Processes Influencing Rainfall Deposition of Mercury in Florida

Considering the pronounced seasonal pattern in rainfall Hg deposition, the relatively uniform summertime rainfall Hg concentrations, and the low conce...
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Environ. Sci. Technol. 2001, 35, 863-873

Processes Influencing Rainfall Deposition of Mercury in Florida JANE L. GUENTZEL* Departments of Marine Science and Chemistry, Coastal Carolina University, P.O. Box 261954, Conway, South Carolina 29528-6054 WILLIAM M. LANDING Department of Oceanography, Florida State University, Tallahassee, Florida 32306-4320 GARY A. GILL Department of Marine Sciences, Texas A&M University, 5007 Avenue U, Galveston, Texas 77551 CURTIS D. POLLMAN Tetra Tech, Inc., 408 West University Avenue, Suite 301, Gainesville, Florida 32601

The primary goal of the Florida Atmospheric Mercury Study (FAMS) was to quantify the atmospheric deposition of Hg throughout Florida. Monthly integrated precipitation and weekly integrated particulate samples were collected at 10 sites in Florida for periods ranging from 2 to 5 yr. The monthly rainfall across the state and the concentrations of Hg in wet-only and bulk deposition increased by a factor of 2-3 during the summertime “wet season” (MayOctober). These parallel increases in rainfall amount and Hg concentration resulted in 5-8-fold increases in rainfall Hg deposition during the wet season. The annual volumeweighted Hg concentrations ranged from 14 ( 2 to 16 ( 2 ng/L across southern Florida, and the annual rainfall Hg fluxes ranged from 20 ( 3 to 23 ( 3 µg m-2 yr-1. The weekly integrated particulate Hg concentrations in southern Florida were low (4.9-9.3 pg/m3) and did not exhibit strong seasonal variability. Considering the pronounced seasonal pattern in rainfall Hg deposition, the relatively uniform summertime rainfall Hg concentrations, and the low concentrations of particulate Hg, we conclude that processes other than particulate Hg transport and scavenging govern rainfall Hg deposition in southern Florida. We hypothesize that long-range transport of reactive gaseous Hg (RGM) species coupled with strong convective thunderstorm activity during the summertime represents >50% of the Hg deposition in southern Florida. Model calculations indicate that local anthropogenic particulate Hg and RGM emissions account for 30-46% of the summertime rainfall Hg deposition across the southern Florida peninsula.

Introduction The discovery of elevated levels of Hg (0.4-4.4 ppm) in large freshwater game fish from southern Florida (1) and the death, potentially attributed to Hg toxicosis, of one endangered * Corresponding author e-mail: [email protected]; phone: (843)349-2374; fax: (843)349-2545. 10.1021/es001523+ CCC: $20.00 Published on Web 01/26/2001

 2001 American Chemical Society

Florida panther (Felis concolor coryi) from a remote region of southern Florida (2) prompted State and Federal agencies to investigate the sources of Hg to Florida’s ecosystems. Previous studies have demonstrated the importance of atmospheric transport and deposition of particulate (aerosol) Hg as a mechanism for the delivery of Hg to remote and pristine ecosystems (3, 4). Mercury in precipitation results from the scavenging of aerosol Hg and reactive gaseous forms of Hg(II) (RGM). Elemental gaseous Hg has a very low solubility in water and must first become oxidized before it is efficiently scavenged by precipitation events. These various forms of Hg in the atmosphere originate from natural processes (25%) and anthropogenic activities (75%) (5). The relatively long atmospheric residence time for elemental gaseous Hg (1 yr) results in an elevated global atmospheric background of anthropogenic Hg (6). In highly urbanized and industrialized regions, the deposition of Hg can be strongly influenced by contributions of aerosol Hg and reactive gaseous Hg (RGM) from local sources. Hg deposition in rural regions is usually significantly lower than in urbanized areas because local anthropogenic emissions are attenuated, by deposition and dilution, as the distance from urban sources increases. This conceptual model has been invoked to describe the deposition of Hg in many regions (6-8). Determining the atmospheric loading of Hg to Florida was therefore a necessary first step toward understanding the processes that govern the cycling of Hg in Florida’s aquatic ecosystems. As a result, the Florida Atmospheric Mercury Study (FAMS) was initiated in the fall of 1992. The primary objective of the FAMS project was to quantify geographical and seasonal gradients in atmospheric Hg deposition across Florida. Other objectives of the study included determining whether subregional gradients in Hg deposition occurred and whether Hg deposition could be correlated with regions (“hotspots”) of alarmingly high Hg concentrations (1-4 ppm) in fish from Everglades Water Conservation Area WCA-3. The project included studying the partitioning between wet and dry deposition; investigating the speciation of Hg in precipitation, throughfall (9), and gaseous Hg (10); and identifying the possible sources of Hg using multiple chemical tracers (11, 12). This paper discusses the concentrations of Hg in aerosols and precipitation and the rates of rainfall Hg deposition at 10 locations in Florida and a site in Barbados. In addition, we propose a model to account for the sources of the elevated rainfall Hg deposition over the southern Florida peninsula. Two meteorological processes that are characteristic of the southern Florida peninsula during the “wet season” (May-October) are the strong synoptic southeast and easterly winds associated with the tropical North Atlantic trade winds and the formation of deep convective thunderstorm cells. The tall (12-16 km) convective thunderstorms are generated when moist air from the sea breeze is carried aloft by the hot air masses rising off the southern peninsula of Florida. In contrast to low altitude frontal storms, tall convective thunderstorms can scavenge particulate Hg and watersoluble RGM from the middle and upper troposphere. We propose that the southeasterly trade winds regularly resupply reactive gaseous Hg species to the atmosphere over the southern peninsula of Florida from May to October. As a result, the tall convective thunderstorms are exposed to a fresh atmospheric burden of “background” RGM on an almost daily basis during the summer months. We present arguments that anthropogenic sources of RGM in Dade and Broward Counties, primarily municipal solid waste and medical waste incinerators, supply less than half of the daily summertime VOL. 35, NO. 5, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Locations of the nine atmospheric sampling towers (LB, FM, FS, EG, TT, CK, AT, EN, and CV) and two ground-based sites (EGG and FL) used in the FAMS project. RGM burden in the southern Florida airshed. We estimate that a greater mass of RGM enters the airshed due to daily ventilation with background air containing scavengable RGM and aerosol Hg. Oxidation of gaseous elemental Hg within the airshed over southern Florida could supply an additional 8% of the daily RGM burden in the boundary layer.

Experimental Methods Site Locations. The FAMS monitoring network consisted of 10 sites in Florida (Figure 1). The Caryville site of the Florida Acid Deposition Network (CV) and Lake Barco (LB) in the Katherine Ordway Reserve represented north and north central Florida, respectively. The sites in southern Florida were located at the Terry Park recreational complex in Ft. Myers (FM), the ranger’s compound of the Fakahatchee Strand State Preserve (FS), the Tamiami Trail Ranger Station (TT) of the Everglades National Park at “40-mile bend” on U.S. Highway 41, the Beard Research Center in the Everglades National Park (EG), the Andytown substation of Florida Power & Light (AT), the South Florida Water Management District Everglades Nutrient Removal Project (EN), and the state park on Little Crawl Key (CK). Each of these sites was equipped with a 15-m aluminum staircase tower with outboard sampling platforms (UpRight Inc.). The samples were collected on top of the towers to minimize the influence of birds and insects and locally generated dust and pollen (13, 14). Two ground-based sites were added to the network in 1995 (Figure 1). The EGG site was established in May 1995 to sample Hg in throughfall and was located under a 4-6-m stand of brazilian pepper trees (Schinus terebinthifolius) approximately 50 m from the EG tower. These trees have foliage year-round. The Ft. Lauderdale site (FL) rain sampler was positioned in July 1995 on a 1.5-m wooden sampling platform located in a grassy field at the University of Florida Institute of Food and Agricultural Sciences regional facility in the Ft. Lauderdale suburb of Davie, FL. This site is approximately 22 km east of the AT site, which borders the eastern edge of the Everglades Water Conservation Area WCA-3, and is less than 2 km from a municipal solid 864

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waste incinerator. Samples were also collected atop the NSF-sponsored Atmosphere-Ocean Chemistry Experiment (AEROCE) sampling tower on remote, windward “Ragged Point” in Barbados (BS) (14). Sample Collection. The success of this project was dependent upon the use of rigorous sample collection protocols coupled with processing and analysis under “clean lab” conditions. In the clean lab at Florida State University, extensive detergent, solvent, and acid washing procedures were used to clean the sampling equipment (15-17). Thorough discussions of the QA/QC procedures, cleaning procedures, equipment design, and sampling methods have been published by Landing et al. (18), Guentzel et al. (15), Pollman et al. (19), Gill et al. (10), Landing et al. (16), and Gill and Fitzgerald (17). Monthly integrated bulk and wet-only deposition samples were collected at the 9 tower sites for periods ranging from 2 to 5 yr. Monthly integrated wet-only deposition samples were collected at the two ground-based sites (EGG and FL). Monthly integrated bulk deposition samples were collected on the Barbados (BS) tower. The FAMS bulk deposition collectors were fabricated using Teflon components, FEP Teflon receiving bottles, and polycarbonate funnels (6, 15, 16). Duplicate bulk deposition samplers were deployed at each site, and the upward-facing polycarbonate funnels were left uncovered for the entire month-long deployment period. Immediately before the samples were recovered, the funnels were each rinsed with two 70-mL aliquots of ultrapure water acidified with triple distilled sub-boiling quartz-distilled HCl (0.045 M 3×Q-HCl). Thus, the bulk deposition samples represent wet deposition plus any dry deposition that may have accumulated on the inner surface of the funnel (16). The wet deposition samples were collected using a modified Aerochem Metrics 301 wet/dry deposition sampler equipped with a polycarbonate roof, splash guard, Tefloncovered seal, and Teflon-coated arms (16). One 1-L polyethylene bottle (pH and major ions) and two 1-L FEP Teflon receiving bottles were nested inside the wet bucket and were attached to polycarbonate funnels using PTFE Teflon couplings. Rainfall amounts exceeding 27 cm/month were not collected due to the volume limitation of our receiving bottles. This loss amounted to less than 5% of the annual rainfall at 10 of the 12 sampling locations (Table 1). Excluding the few periods when the samples overfilled, the excellent agreement between the sample volumes and the rain gauge volumes indicates that the rain events were quantitatively collected over the course of the monthly deployment (sample/gauge ) 1.02 ( 0.01 SD; n ) 822; ref 16). Equipment blanks and field blanks for the 30-day deployment of bulk and wet deposition were negligible (2.5 kg/day) plus in-situ elemental Hg oxidation (0.38 kg/day). Local RGM sources would therefore account for approximately 30 ( 7% of the Hg in wet deposition (2.5 ( 0.5 kg/day out of 8.3 ( 0.9 kg/day total, assuming equivalent thunderstorm entrainment and RGM scavenging efficiency for the boundary layer and the free troposphere). Local sources would account for 46 ( 11% of the Hg in summertime rainfall (2.5 ( 0.5 kg/day out of 5.4 ( 0.7 kg/day total) if one made the extreme assumption that there is no RGM scavenging from the free troposphere. What fraction of the rainfall RGM deposition is expected to come from local sources over the course of a full year? From our data, we see that roughly 16% of the annual rainfall Hg deposition in southern Florida occurs during the November-April dry season. If we make an extreme assumption that long-range transport disappears completely while insitu elemental Hg oxidation matches the wet season value (0.43 ( 0.1 kg/day), then local RGM sources (2.5 ( 0.5 kg/ day) would account for 85 ( 23% of the daily boundary layer RGM supply during the dry season. Since winter (frontal)

storms are less likely to scrub much of the free troposphere, we assign e14% of the annual rainfall Hg flux to local RGM sources during the dry season (85% local source supply times 16% of the annual rainfall Hg deposition). During the wet season, local RGM sources account for 25-39% of the annual rainfall Hg deposition (30-46% local source supply times 84% of the annual rainfall Hg deposition). These conclusions contrast sharply with those reached by Dvonch et al. (35, 45). During the SoFAMMS experiment, they collected over 300 24-h integrated rain samples in the Miami/Ft. Lauderdale area during August 1995 and analyzed them for Hg and a number of other trace elements. Using multi-variate statistical processing and detailed analysis of individual storm events, they concluded that local Hg emission sources had a significant impact on the rainfall Hg concentrations and deposition, accounting for 71-73% of the rainfall Hg deposition in their study domain. According to our calculations, local RGM sources could account for more than 70% of the wet deposition across southern Florida only by making the extreme assumptions that there is no RGM scavenging from the free troposphere and that the scavengable RGM plus aerosol Hg in the background boundary layer air is less than 5 pg/m3. Furthermore, the FAMS data from August 1995 show that the rainfall Hg concentrations and deposition at many of the other FAMS sites in southern Florida (Figures 2-4) were significantly higher than at the SoFAMMS Davie and Andytown sites (45). The Fakahatchee Strand site, over 60 km southwest of the Andytown site, had the highest rainfall Hg deposition of the FAMS sites during August 1995 (18 ng/L; 9.1 µg/m2). The CK site (19 ng/L; 4.37 µg/m2) is even farther away, over 150 km to the south-southwest of the Andytown site (15 ng/L; 4.35 µg/m2). One would expect rainfall washout of particulate Hg and RGM from local emission sources in the Miami/Ft. Lauderdale area to yield progressively lower (not higher) rainfall Hg concentrations and deposition as one moved away from the urban area. The MM5 meteorological model was used to generate forward trajectories from the urban emission sources and back trajectories from the CK site for every rain event during August 1995 at the CK site (46). Combining trajectories with daily rainfall amounts, Green et al. (46) concluded that less than 16% of the rainfall Hg collected at the CK site during August 1995 was associated with air masses that had passed near the RGM emission sources in the Miami/Ft. Lauderdale urban area. Our conclusions also conflict with model results from Bullock (47), who used the EPA Hg emissions inventory and the RELMAP model to predict Hg deposition across the continental United States. In southern Florida, the RELMAP model predicted a very strong east-to-west geographical gradient in rainfall Hg deposition, from >30 µg m-2 yr-1 near urban Miami/Ft. Lauderdale to 17 µg m-2 yr-1 for an incomplete sampling year in 1993 at the EG site and then Hg wet deposition of 24, 24, and 15 µg m-2 yr-1 in 1994, 1995, and 1996, respectively. Data from the MDN network show Hg wet deposition at the EG site (FL-11) of 17 µg m-2 yr-1 in 1996, 27 µg m-2 yr-1 in 1997, 20 µg m-2 yr-1 in 1998, 17 µg m-2 yr-1 in 1999, and 17 µg m-2 yr-1 in 2000 (January-September) (51). It appears that the large reduction in local MSW incinerator Hg emissions has not resulted in a significant decrease in rainfall Hg concentrations or wet deposition of Hg at the EG site. In contrast, it was recently demonstrated that significant decreases in bulk Hg precipitation and deposition coincided with reductions in anthropogenic Hg emissions from a remote site in northern Wisconsin (52). Dry Deposition of RGM. What impact do these various RGM sources have on RGM dry deposition? Previous RGM modeling studies have noted the significance of rapid localized dry deposition, which depletes the RGM reservoir before wet deposition can occur farther downwind (47). If local emission sources of RGM (plus particulate Hg) are in fact responsible for less than half of the daily RGM supply in the summertime boundary layer, then those sources would account for less than half of the RGM dry deposition. During the winter, when long-range RGM transport is perhaps less important, local emission sources could dominate the supply of RGM to the boundary layer and may account for g85% of the wintertime dry deposition. Estimates of RGM dry deposition to Everglades vegetation (16 µg m-2 yr-1; ref 9) are slightly lower than our measured wet deposition rates (15-24 µg m-2 yr-1, Table 1). Extrapolating over the entire 34 × 109 m2 of southern Florida, dry deposition of RGM would come to 1.5 kg/day. Added to the 2.5-3 kg/day summertime wet deposition of Hg (total wet + dry g4 kg/day), it appears that the identified local RGM emission sources (2.5 ( 0.5 kg/day) are simply not adequate to supply all of the necessary RGM, even if one could somehow capture 100% of the emissions each day over the southern Florida peninsula. Implications for the Everglades Region. Our RGM source/ sink inventory described above is a first-order explanation of Hg deposition and the complicated summertime meteorology in southern Florida. Continued efforts to quantify background RGM in the boundary layer and the free troposphere; to quantify local emission sources; and to model Hg emission, transport, and scavenging will further explain and reconcile the FAMS and SoFAMMS data. Lagrangian deposition models (47, 48) and statistical source apportionment models (35) that aspire to explain the deposition of Hg in southern Florida should be able to simulate the strong seasonality we demonstrate with the FAMS rainfall Hg deposition data as well as the uniformity in Hg deposition across the entire southern Florida peninsula. Such modeling efforts must include the impacts of RGM and particulate Hg in background air. The contrast between uniform Hg deposition and geographical “hot spots” in fish Hg concentrations from the Everglades region further suggests that aquatic/terrestrial Hg cycling processes, rather than atmospheric source strength, are responsible for the hot spots. Ecosystem Hg cycling models for the Everglades should be structured to simulate the effects of the summertime pulses of reactive Hg in rainfall. Finally, it is important to recall that 60-70% of the Hg in the modern global atmosphere results from anthropogenic industrial activities (5). From our analysis of the data, we conclude that significant reduction in rainfall Hg deposition 872

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over the Florida Everglades will likely require reductions in local and global Hg emissions.

Acknowledgments This project was supported financially by grants from the Florida Department of Environmental Protection, Florida Power and Light, the Electric Power Research Institute, the South Florida Water Management District, the U.S. Environmental Protection Agency, and the Florida Electric Power Coordinating Group. The authors’ conclusions do not imply endorsement by these funding agencies. We thank the Florida Park Service, the National Park Service, the Keys Marine Laboratory, the Ordway Preserve, and Ken Larson of the Broward County Air Monitoring Program for providing logistical support. We gratefully recognize the invaluable assistance provided by the graduate students and support personnel employed by FSU, Texas A&M, and KBN Engineering. We specifically acknowledge the following: John Cooksey, Laurel Buttermore, Geoffrey Schaefer, Johan Schijf, Jerome J. Perry Jr., Scott Sigler, Stephanie Smith-Moore, Jill Brandenburger, Dave Oliff, Jim Winnie, Paul Ruscher, Melody Owens, Matt Green, Ron Lehman, Dave Bare, Mike Arrants, John and Gayle Swanson, Cornelius Shea, and the NSF/ AEROCE Program.

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Received for review July 26, 2000. Revised manuscript received December 4, 2000. Accepted December 5, 2000. ES001523+

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