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. Optical properties of flame-generated black carbon (BC) containing sootparticles were quantified at multiple wavelengths for particles producedusing two different flames: a methane diffusion flame and an ethylenepremixed flame. Measurements were made for (i) nascent soot particles,(ii) thermally denuded nascent particles, and (iii) particles that werecoated 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 betweenflames for similar conditions. For mature soot, here corresponding toparticles with volume-equivalent diameters > ∼ 160  nm, the MAC andabsorption Angstrom exponent (AAE) values were independent ofparticle collapse while the single-scatter albedo increased. The MAC valuesfor these larger particles were also size-independent. The mean MAC value at532 nm for larger particles was 9.1±1.1  m 2  g −1 , about17 % higher than that recommended by Bond and Bergstrom (2006), and theAAE 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 atall wavelengths for larger particles (with x>0.9 ) independent of thecomplex RI used, while RDG provides good agreement. (The dimensionless sizeparameter x = π d p / λ , where dp is particlediameter and λ is wavelength.) Importantly, this implies that theuse of Mie theory within air quality and climate models, as is common, likelyleads to underpredictions in the absorption by BC, with the extent ofunderprediction depending on the assumed BC size distribution and complex RIused. We suggest that it is more appropriate to assume a constant,size-independent (but wavelength-specific) MAC to represent absorption byuncoated BC particles within models.

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