Environ. Sci. Technol. 2007, 41, 8018–8023
Variability of the Gaseous Elemental Mercury Sea–Air Flux of the Baltic Sea JOACHIM KUSS* AND BERND SCHNEIDER Department of Marine Chemistry, Leibniz Institute for Baltic Sea Research (IOW), Seestrasse 15, D-18119 Rostock-Warnemünde, Germany
Received July 03, 2007. Revised manuscript received September 07, 2007. Accepted September 12, 2007.
The importance of the sea as a sink for atmospheric mercury has been established quantitatively through models based on wet and dry deposition data, but little is known about the release of mercury from sea areas. The concentration of elemental mercury (Hg0) in sea surface water and in the marine atmosphere of the Baltic Sea was measured at high spatial resolution in February, April, July, and November 2006. Windspeed records and the gas-exchange transfer velocity were then used to calculate Hg0 sea–air fluxes on the basis of Hg0 sea–air concentration differences. Our results show that the spatial resolution of the surface water Hg0 data can be significantly improved by continuous measurements of Hg0 in air equilibrated with water instead of quantitative extraction of Hg0 from seawater samples. A spatial and highly seasonal variability of the Hg0 sea–air flux was thus determined. In winter, the flux was low and changed in direction. In summer, a strong emission flux of up to 150 ng m-2 day-1 in the central Baltic Sea was recorded. The total emission of Hg0 from the studied area (235000 km2) was 4300 ( 1600 kg in 2006 and exceeded deposition estimates.
Introduction Between 1972 and 1977, several studies confirmed the hypothesis that air pollutants could travel several thousands of kilometers before deposition and possible harm occurs (1). One of the eight priority elements for monitoring longrange transboundary air pollution is mercury (2) in its elemental form (Hg0). The slow removal of mercury from the atmosphere by the reaction of Hg0 with ozone, a hydroxyl radical, or a bromine radical (3–5) and subsequent particle scavenging leads to its dry and wet deposition (3, 4, 6). Thus, mercury is transported directly into the sea, or it may reach the sea by drainage from rivers. A pioneering study in the Equatorial Pacific (7) supported model predictions of an oceanic source of atmospheric mercury. A subsequent study of the Equatorial Pacific surface waters confirmed enhanced concentrations of volatile Hg0 in cold upwelled water highly abundant in chlorophyll a (8). Overall, only a few studies aimed at determining Hg0 concentrations in sea surface water have been conducted. Measurements have been carried out in the Atlantic Ocean (9–12), in the Pacific Ocean (8, 13), in the Baltic Sea (14), in the North Sea (15), and in the Mediterranean Sea (16). It has been shown that the transformation of reactive mercury to volatile Hg0 is caused by * Corresponding author phone: +49-(0)381-5197-314; fax: +49(0)381-5197-302; e-mail:
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direct photon-induced reactions (17, 18) and by biotic processes (19). A recent modeling effort aimed at improving estimates of oceanic Hg0 emission (20) appears promising because it accounts for the assumed control mechanisms involved in the surface-water transformation of mercury to Hg0. However, that study also made clear that more measurements are necessary to validate the model’s results. In the present study, we used an instrumental setup for the alternate determination of Hg0 in air equilibrated with continuously supplied seawater (Hg0equ) and in the atmosphere (Hg0atm) in order to determine sea–air flux. The hourly measured Hg0equ can additionally be related to the in situ Hg0 concentration (Hg0wat), as described by Henry’s law, and thus allows comparison with literature data. The atmospheric value (Hg0atm) was determined during equilibration times between Hg0equ measurements. The purpose of this study was to better quantify the Hg0 sea–air flux of the Baltic Sea. Specifically, the aims were (i) to determine the spatial and seasonal variability of Hg0 concentrations in surface waters and the atmosphere by carrying out high-resolution measurements, (ii) to discuss Hg0equ variability in terms of seasonal temperature changes and biotic and abiotic Hg0 contributions, (iii) to calculate the Hg0 sea–air fluxes of the Baltic Sea during the annual cycle using the new data set, and finally (iv) to use the improved flux estimates to obtain a Hg0 sea–air flux budget, thereby enabling comparison with anthropogenic Hg emissions and atmospheric depositions in Northern Europe.
Experimental Section Sampling Location and Time. Measurements were carried out in the Baltic Sea, a marginal sea of the North Atlantic. It is the largest brackish water body in the world, with a total surface area of 415300 km2. Its drainage area is 4 times larger than the area of the sea itself. The annual freshwater surplus in surface water generally flows from the Gotland Sea in the northeast via the Bornholm Sea and the Arkona Sea to the Belt Sea in the southwest (21). During four research expeditions, Hg0 in air and water and other selected parameters were measured. The first cruise, with the R/V Maria S. Merian, was conducted from the 16th to the 26th of February 2006. Two cruises with the R/V Alkor followed, the first from the 4th to the 10th of April 2006 and the second from the 3rd to the 10th of July 2006. The fourth cruise was carried out with the R/V Gauss, from the 7th to the 17th of November 2006. Measurements were made in the Belt Sea, Arkona Sea, Bornholm Sea, and the western and eastern Gotland Sea. Hg0 Measurements. Hg0 was determined by an autonomous mercury analyzer (Tekran 2537A), which was calibrated daily. This instrument also detects other volatile Hg species, such as dimethylmercury, but their contributions to total gaseous Hg is small (12, 22) and no differentiation between these volatile Hg speciations was attempted. The operating principle in water measurement is based on the equilibration of Hg0 from a seawater sample with the headspace air that is subsequently measured. This was done using an equilibrator that consisted of a 20-L glass bottle with a shower head and a temperature probe (Figure 1). To relate the Hg0 concentration in equilibrated air (Hg0equ) to the Hg0 concentration in water (Hg0wat), we used the dimensionless Henry’s law constant (H ) Hg0equ/Hg0wat), as given by Sanemasa (23) and recently confirmed (24). There have been previous attempts to improve the spatial resolution of Hg0 measurements. Continuous, high-time-resolution, quantitative Hg0 extraction procedures have been used in freshwater research (25, 26). Also, an equilibrator based on the same 10.1021/es0716251 CCC: $37.00
2007 American Chemical Society
Published on Web 10/26/2007
FIGURE 1. Schematic of the equilibrator coupled to the mercury vapor analyzer. Headspace gas is supplied slightly in excess at rising water level, which was checked occasionally by the bubbling device. After measurement, the water is extruded by pumped clean air. Ship data and temperature recordings were synchronized with mercury measurements by using computer software. principle as that in our study was used in an earlier study (27), but it is important to note that we took additional precautions to minimize contamination: air and water samples were in contact with borosilicate glass or with fluorine polymer only. All components were cleaned with a warm detergent solution (Mucasol, Merz Hygiene Co., Frankfurt, Germany) overnight and then thoroughly rinsed, first with warm tap water and afterward with pure water (Milli-Q system, Millipore Co., Schwalbach, Germany). Furthermore, a supply of air filtered with charcoal (in the following termed “clean air”) was connected to the equilibrator to check for contamination and to facilitate its operation. To obtain the transects, seawater was pumped from a depth of about 3 m by the clean-seawater supply systems of the research vessels and then continuously dispersed into the headspace of the equilibrator (Figure 1). Comparison measurements with a dedicated clean-water supply system (gear pump with a polytetrafluoroethylene (PTFE)/stainless steel pump head and PTFE tubing) confirmed that the ship’s water supply systems were suitable for the analyses, without contamination of any of the research vessels. After 1 h of equilibration, the drain was closed and the rising water level supplied equilibrated air to the mercury analyzer without dilution or contamination by ambient air. The mercury analyzer was programmed for sampling at 5-min intervals, resulting in two consecutive Hg0equ determinations for each equilibration. After the Hg0equ measurements were completed, the mercury analyzer continued with atmospheric measurements by sampling air from an upper deck of the research vessel. Clean air was then pumped into the equilibrator, thus extruding the water in about 5 min. A complete analytical cycle took 75 min. To determine the time necessary for an equilibration between water and air, systematic experiments were performed. Seawater from the same water mass was used for a series of Hg0 measurements, but the equilibration procedure was interrupted after different time spans (5 min to 5 h). The corresponding Hg0 concentration in the headspace reached at a stable level well within 1 h. Hg0equ was corrected for the temperature deviation between in situ values and those of the equilibrator (usually around 1 °C) using the temperature dependence of Henry’s law constant. The values of the replicate measurements agreed well with each other, with 20 °C) in summer.
Acknowledgments We thank H. Kubsch (IOW) for preparing the expedition equipment and for participating in a field expedition, C. Temme and H. H. Kock (GKSS) for their advice concerning gaseous mercury measurement, K. Nagel and K. Bohn (IOW) 8022
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Detailed data of the calculated mercury emissions of the Baltic Sea basins (Table S1), the map of the study area showing cruise tracks and Baltic Sea basins (Figure S1), and the equilibration curve showing Hg0equ measurements versus time of equilibration (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.
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