Soot light absorption and refractive index 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 grows, its morphology changes affecting its optical or radiative properties. Here, the impact of soot maturity on its light absorption is investigated by coupling Discrete Element Modeling (DEM) 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, RI, for mature soot yields constant average and absorption function overestimating the nascent soot light absorption up to 75%. The RI is interpolated between those of nascent and mature soot for wavelengths, lambda = 532 and 1064 nm to account for quantum confinement and evolving number of clustered sp(2)-bonded rings that affect the optical band gap, E-g. This results in excellent agreement of the DEM-derived evolutions of <MAC>, <E> and ratio R= e<E(lambda=532nm)>/<E(lambda=1064nm)> with the corre- sponding LII measurements in methane and ethylene premixed flames. The nascent soot E-g decreases during aging and agglomeration, increasing <MAC> and <E> by 65%. The R decreases from 1.34 to 0.95 by aging and coagulation and slowly converges to the asymptotic 0.89 of mature soot measured in diffusion flames. The good agreement between DEM and LII data confirms that soot dynamics by surface growth and agglomeration strongly correlate with soot maturity, composition and RI that are essential for quantifying soot light absorption and scattering. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.