Title: Measurement and modeling of the multiwavelength optical properties of uncoated flame-generated soot

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 and then thermally denuded, leading to the collapse of the initiallylacy, 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 $\>~160$nm, 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. Mietheory 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 $x=\mathit{\pi }{d}_{\text{p}}/\mathit{\lambda }$, where d_p 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 under predictions 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.

Authors:

Forestieri, Sara D. ^[1]; Helgestad, Taylor M. ^[1]; Lambe, Andrew T. ^[2]; Renbaum-Wolff, Lindsay ^[3]; Lack, Daniel A. ^[4]; Massoli, Paola ^[3]; Cross, Eben S. ^[5]; Dubey, Manvendra Krishna  ^[6];

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Mazzoleni, Claudio  ^[7];

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Olfert, Jason S. ^[8]; Sedlacek III, Arthur J.  ^[9];

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Freedman, Andrew  ^[3];

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Davidovits, Paul ^[10]; Onasch, Timothy B.  ^[2];

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Cappa, Christopher D.  ^[1]

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+ Show Author Affiliations

1. Univ. of California, Davis, CA (United States)
2. Aerodyne Research Inc., Billerica, MA (United States); Boston College, MA (United States)
3. Aerodyne Research Inc., Billerica, MA (United States)
4. NOAA Earth System Research Laboratory, Boulder, CO (United States); Univ. of Colorado, Boulder, CO (United States)
5. Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)
6. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
7. Michigan Technological Univ., Houghton, MI (United States)
8. Univ. of Alberta, Edmonton, AB (Canada)
9. Brookhaven National Lab. (BNL), Upton, NY (United States)
10. Boston College, MA (United States)

Publication Date:

2018-08-22

Research Org.:

Los Alamos National Lab. (LANL), Los Alamos, NM (United States)

Sponsoring Org.:

USDOE Office of Science (SC). Biological and Environmental Research (BER) (SC-23)

OSTI Identifier:

1557768

Report Number(s):

LA-UR-19-22772
Journal ID: ISSN 1680-7324

Grant/Contract Number:  

89233218CNA000001; FG02-05ER63995; SC0006980; SC0011935

Resource Type:

Accepted Manuscript

Journal Name:

Atmospheric Chemistry and Physics (Online)

Additional Journal Information:

Journal Name: Atmospheric Chemistry and Physics (Online); Journal Volume: 18; Journal Issue: 16; Journal ID: ISSN 1680-7324

Publisher:

European Geosciences Union

Country of Publication:

United States

Language:

English

Subject:

54 ENVIRONMENTAL SCIENCES; 58 GEOSCIENCES; Energy Sciences; Soot; Climate; Optical Properties; Refractive Index; Radiative Transfer