Radiocarbon measurements of black carbon in aerosols and ocean sediments

Black carbon (BC) is the combustion-altered, solid residue remaining after biomass burning and fossil fuel combustion. Radiocarbon measurements of BC provide information on the residence time of BC in organic carbon pools like soils and sediments, and also provide information on the source of BC by distinguishing between fossil fuel and biomass combustion byproducts. We have optimized dichromate-sulfuric acid oxidation for the measurement of radiocarbon in BC. We also present comparisons of BC (14)C measurements on NIST aerosol SRM 1649a with previously published bulk aromatic (14)C measurements and individual polycyclic aromatic hydrocarbon (PAH) (14)C measurements on the same NIST standard. Dichromate-sulfuric acid oxidation belongs to the chemical class of BC measurement methods, which rely on the resistance of some forms of BC to strong chemical oxidants. Dilute solutions of dichromate-sulfuric acid degrade BC and marine-derived carbon at characteristic rates from which a simple kinetic formula can be used to calculate concentrations of individual components (Wolbach and Anders, 1989). We show that: ( 1) dichromate-sulfuric acid oxidation allows precise, reproducible (14)C BC measurements: (2) kinetics calculations give more precise BC yield information when per-formed on a % OC basis (vs. a % mass basis); (3) kinetically calculated BC concentrations are similar regardless of whether the oxidation is performed at 23degreesC or 50degreesC; and (4) this method yields (14)C BC results consistent with previously published aromatic (14)C data for an NIST standard. For the purposes of intercomparison, we report % mass and carbon results for two commercially available BC standards. We also report comparative data from a new thermal method applied to SRM 1649a, showing that thermal oxidation of this material also follows the simple kinetic sum of exponentials model, although with different time constants. Copyright (C) 2002 Elsevier Science Ltd.