Atmospheric Environment 35 (2001) 5477–5484

Estimates of air-sea exchange of mercury in the Baltic Sea

Ingvar W.aangberga,*, Stefan Schmolkeb, Peter Schagera, John Munthea,Ralf Ebinghausb, (AAke Iverfeldta

a IVL Swedish Environmental Research Institute, P.O. Box 47086, S-402 58 G .ooteborg, SwedenbGKSS Research Centre, Max-Planck-Str., D-21502 Geesthacht, Germany

Received 15 January 2001; accepted 27 April 2001

Abstract

The concentrations of total gaseous mercury (TGM) in air over the southern Baltic Sea and dissolved gaseousmercury (DGM) in the surface seawater were measured during summer and winter. The summer expedition wasperformed on 02–15 July 1997, and the winter expedition on 02–15 March 1998. Average TGM and DGM values

obtained were 1.70 and 17.6 ngm�3 in the summer and 1.39 and 17.4 ngm�3 in the winter, respectively. Based onthe TGM and DGM data, surface water saturation and air-water fluxes were calculated. The results indicate thatthe seawater was supersaturated with gaseous mercury during both seasons, with the highest values occurring in the

summer. Flux estimates were made using the thin film gas-exchange model. The average Hg fluxes obtained for thesummer and winter measurements were 38 and 20 ngm�2 d�1, respectively. The annual mercury flux from this area wasestimated by a combination of the TGM and DGM data with monthly average water temperatures and wind velocities,resulting in an annual flux of 9.5 mgm�2 yr�1. This flux is of the same order of magnitude as the average wet deposition

input of mercury in this area. This indicates that reemissions from the water surface need to be considered when makingmass-balance estimates of mercury in the Baltic Sea as well as modelling calculations of long-range transboundarytransport of mercury in northern Europe. r 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Gaseous mercury; Dissolved gaseous mercury

1. Introduction

The exchange of mercury between natural surfacesand the atmosphere is an important process for the

atmospheric cycling and environmental turnover of thiselement. Mercury deposited on land or water surfacescan be reemitted into the atmosphere, thus increasing its

environmental lifetime to a great degree. Earlierinvestigations have indicated that emissions of volatilemercury from freshwater surfaces are of the same order

of magnitude as the input via wet deposition (Xiao et al.,1991; Lindberg et al., 1995). From terrestrial surfaces,reemissions are generally lower but may still constitute

an important path of mercury transport (Lindberg et al.,1998).

For the evaluation of long range transport ofmercury, information on point source emissions is notsufficient. In most applications of atmospheric transportmodels, a background concentration of 1–2 ngm�3 is

usually applied to account for the global background ofatmospheric mercury as well as emissions from naturalsurfaces (Petersen et al., 1995). With this approach it is

possible to model air concentrations and wet depositionwith reasonable accuracy, provided that the point sourceemissions are the dominating source of mercury. In

Europe, point source emissions of mercury have beenreduced drastically over the last 5–10 years as shownfrom field measurements of atmospheric mercury

(Iverfeldt et al., 1995) and sediment profile investiga-tions (Munthe et al., 1995). This trend has increased therelative importance of the background concentrations ofmercury, and thus the emissions from natural surfaces.

Further improvement of our concept of the atmospheric*Corresponding author. Fax: +46-31-725-6290.

E-mail address: ingvar.waengberg@ivl.se (I. W.aangberg).

1352-2310/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved.

PII: S 1 3 5 2 - 2 3 1 0 ( 0 1 ) 0 0 2 4 6 - 1

cycling of mercury will include descriptions of not onlyremoval processes such as wet deposition, but also of

reemissions from land and water (Schroeder andMunthe, 1998).Several different experimental approaches for estimat-

ing mercury fluxes over natural surfaces are described inthe literature. Chamber techniques have been extensivelyused over soil and freshwater surfaces (Xiao et al., 1991;Schroeder et al., 1989; Poissant and Casimir, 1998;

Capri and Lindberg, 1998). The main drawback ofchamber techniques is that the chamber creates anenclosed environment with altered air exchange and

sunlight intensity that may affect the fluxes. In water,chambers can only be used during relatively calmconditions, which makes direct surveys of the influence

of wind and waves on the mercury fluxes impossible.This is particularly true for seawater where the exchangeof gases between water and atmosphere often is

governed by storm events with high wind speed andbreaking waves. Micrometeorological techniques havealso been applied for mercury flux measurements overforest canopies, soil and freshwater (Poissant and

Casimir, 1998; Lindberg et al., 1992; Meyers et al.,1996). Another approach for estimating mercury fluxesbetween water surfaces and air is calculating fluxes using

gas exchange models. Using this approach, the dissolvedgaseous mercury (DGM), i.e. dissolved elementalmercury in the surface water, and the elemental mercury

content of the above air, need to be measured. Thismethod provides the opportunity to account for theeffects of wind speed and water temperature, and hasbeen used to estimate mercury fluxes from oceans as well

as from fresh water lakes (Kim and Fitzgerald, 1986;Fitzgerald et al., 1991).In the present investigation, surface sea water samples

for DGM determination were collected in the southernBaltic Sea. One Summer cruise during 02–15 July 1997and one Winter cruise during 02–15 March 1998 were

conducted on the German research vessel Alexander vonHumboldt. The total gaseous mercury (TGM) wascontinuously measured at both expeditions. Based on

the TGM and DGM data, surface water saturation andmercury fluxes were calculated using the thin film gas-exchange model.

2. Methods

2.1. The Baltic Sea

The Baltic Sea is a young interior sea in northern

Europe, formed after the later glacial period. Thesouthern Baltic Sea, or the proper Baltic Sea, has anarea of 214,000 km2, and a mean depth of 62m. The sea

surface of the proper Baltic Sea is about half the totalBaltic Sea, corresponding to 0.05% of the total ocean

surface of the world. The Baltic Sea is connected to theNorth Sea and the Atlantic Sea via the Skagerack, theKattegatt, and narrow passages between the south of

Sweden and the Danish islands. The salinity of thesurface water of the proper Baltic varies from 0.65% inthe north to 1.0% in the southwest. A thermocline

developed during spring and summer separates thewarm surface water from the cold deep water. Thedepth of the thermocline varies from a few metres to

twenty. Due to lower surface temperature levels duringwinter and the effects of winter storms, the surface anddeep water may end up more or less mixed by the end ofthe winter. The sites where DGM samples were collected

are shown in Fig. 1. The routes of the cruises were notspecifically chosen for the mercury measurements andthe sampling sites are therefore somewhat arbitrarily

distributed.

2.2. Ship

Alexander von Humboldt is a large research vesselowned by the Institut f .uur Ostseeforschung War-nem .uunde, Germany. The ship is 64.2m long and

10.6m wide and is operated by a Nautical/technicalstaff of 16 persons. The ship is equipped with winchesand lifting gears for collecting water samples, a data

collecting and distribution system (DATADIS) and anautomatic weather station. It has room for 15 scientistsand is equipped with 115m2 laboratory space.

2.3. TGM measurements

An automatic gas phase mercury analyser (Tekran

Model 2537A) was installed under deck onboard theship. The analyser was equipped with a Teflon samplingline with inlet at the front mast top 20m above the sea

surface and about 15m above the deck of the ship. Theworking principle of the Tekran instruments is the gold

Fig. 1. The proper Baltic Sea.

I. W .aangberg et al. / Atmospheric Environment 35 (2001) 5477–54845478

amalgamation technique. A pre-filtered sample airstream is drawn through Au-traps which are analysed

by thermodesorption followed by cold vapour atomicfluorescence spectrophotometry detection (CVAFS). A47mm diameter Teflon pre-filter protects the Au-traps

against contamination from particulate matter. Theinstrument uses two parallel Au-traps, with alternatingoperation modes (sampling and desorbing/analysing) ona pre-defined time basis of 5min. A sampling flow rate

of 1.5 lmin�1 was used. Under these conditions, TGMvalues with 5min time resolution and a detection limit ofE0.3 ngm�3 was continuously achieved. Hence, TGM

was measured during transport as well as at anchorstations. Since the sampling inlet was placed in the frontmast about 25m from the smokestack, risk of contam-

ination could only occur when the wind came directlyfrom the back or when being close to other ships. Atanchor stations, the ship was always lined up against the

wind thus avoiding the risk of contamination. Simulta-neously measured NOx concentrations constituted asensitive means for detecting risk for contaminationfrom the smokestack. Only TGM values associated with

clean wind conditions have been used for evaluation.

2.4. Dissolved gaseous mercury

Conductivity, temperature and depth measurementswere made before each DGM sampling. The depth of

the thermocline varied between 5 and 16m during thesummer measurements and 30–51m in the winter. Watersamples were collected at 3–10m depths in the well-

mixed surface column above the thermocline, usingGO-FLO, (model 1080) water collectors of 5–10 lvolume.All DGM samples were immediately analysed in the

laboratory onboard the ship. A 2.0 l volume of thesample was poured into an acid cleaned Teflon impinger.The impinger consisted of a tube of 1.80m length and

4.1 cm inner diameter as shown in Fig. 2. The samplewas extracted by introducing a stream of pre-purifiednitrogen via a glass frit in the bottom of the impinger.

The gaseous mercury extracted was collected on an Au-trap connected to an outlet at the top of the impinger.To avoid condensation of water vapour, the Au-trap

was heated to 451C. Each water sample was extractedduring 90min, with a nitrogen flow-rate of 0.31 lmin�1.The Au-traps were analysed using the standard dualamalgamation and CVAFS detection technique (Bros-

set, 1987; Bloom and Fitzgerald, 1988). Betweensamples, the Teflon impinger was kept clean by purgingwith Hg-free nitrogen. The procedural blanks were

3.02 ngm�3, sðn ¼ 5Þ ¼ 0:53 ngm�3, from which adetection limit of 1.1 ngm�3 was calculated. Prior tothe cruises, known DGM amounts were analysed to test

the efficiency and reproducibility of the extractionmethod. From these experiments, it was concluded that

DGM concentrations in the range of 15–20 ngm�3 couldbe determined with a precision of 76%.

2.5. Analysis of total mercury and methylmercury inseawater

All samples for the analysis of total mercury andmethylmercury were collected in 125ml Teflon bottlesand acidified with 0.5ml HCl (Suprapur). The sampleswere stored at +51C in darkness, before being

transferred to the IVL laboratory for analysis. Methyl-mercury was analysed in a few selected samples. In thiscase, an aliquot of the sample (40ml) was analysed using

distillation, aqueous phase ethylation and GC-CVAFSdetection as described by Lee et al. (1994). The detectionlimit was 0.06 mgm�3. Total mercury was analysed after

BrCl treatment, SnCl2 reduction and dual gold amalga-mation, followed by CVAFS detection (Iverfeldt, 1991;Bloom and Fitzgerald, 1988). The detection limit of theanalysis method was 0.06mgm�3 and the statistical

precision was 715% (calculated as 1sðn�1Þ) for samplescontaining 1mgm�3.

3. Results and discussion

DGM and TGM concentrations obtained are shown

in Table 1. The conditions during both seasons weredominated by clean air masses. With some exceptions,1 h average TGM concentrations ranged from 1.4 to

2.0 ngm�3 during the summer expedition. These valuesresemble observations made in southern Sweden duringsummer 1995 (Schmolke et al., 1999) and are common

for conditions with clean air masses in the Scandinavianregion. The winter TGM values are somewhat lower,

Fig. 2. 2.0L volume DGM extractor. The extractor was fed by

pre-purified nitrogen (by means of a freshly desorbed Au-trap).

Hg vapour evolved was trapped on an Au-trap maintained at

451C at the outlet in the top.

I. W .aangberg et al. / Atmospheric Environment 35 (2001) 5477–5484 5479

ranging from 1.2 to 1.6 ngm�3 with an average of

1.39 ngm�3. Summer and winter TGM time series areshown in Figs. 3 and 4.The summer and winter DGM concentrations are

very similar, both ranging between 14 and 22 ngm�3,with average values of 17.6 and 17.4 ngm�3, respec-tively. To our knowledge, this is the first time DGMmeasurements are reported from the Baltic Sea.

Information on DGM concentrations in seawater from

different areas is still somewhat sparse. However, the

present values are comparable to DGM concentrationsmeasured in the open North Sea, 12–54 ngm�3 (Baeyensand Leermakers, 1998), 30–74 ngm�3 (Coquery and

Cossa, 1995). DGM has also been measured in theNorth Atlantic Ocean 30–230 ngm�3 (Mason et al.,1998) and in the Equatorial and North Pacific Ocean,6–220 ngm�3 (Kim and Fitzgerald, 1986; Mason et al.,

1994; Mason and Fitzgerald, 1996).

Table 1

Measured DGM and TGM concentrations and estimated fluxesa

Position

deg min N,

deg min E

Date Time DGM

(ngm�3)

TGM

(ngm�3)

Tw

(oC)

Wind speed

(m s�1)

Degree of

saturation (S)

kw(md�1)

Flux

(ngm�2 d�1)

5600.2, 1749.4 03-07-97 16:35 15.4 1.83 15.9 4.6 1.9 1.6 11

5538.0, 1718.0 04-07-97 13:15 13.9 1.91 16.0 3.7 1.6 1.1 6

5500.0, 1900.0 05-07-97 06:37 21.7 2.03 17.6 5.6 2.6 2.2 30

5500.0, 1900.1 05-07-97 20:45 19.2 1.84 18.2 5.6 2.7 2.3 27

5630.2, 2005.0 07-07-97 09:40 18.8 1.55 17.4 9.3 3.0 5.1 64

5630.0, 1825.5 08-07-97 08:30 15.0 1.58 18.5 2.8 2.5 0.7 7

5620.0, 1819.9 08-07-97 09:50 14.9 1.55 18.5 4.1 2.5 5.2 53

5609.6, 1752.0 09-07-97 09:07 17.2 1.71 17.5 9.3 3.1 1.1 15

5450.0, 1359.9 12-07-97 09:20 20.4 1.56 16.7 3.7 3.5 1.2 16

5450.0, 1359.9 13-07-97 12:35 19.6 1.42 18.0 3.7 4.8 1.2 24

5450.0, 1359.9 13-07-97 12:40 26.2 1.42 18.5 3.7 1.6 10.4 89

Average: 17.6 1.70 17.4 6.2 2.5 3.5 38

5514.4, 1505.9 04-03-98 17:30 22.0 1.50 3.8 18.0 1.6 10.4 89

5440.0, 1600.1 05-03-98 14:30 19.0 1.50 3.9 6.9 1.4 2.2 12

5440.0, 1600.1 06-03-98 09:00 17.2 1.63 3.8 12.1 1.2 5.4 14

5459.3, 1610.6 07-03-98 11:00 22.2 1.37 3.7 5.6 1.8 1.5 15

5530.1, 1859.6 08-03-98 09:25 17.9 1.38 3.7 3.9 1.4 0.8 5

5601.9, 1819.7 09-03-98 14:35 15.0 1.35 2.9 11.8 1.2 5.1 11

5559.9, 1745.0 11-03-98 06:40 14.6 1.33 2.2 4.4 1.1 1.0 1

5600.0, 1820.1 12-03-98 13:10 14.8 1.26 2.5 5.3 1.2 1.3 3

5508.0, 1736.7 13-03-98 13:10 13.7 1.23 3 9.9 1.2 3.8 8

Average: 17.4 1.39 3.3 8.5 1.3 4.5 20

aTw is the surface water temperature. Standard deviation for DGM and TGM measurements are 71 ngm�3 and 70.14 ngm�3,

respectively calculated as 1sðn�1Þ:

Fig. 3. TGM concentrations obtained during the cruise in July

1997.Fig. 4. TGM concentrations obtained during the cruise in

March 1998.

I. W .aangberg et al. / Atmospheric Environment 35 (2001) 5477–54845480

The origin of DGM is likely to be the reductionof Hg(II). Average Hg(tot) concentrations in the

surface water column from the summer and winterexpeditions are 0.7070.4 and 1.270.3mgm�3, respec-tively. Hence, the DGM concentration constitutes a

small fraction, only 1.5–2.5% of the total mercury. Thisis relatively low in comparison to 20% as found inNorth Atlantic waters (Mason et al., 1998). Higherfractions of DGM relative to Hg(tot) were also observed

in the open North Sea, 6–16% (Baeyens and Leer-makers, 1998). According to the literature, Hg(tot) inthe open oceans varies between 0.1 and 1.3 mgm�3

(Mason and Fitzgerald, 1996; Baeyens and Leermakers,1998; Baeyens et al., 1991 and references therein). Therelatively high total mercury values obtained in the

Baltic Sea may reflect the interior character of this seawith a high population density in the surroundingcoastal areas and subsequently higher releases of

pollutants. Seawater samples collected during thesummer expedition were analysed for methylmercury,however, the content of this species was less than thedetection limit (o0.06mgm�3).

3.1. Saturation and flux calculations

The degree of DGM saturation, S, was calculated

using Eq. (1),

S ¼ H 0DGM=TGM; ð1Þ

H 0 ¼ EXPð�4633:3=Tw þ 14:53Þ

ðvalid between 273 and 303KÞ; ð2Þ

where H 0 is the dimensionless Henry’s-Law constant

ðH 0 ¼ ½Hg�g=½Hg�aqÞ; calculated at the appropriatewater temperatures (Tw) using Eq. (2), Clever et al.(1985). Values of S, greater than unity, indicate super-saturation in the water, whereas values lower than unity

mean undersaturation. The measurements show that theseawater is supersaturated during summer as shown bythe degree of saturation in Table 1. The degree of

saturation is significantly lower during winter andsometimes indistinguishable from equilibrium. Sincesummer and winter DGM and TGM concentrations

are very similar, the lower saturation during winter is tothe major part an effect of the lower water temperature.At a given TGM concentration the solubility ofelemental mercury increases with decreasing tempera-

ture. However, the sea is supersaturated during most ofthe time, implying that there will be a net flux of mercuryfrom the sea into the atmosphere.

The flux of mercury, between the seawater surface andair, was estimated using the thin film gas-exchangemodel, according to Eq. (3), (Liss and Slater, 1974).

Flux ¼ kwðDGM� TGM=H 0Þ: ð3Þ

The term kw; is the gas transfer velocity of a species inthe water-air interface. Gas transfer velocities were

calculated using the empirical relation according toEq. (4), (Wanninkhof, 1992),

kw ¼ 0:31 u210ðScHg=600Þ�0:5 ðcm h�1Þ; ð4Þ

where ScHg is the Schmidt number of Hg and u10 is thewind speed at 10m height. Eq. (4) describes the wind

speed dependence of the gas transfer velocity normalisedto a Schmidt number of 600, where 600 corresponds tothe Schmidt number of CO2 in fresh water at 201C. TheSchmidt number is the kinematic viscosity of the water

divided by the aqueous diffusivity of the species beingtransferred. By Eq. (4) the gas transfer velocity ofelemental mercury can be estimated provided that the

ScHg-numbers at the actual temperatures are known. Inthis work ScHg numbers were calculated using tempera-ture corrected viscosities and diffusivities valid for the

brackish water in the Baltic Sea. The diffusivity ofmercury was calculated using the Wilke-Chang methodas described in Reid et al. (1987). Mercury diffusion

constants and Schmidt numbers, calculated at tempera-tures between 11C and 231C are shown in Table 2.The wind speed was measured 22m above the sea-

surface and corrected to u10 (the velocity at 10m

altitude) using the equation (Schwarzenbach et al.,1992),

u10 ¼10:4uz

lnðzÞ þ 8:1ðms�1Þ; ð5Þ

where uz is the wind speed measured at height zm abovethe water surface.

The fluxes presented in Table 1, corresponding todifferent locations, are instantaneous fluxes, obtainedfrom DGM and TGM concentrations, water tempera-

tures (Tw) and wind speeds, at each location and time.

Table 2

Calculated mercury seawater diffusivites and Schmidt numbers

valid in brackish water with a salinity of 7 PSU

Tw (1C) DðHg0Þ (cm2 s�1) ScHg

1.0 1.66E-05 1005

3.0 1.77E-05 889

5.0 1.89E-05 788

7.0 2.01E-05 701

9.0 2.14E-05 625

11.0 2.27E-05 558

13.0 2.40E-05 500

15.0 2.55E-05 449

17.0 2.69E-05 404

19.0 2.85E-05 365

21.0 3.00E-05 330

23.0 3.17E-05 299

I. W .aangberg et al. / Atmospheric Environment 35 (2001) 5477–5484 5481

Gas transfer rates obtained (kw) are also presented.Additionally, average fluxes for each period are given.

These values were calculated using the average DGM,TGM and water temperatures from each periodcombined with the average wind speed of each entire

period. The average summer and winter season mercuryfluxes obtained are 38, and 20 ngm�2 d�1, respectively.The reason for the relative high fluxes during winter is

that the flux apart from DGM concentration and

temperature, is also dependent on the wind speed.According to the model, at very windy occasions duringwinter the mercury flux may exceed that in summer.

The present fluxes are compared with mercury fluxesfrom some other areas in Table 3. As is shown, the BalticSea fluxes are comparable to those from the North Sea.

Fluxes measured at coastal sites e.g., the Scheldt Estuaryat the Belgian Coast and at the Swedish West Coast arehigher than those at the open North Sea and the Baltic

Sea. This seems to be an effect of the higher DGMconcentration in these areas. DGM concentrationsmeasured in the Scheldt Estuary are in the range from42 to 140 ngm�3 with the highest value measured during

summer (Baeyens and Leermakers, 1998). G(aardfeldtet al. (2001) report DGM levels in the range of40–100 ngm�3 from measurements made during sum-

mer. A pronounced diurnal variation in the mercury fluxis reported by several authors performing flux chambermeasurements at coastal sites. The maximum flux

is coincident with maximum insolation (Ferrara et al.,2000; G(aardfeldt et al., 2001).The measurements in the Scheldt Estuary show a

seasonal variation of a factor 2 between winter and

summer DGM concentrations (Baeyens and Leer-makers, 1998). Information on seasonal DGM variation

in large sea areas is scarce. It is, for example, not knownwhether there is a seasonal variation in the open NorthSea. Since DGM production is likely to be dependent on

biological and photolytical processes, one would expectit to be higher during the summer season. On the otherhand, it is possible that a greater DGM productionduring summer is balanced by a higher flux. Evidently,

measurements during all seasons are needed in order tosolve this issue.Disregarding uncertainties concerning the seasonal

variability in DGM, the annual mercury flux fromthe Baltic Sea was calculated based on the averagemonthly wind speeds and water temperatures, and by

assuming constant TGM and DGM concentrations of1.5 ngm�3, and 17 ngm�3, respectively. The result ofthis exercise should only be considered as a first

approximation, but will also demonstrate some featuresof the model. Two different Henry’s Law constants wereused and the result is shown in Fig. 5. The first, madewith Henry’s Law values from Clever et al. (1985),

suggests a pronounced seasonal variation with thehighest flux occurring in the autumn. Sinceboth the TGM and DGM are kept constant the

variation in flux is only due to water temperature andwind speed. The reason for the maximum is therelatively high water temperature in combination with

high wind speeds that occurs in the Baltic Sea during theearly autumn. The annual flux obtained is9.5 mgm�2 yr�1. In the second calculation, values basedon Henry’s Law constants reported by Sanemasa, 1975,

Table 3

Aquatic mercury fluxes reported in the literaturea

Location Flux (ngm�2 d�1) Method Author

Open Sea

Entire North Sea 22–44 GEM Coquery and Cossa, 1995

North Sea, Southern Bight 21–52b GEM Baeyens and Leermakers, 1998

North Sea, German Bight 12b GEM Baeyens and Leermakers, 1998

North Atlantic Ocean 3807260 GEM Mason and Fitzgerald, 1998

N. W. Mediterranean Sea 28 GEM Cossa et al., 1997

Baltic Sea, average summer 38 GEM This work

Baltic Sea, average winter 20 GEM This work

N. Pacific Ocean 6 GEM Mason and Fitzgerald, 1996

Equat. Pacific Ocean 32–228 GEM Mason et al., 1994

Coastal areas

Scheldt Estuary, summer 106–140b GEM Baeyens and Leermakers, 1998

Scheldt Estuary, winter 45–57b GEM Baeyens and Leermakers, 1998

Mediterranean Sea, coastal site at the Thyrrhenian Sea 29–137 FC Ferrara et al., 2000

Coastal site at the Swedish west coast, min-max �24–212 FC G(aardfeldt et al., 2001

aGEM=gas exchange models, FC=flux chamber.bValid at a wind speed of 8.1m s�1.

I. W .aangberg et al. / Atmospheric Environment 35 (2001) 5477–54845482

were used. The temperature dependence of this constantis expressed by Eq. (6),

Log10ðHÞ ¼ �1078=T þ 6:250; ð6Þ

where H has the unit of Atm divided by molar fractionand T is the absolute temperature in Kelvin. Fluxes

calculated using Eq. (6) are much less dependent onwater temperature and gives higher values and a lesspronounced seasonal variation. The annual mercury flux

found using Eq. (6) is 16.3mgm�2 yr�1. Evidently, theflux calculations are sensitive to the choice of Henry’sLaw constant. Unfortunately, this constant is stillunsettled as is shown by Clever et al. (1985).

Although, there are several uncertainties involved inthe estimations of air-sea exchange of mercury, it seemslike the annual mercury flux from the Baltic Sea is

comparable to the annual deposition flux of mercury inthis area. Typical annual wet deposition fluxes ofmercury measured at coastal stations in southern

Scandinavia are in the range 5–15 mgm�2 (Muntheet al., 2001), i.e. the same order of magnitude as theestimated emission fluxes of the sea surface. It can be

concluded that the air-sea exchange of volatile mercuryis an important process in the overall cycling of mercuryin the Baltic Sea. The Henry’s Law constant is animportant parameter needed for mercury flux estimates

and further work on accurate determination of thetemperature dependence of this parameter is needed.

Acknowledgements

This work was supported financially through the EURTD programme MAST-3 within the BASYS project

MAS3-CT96-0058. I.W. thanks ‘‘Stiftelsen f .oor Strate-gisk Forskning’’ for financial support. IOW, Waarne-

muende, is gratefully acknowledged for making theAlexander von Humboldt available for cruises and fortechnical assistance by the ship’s crew. Elsmarie Lord

and Pia Spandow are acknowledged for their analyticalwork at the IVL laboratory.

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Fig. 5. Estimated monthly average mercury fluxes in the

southern Baltic Sea calculated using DGM ¼ 17 ngm�3,

TGM ¼ 1:5 ngm�3 combined with monthly average water

temperatures and wind speeds. The solid bold line was obtained

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The dotted line is from using Henry’s Law constants according

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