The Mobility Diameter of Soot Determines Its Angular Light Scattering Distribution

GEORGIOS A. KELESIDIS, Patrizia Crepaldi, Martin Allemann, Aleksandar Duric, Sotiris Pratsinis, ETH Zurich, Switzerland

     Abstract Number: 170
     Working Group: Carbonaceous Aerosol

Abstract
Characterization of soot agglomerates often relies on the angular distribution of their light scattering cross-section, C, that is based on the structure factor exponent, D_s, and asymmetry parameter, g, and depend on agglomerate mobility, d_m, and constituent primary particle, d_p, size distributions. Here, discrete element modeling (DEM) [1] is interfaced with discrete dipole approximation (DDA) [2] to determine C in the range of d_m= 60 - 450 nm with mean d_p = 9 - 26 nm that are most prevalent in fire detection, air pollution and climate change. Increasing d_m reduces the effective density, ρ_eff, and drastically increases D_s and g (by a factor of about 2-20), while increasing d_p or its geometric standard deviation, σ_g,p, increases ρ_eff but only slightly decreases D_s (10 - 20 %). Thus, the angular light scattering distribution of soot is largely determined by its d_m. Currently, constant D_s and g (obtained for large agglomerates, d_m > 200 nm) are used in laser diagnostics and climate models. This overestimates the d_m for small soot agglomerates by up to a factor of four and underestimates their radiative forcing efficiency by 10 %. So, relations between D_s, g and d_m are derived here and validated with data from our premixed flames [3] and literature diffusion flame and field data. These relations cover the evolution of D_s and g with d_m and nicely converge to the constant D_s for d_m > 200 nm. As such, they can facilitate the characterization of soot agglomerates by light scattering and help to quantify accurately the soot contribution to global warming.

References:
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[2] G.A. Kelesidis, S.E. Pratsinis, Soot light absorption and refractive index during agglomeration and surface growth, Proc. Combust. Inst. 37 (2019) 1177-1184, doi.org/10.1016/j.proci.2018.08.025.
[3] G.A. Kelesidis, M.R. Kholghy, J. Zuercher, J. Robertz, M. Allemann, A. Duric, S.E. Pratsinis, Light scattering from nanoparticle agglomerates, Powder Technol. 365 (2020) 52-59, doi.org/10.1016/j.powtec.2019.02.003.