The mobility diameter of soot determines its angular light scattering distribution

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, Ds, and asymmetry parameter, g, and depend on agglomerate mobility, dm, and constituent primary particle, dp, size distributions. Here, discrete element modeling is interfaced with discrete dipole approximation to determine C in the range of dm = 60–450 nm with mean dp = 9–26 nm that are most prevalent in fire detection, air pollution and climate change. Increasing dm reduces the effective density, ρeff, and drastically increases Ds and g (by a factor of about 2–20), while increasing dp or its geometric standard deviation increases ρeff but only slightly decreases Ds (10–20%). Thus, the angular light scattering distribution of soot is largely determined by its dm. Currently, rather constant Ds and g are obtained for large agglomerates (dm ≥ 250 nm) by laser diagnostics based on the Rayleigh Debye Gans theory and used in climate models. This overestimates the dm for small soot agglomerates by up to a factor of four and underestimates their radiative forcing efficiency by 10%. So, relations between Ds, g and dm are derived here and validated with data from our premixed flames and literature diffusion flame and field data. These relations cover the evolution of Ds and g with dm and nicely converge to the constant Ds for dm ≥ 250 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.