Air-Sea Exchange and Atmospheric Cycling of Mercury in South China Sea

Abstract:

Limited knowledge exists concerning the role of the low-latitude marginal seas in mercury (Hg) emissions on a global scale, especially tropical-subtropical and monsoon-dominated marginal seas in East Asia. To assess this potential mobilization of Hg through air-sea gas exchange, we have determined the dissolved elemental Hg (DEM) and gaseous elemental Hg (GEM) concentrations in surface seawater and atmosphere, respectively, during seasonal oceanographic cruises to the SouthEast Asian Time-series Study (SEATS) station (18 ^oN, 116 ^oE) from 2003 to 2007. The sampling and analysis of GEM and DEM were performed on board ship by using an on-line mercury analyzer (GEMA). Over the SCS, the GEM concentrations are elevated 2-3 times above global background values, with higher enhancements in the winter when the northeast monsoon draws air from China. The impact of long-range transport, as controlled by seasonal monsoons, has on the Hg atmospheric distribution and cycling in the SCS. The DEM concentration varied seasonally, with a high in summer and a low in winter and showed a positive correlation with sea surface temperature (SST). The elevated DEM concentration in summer appears mainly abiologically driven. In winter, the SCS acts as a sink of atmosphere Hg⁰ as a result of low SST and high wind of the year, enhanced vertical mixing and elevated atmospheric gaseous elemental mercury. Annually, the SCS serves as a source of Hg⁰ to the atmosphere of 300±50 pmol m^-2 d^-1 (390±60 kmol Hg y^-1, ~2.6% of global emission in ~1% of global ocean area), suggesting high regional Hg pollution impacts from the surrounding Mainland (mostly China).