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

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.