Revised black carbon assessment using benzene polycarboxylic acids

Black carbon (BC), the ubiquitous stable product of incomplete combustion, is believed to be a potential sink for atmospheric CO2 and therefore a contributor to the Earth’s radiative heat balance. Nevertheless, analytical procedures to measure BC are inconsistent, giving a non-systematic variation by factors of 14–571 for estimates of its content in soil. We hypothesized that the HCl used to isolate benzene polycarboxylic acids (BPCAs) as markers for BC helps form these compounds, which could then cause an overestimation of the BC content of the soil. We found that indeed up to 90% of BPCA yields may be attributed to this HCl pre-treatment. To correct this error we developed a revised method that uses BPCAs as BC markers but which allows us to eliminate any confusion in the results. This aim was achieved by digestion with 4 M trifluoroacetic acid (TFA). After oxidation with HNO3, the BPCAs were purified using a cation exchange resin and derivatized to form the trimethylsilyl derivatives. Analyses were performed using a gas chromatograph (GC) equipped with a flame ionization detector (FID); constant linearity was obtained at 7 ng BPCA injection amount and peak purity was determined using mass spectrometry (MS). The recovery of the BPCAs averaged 93.5 ± 5.1% for pure standards and 95.0 ± 3.6% for spiked charred plant material. The contribution of BPCAs from aspergillin to soil organic carbon was estimated to be negligible. No close correlation between the results obtained with the original method and our revised procedure was observed.