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

Abstract

Aged black carbon (BC) from biomass burning is difficult to identify chemically when it ismixed with other forms of soil organic matter (SOM). As a consequence, the natural abundanceof aged and degraded BC is unknown.We carried out a molecular characterisation of up to 7000 yr old charcoal and NaOHextractableSOM 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 NMRin conjunction with a molecular mixing model (MMM), and the use of BPCAs.Not surprisingly, the charcoal consisted for the most part of chemically distinctive BCmoieties, as concluded from CP and DP NMR–MMM (>72% BC-inherent C) and the BPCAmethod (30–40%). Charcoal produced mainly benzene, toluene and polycyclic aromatichydrocarbons (PAHs) upon pyrolysis and THM. The SOM was a mixture of BC-derived (producingbenzene, toluene, PAHs and benzonitrile on pyrolysis) and non-BC lipid, carbohydrateand 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 suggestedthat the SOM BC was a partially oxidised, N-containing, weakly condensed aromaticnetwork. Apart from increased oxidation with age, 700 and 7000 yr old BC gave similarmolecular fingerprints. The results contribute to our knowledge of the chemistry of BC, apotential climate mediator, and its fate in the terrestrial environment.