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The predictions of atmospheric chemical models are limited by the accuracy of our understanding of the basic physical and chemical processes that underlie the models. In this work we review the current state of our knowledge of the chemical processes that transform atmospheric mercury species via gas and aqueous phase reactions and the physical processes of deposition. We concur with the conclusions of other recent reviews that our understanding of the basic chemistry that controls mercury is incomplete and the experimental data either limited or nonexistent. In spite of this recent experimental and theoretical studies of mercury reaction kinetics have clarified some issues. Observations in Polar Regions suggest that Hg0 can undergo fast oxidation in the presence of elevated levels of bromine compounds. Both experimental and theoretical studies suggest that the recombination of Hg0 with Br atoms is sufficiently fast to initiate this oxidation process. However there is a large uncertainty in the value of the rate coefficient for this recombination reaction and in the fate of the reaction product, HgBr. Most global mercury models incorporate reactions of Hg0 with OH and O3. Based on the most recent high level ab-initio calculations of the stability of HgO it appears that neither of these reactions is likely to play a significant role in mercury oxidation. The most important aqueous oxidation for Hg0 appears to be reaction with O3 however that there has only been one determination of the Hg + O3 reaction rate constant in the aqueous phase. Aqueous phase reduction of oxidized mercury via reaction with HO2 is the only significant reduction reaction in current models but now seems unlikely to be significant. Again this suggests that the chemistry controlling mercury transformation in current models requires significant modification.

The extent of the urban area, the local emission density, and the temporal pattern in the releases govern the local contribution to air pollution levels in urban environments. However, meteorological conditions also heavily affect the actual pollution levels as they govern the dispersion conditions as well as the transport in and out of the city area. The building obstacles play a crucial role in causing generally high pollutant levels in the urban environment, especially inside street canyons where the canyon vortex flow governs the pollution distribution. Of the pollutants dominating urban air pollution climates, particulate pollution in general together with gaseous and particulate polycyclic aromatic hydrocarbons (PAHs) and heavy metals are those where further field measurements, characterization and laboratory studies are urgently needed in order to fully assess the health impact on the urban population and provide the right basis for future urban air pollution management.

Understanding the kinetics and mechanisms associated with the atmospheric chemistry of mercury is of great importance to protecting the environment. This review will focus on theoretical calculations to advance understanding of gas phase oxidation of gaseous elemental mercury (GEM) by halogen species. Understanding the gas phase oxidation process between atmospheric mercury and halogen compounds is particularly important as all studies indicate that this interaction is the primary conversion mechanism in the troposphere leading to deposition of mercury. Theoretically predicting the thermochemistry of mercury containing species in the atmosphere is important because of the lack of experimental results. In this article a review of theoretical calculations of rate constants and reaction products is presented. Available laboratory data are listed and discussed as well in order to highlight the subjects where theoretical calculations in particular can be of value in the future.

Publisher Summary Theoretical techniques have revolutionized the study of chemistry. Due to the parallel development of theoretical methods and computing power, quantum chemistry has become fast and flexible, often yielding detailed insight into chemical processes that cannot be obtained in the laboratory. The earliest report of combining quantum chemistry with the atmosphere deals with the excitation of nitrogen and oxygen molecules as the primary event in the photo-excitation of the atmosphere. Models of atmospheric chemistry have grown with the abilities of computers and the demands of simulating climate change. One example is the simulation of isotopic fractionation in the photolysis of nitrous oxide isotopologues using time-dependent wave-packet propagation. Quantum chemistry provides data that improves understanding of chemical kinetics. The data is further used as input for parameterizing transport and deposition models or chemical reaction schemes in models of various other atmospheric processes. Theoretical techniques are tested through comparison to laboratory measurements and atmospheric observations, and then further applied towards predicting mechanisms and reaction rates which are currently unknown.

There is a qualitative understanding that gaseous elemental mercury (GEM) is oxidized during Arctic spring to reactive gaseous mercury (RGM) that afterwards is removed by fast deposition to snow surfaces. The conditional sampling or relaxed eddy accumulation (REA), technique represents the first opportunity to directly measure fluxes of reactive gaseous mercury (RGM) to the snow pack in the Arctic. Using a micrometeorological method REA system, with a heated sampling system specifically designed for Arctic use, the dry deposition of RGM is measured after polar sunrise, in Barrow, Alaska. Heated KCl-coated manual RGM annular denuders were used as the accumulators with an inlet allowing only fine particles to pass (Cut off diameter 2.5mm). At 3 m above the snow pack significant RGM fluxes were measured each spring in 2001–2004. Both depositions and emissions were observed. The emissions were attributed to chemical formation of RGM at or near the snow surface. The surface resistance, Rc, for RGM was found to be very small and set to zero as a first estimate. r 2006 Elsevier Ltd. All rights reserved.

[1] The load of mercury in the Arctic environment is to a large extent controlled by atmospheric mercury depletion events. At Barrow, Alaska, these depletion events have been linked with the near-surface air formation of reactive gaseous mercury (Hg(II)) (RGM), to a much lesser extent in a particulate-bound form, and the accumulation of total mercury in the snow pack (>100 ng/L in late Spring). This transport of Hg from atmospheric conversion, to deposition, to bio-available forms is likely to be the predominant pathway for mercury into Arctic biota. For the first time we combine flux rate measurements, atmospheric chemistry measurements, and air mass trajectories to give a comprehensive two-week window into the springtime dynamics and mass balance of Arctic mercury. We have conducted polar-sunrise to snowmelt mercury monitoring at Barrow from 1998 to 2004, and the time period March 25th–April 7th (Julian days 84–97), 2003 appears typical for this time of year. A clear link was observed between air of marine origin, the build-up of BrO together with removal of gaseous elemental mercury (GEM)), and the formation of RGM. This provides the most direct evidence so far for Br and Hg chemistry as the direct source of RGM. The fluxes of RGM and GEM were determined and the net flux calculated.

Mercury (Hg) has been used for millennia in many applications, primarily in artisanal mining and as an electrode in the chlor– alkali industry. It is anthropogenically emitted as a pollutant from coal fired power plants and naturally emitted, primarily from volcanoes. Its unique chemical characteristics enable global atmospheric transport and it is deposited after various processes, ultimately ending up in one of its final sinks, such as incorporated into deep sediment or bioaccumulated, primarily in the marine environment. All forms of Hg have been established as toxic, and there have been no noted biological benefits from the metal. Throughout time, there have been notable incidents of Hg intoxication documented, and the negative health effects have been documented to those chronically or acutely exposed. Today, exposure to Hg is largely diet or occupationally dependent, however, many are exposed to Hg from their amalgam fillings. This paper puts a tentative monetary value on Hg polluted food sources in the Arctic, where local, significant pollution sources are limited, and relates this to costs for strategies avoiding Hg pollution and to remediation costs of contaminated sites in Sweden and Japan. The case studies are compiled to help policy makers and the public to evaluate whether the benefits to the global environment from banning Hg and limiting its initial emission outweigh the benefits from its continued use or lack of control of Hg emissions. The cases we studied are relevant for point pollution sources globally and their remediation costs ranged between 2500 and 1.1 million US$ kg �1 Hg isolated from the biosphere. Therefore, regulations discontinuing mercury uses combined with extensive flue gas cleaning for all power plants and waste incinerators is cost effective. © 2005 Elsevier B.V. All rights reserved.

The oxidation of elemental mercury (Hg0) to the divalent gaseous mercury dibromide (HgBr2) has been proposed to account for the removal of Hg0 during depletion events in the springtime Arctic. The mechanism of this process is explored in this paper by theoretical calculations of the relevant rate coefficients. Rice-Ramsberger-Kassel-Marcus (RRKM) theory, together with ab initio quantum calculations where required, are used to estimate the following: recombination rate coefficients of Hg with Br, I, and O; the thermal dissociation rate coefficient of HgBr; and the recombination rate coefficients of HgBr with Br, I, OH, and O2. A mechanism based on the initial recombination of Hg with Br, followed by the addition of a second radical (Br, I, or OH) in competition with thermal dissociation of HgBr, is able to account for the observed rate of Hg 0 removal, both in Arctic depletion events and at lower latitudes.

In this paper, we present the first comprehensive long-term record of preanthropogenic rates of atmospheric mercury accumulation in dated peat deposits for the High Arctic of Canada. Geochemical studies of two peat hummocks from Bathurst Island, Nunavut reveal substantial inputs from soil dust (titanium), marine aerosols (bromine), and mineral-water interactions (uranium). Mercury, however, was supplied to these peat mounds exclusively by atmospheric deposition. Mercury concentration measurements and age dating of the peat profiles indicate rather constant natural "background" mercury flux of ca. 1 microgram per square meter per year from 5900 to 800 calibrated years BP. These values are well within the range of the mercury fluxes reported from other Arctic locations, but also by peat cores from southern Canada that provide a record of atmospheric Hg accumulation extending back 8000 years. Thus, preanthropogenic Hg fluxes in the Arctic were not significantly different from atmospheric Hg fluxes in the temperate zone. In preindustrial times, therefore, the High Arctic was no more important as a sink for global atmospheric mercury than was the temperate zone.

In nearly all domains of Global Change Research (GCR), the role of humans is a key factor as a driving force, a subject of impacts, or an agent in mitigating impacts and adapting to change. While advances have been made in the conceptualisation and practice of interdisciplinary Global Change Research in fields such as climate change and sustainability, approaches have tended to frame interdisciplinarity as actor-led, rather than understanding that complex problems which cut across disciplines may require new epistemological frameworks and methodological practices that exceed any one discipline. GCR studies must involve from their outset the social, human, natural and technical sciences in creating the spaces of interdisciplinarity, its terms of reference and forms of articulation. We propose a framework for funding excellence in interdisciplinary studies, named the Radically Inter- and Trans-disciplinary Environments (RITE) framework. RITE includes the need for a realignment of funding strategies to ensure that national and international research bodies and programmes road-map their respective strengths and identified areas for radical interdisciplinary research; then ensure that these areas can and are appropriately funded and staffed by talented individuals who want to apply their creative scientific talents to broader issues than their own field in the long term, rather than on limited scope (5 year and less) research projects. While our references are mostly to Europe, recommendations may be applicable elsewhere. In nearly all domains of Global Change Research (GCR), the role of humans is a key factor as a driving force, a subject of impacts, or an agent in mitigating impacts and adapting to change. While advances have been made in the conceptualisation and practice of interdisciplinary Global Change Research in fields such as climate change and sustainability, approaches have tended to frame interdisciplinarity as actor-led, rather than understanding that complex problems which cut across disciplines may require new epistemological frameworks and methodological practices that exceed any one discipline.GCR studies must involve from their outset the social, human, natural and technical sciences in creating the spaces of interdisciplinarity, its terms of reference and forms of articulation. We propose a framework for funding excellence in interdisciplinary studies, named the Radically Inter- and Trans-disciplinary Environments (RITE) framework. RITE includes the need for a realignment of funding strategies to ensure that national and international research bodies and programmes road-map their respective strengths and identified areas for radical interdisciplinary research; then ensure that these areas can and are appropriately funded and staffed by talented individuals who want to apply their creative scientific talents to broader issues than their own field in the long term, rather than on limited scope (5 year and less) research projects. While our references are mostly to Europe, recommendations may be applicable elsewhere.

The study of the global cycle of mercury in polar environments is of great importance as mercury appears to be highly accumulated in the snowpack before melting. Exchange rates between snow and atmosphere is of particular interest to better understand the dispersion of this pollutant in polar ecosystems. Continuous monitoring of gaseous mercury (Hg°) in the air of snow and in ambient air at Station Nord, Greenland were performed from 5 to 13 March, 2002. Hg° concentrations in the snow during the day were lower than atmospheric ones as a result of rapid oxidation of Hg° by bromine species (e.g. Br· and BrO·) to form Hg(II) species, subsequently deposited onto snow grains. During 4 successive nights, we recorded a Hg° production in the air of the snow at the depth of 20 cm below the snow surface. Moreover during the 5 following days, we also recorded a Hg° production in the air of the snow at the depth of 40 cm. During these periods, concentrations of Hg° in the snow started to increase after the sunset, and reached a maximum around 2:00 a.m., thereafter decreasing to a minimum just before sunrise. Resulting emission fluxes were however weak in the range of 0.06–0.40 ng m−2 h−1. We postulated that this phenomenon could be the result of the reduction of Hg(II) by HO2· produced in the snow at night.

For detailed reconstructions of atmospheric metal deposition using peat cores from bogs, a comprehensive protocol for working with peat cores is proposed. The first step is to locate and determine suitable sampling sites in accordance with the principal goal of the study, the period of time of interest and the precision required. Using the state of the art procedures and field equipment, peat cores are collected in such a way as to provide high quality records for paleoenvironmental study. Pertinent field observations gathered during the fieldwork are recorded in a field report. Cores are kept frozen at -18 degree C until they can be prepared in the laboratory. Frozen peat cores are precisely cut into 1 cm slices using a stainless steel band saw with stainless steel blades. The outside edges of each slice are removed using a titanium knife to avoid any possible contamination which might have occurred during the sampling and handling stage. Each slice is split, with one-half kept frozen for future studies (archived), and the other half further subdivided for physical, chemical, and mineralogical analyses. Physical parameters such as ash and water contents, the bulk density and the degree of decomposition of the peat are determined using established methods. A subsample is dried overnight at 105 degree C in a drying oven and milled in a centrifugal mill with titanium sieve. Prior to any expensive and time consuming chemical procedures and analyses, the resulting powdered samples, after manual homogenisation, are measured for more than twenty-two major and trace elements using non-destructive X-Ray fluorescence (XRF) methods. This approach provides lots of valuable geochemical data which documents the natural geochemical processes which occur in the peat profiles and their possible effect on the trace metal profiles. The development, evaluation and use of peat cores from bogs as archives of high-resolution records of atmospheric deposition of mineral dust and trace elements have led to the development of many analytical procedures which now permit the measurement of a wide range of elements in peat samples such as lead and lead isotope ratios, mercury, arsenic, antimony, silver, molybdenum, thorium, uranium, rare earth elements. Radiometric methods (the carbon bomb pulse of (14)C, (210)Pb and conventional (14)C dating) are combined to allow reliable age-depth models to be reconstructed for each peat profile.

The Arctic is an indicator of climatic change. This brings opportunities and challenges as the warming climate may facilitate greater harvesting of natural resources. The triadic nexus between globalization, climate change and geopolitics is shaping the future of the Arctic and will bring considerable opportunity at national, regional, and global level which will open a call for a global multi-stakeholder approach. As the ice melts and geopolitics shift, the Arctic is also transformed from a regulatory perspective. Sometimes this transformation is opaque. Research infrastructures (research stations both military and non-military, observation and monitoring networks) are opening access and data to new Arctic and non-Arctic players. Additional logistics hubs than those already existing are and should be established. Countries are sustaining and building new research as well as search and rescue bases/stations. Stations can be used as indicator of this transformation as well as their implications to improve cooperation, engage in multilateral rather than unilateral actions to protect the Arctic infrastructures and to improve military capabilities. These actions have started to attract also non-Arctic actors, such as China and the EU, which are developing new policies. Stations may not be developed and maintained only for the purpose of the scientific understanding of climatic and environmental impacts but also for function as entities that legitimize national or sovereign claims. At the triadic nexus are the scientists that utilize the research bases and their international colleagues. Arctic/Northern bases are primarily military for historical reasons and for reasons of logistics and expertise, as historically indicated through the American presence in Alaska. This is not the same as saying the bases are militarized - or part of some national militarization strategy in the Arctic. New steps to identify the role of stations at national, regional and global level are needed. In this essay we explore the implications and opportunities for these stations to act as pivots between scientific and geopolitical issues. We argue that where there is scientific collaboration there is less risk of military conflict and that the Arctic is not "militarized" based on the international politics and science diplomacy of the Arctic.

There is a growing interest in the atmospheric transport, deposition, and accumulation of anthropogenic Hg in the Arctic. To quantify the impact of industrial Hg emissions, the natural rate of atmospheric Hg accumulation must be known. Mercury concentration measurements and age dating of peat from the Canadian Arctic show that natural background Hg flux rather constant (ca. 1 microgram per sq. m per yr.) throughout the past 6000 years. Mercury concentrations in surface peat layers are much higher, but chronology of these changes cannot be interpreted until more age dates are available. The elevated Hg concentrations in surface layers, however, are out of proportion with Br and Se, suggesting that there has been a significant human impact. Peat cores from southern Canada provide a record of atmospheric Hg accumulation extending back nine thousand years, with similar background fluxes. Thus, pre-anthropagenic Hg fluxes in the High Arctic were not significantly different from atmospheric Hg fluxes in the temperate Zone.