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Open AccessArticle

Refractive Indices of Biomass Burning Aerosols Obtained from African Biomass Fuels Using RDG Approximation

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Department of Physics, North Carolina Agricultural and Technical State University, Greensboro, NC 27411 USA
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Applied Sciences and Technology Program, North Carolina A&T State University, Greensboro, NC 27411, USA
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Department of Chemistry, North Carolina Agricultural and Technical State University, Greensboro, NC 27411, USA
*
Author to whom correspondence should be addressed.
Current Address: Department of Physics and Optical Science, UNC Charlotte, NC 28223, USA
Current Address: Department of Chemistry and Physics, Western Carolina University, Cullowhee, NC 28723, USA.
Atmosphere 2020, 11(1), 62; https://doi.org/10.3390/atmos11010062
Received: 28 November 2019 / Revised: 27 December 2019 / Accepted: 28 December 2019 / Published: 3 January 2020
(This article belongs to the Section Aerosols)
Biomass burning (BB) aerosols contribute to climate forcing, but much is still unknown about the extent of this forcing, owing partially to the high level of uncertainty regarding BB aerosol optical properties. A key optical parameter is the refractive index (RI), which influences the absorbing and scattering properties of aerosols. This quantity is not measured directly, but it is obtained by fitting the measured scattering cross section and extinction cross section to a theoretical model using the RI as a fitting parameter. We used the Rayleigh–Debye–Gans (RDG) approximation to retrieve the complex RI of freshly emitted BB aerosol from two fuels (eucalyptus and olive) from Africa in the spectral range of 500–580 nm. Experimental measurements were carried out using cavity ring-down spectroscopy to measure extinction over the range of wavelengths of 500–580 nm and nephelometry to measure scattering at three wavelengths of 450, 550, and 700 nm for size-selected BB aerosol particles. The fuels were combusted in a tube furnace at a temperature of 800 °C, which is representative of the flaming stage of burning. Filter samples were collected and imaged using tunneling electron microscopy to obtain information on the morphology and size of the particles, which was used in the RDG calculations. The mean radii of the monomers were 27.8 and 31.5 nm for the eucalyptus and the olive fuels, respectively. The components of the retrieved complex RI were in the range of 1.31 ≤ n ≤ 1.56 and 0.045 ≤ k ≤ 0.468. The real and complex parts of the RI increase with increasing particle mobility diameter. The real part of the RI is lower, and the imaginary part is higher than what was recommended in literature for black carbon generated by propane or field measurements from fires of mixed wood samples. Fuel dependent results from controlled laboratory experiments can be used in climate modeling efforts and to constrain field measurements from biomass burning. View Full-Text
Keywords: refractive index; biomass burning aerosols; RDG theory; sub-Saharan African biomass fuels refractive index; biomass burning aerosols; RDG theory; sub-Saharan African biomass fuels
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Sarpong, E.; Smith, D.; Pokhrel, R.; Fiddler, M.N.; Bililign, S. Refractive Indices of Biomass Burning Aerosols Obtained from African Biomass Fuels Using RDG Approximation. Atmosphere 2020, 11, 62.

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