Environ. Sci. Technol. 2002, 36, 4508-4517
Atmospheric Mercury in the Lake Michigan Basin: Influence of the Chicago/Gary Urban Area M A T T H E W S . L A N D I S , * ,† ALAN F. VETTE,† AND GERALD J. KEELER The University of Michigan Air Quality Laboratory, Ann Arbor, Michigan 48109
The relative importance of the Chicago/Gay urban area was investigated to determine its impact on atmospheric mercury (Hg) concentrations and wet deposition in the Lake Michigan basin. Event wet-only precipitation, total particulate, and vapor phase samples were collected for Hg, and trace element determinations from five sites around Lake Michigan from July 1994 through October 1995 as part of the Lake Michigan Mass Balance Study (LMMBS). In addition, intensive over-water measurements were conducted aboard the EPA research vessel Lake Guardian during the summer of 1994 and the winter of 1995 as part of the Atmospheric Exchange Over Lakes and Oceans Study. Atmospheric Hg concentrations were found to be significantly higher in the Chicago/Gary urban area than surrounding sites: Hg in precipitation was a factor of 2 and particulate Hg was a factor of 6 times higher. Overwater measurements found elevated Hg concentrations 19 km off shore of Chicago/Gary suggesting an enhanced near field atmospheric deposition to Lake Michigan. Meteorological transport analyses also determined that local sources in the Chicago/Gary urban area significantly impacted all of the LMMBS sites indicating a broad impact to the entire Lake Michigan basin.
Introduction Mercury (Hg) is a toxic pollutant of concern in aquatic ecosystems because of its ability to bioaccumulate up the food chain and its demonstrated link to human health effects from consuming contaminated fish. Atmospheric deposition is widely recognized as an important link in the cycling of Hg in the environment (1-5) and has been implicated as the primary pathway for inputs of Hg to Lake Michigan (6-8). Consequently, Hg has been identified as a critical pollutant for study and has been specifically targeted in the Great Lakes Water Quality Agreement Amendments of 1987 and Section 112(m) of the Clean Air Act Amendments of 1990 (“Great Waters Provision”). On a global basis, it is estimated that between 50 and 75% of total atmospheric Hg emissions are of anthropogenic origin (9, 10). Natural emissions are typically assumed to be elemental gaseous Hg0 (11). Anthropogenic emissions are primarily Hg0, divalent reactive gaseous mercury (RGM), and particulate Hg (Hg(p)). The dominant form of Hg in the atmosphere is Hg0 (12). Because it is relatively insoluble and deposits very inefficiently, the mean residence time for Hg0 * Corresponding author phone: (919)541-4841; fax: (919)541-1153; e-mail:
[email protected]. † Current address: U.S. EPA, National Exposure Research Laboratory, Research Triangle Park, NC 27711. 4508
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in the atmosphere is estimated to be approximately 1 year (1, 13) allowing for truly global circulation. Any RGM that is directly emitted to the atmosphere is expected to deposit efficiently on a local or regional scale near major sources largely because of its solubility. Atmospheric deposition at any particular location can, therefore, be a complex combination of local, regional, and global emissions and transport/transformation processes (10). The importance of local, regional, and global sources of mercury to observed atmospheric deposition is a topic of contentious debate. Representatives from the Electric Power Research Institute purport that Hg pollution is a global problem in which the U.S. plays a minor role, emitting only 158 tons year-1 of the estimated 2000-6000 tons year-1 emitted globally (14). The contamination of remote lakes and arctic ice cores has been attributed to long range and even global transport of atmospheric Hg (15). However, researchers in the U.S. and Europe have observed significant spatial gradients in atmospheric Hg deposition around urban and industrial areas indicating local anthropogenic influences (7, 16-18). Dated sediment cores from the Great Lakes indicate that annual net deposition rates increased from 7-fold in Lake Erie to over 100-fold in Lake Ontario from preindustrial times to their peak in the mid 1940s (19). Pirrone et al. (19) reported the Hg deposition rate for Lake Michigan increased by a factor of 15 from the early 1800s to its peak in 1946. The relative importance of atmospheric deposition and direct discharges into the Great Lakes or their tributaries contributing to the observed sediment core results are unknown. Most recent estimates of atmospheric deposition of trace elements to the Great Lakes have been made using data from the Integrated Atmospheric Deposition Network (IADN) (20, 21). One sampling site was established on each of the Great Lakes to monitor regional trends and estimate atmospheric deposition. The sites were established in remote locations to avoid being unduly impacted by local sources. The IADN site for Lake Michigan is located in the northeastern part of the lake in the Sleeping Bear Dunes National Lakeshore. The maximum local source density near Lake Michigan occurs along its southwestern shoreline, which is dominated by the greater Chicago, IL and Gary, IN urban area. With a population of over 8 000 000, this is the third largest metropolitan area in the country. A 4-week intensive study demonstrated that the Chicago/Gary urban area had a significant impact on the dry deposition of Hg into southern Lake Michigan (7), and a multisite monitoring study conducted in Michigan suggested the impact of the Chicago/ Gary urban area was also observed when elevated concentrations of Hg in precipitation were found to be associated with air transport from that region (16). Major anthropogenic Hg sources in the Lake Michigan Basin and preliminary estimates of their annual emissions into the atmosphere have recently been reported (22). Sources include fossil fuel utility boilers, municipal and hospital waste incinerators, iron and steel production, coke production, lime production, hazardous waste recycling facilities, and secondary copper and petroleum refining. However, the sources of Hg are numerous and many are not well characterized. As a result, an accurate emissions inventory that includes speciated anthropogenic as well as natural Hg sources is still not available. This reality, coupled with an incomplete understanding of atmospheric processes for Hg, limits the present reliability of deterministic models in predicting the atmospheric behavior and deposition of Hg over short temporal and large spatial scales (11, 16). To investigate 10.1021/es011216j CCC: $22.00
2002 American Chemical Society Published on Web 10/08/2002
FIGURE 1. Location of sampling sites used in the Lake Michigan Mass Balance Study and the Atmospheric Exchange Over Lakes and Oceans Study. transport and deposition of Hg in the Lake Michigan Basin, accurate, long-term measurements at multiple receptor locations were needed. Two studies were conducted concurrently to investigate the role of atmospheric Hg deposition to Lake Michigan: the Lake Michigan Mass Balance Study (LMMBS) from July 1994 through October 1995 and the Atmospheric Exchange Over Lakes and Oceans Study (AEOLOS) from May 1994 through January 1995. This manuscript summarizes our measurements of Hg in precipitation, Hg(p), and total vapor phase Hg (Hg(v)) in the Lake Michigan region from the LMMBS network of land-based sites and over-water measurements made aboard the EPA research vessel Lake Guardian during AEOLOS. In addition, a meteorological cluster analysis will be presented that demonstrates the significant impact of the Chicago/Gary urban area on atmospheric Hg concentrations across the entire Lake Michigan Basin.
Study Design Site Descriptions. Five sampling sites were chosen for the LMMBS (Figure 1). Four sites were located around the lake in Kenosha (KEN), WI (42.50°N, 87.81°W); Chicago (IIT), IL (41.83°N, 87.62°W); Sleeping Bear Dunes (SBD), MI (44.76°N, 86.06°W); and South Haven (SHN), MI (42.46°N, 86.16°W). The fifth site was located in Bondville (BON), IL (40.03°N, 88.22°W). The BON site was considered an “upwind background” location since there were no nearby sources and
the predominant wind direction was from the southwest (23). The IIT site was located on the roof of a four story building on the Illinois Institute of Technology campus in southeast Chicago about 1.6 km west of the lake. This area of the city is mixed commercial and residential. There is heavy urban and industrial development in all directions from the site, with the heaviest concentration 10-20 km to the southeast in Chicago and northwest Indiana (Gary). The KEN site was located approximately 200 m from Lake Michigan in a large open field about 5 km south of Kenosha. The surrounding area was semirural, with mostly residential and some light commercial activity. The SHN site was located 7 km east of Lake Michigan and 12 km northeast of South Haven in an open field. The surrounding area was rural, dominated by general agriculture. The SBD site was located about 1 km east of the lake in a large open field on a secondary dune approximately 5 km south of Empire, MI. The surrounding area is rural, dominated by wooded areas and general agriculture. Site operators were hired and trained to collect samples according to the University of Michigan Air Quality Laboratory (UMAQL) ultraclean protocols (24). The AEOLOS utilized the five LMMBS land-based sites, an additional urban site at George Washington High School (GWS) in Chicago, IL (41.68°N, 87.53°W), and two over-water sites (42.00°N, 87.42°W; 41.77°N, 87.33°W) (Figure 1). The GWS site was located on the roof of the school approximately 18 km southeast of the IIT site in the heavily industrialized area of Chicago. Over-water measurements were made aboard the Lake Guardian from two stations approximately 19 km off shore of Chicago. All ambient samples were collected aboard the Lake Guardian approximately 5 m above the water surface from a retracting boom extended 2 m from the bow of the ship. The ship was stationary during sampling and anchored into the wind to prevent contamination from onboard activities or engine exhaust. Data Description. Precipitation samples were collected from July 1, 1994 through October 31, 1995 during the LMMBS using automatic wet-only precipitation collectors. Samples were collected on an event basis from April through October and on a weekly basis from November through March to constrain analysis costs. The event and weekly precipitation collection methods were previously found to be equivalent (25). Precipitation samples were analyzed for Hg and a suite of trace elements including copper (Cu), zinc (Zn), strontium (Sr), and lead (Pb). Twenty-four h integrated total aerosol and Hg(v) samples were collected every sixth day from July 1, 1994 through October 30, 1995 during the LMMBS. Sampling began at 8:00 a.m. local time and ended the following morning at 8:00 a.m. local time. During AEOLOS, 12-h total Hg(p) and Hg(v) samples were collected at all sites except SBD. In addition, 12-h fine fraction aerosol (
