Speciated atmospheric mercury and sea–air exchange of gaseous mercury in the South China Sea

Abstract

<p><strong>Abstract.</strong> The characteristics of the reactive gaseous mercury (RGM) and particulate mercury (Hg<sup>P</sup>) in the marine boundary layer (MBL) is poorly understood due in part to sparse data from sea and ocean. Gaseous elemental Hg (GEM), RGM and size-fractioned Hg<sup>P</sup> in 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 Hg<sup>P</sup><sub>2.5</sub> were 1.52 ± 0.32 ng m<sup>−3</sup>, 6.1 ± 5.8 pg m<sup>−3</sup> and 3.2 ± 1.8 pg m<sup>−3</sup>, respectively. Low GEM level indicated that the SCS suffered less influence from human activities, which could be due to the majority of air masses coming from the open oceans as modeled by backward trajectories. Atmospheric reactive Hg (RGM + Hg<sup>P</sup><sub>2.5</sub>) 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 were significantly higher than those in the western SCS, and the Hg<sup>P</sup><sub>2.5</sub> and Hg<sup>P</sup><sub>10</sub> levels in the Pearl River Estuary were significantly higher than those in the open waters of the SCS, indicating that the Pearl River Estuary was polluted to some extent. The size distribution of Hg<sup>P</sup> in PM<sub>10</sub> was observed to be bi-modal with a higher peak (5.8–9.0 μm) and a lower peak (0.7–1.1 μm), but the coarse modal was the dominant size, especially in the open SCS. There was no significant diurnal variation of GEM and Hg<sup>P</sup><sub>2.5</sub>, but we found the RGM concentrations were significantly higher in daytime than in nighttime mainly due to the influence of solar radiation. In the northern SCS, the DGM concentrations in nearshore areas were higher than those in the open sea, but this pattern was not significant in the western SCS. The sea–air exchange fluxes of Hg<sup>0</sup> in the SCS varied from 0.40 to 12.71 ng m<sup>−2</sup> h<sup>−1</sup> with a mean value of 4.99 ± 3.32 ng m<sup>−2</sup>  h<sup>−1</sup>. The annual emission flux of Hg<sup>0</sup> from the SCS to the atmosphere was estimated to be 159.6 tons yr<sup>−1</sup>, accounting for about 5.54 % of the global Hg<sup>0</sup> oceanic evasion though 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 Hg<sup>0</sup>, and therefore the dry deposition of atmospheric reactive Hg was an important pathway for the input of atmospheric Hg to the SCS.</p>

Spatial Distribution of Atmospheric Mercury Species in the Southern Ocean

“…The GOM concentrations observed in the oceanic region during this cruise were the lowest compared to coastal Antarctica and sea‐ice regions (Figure 2c and Table 1); the average was similar to the global average (3.1 ± 11 pg·m 3 ) (Soerensen et al., 2010), and observations in seas in the northern hemispheric (the Mediterranean, Bohai, Yellow, and South China seas and off Okinawa) (Chand et al., 2008; Sprovieri et al., 2003; C. Wang et al., 2016, 2019) (Table ). Holmes et al.…”

Critical Observations of Gaseous Elemental Mercury Air‐Sea Exchange

“…However, our mean Hg 0 flux measurements, ranging from 0.3 to 0.6 ng m −2 h −1 , were lower than the average Baltic Sea fluxes reported in spring (0.9 ng m −2 h −1 ) and summer (1.6 ng m −2 h −1 ) over a period from 2006 to 2015 (Kuss et al, 2018). The mean Hg 0 fluxes obtained in this study were also lower compared to recently measured fluxes (May to August, 2010August, -2019 from other border and marginal seas and coastal waters at non-contaminated sites with background Hg 0 air levels, such as Yellow Sea (mean = 1.1 ng m −2 h −1 ), East China Sea (mean = 4.6 ng m −2 h −1 ), South China Sea (mean = 5 ng m −2 h −1 ), Western and Northwestern Mediterranean Sea (mean = 5.3 and 5.0, respectively) (Ci et al, 2015;Cossa et al, 2018;Nerentorp Mastromonaco, Gårdfeldt, & Wängberg 2017;Wang et al, 2016Wang et al, , 2019. When we consider the measurements at a diel resolution, we find that fluxes measured with micrometeorological techniques differ from those calculated with the gas exchange model by the absence of both midday Hg 0 flux peaks and regularly occurring events of seawater Hg 0 uptake (Figures 4d-4g).…”