Amplification of Light Absorption of Black Carbon by Organic Coating

Photo-absorption by black carbon (BC) aerosol is believed to be enhanced by the internal mixing of BC with volatile compounds. We investigate this effect quantitatively with laboratory experiments. Graphite particles are used as a surrogate for BC, with coatings of oleic acid and glycerol. The photo-absorption of coated graphite with known shell and core diameters are directly measured by a photo-acoustic technique. The size distribution and coating thickness are quantified using a single-particle soot photometer (SP2). The absorption is amplified by similar to 30% even with a small coating thickness (shell/core diameter ratio (D-p/D-c) = 1.2). The amplification reaches as high as 2 at D-p/D-c = 2. We calculate the amplification of the absorption using a shell/core model of Mie theory. The model generally reproduces the measured amplification well.