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AMAP Greenland and the Faroe Islands 1997-2001

2 Measurements of gaseous elemental mercury on the Faroe Islands

Henrik Skov
Maria C. Nielsdóttir
Michael E. Goodsite
Jesper Christensen
Carsten A. Skjøth
Gerald L. Geernaert
Ole Hertel
Johanna Olsen¹

National Environment and Research Institute Department of Atmospheric Environment

Summary and conclusions

Gaseous Elemental Mercury (GEM) has been measured on the Faroe Islands from May 2000 through March 2001. The measured data has been analysed together with basic meteorology, trajectories from the Atmospheric Chemistry and Deposition model (ACDEP model), and the modelled GEM concentrations from Danish Eulerian Hemispheric Model (DEHM). The models were subsequently used to determine the most likely source regions, which are associated with the measured concentrations. The air concentration time series shows periods with elevated mercury concentrations (>1.5 ngHg/m³, the generally accepted global background average) which were attributed to two potential causes: local sources and long range transport. After a detailed analysis, it was determined that local sources were not responsible for the elevated levels observed, and it was further determined that the elevated levels were caused by long-range transport from Europe, most notably the UK. Explanations of concentrations lower than the global background average are discussed as well.

1 Measurements of gaseous elemental mercury on the Faroe Islands

1.1 Introduction

Mercury on the Faroe Islands is of both scientific and public concern due to the high concentrations in e.g. pilot whales and higher predators of fish, where up to 3 ppm Hg has been measured (AMAP, 1998). Furthermore, it has been shown that the present levels of mercury in sea animals have a negative effect also on the health of the local populations, when these animals are used as food supply (Grandjean et al. 1998). High concentrations of mercury have also been observed in peat cores taken on the Faroe Islands (Shotyk et al. 2001, see also Chap. 1). However, these levels cannot directly be linked to atmospheric concentrations or deposition, as the levels are not only a function of deposition but also bioaccumulation, the runoff area size and the geochemistry of the profile. It has also been discovered that trout from the Faroe Islands contain high levels of mercury (Larsen and Dam, 1999). In spite of strong indications of high mercury exposure to marine food chains and terrestrial ecosystems (implied by the core data), there has not been any study reported, which explains the sources responsible for the high mercury levels and/or how to mitigate the problem.
The aim of this study is to report a recently collected time series of atmospheric concentrations of gaseous elemental mercury (GEM) collected during roughly a one year period starting in May 2000, and to explore relationships between anthropogenic mercury source regions and deposition to the Faroe Islands. The results are compared with model calculations using the Danish Eulerian Hemispheric Model (DEHM) including scenario calculations. Furthermore trajectory calculations were carried out using the trajectory model developed for the Atmospheric Chemistry and Deposition model (ACDEP, Hertel et al. 1995).

1.2 Experimental

The measurements were performed with a Tekran Model 2537A Mercury Vapour Analyser equipped with an internal permeation source ensuring the stability of the instrument. GEM in ambient air is adsorbed on a gold trap and after sampling the adsorbed mercury is thermally desorbed and detected by Cold Vapour Atomic Fluorescence Spectrophotometry. The monitor is equipped with two gold traps, so continuous samples are taken with 5 minutes resolution, which for practical reasons were averaged to 1 hour mean values. Measurements were carried out based on the Standard Operating Procedure Manual for Total Gaseous Mercury Measurements for the Canadian Atmospheric Mercury Measurement Network (Steffen et al. 1999). The estimated uncertainty from manual calibration, collection efficiency etc. is estimated to 10 % (2 times standard deviation) for values above 1 ng/m³. However, complications might significantly have effected the measurements in the Faroe Islands. The very high relative humidity (above 95%) may have affected the measurements (Matthew Landis, Private communication, 2001, TEKRAN manual p. 2-1) and the high sea spray concentrations have previously been observed to effect the measurements of GEM (Ebinghaus et al. 2000).

Model calculations of the concentrations of GEM were carried out as well by the Danish Hemispheric Model (DEHM) that is a 3 dimensional eulerian model. The model is described in detail elsewhere (Kämäri et al., 1998; Christensen, 1997,1999 and Barrie et al. 2001).
In the current version the emissions of anthropogenic mercury are based on the new global inventory of mercury emissions for 1995 on a 1^ox1^o grid (Pacyna et. al., 2002, private communication), which includes emissions of Hg⁰, reactive gaseous mercury and particulate mercury. There are not any re-emissions from land and oceans, instead a background concentration on 1.5 ng/m³ of Hg⁰ is used as initial concentrations and boundary conditions. The chemical reaction scheme is based on Petersen et al. (1998) and includes 13 mercury species, 3 in the gas-phase (Hg⁰, HgO and HgCl₂), 9 species in the aqueous-phase and 1 in particulate phase.
During the polar sunrise in the Arctic an additional fast oxidation rate of Hg⁰ to HgO is assumed. Inside the boundary layer over sea ice during sunny conditions it is assumed that there is an additional oxidation rate of ¼ hour^-1. The fast oxidation stops, when surface temperature exceeds -4^oC. The removals of Hg⁰ are due to the chemistry and the uptake by cloud water. The dry deposition velocities of the reactive gaseous mercury species are based on the resistance method, where the surface resistance similar to HNO₃ is used. Dry deposition velocities for RGM have been measured and reported The from Barrow and are similar to those for HNO₃ (Lindberg et al, 2002). The wet deposition of reactive and particulate mercury is parameterized by using a simple scavenging coefficients formulation with different in-cloud and below-cloud scavenging coefficients (see Christensen, 1997).

1.3 Site

The monitor was set up on the east part of Tórshavn, the capital on Faroe Islands, located 62^o01’N and 6^o47’W. This location is approximately 400 km north of Scotland, 600 km west of Norway and 500 km east of Iceland. The monitor was placed in a residential area. Because of this location, we tested the influence of possible local sources by examining the time series based on wind direction and concentration. Our results did not show any correlation between these quantities, both during quiescent periods and during episodes, thus allowing us to conclude that any local sources were insignificant.

1.4 Results

The results of the one year of measurements, i.e., from May 2000 to March 2001, of GEM are shown in figure 1.

View the image in full size
Figure 1.
GEM concentrations at Faroe Islands from May 2000 to March 2001. Values are 1 hour averages.

The data varies between a general level of about 2 ng/m³ in the beginning of the period (in May) with a decrease down to about 0.5 ng/m³ in July, August with a slight increase to about 1.7 ng/m³ in January and February. On top of this, some peaks are seen with values up to 3.4 ng/m³, i.e., most notably in an episode from June 21 to 25. During this episode a low-pressure system above the British Islands forced air masses to be transported from Europe up to the Faroe Islands, with the UK being the most recent emission region loading the air mass. In the following period the dominant wind direction was from west (from the open Ocean), although there are also days where there were air mass trajectories originated from the UK. The concentration levels in the period of generally westerly winds varied between 0.8 to 1.3 ng/m³. The daily mean temperature in the periods varied from 5.7 ⁰C in May and June to 12.1 ⁰C in August. The relative humidity was always close to 95%.

Through the rest of the year the concentrations slowly rise to a level of 1.7 ng/m³ in January and February 2001.

The model calculations of daily average Hg⁰ concentrations together with measured daily average GEM concentrations are shown in figure 2.

View the image in full size
Figure 2.
Comparison of daily mean GEM concentrations at Faroe Islands measured by a TEKRAN Hg analyser and Hg0 obtained by DEHM.

The calculated concentrations are close to a constant level of 1.6 ng/m³ throughout the one year period with only a minor decrease in April and May noted in both 2000 and 2001. The decrease is caused by mercury depletion episodes in the Arctic (MDE’s) as documented in scenario calculations with (shown here) and without MDE, see also Chap. 3 Atmospheric Modelling.

1.5 Discussion

The measurements of GEM on the Faroe Islands are comparable in magnitude with the levels in 1999 from Mace Head on the west coast of Ireland (Ebinghaus et al. 2000) and with the values from Harwell an inland location in southern England in June 1995 to May 1996 (Lee et al. 2000). The average concentrations at the three localities are 1.68 ng/m³ at Mace Head, 1.7 ng/m³ at Harwell and 1.4 ng/m³ on the Faroe Islands. The largest concentration levels at Harwell and at Mace Head are practically identical, with the concentrations on Faroe Islands somewhat smaller. It is generally believed that the background concentration of mercury in the Northern Hemisphere is about 1.5 ng/m³ and fairly constant. The central question is of course if these measurements reflect the actual ambient concentrations or if they are lowered due to passivation of the gold trap.
Ebinghaus et al. mentioned passivation of the gold traps as a problem measuring at Mace Head caused by sea spray (the TEKRAN manual notes that low values are generally due to trap passivation too). A similar complication could be the reason for the relative low values at Faroe Islands. However Lee et al. reported similar low values at the inland station where sea spray should be of minor importance. If the low values are real they might be explained by marine air containing significant amounts of Clx and/or Brx that convert GEM to reactive gaseous mercury (RGM) by a mechanism similar to those observed in the high Arctic during polar spring (Schroeder et al. 1998, Berg et al. 2001, Brooks et al. 2002, Lindberg et al, 2002, Skov et al. 2001).

However, initiatives should be taken to ensure that passivation of sample gold cartridges do not occur in the future. Therefore all future GEM measurements should be carried out, by sampling through a heated line and through a soda-lime trap (recommended by Matthew S. Landis private communication, 2001 and the Canadian Catnet protocol).

The reason for the high concentrations of up to 3.5 ng/m³ was examined. The high levels of GEM observed on Faroe Islands was suggested to originate from local anthropogenic sources but a questionnaire survey demonstrated that practically all mercury containing waste was collected and sent to Denmark for further treatment so this possibility can be ruled out.
Alternatively, long range transport of GEM from the source regions on the British Isles and Europe could cause the episodic increase in the GEM concentrations. In fact the air masses in the episode from June 21 to 25 were calculated to arrive from British Isles and the European Continent bringing in air masses with generally elevated air pollution (Kemp et al. 1993), i.e. including mercury. Afterwards the wind was dominated by more westerly wind with resulting lower concentrations.

The model calculations gave a close to constant level of GEM throughout the period in accordance with the general belief that GEM has a long atmospheric lifetime of about 1 year (Lin and Pekhonen, 1999). If Clx and/or Brx are important sinks in the marine boundary layer then, this will shorten the lifetime of GEM, significantly creating a more varying time series of GEM in accordance with our observations.

1.6 Conclusion

The analysis of the concentrations of GEM at Faroe Islands shows that the levels are slightly lower than those observed at Harwell and at Mace Head. In short episodes, high concentrations were observed but the direct transport of GEM cannot explain the high levels observed in e.g. trout and peat since the transfer function for mercury from the atmosphere to the terrestrial system and further to the biosphere, is simply not known, but assumed to be primarily by wet depostion, upon conversion of GEM to RGM.
The highest GEM concentrations observed on the Faroe Islands can be explained by long-range transport from the British Isles and Europe. Whereas there is not any direct explanation of the low values, they might be explained by interference from humidity and/or sea spray. Therefore future measurements at coastal sites should be carried out through a heated line and through a soda lime trap. Comparison with the results from DEHM could not at present explain the GEM concentrations in Faroe Islands, but the model is under development, especially with respect to the chemical scheme.

Further atmospheric studies are needed in order to definitively answer the question of the high Hg levels measured in peat and in marine mammals near the Faroe Islands. In addition, if the low atmospheric concentrations observed on Faroe Islands are real then they indicate that GEM has a shorter lifetime within the marine boundary layer believed and thus deposition of atmospheric mercury to the marine system may be more important than previously believed.

2 Acknowledgements

We wish to thank Bjarne Jensen and Hanne Langberg, NERI for technical support, and the National Historic Museum for lending us their laboratory. Per Løfstrøm is acknowledged for his advice concerning meteorology and Niels Zeuthen Heidam for administration of the Danish contribution to the Arctic Monitoring and Assessment programme. The Danish Environmental Protection Agency financially supported this work with means from the MIKA/DANCEA funds for Environmental Support to the Arctic Region. The results and conclusions presented are those of the authors alone and do not necessarily reflect the opinions of our employers or grant agencies. 

3 References

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Larsen, R.B. and Dam, M. (1999). AMAP phase I report, The Faroe Islands. The Food and Environmental Agency, Faroe Islands, 1999:1, pp 43.

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Petersen, G. Munthe, J. Pleijel, K. Bloxam, R. and Vinod Kumar, A.; A comprehensive eulerian modeling framework for airborne mercury species: development and testing of the tropospheric chemistry module (TCM). Atm. Env., 32. (1998) 829-843.

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Shotyk, W. Goodsite, M.E. Roos-Barraclough, F. Givelet, N. and Knudsen, K. Millennium-scale records of atmospheric mercury deposition revealed by peat bogs on the Faroe Islands. Under preparation (2001).

Skov, H. Goodsite, M.E. and Christensen, J. Atmospheric mercury in Arctic, future challenges with respect to chemical kinetics. Oral presentation at The Second Informal Conference on Reaction Kinetics and Atmospheric Chemistry; NORFA and COGCI, Helsingør, Denmark, June, 2001.

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1 Food and Environmental Agency, Faroe Islands

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