Knowledge of the optical properties of soot black carbon (BC) is required for the prediction of the radiative effects of freshly-emitted and aged BC particles. Here we review BC mass absorption cross section (MAC) and absorption function E(m) measurements, focusing on freshly-emitted BC. First, we review recently reported MACs at 550 nm wavelength as obtained from direct measurements of particulate absorption and mass concentration; we find an average of 8.0 ± 0.7 m2/g from ten measurements, not significantly higher (p > 0.26) than the widely used MAC of 7.5 ± 1.2 m2/g recommended by Bond and Bergstrom [Bond, T. C., and R. W. Bergstrom. 2006. Light absorption by carbonaceous particles: An investigative review. Aerosol Sci. Technol. 40(1):27–67]. Second, we review recently reported E(m), whose retrieval is more complex due to the need to combine measurements with numerical models to estimate the contribution of scattering to extinction. Third, we review recent numerical studies that have aimed to predict the BC MAC using various complex refractive indices (m = n + ik). Most of these studies have used m = 1.95 + 0.79i recommended by Bond and Bergstrom (2006), yet failed to predict a MAC as high as 7.5 or 8.0 m2/g at 550 nm wavelength. Fourth, we summarize a selected range of alternative values of m that has been reported by recent studies and place them in the context of measurements using a contour plot of E(m) on the n–k plane. We show that the widely used m = 1.95 + 0.79i corresponds to an E(m) that is too low to be consistent with the measured MAC values. We conclude that the E(m) of BC in the visible and near infrared should be greater than 0.32, and that the commonly used BC models or the refractive index, or both, are still in need of improvement.

Review of recent literature on the light absorption properties of black carbon: Refractive index, mass absorption cross section, and absorption function

We performed a study of the evolution of soot composition and fine structure, i.e., maturity level, in an atmospheric ethylene-air diffusion flame. We used laser-induced incandescence (LII) to provide information about maturity level of the bulk primary particle and X-ray photoelectron spectroscopy (XPS) to provide complementary information about particle-surface-maturity level. The results demonstrate that the bulk material and the particle surface evolve separately in the flame. Increased soot-maturity level is associated with increased long-range order of the particle fine structure. This increased order leads to an increase in the absorption cross-section in the visible and near-infrared and a shift of the absorption to longer wavelengths with increasing maturity level of the bulk particle. These trends result in a decrease in the dispersion exponent (ξ) and increase in the absorption cross-section scaling factor (β), as inferred from LII measurements. LII measurements demonstrate that bulk-ma...

https://www.tandfonline.com/doi/pdf/10.1080/02786826.2017.1355047

Evolution of maturity levels of the particle surface and bulk during soot growth and oxidation in a flame

This paper addresses how well and under what conditions the Rayleigh-Debye-Gans (RDG) approximation describes scattering and absorption of light by fractal aggregates (FA) including soot. The RDGFA theory, which is the prevailing, first order description of this problem, has two assumptions: the monomers, or primary particles, of the aggregate scatter and absorb in the Rayleigh regime, and the aggregate scatters in the diffraction limit weighted by this Rayleigh scattering and absorbs as a system of independent monomer particles. The aggregates studied here are formed via Diffusion Limited Cluster Aggregation (DLCA) and have a fractal dimension D = 1.78 ± 0.04 and prefactor of k0 = 1.35 ± 0.10. The aggregates are a collection of monodisperse spherical monomers with point contacts. Optical calculations were performed with the multiple sphere T-matrix (MSTM) and DDSCAT codes for incident light polarized perpendicular to the scattering plane. The scattering considered is the forward scattering intensity and the angular scattering as parameterized by the scattering wave vector. The total absorption cross section for aggregates is also calculated. This work stresses the systematic study of the effects of the variables of monomers per aggregate, which ranged from one to 502, two monomer size parameters of 0.157 and 0.314, and a wide range of refractive index real and imaginary parts. It also considers soot refractive indices with three representative dispersions. A summary of results for both scattering and absorption includes deviations from RDGFA theory ranging as large as 35% with positive deviations increasing with the real part of the refractive index and negative deviations growing with the imaginary part. These deviation from the RDG limit are shown to be similar to deviations for spheres.

Light scattering and absorption by fractal aggregates including soot

Revealing soot maturity based on multi-wavelength absorption/emission measurements in laminar axisymmetric coflow ethylene diffusion flames

In this work, a mini-CAST soot generator was used to produce soot with different optical and physicochemical characteristics. Absorption Angstrom exponents (AAE) expressing the absorption wavelength dependence were assessed by multiwavelength in-situ and filter-based (aethalometer) laser extinction. The two optical techniques showed good agreement. For the chosen mini-CAST operating conditions, AAEs between 1 and 3.5 were found. Soot with high mass-fractions of organic carbon (OC) and pyrolytic carbon (PC) determined with thermal optical analysis were associated with AAEs significantly higher than 1. Heating to 250 and 500°C removed the majority of polycyclic aromatic hydrocarbons. However, the thermal-optical analysis revealed that OC and PC were abundant in the soot with AAE > 2 also after heating the aerosol. Analysis of mass absorption cross section ratios for elemental carbon and OC indicated that elevated AAEs also after heating to 500°C could be related to persistent OC and PC components and/or the refractory soot. By comparing the mini-CAST soot optical properties with soot properties derived from in-situ extinction measurements in a premixed flame, mini-CAST soot with a higher AAE could be identified as less mature soot.

https://www.tandfonline.com/doi/pdf/10.1080/02786826.2018.1457767

Investigation of the absorption Ångström exponent and its relation to physicochemical properties for mini-CAST soot

Soot optical properties determined by analyzing extinction spectra in the visible near-UV: Toward an optical speciation according to constituents and structure

C. Rozé

Papers

Soot optical properties determined by analyzing extinction spectra in the visible near-UV: Toward an optical speciation according to constituents and structure