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Vertical profiles of atmospheric mercury concentrationR. Ferrara a , B.E. Maserti a , A. De Liso b , H. Edner c , P. Ragnarson c , S. Svanberg c & E.Wallinder ca Istituto di Biofisica , CNR , Via S. Lorenzo 26, Pisa, 56100, Italyb Centro Interuniversitario di Biologia Marina , P.le Mascagni 1, Livorno, 57100, Italyc Department of Physics , Lund Institute of Technology , P.O. Box 118, Lund, S‐221 00,SwedenPublished online: 17 Dec 2008.

To cite this article: R. Ferrara , B.E. Maserti , A. De Liso , H. Edner , P. Ragnarson , S. Svanberg & E. Wallinder(1992) Vertical profiles of atmospheric mercury concentration, Environmental Technology, 13:11, 1061-1068, DOI:10.1080/09593339209385243

To link to this article: http://dx.doi.org/10.1080/09593339209385243

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Eiwironmental Technology. VoL 13, pp 1061-1068©Publications Division, Selper Ltd. 1992

VERTICAL PROFILES OF ATMOSPHERIC MERCURYCONCENTRATION

R. FERRARA 1*, B.E. MASERTI 1 , A. DE LISO2, H. EDNER3, P. RAGNARSON3, S. SVANBERG3 AND E. WALLINDER3

1CNR- Istituto di Biofisica, Via S. Lorenzo 26, 56100 Pisa, Italy2Centro Interuniversitario di Biologia Marina, P.le Mascagni 1, 57100 Livorno, Italy

3Department of Physics, Lund Institute of Technology, P.O. Box 118, S- 221 00 Lund, Sweden

(Received 31 March 1992; Accepted 27 May 1992)

ABSTRACT

Vertical profiles of atmospheric mercury concentration determined with a lidar and pointmonitor systems in the mineralized region of Mt. Amiata (Italy) are reported. Measurementswere performed over a large flat area, without arboreal and herbaceous vegetation, constituting aroasted cinnabar deposit which still contains about two parts per thousand of mercury. Thedeterminations carried out with the two techniques yielded comparable results for the workingconditions used. Data demonstrate the presence of a vertical gradient of atmospheric mercuryconcentration, which is particularly large in the layers of air nearest the soil. The highest values(45-1000 ng m-3) were measured a few centimeters from the soil, while background values (2-3 ngm13) were reached at heights of 10-20 m. The vertical gradient proved to be strongly dependent onambient temperature.

Keywords: Mercury, lidar, atmosphere

INTRODUCTION

Mercury is present in the air in the vapourphase in concentrations ranging fromnanograms to a few micrograms per m3 (1). It isgenerally recognized that the elemental form ofmercury makes up 70-90% of the total (2). Manystudies have been performed on mercury levelsin densely inhabited areas and in highlymineralized or rural zones (3,4).

The instrumentat ion available todaypermits accurate determinations even of thelowest background levels observable in rural ormarine areas. Usually, the determination ismade with point monitors through the intake ofconsiderable volumes of air (200-300 liters) overa period of 2-4 hours and preconcentrating themercury contained in it on gold traps (5).

Little, however, is known of the verticaldistribution of the mercury concentration despitethe usefulness of this information forunderstanding the transport and diffusionmechanisms of mercury in the atmosphere. Thisinformation is particularly useful when the

presence of mercury in the air is due to thedegassing processes of the earth's crust whichare known to constitute the main natural sourceof atmospheric mercury. Before 1969 it wasbelieved that the mercury concentration in airwas not dependent on the height from the ground;this consideration makes it likely that firstestimates of the atmospheric mercury pool havebeen overestimating (6).

The only papers dealing with the verticaldistribution of gaseous mercury are those byMcCarthy et al. (7) and Johnson and Braman (8).The first authors reported that the verticaldistribution in the atmosphere is highly variableand depends on the degree of turbulence in theair. Johnson and Braman (8) measured totalmercury in 26 vertical profiles over the range of0.1 to 10 m above the ground, finding a decreasingconcentration of mercury as a function of height.

A few estimates of the magnitude anddirection of the mercury transfer between soiland air have been reported by Schroeder et al. (9)and Xiao et al. (10) for some Swedish andCanadian sites.

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The determination of vertical profilespresen t s considerable analytical andmethodological problems. First, it requires astructure (tower, crane, etc.) capable ofsuspending the sampling devices atpredetermined heights; moreover these devicesmust suck in a small volume of air slowly so asnot to perturb the mercury distribution. In theseconditions the amount of mercury accumulated inthe trap may prove to be too small for an accuratedetermination. A partial solution to this problemmight be to have the sampling devices suspendedout of alignment, at the different heightsstaggered with respect to the vertical axis.

In any case, the fact of having to suck in acertain volume of air presents a perturbation ofthe gradient, and the fact that a certain period oftime is taken up implies that the resultrepresents an average value of the fluctuatingmercury concentration in the air.

Another technique recently used to measureatmospheric mercury is the lidar (light detectionand ranging). This technique, already knownfrom the study of some molecular pollutants (SO2,NOx), was developed by Edner et al. (11) for thedetermination of elemental mercury, the onlypollutant in the atomic state in the troposphere.The considerable potential of the lidar emergedin some research projects on the distribution ofmercury in mineralized areas (12), ingeothermal fields (13) and in industrializedareas (14).

This optoelectronic system can performatmospheric mercury determinations in a shorttime (~ min) without altering the distribution,and this makes it possible to detect and follow thetemporal trend of the gradient almost in realt ime.

The possibility of making a comparison ofthe results obtained with the two techniquesallows an evaluation of the possible error thatmight be associated with the use of pointmonitors.

A first approach to the study of verticalprofiles of atmospheric mercury concentrationswas made with the two measuring techniques overa large area of roasted cinnabar depositsderiving from past mining activity on Mt.Amiata (Italy). The choice of this area with highmercury concentra t ion, permits easyobservation of interesting diurnal and seasonalvariations in the vertical distribution ofatmospheric mercury reflecting the strongdependence of the soil degassing on the ambienttemperature.

MATERIALS AND METHODS

A detailed description of the point monitorshas been given by Ferrara et al. (5). Air wassucked at a constant flow rate of 0.5-11 min'1 for1-2 hours by means of a membrane pump,powered with a battery, through two parallel goldtraps. Each trap is made of a quartz tube (internaldiameter 3 mm) with a length of 150 mm,containing a coil of pure gold (0.8 g - filamentdiameter 0.4 mm). A timer allows performingthe sampling at set times. Electrothermallydesorbed mercury is determined with a modifiedatomic absorption spectrometer. Standardadditions of mercury, obtained by sucking themercury from a mercury generator through amicrosyringe (15), are deposited on thecollection traps to calibrate the system. Thedetection limit is 0.01 ng of mercury.

Six point monitors were suspended from acrane over the examined area by means of aKevlar wire up to a height of 12 meters from thesoil.

The temperature of the air at a height of 1 mand that of the soil (1-2 cm depth) was measuredwith thermometers, with a precision of ± 0.1°C.

In order to observe the diurnal variation ofmercury concentration, an automatic device with10 gold traps was used. During these experimentsthe air humidity and the ambient temperaturewere measured with a recording weather station.

The lidar system, extensively described byEdner et aL (11), is housed in a mobile truck. Thelaser radar system is capable of generating pulseenergies of up to 5 mJ with a linewidth of 0.001nm at the mercury resonance line (253.6 nm) andwith a repetition rate of 10 Hz. The laser beam isdirected out into the atmosphere via a largecomputer-controlled mirror, which can berotated around both the horizontal and verticalaxes. Back-scattered light from the atmosphereis reflected via the same mirror down into avertical Newtonian telescope and the lidar signalis detected by a photomultiplier tube. The laser istuned on and off the resonance line of mercuryevery second laser shot, allowing differentialabsorption measurements. A transient recorderperforms A/D conversion of the signal with atime resolution of 10 ns, giving a theoreticalspatial resolution of about 1 m. The detectionlimit is of the order of 2 ng m-3 of elementalmercury. Mercury compounds possibly presentin the atmosphere cannot be detected with thistechnique.

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STUDIED AREA

The volcanic region of Mt. Amiata, locatedin central Italy, covers an area of 400 km2 and isone of the largest cinnabar deposits in the world.Intensive mining activity was carried out forabout a century and reached a maximumproduction of 3xlO6 flasks (34.5 kg) of mercuryin 1969; the activity practically ceased in 1980.

Vertical profiles of atmospheric mercuryconcentration were performed on a large flatarea of about 4 hectares, without arboreal andherbaceous vegetation, constituting a roastedcinnabar deposit. The roasted ore still containsabout two parts per thousand of mercury.

The lidar was placed at the edge of the area;at the opposite edge of the deposit there was a steepdownhill slope of about 20 m to a new level ofthe cinnabar deposit. Measurements wereperformed in a first section of terrain^averaging the values of the concentrationsobtained at a distance of 75-180 m from the lidar,and in a second section at greater distance wherethe hill rapidly declines.

The point monitors, suspended from acrane, were positioned at a distance of about 130m from the lidar, at the center of the interval ofthe distances of the first section between whichthe determinations of atmospheric mercury weremade with the lidar. The maximum height fromthe ground was 12 m for the point monitors. Thedeterminations of the mercury concentration inthe air were performed on windless days or dayswith very light winds (1-2 m s"1).

RESULTS AND DISCUSSION

A first series of determinations of thevertical profile of the atmospheric mercury-concentration was made in September 1990 bothwith the lidar and with the point monitors in thearea described above. The results obtained aregiven in Figure 1, where also the lidarmeasurement intervals are given (Figure 1:A,B),as well as the positions with respect to the groundof the point monitors (Figure 1:C,D,E). Themercury concentrations observed are giveninside the circles.

As regards the determinations made with thelidar, in Figure 1 the direction of the laser beamis indicated as well as the length of the opticalpath (included between two dashes) for which themean value of the atmospheric mercuryconcentration was calculated. Although themeasurements were made on different days and

with two different techniques, the followinggeneral observations can be made:- The mercury concentration in the air rapidly

decreases in the first 2-3 m above the ground.- Minimum concentration values, comparable to

the background ones for rural areas, are reachedat heights of 10-20 m.- The maximum values (45-65 ng m"3) were

determined with the point monitors a fewcent imeters from the ground. Thesemeasurements cannot be made with the lidarbecause of the unevenness of the ground.

The mercury concentrations determinedwith the lidar technique seem to be slightly higherthan the ones observed with the point monitors.This could be due to the fact that the former referto measurements made in a short period of timeand represent a mean (spatial) value of theconcentrations over a long section of air nearlyparallel to the ground (the one included betweenthe two dashes), whereas the point monitorperforms a temporal average over a much longertime period on an air volume of approximately 60liters. This air coming from the spacesurrounding the intake position causes aperturbation of the concentration gradient and, inthe final analysis, has a slight diluting effect onit. However the vertical gradients of the mercuryconcentration determined with the two techniquescan be considered comparable, and confirm that,for the described working conditions, pointmonitors can also be used for observations ofthis type.

To evaluate the effect of the ambienttemperature on the concentration and verticaldistribution of mercury in the air, a secondseries of determinations was made in the samearea in July 1991 using only the point monitors.The higher temperatures reached by the soil inthe summer considerably influence thedegassing processes of mercury. Extremely highconcentrations (around 1000 ng m*3) at a height ofa few centimeters above the soil were found. Theconcentration rapidly decreases with height and,in this case as well, at 10-12 m it is already quitecomparable to the concentration observed in themonth of September. The wide range of theatmospheric mercury concentration makes agraphic representation of the results obtainedextremely difficult; for this reason they aresummarized in Table 1.

The dependence of the atmospheric mercuryconcentration on ambient temperature wasshown by Dumarey and Dams (16) in a series ofdeterminations performed between February andJuly 1980 at the Institute for Nuclear Science of

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Ghent (Belgium). We found that this dependence determined 30 cm from the ground by means ofis present not only in the different seasons, but the automatic sampling device above the samealso daily. Figure 2A shows diurnal variation deposit of roasted cinnabar. The fact that the

- f&aste_d_cirmabar deposit V i - > » .

26-09-90 ;16/j;rs.18"C

cinnabar^-^-y depositï^gr7

m10-

10

D 28-O9-9O.-13/JTs'22'C

300 m

5-

28-09-96.16/)rs.14«C

"deposit 2> i .~

Figure 1. Vertical profiles of atmospheric mercury concentration (ng m-3) measured over aroasted cinnabar deposit, with the lidar (A,B) and the point monitors (C.D.E) (Ts = soiltemperature).

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Table 1. Atmospheric mercury concentration (ng m"3) as a function of the height (m) measured atdifferent times over a roasted cinnabar deposit, during July 25-26, 1991. (Tg =temperature of the soil; Ta = temperature of the air).

Heightm

11.507.504.501.500.300.05

Time 11.25TB-33°CT t t- 29°C

3.915.627.445.3

580.31500.0

Time 15.08T 8 - 338CTa-27°C

8.418.326.027.4

321.11100.0

Time 17.30TB- 28°CT a - 23°C

6.515.87.2

31.5576.3915.5

Time 20.00Ta-25°CT a - 19°C

3.8 •26.4

8.447.8

346.0997.0

Time 7.30T a - 20°CT a - 20"C

12.626.947.539.5

186.5806.4

Hgng/m1

200

160

120

80

40

0

HgH%

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• - , •

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40-

30-

20-

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200 -

160 -

120

80

40 h

18 24 6 12 h

Figure 2. Daily behaviour of atmospheric mercury concentrations, temperature and relativehumidity measured 30 cm above a roasted cinnabar soil on two different days (A-B) inMay 1987.

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mercury concentration follows the trend of theambient temperature without any appreciabledelay means that only the shallow layers of thesoil are involved in the degassing processes.This is also confirmed by the plot in Figure 3,which shows the temperature of the soil as afunction of depth for a zone exposed to the sun,and for a shaded zone, respectively. As can beseen, the soil layers at a depth greater than 10 cmmaintain a fairly constant temperaturethroughout the day.

Another example of daily behavior isreported in Figure 2B, which for a different day,shows an unexpected increase of gaseousmercury levels during the night, even though thetemperature was lower. This is explained by theatmospheric accumulation of the mercuryemitted from the soil due to a complete lack ofwind and by a limited vertical atmosphericexchange as denoted by the high levels ofhumidity.

CONCLUSIONS

The assumption that mercuryconcentrations in air are constant withincreasing altitude became questionable after thedata reported by McCarthy et al. (7) and Johnsonand Braman (8). When the presence of mercury

in the air is due to the degassing of the earth'smantle, Abramovskiy et al. (17) showed that inthe absence of disturbing phenomena, themercury concentration decreases exponentiallywith increasing altitude, following therelationship Cz = Co e'Kz, where Co = groundconcentration, Cz = concentration at any altitude zand K is a constant, approximately equal to 10'3

Other reliable data do not exist in theliterature, probably because of the difficultyinherent in determining the concentrationgradients of mercury in the air. Thedeterminations reported in this work, obtainedboth with the lidar technique and with pointmonitors, yielded comparable values in theworking conditions used, suggesting the validityof both measurement techniques. The datareported demonstrate the presence of a verticalgradient in the atmospheric mercuryconcentration, which in the highly mineralizedarea examined is particularly large in the layersof air nearest the ground. However, it seems to usthat these variations cannot be described byAbramovskiy's function, which predicts a rathermoderate decrease of the concentration withheight.

The presence of high mercury concentrationin the superficial soil makes this area anoticeable source of atmospheric mercury. Some

T(°C)

50 -

40 —

30 -

20 -

10

ill*••• • •

• • «• • <• • <

! • • !

! mt

j

m

|; M

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Â

m

L

mm

• •

ill

I I I I

1 2 3 4 5 6 7 8 9 10 cm

Figure 3. Soil temperature as a function of depth: I Ishaded soil (Tair = 24°C); 00 sunny soil (Ta i r

32°C) observed on July 1987.

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preliminary measurements of vertical fluxes pine needles, leaves), particular attention mustperformed by us (unpublished data) with the flux be paid in trying to collect samples atchamber described by Schroeder et al. (9), show a comparable heights, not to make evaluationmercury emission rate ranging from 20 to 80 ng errors, especially in mineralized areas.m ^ h r 1 . The high atmospheric mercury

The vertical gradient proved to be strongly concentration values existing a few centimetersdependent on the ambient temperature; the from the soil may have effects on all thedegassing processes of the mercury from the organisms that live in this microenvironment;soil, responsible for the presence of the metal in infact during some preliminary measurementsthe air, seem to occur mainly in the top shallow we observed high mercury levels in the leaves oflayers of the soil, layers that rapidly follow the Graminae (6 - 850 ug g*1 dw) with respect to thevariation of temperature. same species sampled on non mineralized soils

The existence of a strong vertical gradient (0.03-0.1 jigg'1).of mercury concentration makes it necessary to We feel that it would be particularlyestablish the measurement procedures for interesting to study the vertical gradient ofquoting the mercury concentration in a given atmospheric mercury in a forest, where thearea, in particular to standardize the height of the canopy influences the mercury diffusion andsampling point above the ground. interacts with the natural distribution through the

In the case of the use of bioindicators of processes of uptake and release of the metal inatmospheric mercury levels (such as lichens, the leaf apparatus.

REFERENCES

1. Schoeder W.H., Sampling and analysis of mercury and its compounds in the atmosphere.Environ. Sci. Technol., 16, 394-400 (1982).

2. Lindqvist O. and Rhode H., Atmospheric mercury - a review. Tellus, 37B, 136-159 (1985).3 . Breder R. and Flucht R., Mercury levels in the atmosphere of various regions and locations in

Italy. Sci. Total Environ., 40, 231-244 (1984).4. Perrara R., Maserti B.E. and Breder R., Mercury in abiotic and biotic compartments of an area

affected by a geochemical anomaly (Mt. Amiata, Italy). Water Air and Soil Pollut., 56, 219-233(1991).

5. Ferrara R., Maserti B.E., Edner H., Ragnarson P., Svanberg S. and Wallinder E., Atmosphericmercury determinations by lidar and point monitors in environmental studies. Fourth IAEACWorkshop on Toxic Metal Compounds, Les Diablerets (Switzerland) March 4-8, 1991. Specialsupplement to Chemical Speciation and Bioauailability, 29-37 (1992).

6. Andren A.W. and Nriagu J.O., The global cycle of mercury. In: The Biogeochemistry of Mercuryin the Environment, Nriagu J.O. (ed.) Elsevier/North Holland Biomedical Press, Amsterdam,New York, Oxford, 1-22 (1975).

7. Mc Carthy J.R. Jr, Vaughn W.W., Learned R.E. and Mueschke J.L., Mercury in soil, gas and air- a potential tool in mineral exploration. U.S. Geol. Surv. Circ. 609. Washington, D.C., (1969).

8. Johnson D.L. and Braman R.S., Distribution of atmospheric mercury species near ground.Environ. Sci. Technol., 8, 1003-1009 (1974).

9. Schroeder W.H., Munthe J. and Lindqvist O., Cycling of mercury between water, air, and soilcompartments of the environment. Water Air Soil Pollut, 48, 337-347 (1989).

10. Xiao Z.F., Munthe J., Schroeder W.H. and Lindqvist O., Vertical fluxes of volatile mercury overforest soil and lake surfaces in Sweden. Tellus, 43B, 267-279 (1991).

11. Edner H., Faris G. V., Sunesson A. and Svanberg S., Atmospheric atomic mercury monitoringusing differential absorption lidar techniques. Applied Optics, 28, 921-930 (1989).

12. Edner H., Ragnarson P., Svanberg S., Wallinder E., Ferrara R., Maserti B.E. and Bargagli R.,Atmospheric mercury mapping in a cinnabar mining area. Sci. Total Environ, (in press).

13. Edner H., Ragnarson P., Svanberg S., Wallinder E., De Liso A., Ferrara R. and Maserti B.E.,Differential absorption lidar mapping of atmospheric atomic mercury in Italian geothermalfields. J. Geophys. Res., 97, 3779-3786 (1992).

14. Ferrara R., Maserti B.E., Edner H., Ragnarson P., Svanberg S. and Wallinder E., Mercuryemission into the atmosphere from a chlor-alkali complex measured with the lidar technique.Atmos. Environ., 26A, 1253-1258 (1992).

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15. Schroeder V.H., Report ARQA-73-79 Atmospheric Environment Service, Downsview, Ontario,Canada, (1979).

16. Dumarey R. and Dams R., The influence of meteorological parameters on atmospheric volatileand particulate mercury distribution. Environ. Pollut., 10, 277-285 (1985).

17. Abramovskiy B.P., Anokhin Yu. A., Ionov V.A., Nazarov E.M. and Ostromogil'skiy A.Kh.,Global balance and maximum permissible mercury emissions into the atmosphere. In: SecondJoint US/USSR Symposium on the Comprehensive Analysis of the Environment. October 1975. USEnvironmental Protection Agency, Honolulu, 14-21 (1975).

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