Challenges in the analysis of dissolved black carbon in natural waters

Black carbon (BC) is the organic residue produced from the incomplete combustion of biomass and fossil fuels. Dissolved black carbon (DBC) is operationally defined as the BC fraction that is water-soluble and able to pass through a filter of 0.1-0.70 µm. DBC acts as a crucial flux linking the two primary end-member BC pools-the soils and the ocean sediments, and is also an important component of dissolved organic carbon. Therefore, analysis of the molecular structures, concentrations, and sources of DBC in natural environments is essential for assessing the global carbon cycle. However, the chemical heterogeneity of DBC makes it challenging to discern and quantify in natural waters. Here, we reviewed the major analytical techniques for DBC and outlined promising methodological frameworks for future research. The contributions of nuclear magnetic resonance and Fourier Transform-Ion Cyclotron Resonance-Mass Spectrometry (FT-ICR-MS) in the structural characterization of DBC were evaluated. As the two primary methods for quantifying the condensed aromatic fraction of DBC, the chemo-thermal oxidation and the digestion-based benzenepolycarboxylic acids methods were comprehensively introduced. Intrinsic benzenepolycarboxylic acids in DBC have the potential to function as digestion-free markers for quantifying total DBC and assessing its cycling. Developing complementary techniques for FT-ICR-MS and identifying robust molecular markers for DBC in future research will be crucial for advancing DBC analysis and elucidating its global cycling.