Dissolved black carbon concentrations in suspended particulate matter from the Bohai Sea

Dissolved black carbon (DBC) includes the water-soluble component of black carbon and/or its degradation products. It is a hydrophobic organic compound due to its thick cyclic aromatic structure and can therefore be present as particulate DBC (PDBC) on suspended particulate matter or particulate organic matter. PDBC is not equivalent to particulate black carbon (PBC) and is thus often overlooked when researchers measure PBC in particulate matter using current conventional methods. Consequently, the amount of DBC in nature is likely underestimated in many studies and the reliability of current findings on the geochemical behavior of DBC is subject to challenge. In this study, we attempted to draw the solid-liquid partition coefficient (K_d, PDBC/DBC) based on detailed data on PBC, DBC, particulate organic carbon, total suspended solids, dissolved polycyclic aromatic hydrocarbons, and particulate polycyclic aromatic hydrocarbons in the surface waters of the Bohai Sea. The obtained K_d was in the range of 0.018–0.072. Accordingly, the concentrations of PDBC were 1.637–6.449 µg C L⁻¹. The estimated inventory of PDBC on suspended particulate matter in the Bohai Sea was 2.049–8.194 Gg, which was lower than that of PBC (15.16 Gg)but of the same order of magnitude. These results indicate that PDBC is an important form of DBC in the Bohai Sea. Quantitative results of DBC fluxes between the dissolved and particulate phases will benefit our understanding of DBC transport and fates in the ocean.