Combustion-driven particulate black carbon (BC) is a crucial slow-cycling pool in the organic carbon flux from rivers to oceans. Since the refractoriness of BC stems from the association of non-homologous char and soot, the composition and source of char and soot must be considered when investigating riverine BC. Samples along the Yangtze river continuum during different hydrological periods were collected in this study to investigate the association and asynchronous combustion drive of char and soot in BC. The results revealed that BC in Yangtze River, with higher refractory nature, accounts for 13.73±6.89% of organic carbon, and soot occupies 37.53±11.00% of BC. The preponderant contribution of fossil fuel combustion to soot (92.57±3.20%) compared to char (27.55±5.92%), suggested that fossil fuel combustion is a crucial driver for BC with high soot percentage. Redundancy analysis and structural equation modeling confirmed that the fossil fuel energy used by anthropogenic activities promoting soot is the crucial reason for high refractory BC. We estimated that the Yangtze River transported 0.15~0.23 Tg soot and 0.15~0.25 Tg char to ocean annually, and the export of large higher refractory BC to the ocean can form a long-term sink and prolong the residence time of terrigenous carbon.
 

Black carbon,Carbon cycle,Anthropogenic activities,Fossil Fuel Combustion