28 Jan 2022
28 Jan 2022
Status: a revised version of this preprint is currently under review for the journal ACP.

Characteristics and Evolution of Brown Carbon in Western United States Wildfires

Linghan Zeng1, Jack Dibb2, Eric Scheuer2, Joseph M. Katich3,4, Joshua P. Schwarz4, Ilann Bourgeois3,4, Jeff Peischl3,4, Tom Ryerson3,4,a, Carsten Warneke4, Anne E. Perring5, Glenn S. Diskin6, Joshua P. DiGangi6, John B. Nowak6, Richard H. Moore6, Elizabeth B. Wiggins6, Demetrios Pagonis3,7,b, Hongyu Guo3,7, Pedro Campuzano-Jost3,7, Jose L. Jimenez3,7, Lu Xu8,c,d, and Rodney J. Weber1 Linghan Zeng et al.
  • 1Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA
  • 2College of Engineering and Physical Sciences, University of New Hampshire, Durham, NH, USA
  • 3Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO, USA
  • 4Chemical Sciences Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO, USA
  • 5Department of Chemistry, Colgate University, Hamilton, NY, USA
  • 6NASA Langley Research Center, Hampton, VA, USA
  • 7Department of Chemistry, University of Colorado Boulder, Boulder, CO, USA
  • 8Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
  • anow at: Scientific Aviation, Boulder, CO, USA
  • bnow at: Department of Chemistry and Biochemistry, Weber State University, Ogden, UT, USA
  • cnow at: Chemical Sciences Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO, USA
  • dnow at: Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO, USA

Abstract. Brown carbon (BrC) associated with aerosol particles in western United States wildfires was measured between Jul. and Aug. 2019 onboard the NASA DC-8 research aircraft during the Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) study. Two BrC measurement methods are investigated; highly spectrally-resolved light absorption in solvent (water and methanol) extracts of particles collected on filters and in-situ bulk aerosol particle light absorption measured at three wavelengths (405, 532, 664 nm) with a photo acoustic spectrometer (PAS). A light absorption closure analysis for wavelengths between 300 and 700 nm was performed. The combined light absorption of particle pure black carbon material, including enhancements due to internally mixed materials, plus soluble BrC and a Mie-predicted factor for conversion of soluble BrC to aerosol particle BrC, was compared to absorption spectra from a power law fit to the three PAS wavelengths. For the various parameters used, at a wavelength of roughly 400 nm they agreed, at lower wavelengths the individual component-predicted particle light absorption significantly exceeded the PAS and at higher wavelengths the PAS absorption was consistently higher, but more variable. Limitations with extrapolation of PAS data to wavelengths below 405 nm and missing BrC species of low solubility that more strongly absorb at higher wavelengths may account for the differences. Based on measurements closest to fires, the emission ratio of PAS measured BrC at 405 nm relative to carbon monoxide (CO) was on average 0.13 Mm−1 ppbv−1, emission ratios for soluble BrC are also provided. As the smoke moved away from the burning regions the evolution over time of BrC was observed to be highly complex; BrC enhancement, depletion, or constant levels with age were all observed in the first 8 hours after emission in different plumes. Within 8 hours following emissions, 4-nitrocatechol, a well characterized BrC chromophore commonly found in smoke particles, was largely depleted relative to the bulk BrC. In a descending plume where temperature increased by 15 K, 4-nitrocatechol dropped possibly due to temperature-driven evaporation, but bulk BrC remained largely unchanged. Evidence was found for reactions with ozone, or related species, as a pathway for secondary formation of BrC under both low and high oxides of nitrogen (NOx) conditions, while BrC was also observed to be bleached in regions of higher ozone and low NOx, consistent with complex behaviors of BrC observed in laboratory studies. Although the evolution of smoke in the first hours following emission is highly variable, a limited number of measurements of more aged smoke (15 to 30 hours) indicate a net loss of BrC. It is yet to be determined how the near-field BrC evolution in smoke affects the characteristics of smoke over longer time and spatial scales, where its environmental impacts are likely to be greater.

Linghan Zeng et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-70', Rawad Saleh, 20 Feb 2022
  • RC2: 'Comment on acp-2022-70', Anonymous Referee #2, 21 Apr 2022

Linghan Zeng et al.

Linghan Zeng et al.


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Short summary
Wildfires emit aerosol particles containing brown carbon material that affects visibility, global climate, and are toxic. Brown carbon is poorly characterized due to measurement limitations and their evolution in the atmosphere is not well known. We report on aircraft measurements of brown carbon from large wildfires in the western United States. We compare two methods for measuring brown carbon and study the evolution of brown carbon in the smoke as it moved away from the burning regions.