| Dissolved black carbon(DBC) is an important component of dissolved organic carbon and ubiquitous in natural waters. Its molecular structure is consisted of a condensed aromatic core usually contains four to seven aromatic rings and peripherally substituted by hydrophilic function groups. Dissolved black carbon is carbon-rich and hydrogen-deficient substances and its ratios of H/C and O/C are distinctly different from the common components derived from biomass and humic substances. The condensed aromatic structures are the characteristical molecular structures derived from heating-processes(commonly combustion), whose by-production is well known as “black carbon”. Black carbon is hydrophobic and carbon-rich particles. A series of compounds containing the characteristic structures of black carbon in dissolved organic carbon is given the name of “dissolved black carbon”. It has been proven that the precursor of dissolve black carbon is black carbon. Therefore, dissolved black carbon is a term to describe a series of condensed aromatic compounds which are derived from heating-processes(combustion) and are able to dissolve in water because of containing hydrophilic function groups. Dissolved black carbon has potential impacts on black carbon and carbon cycle, the translocation and biological availability of pollutants, the primary production of aquatic biological system, the quality of natural water and something-like these issues on the climate change, environment, biology and ecology. Dissolved black carbon has been a hot spot nowadays.Dissolved black carbon is the intermediate production of black carbon decomposition and water is the connector of the two major pools of black carbon, terrestrial soil and marine sediment, thus hydrosphere should be an important intermediate pool of black carbon. The North China Plane has high density emission of black carbon and the Bohai Sea is the major pool of pollutions including black carbon emitted from the North China Plane. To investigate the concentrations, distributions, spatial-temporal variations, source-sink and its control factors of dissolve black carbon in the river-sea water system of the Bohai sea, 1155 seawater samples were taken in different seasons and water depth in the Bohai Sea and the Yellow Sea from 2010 to 2014, meanwhile 119 river water samples were taken in 45 rivers entering the Bohai Sea and the North Yellow Sea during the high flow, low flow and normal flow seasons. The concentrations of dissolved black carbon were measured by the benzene polycarboxylic acid method, a molecular marker method after establishment and optimization in our laboratory. The concentrations of organic carbon, particulate black carbon, particulate organic carbon and suspended particulate matter were also measured. The geochemistry dynamics of dissolved black carbon was discussed according the seawater temperature and salinity, water currents, river hydrodynamics, precipitation and something like these physical parameters. The river fluxes and the Bohai Sea inventory of dissolved black carbon were also estimated.The concentrations of dissolved black carbon in the rivers were ranged from 79.5 to 1328.0 μg L-1, with mean value of 341 ± 209.3 μg L-1 and median value of 303.3 μg L-1(n = 115), within the concentrations ranges of dissolved black carbon among the world rivers. And the concentrations of dissolved black carbon in the Bohai sea were ranged from 36.5 to 28.3 μg L-1, with mean value of 85.5 ± 27.1 μg L-1 and median value of 82.9 μg L-1(n = 846),higher than that in the Yellow Sea water, comparable to the coastal zone area seawater of America, but much higher than that in the open ocean seawater. The concentrations of dissolved black carbon in the rivers and the sea both had three-dimensional spatial-temporal variations, decreasing from river to coastal zone, offshore and to outer sea. The concentrations of dissolved black carbon in different river catchments had distinct spatial variations. It was obvious that the concentrations of dissolved black carbon in the rivers entering the Bohai Bay and the Laizhou Bay were much higher than those rivers entering the Liaodong Bay and North Yellow Sea along the Liaonig Province coastal line. And the average concentrations of dissolved black carbon in the rivers entering the Bohai Bay, Laizhou Bay, Liaodong Bay and the North Yellow Sea along the Liaonig Province coastal line were 488.0 ± 219.1, 385.7 ± 192.1, 218.3 ± 87.0 and 124.7 ± 25.5 μg L-1 respectively. The concentrations of dissolved black carbon in the Bohai Bay and Laizhou Bay seawater were higher than that in the Liaodong Bay seawater, consistent with the concentrations difference of dissolved black carbon among the rivers. The average concentrations of dissolved black carbon were 105.1 ± 24.7、117.6 ± 33.2 and 90.9 ± 14.3 μg L-1 in the Bohai Bay, Laizhou Bay and Liaodong Bay respectively. What’s more, the concentrations of dissolved black carbon were higher in the Bays than the Central Bohai Sea and the Yellow Sea whose average concentrations of dissolved black carbon were 75.1 ± 19.5 and 56.4 ± 14.3 μg L-1 respectively. On contrast, the concentractions of dissolved black carbon had no significant difference in the vertical water column and the main reason was that the Bohai Sea is a shallow sea where the water can be easily vertically mixed well by wave and tide.On the temporal view, the concentractions of dissolved black carbon in the rivers were the highest in the normal flow season, second higher in the high flow season and the lowest in the low flow season. In late summer and early autumn season, the concentractions of dissolved black carbon in the Bohai Bay and the Laizhou Bay were higher in 2013( average concentractions were 136.2 ± 34.3 and 131.2 ± 30.7 μg L-1 respectively) than 2014( average concentractions were 107.4 ± 28.0 and 95.5 ± 12.1 μg L-1 respectively) mainly because that the river fluex of dissolved black carbon had interannual variation. The concentractions of dissolved black carbon in the Central Bohai Sea were significant higher in winter than the other three seasons. The average concentractions of dissolved black carbon in winter, spring, summer, and autumn were 87.0 ± 22.1, 73.9 ± 17.3, 73.7 ± 19.2 and 79.8 ± 21.9 μg L-1 respectively from the year of 2010 to 2011 in the Central Bohai Sea. These was the results of the enhancement of the prevailing northward wind of monsoon in winter which strengthened the coastal currents and speeded up the water exchange rate between the Bays and the Central Bohai Sea.The river fluxes of dissolved black carbon was estimated to 7.43 ± 1.51 Gg in the year of 2014, while the Yellow River contributed to 25.9%. The total water fluxes of the rivers entering the Boahai Sea and the North Yellow Sea was account for ca 0.84‰ of the whole world river water fluxes, carriling ca 0.28‰ of dissolved black carbon to the ocean transported by the whole rivers. The volume of dissolved black carbon in the Bohai Sea was estimated to 113.81 Gg. |
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