Light Absorption and Scattering by Coated Combustion Soot and Its Surrogates
Egor Demidov, Ali Hasani, Ogochukwu Enekwizu, Chong Qiu, and Alexei Khalizov
Atmospheric soot (or black carbon, BC) affects climate through solar light absorption and scattering, which depend strongly on the particle morphology and composition. Initially, BC particles are fractal aggregates of loosely connected spheres made of elemental carbon. Condensation of vapors on BC particles in the atmosphere not only changes their composition, but also induces compaction. Hence, processing of BC by vapor condensation changes its optical properties in two ways: particle volume increase by coating addition and restructuring of the BC core. In laboratory studies of BC optics, surrogates are often used in place of real flame-generated BC, including carbon black (CB) and nigrosin. CB is an industrial product, with particles made of elemental carbon spheres, but arranged in a compact configuration. Nigrosin is a water-soluble dye that upon nebulization produces spherical particles. We generated BC, compact CB, agglomerated CB (produced via coagulation of compact CB), and nigrosin aerosols, subjected them to supersaturated vapor of dioctyl sebacate (DOS) to form a coating layer, and investigated the morphological response of these four particle types to coating addition and removal. Using coated and coated-denuded aerosol particles with known composition and morphology, we also investigated light scattering and absorption enhancements to separately quantify the contributions of volume increase and restructuring. We compared experimental measurements against different particle optics models and found that it was crucial to account for larger, multiply charged particles present in the “monodisperse” aerosol. Producing a disproportionately large contribution to absolute values of optical cross sections, such particles also result in lesser optical enhancements due to slower growth of larger particles by vapor condensation. Absorption increases only due to increasing volume for all particles. Scattering increases not only from additional volume, but also due to restructuring for BC (strongly) and agglomerated CB (weakly). Finally, simple optical models like Mie are often sufficient to provide closure with experimental results, but only after accounting for multiply charged particles.