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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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Microscopic particles interact with sunlight and affect the earth's climate in ways that are not fully understood. Aerosols from wildfire smoke present particular challenges due to their complexity in shape and composition. We demonstrate that we can experimentally measure aerosol optical properties for many types of smoke particles, using measurements of smoke from controlled burns, but that the method does not work well for smoke with high soot content.
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Preprints
https://doi.org/10.5194/acp-2020-1200
https://doi.org/10.5194/acp-2020-1200

  24 Nov 2020

24 Nov 2020

Review status: this preprint is currently under review for the journal ACP.

Complex refractive indices in the ultraviolet and visible spectral region for highly absorbing non-spherical biomass burning aerosol

Caroline C. Womack1,2, Katherine M. Manfred1,2,a, Nicholas L. Wagner1,2, Gabriela Adler1,2,b, Alessandro Franchin1,2,c, Kara D. Lamb1,2,d, Ann M. Middlebrook2, Joshua P. Schwarz2, Charles A. Brock2, Steven S. Brown2,3, and Rebecca A. Washenfelder2 Caroline C. Womack et al.
  • 1Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309, USA
  • 2Chemical Sciences Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO 80305, USA
  • 3Department of Chemistry, University of Colorado, Boulder, CO 80309, USA
  • anow at: Wolfson Atmospheric Chemistry Laboratories, Department of Chemistry, University of York, York, UK
  • bnow at: Breezometer, Haifa, Israel
  • cnow at: the National Center for Atmospheric Research, Boulder, CO 80305, USA
  • dnow at: Department of Earth and Environmental Engineering, Columbia University, New York, NY 10027, USA

Abstract. Biomass burning aerosol is a major source of PM2.5, and significantly affects Earth's radiative budget. The magnitude of its radiative effect is poorly quantified due to uncertainty in the optical properties of aerosol formed from biomass burning. Using a broadband cavity enhanced spectrometer with a recently increased spectral range (360–720 nm) coupled to a size-selecting aerosol inlet, we retrieve complex refractive indices of aerosol throughout the near-ultraviolet and visible spectral region. We demonstrate refractive index retrievals for two standard aerosol samples: polystyrene latex spheres and ammonium sulfate. We then retrieve refractive indices for biomass burning aerosol from 13 controlled fires during the 2016 Missoula Fire Science Laboratory Study. We demonstrate that the technique is highly sensitive to the accuracy of the aerosol size distribution method, and find that while we can constrain the optical properties of brown carbon aerosol for many fires, fresh smoke dominated by fractal-like black carbon aerosol presents unique challenges and is not well-represented by Mie theory. For the 13 fires, we show that the accuracy of Mie theory retrievals decreases as the fraction of black carbon mass increases. At 475 nm, the average refractive index is (1.635 ± 0.056) + (0.06 ± 0.12)i.

Caroline C. Womack et al.

 
Status: open (until 02 Feb 2021)
Status: open (until 02 Feb 2021)
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Caroline C. Womack et al.

Caroline C. Womack et al.

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Short summary
Microscopic particles interact with sunlight and affect the earth's climate in ways that are not fully understood. Aerosols from wildfire smoke present particular challenges due to their complexity in shape and composition. We demonstrate that we can experimentally measure aerosol optical properties for many types of smoke particles, using measurements of smoke from controlled burns, but that the method does not work well for smoke with high soot content.
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