Black Carbon, Aerosols, and the Tooth Fairy

Black carbon (BC) is widely cited in the atmospheric literature as a major aerosol particle type with significant effects on climate warming. Several analytical techniques are used for its determination, primarily through optical absorption measurements. A recently developed and widely used method is single particle soot photometry (SP2). During attempts to obtain reliable BC samples for study using transmission electron microscopy (TEM), it became apparent that no such samples exist. Instead, surrogate materials such as graphite, fullerene, Aquadag, and perhaps other things are used as calibration standards. It became rapidly evident that BC is an inferred rather than actual, identifiable substance with distinct material properties other than its absorption spectrum and refractory character (accounting for the subset of refractory black carbon, or rBC). Since climate effects depend on optical properties, and these are estimated for BC, it may not be critical at this time whether or not it is a discrete material. However, the same term is also used by other environmental communities for things that are distinctly different. Such imprecision in terms can lead to unnecessary confusion. The situation is summarized in the Table. We propose that 1) the term BC should be restricted to light-absorbing refractory carbonaceous matter of uncertain character and 2) the uncertainty be stated explicitly. We also propose a more precise definition for soot as a specific material, which we call ns-soot, where "ns" refers to carbon nanospheres. We define ns-soot as particles that consist of nanospheres, typically with diameters <100 nm, that possess distinct structures of concentrically wrapped, graphene-like layers of carbon and with grape-like (acinoform) morphologies.;