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 ig is used by many as a standard gram-specific absorption cross sec tion fur atmospheric soot. In order to better understand the possible varia tions in absorption by atmospheric carbon, we reevaluated its optical prope rties in terms of the material composition and morphology of soot and the e lectrodynamics of spherules agglomerated into loose (ramiform) aggregates. Primary particles ranging in composition from paracrystalline graphite to l ow-density air/graphite volume mixtures are considered. The effects on exti nction efficiency of aggregation and of internal mixing of carbon with sulf ate are considered in detail. We also compare our results with estimates of specific absorption of internally mixed soot that are based on several hom ogeneous mixing rules (effective medium approximations). On the basis of ou r modeling of the optical properties of aggregates of graphitic carbon grai ns, 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 car bon in diesel soot at a wavelength of 0.550 mu m may often be less than 7 m (2)/g, although variations in optical constants and, especially, the specif ic 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 random ly positioned within droplets, however, can display averaged absorption enh ancement factors of about 2.5-4 for hosts with refractive indices ranging f rom 1.33-1.53, respectively, and radii greater than or similar to 0.20 mu m . Nonetheless, calculations indicate that for realistic dry particle popula tions, cu, < 10 m(2)/g for graphitic carbon in the atmosphere unless (1) mo st of it is encapsulated, and (2) the geometric mean radius of the hosts is larger than about 0.06 mu m (which corresponds to a mass median diameter o f 0.34 mu m). These results suggest the importance of the determination of the physical state of the soot particles and their immediate environment wh en ascribing characteristic values for their absorption and scattering effi ciencies.