(198aa) Carbon Black Morphology, Light Scattering and Direct Radiative Forcing

Carbon black (soot) nanoparticles generated by combustion sources involving transportation, power generation and fires form fractal-like, porous clusters (agglomerates).¹ Yet, their optical properties are calculated using the Mie theory for spheres neglecting the ramified agglomerate morphology and impeding the accurate estimation of carbon black environmental impact, characterization by optical diagnostics and selective detection with fire sensors.

Here, the carbon black morphology and radiative properties are investigated experimentally and simulated during surface growth and agglomeration by Discrete Element Modeling (DEM)² coupled with Discrete Dipole Approximation (DDA).³ The DEM-derived number of constituent primary particles (PPs), effective density and differential scattering cross-sections for vertically-, C_v, and horizontally-polarized incident light, C_h, are in excellent agreement with those measured here in different premixed ethylene flame conditions. In contrast, the Rayleigh Debye Gans (RDG) theory coupled with a scaling law for agglomerates with monodisperse, single, non-aggregated PPs⁴ underestimates the DEM-derived C_v and C_h by 60 %. The integral mass scattering (MSC) and absorption cross-sections (MAC) estimated by DEM-DDA account for the detailed carbon black fractal-like morphology, yielding average direct radiative forcing of 0.70 ± 0.07 W/m². This is 45 % smaller than the maximum global climate model predictions using the Mie theory for spheres. The C_v, C_h, MSC and MAC derived by RDG theory coupled with a DEM-derived scaling law accounting for PP aggregation and polydispersity¹ are in good agreement with those estimated by DDA. Thus, both DEM-DDA and the revised RDG theory can be used for the optimization of climate forcing estimations and carbon black optical diagnostics, as well as for selective detection by fire sensors.

References:

[1] Kelesidis, G. A.; Goudeli, E.; Pratsinis, S. E., Morphology and mobility diameter of carbonaceous aerosols during agglomeration and surface growth. Carbon 2017, 121, 527-535.

[2] Kelesidis, G. A.; Goudeli, E.; Pratsinis, S. E., Flame synthesis of functional nanostructured materials and devices: Surface growth and aggregation. Proc Combust Inst 2017, 36, 29-50.

[3] Kelesidis, G.A.; Pratsinis, S.E., From nascent to mature soot light absorption during agglomeration and surface growth. Proc Combust Inst 2018, under review.

[4] Sorensen, C. M., The Mobility of Fractal Aggregates: A Review. Aerosol Sci Technol 2011, 45, 765-779.