Black carbon assessment using benzene polycarboxylic acids: Limitations for organic-rich matrices

For the assessment of black carbon (BC), its oxidation to benzene polycarboxylic acids (BPCAs) is an established method. However, doubts about biological precursors remain and not all published data were obtained at low carbon concentration. We hypothesised that a considerable proportion of BC may be produced during sample treatment in the presence of a high amount of organic carbon (OC). We therefore tested whether and to which degree (i) BC-free material from stems of Zea mays L. (maize straw) and leaves of Capsicum annuum L. (bell pepper), as well as (ii) cyclic and non-cyclic carbon forms (chlorophyllin, ellagic acid and β-carotene) afford BPCAs when method protocols are overloaded with a sample above the recommended amount of 5 mg OC. The results showed that small amounts (< 2 g/kg OC) of BPCAs with three and four carboxyl groups may be formed even at low sample weight (< 5 mg OC), thereby falsely representing biological BC production. When this threshold was exceeded, all BPCA forms were detected. The artificial BPCA production yield in g OC increased with increasing amount of OC (R2 ⩾ 0.81), adding up to 8.7 g/kg OC (19.7 g BC/kg OC) artificial production. We therefore strongly recommend that a threshold of 5 mg OC sample concentration be maintained in future studies and that future BC assessments be restricted to BPCAs with five and six carboxyl groups. This constrains the application of the BPCA method for organic rich samples and for samples expected to contain a relatively low amount of BC.