Molecular Characterization by Ultrahigh Resolution Mass Spectrometry of Dissolved Black Carbon-like Molecules in Summer Along the Pearl River Estuary, China

Significant amounts of organic carbon derived from black carbon (BC) are transported annually through the Pearl River Estuary (PRE) in Southern China, into the continental shelf of the South China Sea. However, molecular composition and distribution of dissolved black carbon (DBC) compounds along the PRE are still rarely studied. In this work, dissolved organic matter isolated and extracted by solid phase extraction from water samples collected in summer along the PRE are studied and characterized by negative electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), and with special attention paid to DBC molecules. The numbers and relative intensities of identified DBC molecules (CHOx class) decrease gradually with the salinity gradient from the upper reach to the open water. The DBC molecules accumulated with relatively high intensities in O7 and O9 classes in Site A and slightly shifted to O5, O7-8 classes in Site E along the PRE. Possible three or four rings of aromatic ring structure of DBCs, with hydroxyl and carboxy substituent groups are proposed. The relatively smaller sizes of DBCs are likely to have originated from relatively larger oxidized condensed aromatic structures of black carbon. The trend of DBC distribution along the estuary could be explained by DOM degradation in the mixing zone.