(494f) From Nascent to Mature Soot Light Absorption during Agglomeration and Surface Growth

Optical characterization of soot (e.g. Laser Induced Incandescence, LII) and its impact on climate largely depend on light absorption. As soot is incepted by reactive dimerization of small aromatics¹ and grows by acetylene surface reactions² and agglomeration,³ its morphology changes affecting its optical properties. In the early stages of soot formation, small and compact nascent soot agglomerates² are made that are almost transparent to visible and near-infrared. Large, ramified mature soot agglomerates formed at longer residence times by coagulation in the absence of surface growth³ strongly absorb light from the visible to the near-infrared.

Here, the impact of soot morphology on its light absorption is investigated by coupling Discrete Element Modeling (DEM)² with Discrete Dipole Approximation (DDA) during soot surface growth and agglomeration. The Mass Absorption Cross-section, MAC, of nascent and mature soot agglomerates is estimated by DDA and validated against atomistic point dipole interactions and mesoscale DDA calculations. Using a refractive index, m, for mature soot yields constant average <MAC> and absorption function <E>, that overestimate the nascent soot light absorption up to 60 %. Interpolating m between those of nascent and mature soot for wavelengths, λ = 532 and 1064 nm results in excellent agreement of the DEM-derived evolution of <MAC> and <E> with LII measurements in premixed flames. The nascent soot mass-mobility exponent, D_fm, decreases by agglomeration, doubling the <MAC> and increasing <E> by 65 %. A quasi-asymptotic D_fm of 2.45 is attained faster by larger soot volume fractions, accelerating the increase of <MAC> and reaching nearly constant m = 1.61 – 0.63·i and 1.69 – 0.70·i at λ = 532 and 1064 nm, respectively, for residence time larger than 30 ms. Thus, accounting for the soot morphology dynamics during surface growth and agglomeration are essential to characterize soot by light absorption.

References:

[1] Kholghy, M. R.; Kelesidis, G.A.; Pratsinis, S.E., Reactive polycyclic aromatic hydrocarbon dimerization drives soot nucleation. Phys Chem Chem Phys 2018, in press.

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