scholarly journals Ten years trends in atmospheric mercury concentrations, meteorological effects and climate variables at Zeppelin, Ny-Ålesund

2013 ◽  
Vol 13 (1) ◽  
pp. 2273-2312
Author(s):  
T. Berg ◽  
K. A. Pfaffhuber ◽  
A. S. Cole ◽  
O. Engelsen ◽  
A. Steffen
Keyword(s):  
Relative Humidity ◽  
Wind Direction ◽  
Elemental Mercury ◽  
Atmospheric Mercury ◽  
The Arctic ◽  
Climate Indices ◽  
Atmospheric Conditions ◽  
Gaseous Elemental Mercury ◽  
Surface Emission ◽  
Wind Speeds

Abstract. Results from ten years of gaseous elemental mercury (GEM) measurements at Zeppelin Station, Ny-Ålesund, Svalbard, show no overall annual trend between 2000 and 2009. Seasonal trend analysis showed significantly decreasing trends in January, February, March and June and significantly increasing trends in May and July through December. Results showed that atmospheric mercury depletion events (AMDEs) were equally distributed between April and May with only a few having been observed in March and June. A negative correlation between AMDEs and temperature is reported and supports earlier observations that AMDEs tend to occur at low temperatures. Lower concentrations of GEM were seen at lower temperatures below a threshold of 0°C. The occurrence of AMDEs and wind direction were well correlated with the lowest GEM measured when the wind direction was from the Arctic Ocean region. Wind speed was found to not correlate with AMDEs, but the lowest GEM concentrations were observed at low wind speeds between 4 and 11 m s−1. AMDEs and relative humidity did not correlate well, but the lowest GEM levels appeared when the relative humidity was between 80 and 90%. Diurnal variation was observed especially during the month March and is likley due to daytime snow surface emission induced by solar radiation. Relationships between GEM concentration and the Northern Hemisphere climate indices were investigated to assess if these climate parameters might reflect different atmospheric conditions that enhance or reduce spring AMDE activity. No consistent pattern was observed.

scholarly journals Ten-year trends in atmospheric mercury concentrations, meteorological effects and climate variables at Zeppelin, Ny-Ålesund

2013 ◽  
Vol 13 (13) ◽  
pp. 6575-6586 ◽  
Author(s):  
T. Berg ◽  
K. A. Pfaffhuber ◽  
A. S. Cole ◽  
O. Engelsen ◽  
A. Steffen
Keyword(s):  
Relative Humidity ◽  
Wind Direction ◽  
Elemental Mercury ◽  
Atmospheric Mercury ◽  
The Arctic ◽  
Climate Indices ◽  
Atmospheric Conditions ◽  
Gaseous Elemental Mercury ◽  
Surface Emission ◽  
Wind Speeds

Abstract. Results from ten years of gaseous elemental mercury (GEM) measurements at Zeppelin station, Ny-Ålesund, Svalbard, show no overall annual trend between 2000 and 2009. Seasonal trend analysis showed significantly decreasing trends in January, February, March and June (−4.5 to −14.9 pg m−3 yr−1) and significantly increasing trends in May and July through December (1.5 to 28.7 pg m−3 yr −1). Results showed that atmospheric mercury depletion events (AMDEs) were equally distributed between April and May with only a few having been observed in March and June. A negative correlation between AMDEs and temperature is reported and supports earlier observations that AMDEs tend to occur at low temperatures. Lower concentrations of GEM were seen at lower temperatures below a threshold of 0 °C. The occurrence of AMDEs and wind direction were well correlated with the lowest GEM measured when the wind direction was from the Arctic Ocean region. Wind speed was found to not correlate with AMDEs, but the lowest GEM concentrations were observed at low wind speeds between 4 and 11 m s−1. AMDEs and relative humidity did not correlate well, but the lowest GEM levels appeared when the relative humidity was between 80 and 90%. Diurnal variation was observed especially during the month of March and is probably due to daytime snow surface emission induced by solar radiation. Relationships between GEM concentration and the Northern Hemisphere climate indices were investigated to assess if these climate parameters might reflect different atmospheric conditions that enhance or reduce spring AMDE activity. No consistent pattern was observed.

scholarly journals Trends in long-term gaseous mercury observations in the Arctic and effects of temperature and other atmospheric conditions

2010 ◽  
Vol 10 (10) ◽  
pp. 4661-4672 ◽  
Author(s):  
A. S. Cole ◽  
A. Steffen
Keyword(s):  
Polar Vortex ◽  
Elemental Mercury ◽  
Early Spring ◽  
Atmospheric Mercury ◽  
The Arctic ◽  
Atmospheric Conditions ◽  
Climate Data ◽  
Gaseous Elemental Mercury ◽  
Teleconnection Patterns ◽  
Kinetic Calculations

Abstract. Gaseous elemental mercury (GEM) measurements at Alert, Canada, from 1995 to 2007 were analyzed for statistical time trends and for correlations with meteorological and climate data. A significant decreasing trend in annual GEM concentration is reported at Alert, with an estimated slope of −0.0086 ng m−3 yr−1 (−0.6% yr−1) over this 13-year period. It is shown that there has been a shift in the month of minimum mean GEM concentration from May to April due to a change in the timing of springtime atmospheric mercury depletion events (AMDEs). These AMDEs are found to decrease with increasing local temperature within each month, both at Alert and at Amderma, Russia. These results support the temperature dependence suggested by previous experimental results and theoretical kinetic calculations on both bromine generation and mercury oxidation and highlight the potential for changes in Arctic mercury chemistry with climate. A correlation between total monthly AMDEs at Alert and the Polar/Eurasian Teleconnection Index was observed only in March, perhaps due to higher GEM inputs in early spring in those years with a weak polar vortex. A correlation of AMDEs at Alert with wind direction supports the origin of mercury depletion events over the Arctic Ocean, in agreement with a previous trajectory study of ozone depletion events. Interannual variability in total monthly depletion event frequency at Alert does not appear to correlate significantly with total or first-year northern hemispheric sea ice area or with other major teleconnection patterns. Nor do AMDEs at either Alert or Amderma correlate with local wind speed, as might be expected if depletion events are sustained by stable, low-turbulence atmospheric conditions. The data presented here – both the change in timing of depletion events and their relationship with temperature – can be used as additional constraints to improve the ability of models to predict the cycling and deposition of mercury in the Arctic.

scholarly journals Effects of temperature and other atmospheric conditions on long-term gaseous mercury observations in the Arctic

2009 ◽  
Vol 9 (6) ◽  
pp. 27167-27194
Author(s):  
A. S. Cole ◽  
A. Steffen
Keyword(s):  
Polar Vortex ◽  
Elemental Mercury ◽  
Early Spring ◽  
Atmospheric Mercury ◽  
The Arctic ◽  
Atmospheric Conditions ◽  
Climate Data ◽  
Gaseous Elemental Mercury ◽  
Teleconnection Patterns ◽  
Kinetic Calculations

Abstract. Gaseous elemental mercury (GEM) measurements at Alert, Canada, from 1995 to 2007 were analyzed for statistical time trends and for correlations with meteorological and climate data. A significant decreasing trend in annual GEM concentration is reported at Alert, with an estimated slope of −0.0086 ng m−3 yr−1 (−0.6% yr−1) over this 13-year period. It is shown that there has been a shift in the month of minimum mean GEM concentration from May to April due to a change in the timing of springtime atmospheric mercury depletion events (AMDEs). These AMDEs are found to decrease with increasing local temperature within each month, both at Alert and at Amderma, Russia. These results agree with the temperature dependence suggested by previous experimental results and theoretical kinetic calculations and highlight the potential for changes in Arctic mercury chemistry with climate. A correlation between total monthly AMDEs at Alert and the Polar/Eurasian Teleconnection Index was observed only in March, perhaps due to higher GEM inputs in early spring in those years with a weak polar vortex. A correlation of AMDEs at Alert with wind direction supports the origin of mercury depletion events over the Arctic Ocean, in agreement with a previous trajectory study of ozone depletion events. Interannual variability in total monthly depletion event frequency at Alert does not appear to correlate significantly with total or first-year northern hemispheric sea ice area or with other major teleconnection patterns. Nor do AMDEs at either Alert or Amderma correlate with local wind speed, as might be expected if depletion events are sustained by stable, low-turbulence atmospheric conditions. The data presented here – both the change in timing of depletion events and their relationship with temperature – can be used as additional constraints to improve the ability of global models to predict the cycling and deposition of mercury in the Arctic.

scholarly journals A synthesis of atmospheric mercury depletion event chemistry linking atmosphere, snow and water

2007 ◽  
Vol 7 (4) ◽  
pp. 10837-10931 ◽  
Author(s):  
A. Steffen ◽  
T. Douglas ◽  
M. Amyot ◽  
P. Ariya ◽  
K. Aspmo ◽  
...  
Keyword(s):  
Sea Ice ◽  
Current Knowledge ◽  
Kinetic Studies ◽  
Elemental Mercury ◽  
Atmospheric Mercury ◽  
The Arctic ◽  
Polar Regions ◽  
Environmental Media ◽  
Gaseous Elemental Mercury ◽  
History Of

Abstract. It was discovered in 1995 that, during the spring time, unexpectedly low concentrations of gaseous elemental mercury (GEM) occurred in the Arctic air. This was surprising for a pollutant known to have a long residence time in the atmosphere; however conditions appeared to exist in the Arctic that promoted this depletion of mercury (Hg). This phenomenon is termed atmospheric mercury depletion events (AMDEs) and its discovery has revolutionized our understanding of the cycling of Hg in Polar Regions while stimulating a significant amount of research to understand its impact to this fragile ecosystem. Shortly after the discovery was made in Canada, AMDEs were confirmed to occur throughout the Arctic, sub-Artic and Antarctic coasts. It is now known that, through a series of photochemically initiated reactions involving halogens, GEM is converted to a more reactive species and is subsequently associated to particles in the air and/or deposited to the polar environment. AMDEs are a means by which Hg is transferred from the atmosphere to the environment that was previously unknown. In this article we review the history of Hg in Polar Regions, the methods used to collect Hg in different environmental media, research results of the current understanding of AMDEs from field, laboratory and modeling work, how Hg cycles around the environment after AMDEs, gaps in our current knowledge and the future impacts that AMDEs may have on polar environments. The research presented has shown that while considerable improvements in methodology to measure Hg have been made the main limitation remains knowing the speciation of Hg in the various media. The processes that drive AMDEs and how they occur are discussed. As well, the roles that the snow pack, oceans, fresh water and the sea ice play in the cycling of Hg are presented. It has been found that deposition of Hg from AMDEs occurs at marine coasts and not far inland and that a fraction of the deposited Hg does not remain in the same form in the snow. Kinetic studies undertaken have demonstrated that bromine is the major oxidant depleting Hg in the atmosphere. Modeling results demonstrate that there is a significant deposition of Hg to Polar Regions as a result of AMDEs. Models have also shown that Hg is readily transported to the Arctic from source regions, at times during springtime when this environment is actively transforming Hg from the atmosphere to the snow and ice surfaces. The presence of significant amounts of methyl Hg in snow in the Arctic surrounding AMDEs is important because this species is the link between the environment and impacts to wildlife and humans. Further, much work on methylation and demethylation processes have occurred but are not yet fully understood. Recent changes in the climate and sea ice cover in Polar Regions are likely to have strong effects on the cycling of Hg in this environment; however more research is needed to understand Hg processes in order to formulate meaningful predictions of these changes. Mercury, Atmospheric mercury depletion events (AMDE), Polar, Arctic, Antarctic, Ice

scholarly journals Investigation of processes controlling GEM oxidation at mid-latitudinal marine, coastal, and inland sites

2016 ◽  
Author(s):  
Z. Ye ◽  
H. Mao ◽  
C.-J. Lin ◽  
S. Y. Kim
Keyword(s):  
Marine Boundary Layer ◽  
State Of The Art ◽  
Elemental Mercury ◽  
Atmospheric Mercury ◽  
Chemical Mechanism ◽  
Atmospheric Conditions ◽  
Gaseous Elemental Mercury ◽  
Mixing Ratios ◽  
Marine Site ◽  
Study Sites

Abstract. A box model incorporating a state-of-the-art chemical mechanism for atmospheric mercury (Hg) cycling was developed to investigate oxidation of gaseous elemental mercury (GEM) at three locations in the northeastern United States: Appledore Island (marine), Thompson Farm (coastal, rural), and Pack Monadnock (inland, rural, elevated). The chemical mechanism improved model's ability to simulate the formation of gaseous oxidized mercury (GOM) at the study sites. At the coastal and inland sites, GEM oxidation was predominated by O3 and OH, contributing 80–99 % of total GOM production during daytime. H2O2 initiated GEM oxidation was significant (~ 33 % of the total GOM) at the inland site during nighttime. In the marine boundary layer (MBL), Br and BrO were dominant GEM oxidants contributing ~ 70 % of the total GOM production during mid-day, while O3 dominated GEM oxidation (50–90 % of GOM production) over the remaining day. Following the production of HgBr from GEM + Br, HgBr was oxidized by BrO, HO2, OH, ClO, and IO to form Hg(II) brominated GOM species. However, under atmospheric conditions, the prevalent GEM oxidants in the MBL could be Br / BrO or O3 / OH depending on Br and BrO mixing ratios. Relative humidity and products of the CH3O2 + BrO reaction possibly affected significantly the mixing ratios of Br or BrO radicals and subsequently GOM formation. Gas-particle partitioning could be potentially important in the production of GOM as well as Br and BrO at the marine site.

scholarly journals A synthesis of atmospheric mercury depletion event chemistry in the atmosphere and snow

2008 ◽  
Vol 8 (6) ◽  
pp. 1445-1482 ◽  
Author(s):  
A. Steffen ◽  
T. Douglas ◽  
M. Amyot ◽  
P. Ariya ◽  
K. Aspmo ◽  
...  
Keyword(s):  
Sea Ice ◽  
Current Knowledge ◽  
Kinetic Studies ◽  
Elemental Mercury ◽  
Atmospheric Mercury ◽  
The Arctic ◽  
Polar Regions ◽  
Environmental Media ◽  
Gaseous Elemental Mercury ◽  
Low Concentrations

Abstract. It was discovered in 1995 that, during the spring time, unexpectedly low concentrations of gaseous elemental mercury (GEM) occurred in the Arctic air. This was surprising for a pollutant known to have a long residence time in the atmosphere; however conditions appeared to exist in the Arctic that promoted this depletion of mercury (Hg). This phenomenon is termed atmospheric mercury depletion events (AMDEs) and its discovery has revolutionized our understanding of the cycling of Hg in Polar Regions while stimulating a significant amount of research to understand its impact to this fragile ecosystem. Shortly after the discovery was made in Canada, AMDEs were confirmed to occur throughout the Arctic, sub-Artic and Antarctic coasts. It is now known that, through a series of photochemically initiated reactions involving halogens, GEM is converted to a more reactive species and is subsequently associated to particles in the air and/or deposited to the polar environment. AMDEs are a means by which Hg is transferred from the atmosphere to the environment that was previously unknown. In this article we review Hg research taken place in Polar Regions pertaining to AMDEs, the methods used to collect Hg in different environmental media, research results of the current understanding of AMDEs from field, laboratory and modeling work, how Hg cycles around the environment after AMDEs, gaps in our current knowledge and the future impacts that AMDEs may have on polar environments. The research presented has shown that while considerable improvements in methodology to measure Hg have been made but the main limitation remains knowing the speciation of Hg in the various media. The processes that drive AMDEs and how they occur are discussed. As well, the role that the snow pack and the sea ice play in the cycling of Hg is presented. It has been found that deposition of Hg from AMDEs occurs at marine coasts and not far inland and that a fraction of the deposited Hg does not remain in the same form in the snow. Kinetic studies undertaken have demonstrated that bromine is the major oxidant depleting Hg in the atmosphere. Modeling results demonstrate that there is a significant deposition of Hg to Polar Regions as a result of AMDEs. Models have also shown that Hg is readily transported to the Arctic from source regions, at times during springtime when this environment is actively transforming Hg from the atmosphere to the snow and ice surfaces. The presence of significant amounts of methyl Hg in snow in the Arctic surrounding AMDEs is important because this species is the link between the environment and impacts to wildlife and humans. Further, much work on methylation and demethylation processes has occurred but these processes are not yet fully understood. Recent changes in the climate and sea ice cover in Polar Regions are likely to have strong effects on the cycling of Hg in this environment; however more research is needed to understand Hg processes in order to formulate meaningful predictions of these changes.

scholarly journals Atmospheric mercury over sea ice during the OASIS-2009 campaign

2013 ◽  
Vol 13 (14) ◽  
pp. 7007-7021 ◽  
Author(s):  
A. Steffen ◽  
J. Bottenheim ◽  
A. Cole ◽  
T. A. Douglas ◽  
R. Ebinghaus ◽  
...  
Keyword(s):  
Sea Ice ◽  
Elemental Mercury ◽  
Atmospheric Mercury ◽  
The Arctic ◽  
Mercury Deposition ◽  
International Polar Year ◽  
Bromine Oxide ◽  
Gaseous Elemental Mercury ◽  
Significant Difference ◽  
Mercury Cycle

Abstract. Measurements of gaseous elemental mercury (GEM), reactive gaseous mercury (RGM) and particulate mercury (PHg) were collected on the Beaufort Sea ice near Barrow, Alaska, in March 2009 as part of the Ocean-Atmosphere-Sea Ice-Snowpack (OASIS) and OASIS-Canada International Polar Year programmes. These results represent the first atmospheric mercury speciation measurements collected on the sea ice. Concentrations of PHg averaged 393.5 pg m−3 (range 47.1–900.1 pg m−3) and RGM concentrations averaged 30.1 pg m−3 (range 3.5–105.4 pg m−3) during the two-week-long study. The mean concentration of GEM during the study was 0.59 ng m−3 (range 0.01–1.51 ng m−3) and was depleted compared to annual Arctic ambient boundary layer concentrations. It is shown that when ozone (O3) and bromine oxide (BrO) chemistry were active there is a positive linear relationship between GEM and O3, a negative one between PHg and O3, a positive correlation between RGM and BrO, and none between RGM and O3. For the first time, GEM was measured simultaneously over the tundra and the sea ice. The results show a significant difference in the magnitude of the emission of GEM from the two locations, with significantly higher emission over the tundra. Elevated chloride levels in snow over sea ice are proposed to be the cause of lower GEM emissions over the sea ice because chloride has been shown to suppress photoreduction processes of RGM to GEM in snow. Since the snowpack on sea ice retains more mercury than inland snow, current models of the Arctic mercury cycle may greatly underestimate atmospheric deposition fluxes because they are based predominantly on land-based measurements. Land-based measurements of atmospheric mercury deposition may also underestimate the impacts of sea ice changes on the mercury cycle in the Arctic. The predicted changes in sea ice conditions and a more saline future snowpack in the Arctic could enhance retention of atmospherically deposited mercury and increase the amount of mercury entering the Arctic Ocean and coastal ecosystems.

scholarly journals Natural and anthropogenic atmospheric mercury in the European Arctic: a fractionation study

2011 ◽  
Vol 11 (13) ◽  
pp. 6273-6284 ◽  
Author(s):  
A. O. Steen ◽  
T. Berg ◽  
A. P. Dastoor ◽  
D. A. Durnford ◽  
O. Engelsen ◽  
...  
Keyword(s):  
Elemental Mercury ◽  
Atmospheric Mercury ◽  
Seasonal Distribution ◽  
The Arctic ◽  
Mercury Distribution ◽  
Gaseous Elemental Mercury ◽  
Reactive Gaseous Mercury ◽  
Complete Dataset ◽  
European Arctic ◽  
Non Local

Abstract. Gaseous elemental mercury (GEM) is converted to reactive gaseous mercury (RGM) during springtime Atmospheric Mercury Depletion Events (AMDE). This study reports the longest time series of GEM, RGM and particle-bound mercury (PHg) concentrations from a European Arctic site. From 27 April 2007 until 31 December 2008 composite GEM, RGM and PHg measurements were conducted in Ny-Ålesund (78° 54′ N, 11° 53′ E). The average concentrations of the complete dataset were 1.6 ± 0.3 ng m−3, 8 ± 13 pg m−3 and 8 ± 25 pg m−3 for GEM, RGM and PHg, respectively. For the complete dataset the atmospheric mercury distribution was 99 % GEM, whereas RGM and PHg constituted <1 %. The study revealed a seasonal distribution of GEM, RGM and PHg previously undiscovered in the Arctic. Increased concentrations of RGM were observed during the insolation period from March through August, while increased PHg concentrations occurred almost exclusively during the spring AMDE period in March and April. The elevated RGM concentrations suggest that atmospheric RGM deposition also occurs during the polar summer. RGM was suggested as the precursor for the PHg existence, but long range transportation of PHg has to be taken into consideration. Still there remain gaps in the knowledge of how RGM and PHg are related in the environment. RGM and PHg accounted for on average about 10 % of the depleted GEM during AMDEs. Although speculative, the fairly low RGM and PHg concentrations supported by the predominance of PHg with respect to RGM and no clear meteorological regime associated with these AMDEs would all suggest the events to be of non-local origin. With some exceptions, no clear meteorological regime was associated with the GEM, RGM and PHg concentrations throughout the year.

scholarly journals Overview of mercury measurements in the Antarctic troposphere

2010 ◽  
Vol 10 (7) ◽  
pp. 3309-3319 ◽  
Author(s):  
A. Dommergue ◽  
F. Sprovieri ◽  
N. Pirrone ◽  
R. Ebinghaus ◽  
S. Brooks ◽  
...  
Keyword(s):  
Elemental Mercury ◽  
Reaction Rate Constant ◽  
Atmospheric Mercury ◽  
Water Soluble ◽  
The Arctic ◽  
Polar Regions ◽  
Priority List ◽  
Gaseous Elemental Mercury ◽  
Modelling Studies ◽  
The Antarctic

Abstract. Polar ecosystems are considered to be the last pristine environments of the earth relatively uninfluenced by human activities. Antarctica in particular, compared to the Arctic is considered to be even less affected by any kind of anthropogenic influences. Once contaminants reach the Polar Regions, their lifetime in the troposphere depends on local removal processes. Atmospheric mercury, in particular, has unique characteristics that include long-range transport to Polar Regions and the transformation to more toxic and water-soluble compounds that may potentially become bioavailable. These chemical-physical properties have placed mercury on the priority list of an increasing number of International, European and National conventions, and agreements, aimed at the protection of the ecosystems including human health (i.e. GEO, UNEP, AMAP, UN-ECE, HELCOM, OSPAR). This interest, in turn, stimulates a significant amount of research including measurements of gaseous elemental mercury reaction rate constant with atmospheric oxidants, experimental and modelling studies in order to understand the cycling of mercury in Polar Regions, and its impact to these ecosystems. Special attention in terms of contamination of Polar Regions is paid to the consequences of the springtime phenomena, referred to as "Atmospheric Mercury Depletion Events" (AMDEs), during which elemental mercury through a series of photochemically-initiated reactions involving halogens, may be converted to a reactive form that may accumulate in polar coastal, or sea ice, ecosystems. The discovery of the AMDEs, first noted in the Arctic, has also been observed at both poles and was initially considered to result in an important net input of atmospheric mercury into the polar surfaces. However, recent studies point out that complex processes take place after deposition that may result in less significant net-inputs from the atmosphere since a fraction, sometimes significant, of deposited mercury may be recycled. Therefore, the contribution of this unique reactivity occurring in polar atmospheres to the global budget of atmospheric mercury, and the role played by snow and ice surfaces of these regions, are important issues. This paper presents a review of atmospheric mercury studies conducted in the Antarctic troposphere, both at coastal locations and on the Antarctic Plateau since 1985. Our current understanding of atmospheric reactivity in this region is also presented.

scholarly journals The variability in Gaseous Elemental Mercury at Villum Research Station, Station Nord in North Greenland from 1999 to 2017

2020 ◽  
Author(s):  
Henrik Skov ◽  
Jens Hjorth ◽  
Claus Nordstrøm ◽  
Bjarne Jensen ◽  
Christel Christoffersen ◽  
...  
Keyword(s):  
Relaxation Time ◽  
Elemental Mercury ◽  
Atmospheric Mercury ◽  
The Arctic ◽  
Back Trajectory ◽  
Research Station ◽  
Emission Reductions ◽  
Gaseous Elemental Mercury ◽  
Mercury Analysis ◽  
North Greenland

Abstract. Mercury is ubiquitous in the atmosphere and atmospheric transport is an important source for this element in the Arctic. Measurements of gaseous elemental mercury (GEM) have been carried out at the Villum Research Station (Villum) at Station Nord, situated in north Greenland. The measurements cover the period 1999–2017 with a gap in the data for the period 2003–2008 (for a total of 11 years). The measurements were compared with model results from the Danish Eulerian Hemispheric Model (DEHM) model that describes the contribution from direct anthropogenic transport, marine emission and general background concentration. The percentage of time spent over different surfaces was calculated by back-trajectory analysis and the reaction kinetics was determined by comparison with ozone. The GEM measurements were analysed for trends, both seasonal and annually. The only significant trend found was a negative one for the winter months. Comparison of the measurements to simulations using the Danish Eulerian Hemispheric Model (DEHM) indicated that direct transport of anthropogenic emissions of mercury accounts for between 14 and 17 % of the measured mercury. Analysis of the kinetics of the observed Atmospheric Mercury Depletion Events (AMDEs) confirms the results of a previous study at Villum of the competing reactions of GEM and ozone with Br, which suggests a lifetime of GEM on the order of a month. However, a GEM lifetime of 12 months gave the best agreement between model and measurements. The chemical lifetime is shorter and thus the apparent lifetime appears to be the result of deposition followed by reduction and reemission; for this reason the term relaxation time is preferred to lifetime for GEM. The relaxation time for GEM causes a delay between emission reductions and the effect on actual concentrations. No annual trend was found for the measured concentrations of GEM over the measurement period despite emission reductions. This is interesting and together with low direct transport of GEM to Villum, as found by the DEHM model, it shows that the dynamics of GEM is very complex. Therefore in the coming years, intensive measurement networks is highly needed to describe the global distribution of mercury in the environment as the use of models to predict future levels will still be highly uncertain. The situation is increasingly complex due to global change that most likely will change the transport patterns of mercury not only in the atmosphere but also between matrixes.

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