Benzene polycarboxylic acids as molecular markers of black carbon: Progresses and challenges

Black carbon (BC) is generated as a result of the pyrolysis of biomass and fossil fuels. Different approaches have been taken to analyse BC in the environment, including thermal, optical and chemical methods. The chemical approach which uses benzene polycarboxylic acids (BPCAs) as molecular markers of BC has gained popularity within the scientific community recently. These pyrogenic molecular markers can be used to reconstruct ancient fire history and human presence. Here we review the development of the BPCA protocols for the analysis of BC and the previous studies that have used these methods. Additionally, this review explores the biogeochemical factors that influence the content and composition of BPCAs, which in turn affect the sources attributed to BC. These factors include the generation temperature of char, photodegradation, biodegradation and the interference of non-pyrogenic organic matter (OM) in BPCA-BC analysis. Different combustion temperatures can yield charred BC with varying degrees of aromatic condensation throughout the BC continuum, while aged soot-BC undergoes photochemical degradation, causing the loss of its original condensed aromatic structure. Photodegradation reduces the degree of BC condensation by preferentially breaking down the most condensed forms, whereas biodegradation primarily mineralizes the smaller and more biolabile BC. Non-pyrogenic sources, such as humic acids (HAs), have been found to contribute up to 25% of BPCA-BC in soil, and their presence can lead to overestimations of BC. Future research should focus on calibrating contemporary BPCA protocols using known reference materials and investigating the role of non-pyrogenic OM in BPCA-BC analysis.