Air–sea exchange of volatile mercury in the North Atlantic Ocean

Part of the Beringia 2005 expedition covered the North Atlantic Ocean. During this passage, dissolved gaseous mercury (DGHg) in seawater and atmospheric concentrations of total gaseous mercury (TGHg), carbon monoxide (CO), and ozone (O3) were measured continuously at high resolution. In addition, meteorological parameters such as wind speed and water temperature were measured continuously by the ship's meteorological instrumentation. The DGHg concentration was measured using a continuous equilibrium system where the elemental mercury in the sea water was equilibrated with a stream of gas. The DGHg concentration was calculated using DGHg = Hgeq/kH′ where Hgeq is the concentration of elemental mercury in the equilibrated air and kH′ is the dimensionless Henry's law constant. The degree of saturation was determined directly from the measurements S = Hgeq/TGHg. The water sampled had an average DGHg concentration of 58 ± 10 fM and the average TGHg concentration was determined to 1.7 ± 0.1 ng m− 3. The water sampled was under- and super-saturated with respect to elemental mercury covering a large range of saturation of 70 to 230% resulting in an average degree of saturation of 150 ± 30%. Therefore, both evasion and deposition of elemental mercury were observed in the sampled water. In the light of the average flux, 2.1 ± 1.8 pmol m−2 h− 1, it is concluded that mostly evasion occured during the sampling period.

Research Highlights
► For the first time, both TGHg and DGHg were measured continuously with a high time resolution along a cruise track in the North Atlantic Ocean.
► The DGHg concentration measured during the expedition ranged from 29 to 90 fM, the average concentration was 58 ± 10 fM, and the TGHg concentration ranged from 1.2 to 2.2 ng m− 3 with an average of 1.7 ± 0.1 ng m− 3.
► The average flux was calculated to 2.1 ± 1.8 pmol m−2 h− 1. Depending on the degree of saturation and wind speed, both deposition (− 3.2 pmol m−2 h− 1) and evasion (12.6 pmol m−2 h− 1) of elemental mercury occurred.