The role of oceans in the global mercury cycling. Introduction to the special issue in the marine Environment.

With the signing of the UNEP Minamata Convention in 2013, the world's governments have accepted that mercury (Hg) is toxic and of global relevance; scientific needs will therefore shift towards best implementation practices of the Convention. Because majority of emissions emanate from the energy and industrial sectors governments will have to balance economic and environmental interests. Despite decades of Hg science, we still lack answers to the most basic questions concerning the fundamental Hg transformations and fluxes. Improvement in such knowledge is essential in order to model future scenarios in the light of climate change in combination with implementation of various reducing practices according to the Convention. Hg pollution poses global human health and environmental risks. Although Hg is naturally present in the environment, human activities, such as coal burning, have increased the amount of Hg cycling among the land, atmosphere, and ocean by a factor of three to five. Emitted to the atmosphere in its elemental form, Hg travels worldwide before oxidizing to a form that deposits to ecosystems. In aquatic systems, Hg can convert to monomethyl mercury (MMHg), a potent neurotoxin. People and wildlife are exposed to MMHg as it bioaccumulates up the food chain. Of great concern is the interaction of Hg between oceans and atmosphere. Oceans can act as either a net sink or source of Hg to the atmosphere. This process is largely driven by the dynamics of dissolved gaseous Hg (DGM) in oceans. There are several sources and processes contributing to the DGM budget in the oceans such as the dissolution and deposition of atmospheric Hg, demethylation of MMHg and dimethyHg (DMHg) and the reduction of Hg(II). The balance between reduction and oxidation reactions results in diurnal variations of Hg(0) in waters while atmosphere-water exchange remains relatively constant. The extent and dynamics of these processes differ between various environmental settings and remain mostly unknown. In tectonically active environments, DGM can enter oceans, for example, through tectonic faults and volcanoe. Alternatively, in Polar Regions it is believed that sea ice acts as a cap for DGM in sea water prohibiting Hg evasion from sea surfaces to the atmosphere. In such environments, atmospheric Hg depletion events (AMDE) also occur. During such an event, Hg is oxidized from a relatively inert gaseous form to a water-soluble fraction, which causes deposition of tens of Mg (90-200) of Hg in the Arctic during spring. Depletion of both ozone and elemental Hg occurs during such events since both substances are involved in the same reaction chain caused by halogen species evading from sea surfaces. Until recently, the AMDE was thought to be a result of a series of atmospheric reactions occurring during the polar spring, but it has recently been shown that such depletion events also occur during polar nights. This Special Issue collects results over ocean measurements obtained during the EU-funded Project "Global Mercury Observation System (GMOS)" within the 7th framework program. The research work summarized in this special issue will contribute to the GMOS data base and will improve mass balances and inventories of global mercury cycle. Moreover, the work included in this special issue has strong ties to the UNEP Global Partnership on Atmospheric Mercury Transport and Fate Research (UNEP F & T), the Task Force on Hemispheric Transport of Air Pollutants (TF HTAP) of the UNECE-LRTAP convention, and GEO Task HE-02: Tracking Pollutants related to Hg and POPs as well as the Arctic Monitoring and Assessment Program (AMAP).