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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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Volume 17, issue 8 | Copyright
Atmos. Chem. Phys., 17, 5063-5078, 2017
https://doi.org/10.5194/acp-17-5063-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 19 Apr 2017

Research article | 19 Apr 2017

Relative importance of black carbon, brown carbon, and absorption enhancement from clear coatings in biomass burning emissions

Rudra P. Pokhrel1, Eric R. Beamesderfer1,a, Nick L. Wagner2, Justin M. Langridge3, Daniel A. Lack4, Thilina Jayarathne5, Elizabeth A. Stone5, Chelsea E. Stockwell6, Robert J. Yokelson6, and Shane M. Murphy1 Rudra P. Pokhrel et al.
  • 1Department of Atmospheric Science, University of Wyoming, Laramie, Wyoming, USA
  • 2NOAA Earth System Research Laboratory, Chemical Sciences Division, Boulder, Colorado, USA
  • 3Observation Based Research, Met Office, Fitzroy Road, Exeter, EX1 3PB, UK
  • 4Transport Emissions, Air Quality and Climate Consulting, Brisbane, Australia
  • 5Department of Chemistry, University of Iowa, Iowa City, Iowa, USA
  • 6Department of Chemistry, University of Montana, Missoula, Montana, USA
  • anow at: School of Geography and Earth Science, McMaster University, Hamilton, Ontario, Canada

Abstract. A wide range of globally significant biomass fuels were burned during the fourth Fire Lab at Missoula Experiment (FLAME-4). A multi-channel photoacoustic absorption spectrometer (PAS) measured dry absorption at 405, 532, and 660nm and thermally denuded (250°C) absorption at 405 and 660nm. Absorption coefficients were broken into contributions from black carbon (BC), brown carbon (BrC), and lensing following three different methodologies, with one extreme being a method that assumes the thermal denuder effectively removes organics and the other extreme being a method based on the assumption that black carbon (BC) has an Ångström exponent of unity. The methodologies employed provide ranges of potential importance of BrC to absorption but, on average, there was a difference of a factor of 2 in the ratio of the fraction of absorption attributable to BrC estimated by the two methods. BrC absorption at shorter visible wavelengths is of equal or greater importance to that of BC, with maximum contributions of up to 92% of total aerosol absorption at 405nm and up to 58% of total absorption at 532nm. Lensing is estimated to contribute a maximum of 30% of total absorption, but typically contributes much less than this. Absorption enhancements and the estimated fraction of absorption from BrC show good correlation with the elemental-carbon-to-organic-carbon ratio (ECOC) of emitted aerosols and weaker correlation with the modified combustion efficiency (MCE). Previous studies have shown that BrC grows darker (larger imaginary refractive index) as the ratio of black to organic aerosol (OA) mass increases. This study is consistent with those findings but also demonstrates that the fraction of total absorption attributable to BrC shows the opposite trend: increasing as the organic fraction of aerosol emissions increases and the ECOC ratio decreases.

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This study investigates enhancement of black carbon (BC) absorption in biomass burning emissions due to absorbing and non-absorbing coatings. The fraction of absorption due to BC, brown carbon (BrC), and lensing is estimated using different approaches. The similarities and differences between the results from these approaches are discussed. Absorption by BrC is shown to have good correlation with the elemental to organic carbon ratio (EC / OC) and AAE.
This study investigates enhancement of black carbon (BC) absorption in biomass burning emissions...
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