Global mercury circulation
Mercury fluxes between the atmosphere, soil and water consist of contributions from natural and human releases. The global mercury budget involves emissions from local and regional sources and re-emissions from land, fresh water, and evasion from the oceans. In some cases, local sources also include direct water discharges. Re-emissions occur from previously deposited mercury reservoirs of either human or natural origin. The figure below illustrates the conceptual cycling of mercury between its various pools and the transportation and transformation processes that are thought to occur.
Since the advent of industrialization, the amount of mercury mobilized and released to the environment has increased significantly due to human activities. Current ambient levels are approximately 1.6 nanograms per cubic meter of air, versus pre-industrial estimates of 0.5 to 0.8 (a nanogram is equal to one billionth of a gram). It has also been suggested that anthropogenic releases of mercury to the atmosphere have caused a two to four-fold increase in its concentration in air and marine surface waters since the pre-industrial era (circa 1850).
The ratio of anthropogenic to natural mercury in the atmosphere has been stated as being about 1.4:1 or about 2 900 tonnes from human activity and 2 100 tonnes from natural sources on an annual basis. However, the magnitude of human emissions and resulting mercury deposition are greatly debated. Research attempting to describe and quantify the movement and distribution of mercury in the environment has been complicated due to the difficulty in separating new, natural sources from re-mobilized past and current human releases. It is thought that 95% of the estimated 200 000 tonnes of mercury emitted to the atmosphere since 1890 is currently in terrestrial soil stocks or part of the oceanic sink. However, more research is needed to conclusively differentiate between natural releases and re-emissions of anthropogenic mercury loadings from these pools.
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