Quantification of black carbon in marine systems using the benzene polycarboxylic acid method: a mechanistic and yield study

Abstract

Quantification of black carbon (BC), carbonaceous material of pyrogenic origin, has typically required either chemical or thermal oxidation methods for isolation from heterogeneous matrices, such as sediment or soil. The benzene polycarboxylic acid (BPCA) method involves chemical oxidation of aromatic structures, such as those in BC, into BPCAs. We revised the BPCA method with the intent to quantify BC in marine dissolved organic carbon (DOC). As part of this work, we evaluated the mechanism and yield of the method using nine polycyclic aromatic hydrocarbons (PAHs) and six BC reference materials. After 8 h of oxidation at 180°C, the average carbon yield was 26 ± 7% C and was not correlated to the molecular weight of the PAH oxidized. The majority of observed BPCAs were nitrated, which has serious implications for the quantification of BC. Smaller PAHs favor the formation of less substituted BPCAs, whereas larger PAHs, such as coronene, favor the formation of more fully substituted BPCAs. Time‐course experiments revealed variations of BPCA distributions over time, favoring less substituted BPCAs with longer oxidation times, whereas the carbon yield exhibited little variation. No decarboxylation of fully substituted mellitic acid (B6CA) was observed during time course experiments. Using the model compound anthracene, a potential internal standard, we propose a mechanism for the oxidation reaction based on time‐course experiment data. Quantification of BC in reference materials revealed that this revision of the BPCA method is significantly more efficient than previous versions and is effective for quantifying both char and soot BC.