Critical Observations of Gaseous Elemental Mercury Air‐Sea Exchange

Abstract

On a global scale 3,800 Mg a −1 mercury (Hg) enter the ocean through atmospheric deposition and 300 Mg a −1 through riverine input (UNEP, 2019). Atmospheric wet deposition of divalent mercury (Hg II ) constitutes the main deposition pathway of total atmospheric Hg to the surface ocean (Zhang et al., 2014), while dry deposition of Hg II constitutes a minor fraction in the mid-latitude marine boundary layer (Holmes et al., 2009). Reduction of Hg II to gaseous elemental mercury (Hg 0 ) in the surface ocean leads to re-emission of Hg 0 from the ocean of approximately 2,900 Mg a −1 (Horowitz et al., 2017). However, these current air-sea Hg exchange estimates are associated with high uncertainty and a better constraint on the Hg 0 flux is crucial for two reasons: first, ocean emissions reduce the reservoir of Hg II available for methylation in the water column and subsequent bioaccumulation in marine biota (Lavoie et al., 2013). Second, ocean emissions increase the amount of Hg actively cycling between the atmosphere, marine and terrestrial ecosystems (Amos et al., 2013;Selin, 2009). Air-sea exchange of Hg 0 is a diffusion process driven by the concentration gradient between Hg 0 in the atmosphere (Hg 0 air ) and dissolved gaseous elemental Hg in seawater (DGM, hereinafter referred to as Hg 0 aq ). The two key factors that control the Hg 0 air-sea exchange are (a) the saturation of Hg 0 in surface water relative to equilibrium conditions expressed by Henry's law constant for Hg 0 and (b) the gas transfer velocity (k) (Qureshi et al., 2011). The Hg 0 flux is typically estimated based on a thin film gas exchange model (Liss & Merlivat, 1986;Wanninkhof, 1992) that uses in situ measurements of Hg 0 air and Hg 0 aq together with a wind speed dependent parameterization of k that was developed based on field experiments with volatile

Mercury Reduction, Uptake, and Species Transformation by Freshwater Alga Chlorella vulgaris under Sunlit and Dark Conditions

“…The evasion of dissolved elemental Hg(0) aq to the atmosphere is an important source of atmospheric gaseous Hg(0), which depends on the conversion of Hg(II) to Hg(0) in aquatic systems. 59,60 While DOM and mer-mediated reduction of Hg(II) in aquatic systems have been extensively studied, 11,12,61 we show that algae and algae-derived organic matter could be important contributors to Hg(II) reduction or immobilization.…”

“…The evasion of dissolved elemental Hg(0) aq to the atmosphere is an important source of atmospheric gaseous Hg(0), which depends on the conversion of Hg­(II) to Hg(0) in aquatic systems. , While DOM and mer -mediated reduction of Hg­(II) in aquatic systems have been extensively studied, ,, we show that algae and algae-derived organic matter could be important contributors to Hg­(II) reduction or immobilization. Algae-mediated Hg­(II) reduction and immobilization in surface water, depending on the dark or sunlit conditions, could play an important role in modulating the air–water exchange of Hg, thus affecting aquatic Hg­(II) concentrations for subsequent MeHg production.…”

Using Mercury Stable Isotopes to Quantify Bidirectional Water–Atmosphere Hg(0) Exchange Fluxes and Explore Controlling Factors

“…Exchange of gaseous elemental mercury (Hg(0)) between water and atmosphere plays a pivotal role in mercury biogeochemical cycling. The current exchange flux estimates are however associated with high uncertainties, and a better constraint is crucial for assessing mercury sources and sinks and associated atmospheric and biogeochemical cycling . The surface layer of sea and fresh water bodies is usually in a state of supersaturation with respect to dissolved Hg(0) (DGM) as shown by extensive surveys using automatic continuous equilibrium systems .…”

“…Despite the fact that concentration gradients of Hg(0) between water and air are in favor of emissions, several studies report intermittent periods of net Hg(0) uptake in the flux dynamics possibly driven by high primary production (e.g. algal blooms). , Generally, net atmosphere-surface exchange of Hg(0) is a result of simultaneously occurring emissive and depositional flux vectors . Atmospheric Hg(0) deposited to surface water can increase the Hg­(II) pool in aquatic systems, which can subsequently convert to neurotoxic methylmercury (MeHg) that bioaccumulates in food webs .…”

“…The current exchange flux estimates are however associated with high uncertainties, and a better constraint is crucial for assessing mercury sources and sinks and associated atmospheric and biogeochemical cycling. 1 The surface layer of sea and fresh water bodies is usually in a state of supersaturation with respect to dissolved Hg(0) (DGM) as shown by extensive surveys using automatic continuous equilibrium systems. 2 Despite the fact that concentration gradients of Hg(0) between water and air are in favor of emissions, several studies report intermittent periods of net Hg(0) uptake in the flux dynamics possibly driven by high primary production (e.g.…”