Chapter 12
Speciation and Distribution of Atmospheric Mercury over the Northern Chesapeake Bay Guey-Rong Sheu, Robert P. Mason, and Nicole M. Lawson Chesapeake Biological Laboratory, Center for Environmental Science, University of Maryland, P.O. Box 38, Solomons, MD 20688-0038
Baltimore's urban air is a potentially important source of mercury (Hg) to the northern Chesapeake Bay. Elevated total atmospheric H g (THg) concentrations were detected at a sampling site in downtown Baltimore (4.4±2.7 ng/m ), as compared to a rural site (1.7±0.5 ng/m ). The urban air was also enriched with reactive gaseous Hg (RGHg) and particulate-bound Hg (Hg-P). The annual dry depositional fluxes of R G H g and Hg-P at sites around the northern Chesapeake Bay have been determined, with the fluxes of R G H g ranging from 7 to 121 μg/m and the fluxes of Hg-P from 1 to 34 μg/m . These values were the same magnitude as the wet depositional fluxes of Hg measured at the same sites. Local wind direction influenced the concentration of atmospheric Hg detected at the urban sampling site. When air came from the SE, S and S W directions, the urban sampling site tended to be impacted by the local emission sources, with higher THg and Hg-P concentrations detected. 3
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© 2002 American Chemical Society
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224 More than 40 states in the U S A have issued advisories regarding the consumption of fish containing elevated levels of Hg, which is mostly as the highly toxic monomethylmercury ( M M H g ; 1-2). Recent studies have demonstrated that the atmosphere is an important source of Hg to many surface waters and terrestrial environments (3-8), especially in areas remote from the point sources. According to Mason et al.'s estimate, for example, the combination of wet and dry particulate atmospheric depositions currently make up 90% of the total Hg input to the open ocean (9). However, typically less than 1% of the total atmospheric Hg deposition is M M H g (6, 10-12). Simple mass balance calculations have demonstrated that the amount of deposited M M H g is not enough to support the M M H g levels found in aquatic systems and in situ production is therefore essential (9-10). Although the wet deposition is undoubtedly an important pathway of atmospheric pollutants to surface waters (6, 13-14), recent studies also suggest the importance of dry deposition (13-14). The intensity of dry deposition is directly related to the concentration and speciation of pollutants in the atmosphere. Thus, monitoring the concentration and speciation of atmospheric Hg and understanding its transformation and transport are imperative to the study of H g biogeochemistry and the development of controlling and remediation strategies. Three types of H g are identified in the atmosphere based on their physical and chemical properties: elemental gaseous H g (Hg°), reactive gaseous Hg (RGHg, divalent forms), and particulate-bound H g (Hg-P). The speciation of R G H g is currently unknown, but it is usually assumed to be gaseous H g C l or other mercuric halides, which have a higher surface reactivity and water solubility than gaseous Hg°. Unlike other metal pollutants that tend to exist in the particulate phase in the atmosphere, Hg exists mainly in the gaseous phase because of its anomalous physicochemical properties. For example, elemental Hg has a relatively high vapor pressure of 0.246 Pa at 25°C (15) and a low Henry's law constant of 0.11 M/atm at 20°C (16). This low Henry's law constant, which means low water solubility, makes the removal of gaseous Hg° by rainwater less efficient compared to the more water-soluble R G H g and to the scavenging of Hg-P. Both R G H g and Hg-P are effectively removed from the atmosphere by wet and dry deposition with an atmospheric residence time from a few minutes to weeks. On the other hand, Hg° is more stable in the atmosphere with a residence time between 6 months and 2 years (17-18). Consequently, gaseous Hg° is the dominant form of Hg in the atmosphere with only a few percent of the total present as R G H g and Hg-P (19-21). Nevertheless, air could be enriched with R G H g and Hg-P in the close proximity of point sources. Although Hg° is the dominant form in the atmosphere, R G H g may still be a significant component of the total depositional fluxes. However, due to the poor understanding of its speciation, its usually low concentration in the atmosphere, 2
225 and the lack of proper sampling techniques, the importance of R G H g to Hg biogeochemistry has been neglected until recently. This situation has been improved with several R G H g sampling techniques being developed in the past few years, including a multi-stage filter pack method (22), a refluxing mist chamber method (23-26), and a KCl-coated annular dénuder method (27). A n examination of these methods has been published elsewhere (21). These sampling techniques greatly enhance our ability to evaluate the significance of R G H g to the local and regional H g biogeochemical cycles. Urban air usually contains elevated concentrations of trace elements and pollutants, which could enhance depositional fluxes to nearby great waters, such as the Chicago metropolitan area to Lake Michigan (28) and the Baltimore metropolitan area to Chesapeake Bay (29-30). The Chesapeake Bay is the largest estuary in the United States and the Baltimore metropolitan area, one of the heavily industrialized areas on the east coast, is located on the western shore of the Chesapeake Bay. The Chesapeake Bay has a large surface-to-volume ratio (mean depth is 7 m), and it is therefore particularly vulnerable to the influence of atmospheric deposition (75). Previous studies have demonstrated that Baltimore's urban atmosphere is an important source of P A H s and PCBs to the Chesapeake Bay (29-30). A mass balance for Hg in the Chesapeake Bay also showed that atmospheric deposition is the major source of H g with an annual flux of 1300 moles (3, 6). This number was derived mainly from the wet and dry particulate deposition data collected in Maryland. The annual wet depositional flux measured in 1997/98 at a downtown Baltimore site was higher than those measured at rural sites, by a factor of 2-3 (31), which demonstrated the importance of urban areas as Hg sources through wet deposition to the adjacent waters. However, dry deposition of atmospheric R G H g was not fully considered in this mass balance calculation and the influence of urban air on the dry depositional flux to the Chesapeake Bay was not discussed. Hence, the primary objectives of this study were to characterize the speciation and distribution of atmospheric H g in rural and urban sites around the northern Chesapeake Bay and to estimate the associated dry depositional fluxes. The significance of R G H g to the air-surface exchange of Hg was of special concern since its role usually has been neglected in the early mass balance studies. The impact of Baltimore's urban air on the dry depositional flux of H g to the Chesapeake Bay was also evaluated.
Sampling Sites and Methods Both intensive and long-term atmospheric H g sampling was conducted in the city and in the northern Chesapeake Bay downwind of the Baltimore metropolitan area. A i r was sampled aboard the E P A OSV Anderson (SHIP)
226 over the northern Chesapeake Bay and at four land-based sites (Fig. 1): the first site was on the roof of the Maryland Science Center (SC) at downtown Baltimore next to the Inner Harbor, the second site was 9 km south-southeast of SC at the U S C G Station Curtis Creek (CC), the third was 17 km east of SC by Hart-Miller Island (HMI), and the fourth was about 42 km east-northeast of SC at U S C G Station Stillpond (STP) on the Eastern Shore of Maryland. Sampling started in February 1997. Two intensive sampling events were conducted aboard the ship over the bay and at all the land sites in February (18th-26th) and July (22nd-31st) 1997 with 24-hr samples being collected daily. Samples were also collected every 9 days at SC (stopped on 12/21/1998) and at STP (stopped on 03/12/1998). Usually the sampling time was 24-h at all the sites except aboard the ship over the bay, where weather and the operation of the ship restricted the air sampling. A total of 70 and 50 samples were collected at SC and STP, respectively. Sixteen samples were collected at all sites during the two intensive sampling events. Total atmospheric Hg (THg), R G H g , and Hg-P were all measured. Total atmospheric H g was sampled by pumping air through 3 gold-coated quartz sand traps (32) and quantified by cold vapor fluorescence spectrometry ( C V A F S ; 33). As the gold traps retained R G H g and Hg-P in addition to Hg°, since they were not pre-removed before entering the traps, the T H g represents the sum of all the atmospheric H g species. Some of the T H g data were less than 1 ng/m , which is the lower limit of the background value for this latitude (18, 34). This could be due to power failure of the pump or to a gas leak from the sampling tubing and hence these low T H g data were excluded from further analysis. 3
Both R G H g and Hg-P were sampled using the filter pack method and the details of this method has been described elsewhere (21-22). Acid-cleaned open face 5-stage Teflon filter holders were used. Two 47 mm 0.45 pm Teflon filters and three 47 mm cation exchange membranes were housed in the Teflon filter holder. The first Teflon filter removed the particles while the second filter acted as a backup (or blank). Similarly, the first ion exchange membrane collected the R G H g sample with the second membrane acting as a backup. The third membrane was to prevent backward diffusion of R G H g from the pump. Incoming air first flowed through the Teflon filters to remove particles prior to the ion exchange membranes that were positioned behind the Teflon filters. Filter samples were first oxidized by 0.2 Ν B r C l solution for at least 30 minutes, then pre-reduced using hydroxylamine hydrochloride, prior to quantification by SnCl reduction-CVAFS (6). The analytical detection limits (3 times the S.D. of blank filters divided by the average air volume sampled in 24-hr) for R G H g and Hg-P were 7 pg/m and 9 pg/m , respectively. 2
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Local wind directions were recorded every 10 minutes at the SC site by our collaborators in 1997. This information enabled us to explore how the local wind direction influenced the atmospheric H g measured at sites near potential emission sources, such as the SC sampling site.
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Figure 1. Map of the sampling sites.
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Results and Discussion A l l the atmospheric Hg data collected for this study are summarized in Table I. Atmospheric Hg values reported by other groups are also included for comparison. The atmospheric H g data measured at C C and SC were similar to those measured at other urban or industrial areas. A t a certain site, both R G H g and Hg-P exhibited higher temporal variability compared to T H g , as indicated by the ratio between the standard deviation and the mean value. Detailed analyses of our data are presented in the following sections.
Table I. Summary of O u r Data and Published Values Sampling Sites
THg (ng/m ) 6.1±1.9 3.612.3 2.210.7 2.210.8 1.710.4 4.412.7 1.710.5 1.4-23.2 0.7-23,6 2.7-12.2 1.9-2.4 3.3-4.7 1.2-1.8 1.8-3.3 3
C C (intensive)* SC (intensive)* H M I (intensive)* SHIP (intensive)* STP (intensive)* SC (all data)* STP (all data)* Seoul, Korea Bordeaux, France Guizhou, China Tennessee Indiana Northern Wisconsin Florida Detroit, Michigan Rural Michigan Toronto, Canada Urban U . K . a.
RGHg (pg/m ) 3851495 1201200 22119 21126 21115 891150 24122 3
Hg-P (pg/m ) 71511285 1421392 44164 25122 24129 741197 42150
References
3
150-1460 50-257 83-156 6-63 34-51 94 10-22 3-91 100 pg/m ) were associated with wind from Ν and N E . These rare high R G H g events suggest there are unidentified R G H g sources located on the Ε and N E of the SC site, or may reflect in situ production of R G H g under specific environmental conditions, which are still not well understood. 3
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SE-S-SW
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Wind Direction Figure 7. Statistical distribution of atmospheric Hg concentrations based on wind directions.
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Conclusions Our 1997/98 study in the northern Chesapeake Bay region has demonstrated that Baltimore's urban air is an important source of H g to the Chesapeake Bay. A i r sampling was conducted at four land-based sites (CC, H M I , SC and STP) and aboard a research vessel over the surface water (SHIP). The C C , H M I and SC sites are on the western shore of the bay: SC is in downtown Baltimore, C C is in an industrial area SE of the city, and H M I is on the east of the city. Some potential stationary sources of Hg, including power plants and waste incinerators, are located on the SW-S-SE region of the city. The STP site is located in the rural area of the Eastern Shore, which is downwind of the Baltimore metropolitan area. The intensive sampling data showed higher atmospheric H g concentrations at C C and SC (Table I). These values are similar to those reported for other urban/industrial areas. Intermediate values of atmospheric H g were detected at H M I and SHIP, while the lowest values were always found at STP (Table I). As the urban air moved eastward across the Chesapeake Bay, both R G H g and Hg-P were depleted faster than THg, suggesting mechanisms other than mixing and dilution were acting on these two species. Dry deposition to surface water is probably the main mechanism responsible for this quick depletion of R G H g and Hg-P. The composition of atmospheric Hg also changed dramatically as the air moved eastward, with a high fraction of THg as R G H g and Hg-P at C C and SC (Table II). The changing speciation supports the notion of the removal of R G H g and Hg-P by dry deposition during transport. The annual dry depositional fluxes of R G H g and Hg-P were estimated using the intensive sampling data (Table III). The dry depositional flux was greater than the wet flux at SC and was about half of the wet flux at STP. Therefore, it is clear that the dry deposition is an important pathway for H g to enter the Chesapeake Bay, with higher fluxes associated with urban air. Long-term data were collected at SC and STP. The STP data did not show significant differences in atmospheric Hg concentration and speciation between 1997 and 1998 (Figure 3). This suggests that the STP site may represent the regional background signal and thus is not showing much annual variation of atmospheric Hg. No evident seasonal pattern of THg and R G H g were observed at both sites (Figures 4-5); however, a distinct seasonal pattern was observed for Hg-P, with higher ambient Hg-P in colder months (Figure 6). This may be due to the higher anthropogenic emission intensity of Hg-P in cold seasons or enhanced Hg° deposition onto ambient aerosols at low temperatures. Concentrations of atmospheric Hg measured at SC were influenced by local wind directions. With potential Hg sources on the SW-S-SE directions of the site, elevated T H g and Hg-P were detected with air originating from these directions (Table V ) . However, air that came from the same directions
240 contained only about 50% of the RGHg, on average, as measured in air from other directions (Table V ) . This was due to few high R G H g concentrations measured with air from the Ε and N E directions. The exact causes responsible for these irregular measurements are not clear, however. In summary, atmospheric Hg showed great spatial and temporal variability around the northern Chesapeake Bay region, with elevated H g values usually measured in Baltimore's urban air. This urban plume is an important source of Hg to the Chesapeake Bay through wet and dry deposition. The dry deposition of R G H g is of special concern since it was usually neglected in the early mass balance calculations. High R G H g values were detected occasionally at SC with air coming from the directions where no sources have been identified. This could be due to unknown sources or the in situ production in the air under specific environmental conditions, which is still not well understood and needs further investigation.
Acknowledgements We thank Joe Steinbacher and Bernie Crimmins and others from C B L for help with sample collection. We also thank the captain and crew of the OSV Anderson for their help. We also thank Dr. Ondov of University of Maryland for providing the local wind directions. This study was funded by an E P A S T A R grant. This is Contribution No. 3464, Chesapeake Biological Laboratory, Center for Environmental Science, University of Maryland.
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