Mercury in Air

Hg is a persistent pollutant enriched in the environment by human activities, and cycling between different environmental compartments. It is emitted into the atmosphere from a variety of primary and secondary emission sources in different chemical and physical forms. Primary sources, both natural and anthropogenic, transfer Hg from surface reservoirs to the atmosphere where once oxidized deposits to aquatic and terrestrial ecosystems. Deposited Hg can be partially reduced to Hg0 and reemitted back. Re-emission processes represent secondary sources that exchange Hg among environmental media using the atmosphere as a vehicle. The temporal and spatial scale of Hg transport in the atmosphere and its transfer to environmental receptor bodies depend primarily on its chemical and physical characteristics. Hg0 is relatively inert under atmospheric conditions, only slightly soluble and also quite volatile, thus it can be transported long distances before oxidation and removal by particle and gas-phase dry deposition or scavenging by precipitation. Although there is still some debate over whether some of the oxidants oxidize Hg in the atmosphere, the reaction which is unanimously accepted to date is that of Hg with Br and Br-containing compounds. This reaction is responsible for the phenomenon, named as "atmospheric mercury depletion event" (AMDE), during which Hg0 may be rapidly converted to a reactive form that deposits in polar ecosystems resulting in an important removal pathway for atmospheric Hg during polar spring. As in polar regions during the AMDEs, O3 and Hg0 destruction after sunrise has been observed during the last decade also in the MBL at mid-latitudes with consequent production of appreciable HgII concentrations measured also in remote MBL of oceans (i.e., Atlantic, Pacific) and seas, including the Mediterranean Basin. All the MBL measurements of HgII performed over marine water have in common a diurnal cycle, increasing after sunrise and decreasing towards evening with the highest HgII concentrations occurring at the solar irradiation maximum, indicative of photolytically produced oxidants responsible of the gas phase oxidation of Hg0 to HgII. Previous modeling studies of Hg chemistry in the MBL showed that the inclusion of the reaction between Hg0 and OH to eventually form HgO could account for at least a part of the measured HgII concentrations when cycling of HgO to HgCl2 via the sea salt aerosol was included. The inclusion of the reactions between halogen molecules and atoms and Hg0 means that such reactions affect significantly the atmospheric residence time of Hg not only at both poles, but possibly also in the MBL and in the upper troposphere, where those radicals are present in sufficient concentrations. In the presence of sunlight and low temperature, rapid oxidation of Hg0 by halogen species may occur in the open ocean MBL leading to HgII formation and deposition to the water surface. Hg can be later emitted again as Hg0, thereby leading to a dynamic exchange of Hg between the MBL and marine waters. In warm coastal waters, HgII formation may not be as significant, possibly because the kinetics of HgII formation via halogen chemistry is favored at lower temperatures. Therefore, Hg chemistry within the MBL is likely to vary spatially and temporally, depending on meteorological and sea conditions. One of the most critical issue to understand the Hg biogeochemical cycle is the atmospheric Hg deposition rate which is directly linked to the reduction/oxidation processes that govern the speciation of Hg in the atmosphere. The dominant form of Hg in wet deposition (rain and snow) is dissolved and particulate HgII. Except for the polar regions, and possibly the MBL, the primary mechanisms for wet deposition are in-cloud oxidation of Hg0 by O3, and the gas-phase oxidation of Hg0 by OH and O3 followed by cloud-droplet uptake. Hgp also contributes to wet deposition through cloud-droplet and precipitation scavenging. Wet deposition thus provides an efficient sink for HgII. Currently, accepted standardized methods are used to assess Hg wet deposition while measurements and modeling applications of Hg dry deposition are still problematic and represent one of the most challenging gap in our understanding of Hg depositions. Further research needs to improve measurements, modeling, and understanding on Hg fluxes across terrestrial surface emission and air-water interface. All chemical and physical processes characterizing the atmospheric Hg cycle are directly and/or indirectly linked to the climate change phenomenon which could be the primary factor influencing the future distribution of Hg on regional and global scales. Spatial and temporal variation of Hg emission and deposition to receptor bodies can result from climate change. The effects of climate change are in fact more visible in cold remote environments such as Arctic and Antarctica, where the reduction of sea-ice extend, in addition, has as direct effect the albedo changes which lead to heat accumulation in polar water bodies with a large re-emission of Hg0 from the top water micro-layer to the atmosphere causing an increased global spread. Another fundamental effect of climate change, also at mid latitudes, is the enhancement of solar radiation amount which determines the increasing of photo-chemical reactions in the MBL as well as in the sea water, leading to changes in the primary production of Hg and its conversion into organic compounds enhancing environmental risks for human health. The risk to humans and wildlife related to Hg released into the global environment and ultimately converted to more bioavailable forms, which accumulate reaching high toxic levels at the top of the aquatic food chain has gained growing attention during the last 15 year with a number of concerted initiatives started at international levels aimed to develop a policy framework to control and possibly reduce the use of Hg for decreasing human health and environmental risk from the release of Hg and its compounds. In this context, several international programs, such as the UNEP Global Partnership for Mercury Air Transport and Fate Research and Conventions, particularly the Minamata Convention (MC) on Mercury have recognized that Hg is toxic and of global relevance; scientific needs will therefore shift towards best implementation practices of the Convention. Improvement in Hg knowledge is therefore essential to model future scenarios in combination with implementation of various reducing practices according to the MC.