PAHs in the Northern South China Sea: Horizontal transport and downward export on the continental shelf

To investigate horizontal transport and particulate export of semivolatile pollutants on the continental shelf or marginal sea, surface and depth water samples were collected across the northern South China Sea (SCS) and analyzed for polycyclic aromatic hydrocarbons (PAHs). The total concentrations of Σ15PAH in seawater were in the range of 1.6-14 ng L−1 (mean 5.5 ng L−1), and the offshore decreasing trend of PAHs revealed intense terrigenous transport on the continental shelf, which was originated from combustion activity. Vertically, PAHs in the particulate phase followed surface-enrichment and depth-depletion pattern with a concentration maximum (2.9 ng L−1) at the biomass maximum, while the dissolved PAH concentrations showed a minimum (1.5 ng L−1) at the same depth. Particulate export fluxes of PAHs were between 24 and 195 ng m−2 d−1 in the northern SCS based on 238U/234Th equilibria, which is suggested appropriate to quantify their downward flux in the upper water column. Efficient oceanic biological pump drives PAHs downward export to the deep ocean in the northern SCS.