Effects of mixing on extinction by carbonaceous particles

Reported values for the absorption cross section of particulate carbon per unit mass range from under 4 to over 20 m 2 /g, and the intermediate value of 10 m 2 /g is used by many as a standard gram‐specific absorption cross section for atmospheric soot. In order to better understand the possible variations in absorption by atmospheric carbon, we reevaluated its optical properties in terms of the material composition and morphology of soot and the electrodynamics of spherules agglomerated into loose (ramiform) aggregates. Primary particles ranging in composition from paracrystalline graphite to low‐density air/graphite volume mixtures are considered. The effects on extinction efficiency of aggregation and of internal mixing of carbon with sulfate are considered in detail. We also compare our results with estimates of specific absorption of internally mixed soot that are based on several homogeneous mixing rules (effective medium approximations), On the basis of our modeling of the optical properties of aggregates of graphitic carbon grains, we conclude that 10 m 2 /g may be over 50% too high in many cases, and we suggest that the mass absorption coefficient for the light‐absorbing carbon in diesel soot at a wavelength of 0.550 μm may often be less than 7 m 2 /g, although variations in optical constants and, especially, the specific gravity of the absorbing material make it difficult to assign a specific numerical value. Adhesion of carbon grains to sulfate droplet surfaces is expected to enhance their absorption by no more than about 30%. Soot randomly positioned within droplets, however, can display averaged absorption enhancement factors of about 2.5–4 for hosts with refractive indices ranging from 1.33–1.53, respectively, and radii ≳0.20 μm. Nonetheless, calculations indicate that for realistic dry particle populations, α a < 10 m 2 /g for graphitic carbon in the atmosphere unless (1) most of it is encapsulated, and (2) the geometric mean radius of the hosts is larger than about 0.06 μm (which corresponds to a mass median diameter of 0.34 μm). These results suggest the importance of the determination of the physical state of the soot particles and their immediate environment when ascribing characteristic values for their absorption and scattering efficiencies.