function of hydroxyl radical (OH) exposure, yielding an effective photochemical rate constant of 9.7–1.6+4.8 × 10–12 cm3 molecule–1 s–1. The relatively slow reaction rate results in small BrC decay within the first few hours after emission, making it difficult to distinguish from source variability. This helps explain the absence of clear evolutionary trends in near-field studies. Assuming an OH concentration of 1.26 × 106 molecules cm–3, this rate constant corresponds to an e-folding lifetime of approximately 23 h. After extensive photooxidation (OH exposure ∼1012 molecules cm–3 s), 4 ± 2% of the emitted BrC persists, representing a recalcitrant fraction with potential long-term climate impacts. These results improve our understanding of BrC variability and photochemical processing and provide critical constraints for modeling its impacts on climate.

Copyright and License

© 2025 American Chemical Society.

Acknowledgement

J.H.C., U.P., and L.X. were supported by NASA (80NSSC21K1704) and Simons Foundation (SFI-MPS-SRM-00005174); R.J.W. by NOAA AEROMMA project grant NA21OAR4310126-T1-01; and R.J.Y. and V.S. by NOAA (NA16OAR4310100) and NSF (1748266). This research was supported by NOAA cooperative agreement NA22OAR4320151 and NA17OAR4320101, by the European Research Council (ERC) under the European Union’s Horizon 2022 research and innovation program (Grant agreement No. 101076276) and by the Klaus Tschira Boost Fund (grant no. KT28).

Supplemental Material

Description of field campaigns and instruments; wildfire plume identification; back trajectory analysis; calculations of NEMR, OH exposure, and AbsBrC; parameterization of emission ratios; BrC evolution (PDF)

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Additional details

Identifiers

PMID
40772467

Related works

Describes
Journal Article: 40772467 (PMID)

Funding

National Aeronautics and Space Administration
80NSSC21K1704
Simons Foundation
SFI-MPS-SRM-00005174
National Oceanic and Atmospheric Administration
NA21OAR4310126-T1-01
National Oceanic and Atmospheric Administration
NA16OAR4310100
National Science Foundation
AGS-1748266
National Oceanic and Atmospheric Administration
NA22OAR4320151
National Oceanic and Atmospheric Administration
NA17OAR4320101
European Research Council
101076276
Klaus Tschira Foundation
KT28

Dates

Accepted
2025-07-31
Available
2025-08-07
Published online

Caltech Custom Metadata

Caltech groups
Division of Engineering and Applied Science (EAS), Division of Geological and Planetary Sciences (GPS)
Publication Status
Published