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Formation of refractory black carbon by SP2-induced charring of organic aerosol

Black carbon (BC) in the atmosphere continues to be a focus of research because its light-absorptive properties put it second only to CO2 as a warming agent of Earth’s climate. Towards this end, the measurement of ambient BC has been aided greatly by the development of the Single Particle Soot Photometer (SP2)—an instrument that detects refractory black carbon (rBC) through laser-induced incandescence (Schwarz et al., 2006). Potential interference from other substances that can incandesce under 1064 nm illumination (e.g., some metals and minerals) is mitigated through the use of spectral bandpass filters that allow determination of the color temperature of incandescence to ensure that the SP2 remains highly selective to rBC. Here, we report on the detection of rBC that is produced through SP2 laserinduced charring (i.e., carbonization) of organic aerosols. Nigrosin and fulvic and humic acids—non-BCcontaining materials—were used as a surrogates for light-absorbing organic aerosols. The color temperature of the detected particles originating from these charred organic aerosols is near that of carbon black, fullerene soot, and ethylene soot, indicating that the incandescent particles are composed of rBC. Failure to properly account for this heretofore unidentified artifact of the SP2 will lead to an overestimate of rBC loadings, which could, in turn, impact aerosol radiative forcing model predictions. Methods