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Atmospheric mercury species in the European Arctic: Measurements and modelling
Atmospheric Environment 35 (2001) 2569}2582
Atmospheric mercury species in the European Arctic: measurements and modelling Torunn Berg *, Jerzy Bartnicki, John Munthe, Heikki Lattila, Jaroslaw Hrehoruk, Andrzej Mazur Norwegian Institute for Air Research (NILU), P.O. Box 100, N-2007 Kjeller, Norway Norwegian Meteorological Institute (DNMI), P.O. Box 43 Blindern, N-0313 Oslo, Norway Swedish Environmental Research Institute (IVL), P.O. Box 47086, S-402 58 Go( teborg, Sweden Finnish Meteorological Institute (FMI), Sahaajankatu 20E; FIN-00810 Helsinki, Finland Institute for Meteorology and Water Management (IMGW), Ul. Podlesna 61, PL-01-673 Warsaw, Poland Received 18 January 2000; received in revised form 30 June 2000; accepted 14 July 2000
Abstract Concentrations of di!erent species of mercury in arctic air and precipitation have been measured at Ny-As lesund (Svalbard) and Pallas (Finland) during 1996}1997. Typical concentrations for vapour phase mercury measured at the two stations were in the range of 0.7}2 ng m\ whereas particulate mercury concentrations were below 5 pg m\. Total mercury in precipitation was in the range 3}30 ng l\. In order to evaluate the transport and deposition of mercury to the arctic from European anthropogenic sources, the Eulerian transport model HMET has been modi"ed and extended to also include mercury species. A scheme for chemical conversion of elemental mercury to other species of mercury and deposition characteristics of di!erent mercury species have been included in the model. European emission inventories for three di!erent forms of Hg (Hg, HgCl and Hg ) have been implemented in the numerical grid system for the HMET model. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Mercury; Modelling; Mercury species; Vapour-phase mercury; Particulate-phase mercury; Precipitation; Air
1. Introduction The high level of mercury contamination observed in the once pristine Arctic environment is largely connected to the unique properties of mercury as a metal. Mercury in the atmosphere exists predominantly in the gaseous state, which, in remote areas such as the Arctic, consists almost exclusive of the elemental form, Hg. The particle phase makes up only a small fraction (&1%) of total airborne concentrations. This fraction, however, still plays an important role for determining the deposition #uxes of mercury (Lindberg et al., 1991; Lu and Schroeder, 1999). The elemental form of mercury is rela-
* Corresponding author. Tel.: #47-63-89-82-49; fax: #4763-89-80-50. E-mail address: [email protected] (T. Berg).
tively unreactive, has a high volatility and low solubility in natural waters, which imparts a long atmospheric residence time of &6}24 months (Slemr et al., 1985; Lindqvist and Rodhe, 1985). Once introduced into the atmosphere, mercury can circulate for long periods of time prior to being transferred to a permanent sink reservoir. Similar to many persistent, semi-volatile organic compounds it can be re-emitted into the atmosphere and re-enter the global atmospheric mercury cycle. The cold Arctic climate may favour a "nal deposition here rather than in warmer climates. Mercury is removed from the atmosphere through both wet- and dry-deposition processes of species like Hg, Hg(II) and Hg(p) (Schroeder and Munthe, 1998). Recently, frequent episodic depletions in mercury vapour concentrations at Alert in the Canadian high Arctic, closely resembling ozone depletions in Arctic surfacelevel air during the 3-month period following polar
1352-2310/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 0 ) 0 0 4 3 4 - 9
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sunrise, have been observed (Schroeder et al., 1997, 1998). Preliminary observations have indicated that this atmospheric Hg depletion mechanism involves conversion of mercury from the gas phase to the particulate phase (Lu et al., 1998). This transformation process is unique to the Arctic and may constitute an important pathway for the entry of mercury to this sensitive ecosystem. At Alert the highest mercury vapour concentrations are observed during summer, probably due to a temperature and/or sunlight-induced emission/re-emission of volatile mercury species from the Earth's surface during that season of the year (Xiao et al., 1991), and not solely due to atmospheric transport from any speci"c sector with large mercury emissions (Schroeder et al., 1997). Measurements of atmospheric mercury over the Atlantic Ocean in the years 1977}1980, 1990 and 1994 suggest an increase between 1977 and 1990 and a decrease after 1990 (Slemr and Langer, 1992; Iverfeldt et al., 1995; Slemr, 1996; Slemr and Scheel, 1998). Numerical models of atmospheric transport and deposition are essential tools for the understanding of individual mercury processes and their interaction with the atmospheric system, and for the interpretation of "eld measurements data. Modelling of mercury has undergone rapid development during the last decade and several large-scale models are in operation today (Petersen et al., 1995, 1998; Galperin et al., 1995; Shannon and Voldner, 1995; Ryaboshapko et al., 1999). The aims of this study were to determine di!erent species of mercury in Arctic air and precipitation, and to estimate the role of long-range transport of mercury to the Arctic from European sources.
2. Experimental 2.1. Site locations and measurement programme Monitoring of atmospheric mercury in the Arctic was initiated as a part of the Arctic Monitoring and Assessment Programme's (AMAP) "rst phase in 1994. Measurements of vapour-phase mercury are carried out by NILU at Ny-As lesund, Svalbard (78354N, 11353E), and measurements of vapour phase mercury and total mercury in precipitation are carried out by IVL at Pallas, Finland (67322N, 26339E). Ny-As lesund is a small settlement near sea-level on the western coast of Spitzbergen (Fig. 1). There are surrounding mountains with tops about 1000}1500 m altitude in the region. Precipitation sampling is carried out close to the settlement, whereas air sampling is performed at the research station on the nearby Zeppelin mountain (474 m a.s.l.), accesible from Ny-As lesund by cable car. Pallas (Fig. 1) is a national park in Northern Finland with marked hills up to 727 m a.s.l. The research station is situated far from any settlement, in a hillside forest clearing, 566 m a.s.l.
Fig. 1. Location of the sampling stations.
2.2. Vapour-phase mercury At Ny-As lesund, one 24 h sample of vapour-phase mercury was collected manually each week by amalgamation on gold traps. The sampling system consisted of a sample line (Te#on), a quartz-wool plug, two goldtraps in series, a gas meter and a pump. The gold traps consisted of 12 cm quartz tubes "lled with a wire made of gold and platinum. The thickness of the wire was about 10 m. Usually about 1000 l of air was sampled at a #ow rate of about 0.7 l min\. At the Zeppelin mountain in the high Arctic there are probably less chemical species in the air, which can a!ect the collection ability of the gold traps than at lower latitutes. Two parallel sampling systems were operated. The system was somewhat modi"ed during the period of measurements. Until April 1996, a paper "lter (Whatman 40) was used instead of the quartz-wool plug. During the period May 1996 to March 1997, the Au traps were located outside on the roof of the research station. Since then a heating tube have been used. The time between sampling and analysis was generally 14 days. The mercury collected on the goldtraps and quartz-wool plug was analysed using thermal desorption/cold vapour atomic #uorescence spectrometry (CVAFS). The sampling trap was mounted in series with an analytical trap. Heating was achieved with a heating wire (NiCr). In the "rst analysis step the mercury is desorbed from the sampling trap onto the analytical trap. In the second step mercury is desorbed from the analytical trap and transported to the CV-AFS detector. The desorption
T. Berg et al. / Atmospheric Environment 35 (2001) 2569}2582
temperature is about 7003C for both steps. Argon was used as carrier gas. Calibration was performed by injecting air, saturated with Hg vapour (25, 50 and 100 l) on the analysis trap with gas tight syringes several times each day. The Hg-vapour source consisted of Hg vapour-saturated air in equilibrium with a pool of liquid mercury contained in an enclosed vessel. The reported value is the sum of the three constituents: quartz wool plug and two gold traps, which in fact is `total airborne mercurya. At Pallas, one 12 h sample of vapour-phase mercury was collected manually each week with gold traps. The sampling system consisted of two goldtraps in series, a gas meter and a pump. Gold coated pieces of crushed quartz were used as adsorbent. About 500 l of air was sampled at a #ow rate of about 0.5 l min\. The gold traps were exposed directly to the air on the roof of the station. Mercury was analysed by thermal desorption/CV-AFS at IVL. The reported value is also here `total airborne mercurya. The method has been described in detail elsewhere (Brosset and Iverfeldt, 1989; Iverfeldt, 1991). 2.3. Particulate mercury Mercury associated with airborne particulate matter was sampled on glass "ber "lters (Gelman Type AE, 61635, 142 mm) at Ny-As lesund by NILU using two parallel high volume samplers. These are traditionally `HiVola samplers operated at much reduced #ow rates for mercury (&160 l min\). The "lters had been preheated at 4503C for 8 h to reduce the blank content of mercury. The "lters were changed every day or every two days. Sample treatment and analyses were performed in a clean-room (class 300). The "lters were digested with 8 ml concentrated nitric acid (Merck Suprapur) in closed te#on vessels at 2003C for 6}8 h. After cooling to room temperature the samples were transfered to glass bottles, further oxidized with BrCl and then demineralized water was added to a total volume of 25 ml. The samples were analysed with a modi"ed cold vapour atomic #uorescence spectrometer (CV-AFS) from PSAnalytical, using SnCl as reduction agent. Calibration was accomplished using standard solutions (at 5, 10 and 20 pg ml\), prepared by appropriate dilution of a stock solution (Spex). Field blanks, in general, contained less than 10% of the mercury concentrations measured in the samples. Weekly-integrated samples of particulate mercury were collected on te#on "lters (47 mm diameter) at Pallas by IVL using a low volume sampler (&7 l min\). A number of samples from Ny-As lesund were sampled in a similar way for intercomparison. The "lters were extracted for about 3 h with 3 ml 7 : 3 HNO /H SO acids. Mercury in the samples was completely oxidated with BrCl, pre-reduced with NH Cl followed by reduction of
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the aqueous Hg to Hg (dissolved gaseous mercury, DGM). The DGM was purged onto gold traps with Ar as carrier gas followed by thermal desorption and chemical analysis using CV-AFS. 2.4. Total mercury in precipitation and surface snow Precipitation at Ny-As lesund was sampled on a monthly basis using two parallel IVL bulk samplers, each consisting of a glass funnel, glass bottle (0.5 l) and a heating system to prevent freezing. Because representative samples of snowfall in the High Arctic are di$cult to obtain due to the presence of blowing snow, precipitation was sampled only during the summer season. Surfacesnow samples were collected in te#on bottles immediately after a snowfall close to the base of the Zeppelin mountain. All glass equipment and te#on bottles used were rinsed in a BrCl solution for at least 24 h before use. For preservation, the collection bottles used were preacidi"ed with 2.5 ml HCl (suprapur). The samples were stored in the dark at #53C for up to 3 months . Before analysis the samples were oxidized with BrCl, converting stable mercury forms to water-soluble species, which in turn were reduced to Hg with SnCl Analysis was performed using CV-AFS. At Pallas, monthly-integrated samples of precipitation were carried out during the whole year of 1996 by IVL similarly to what is described for Ny-As lesund. Pretreatment and analysis of the samples were the same as described above. Details can be found elsewhere (Iverfeldt, 1988; Munthe, 1996). 2.5. Methyl mercury in precipitation Parts of two precipitation samples at Pallas were used for methyl mercury determination. The analysis was performed at IVL by aqueous ethylation of mercury species (organo- and oxidized) in the precipitation sample whereby Hg> was converted to diethylmercury and CH Hg> to methyl-, ethylmercury. The two species were separated in a GC column and detected with the use of CV-AFS after conversion to Hg (Lee et al., 1994).
3. Measurement campaigns 3.1. Vapour-phase mercury in air Fig. 2 shows the time series of vapour-phase mercury concentrations for the years 1996}1997 for Ny-As lesund and Pallas. The frequency distributions of these measurements are shown in Fig. 3, and a statistical summary of the vapour-phase data from Ny-As lesund and Pallas is given in Table 1. Concentrations for gaseous mercury measured at Ny-As lesund were in the range of 0.63} 3.55 ng m\, with an annual mean of 1.43 ng m\. At
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Fig. 2. Time series of monthly averaged vapour-phase mercury concentrations (ng m\) at Ny-As lesund and Pallas, 1996}1997.
Table 1 Statistical summary of the measurment data at Ny-As lesund and Pallas for both the years 1996}1997 Ny-As lesund Pallas Vapour-phase mercury
Fig. 3. Frequency distributions of vapour-phase mercury concentrations at Ny-As lesund and Pallas, 1996}1997.
Pallas the concentrations were between 0.15 and 1.80, and the annual mean 1.26 ng m\. The concentrations are relatively constant at Pallas during 1996 and 1997, whereas they have decreased during the same time at Ny-As lesund. An annual average of about 1.5 ng m\ is comparable to that observed in the northern Greenland Sea and Fram Strait (Schroeder et al., 1995) and at Alert in the Canadian High Arctic (Schroeder et al., 1995,
No. of samples 125 Average (ng m\) 1.43 Median (ng m\) 1.40 Std. dev. (ng m\) 0.49 Range (ng m\) 0.63}3.55 Particulate mercury No. of samples 19 Average (pg m\) 2.67 Median (pg m\) 1.20 Std. dev. (pg m\) 4.5 Range (pg m\) 0.10}20 Total mercury in prec. No. of samples 6 Average (ng l\) 14.2 Median (ng l\) 14.5 Std. dev. (ng l\) 9.9 Range (ng l\) 3.8}31 Total mercury in snow No. of samples 2 Range (ng 1\) 3.7}4.0 Methyl mercury in prec. No. of samples Range (ng 1\)
92 1.26 1.29 0.28 0.15}1.80 33 1.44 1.03 1.06 0.33}5.8 21 15.5 9.6 12.7 3.9}51.7
1.3}2.1
1997). These concentrations are however three times higher than those reported by De Mora et al. (1993) for Antarctica during the period 1987}1989. In contrast to the results from Alert in the Canadian High Arctic (Schroeder et al., 1997, 1998), no mercury depletion episodes after polar sunrise were observed at Ny-As lesund and Pallas. This is probably due to the fact that manual gold traps were used in this study and a high-resolution automatic monitor (giving either 5-min or 30-min.
T. Berg et al. / Atmospheric Environment 35 (2001) 2569}2582 Table 2 Intercomparison of concentrations of vapour phase mercury and particulate mercury at Ny-As lesund obtained by applying NILU and IVL analytical methods
Vapour-phase mercury (ng m\)
Particulate mercury (pg m\)
Date
NILU
IVL
5}6 Nov 97
0.77
0.83
6}7 Nov 97 12}13 Nov 97 13}14 Nov 97 19}20 Nov 97 6}7 Jun 97
0.85 0.80 0.76 0.78 5.2
0.58 0.75 0.75 0.86 3.6
9}11 Jun 97 13}15 Jun 97 15}17 Jun 97 17}18 Jun 97 22}24 Jun 97
3.8 1.2 2.2 4.9 2.5
1.9 0.8 1.2 4.1 1.9
means) was used at Alert. Incidentally, the depletion episodes did not become evident at Alert until the manual gold trap methodology was replaced with the automated monitor (Tekran Inc.) capable of much higher resolution (Schroeder et al., 1995, 1997, 1998). Recent measurements with a similar monitor have shown that mercury depletion episodes also take place at NyAs lesund (Berg T. ) as well as depletion episodes in tropospheric surface-level ozone (Solberg et al., 1996). The depletion episodes in tropospheric ozone, however, do not reach as far south as Pallas (Hjellbrekke, 1998). The
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di!erent atmospheric chemistry going on at the two stations after polar sunset may also partly explain why the vapour-phase data at Ny-As lesund and Pallas seem to correlate during autumn}winter (Fig. 2), but not during spring and summer. The results from an intercomparison at Ny-As lesund between the two participating institutes are presented in Table 2. The levels are very low, but with the exception of one period, comparable. 3.2. Particulate mercury With the exception of one single measurement, particulate mercury concentrations, were in the range 0.5}5 pg m\ at both Ny-As lesund and Pallas (Fig. 4), which is less than 0.2% of the vapour-phase mercury concentrations at the same sites. A statistical summary of the particulate phase data from Ny-As lesund and Pallas are given in Table 1. The concentrations measured at Ny-As lesund during September}December 1996 by both NILU and IVL (Table 2) were lower (less than 1 pg m\) than those measured by the same laboratories in June 1997 (1}5 pg m\). The high volume sampler (NILU) showed a little, but consistently higher concentrations than the low volume sampler used at IVL. In the last campaign also, the Meteorological Service of Canada (MSC), formerly Atmospheric Environment Service (AES), Environment Canada participated with a newly developed technique for sampling and analysis of particulate mercury (Lu et al., 1998). The maximum di!erences between the three techniques, were less than a factor of three times, except for two ozone depletion episodes when AES's concentrations were considerably
Fig. 4. Mercury associated with particles (pg m\) at Ny-As lesund and Pallas during four measurment campaigns 1996}1997.
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higher (IVL did not have any measurements during these episodes). During tropospheric ozone depletion periods, it has been suggested that halogens (or other strong oxidants) convert Hg to Hg>, resulting in reaction products which are much less volatile than Hg and more readily deposited from the atmosphere to polar ecosystems (Schroeder et al., 1998; Lu et al., 1998). The lower levels determined with the high volume sampler at the same time could conceivably be due to evaporation (`strippinga) of mercury from the "lter surface or from airborne particulate matter collected on the "lter while sampling for extended time periods at the high sampling rate. Much more work, however, needs to be carried out to fully understand this potential sampling artifact. At Ny-As lesund quartz-wool plugs were analysed routinely as a part of the vapour-phase measurements. The quartz-wool results were generally in the range 20} 200 pg m\ which probably also includes interference by adsorbed vapour-phase mercury. This situation is also consistent with the result from an international intercalibration study conducted at Mace Head on the Irish West coast, where particulate mercury concentrations were reported to be in the range of 4.5}26 pg m\ when disc "lters were used, and between 28 and 115 pg m\ when quartz wool, or gold-coated glass beads preceded by an Au-denuder to remove vapour-phase mercury (Ebinghaus et al., 1999). 3.3. Total mercury in precipitation and surface snow Concentrations of total mercury in precipitation were in the range 4}31 ng l\ during the summers of 1996 and
1997 at Ny-As lesund, whereas the corresponding data for Pallas were in the range 4}52 ng l\ (Fig. 5). A statistical summary of the precipitation data from Ny-As lesund and Pallas is given in Table 1. The concentration level is comparable to that observed at remote sites in Europe, like Lista in the southernmost part of Norway (T+rseth et al., 1999) and Mace Head on the western coast of Ireland (Ebinghaus et al., 1999), but a little lower than that observed at background sites in Germany (Berg and Hjellbrekke, 1998). The uncertainty of two samplers operated at Ny-As lesund is in the range 25}100%, which is much worse than for stations operated further south, which normally have a uncertainty lower than 25%. This is probably due to the di$cult weather conditions in the high Arctic, making it di$cult to collect representative samples. Total mercury concentrations in two samples of fresh-surface snow collected at Ny-As lesund in December 1996 were 3.7 and 4.0 ng l\, respectively. 3.4. Methylmercury in precipitation Methyl mercury concentrations in two precipitation samples collected in September 1996 at Pallas were 1.3 and 2.1 ng l\, respectively, which is considerably higher than the concentration level of 75}98 pg l\ reported from Mace Head (Ebinghaus et al., 1999), as well as monthly data from four stations within the the Swedish National Monitoring Network which range from (0.03 to 1.06 ng l\ (average 0.18 ng l\) for the years 1995}1997 (Kindbom et al., 1997, 1998).
Fig. 5. Total mercury in precipitation (ng l\) at Ny-As lesund and Pallas, 1996}1997.
T. Berg et al. / Atmospheric Environment 35 (2001) 2569}2582
4. Modelling of the atmospheric transport of mercury 4.1. Model structure The latest version of the Heavy Metals Eulerian Transport (HMET) model has been developed at the Norwegian Meteorological Institute in Oslo (Bartnicki, 1998) in order to include mercury compounds. The vertical structure of the HMET model is shown in Fig. 6. The lower part of the troposphere is represented by two dynamic layers: the mixing layer and the residence layer. The "rst model assumption states that mercury is emitted into the mixing layer. Immediately after emission, Hg is well mixed in this layer, so that the initial concentration is homogeneous in vertical direction below the mixing height h. The mixing height is variable in space and time and is represented by the maximum of its daily value and minimum of its night value calculated from the vertical pro"les of temperature at 12:00 GMT and 0:00 GMT, respectively. The range of values for the mixing height is 200}2500 m. Mercury in the mixing layer is advected horizontally with the wind "eld from the -level"0.925, which corresponds to approximately 570 m. It is assumed that the transport wind is uniform within the mixing layer. The thickness of the mixing layer is variable in space and time depending on the current value of the mixing height interpolated in time between 12:00 GMT and 0:00 GMT, as shown in Fig. 6. Diurnal variation of the mixing height is the main mechanism for the vertical exchange of mercury between the mixing layer and the residence layer parameterized by the vertical exchange #ux F in Fig. 5. Usually there is an increase of the mixing height during the "rst part of the day and mercury from the residence layer is captured by the mixing layer during this time (F (0). Shortly after sunset, a new inversion is established close to the surface, which means that the mixing height is descend-
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ing and mercury from the mixing layer is captured by the residence layer (F '0). The top of the residence layer (H"3 km) was chosen in such a way that the maximum of the mixing height is still below the top of the residence layer. Horizontal transport in this layer is calculated using the same wind as in the mixing layer. It is assumed that mercury is well mixed within the residence layer, so that concentration is homogenous in vertical direction. During transport, mercury in the residence layer is only a!ected by deposition processes which are parameterized in a similar way as in the mixing layer, although, the scavenging ratio is higher at this level because of the presence of the clouds. A full description of the HMET model used in the present work is given in Bartnicki (1998). 4.2. Implementation of the chemistry of mercury species in the model Transformations between di!erent species is an important factor regulating the atmospheric transport and deposition of mercury. Di!erent chemical schemes have been employed in di!erent models ranging from detailed descriptions of individual gas and aqueous phase processes in plume or cloud models (Seigneur, et al., 1994; Pleijel and Munthe, 1995) to simple parameterisations of overall oxidation rate of elemental mercury vapour in global models (Bergan and Rodhe, 1999). For this application a chemical scheme similar to that developed by Petersen et al. (1995) was used, with some modi"cations. The main components of the scheme are: E oxidation of elemental mercury (Hg) to divalent mercury (Hg>) by ozone; E immediate complexation and subsequent reduction of Hg> by sulphite ions. Combination of the above processes leads to the following equation: k [Hg>] " [O ] [Hg] , k H where k "4.7;10s\ is the second-order rate con stant for the oxidation reaction; k "4.0;10\ s\ the rate constant for the decomposition reaction; and H "0.112 mol l\ atm\ Henry's law coe$cient for Hg. [O ] "H [O ] , where H "0.013 mol l\ atm\ is Henry's law coe$ cient for ozone. Divalent aqueous mercury may also be adsorbed on soot particles in cloud water or rain according to the following formula (Petersen et al., 1995):
Fig. 6. Vertical structure of the HMET model.
[Hg>]
k "[Hg>] [soot] , r
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T. Berg et al. / Atmospheric Environment 35 (2001) 2569}2582
where k "5;10\ m g\ is the soot adsorption equi librium constant and r"1;10\ m is the radius of soot particles. Hence, the total concentration of aqueous divalent mercury is equal to
k k [Hg>] " [O ] [Hg] 1# [soot] k H r and a washout ratio for elemental mercury, Hg can be written: [Hg>] . =[Hg]" [Hg] For divalent and particulate mercury, the following washout ratios were used: W(Hg>)"1.6;10 (as for nitric acid), W(Hg )"0.5;10 (as for lead). The above set of equations was incorporated into the HMET model.
The data for gaseous ozone concentrations were obtained from the DNMI photochemical model (Malik et al., 1996). These data cover only the warm season of 1990 (April}September) with temporal resolution of 6 h. Thus, appropriate background values had to be set for the rest of the year. Following the data presented in Petersen (1992), a constant background value equal to 20 ppb was set for the rest of the year. Since no data for air concentrations of soot (carbon) particles were available during this work, some estimations and assumptions had to be made. According to Petersen (1992), soot particle emissions are quite well correlated with SO emissions. Taking this into account, a similar correlation in concentrations may be expected. Novakov (1991) presented some geographically averaged multiplicative factors, based on which it is possible to calculate soot particles concentrations from SO data. Since for Eastern Europe the factor is equal to 0.22 and for Western Europe } 0.30, an average factor of 0.25 was applied in the current work. The SO concentra tion data (daily averages for the whole year 1987) were
Fig. 7. Computed, annual mean concentration map for Hg in 1996 (unit, ng m\).
T. Berg et al. / Atmospheric Environment 35 (2001) 2569}2582
obtained from the EMEP Lagrangian model (Barrett et al., 1995). In the previous work (Petersen et al., 1995) dry deposition velocities for all mercury forms were assumed constant and not equal E for Hg 0.0 cm s\ (no dry deposition), E for Hg 0.2 cm s\, E for Hg> 4.0 cm s\. In the HMET model a very small (1;10\ cm s\) dry deposition velocity for Hg has been accepted, since there is some evidence that elemental mercury is deposited via dry processes, even though it is not very soluble in water (Shannon and Voldner, 1995; Xiao et al., 1991). In forested ecosystems this dry deposition process is controlled by leaves/needles biochemistry and should be treated more accurately in future modelling attempts. Regarding certain similarities between Hg and lead, a scheme used in the HMET model for lead has been
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adopted to calculate dry deposition of particulate mercury. In this scheme, dry deposition velocity is treated as a function of turbulent stress and terrain roughness (Bartnicki, 1994). For gaseous Hg> the value of 4.0 cm s\, as used by Petersen et al. (1995), has been accepted and is included into model calculations. 4.3. Emissions and meteorological data In the model computations, only anthropogenic mercury emission were taken into account. Annual emission inventories for 1987 have been used in the model grid system for three components: elemental, divalent inorganic and particulate mercury. Total European emissions of mercury in 1987 were 726 t. Since the latest emissions inventories for mercury available at the time of this study represented 1987, these inventories were used to simulate transport from Europe to the Arctic in 1996. As meteorological input: wind, precipitation, mixing height and other meteorological "elds for 1996 were used
Fig. 8. Computed, annual mean concentration map for Hg(p) in 1996 (unit, ng m\).
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in the simulations. These meteorological data are available at the Norwegian Meteorological Institute with 6-h resolution. The annual emission sums were recalculated to estimate the seasonal changes. It was made in a very simple way: (a) The total emissions for each mercury form were divided into two parts: E the variable part including emissions from power plants, and commercial, residential and industrial heat generation (coal combustion), E the constant part including emissions from industrial processes, waste incineration, crematories etc., (b) The seasonal changes for the "rst part were treated as a cosine function with amplitude 0.34, maximum during winter and minimum during summer (as for other heavy metals in the HMET model). According to data prepared by Axenfeld et al. (1992) the following split factors for mercury emissions were
included into model equations: Mercury form
Variable part
Constant part
Hg Hg> Hg
0.50 0.60 0.90
0.50 0.40 0.10
4.4. Model results Computed concentration maps for Hg3, Hg and Hg> in the mixing layer for 1996 are shown in Figs. 7}9, respectively. Hg varied from about 1.85 ng m\ in the Arctic to 12.3 ng m\ in Central Europe. Computed levels of Hg were in the range (0.01}0.767 ng m\ for the same areas, whereas Hg> showed concentrations from (0.01 to 3.2 ng m\ in Central Europe. A computed deposition map for Hg(tot) is shown in Fig. 10 with values ranging from 0.3 in the Arctic to 628 g m\ yr\ in Central Europe.
Fig. 9. Computed, annual mean concentration map for Hg'' in 1996 (unit, ng m\).
T. Berg et al. / Atmospheric Environment 35 (2001) 2569}2582
5. Comparison of model results with measurements Computed model concentrations of mercury were compared with observations at Ny-As lesund and Pallas for 1996 (Table 3). In the case of gaseous mercury, average measured values for 1996 at Ny-As lesund and Pallas were 1.59 and 1.25 ng m\, respectively. Computed concentrations from the HMET model were 1.9 and 2.17 ng m\ for the two sites, respectively. Agreement between average measured and computed concentrations is thus relatively good considering that the emission inventory used represents the year 1987 and emissions have been reduced since this date. It is also of interest to examine the in#uence of the global background concentration of mercury. In the model simulations, a background value of 1.8 ng m\ was applied in accordance with previous model studies (Petersen et al., 1995). This value represents the boundary concentrations of elemental mercury with a long-atmospheric residence time in the model domain. Without adding this background value, the model underestimates measured concentra-
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tions of gaseous mercury by almost two orders of magnitude. These results indicate that direct atmospheric transport of elemental mercury from source areas in central and northern Europe to the Arctic is small. Mercury contamination of the Arctic via atmospheric processes must therefore be considered on a global or hemispherical scale and the long-term atmospheric cycling of mercury must be taken into account. Furthermore, models need to take into account re-emission processes occurring over land and water in order to fully assess the impact of European mercury emissions on the Arctic environment. The development of this type of model requires information on re-emission #uxes representative of the European and Arctic regions. Very little information is today available on this topic. Obviously, this approach also requires detailed information on emissions (both natural and anthropogenic) on the same global or hemispherical scale. Computed air concentrations of particulate mercury in air in 1996 (2.3 pg m\:Ny-As lesund, 7.1 pg m\: Pallas), are close to those measured in 1996 (1.5}3 pg m\) at the
Fig. 10. Computed, annual mean deposition map of total (wet#dry) Hg in 1996 (unit: mg m\).
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Table 3 Modelled vs. measured values at Ny-As lesund and Pallas (1996) Source
Model
Station
Ny-As lesund Pallas
Measurements
Ny-As lesund Pallas
Compound Cons Hg (0) (ng m\)
Cons Hg (II) (pg m\)
Cons Hg (p) (pg m\)
Dep Hg (t) (ug m\)
Dep Hg (0) (ug m\)
1.9 1.85}1.91 2.07 2.02}2.17 1.59 0.68}3.55 1.25 0.73}1.80
1.4 0.9}1.9 8.2 6.6}10.6
2.3 1.5}3.3 7.1 6.6}9.6 2.37 0.1}20 1.5 0.33}5.8
0.8 0.6}1.1 3.5 2.5}7.2
0.2 0.2}0.3 1.2 0.8}2.0
two stations with some overestimation of the Pallas values, again possibly due to the high emission values in the applied inventory. Particulate Hg is emitted from anthropogenic sources, but is also formed in the atmosphere via gas/solid interactions and/or evaporation of cloud droplets. These processes are not well understood and quantitative information is not available making inclusion into models di$cult. Since the atmospheric lifetime of particulate species is considerable shorther than that of elemental mercury vapour, no global background value is used in the model and the relatively good agreement between modelled and measured concentrations indicates that a direct transport from continental Europe to the Arctic occurs. Computed deposition of Hg(tot) at Pallas is 3.5 g m\ yr\, which is &60% higher than measured (2.1 g m\ yr\). This may partly be due to the overestimation of particulate mercury concentrations at Pallas which will also in#uence the wet deposition via wash-out processes. In addition to the transformations and removal processed described in this model, recent studies have revealed that enhanced dry deposition of mercury can occur in the Arctic environment during polar sunrise via rapid oxidation of elemental mercury to particulate form (Schroeder et al., 1997). The processes involved are yet to be identi"ed but the observed rapid #uctuations in gaseous mercury concentrations coincide with ozone depletion events indicating that halogen-containing radicals are involved (Schroeder et al., 1997). The overall signi"cance of this mercury depletion and deposition is not possible to quantify but should clearly be included in future modelling e!orts aimed at the Arctic environment.
6. Conclusions Representative concentrations of total vapour-phase mercury (Average: 1.3 and 1.4 ng m\), particulate mer-
2.1
cury (Average: 1.4 and 2.7 pg m\) and wet deposition mercury (Average: 14 and 15 ng l\) have been determined at two Arctic locations. The HMET atmospheric transport model has been modi"ed with parameterisation for mercury chemistry and removal processes. The model simulations agree reasonably well with measured values with some overestimation probably caused by lower European emissions of mercury during the measurement period in comparison to the inventory used for the modelling. The model results indicate that direct transport of elemental mercury to the Arctic is small. The concentrations at the Arctic locations are mainly in#uenced by the background values applied in the model. Thus, global or hemispherical models for mercury, capable of simulating periods of several years, should be developed for assessment of anthropogenic in#uence on mercury levels in the Arctic. In addition to the larger scale models, further information on re-emissions from soil, snow and water is needed for improvement of model results. Identi"cation of chemical processes involved in the Arctic spring depletion events is also needed as well as determination of the net input of mercury during these events.
Acknowledgements We wish to thank the Nordic Council of Ministers (NMR) for "nancial support. The sta! at the Norwegian Polar Research Institute, Ny-As lesund and FMI, Pallas are gratefully acknowledged for collecting samples. We would also wish to thank Marit Vadset, NILU, and Elsmarie Lord, Peter Schager and Pia Carlsson, IVL, for assistance with sample analysis. We wish to acknowledge Dr. G. Petersen and Dr. O. KruK ger from the GKSS Research Centre, Geestadt, Germany who provided European emission data for mercury. In addition, we would like to thank the peer reviewers for insightful
T. Berg et al. / Atmospheric Environment 35 (2001) 2569}2582
comments and helpful suggestions which have substantially improved the manuscript.
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nederboK rdskemi. OG vervakning av svavel- och kvaK vefoK reningar, ozon, baskatjoner, tungmetaller och kvicksilver i bakgrundsmiljoK . Rapportering av 1995 a rs maK tresultat inom EMEP och Luft-och nederboK rdskemiska naK tet samt spridnings-och depositionsberaK kningar med MATCHSverige. IVL B-1252. Swedish Environmental Research Institute (IVL), PO Box 21060, S-100 31 Stockholm, Sweden. Kindbom, K., SjoK berg, K., Munthe, J., Peterson, K., Persson, C., Roos, E., BergstroK m, R., 1998. Nationell miljoK oK vervakning av luft-och nederboK rdskemi 1996. IVL Report B 1289; Swedish Environmental Research Institute (IVL), PO Box 21060, S-100 31 Stockholm, Sweden. Lee, Y.-H., Munthe, J., Iverfeldt, As ., 1994. Experiences on analytical procedures for the determination of methylmercury in environmental samples. Applied Organometallic Chemistry 8, 659}664. Lindberg, S.E., Turner, R.R., Meyers, T.P., Taylor Jr., G.E., Schroeder, W.H., 1991. Atmospheric concentrations and deposition of Hg to a deciduous forest at Walker Branch Watershed, Tennessee, USA. Water, Air and Soil Pollution 56, 577}594. Lindqvist, O., Rodhe, H., 1985. Atmospheric mercury } a review. Tellus 37B, 136}159. Lu, J.Y., Schroeder, W.H., Berg, T., Munthe, J., Schneeberger, D., Schaedlich, F., 1998. A device for sampling and determination of total particulate mercury in ambient air. Analytical Chemistry 70, 2403}2408. Lu, J.Y., Schroeder, W.H., 1999. Sampling and determination of particulate mercury in ambient air: a review. Water, Air and Soil Pollution 112, 279}295. Malik, S., Simpson, D., Hjellbrekke, A.-G., ApSimon, H., 1996. Photochemical model for .calculations over Europe for summer 1990. Model results and comparison with observations. EMEP/MSC-W Report 2/96. Norwegian Meteorological Institute, Oslo, Norway. Munthe, J., 1996. Guidelines for the sampling and analysis of mercury in air and precipitation. Report to the Oslo and Paris Commissions. Swedish Environmental Research Institute. IVL-Report L 96/204. Novakov, T., 1991. A global inventory of soot emissions from fossil fuel combustion. 44th Conference on Carbonaeous Particles in the Atmosphere, Vienna, Austria. Petersen, G., 1992. Belastung von Nord- and Ostsee durch oK kologisch gefaK hrlische Sto!e am Beispiel atmospha rischer Quecksilververbindungen. Abschlussbericht des Forchungsforhabens 104 02 726 des Bundesministers fuK r Umwelt, Naturschutz und Reaktorsicherheit. Im Auftrag des Umweltreaktorsicherheitbundesamt, Berlin. GKSS 92/E/111 External report, GKSS Research Centre, Geesthacht, Germany. Petersen, G., Iverfeldt, As ., Munthe, J., 1995. Atmospheric mercury species over Central and Northern Europe. Model calculations and comparison with observations from the Nordic air and precipitation network. Atmospheric Environment 29, 47}67. Petersen, G., Munthe, J., Pleijel, K., Bloxam, R., Vinod Kumar, A., 1998. A comprehensive Eulerian modeling framework for airborne mercury species: development and testing of the tropospheric chemistry module (TCM). Atmospheric Environment 32, 829}843.
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Pleijel, K., Munthe, J., 1995. Modelling the atmospheric mercury cycle } chemistry in fog droplets. Atmospheric Environment 29, 1441}1457. Ryaboshapko, A., Ilyin, I., Gusev, A., A"nogenova, O., Berg ,T., and Hjellbrekke, A.-G., 1999. Monitoring and modelling of lead, cadmium and mercury transboundary transport in the atmosphere of Europe. Joint report of EMEP Centres: MSCE and CCC. EMEP/MSC-E Technical Report 1/99, EMEP Meteorological Synthesizing Center-East, Kedrova str. 8}1, Moscow, Russia. Schroeder, W.H., Anlauf, K., Barrie, L.A., Berg, T., Schneeberger, D.R., 1997. Atmospheric mercury and polar sunrise tropospheric ozone depletion at Alert in the Canadian high arctic. The AMAP International Symposium on Environmental Pollution in the Arctic, Troms+, Norway June 1}5, 1997, pp. 354}356. Schroeder, W.H., Anlauf, K., Barrie, L.A., Lu, J.Y., Ste!en, A., Schneeberger, D.R., Berg, T., 1998. Arctic springtime depletion of mercury. Nature 394, 331}332. Schroeder, W.H., Ebinghaus, R., Shoeib, M., Timoschenko, K., Barrie, L.A., 1995. Atmospheric mercury measurements in the northern hemisphere from 563 to 82.53N latitude. Water, Air and Soil Pollution 80, 1227}1236. Schroeder, W.H., Munthe, J., 1998. Atmospheric mercury } an overview. Atmospheric Environment 32, 809}822. Shannon, J.D., Voldner, E.C., 1995. Modeling atmospheric concentrations of mercury and deposition to the Great Lakes. Atmospheric Environment 29, 1649}1662.
Slemr, 1985. Distribution, speciation, and budget of atmospheric mercury. Journal of Atmospheric Chemistry 3, 407}434. Slemr, F., 1996. Trends in atmospheric mercury concentrations over the Atlantic Ocean and at the Wank summit and the resulting constraints on the budget of atmospheric mercury. In: Baeyens, W., Ebinghaus, R., Vasiliev, O. (Eds.), Global and Regional Cycles. Sources, Fluxes and Mass Balances, NATO-ASI-Series. Kluwer, Academic Publishers, Dordrecht, The Netherlands, pp. 33}84. Slemr, F., Langer, E., 1992. Increase in global atmospheric concentrations of mercury inferred from measurements over the Atlantic Ocean. Nature 355, 434}436. Slemr, F., Scheel, 1998. Trends in atmospheric mercury concentrations at the summit of the Wank mountain, Southern Germany. Atmospheric Environment 32, 845}853. Solberg, S., Schmidbauer, N., Semb, A., Stordal, F., Hov, "., 1996. Boundary-layer ozone depletion as seen in the Norwegian Arctic in spring. Journal of Atmospheric Chemistry 23, 301}332. T+rseth, K., Berg, T., Hanssen, J.E., Man+, S., 1999. Overva king av langtransportert forurenset luft og nedb+r. Atmosfarisk tilf+rsel, 1998. Kjeller (OR 27/99). Xiao, Z.F., Munthe, J., Schroeder, W.H., Lindqvist, O., 1991. Vertical #uxes of volatile mercury over forest soil and lake surfaces in Sweden. Tellus 43B, 267}279.
Atmospheric mercury species in the European Arctic: measurements and modelling Torunn Berg *, Jerzy Bartnicki, John Munthe, Heikki Lattila, Jaroslaw Hrehoruk, Andrzej Mazur Norwegian Institute for Air Research (NILU), P.O. Box 100, N-2007 Kjeller, Norway Norwegian Meteorological Institute (DNMI), P.O. Box 43 Blindern, N-0313 Oslo, Norway Swedish Environmental Research Institute (IVL), P.O. Box 47086, S-402 58 Go( teborg, Sweden Finnish Meteorological Institute (FMI), Sahaajankatu 20E; FIN-00810 Helsinki, Finland Institute for Meteorology and Water Management (IMGW), Ul. Podlesna 61, PL-01-673 Warsaw, Poland Received 18 January 2000; received in revised form 30 June 2000; accepted 14 July 2000
Abstract Concentrations of di!erent species of mercury in arctic air and precipitation have been measured at Ny-As lesund (Svalbard) and Pallas (Finland) during 1996}1997. Typical concentrations for vapour phase mercury measured at the two stations were in the range of 0.7}2 ng m\ whereas particulate mercury concentrations were below 5 pg m\. Total mercury in precipitation was in the range 3}30 ng l\. In order to evaluate the transport and deposition of mercury to the arctic from European anthropogenic sources, the Eulerian transport model HMET has been modi"ed and extended to also include mercury species. A scheme for chemical conversion of elemental mercury to other species of mercury and deposition characteristics of di!erent mercury species have been included in the model. European emission inventories for three di!erent forms of Hg (Hg, HgCl and Hg ) have been implemented in the numerical grid system for the HMET model. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Mercury; Modelling; Mercury species; Vapour-phase mercury; Particulate-phase mercury; Precipitation; Air
1. Introduction The high level of mercury contamination observed in the once pristine Arctic environment is largely connected to the unique properties of mercury as a metal. Mercury in the atmosphere exists predominantly in the gaseous state, which, in remote areas such as the Arctic, consists almost exclusive of the elemental form, Hg. The particle phase makes up only a small fraction (&1%) of total airborne concentrations. This fraction, however, still plays an important role for determining the deposition #uxes of mercury (Lindberg et al., 1991; Lu and Schroeder, 1999). The elemental form of mercury is rela-
* Corresponding author. Tel.: #47-63-89-82-49; fax: #4763-89-80-50. E-mail address: [email protected] (T. Berg).
tively unreactive, has a high volatility and low solubility in natural waters, which imparts a long atmospheric residence time of &6}24 months (Slemr et al., 1985; Lindqvist and Rodhe, 1985). Once introduced into the atmosphere, mercury can circulate for long periods of time prior to being transferred to a permanent sink reservoir. Similar to many persistent, semi-volatile organic compounds it can be re-emitted into the atmosphere and re-enter the global atmospheric mercury cycle. The cold Arctic climate may favour a "nal deposition here rather than in warmer climates. Mercury is removed from the atmosphere through both wet- and dry-deposition processes of species like Hg, Hg(II) and Hg(p) (Schroeder and Munthe, 1998). Recently, frequent episodic depletions in mercury vapour concentrations at Alert in the Canadian high Arctic, closely resembling ozone depletions in Arctic surfacelevel air during the 3-month period following polar
1352-2310/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 0 ) 0 0 4 3 4 - 9
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sunrise, have been observed (Schroeder et al., 1997, 1998). Preliminary observations have indicated that this atmospheric Hg depletion mechanism involves conversion of mercury from the gas phase to the particulate phase (Lu et al., 1998). This transformation process is unique to the Arctic and may constitute an important pathway for the entry of mercury to this sensitive ecosystem. At Alert the highest mercury vapour concentrations are observed during summer, probably due to a temperature and/or sunlight-induced emission/re-emission of volatile mercury species from the Earth's surface during that season of the year (Xiao et al., 1991), and not solely due to atmospheric transport from any speci"c sector with large mercury emissions (Schroeder et al., 1997). Measurements of atmospheric mercury over the Atlantic Ocean in the years 1977}1980, 1990 and 1994 suggest an increase between 1977 and 1990 and a decrease after 1990 (Slemr and Langer, 1992; Iverfeldt et al., 1995; Slemr, 1996; Slemr and Scheel, 1998). Numerical models of atmospheric transport and deposition are essential tools for the understanding of individual mercury processes and their interaction with the atmospheric system, and for the interpretation of "eld measurements data. Modelling of mercury has undergone rapid development during the last decade and several large-scale models are in operation today (Petersen et al., 1995, 1998; Galperin et al., 1995; Shannon and Voldner, 1995; Ryaboshapko et al., 1999). The aims of this study were to determine di!erent species of mercury in Arctic air and precipitation, and to estimate the role of long-range transport of mercury to the Arctic from European sources.
2. Experimental 2.1. Site locations and measurement programme Monitoring of atmospheric mercury in the Arctic was initiated as a part of the Arctic Monitoring and Assessment Programme's (AMAP) "rst phase in 1994. Measurements of vapour-phase mercury are carried out by NILU at Ny-As lesund, Svalbard (78354N, 11353E), and measurements of vapour phase mercury and total mercury in precipitation are carried out by IVL at Pallas, Finland (67322N, 26339E). Ny-As lesund is a small settlement near sea-level on the western coast of Spitzbergen (Fig. 1). There are surrounding mountains with tops about 1000}1500 m altitude in the region. Precipitation sampling is carried out close to the settlement, whereas air sampling is performed at the research station on the nearby Zeppelin mountain (474 m a.s.l.), accesible from Ny-As lesund by cable car. Pallas (Fig. 1) is a national park in Northern Finland with marked hills up to 727 m a.s.l. The research station is situated far from any settlement, in a hillside forest clearing, 566 m a.s.l.
Fig. 1. Location of the sampling stations.
2.2. Vapour-phase mercury At Ny-As lesund, one 24 h sample of vapour-phase mercury was collected manually each week by amalgamation on gold traps. The sampling system consisted of a sample line (Te#on), a quartz-wool plug, two goldtraps in series, a gas meter and a pump. The gold traps consisted of 12 cm quartz tubes "lled with a wire made of gold and platinum. The thickness of the wire was about 10 m. Usually about 1000 l of air was sampled at a #ow rate of about 0.7 l min\. At the Zeppelin mountain in the high Arctic there are probably less chemical species in the air, which can a!ect the collection ability of the gold traps than at lower latitutes. Two parallel sampling systems were operated. The system was somewhat modi"ed during the period of measurements. Until April 1996, a paper "lter (Whatman 40) was used instead of the quartz-wool plug. During the period May 1996 to March 1997, the Au traps were located outside on the roof of the research station. Since then a heating tube have been used. The time between sampling and analysis was generally 14 days. The mercury collected on the goldtraps and quartz-wool plug was analysed using thermal desorption/cold vapour atomic #uorescence spectrometry (CVAFS). The sampling trap was mounted in series with an analytical trap. Heating was achieved with a heating wire (NiCr). In the "rst analysis step the mercury is desorbed from the sampling trap onto the analytical trap. In the second step mercury is desorbed from the analytical trap and transported to the CV-AFS detector. The desorption
T. Berg et al. / Atmospheric Environment 35 (2001) 2569}2582
temperature is about 7003C for both steps. Argon was used as carrier gas. Calibration was performed by injecting air, saturated with Hg vapour (25, 50 and 100 l) on the analysis trap with gas tight syringes several times each day. The Hg-vapour source consisted of Hg vapour-saturated air in equilibrium with a pool of liquid mercury contained in an enclosed vessel. The reported value is the sum of the three constituents: quartz wool plug and two gold traps, which in fact is `total airborne mercurya. At Pallas, one 12 h sample of vapour-phase mercury was collected manually each week with gold traps. The sampling system consisted of two goldtraps in series, a gas meter and a pump. Gold coated pieces of crushed quartz were used as adsorbent. About 500 l of air was sampled at a #ow rate of about 0.5 l min\. The gold traps were exposed directly to the air on the roof of the station. Mercury was analysed by thermal desorption/CV-AFS at IVL. The reported value is also here `total airborne mercurya. The method has been described in detail elsewhere (Brosset and Iverfeldt, 1989; Iverfeldt, 1991). 2.3. Particulate mercury Mercury associated with airborne particulate matter was sampled on glass "ber "lters (Gelman Type AE, 61635, 142 mm) at Ny-As lesund by NILU using two parallel high volume samplers. These are traditionally `HiVola samplers operated at much reduced #ow rates for mercury (&160 l min\). The "lters had been preheated at 4503C for 8 h to reduce the blank content of mercury. The "lters were changed every day or every two days. Sample treatment and analyses were performed in a clean-room (class 300). The "lters were digested with 8 ml concentrated nitric acid (Merck Suprapur) in closed te#on vessels at 2003C for 6}8 h. After cooling to room temperature the samples were transfered to glass bottles, further oxidized with BrCl and then demineralized water was added to a total volume of 25 ml. The samples were analysed with a modi"ed cold vapour atomic #uorescence spectrometer (CV-AFS) from PSAnalytical, using SnCl as reduction agent. Calibration was accomplished using standard solutions (at 5, 10 and 20 pg ml\), prepared by appropriate dilution of a stock solution (Spex). Field blanks, in general, contained less than 10% of the mercury concentrations measured in the samples. Weekly-integrated samples of particulate mercury were collected on te#on "lters (47 mm diameter) at Pallas by IVL using a low volume sampler (&7 l min\). A number of samples from Ny-As lesund were sampled in a similar way for intercomparison. The "lters were extracted for about 3 h with 3 ml 7 : 3 HNO /H SO acids. Mercury in the samples was completely oxidated with BrCl, pre-reduced with NH Cl followed by reduction of
2571
the aqueous Hg to Hg (dissolved gaseous mercury, DGM). The DGM was purged onto gold traps with Ar as carrier gas followed by thermal desorption and chemical analysis using CV-AFS. 2.4. Total mercury in precipitation and surface snow Precipitation at Ny-As lesund was sampled on a monthly basis using two parallel IVL bulk samplers, each consisting of a glass funnel, glass bottle (0.5 l) and a heating system to prevent freezing. Because representative samples of snowfall in the High Arctic are di$cult to obtain due to the presence of blowing snow, precipitation was sampled only during the summer season. Surfacesnow samples were collected in te#on bottles immediately after a snowfall close to the base of the Zeppelin mountain. All glass equipment and te#on bottles used were rinsed in a BrCl solution for at least 24 h before use. For preservation, the collection bottles used were preacidi"ed with 2.5 ml HCl (suprapur). The samples were stored in the dark at #53C for up to 3 months . Before analysis the samples were oxidized with BrCl, converting stable mercury forms to water-soluble species, which in turn were reduced to Hg with SnCl Analysis was performed using CV-AFS. At Pallas, monthly-integrated samples of precipitation were carried out during the whole year of 1996 by IVL similarly to what is described for Ny-As lesund. Pretreatment and analysis of the samples were the same as described above. Details can be found elsewhere (Iverfeldt, 1988; Munthe, 1996). 2.5. Methyl mercury in precipitation Parts of two precipitation samples at Pallas were used for methyl mercury determination. The analysis was performed at IVL by aqueous ethylation of mercury species (organo- and oxidized) in the precipitation sample whereby Hg> was converted to diethylmercury and CH Hg> to methyl-, ethylmercury. The two species were separated in a GC column and detected with the use of CV-AFS after conversion to Hg (Lee et al., 1994).
3. Measurement campaigns 3.1. Vapour-phase mercury in air Fig. 2 shows the time series of vapour-phase mercury concentrations for the years 1996}1997 for Ny-As lesund and Pallas. The frequency distributions of these measurements are shown in Fig. 3, and a statistical summary of the vapour-phase data from Ny-As lesund and Pallas is given in Table 1. Concentrations for gaseous mercury measured at Ny-As lesund were in the range of 0.63} 3.55 ng m\, with an annual mean of 1.43 ng m\. At
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Fig. 2. Time series of monthly averaged vapour-phase mercury concentrations (ng m\) at Ny-As lesund and Pallas, 1996}1997.
Table 1 Statistical summary of the measurment data at Ny-As lesund and Pallas for both the years 1996}1997 Ny-As lesund Pallas Vapour-phase mercury
Fig. 3. Frequency distributions of vapour-phase mercury concentrations at Ny-As lesund and Pallas, 1996}1997.
Pallas the concentrations were between 0.15 and 1.80, and the annual mean 1.26 ng m\. The concentrations are relatively constant at Pallas during 1996 and 1997, whereas they have decreased during the same time at Ny-As lesund. An annual average of about 1.5 ng m\ is comparable to that observed in the northern Greenland Sea and Fram Strait (Schroeder et al., 1995) and at Alert in the Canadian High Arctic (Schroeder et al., 1995,
No. of samples 125 Average (ng m\) 1.43 Median (ng m\) 1.40 Std. dev. (ng m\) 0.49 Range (ng m\) 0.63}3.55 Particulate mercury No. of samples 19 Average (pg m\) 2.67 Median (pg m\) 1.20 Std. dev. (pg m\) 4.5 Range (pg m\) 0.10}20 Total mercury in prec. No. of samples 6 Average (ng l\) 14.2 Median (ng l\) 14.5 Std. dev. (ng l\) 9.9 Range (ng l\) 3.8}31 Total mercury in snow No. of samples 2 Range (ng 1\) 3.7}4.0 Methyl mercury in prec. No. of samples Range (ng 1\)
92 1.26 1.29 0.28 0.15}1.80 33 1.44 1.03 1.06 0.33}5.8 21 15.5 9.6 12.7 3.9}51.7
1.3}2.1
1997). These concentrations are however three times higher than those reported by De Mora et al. (1993) for Antarctica during the period 1987}1989. In contrast to the results from Alert in the Canadian High Arctic (Schroeder et al., 1997, 1998), no mercury depletion episodes after polar sunrise were observed at Ny-As lesund and Pallas. This is probably due to the fact that manual gold traps were used in this study and a high-resolution automatic monitor (giving either 5-min or 30-min.
T. Berg et al. / Atmospheric Environment 35 (2001) 2569}2582 Table 2 Intercomparison of concentrations of vapour phase mercury and particulate mercury at Ny-As lesund obtained by applying NILU and IVL analytical methods
Vapour-phase mercury (ng m\)
Particulate mercury (pg m\)
Date
NILU
IVL
5}6 Nov 97
0.77
0.83
6}7 Nov 97 12}13 Nov 97 13}14 Nov 97 19}20 Nov 97 6}7 Jun 97
0.85 0.80 0.76 0.78 5.2
0.58 0.75 0.75 0.86 3.6
9}11 Jun 97 13}15 Jun 97 15}17 Jun 97 17}18 Jun 97 22}24 Jun 97
3.8 1.2 2.2 4.9 2.5
1.9 0.8 1.2 4.1 1.9
means) was used at Alert. Incidentally, the depletion episodes did not become evident at Alert until the manual gold trap methodology was replaced with the automated monitor (Tekran Inc.) capable of much higher resolution (Schroeder et al., 1995, 1997, 1998). Recent measurements with a similar monitor have shown that mercury depletion episodes also take place at NyAs lesund (Berg T. ) as well as depletion episodes in tropospheric surface-level ozone (Solberg et al., 1996). The depletion episodes in tropospheric ozone, however, do not reach as far south as Pallas (Hjellbrekke, 1998). The
2573
di!erent atmospheric chemistry going on at the two stations after polar sunset may also partly explain why the vapour-phase data at Ny-As lesund and Pallas seem to correlate during autumn}winter (Fig. 2), but not during spring and summer. The results from an intercomparison at Ny-As lesund between the two participating institutes are presented in Table 2. The levels are very low, but with the exception of one period, comparable. 3.2. Particulate mercury With the exception of one single measurement, particulate mercury concentrations, were in the range 0.5}5 pg m\ at both Ny-As lesund and Pallas (Fig. 4), which is less than 0.2% of the vapour-phase mercury concentrations at the same sites. A statistical summary of the particulate phase data from Ny-As lesund and Pallas are given in Table 1. The concentrations measured at Ny-As lesund during September}December 1996 by both NILU and IVL (Table 2) were lower (less than 1 pg m\) than those measured by the same laboratories in June 1997 (1}5 pg m\). The high volume sampler (NILU) showed a little, but consistently higher concentrations than the low volume sampler used at IVL. In the last campaign also, the Meteorological Service of Canada (MSC), formerly Atmospheric Environment Service (AES), Environment Canada participated with a newly developed technique for sampling and analysis of particulate mercury (Lu et al., 1998). The maximum di!erences between the three techniques, were less than a factor of three times, except for two ozone depletion episodes when AES's concentrations were considerably
Fig. 4. Mercury associated with particles (pg m\) at Ny-As lesund and Pallas during four measurment campaigns 1996}1997.
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higher (IVL did not have any measurements during these episodes). During tropospheric ozone depletion periods, it has been suggested that halogens (or other strong oxidants) convert Hg to Hg>, resulting in reaction products which are much less volatile than Hg and more readily deposited from the atmosphere to polar ecosystems (Schroeder et al., 1998; Lu et al., 1998). The lower levels determined with the high volume sampler at the same time could conceivably be due to evaporation (`strippinga) of mercury from the "lter surface or from airborne particulate matter collected on the "lter while sampling for extended time periods at the high sampling rate. Much more work, however, needs to be carried out to fully understand this potential sampling artifact. At Ny-As lesund quartz-wool plugs were analysed routinely as a part of the vapour-phase measurements. The quartz-wool results were generally in the range 20} 200 pg m\ which probably also includes interference by adsorbed vapour-phase mercury. This situation is also consistent with the result from an international intercalibration study conducted at Mace Head on the Irish West coast, where particulate mercury concentrations were reported to be in the range of 4.5}26 pg m\ when disc "lters were used, and between 28 and 115 pg m\ when quartz wool, or gold-coated glass beads preceded by an Au-denuder to remove vapour-phase mercury (Ebinghaus et al., 1999). 3.3. Total mercury in precipitation and surface snow Concentrations of total mercury in precipitation were in the range 4}31 ng l\ during the summers of 1996 and
1997 at Ny-As lesund, whereas the corresponding data for Pallas were in the range 4}52 ng l\ (Fig. 5). A statistical summary of the precipitation data from Ny-As lesund and Pallas is given in Table 1. The concentration level is comparable to that observed at remote sites in Europe, like Lista in the southernmost part of Norway (T+rseth et al., 1999) and Mace Head on the western coast of Ireland (Ebinghaus et al., 1999), but a little lower than that observed at background sites in Germany (Berg and Hjellbrekke, 1998). The uncertainty of two samplers operated at Ny-As lesund is in the range 25}100%, which is much worse than for stations operated further south, which normally have a uncertainty lower than 25%. This is probably due to the di$cult weather conditions in the high Arctic, making it di$cult to collect representative samples. Total mercury concentrations in two samples of fresh-surface snow collected at Ny-As lesund in December 1996 were 3.7 and 4.0 ng l\, respectively. 3.4. Methylmercury in precipitation Methyl mercury concentrations in two precipitation samples collected in September 1996 at Pallas were 1.3 and 2.1 ng l\, respectively, which is considerably higher than the concentration level of 75}98 pg l\ reported from Mace Head (Ebinghaus et al., 1999), as well as monthly data from four stations within the the Swedish National Monitoring Network which range from (0.03 to 1.06 ng l\ (average 0.18 ng l\) for the years 1995}1997 (Kindbom et al., 1997, 1998).
Fig. 5. Total mercury in precipitation (ng l\) at Ny-As lesund and Pallas, 1996}1997.
T. Berg et al. / Atmospheric Environment 35 (2001) 2569}2582
4. Modelling of the atmospheric transport of mercury 4.1. Model structure The latest version of the Heavy Metals Eulerian Transport (HMET) model has been developed at the Norwegian Meteorological Institute in Oslo (Bartnicki, 1998) in order to include mercury compounds. The vertical structure of the HMET model is shown in Fig. 6. The lower part of the troposphere is represented by two dynamic layers: the mixing layer and the residence layer. The "rst model assumption states that mercury is emitted into the mixing layer. Immediately after emission, Hg is well mixed in this layer, so that the initial concentration is homogeneous in vertical direction below the mixing height h. The mixing height is variable in space and time and is represented by the maximum of its daily value and minimum of its night value calculated from the vertical pro"les of temperature at 12:00 GMT and 0:00 GMT, respectively. The range of values for the mixing height is 200}2500 m. Mercury in the mixing layer is advected horizontally with the wind "eld from the -level"0.925, which corresponds to approximately 570 m. It is assumed that the transport wind is uniform within the mixing layer. The thickness of the mixing layer is variable in space and time depending on the current value of the mixing height interpolated in time between 12:00 GMT and 0:00 GMT, as shown in Fig. 6. Diurnal variation of the mixing height is the main mechanism for the vertical exchange of mercury between the mixing layer and the residence layer parameterized by the vertical exchange #ux F in Fig. 5. Usually there is an increase of the mixing height during the "rst part of the day and mercury from the residence layer is captured by the mixing layer during this time (F (0). Shortly after sunset, a new inversion is established close to the surface, which means that the mixing height is descend-
2575
ing and mercury from the mixing layer is captured by the residence layer (F '0). The top of the residence layer (H"3 km) was chosen in such a way that the maximum of the mixing height is still below the top of the residence layer. Horizontal transport in this layer is calculated using the same wind as in the mixing layer. It is assumed that mercury is well mixed within the residence layer, so that concentration is homogenous in vertical direction. During transport, mercury in the residence layer is only a!ected by deposition processes which are parameterized in a similar way as in the mixing layer, although, the scavenging ratio is higher at this level because of the presence of the clouds. A full description of the HMET model used in the present work is given in Bartnicki (1998). 4.2. Implementation of the chemistry of mercury species in the model Transformations between di!erent species is an important factor regulating the atmospheric transport and deposition of mercury. Di!erent chemical schemes have been employed in di!erent models ranging from detailed descriptions of individual gas and aqueous phase processes in plume or cloud models (Seigneur, et al., 1994; Pleijel and Munthe, 1995) to simple parameterisations of overall oxidation rate of elemental mercury vapour in global models (Bergan and Rodhe, 1999). For this application a chemical scheme similar to that developed by Petersen et al. (1995) was used, with some modi"cations. The main components of the scheme are: E oxidation of elemental mercury (Hg) to divalent mercury (Hg>) by ozone; E immediate complexation and subsequent reduction of Hg> by sulphite ions. Combination of the above processes leads to the following equation: k [Hg>] " [O ] [Hg] , k H where k "4.7;10s\ is the second-order rate con stant for the oxidation reaction; k "4.0;10\ s\ the rate constant for the decomposition reaction; and H "0.112 mol l\ atm\ Henry's law coe$cient for Hg. [O ] "H [O ] , where H "0.013 mol l\ atm\ is Henry's law coe$ cient for ozone. Divalent aqueous mercury may also be adsorbed on soot particles in cloud water or rain according to the following formula (Petersen et al., 1995):
Fig. 6. Vertical structure of the HMET model.
[Hg>]
k "[Hg>] [soot] , r
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where k "5;10\ m g\ is the soot adsorption equi librium constant and r"1;10\ m is the radius of soot particles. Hence, the total concentration of aqueous divalent mercury is equal to
k k [Hg>] " [O ] [Hg] 1# [soot] k H r and a washout ratio for elemental mercury, Hg can be written: [Hg>] . =[Hg]" [Hg] For divalent and particulate mercury, the following washout ratios were used: W(Hg>)"1.6;10 (as for nitric acid), W(Hg )"0.5;10 (as for lead). The above set of equations was incorporated into the HMET model.
The data for gaseous ozone concentrations were obtained from the DNMI photochemical model (Malik et al., 1996). These data cover only the warm season of 1990 (April}September) with temporal resolution of 6 h. Thus, appropriate background values had to be set for the rest of the year. Following the data presented in Petersen (1992), a constant background value equal to 20 ppb was set for the rest of the year. Since no data for air concentrations of soot (carbon) particles were available during this work, some estimations and assumptions had to be made. According to Petersen (1992), soot particle emissions are quite well correlated with SO emissions. Taking this into account, a similar correlation in concentrations may be expected. Novakov (1991) presented some geographically averaged multiplicative factors, based on which it is possible to calculate soot particles concentrations from SO data. Since for Eastern Europe the factor is equal to 0.22 and for Western Europe } 0.30, an average factor of 0.25 was applied in the current work. The SO concentra tion data (daily averages for the whole year 1987) were
Fig. 7. Computed, annual mean concentration map for Hg in 1996 (unit, ng m\).
T. Berg et al. / Atmospheric Environment 35 (2001) 2569}2582
obtained from the EMEP Lagrangian model (Barrett et al., 1995). In the previous work (Petersen et al., 1995) dry deposition velocities for all mercury forms were assumed constant and not equal E for Hg 0.0 cm s\ (no dry deposition), E for Hg 0.2 cm s\, E for Hg> 4.0 cm s\. In the HMET model a very small (1;10\ cm s\) dry deposition velocity for Hg has been accepted, since there is some evidence that elemental mercury is deposited via dry processes, even though it is not very soluble in water (Shannon and Voldner, 1995; Xiao et al., 1991). In forested ecosystems this dry deposition process is controlled by leaves/needles biochemistry and should be treated more accurately in future modelling attempts. Regarding certain similarities between Hg and lead, a scheme used in the HMET model for lead has been
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adopted to calculate dry deposition of particulate mercury. In this scheme, dry deposition velocity is treated as a function of turbulent stress and terrain roughness (Bartnicki, 1994). For gaseous Hg> the value of 4.0 cm s\, as used by Petersen et al. (1995), has been accepted and is included into model calculations. 4.3. Emissions and meteorological data In the model computations, only anthropogenic mercury emission were taken into account. Annual emission inventories for 1987 have been used in the model grid system for three components: elemental, divalent inorganic and particulate mercury. Total European emissions of mercury in 1987 were 726 t. Since the latest emissions inventories for mercury available at the time of this study represented 1987, these inventories were used to simulate transport from Europe to the Arctic in 1996. As meteorological input: wind, precipitation, mixing height and other meteorological "elds for 1996 were used
Fig. 8. Computed, annual mean concentration map for Hg(p) in 1996 (unit, ng m\).
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in the simulations. These meteorological data are available at the Norwegian Meteorological Institute with 6-h resolution. The annual emission sums were recalculated to estimate the seasonal changes. It was made in a very simple way: (a) The total emissions for each mercury form were divided into two parts: E the variable part including emissions from power plants, and commercial, residential and industrial heat generation (coal combustion), E the constant part including emissions from industrial processes, waste incineration, crematories etc., (b) The seasonal changes for the "rst part were treated as a cosine function with amplitude 0.34, maximum during winter and minimum during summer (as for other heavy metals in the HMET model). According to data prepared by Axenfeld et al. (1992) the following split factors for mercury emissions were
included into model equations: Mercury form
Variable part
Constant part
Hg Hg> Hg
0.50 0.60 0.90
0.50 0.40 0.10
4.4. Model results Computed concentration maps for Hg3, Hg and Hg> in the mixing layer for 1996 are shown in Figs. 7}9, respectively. Hg varied from about 1.85 ng m\ in the Arctic to 12.3 ng m\ in Central Europe. Computed levels of Hg were in the range (0.01}0.767 ng m\ for the same areas, whereas Hg> showed concentrations from (0.01 to 3.2 ng m\ in Central Europe. A computed deposition map for Hg(tot) is shown in Fig. 10 with values ranging from 0.3 in the Arctic to 628 g m\ yr\ in Central Europe.
Fig. 9. Computed, annual mean concentration map for Hg'' in 1996 (unit, ng m\).
T. Berg et al. / Atmospheric Environment 35 (2001) 2569}2582
5. Comparison of model results with measurements Computed model concentrations of mercury were compared with observations at Ny-As lesund and Pallas for 1996 (Table 3). In the case of gaseous mercury, average measured values for 1996 at Ny-As lesund and Pallas were 1.59 and 1.25 ng m\, respectively. Computed concentrations from the HMET model were 1.9 and 2.17 ng m\ for the two sites, respectively. Agreement between average measured and computed concentrations is thus relatively good considering that the emission inventory used represents the year 1987 and emissions have been reduced since this date. It is also of interest to examine the in#uence of the global background concentration of mercury. In the model simulations, a background value of 1.8 ng m\ was applied in accordance with previous model studies (Petersen et al., 1995). This value represents the boundary concentrations of elemental mercury with a long-atmospheric residence time in the model domain. Without adding this background value, the model underestimates measured concentra-
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tions of gaseous mercury by almost two orders of magnitude. These results indicate that direct atmospheric transport of elemental mercury from source areas in central and northern Europe to the Arctic is small. Mercury contamination of the Arctic via atmospheric processes must therefore be considered on a global or hemispherical scale and the long-term atmospheric cycling of mercury must be taken into account. Furthermore, models need to take into account re-emission processes occurring over land and water in order to fully assess the impact of European mercury emissions on the Arctic environment. The development of this type of model requires information on re-emission #uxes representative of the European and Arctic regions. Very little information is today available on this topic. Obviously, this approach also requires detailed information on emissions (both natural and anthropogenic) on the same global or hemispherical scale. Computed air concentrations of particulate mercury in air in 1996 (2.3 pg m\:Ny-As lesund, 7.1 pg m\: Pallas), are close to those measured in 1996 (1.5}3 pg m\) at the
Fig. 10. Computed, annual mean deposition map of total (wet#dry) Hg in 1996 (unit: mg m\).
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Table 3 Modelled vs. measured values at Ny-As lesund and Pallas (1996) Source
Model
Station
Ny-As lesund Pallas
Measurements
Ny-As lesund Pallas
Compound Cons Hg (0) (ng m\)
Cons Hg (II) (pg m\)
Cons Hg (p) (pg m\)
Dep Hg (t) (ug m\)
Dep Hg (0) (ug m\)
1.9 1.85}1.91 2.07 2.02}2.17 1.59 0.68}3.55 1.25 0.73}1.80
1.4 0.9}1.9 8.2 6.6}10.6
2.3 1.5}3.3 7.1 6.6}9.6 2.37 0.1}20 1.5 0.33}5.8
0.8 0.6}1.1 3.5 2.5}7.2
0.2 0.2}0.3 1.2 0.8}2.0
two stations with some overestimation of the Pallas values, again possibly due to the high emission values in the applied inventory. Particulate Hg is emitted from anthropogenic sources, but is also formed in the atmosphere via gas/solid interactions and/or evaporation of cloud droplets. These processes are not well understood and quantitative information is not available making inclusion into models di$cult. Since the atmospheric lifetime of particulate species is considerable shorther than that of elemental mercury vapour, no global background value is used in the model and the relatively good agreement between modelled and measured concentrations indicates that a direct transport from continental Europe to the Arctic occurs. Computed deposition of Hg(tot) at Pallas is 3.5 g m\ yr\, which is &60% higher than measured (2.1 g m\ yr\). This may partly be due to the overestimation of particulate mercury concentrations at Pallas which will also in#uence the wet deposition via wash-out processes. In addition to the transformations and removal processed described in this model, recent studies have revealed that enhanced dry deposition of mercury can occur in the Arctic environment during polar sunrise via rapid oxidation of elemental mercury to particulate form (Schroeder et al., 1997). The processes involved are yet to be identi"ed but the observed rapid #uctuations in gaseous mercury concentrations coincide with ozone depletion events indicating that halogen-containing radicals are involved (Schroeder et al., 1997). The overall signi"cance of this mercury depletion and deposition is not possible to quantify but should clearly be included in future modelling e!orts aimed at the Arctic environment.
6. Conclusions Representative concentrations of total vapour-phase mercury (Average: 1.3 and 1.4 ng m\), particulate mer-
2.1
cury (Average: 1.4 and 2.7 pg m\) and wet deposition mercury (Average: 14 and 15 ng l\) have been determined at two Arctic locations. The HMET atmospheric transport model has been modi"ed with parameterisation for mercury chemistry and removal processes. The model simulations agree reasonably well with measured values with some overestimation probably caused by lower European emissions of mercury during the measurement period in comparison to the inventory used for the modelling. The model results indicate that direct transport of elemental mercury to the Arctic is small. The concentrations at the Arctic locations are mainly in#uenced by the background values applied in the model. Thus, global or hemispherical models for mercury, capable of simulating periods of several years, should be developed for assessment of anthropogenic in#uence on mercury levels in the Arctic. In addition to the larger scale models, further information on re-emissions from soil, snow and water is needed for improvement of model results. Identi"cation of chemical processes involved in the Arctic spring depletion events is also needed as well as determination of the net input of mercury during these events.
Acknowledgements We wish to thank the Nordic Council of Ministers (NMR) for "nancial support. The sta! at the Norwegian Polar Research Institute, Ny-As lesund and FMI, Pallas are gratefully acknowledged for collecting samples. We would also wish to thank Marit Vadset, NILU, and Elsmarie Lord, Peter Schager and Pia Carlsson, IVL, for assistance with sample analysis. We wish to acknowledge Dr. G. Petersen and Dr. O. KruK ger from the GKSS Research Centre, Geestadt, Germany who provided European emission data for mercury. In addition, we would like to thank the peer reviewers for insightful
T. Berg et al. / Atmospheric Environment 35 (2001) 2569}2582
comments and helpful suggestions which have substantially improved the manuscript.
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