A review of global environmental mercury processes in response to human and natural perturbations: Changes of emissions, climate, and land use

Abstract

We review recent progress in our understanding of the global cycling of mercury (Hg), including best estimates of Hg concentrations and pool sizes in major environmental compartments and exchange processes within and between these reservoirs. Recent advances include the availability of new global datasets covering areas of the world where environmental Hg data were previously lacking; integration of these data into global and regional models is continually improving estimates of global Hg cycling. New analytical techniques, such as Hg stable isotope characterization, provide novel constraints of sources and transformation processes. The major global Hg reservoirs that are, and continue to be, affected by anthropogenic activities include the atmosphere (4.4–5.3 Gt), terrestrial environments (particularly soils: 250–1000 Gg), and aquatic ecosystems (e.g., oceans: 270–450 Gg). Declines in anthropogenic Hg emissions between 1990 and 2010 have led to declines in atmospheric Hg0 concentrations and HgII wet deposition in Europe and the US (− 1.5 to − 2.2% per year). Smaller atmospheric Hg0 declines (− 0.2% per year) have been reported in high northern latitudes, but not in the southern hemisphere, while increasing atmospheric Hg loads are still reported in East Asia. New observations and updated models now suggest high concentrations of oxidized HgII in the tropical and subtropical free troposphere where deep convection can scavenge these HgII reservoirs. As a result, up to 50% of total global wet HgII deposition has been predicted to occur to tropical oceans. Ocean Hg0 evasion is a large source of present-day atmospheric Hg (approximately 2900 Mg/year; range 1900–4200 Mg/year). Enhanced seawater Hg0 levels suggest enhanced Hg0 ocean evasion in the intertropical convergence zone, which may be linked to high HgII deposition. Estimates of gaseous Hg0 emissions to the atmosphere over land, long considered a critical Hg source, have been revised downward, and most terrestrial environments now are considered net sinks of atmospheric Hg due to substantial Hg uptake by plants. Litterfall deposition by plants is now estimated at 1020–1230 Mg/year globally. Stable isotope analysis and direct flux measurements provide evidence that in many ecosystems Hg0 deposition via plant inputs dominates, accounting for 57–94% of Hg in soils. Of global aquatic Hg releases, around 50% are estimated to occur in China and India, where Hg drains into the West Pacific and North Indian Oceans. A first inventory of global freshwater Hg suggests that inland freshwater Hg releases may be dominated by artisanal and small-scale gold mining (ASGM; approximately 880 Mg/year), industrial and wastewater releases (220 Mg/year), and terrestrial mobilization (170–300 Mg/year). For pelagic ocean regions, the dominant source of Hg is atmospheric deposition; an exception is the Arctic Ocean, where riverine and coastal erosion is likely the dominant source. Ocean water Hg concentrations in the North Atlantic appear to have declined during the last several decades ...

Impact of Water-Induced Soil Erosion on the Terrestrial Transport and Atmospheric Emission of Mercury in China

“…59 However, the mechanism of erosion-induced THg release is also linked closely with sedimentary function and form, which may be influenced by other factors such as tectonics, topography, soil type, land cover, etc. 4,57,58,60,61 …”

“…1–4 The global release of total mercury (THg) to the environment is estimated to have increased 1.5- to 3-fold since the industrial revolution. 5,6 Quantification of the THg amount released into the environment is essential to assess Hg global biogeochemical cycling accurately, and such evaluations have been performed at the global scale.…”

How closely do mercury trends in fish and other aquatic wildlife track those in the atmosphere? – Implications for evaluating the effectiveness of the Minamata Convention

“…The atmospheric and aquatic chemistry of Hg is one of the most complex of all trace metals. The Hg that is emitted from anthropogenic (and natural) sources to air is almost exclusively inorganic as elemental (Hg 0 ) and divalent compounds (Hg II ) in gaseous and particulate forms (Obrist et al, 2018). Redox reactions between Hg 0 and Hg II in the atmosphere are primarily photochemically driven (Ariya et al, 2015;Saiz-Lopez et al, 2018), with the bulk of the atmospheric Hg deposited onto the Earth's surface (oceans, land, and freshwaters) being in the form of inorganic Hg II .…”

Eddy covariance flux measurements of gaseous elemental mercury over a grassland

“…The EC technique has been under development since the late 1940s to measure the surface-atmosphere exchange of heat, mass and momentum in the surface boundary layer, which refers to the lowest 20-50 m of the atmosphere (Montgomery, 1948;Obukhov, 1951;Swinbank, 1951). In order to estimate a vertical turbulent flux, the covariance of two concurrently measured variables is calculated, (1) the scalar quantity of interest (in our case Hg 0 ) and (2) the turbulent fluctuations of the vertical wind velocity, which are both measured at high temporal resolution.…”