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Discussion papers | Copyright
https://doi.org/10.5194/acp-2018-306
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 11 Apr 2018

Research article | 11 Apr 2018

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This discussion paper is a preprint. A revision of this manuscript was accepted for the journal Atmospheric Chemistry and Physics (ACP) and is expected to appear here in due course.

Measurement and modeling of the multi-wavelength optical properties of uncoated flame-generated soot

Sara D. Forestieri1,a, Taylor M. Helgestad1,a, Andrew Lambe2,3, Lindsay Renbaum-Wolff2, Daniel A. Lack4,5,b, Paola Massoli2, Eben S. Cross6,c, Manvendra K. Dubey7, Claudio Mazzoleni8, Jason Olfert9, Andrew Freedman2, Paul Davidovits3, Timothy B. Onasch2,3, and Christopher D. Cappa1 Sara D. Forestieri et al.
  • 1Department of Civil and Environmental Engineering, University of California, Davis, CA 95616
  • 2Aerodyne Research Inc., Billerica, Massachusetts, USA, 01821
  • 3Chemistry Department, Boston College, Boston, MA, USA, 02467
  • 4NOAA Earth System Research Laboratory, Boulder, CO, USA, 80305
  • 5University of Colorado, Cooperative Institute for Research of the Environmental Sciences, Boulder, CO, USA, 80305
  • 6Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA
  • 7Los Alamos National Laboratory, Los Alamos, NM, USA
  • 8Deparment of Physics and Atmospheric Sciences Program, Michigan Technological University, Houghton, MI, USA
  • 9Department of Mechanical Engineering, University of Alberta, Edmonton, Alberta, Canada
  • anow at: California Air Resources Board, Sacramento, CA, USA
  • bnow at: Transport Emissions, Air Quality and Climate Consulting, Brisbane, Australia
  • cnow at: Aerodyne Research Inc., Billerica, Massachusetts, USA, 01821

Abstract. Optical properties of flame-generated black carbon (BC) containing soot particles were quantified at multiple wavelengths for particles produced using two different flames, a methane diffusion flame and an ethylene premixed flame. Measurements were made for: (i) nascent soot particles, (ii) thermally denuded nascent particles, and (iii) particles that were coated then thermally denuded, leading to collapse of the initially lacy, fractal-like morphology. The measured mass absorption coefficients (MAC) depended on soot maturity and generation, but were similar between flames for similar conditions. For mature soot, here corresponding to particles with volume-equivalent diameters >~160nm, the MAC and absorption Angstrom exponent values were independent of particle collapse while the single scatter albedo increased. The MAC values for these larger particles were also size-independent. Effective, theory-specific complex refractive index (RI) values are derived from the observations with two widely-used methods: Lorenz-Mie theory and the Rayleigh-Debye-Gans (RDG) approximation. Mie theory systematically under-predicts the observed absorption cross-sections at all wavelengths for larger particles (with x>0.9) independent of the complex RI used, while RDG provides good agreement. Importantly, this implies that the use of Mie theory within air quality and climate models, as is common, likely leads to under-predictions in the absorption by BC, with the extent of under-prediction depending on the assumed BC size distribution and complex RI used. We suggest that it is more appropriate to assume a constant, size-independent (but wavelength-specific) MAC to represent absorption by uncoated BC particles within models.

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Sara D. Forestieri et al.
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Measurement and modeling of the multi-wavelength optical properties of uncoated flame-generated soot: Data from the BC2, BC3, BC3+ and BC4 studies S. D. Forestier and C. D. Cappa https://doi.org/10.25338/B8JP4V

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Short summary
We characterized optical properties of flame-derived black carbon particles and interpret our observations through the use of Mie theory and Rayleigh-Debye-Gans theory. We determined that the mass absorption coefficient is independent of particle collapse and use these to derive theory- and wavelength-specific refractive indices for BC. We demonstrate the inadequacy of Mie theory, and suggest an alternative approach that atmospheric models can take to better represent light absorption by BC.
We characterized optical properties of flame-derived black carbon particles and interpret our...
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