Abstract
Emissions of black carbon (BC), a product of incomplete combustion of fossil fuels, biofuels and biomass, are high in the Asia-Pacific region, yet input pathways and rates to the ocean are not well constrained. Atmospheric and riverine inputs of BC in Halong Bay (Vietnam), a hotspot of atmospheric BC, were studied at monthly intervals during one year. Climate in Halong Bay is governed by the monsoon regime, characterized by a northeast winter monsoon (dry season) and southeast summer monsoon (wet season). During the dry season, atmospheric BC concentrations averaged twice those observed during the wet season. In the sea surface microlayer (SML) and underlying water (ULW), concentrations of particulate BC (PBC) averaged 539 and 11 µmol C L–1, respectively. Dissolved BC (DBC) concentrations averaged 2.6 µmol C L–1 in both the SML and ULW. Seasonal variations indicated that PBC concentration in the SML was controlled by atmospheric deposition during the dry season, while riverine inputs controlled both PBC and DBC concentrations in ULW during the wet season. Spatiotemporal variations of PBC and DBC during the wet season suggest that river runoff was efficient in transporting PBC that had accumulated on land during the dry season, and in mobilizing and transporting DBC to the ocean. The annual river flux of PBC was about 3.8 times higher than that of DBC. The monsoon regime controls BC input to Halong Bay by favoring dry deposition of BC originating from the north during the dry season, and wet deposition and river runoff during the wet season. High PBC concentrations seem to enhance the transfer of organic carbon from dissolved to particulate phase by adsorbing dissolved organic carbon and stimulating aggregation. Such processes may impact the availability and biogeochemical cycling of other dissolved substances, including nutrients, for the coastal marine ecosystem.
Highlights
Black carbon (BC) is the product of incomplete combustion of fossil fuels, biofuels and biomass, and is a major component of soot which encompasses both black and organic carbon (OC)
Weather conditions and characteristics of aerosols The seasonal rainfall regime recorded from October 2012 to October 2013 only differed from the typical monsoon regime of the region by sporadic heavy rainfall events that occurred in November–December, during the early phase of the dry season (Figure 3)
Our study shows that BC can be highly enriched in coastal systems located in regions experiencing high atmospheric BC concentrations, as in North Vietnam
Summary
Black carbon (BC) is the product of incomplete combustion of fossil fuels, biofuels and biomass, and is a major component of soot which encompasses both black and organic carbon (OC). BC is associated with other aerosols such as sulfates, nitrates, metals and organic acids (Guazzotti et al, 2001). All together, these aerosols form what are called atmospheric brown clouds, which are known to concentrate in specific regional hot-spots, the most important being located in Asia, in accordance with hot-spots of BC emissions. These aerosols form what are called atmospheric brown clouds, which are known to concentrate in specific regional hot-spots, the most important being located in Asia, in accordance with hot-spots of BC emissions These hot-spots of increased atmospheric BC concentration give rise to widespread atmospheric plumes allowing long range transport and deposition over adjacent oceans (Hadley et al, 2007; Ramanathan et al, 2007; Ramanathan and Carmichael, 2008; Lin et al, 2014)
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