In addition to evaluating mercury's biogeochemical interactions, scientists must assess the transport and the global, regional and local fluxes of mercury in order to predict the sources, environmental receptors and the impacts of mercury emissions. The global mercury cycle is complicated due to the volatility of elemental mercury (Hg0), which allows mercury to travel in a multi-step sequence of emission to the atmosphere, transportation, deposition and re- emission. As a result, mercury from point source emissions may remain localized in the environment, or may be transported regionally and even globally.
Atmospheric transport is likely the primary mechanism by which Hg0 is distributed throughout the environment, unlike many pollutants that follow erosion or leaching pathways. Mercury can enter the atmosphere as a gas or bound to other airborne particles and circulates until removal. Removal occurs primarily through the "wet" deposition of Hg2+ in rainfall, however it can also occur in the presence of snow, fog, or through direct, or "dry", deposition.
Approximately 98% of the estimated 5,000 tons of mercury in the atmosphere is Hg0 vapor, emitted from human activities, contaminated soils and water as well as natural sources. This gas is readily transported and has a mean atmospheric residence time of about one year to one and a half years. The transformation of insoluble Hg0 to its more reactive and water-soluble form, Hg2+, is thought to provide the mechanism for the deposition of Hg0 emissions to land and water. Hg0 oxidation may also be affected by concentrations of other atmospheric pollutants such as ozone, sulphur dioxide and soot; however, additional research is needed in order to predict corresponding mercury deposition rates.
Following release to the atmosphere and depending on its physical/chemical form, mercury can be either deposited in the vicinity of the emission source, or subjected to long-range atmospheric transport via air masses. Because the uptake of Hg0 in cloud water is relatively slow, this process may be responsible for the deposition of mercury far from its source and may be important when considering global mercury pollution. Gaseous Hg+2 and particulate mercury (Hg(p), mercury adsorbed onto other particulate matter) emissions generally undergo direct wet or dry deposition to the earth's surface locally. These species have relatively short residence times in the atmosphere ranging from hours to months. Gaseous Hg+2 has a residence time of just 5 to 14 days in the atmosphere, and may travel tens to hundreds of kilometers. Particulate forms of mercury (Hg(p))tend to fall out closer to the source of emissions, with larger particles falling out faster than smaller ones. The site-specific deposition of mercury is variable, and is affected by conditions like meteorology, temperature and humidity, solar radiation and emission characteristics (speciation, source, stack height, etc.).
Atmospheric circulation processes may play an important role in determining where airborne mercury is eventually deposited. Relatively high concentrations of mercury found in the Arctic, an area with no significant sources of mercury, may be linked to the long-range transport of pollutants on air currents from Asia and Europe. Mercury, like other semi-volatile compounds such as PCBs, is thought to participate in a global distillation phenomenon that transfers chemical emissions from equatorial, subtropical and temperate regions to the Polar Regions via the "grasshopper effect".
When this phenomenon takes place, an emitted compound re-enters the atmosphere by volatilizing after initial deposition, and continues over time to "hop" through the environment in the direction of the prevailing winds, favoring accumulation in the colder regions of the planet. During the summer months, major air currents in the Northern hemisphere lead to the Arctic, and once there, a contaminant can no longer gain enough heat energy for another "hop" out of the Arctic. The net result is a concentration of contaminants in the Arctic at odds with the relative sparsity of emissions sources in the region.
A process known as "mercury depletion" further compounds the problem of mercury contamination in the Arctic. A Canadian researcher, Dr. William Shroeder, discovered that following polar sunrise in the spring, atmospheric Hg0 can be rapidly oxidized to the reactive and water soluble form of Hg2+, and deposited on snow and ice surfaces. This reaction is thought to occur photochemically (in the presence of sunlight) and in the presence of reactive chemicals released from sea salt (for example, bromine and chlorine ions). As a result, a pulse of reactive mercury enters the Arctic environment when the short lived growing season is beginning. It remains a research question what fraction of this reactive mercury is converted to toxic methylmercury through biogeochemical reactions and taken up by animals and plants.
In addition to atmospheric pathways, mercury can be transported through river systems in their sediment loads, or in aqueous solution. The distance traveled may be long or short. Where mercury is carried on particles, the distance traveled is limited by sedimentation. Transport of contaminants via particles tends to halt at riverine lakes or reservoirs since heavy sedimentation occurs there. Transport also occurs along ocean currents.
The oceans are considered the ultimate sink for mercury because Hg2+ deposited from the atmosphere can settle to oceanic depths where it is reduced and precipitates as insoluble mercuric sulfide. It is thought that approximately one third of the total current mercury emissions cycle between the oceans and the atmosphere, and that 20 to 30% of oceanic emissions are re-emitted from prior anthropogenic sources. Differentiating between natural and recycled anthropogenic emissions is one of the challenges facing researchers attempting to describe and quantify the global mercury budget.