Characterisation of aged black carbon using pyrolysis-GC/MS, thermally assisted hydrolysis and methylation (THM), direct and cross-polarisation 13C nuclear magnetic resonance (DP/CP NMR) and the benzenepolycarboxylic acid (BPCA) method

Aged black carbon (BC) from biomass burning is difficult to identify chemically when it is mixed with other forms of soil organic matter (SOM). As a consequence, the natural abundance of aged and degraded BC is unknown. We carried out a molecular characterisation of up to 7000 yr old charcoal and NaOH-extractable SOM obtained from a colluvial soil in NW Spain using Curie point pyrolysis-GC/MS and THM. Black C was tentatively quantified using solid state 13C CP and DP NMR in conjunction with a molecular mixing model (MMM), and the use of BPCAs. Not surprisingly, the charcoal consisted for the most part of chemically distinctive BC moieties, as concluded from CP and DP NMR–MMM (>72% BC-inherent C) and the BPCA method (30–40%). Charcoal produced mainly benzene, toluene and polycyclic aromatic hydrocarbons (PAHs) upon pyrolysis and THM. The SOM was a mixture of BC-derived (producing benzene, toluene, PAHs and benzonitrile on pyrolysis) and non-BC lipid, carbohydrate and protein-derived OM. Benzenes, PAHs and benzonitrile accounted for 41–54% (pyrolysis-GC/MS) and 34–58% (THM) of total identified peak area in extractable SOM, comparable with the 32–41% BC obtained using DP NMR–MMM. Combined results suggested that the SOM BC was a partially oxidised, N-containing, weakly condensed aromatic network. Apart from increased oxidation with age, 700 and 7000 yr old BC gave similar molecular fingerprints. The results contribute to our knowledge of the chemistry of BC, a potential climate mediator, and its fate in the terrestrial environment.