The characteristics of the reactive gaseous mercury (RGM) and particulate mercury (HgP) in the marine boundary layer (MBL) are poorly understood, due in part to sparse data from the sea and ocean. Gaseous elemental Hg (GEM), RGM, and size-fractionated HgP in the marine atmosphere, and dissolved gaseous Hg (DGM) in surface seawater, were determined in the South China Sea (SCS) during an oceanographic expedition (3–28?September?2015). The mean concentrations of GEM, RGM, and Hg2.5P were 1.52±0.32ngm?3, 6.1±5.8pgm?3, and 3.2±1.8pgm?3, respectively. A low GEM level indicated that the SCS suffered less influence from fresh emissions, which could be due to the majority of air masses coming from the open oceans, as modeled by back trajectories. Atmospheric reactive Hg (RGM + Hg2.5P) represented less than 1% of total atmospheric Hg, indicating that atmospheric Hg existed mainly as GEM in the MBL. The GEM and RGM concentrations in the northern SCS (1.73±0.40ngm?3 and 7.1±1.4pgm?3, respectively) were significantly higher than those in the western SCS (1.41±0.26ngm?3 and 3.8±0.7pgm?3), and the Hg2.5P and Hg10P levels (8.3 and 24.4pgm?3) in the Pearl River estuary (PRE) were 0.5–6.0 times higher than those in the open waters of the SCS, suggesting that the PRE was polluted to some extent. The size distribution of HgP in PM10 was observed to be three-modal, with peaks around 0.4, 0.7–1.1, and 5.8–9.0μm, respectively, but the coarse modal was the dominant size, especially in the open SCS. There was no significant diurnal pattern of GEM and Hg2.5P, but we found that the mean RGM concentration was significantly higher in daytime (8.0±5.5pgm?3) than in nighttime (2.2±2.7pgm?3), mainly due to the influence of solar radiation. In the northern SCS, the DGM concentrations in the nearshore area (40–55pgL?1) were about twice as high as those in the open sea, but this pattern was not significant in the western SCS. The sea–air exchange fluxes of Hg0 in the SCS varied from 0.40 to 12.71ngm?2h?1 with a mean value of 4.99±3.32ngm?2h?1. The annual emission flux of Hg0 from the SCS to the atmosphere was estimated to be 159.6tyr?1, accounting for about 5.54% of the global Hg0 oceanic evasion, although the SCS only represents 1.0% of the global ocean area. Additionally, the annual dry deposition flux of atmospheric reactive Hg represented more than 18% of the annual evasion flux of Hg0, and therefore the dry deposition of atmospheric reactive Hg was an important pathway for the input of atmospheric Hg to the SCS.
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