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

Abstract

The characteristics of the reactive gaseous mercury (RGM) and particulate mercury (Hg P ) 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 Hg P 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 Hg P 2.5 were 1.52 ± 0.32 ng m −3 , 6.1 ± 5.8 pg m −3 , and 3.2 ± 1.8 pg m −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 + Hg P 2.5 ) 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 ± 0.40 ng m −3 and 7.1 ± 1.4 pg m −3 , respectively) were significantly higher than those in the western SCS (1.41 ± 0.26 ng m −3 and 3.8 ± 0.7 pg m −3 ), and the Hg P 2.5 and Hg P 10 levels (8.3 and 24.4 pg m −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 Hg P in PM 10 was observed to be three-modal, with peaks around < 0.4, 0.7-1.1, and 5.8-9.0 µm, respectively, but the coarse modal was the dominant size, especially in the open SCS. There was no significant diurnal pattern of GEM and Hg P 2.5 , but we found that the mean RGM concentration was significantly higher in daytime (8.0 ± 5.5 pg m −3 ) than in nighttime (2.2 ± 2.7 pg m −3 ), mainly due to the influence of solar radiation. In the northern SCS, the DGM concentrations in the nearshore area (40-55 pg L −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 Hg 0 in the SCS varied from 0.40 to 12.71 ng m −2 h −1 with a mean value of 4.99 ± 3.32 ng m −2 h −1 . The annual emission flux of Hg 0 from the SCS to the atmosphere was estimated to be 159.6 t yr −1 , accounting for about 5.54 % of the global Hg 0 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 Hg 0 , and therefore the dry deposition of atmospheric reactive Hg was an important pathway for the input of atmospheric Hg to the SCS.

Interannual Variability of Air–Sea Exchange of Mercury in the Global Ocean: The “Seesaw Effect” in the Equatorial Pacific and Contributions to the Atmosphere

“…Modeled multiple-year mean Hg(0) fluxes in the global ocean during 1994–2016. Observed values are shown as color-coded circles. ,,− The black box represents the region of tropical Pacific, while the North Atlantic is marked in the red box.…”

An improved model for air–sea exchange of elemental mercury in MITgcm-ECCO v4-Hg: the role of surfactants and waves

“…In addition, since only the ocean part is considered, the atmospheric Hg 0 concentration and deposition remain constant over time, which affect flux calculations to a substantially higher degree (Soerensen et al, 2013). To address this limitation, employing a coupled online model (Zhang et al, 2019), proves to be a valuable strategy for achieving a more accurate simulation of Hg 0 flux.…”

“…The air-sea exchange flux of Hg 0 is generally controlled by both kinetic (gas transfer velocity, kHg) and thermodynamic (partial pressure related concentration gradients) forcing (Wanninkhof, 1992;Wanninkhof et al, 2009;Kuss et al, 2011). However, the lack of direct measurements of Hg 0 transfer velocity results in substantial uncertainty in estimating large-scale air-sea Hg 0 exchange (Zhang et al, 2019). Considering that wind is the primary force driving turbulence in the upper ocean, the transfer velocity is typically parameterized with wind speed through linear (Jähne et al, 1979;Liss and Merlivat, 1986), quadratic (Wanninkhof et al, 1992;Nightingale et al, 2000), or cubic relationships (McGillis et al, 2001;Edson et al, 2011).…”

Organophosphate Esters in Air and Seawater of the South China Sea: Spatial Distribution, Transport, and Air–Sea Exchange

“…These countries are becoming the world's factories due to dense populations, effective resources, and economic development policies. 15 Accompanying the blooming of industry, economy, and metropolitan areas, the South China Sea is facing extensive anthropogenic pressure caused by increasing human activities. 16,17 Large rivers such as the Pearl River and the Mekong act as important vectors for transporting numbers of chemicals into the South China Sea.…”

“…The South China Sea is one of the largest marginal seas in the world, with an area of about 3.5 million km 2 surrounded by some tropical–subtropical developing countries in Southeast Asia, such as China, Vietnam, Philippines, Thailand, Brunei, Singapore, Malaysia, and Indonesia. These countries are becoming the world’s factories due to dense populations, effective resources, and economic development policies . Accompanying the blooming of industry, economy, and metropolitan areas, the South China Sea is facing extensive anthropogenic pressure caused by increasing human activities. , Large rivers such as the Pearl River and the Mekong act as important vectors for transporting numbers of chemicals into the South China Sea. − As the South China Sea is a semienclosed marginal sea of the Northwest Pacific, it is affected by frequent Pacific water intrusion, abundant runoff, and seasonally changing monsoon winds .…”