Light absorption by carbonaceous particles: An investigative review

The optical properties of the light-absorbing, carbonaceous substance often called "soot," "black carbon," or "carbon black" have been the subject of some debate. These properties are necessary to model how aerosols affect climate, and our review is targeted specifically for that application. We recommend the term light-absorbing carbon to avoid conflict with operationally based definitions. Absorptive properties depend on molecular form, particularly the size of sp²-bonded clusters. Freshly-generated particles should be represented as aggregates, and their absorption is like that of particles small relative to the wavelength. Previous compendia have yielded a wide range of values for both refractive indices and absorption cross section. The absorptive properties of light-absorbing carbon are not as variable as is commonly believed. Our tabulation suggests a mass-normalized absorption cross section of 7.5 ± 1.2 m²/g at 550 nm for uncoated particles. We recommend a narrow range of refractive indices for strongly-absorbing carbon particles, of which the highest is 1.95-0.79i. Our refractive indices are consistent with most measurements reported in the literature, and values used in present-day climate modeling are in error. Realistic refractive indices underpredict measured absorption by about 30% when used with common theories for spherical particles or aggregates. Field programs since about 1970 have measured quantities relevant to light absorption, but have only recently made enough measurements to isolate the light-absorbing carbonaceous component and determine its absorptive properties.