Impact of Wet Deposition of Black Carbon on Particle Dynamics in Surface Waters of Halong Bay, North Vietnam

Xavier Mari, Institute of Research for Development, Mediterranean Institute of Oceanography (MIO), Marseille, France, Benjamin P Guinot, CNRS, Paris Cedex 16, France, CHU Van Thuoc, Institute of Marine Environment and Resources, Haiphong, Vietnam, Justine Brune, Institute of Research for Development, Laboratoire Ecologie des Systèmes Marins Côtiers (ECOSYM), Montpellier, France, Jean-Pierre Lefebvre, Institte of Research for Development, Laboratoire d’Etudes en Géophysique et Océanographie Spatiales (LEGOS), Toulouse, France, Patrick Raimbault, Mediterranean Institute of Oceanography, Marseille Cedex 09, France, Jutta Niggemann, University of Oldenburg, Research Group for Marine Geochemistry (ICBM-MPI Bridging Group), Oldenburg, Germany and Thorsten Dittmar, Carl von Ossietzky University Oldenburg, Institute for Chemistry and Biology of the Marine Environment (ICBM), Oldenburg, Germany
Abstract:
Black Carbon (BC) is an aerosol emitted during biomass burning and fossil fuel combustion. The atmospheric lifetime of Black Carbon (BC) ranges from a few days in rainy climates up to one month in dry regions, and on a global scale wet deposition of atmospheric BC accounts for about 80% of the BC input to the ocean. The rain-mediated input of BC to the ocean was studied in a coastal site located in a regional hotspot of atmospheric BC concentration, North Vietnam. We monitored changes in atmospheric and marine BC during a 24-h cycle impacted by a short and heavy rainfall event. During the rainfall event, atmospheric BC concentration decreased by a factor of 8 (i.e. from 5230 to 660 µg BC m-3). This cleaning of the air column was immediately followed by a significant increase (by a factor of 2 to 4) of particulate BC (PBC) and POC concentrations in the surface microlayer (SML) and at 1.5 m depth. In the SML, this event was also followed by a significant increase of DOC and dissolved BC (DBC) concentrations. Interestingly, the concentration of DOC decreased by >10% after the rainfall at 1.5 m depth, suggesting an adsorption of DOC onto sinking PBC. Concomitantly with the increase in particulate BC, nutrient concentrations increased by a factor of 2 in the SML, while no change was observed in the underlying water column. After the rainfall, the particle size spectra, measured along the water column with a LISST (Laser In-Situ Scattering and Transmissometry probe), changed in that the concentration of small particles (<5 µm) decreased and the concentration of large particles (>100 µm) increased. This alteration of the particle size spectra was restricted to a thin layer of about 20 cm thickness, probably corresponding to a BC-enriched layer adsorbing DOC and small particles, and stimulating aggregation during sinking from the surface to deeper water layers. The concentrations of POC, DOC, PBC, DBC and nutrients reached pre-rainfall levels 4 hours after the event.
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