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Measurement and modeling of the multiwavelength optical properties of uncoated flame-generated soot
Authors
SD Forestieri
TM Helgestad
+13 more
AT Lambe
L Renbaum-Wolff
DA Lack
P Massoli
ES Cross
MK Dubey
C Mazzoleni
JS Olfert
AJ Sedlacek
A Freedman
P Davidovits
TB Onasch
CD Cappa
Publication date
1 January 2013
Publisher
eScholarship, University of California
Abstract
© Author(s) 2018. 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 and then thermally denuded, leading to the collapse of the initially lacy, fractal-like morphology. The measured mass absorption coefficients (MACs) 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 Ångström exponent (AAE) values were independent of particle collapse while the single-scatter albedo increased. The MAC values for these larger particles were also size-independent. The mean MAC value at 532nm for larger particles was 9.1±1.1m2g-1, about 17% higher than that recommended by Bond and Bergstrom (2006), and the AAE was close to unity. 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 underpredicts 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. (The dimensionless size parameter x = π dp/λ, where dp is particle diameter and λ is wavelength.) Importantly, this implies that the use of Mie theory within air quality and climate models, as is common, likely leads to underpredictions in the absorption by BC, with the extent of underprediction 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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The University of Utah: J. Willard Marriott Digital Library
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The University of Utah: J. Willard Marriott Digital Library
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The University of Utah: J. Willard Marriott Digital Library
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The University of Utah: J. Willard Marriott Digital Library
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The University of Utah: J. Willard Marriott Digital Library
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The University of Utah: J. Willard Marriott Digital Library
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Last time updated on 01/01/2020