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

Forestieri, Sara D.; Helgestad, Taylor M.; Lambe, Andrew T.; Renbaum-Wolff, Lindsay; Lack, D. A.; Massoli, P.; Cross, Eben S.; Dubey, Manvendra K.; Mazzoleni, Cláudio; Olfert, Jason S.; Freedman, Andrew; Davidovits, P.; Onasch, T. B.; Cappa, Christopher D.

doi:10.5194/acp-2018-306

Cited by 3 publications

(4 citation statements)

References 33 publications

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“…We suggest that a reasonable precision‐limited uncertainty on the MAC BC,ref values for both campaigns is 10%. It is noteworthy that the derived campaign‐specific MAC BC,ref values are in reasonable agreement with laboratory measurements (Cross et al, ; Forestieri et al, ) and literature assessments (Bond & Bergstrom, ). They are, however, substantially smaller than recently determined from filter‐based measurements made across Europe for ambient particles, where the MAC at 637 nm was reported as 10.0 m 2 /g, which would correspond to an MAC at 532 nm of 12 m 2 /g assuming an AAE = 1.…”

Section: Resultssupporting

confidence: 83%

Light Absorption by Ambient Black and Brown Carbon and its Dependence on Black Carbon Coating State for Two California, USA, Cities in Winter and Summer

et al. 2019

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Observations from a wintertime and summertime field campaign are used to assess the relationship between black and brown carbon (BC and BrC, respectively) optical properties and particle composition and coating state. The wintertime campaign, in Fresno, CA, was impacted by primary emissions from residential wood burning, secondary organic and inorganic particle formation, and BC from motor vehicles. Two major types of BrC were observed in wintertime. One occurred primarily at night—the result of primary biomass burning emissions. The second was enhanced in daytime and strongly associated with particulate nitrate and the occurrence of fog. The biomass‐burning‐derived BrC absorbed more strongly than the nitrate‐associated BrC but had a weaker wavelength dependence. The wintertime BC‐specific mass absorption coefficient (MACBC) exhibited limited dependence on the ensemble‐average coating‐to‐BC mass ratio (Rcoat‐rBC) at all wavelengths, even up to Rcoat‐rBC of ~5. For the summertime campaign, in Fontana, CA, BC dominated the light absorption, with negligible BrC contribution even after substantial photochemical processing. The summertime MACBC exhibited limited dependence on Rcoat‐rBC, even up to ratios of >10. Based on the four classes of BC‐containing particles identified by Lee et al. (2017, https://doi.org/10.5194/acp-17-15055-2017) for the summertime measurements, the general lack of an absorption enhancement can be partly—although not entirely—attributed to an unequal distribution of coating materials between the BC‐containing particle types. These observations demonstrate that in relatively near‐source environments, even those impacted by strong secondary aerosol production, the ensemble‐average, mixing‐induced absorption enhancement for BC due to coatings can be quite small.

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Section: Resultssupporting

confidence: 83%

Light Absorption by Ambient Black and Brown Carbon and its Dependence on Black Carbon Coating State for Two California, USA, Cities in Winter and Summer

et al. 2019

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“…The data associated with this paper are archived at the UC Davis DASH data repository and are available for download from https://doi.org/10.25338/B8JP4V (Forestieri and Cappa, 2018).…”

Section: Resultsmentioning

confidence: 99%

“…Here, we have assumed that N spherule = m p /m spherule , where m p is the measured per-particle mass and m spherule is the mass of an individual spherule with d p = 20 nm (ethylene premixed flame) (Cross et al, 2010) or d p = 37 nm (methane diffusion flame) (Ghazi et al, 2013), again assuming a material density of 1.8 g cm −3 . Optimal (best-fit), theory-specific values of m and k were established by comparing the size-dependent observations of σ abs to calculations from either Mie theory or the RDG approximation by varying these parameters over the ranges 1.3 < m < 2.2 and 0.1 < k < 1.5 and determining the minimum value of the reduced chi-square statistic:…”

Section: Refractive Index Retrievalmentioning

confidence: 99%

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

et al. 2018

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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 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 >∼ 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 532 nm for larger particles was 9.1 ± 1.1 m 2 g −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 d p /λ, 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 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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“…Data availability. The data associated with this paper are archived at the UC Davis DASH data repository and are available for download from https://doi.org/10.25338/B8JP4V (Forestieri and Cappa, 2018).…”

Section: Resultsmentioning

confidence: 99%

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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 >∼ 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 532 nm for larger particles was 9.1 ± 1.1 m 2 g −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 d p /λ, 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 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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Measurement and modeling of the multi-wavelength optical properties of uncoated flame-generated soot

Cited by 3 publications

References 33 publications

Light Absorption by Ambient Black and Brown Carbon and its Dependence on Black Carbon Coating State for Two California, USA, Cities in Winter and Summer

Light Absorption by Ambient Black and Brown Carbon and its Dependence on Black Carbon Coating State for Two California, USA, Cities in Winter and Summer

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

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