Buseck et al. (Atmos. Chem. Phys. Discuss., 12, 24821-24846, 2012 www.atmos-chem-phys-discuss.net/12/24821/2012/) distinguish among substances that have been variously called "black carbon," "elemental carbon," and "soot." We suggest that these names refer to different attributes of substances, respectively, optical properties, composition, and formation process. Black carbon is a category of substances consisting mostly of carbon and having a very low reflectivity throughout the visible spectrum and thus appearing black. Elemental carbon denotes a set of substances consisting entirely (or almost entirely) of carbon, not all of which are highly absorbing throughout the visible. Soot, which encompasses a broad set of substances produced by incomplete combustion of carbonaceous fuel, is defined by formation mechanism, not optical properties, composition, or structure. The term "nanosphere soot" proposed by Buseck et al. to denote nanospheres of concentrically wrapped, graphene-like layers of carbon is neither comprehensive nor exclusive.
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