2003 Progress Report: Natural and Anthropogenic Sources of Mercury to the Atmosphere: Global and Regional Contributions

EPA Grant Number: R829796
Title: Natural and Anthropogenic Sources of Mercury to the Atmosphere: Global and Regional Contributions
Investigators: Fitzgerald, William F. , Engstrom, Daniel
Institution: University of Connecticut , Science Museum of Minnesota
EPA Project Officer: Chung, Serena
Project Period: January 1, 2003 through December 31, 2005
Project Period Covered by this Report: January 1, 2003 through December 31, 2004
Project Amount: $897,219
RFA: Mercury: Transport, Transportation, and Fate in the Atmosphere (2001) RFA Text |  Recipients Lists
Research Category: Mercury , Air Quality and Air Toxics , Safer Chemicals , Air


The importance of the atmosphere in transporting mercury (Hg) intra- and interhemispherically (elemental Hg [Hg0] is the major species transported) has been established, and knowledge of the behavior and fate of Hg in the atmosphere is increasing. However, the relative contribution of natural and anthropogenic sources is uncertain, the mechanisms by which Hg is removed from the atmosphere are poorly understood, and the processes and reactions linking inputs of anthropogenic Hg, especially from the atmosphere, and the bioaccumulation of methylmercury in sensitive aquatic ecosystems have not yet been established.

This project addresses questions relating to natural and anthropogenic contributions from global and localized sources, the identification of Hg deposition with a regional origin (e.g., United States), and the examination of spatial and temporal trends (e.g., increases, declines) in atmospheric Hg deposition for predictive/modeling purposes. The objectives of this research project are to address the following hypotheses: (1) atmospheric Hg deposition in southeastern Alaska can be viewed as an integrated sample of global Hg pollution in the Northern Hemisphere, and it provides a refined estimate for the hemispheric component of Hg deposition and a baseline for sites closer to local and regional emission sources; (2) atmospheric Hg deposition in Canada’s maritime provinces is elevated above the northern hemispheric average by regional contributions from the industrialized Northeast/Midwest United States and Southeast Canada, and can be separated into global and regional components by a comparison of sedimentary archives; (3) the linear correlation between Hg and 210Pb found in rainwater from other remote and semiremote locations is observed in southeastern Alaska, and this behavior can be used to constrain the global-scale wet atmospheric flux of Hg to lakes and watersheds of temperate North America; (4) at less remote sites, enhanced atmospheric Hg deposition that is locally/regionally derived is indicated by deviations from the Hg and 210Pb relationship observed in southeastern Alaska; (5) sediment archives will show, when corrected for climatology using 210Pb, that the Pacific and Atlantic seaboards of North America received equivalent preindustrial atmospheric Hg fluxes and provide a baseline for assessing the global component of anthropogenic Hg deposition at any given locality. This research will yield the high-quality biogeochemical data needed for quantitative assessment of the scale and historical record of potentially enhanced atmospheric Hg deposition related to increased human-related Hg emissions over the past 150 years. This information will be especially useful in improving models of the global and regional biogeochemical and atmospheric cycling of Hg, and in assessing the impact associated with atmospherically transported pollutant-derived Hg in the environment.

Progress Summary:

Experimental Design

This research is focusing on the modern and historical variation in atmospheric Hg fluxes associated with the mid- and subtropical latitudes of North America. We are combining two research strategies: (1) archives provided by lake sediment cores; and (2) collocated Hg and 210Pb deposition collectors to derive precise estimates of the modern Hg flux to eastern and western North America.

Lake Sediment Archives. Contemporary and historical information provided by careful measurements and scrupulous examination of natural archives (Electrical Power Research Institute, 1996) has been critically important to enhancing knowledge, understanding, environmental perspective, assessment of anthropogenic interferences, and modeling of major biogeochemical cycles. Lake sediments, peat bogs, and ice cores have been used successfully for regional and global studies of modern and historical Hg atmospheric depositional patterns, and we have much experience with this scientific approach (e.g., Swain, et al., 1992; Fitzgerald, et al., 1998; Engstrom and Swain, 1997; Engstrom, et al., 1994; Benoit, et al., 1998; Lamborg, et al., 2002). Lake sediments are especially valuable because they occur over broad geographic regions. These natural archives are particularly well suited to examine the global/regional nature of atmospheric Hg dispersion and deposition, and to assess the impact of human-related Hg emissions on the natural Hg cycle as recorded in accumulating lakes sediment.

We are evaluating recent changes in atmospheric Hg deposition for two regions of North America over which different Hg emission sources should dominate. Trends in atmospheric deposition will be inferred from Hg accumulation rates in 210Pb-dated sediment cores. Our plan is to use the archives provided by lake sediments to make an assessment of the extent to which atmospherically transported Hg has impacted southeastern Alaska, a region remote from localized emission sources (hypothesis 1. The magnitude of increase in Hg accumulation in these sedimentary records will provide a global reference against which changes in Hg deposition from other regions may be compared. There are no significant anthropogenic Hg sources anywhere in coastal southeastern Alaska, and there is limited human development within several hundred kilometers of lakes in the region. Historical records from lake sediments and peat indicate that atmospheric deposition of Hg has increased two to four times in the Northern Hemisphere since industrialization (i.e., Swain, et al., 1992; Benoit, et al., 1998; Fitzgerald, et al., 1998). However, our preliminary data from Glacier Bay National Park suggest that the increase in southeastern Alaska is at the lower end of this range (ca. 1.8-2.1 times; Engstrom and Swain, 1997). We will use lake sediment cores collected in Newfoundland to assess the increase in atmospheric Hg deposition experienced by a region "downwind" of the relatively industrialized eastern North America (hypothesis 2). The magnitude of change in these sedimentary archives should be greater than that measured in southeastern Alaska, with the difference attributable to regional emission sources affecting the Atlantic coast. Our previous efforts indicate that in Nova Scotia, and by extension in Newfoundland, as much as 40 percent could be due to regional source and/or enhanced oxidation phenomena (Lamborg, et al., 2002). Cores from Nova Scotia lakes suggest that present day Hg deposition in maritime Canada is elevated over preindustrial rates by a factor of 4.3-5.4 times (Lamborg, et al., 2002).

Southeastern Alaska. Sediment cores were collected from four carefully chosen study lakes near Point Adolphus on Chichagof Island in Tongass National Forest, southeastern Alaska. Six cores were collected from widely spaced locations in the deeper regions of each lake by means of a HTH-Teknik gravity corer equipped with 7 cm polycarbonate core tubes. Collected core lengths range from 26 to 40 cm. Lake work was conducted from a small Zodiac inflatable boat powered by an electric trolling motor. The selected lakes occur in a relatively tight cluster and are located within 25 km of our atmospheric Hg collectors at Bartlett Cove, Glacier Bay National Park headquarters. The lakes are located at low elevation (230-335 m above sea level) in the coastal foothills of the Chichagof central massif. They are small in surface area (5-10 ha) and possess a simple (single basin) flat-bottom bathymetry; maximum depths range from 6.7 to 15.3 m. Based on topographic maps and field observations, the lakes appear to have relatively small watersheds. Local vegetation is comprised of undisturbed (pristine) coastal rainforest (Sitka spruce, western hemlock) and open muskeg (peatlands) on more gently sloping surfaces. Geographic information system (GIS) analyses, currently in progress, will determine precise surface areas, bathymetry, and catchment characteristics.

Cores were sectioned into 1 to 2 cm increments in Juneau, and later shipped to the St. Croix Watershed Research Station laboratories (MN) for processing and 210Pb dating. All core intervals were analyzed for water content, bulk density, and loss-on-ignition (organic content) and then freeze dried. 210Pb dating of all 24 cores has been in progress since fall of 2003, and it is now nearly completed (20 of 24 cores are done). Selected samples (based on the 210Pb results) from roughly one-half of the cores have been sent to the University of Connecticut for Hg analysis. Pb-210 dating parameters show a remarkable uniformity in sedimentation rates, 210Pb fluxes, and other core characteristics within and among lakes. Most importantly, virtually every core shows a near-constant sediment accumulation rate, which should make for straightforward interpretation of Hg accumulation trends. Overall, sedimentation rates are quite slow, ranging from 0.0027 to 0.0062 g cm-2 yr-1, and the depth at which unsupported 210Pb becomes undetectable (dating horizon) is between 6 and 12 cm. The extrapolated basal ages of the cores range from 800 to 1,600 years BP (before present). Despite the slow accumulation rate, the sample resolution in the upper part of all of the cores is sufficient to characterize changes in Hg flux on the order of a decade or less.

Current Hg and 210Pb Deposition

The accuracy of contemporary and historic reconstructions of atmospheric depositional patterns for Hg is aided greatly by incorporating the constraint of real time, independent, and collocated deposition determinations into the experimental design. In the current research, we are measuring Hg deposition at Glacier Bay, and we make use of data from relevant Mercury Deposition Network (MDN) Stations. The modern estimated atmospheric Hg flux using archival data from lake sediments will be compared against these "ground truthing" measurements. Biogeochemical investigations benefit from the availability of tracers for processes, reactions, and spatial/temporal scales. Indeed, significant environmental insight can be gained by examining substances of interest (e.g., Hg) relative to specific environmental indicators. We are exploring the utility of using 210Pb:Hg relationships as means of: (1) determining Hg deposition associated with precipitation; and (2) identifying and quantifying the contributions of global and localized emissions to atmospheric Hg deposition. We have found a linear correlation between Hg and 210Pb in rainwater from remote and semiremote locations (see Figure 1; Lamborg, et al., 1999; Lamborg, et al., 2000; Tseng, et al., 2004). Although preliminary, this relationship yields information and scientific insight regarding the transformation mechanisms of Hg in the atmosphere and is an indicator of the global-scale Hg flux.

Figure 1. The Linear Correlation of Hg and 210Pb in Precipitation, Illustrating 210Pb Acting as a Tracer for Hg (Lamborg, et al., 2000). The circles are data from north-central Wisconsin, triangles are data from the south and equatorial Atlantic Ocean, and squares are data from arctic Alaska.

The correlation between Hg and 210Pb in rainwater suggests analogous removal mechanisms for these two species; namely, gas-phase conversion from a more volatile (Hgº-222Rn) form to a particle-reactive form (Hg2+-210Pb). The correlation appears to be nearly universal with observations from mid-continent, coastal, and mid-ocean locations agreeing (Lamborg, et al., 2000). The Hg:210Pb relationship from remote locations, such as southeastern Alaska, should provide a robust measure of global-source Hg deposition (hypothesis 3), one that would be independent of local climatological conditions and broadly applicable across large geographic regions. Differences in volume-weighted Hg concentrations in precipitation at the MDN sites raise important questions relating to the identification of sources and reactions in our developing understanding and modeling of the atmospheric cycling of Hg. For example, do higher levels of Hg in precipitation reflect localized emissions of Hg (hypothesis 4), and are these localized sources natural (e.g., enhanced particle scavenging) or anthropogenic? We are addressing such questions by developing the Hg and 210Pb relationship in precipitation to examine the apportionment of Hg sources in atmospheric Hg deposition for distinct and significant geographic and demographic regions.

Preliminary Results. The uniformity of the empirically established 210Pb flux among core sites is an important and useful result. The grand mean for all 20 cores thus far dated is 0.34 ± 0.06 pCi cm-2 yr-1. Based on the empirical relationship between atmospheric Hg and 210Pb deposition in remote regions (Lamborg, et al., 2000 and Figure 1) and rainfall depths of 1.4 - 1.8 m y-1, this 210Pb flux would predict a contemporary Hg deposition of 11 ± 3 µg m-2 yr-1. Atmospheric Hg and 210Pb collections in nearby Glacier Bay National Park, now underway, will provide a critical test of this prediction as well as constrain/refine the use of 210Pb as a tracer for atmospheric Hg.

Atmospheric Hg and 210Pb Deposition Collections. As outlined, the current wet deposition of Hg associated with precipitation is being obtained through approximately weekly sampling using an "ultra-clean" wet only Hg collector (Lindberg and Vermette, 1995) at Glacier Bay National Park (which currently is not included in MDN activities), in close proximity to the lake study region within the Tongass National Forest of southeastern Alaska. Currently, all of the rain samples collected through December 2003 have been analyzed for Hg. Total Hg is analyzed using methods designed for seawater (Bloom and Crecelius, 1983; Fitzgerald and Gill, 1979; Gill and Fitzgerald, 1985) that are similar to U.S. Environmental Protection Agency method 1631. This sampling program is planned for at least a 12- to 14-month period. We have run a similar and successful site in the environs of the Toolik Lake Arctic Long-Term Ecological Research Station (Tseng, et al., 2004). A 210Pb collector is collocated at Glacier Bay, and weekly collections are expected to provide sufficient precipitation for radioisotopic analysis. Because 210Pb samples must be held for 1 year to allow the establishment of secular equilibrium of 210Pb with its radioactive progeny, 210Po, early emplacement of the collocated collectors was necessary to obtain 12-14 months of data during the course of study. Precipitation is the principal means of removal for particle-bound species from the atmosphere, and contamination is virtually impossible, so a simple bulk precipitation collector with an evaporation trap suffices for the collection of 210Pb samples. These same bulk 210Pb collectors have been installed at the selected MDN sites (specific sites listed below), and samples are retrieved by the operators of the sites on the Hg sampling schedule.

210Pb collectors have been collocated at selected MDN sites that display a range in Hg deposition. The specific regions are West Coast North America, Mid-Continent United States, East Coast North America, and Southeastern United States. The West Coast stations are at Glacier Bay (non-MDN) and King County, Washington, near Seattle (WA18, MDN station abbreviation; see Figure 2). As outlined, Glacier Bay will provide a measure of Hg and 210Pb associated with incoming open ocean-global circulation. The MDN site in Seattle is an urban site, and the Hg and 210Pb ratios in precipitation should reflect the presence of significant locally generated Hg contributions. The MidContinent stations are in Minnesota at the Marcell Experimental Forest (MN16) in a northern forested region and Lamberton (MN27) in the agricultural region in the southwestern part of the state. Minnesota provides a location that is in the prevailing west to east air transport across the United States, which will be influenced by localized and regional emissions and deposition. The East Coast sites are at Acadia National Park, Maine (ME96), and at Cormack, Newfoundland (NF09). Although the precipitation volume-weighted values for Hg are low in Maine and Newfoundland, local/regional emissions and deposition are expected to be pronounced. Such sources should be evident in the Hg and 210Pb distribution in precipitation. In the Southeastern United States, the stations selected are in Florida at Andytown (near Miami, FL04) and the more rural and less populated Everglades Nutrient Removal Project (FL34). The precipitation volume-weighted Hg levels in South Florida precipitation are high, and the sources are unclear. This may be a locally derived phenomenon, or it may be associated with longer range transported Hg that may be scavenged efficiently as a consequence of upper level air-oxidation processes.

Figure 2. The MDN of the National Atmospheric Deposition Program. The sites included in this study are WA18, NF09, ME98, MN16, MN 27, FL04, and FL34. Additionally, a new site has been established at Glacier Bay National Park, as noted in the figure.


Benoit JM, Fitzgerald WF, Damman AWH. The biogeochemistry of an ombrotrophic bog: evaluation of use as an archive of atmospheric mercury deposition. Environmental Research 1998;78:118-133.

Benoit G, Hemond HF. Improved methods for the measurement of 210Po, 210Pb and 226Ra. Limnology and Oceanography 1988;33:1618-1622.

Bloom NS, Crecelius EA. Determination of mercury in seawater at subnanogram per liter levels. Marine Chemistry 1983;14:49-59.

Engstrom DR, Swain EB, Henning TA, Brigham ME, Brezonik PL. Atmospheric mercury deposition to lakes and watersheds: a quantitative reconstruction from multiple sediment cores. In: Baker LA, ed. Environmental Chemistry of Lakes and Reservoirs. Washington, DC: American Chemical Society, 1994, pp. 33-66.

Engstrom DR, Swain EB. Recent declines in atmospheric mercury deposition in the upper Midwest. Environmental Science and Technology 1997;312:960-967.

Electric Power Research Institute. Protocol for Estimating Historic Atmospheric Mercury Deposition, EPRI/TR-106768. Palo Alto, CA, 1996.

Fitzgerald WF, Gill GA. Sub-nanogram determination of mercury by two-stage gold amalgamation applied to atmospheric analysis. Analytical Chemistry 1979;51:1714-1720.

Fitzgerald WF, Engstrom DR, Mason RP, Nater EA. The case for atmospheric mercury contamination in remote areas. Environmental Science and Technology 1998;32:1-7.

Flynn WW. The determination of low levels of polonium-210 in environmental materials. Analytica Chimica Acta 1968;43:221-227.

Gill GA, Fitzgerald WF. Mercury sampling of open ocean waters at the picomolar level. Deep Sea Research 1985;32:287-297.

Lamborg CH, Rolfhus KR, Fitzgerald WF, Kim G. The atmospheric cycling and air-sea exchange of mercury species in the South and equatorial Atlantic Ocean. Deep Sea Research Part II: Topical Studies in Oceanography 1999;46(5):957-977.

Lamborg CH, Fitzgerald WF, Graustein WC, Turekian KK. An examination of the atmospheric chemistry of mercury using 210Pb and 7Be. Journal of Atmospheric Chemistry 2000;36:325-338.

Lamborg CH, Fitzgerald WF, Damman AWH, Benoit JM, Balcom PH, Engstrom DR. Modern and historic atmospheric mercury fluxes in both hemispheres: global and regional mercury cycling implications. Global Biogeochemical Cycles 2002;16:1104.

Lindberg SE, Vermette S. Workshop on sampling mercury in precipitation for the national atmospheric deposition program. Atmospheric Environment 1995;29:1219-1220.

Swain EB, Engstrom DR, Brigham ME, Henning TA, Brezonik PL. Increasing rates of atmospheric mercury deposition in midcontinental North America. Science 1992;257:784-787.

Future Activities:

We will conduct Hg analyses of sediment cores collected from the Tongass National Forest of Southeastern Alaska during the spring of 2004 at the University of Connecticut. Hg analysis will be conducted using an automated direct Hg analyzer (Milestone DMA 80), which requires no sample preparation before analysis. Additionally, comparison analyses will be conducted using a microwave/acid digestion procedure with cold vapor atomic fluorescence detection (Lamborg, et al., 2002). 210Pb analysis of rain samples (following a suitable grow-in period) collected at the MDN sites and Glacier Bay will begin in the summer of 2004 at the University of Connecticut and will be ongoing through the study. 210Pb determinations will be conducted using alpha-spectrometry of the daughter, 210Po, spontaneously deposited on silver foil (Flynn, 1968). Approximately 400-500 210Pb precipitation samples will be received from the eight collection locations. Hg analysis of precipitation samples from Glacier Bay will be ongoing at the University of Connecticut. Approximately 50-60 Hg precipitation samples will be obtained for analysis. Lake coring in Newfoundland is expected to occur during the fall of 2004. Each lake-coring campaign generates approximately 350-400 sediment samples for 210Pb and Hg analysis.

Journal Articles on this Report : 1 Displayed | Download in RIS Format

Other project views: All 13 publications 3 publications in selected types All 3 journal articles
Type Citation Project Document Sources
Journal Article Tseng CM, Lamborg C, Fitzgerald WF, Engstrom DR. Cycling of dissolved elemental mercury in Arctic Alaskan lakes. Geochimica et Cosmochimica Acta 2004;68(6):1173-1184. R829796 (2003)
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  • Supplemental Keywords:

    chemical transport, heavy metals, scaling, environmental chemistry, Northeast, Southeast, Midwest, Pacific Northwest, atmospheric mercury, United States, predictive modeling, deposition, quantitative assessment, air toxics, air pollutants, atmospheric chemistry, atmospheric deposition, atmospheric mercury chemistry, atmospheric mercury cycling, chemical kinetics, contaminant transport models, fate and transport, mercury, Hg, mercury chemistry, mercury cycling, mercury emissions, mercury vapor., Scientific Discipline, Air, INTERNATIONAL COOPERATION, Waste, Ecosystem Protection/Environmental Exposure & Risk, POLLUTANTS/TOXICS, Air Quality, air toxics, Environmental Chemistry, Chemicals, Fate & Transport, Environmental Monitoring, Atmospheric Sciences, Chemistry and Materials Science, fate and transport, air pollutants, Hg, mercury, mercury emissions, modeling, mercury cycling, chemical kinetics, atmospheric mercury chemistry, mercury chemistry, atmospheric chemistry, atmospheric mercury cycling, atmospheric deposition, contaminant transport models, heavy metals, mercury vapor

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