Environ. Sci. Technol. 2000, 34, 3069-3078
Mercury Water-Air Exchange over the Upper St. Lawrence River and Lake Ontario
Environ. Sci. Technol. 2000.34:3069-3078. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 01/10/19. For personal use only.
L A U R I E R P O I S S A N T , * ,† M A R C A M Y O T , ‡ MARTIN PILOTE,† AND DAVID LEAN§ Meteorological Service of Canada, Atmospheric Toxic Processes Section, Environment Canada, 100 Boulevard Alexis Nihon, Suite 300, Ville St. Laurent, Que´bec, Canada, H4M 2N8, INRS-EAU, Universite´ du Que´bec, 2800 Rue Einstein, C.P. 7500, Sainte-Foy, QC, G1V 4C7, Canada, and Department of Biology, University of Ottawa, P.O. Box 450 Station A, Ottawa, K1N 6N5, Canada
The paper presents an analysis of a set of data that includes both Hg profiles and modeled fluxes over water surface (using the two-layer model) and develops some insights regarding mercury water-air exchanges in Lake Ontario and the Upper St. Lawrence River (USLR). This paper presents the first in situ estimation of the water-air Hg gas exchange in Lake Ontario. It shows that waterair exchange fluxes over Lake Ontario and the Upper St. Lawrence River contributed to atmospheric mercury builtup over water bodies. TGM concentrations over Lake Ontario were significantly higher on the South shore than on the North shore, whereas the degree of saturation was higher on the North shore. Moreover TGM concentrations over water bodies were significantly higher than the reference station located on the ground surface (St. Anicet) indicated. TGM gradient measurements suggested that the South shore of Lake Ontario was mostly in evasion mode, whereas on the North shore and the Upper St. Lawrence River, both evasion and deposition were observed. However, Hg gas evasion should be far larger than gas deposition. Hg fluxes were calculated through the twolayer model. Model calculation indicated Hg supersaturation (476-2163%) and Hg fluxes between ∼0 and 9.28 ng/ m2/h (median 2.88 ng/m2/h). Hg gradients were weakly related to solar radiation, whereas the calculated Hg fluxes were not. Modeled Hg fluxes are related to Henry’s law and wind speed. It is suggested that the two-layer model gives the order of magnitude of the Hg flux but cannot express adequately the fine structure of Hg water-air fluxes. Further research is needed to elucidate the fine structure of the Hg water-air gas exchange flux. This study points out the complexity of Hg water-air gas exchange flux processes over large lakes or rivers and that Hg gas exchange is dynamic, changing in space and time.
atmosphere. Near-ground micrometeorological conditions impact the diurnal variation of total mercury in vertical profile (1) and its further transfer and transport in the global environment. Understanding the global budgets and cycles of atmospheric mercury requires an assessment of airsurface exchange processes and rates. Cycling of atmospheric mercury might proceed by gas exchange (2, 3), particle settling (4), or by rain scavenging (5, 6). Wet deposition and particle deposition are considered to be unidirectional processes, whereas Hg gas exchange is a bidirectional process (7). Wet and particle Hg deposition is mainly under oxidized form, i.e., the Hg(II) species which is relatively immobile. Chemical, photolytic, or biological reduction to elemental form can increase the mobility of mercury. Transport of mercury from water bodies to the atmosphere (volatilization) or atmospheric deposition are significant components for mercury budgets in lakes and rivers (8). Aquatic and marine studies have demonstrated that in situ synthesis of volatile Hg and its subsequent evasion at the water-air interface are major features of the global Hg cycle (9-11). Elemental mercury (Hg°) and dimethyl mercury are the most volatile mercury forms. In general, the dissolved gaseous mercury (DGM) fraction in lake water consists principally of Hg°, with no significant contribution from volatile organic Hg species (12). The dominant form of mercury in the atmosphere is gaseous elemental mercury (Hg°) (∼97%). In-lake biological and chemical processes for Hg° and methyl Hg compete for the reactive Hg. Hg(II) is methylated and accumulates in the food chain. Once Hg° is produced in the aqueous phase, it is unreactive, but when the level exceeds saturation it is lost from the system (12). Some recent results from Amyot et al. (13) suggest that Hg° oxidation in water might be significant. The large majority of aquatic ecosystems studied so far have been found to contain dissolved gaseous mercury at concentrations which are supersaturated relative to the equilibrium values predicted by Henry’s law. Evasion of elemental mercury was suggested to occur over ocean and from inland waters but was only measured directly in a few cases (3, 14-16). A few instances of net deposition were observed with flux chamber studies over inland waters (3, 14). Atmospheric input and evasion fluxes of mercury in Lake Ontario were estimated to be significant (17), but no direct measurement of mercury water-air gas exchange was measured previously. Poissant and Casimir (3) made the first extensive application of a real time-high time resolution technique for mercury fluxes applied over the water-air interface in Lake St. Francis (a fluvial lake of the St. Lawrence River). The purpose of this paper is to present an analysis of a set of data that includes both Hg profiles and modeled fluxes over a water surface (using the two-layer model) and to develop some insights regarding mercury water-air exchange in Lake Ontario and the Upper St. Lawrence River (USLR).
Introduction
Experimental Methods
Because of the high volatility of elemental mercury and some of its compounds, mercury is widely dispersed in the Earth’s
Sampling Strategy. Total gaseous mercury (TGM) and dissolved gaseous mercury (DGM) sampling in Lake Ontario and the USLR were performed aboard the Canadian Coast Guard Ship CCGS Limnos. The cruise was from July 27th to 31st, 1998. Figure 1 gives the cruise track. The Limnos carried on eastward along the 40 and 30 m South shore depth contours and stopped for stations before reaching the eastern
* Corresponding author phone: (514)283-1140; (fax) (514)2837149; e-mail:
[email protected]. † Environment Canada. ‡ Universite ´ du Que´bec. § University of Ottawa. 10.1021/es990719a CCC: $19.00 Published on Web 06/21/2000
2000 American Chemical Society
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FIGURE 1. Lake Ontario and St. Lawrence River track plot during the water-air exchange of toxic contaminants cruise on July 27-31, 1998. Also location of the surface back ground reference site in St. Anicet (Qc).
TABLE 1. Atmospheric Instrumentation Onboard the Limnos parameter TGM wind speed wind direction net solar radiation water temp air temp humidity atm pressure
height above the water surface (m) 2.7 and 4.2 8 8 7.5 water-air interface 2.7, 4.2, and 5.7 2.7, 4.2, and 5.7 8
end of the lake. On July 28th, the Limnos entered into the St. Lawrence River. The Limnos stopped at station 66 just upstream of Brockville (44°30′21" latitude North and 75°46′51" longitude West) for a 24 h period until early evening of July 29th. The Limnos returned to Lake Ontario and proceeded westward along the 30 m North shore depth contour for samples. TGM concentrations were also measured in parallel at a surface research station in St. Anicet (Que´bec) which reflects the continental background. The station is located about 3 km inland on the South shore of the St. Lawrence River between Cornwall (Ontario) and Montre´al (Que´bec) at 45°07′ latitude North and 74°17′ longitude West. Selected weather stations were also used for wind speed and wind direction records. Wind speed measurements from Toronto Islands and from aboard were used in the model application. Analytical Devices for TGM. The total gaseous mercury (TGM) analysis was achieved with an automatic analyzer (Tekran 2537A). In short, the analytical train of this instrument is based on amalgamation of mercury onto a pure gold surface followed by a thermodesorption and analysis by cold vapor atomic fluorescence spectrophotometry (CVAFS) (λ ) 253.7 nm) (18). Dual cartridge designs allowed alternate sampling and desorption, resulting in continuous measurements of mercury in the air stream. Particulate matter was removed by a 47 mm diameter Teflon filter (0.45 µm). The calibration 3070
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instruments
accuracy
Tekran 2737A R. M Young model 03102 R. M. Young model 03302 Q-7.1 net radiometer sensor (REBS product) CS model 107 HMP35 (CS) HMP35 (CS) CS105 barometric pressure sensor
South shore). The modulation of the gradient trends seemed to be environmentally related (with maximum values during midday period and minimum values during night-time for TGM and vapor pressure gradients whereas this was reversed for air temperature gradients). In some circumstances, flux/gradient may be an inappropriate framework for characterizing turbulent transport. That is, the sign of the gradient is not always indicative of the direction of the turbulence flux. A counter-gradient transport structure can arise due to the nonlocal nature of turbulent transport within the surface layer (38). This scenario was not very probable during the cruise since the synoptic atmospheric conditions across the domain did not show major atmospheric Hg advection (measured from aboard as well as over continental reference site (St. Anicet)). Poissant et al. (39) pointed out during an international intercomparison of micrometeorological techniques applied to measure mercury fluxes during the Nevada Study and Tests of the Release of Mercury from Soils (STORMS) in Reno, NV and using principal component analysis that Hg fluxes were more related to turbulence than Hg concentrations (and gradients) in the air masses (i.e., FHg ) K‚∆Hg). Hence, the Hg gradient would not necessarily be an indicator of the flux intensities but an indicator of its direction. Tables 3 and 4 give DGM concentrations in the epilimnion along the transect at selected sites. DGM concentrations were significantly higher North shore > South shore > USLR (details in ref 25). These DGM spatial profiles, especially
time
depth (m)
Hg(0) (pg/L)
SD
notes
20h05 21h20 12h00 2h00 4h00 6h00 8h00 10h00 10h00 10h00 12h00 12h00 12h00 14h00 14h00 14h00 14h00 16h00 18h00 20h00
2 2 2 2 2 2 2 1 2 3 1 2 3 2 4 8 12 2 2 2
30 31 34 34 37 50 35 35 32 34 31 32 28 34 31 31 31 31 32 29
12 1 5 1 3 0 0
heavy storm
no replicates no replicates no replicates no replicates no replicates no replicates no replicates no replicates no replicates no replicates no replicates no replicates no replicates
regarding the South shore and the North shore, were opposite to TGM concentrations and gradient profiles. The observed spatial and temporal patterns might be related to different environmental characteristics such as water temperature (Henry’s law constant-temperature dependency); solar radiation (photoreduction potential); wind speed (turbulence and volatilization); microbial processes; mercury sources; Hg speciation; mixing depth; and the chemical Hg competitors in water (e.g., DOC). To provide some insights into the factors which influenced water-air exchange, both concentrations, namely TGM and DGM, were expressed in terms of the degree of saturation for elemental mercury, expressed as
S ) [(CwH′)/Ca]100
(9)
where S > 100 indicates supersaturation. Figure 5 shows the degree of saturation over Lake Ontario and the USLR water bodies during the cruise. Supersaturation was between 476% to 2163%. The maximum value was measured along the North shore, whereas the lowest value was collected along the South shore (Table 2). Interestingly, a peak in the degree of saturation was recorded in the USLR between 4:00 and 8:00 in the morning on July 29th. The degree of saturation rose from 680% to 925% and went back to around 620% for subsequent periods and was concomitant with sunrise. During the night around 1:00 A.M., a storm impacted the site with heavy rain. This peak suggested that photoreduction would be involved in formation of DGM. Photoreduction was measured in the incubation experiments conducted in parallel (see ref 25). Photoreduction of Hg(II) to Hg° in rainwater has been pointed out to be very effective, especially for inorganic complexes (e.g., HgCl2) (40). It is possible that the surface water was enriched with rainwater. The rate of photoreduction was very fast > 2% per min. This rate was comparable to Amyot et al. (41) who reported a midday DGM production rate of 36.4 pg/h in surface water from Ranger Lake. Figure 6 shows the mass transfer coefficient (Kw) values calculated from eq 3. As discussed above the Kw values were temperature-corrected and wind speed-adjusted. The overall median Kw value was 8.38 cm/h. The Kw was minimum on the North shore and during the last part on the St. Lawrence River (∼0 cm/h) and maximum over the St. Lawrence River during the storm event (33.0 cm/h). Kw values were signifiVOL. 34, NO. 15, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. Degree of saturation in Lake Ontario and the Upper St. Lawrence River according to in situ TGM and DGM concentrations. The supersaturation was between 476% and 2163%. The maximum value was measured along the North shore of the Lake Ontario.
FIGURE 6. Mass transfer coefficient (Kw) over Lake Ontario and the Upper St. Lawrence River. Kw was estimated according to eq 3. Wind speed correlates the Kw values.
FIGURE 7. Hg air-water flux based on the two-layer model calculated during the cruise. The Hg flux time series correlates the Kw, whereas it is not for percent of saturation. cantly higher along the Lake Ontario South shore (1.5-19.3 cm/h) than the Lake Ontario North shore (∼0 to 2.8 cm/h). Figure 7 presents the Hg water-air fluxes in Lake Ontario and the USLR based on the two-layer model (eq 1). The overall median calculated flux during the cruise was 2.88 ng/m2/h. The minimum Hg flux value was ∼0 ng/m2/h, and the maximum Hg flux value was 9.28 ng/m2/h. Hg flux values were significantly higher along the Lake Ontario South shore (0.91-9.07 ng/m2/h) than the Lake Ontario North shore (∼0 to 2.79 ng/m2/h). Hg fluxes were between 0.02 and 9.28 ng/ m2/h over the St. Lawrence River location. It is well demonstrated from Figures 5 to 7 that Hg fluxes under supersaturation conditions were driven (modulated) by Kw and not by the degree of saturation. This finding is very important since most of the previous literature applications of the two-layer model assigned the degree of saturation as the main modulating terms of the Hg fluxes by applying a constant Kw value in eq 1 (12, 13). Moreover, the results suggested that the degree of saturation might be partly modulated by Kw since both are 3076
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FIGURE 8. Degree of Hg saturation might be modulated by Kw. Kw is the transfer coefficient within the water layer which offer the larger resistance to Hg water-gas exchange. negatively correlated through a power law (R 2 ) 0.55 (F < 0.001)) (Figure 8). The liquid layer acting as a barrier for Hg evasion may explain the higher degree of saturation where lower Hg mass transfer coefficients were observed. However, other explanations for the higher degree of saturation along the Lake Ontario North shore might be related to larger Hg
which are supersaturated relative to the equilibrium values predicted by Henry’s law (7). Is the evaluation of mercury supersaturation accurate? (4) The supersaturation was reported to be between 476% to 2163% (based on 1-2 m water depth sampling for DGM concentrations). Obviously the water cannot be supersaturated if deposition of elemental mercury occurs. Hence, are the 1-2 m water depth DGM concentrations representative of the liquid film DGM concentrations? (DGM concentration in the liquid film should be lower to permit Hg° deposition.) FIGURE 9. Correlation between median TGM gradient and NSR. Results measured over the USLR.
Further research is needed to elucidate the fine structure of the Hg water-air gas exchange flux. This study points out the complexity of Hg water-air gas exchange flux processes over a large lake or river and that Hg gas exchange is dynamic, changing over space and time.
Acknowledgments
FIGURE 10. Correlation between median Hg° flux and NSR. Results measured over the USLR. sources (e.g., nuclear power plant), Hg° production, or deeper mixing depth on the South shore (∼20 m South shore vs 5 m North shore). However, DGM concentrations at 20 m depth are three times greater on the North shore than on the South shore (25). The latter suggests a mitigated effect due to mixing depth. To focus on specific aspects and determine the processes involved, results measured over the USLR during the intensive measurements (July 28th-29th) were further used. Figure 9 shows the correlation between the NSR and the TGM gradient (R 2 ∼ 0.1), and Figure 10 presents the correlation between NSR and Hg fluxes obtained through the two-layer model calculation (eq 1) (R 2 ∼ 0). It appears that the TGM gradients are better related to the solar radiation than to the Hg flux from eq 1. Although Hg fluxes estimated from the two-layer model do not express any deposition, the TGM gradient suggests that such mechanisms appeared during the measurements, especially during night-time. Poissant and Casimir (3) using a dynamic flux chamber found a diel mercury pattern with deposition as well as evasion over Lake St. Francis (a fluvial lake of the St. Lawrence River close to St. Anicet). The magnitude was respectively -0.5 to 1.0 ng/m2/h, and the pattern was related to solar radiation and wind speed. The median Hg flux estimated over the USLR was 2.98 ng/m2/h (this study). Hence, since reversed gradients as well as wind speed were low and observed during night-time, it is assumed that Hg deposition was low. From this analysis of a set of data that included both Hg profile and modeled flux over the water surface (using the two-layer model) some comments may be brought forward regarding the contradiction between the two applications: (1) The two-layer model might express the order of magnitude of Hg flux but cannot adequately reveal the fine structure of the fluxes, whereas the micrometeorological approach (in situ) is more sensitive and better expresses the integrated factors in the TGM water surface-air exchange flux. (2) Some redox process (e.g., oxidation of elemental mercury in the aqueous phase by ozone within the surface layer) might be important in water-surface gas exchange as suggested by Poissant and Casimir (3). (3) It is curious that the large majority of aquatic ecosystems studied so far have been found to contain dissolved gaseous mercury at concentrations
The cruise was supported by NSERC Ship Grant to D.L., and the authors would like to thank the Technical Operations Group and the crew on the Limnos. L.P. would like to thank SLAP 2000 for funding. Part of this research was supported by an NSERC research grant and an NSERC Industrial Chair grant to D.L. and M.A. INRS grant.
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Received for review June 27, 1999. Revised manuscript received April 10, 2000. Accepted April 11, 2000. ES990719A
