Measurement of Atmospheric Mercury: Current Limitations and Suggestions for Paths Forward.

Mercury (Hg) researchers have made progress in understanding atmospheric Hg, especially with respect to oxidized Hg (HgII) that can represent 2 to 20% of Hg in the atmosphere. Knowledge developed over the past ∼10 years has pointed to existing challenges with current methods for measuring atmospheric Hg concentrations and the chemical composition of HgII compounds. Because of these challenges, atmospheric Hg experts met to discuss limitations of current methods and paths to overcome them considering ongoing research. Major conclusions included that current methods to measure gaseous oxidized and particulate-bound Hg have limitations, and new methods need to be developed to make these measurements more accurate. Developing analytical methods for measurement of HgII chemistry is challenging. While the ultimate goal is the development of ultrasensitive methods for online detection of HgII directly from ambient air, in the meantime, new surfaces are needed on which HgII can be quantitatively collected and from which it can be reversibly desorbed to determine HgII chemistry. Discussion and identification of current limitations, described here, provide a basis for paths forward. Since the atmosphere is the means by which Hg is globally distributed, accurately calibrated measurements are critical to understanding the Hg biogeochemical cycle.

Observations of the chemistry and concentrations of reactive Hg at locations with different ambient air chemistry.

The Hg research community needs methods to more accurately measure atmospheric Hg concentrations and chemistry. The Reactive Mercury Active System (RMAS) uses cation exchange, nylon, and PTFE membranes to determine reactive mercury (RM), gaseous oxidized mercury, and particulate-bound mercury (PBM) concentrations and chemistry. New data for Atlanta, Georgia (NRGT) demonstrated that particulate-bound Hg was dominant and the chemistry was primarily N and S HgII compounds. At Great Salt Lake, Utah (GSL), RM was predominately PBM, with NS > organics > halogen > O HgII compounds. At Guadalupe Mountains National Park, Texas (GUMO), halogenated compound concentrations were lowest when air interacting with the site was primarily derived from the Midwest, and highest when the air was sourced from Mexico. At Amsterdam Island, Southern Indian Ocean, compounds were primarily halogenated with some N, S, and organic HgII compounds potentially associated with biological activity. The GEOS-Chem model was applied to see if it predicted measurements at five field sites. Model values were higher than observations at GSL, slightly lower at NRGT, and observations were an order of magnitude higher than modeled values for GUMO and Reno, Nevada. In general, data collected from 13 locations indicated that N, S, and organic RM compounds were associated with city and forest locations, halogenated compounds were sourced from the marine boundary layer, and O compounds were associated with long-range transport. Data being developed currently, and in the past, suggest there are multiple forms of RM that modelers must consider, and PBM is an important component of RM.

Tree rings as historical archives of atmospheric mercury: A critical review.

Historical concentrations of atmospheric mercury (Hg) are uncertain, as monitoring only began a few decades ago. Tree rings can serve as historical archives of Hg, providing centennial trends. The vast majority of tree-ring Hg studies have been published in the last decade, demonstrating the growing use of tree rings for Hg dendrochemistry. Thus, there is a need for a systematic review on current knowledge of tree rings as archives of atmospheric Hg. In this review, the predominant pathways of Hg uptake to tree rings are discussed, including the initial Hg uptake from the surrounding environment, fixation, and subsequent translocation. Foliar uptake of Hg was found to be the most important uptake route for Hg in tree rings, the root and bark route being negligible. Our summary of the suitability of different tree species indicates that radial translocation is the biggest limiting factor for Hg dendrochemistry, shifting and blurring historical Hg trends. Based on the review findings, Picea (spruce) and Larix (larch) are the most promising genera for Hg dendrochemistry. Additionally, the use of tree-ring Hg archives in combination with other co-located archives, namely lake sediments, peat, and ice, is suggested as it enhances the viability of observed tree-ring historical Hg trends. Finally, we propose future directions and recommendations for research using tree-ring Hg, including sampling protocols, experimental designs, and tree selection.

Comparison and calibration of methods for ambient reactive mercury quantification.

Gaseous oxidized mercury (GOM) is the dominant form of atmospheric mercury (Hg) deposited and sequestered within ecosystems. Thus, accurate, calibrated measurements of GOM are needed. Here, two active membrane-based collection systems (RMAS) were used to determine GOM and particulate-bound Hg (PBM), as well as reactive Hg (RM = GOM + PBM), and compared with two dual-channel systems (DCS) and a Tekran 2537/1130 speciation system. The DCS measured operationally defined GOM by difference, using concentrations of gaseous elemental Hg (GEM) and total gaseous Hg. One DCS was linked to a custom-built, automated calibration system that permeated GEM, HgBr2, or HgCl2. The five systems were co-located for one-year to develop a dataset that would allow for understanding limitations of each system, and assessing measurement accuracy and long-term precision of the calibrator. The Tekran system measured ~14.5 % of the GOM measured by the other systems. The USU and UNR DCS and RMAS were significantly correlated, but the DCS was 50 and 30 % higher, respectively, than the RMAS. The calibrator performed consistently in the field and lab, and the DCS fully recovered GOM injected by the calibrator. Since the uncalibrated DCS measured the same concentrations as the calibrated DCS, they are both accurate methods for measuring RM and/or GOM. Some loss occurred from the RMAS membranes. SYNOPSIS: Accurate and calibrated measurements of atmospheric reactive mercury using membranes and two dual-channel systems.

Investigation of the biochemical controls on mercury uptake and mobility in trees.

Atmospheric elemental mercury (Hg(0)) enters plant stomata, becomes oxidized, and is then transferred to annual growth rings providing an archive of air Hg(0) concentrations. To better understand the processes of Hg accumulation and translocation, the foliage of quaking aspen and Austrian pine were exposed to Hg(0), and methylmercury (MeHg) or Me198Hg via roots, in controlled exposures during the summer. Isotopic measurements demonstrated, in a laboratory setting, that the natural mass-dependent fractionation observed was the same as that measured in field studies, with the lighter isotopes being preferentially taken up by the leaves. Hg was measured in plant tissues across seasons. Aspen trees moved Hg into new growth immediately after exposure, resorbed Hg in the fall, and then distributed Hg to new growth tissues in the spring. Austrian pine did not reallocate Hg. Mercury measured in aspen leaf fractions of trees exposed to Hg(0) demonstrated that 85 % of Hg was in the cell wall. It was also found that redox-active molecules, such as H2O2, could potentiate the release of cell wall-bound Hg from aspen leaves, providing a potential mechanism for remobilization. Regardless of the mechanism, the ability of aspen to reallocate Hg to new tissues indicates that Hg distribution in tree rings from aspen do not provide a reliable record of yearly changes in atmospheric Hg(0).

Evidence against Rapid Mercury Oxidation in Photochemical Smog.

Mercury pollution is primarily emitted to the atmosphere, and atmospheric transport and chemical processes determine its fate in the environment, but scientific understanding of atmospheric mercury chemistry is clouded in uncertainty. Mercury oxidation by atomic bromine in the Arctic and the upper atmosphere is well established, but less is understood about oxidation pathways in conditions of anthropogenic photochemical smog. Many have observed rapid increases in oxidized mercury under polluted conditions, but it has not been clearly demonstrated that these increases are the result of local mercury oxidation. We measured elemental and oxidized mercury in an area that experienced abundant photochemical activity (ozone >100 ppb) during winter inversion (i.e., cold air pools) conditions that restricted entrainment of air from the oxidized mercury-rich upper atmosphere. Under these conditions, oxidized mercury concentrations decreased day-upon-day, even as ozone and other pollutants increased dramatically. A box model that incorporated rapid kinetics for reactions of elemental mercury with ozone and OH radical overestimated observed oxidized mercury, while incorporation of slower, more widely accepted reaction rates did not. Our results show that rapid gas-phase mercury oxidation by ozone and OH in photochemical smog is unlikely.

Using a next-generation sequencing approach to DNA metabarcoding for identification of adulteration and potential sources of mercury in commercial cat and dog foods.

Studies have demonstrated that some commercial pet (i.e., cat and dog) food products contain high concentrations of mercury (Hg), and some products have Hg concentrations that are higher than expected based on the ingredients included in the package ingredient list. Additionally, concentrations of methylmercury, a particularly toxic form of Hg commonly associated with fish-based ingredients, are largely unstudied despite the widespread use of such ingredients in pet food products. This study aimed to quantify total Hg and methylmercury in a variety of commercial pet food products (n = 127), and use genetic tools to determine if specific ingredients contributed to high Hg concentrations in the final product. Results indicate that total Hg concentrations were above suggested maximum tolerable limits in three of the tested pet food products, and that methylmercury concentrations were at safe levels in all tested products. Next-generation amplicon sequencing using ten barcode primers was conducted to target distinct taxa and to determine if one primer set outperformed the others in amplifying the often heavily degraded DNA found in pet food products. The 16sUniF_16sUniR primer set generated a relatively higher number of reads across the broadest set of taxa, although several of the primer sets were useful in identifying common animal- and plant-based ingredients in commercial pet food products. Combined with the Hg results, it was demonstrated that pet food product ingredients are consistent among and between product lots. However, these results also revealed that adulteration is prevalent in pet food products.

Use of Membranes and Detailed HYSPLIT Analyses to Understand Atmospheric Particulate, Gaseous Oxidized, and Reactive Mercury Chemistry.

The atmosphere is the primary pathway by which mercury enters ecosystems. Despite the importance of atmospheric deposition, concentrations and chemistry of gaseous oxidized (GOM) and particulate-bound (PBM) mercury are poorly characterized. Here, three membranes (cation exchange (CEM), nylon, and poly(tetrafluoroethylene) (PTFE) membranes) were used as a means for quantification of concentrations and identification of the chemistry of GOM and PBM. Detailed HYSPLIT analyses were used to determine sources of oxidants forming reactive mercury (RM = PBM + GOM). Despite the coarse sampling resolution (1-2 weeks), a gradient in chemistry was observed, with halogenated compounds dominating over the Pacific Ocean, and continued influence from the marine boundary layer in Nevada and Utah with a periodic occurrence in Maryland. Oxide-based RM compounds arrived at continental locations via long-range transport. Nitrogen, sulfur, and organic RM compounds correlated with regional and local air masses. RM concentrations were highest over the ocean and decreased moving from west to east across the United States. Comparison of membrane concentrations demonstrated that the CEM provided a quantitative measure of RM concentrations and PTFE membranes were useful for collecting PBM. Nylon membranes do not retain all compounds with equal efficiency in ambient air, and an alternate desorption surface is needed.

Improvements to the Accuracy of Atmospheric Oxidized Mercury Measurements.

We developed a cation-exchange membrane-based dual-channel system to measure elemental and oxidized mercury and deployed it with an automated calibration system and the University of Nevada, Reno-Reactive Mercury Active System (UNR-RMAS) at a rural/suburban field site in Colorado during the summer of 2018. Unlike oxidized mercury measurements collected via the widely used KCl denuder method, the dual-channel system was able to quantitatively recover HgCl2 and HgBr2 injected by the calibrator into the ambient sample air and compared well with the UNR-RMAS measurements. The system measured at 10 min intervals and had a 3-h average detection limit for oxidized mercury of 33 pg m-3. It was able to detect day-to-day variability and diel cycles in oxidized mercury (0 to 200 pg m-3) and will be an important tool for future studies of atmospheric mercury. We used a gravimetric method to independently determine the total mercury permeation rate from the permeation tubes. Permeation rates derived from the gravimetric method matched the permeation rates observed via mercury measurement devices to within 25% when the mercury permeation rate was relatively high (up to 30 pg s-1), but the agreement decreased for lower permeation rates, probably because of increased uncertainty in the gravimetric measurements.

Mercury biogeochemical cycling: A synthesis of recent scientific advances.

The focus of this paper is to briefly discuss the major advances in scientific thinking regarding: a) processes governing the fate and transport of mercury in the environment; b) advances in measurement methods; and c) how these advances in knowledge fit in within the context of the Minamata Convention on Mercury. Details regarding the information summarized here can be found in the papers associated with this Virtual Special Issue of STOTEN.

Comparison of co-located ice-core and tree-ring mercury records indicates potential radial translocation of mercury in whitebark pine.

Tree-ring records are a potential archive for reconstructing long-term historical trends in atmospheric mercury (Hg) concentrations. Although Hg preserved in tree rings has been shown to be derived largely from the atmosphere, quantitative relationships linking atmospheric concentrations to those in tree rings are limited. In addition, few tree-ring-based Hg records have been evaluated against co-located proxies of atmospheric Hg deposition or direct atmospheric measurements. Here we develop long-term Hg records extending from 1800 to 2018 CE using cores collected from two stands of whitebark pine located near the Upper Fremont Glacier in the Wind River Range, Wyoming, where a long-term record of atmospheric Hg deposition previously was developed from an ice core. The tree ring record showed that Hg concentrations increased beginning in 1800 CE to a broad peak centered at ~1960 CE, before decreasing to present, generally paralleling the ice-core record of Hg deposition. The exact timing and magnitude of the Hg increases in the trees, however, is offset earlier relative to the ice-core record. These discrepancies potentially arise from biotic processes that impact Hg uptake and preservation in whitebark pine, and results from an advection-diffusion model indicate that the temporal differences are consistent with radial movement of Hg within the trees. The forms of atmospheric Hg and seasonality may also impact the Hg record preserved by each archive, but are less likely to affect long-term trends. Further work is needed to assess radial Hg translocation in more controlled studies with larger sample sizes.

Use of Multiple Lines of Evidence to Understand Reactive Mercury Concentrations and Chemistry in Hawai'i, Nevada, Maryland, and Utah, USA.

To advance our understanding of the mercury (Hg) biogeochemical cycle, concentrations and chemistry of gaseous oxidized Hg (GOM), particulate-bound Hg (PBM), and reactive Hg (RM = GOM + PBM) need to be known. The UNR-RMAS 2.0 provides a solution that will advance knowledge. From 11/2017 to 02/2019, the RMAS 2.0 was deployed in Hawai'i, Nevada, Maryland, and Utah to test system performance and develop an understanding of RM at locations impacted by different atmospheric oxidants. Mauna Loa Observatory, Hawai'i, impacted by the free troposphere and the marine boundary layer, had primarily -Br/Cl RM compounds. The Nevada location, directly adjacent to a major interstate highway and experiences inputs from the free troposphere, exhibited -Br/Cl, -N, -S, and organic compounds. In Maryland, compounds observed were -N, -S, and organic-Hg. This site is downwind of coal-fired power plants and located in a forested area. The location in Utah is in a basin impacted by oil and natural gas extraction, multiday wintertime inversion episodes, and inputs from the free troposphere. Compounds were -Br/Cl or -O, -N, and -Br/Cl. The chemical forms of RM identified were consistent with the air source areas, predominant ion chemistry, criterion air pollutants, and meteorology.

Comparison of 4 Methods for Measurement of Reactive, Gaseous Oxidized, and Particulate Bound Mercury.

The atmosphere is an important (1) pathway by which mercury (Hg) is transported around the globe and (2) source of Hg to ecosystems. Thus, understanding Hg atmospheric chemistry is critical for understanding the biogeochemical cycle and impacts to human and ecosystem health. Work over the past 13 years has demonstrated that the standard instrument used to measure atmospheric Hg does not accurately quantify gaseous oxidized mercury (GOM) or particulate bound mercury (PBM). This study focused on comparing four methods for quantifying atmospheric Hg and identifying Hg(II) compounds. Data from two automated systems, the Tekran 2537/1130 system and the University of Nevada, Reno-Dual Channel System (DCS), were compared with two University of Nevada, Reno-Reactive Mercury Active Systems (RMAS 2.0). One RMAS 2.0 included cation exchange membranes (CEMs) and nylon membranes, and the second included a polytetrafluoroethylene (PTFE) membrane upstream of the CEM and nylon membranes. The Tekran system and the DCS underestimated GOM concentrations with respect to that measured using the RMAS 2.0. The RMAS 2.0 with the upstream PTFE provided a means of distinguishing GOM and PBM. Thermal desorption of nylon membrane data identified a variety of GOM and PBM compounds present.

What is in commercial cat and dog food? The case for mercury and ingredient testing.

Commercial pet foods should be safe for long-term feeding. However, recent recalls and lawsuits have eroded public trust in pet food companies and products. Recent studies have identified high concentrations of mercury, a potent neurotoxin, in pet food products. Here we posit that pet food products require independent testing to verify safety and compliance with developed Food and Drug Administration and Association of American Feed Control Officials standards, and initiate a discussion as to why including quantification of mercury and methylmercury, as well as the identification of adulteration, are important to such testing protocols. The outcomes of these discussions will be multi-faceted: initiating the impetus to investigate the quality and label accuracy of pet foods; ensuring product safety; promoting transparency within the pet food industry; informing veterinary practices regarding pet food recommendations; providing data for evidence-based policy and regulatory enforcement; and working toward fulfilling the National Research Council's call for research that identifies levels of contaminants in animal feeds and residues in human foods.

Assessment of the Suitability of Tree Rings as Archives of Global and Regional Atmospheric Mercury Pollution.

This study investigated the methodology and utility of dendrochemistry in the assessment of spatial and temporal concentrations of gaseous elemental mercury. Tree cores from the Pinus species in California and Nevada, U.S.A. were collected from previously sampled areas to test the stability of tree ring concentrations over time. Cores were collected from 2 new locations to assess spatial variability among trees within and between stands located at two elevations in the same watershed. Results indicated that using 2 to 3 cores from ∼10 or more trees provided the best framework for understanding tree ring concentrations within a population of trees located in uncontaminated areas. At the least 2 sides of a tree should be cored to account for radially asymmetric variations associated with growing conditions or injury. An agreement of concentrations and trends measured in trees cored with previous research indicated that tree rings are suitable proxies for historical air mercury concentrations and that mercury concentrations have increased since the Industrial Revolution. Data collected demonstrate that tree rings record regional gradients in GEM concentrations. In addition, temporal consistency may vary within a geographic location due to differing biotic and abiotic factors influencing ring growth since trees are active samplers of atmospheric Hg.

Use of multiple tools including lead isotopes to decipher sources of ozone and reactive mercury to urban and rural locations in Nevada, USA.

Ambient air particulate matter (<2.5µm in diameter) samples were collected on two different filter types in 2014 and 2015 over 24h periods and analyzed for reactive mercury (gaseous oxidized mercury+particulate bound mercury) concentrations and lead isotopes to determine sources of pollution to three sites in Nevada, USA. Two sites were located on the western edge of Nevada (Reno, urban, 1370m and Peavine Peak, rural, high elevation, 2515m); the third location was ~485km east in rural Great Basin National Park, NV (2061m). Reactive mercury samples were collected on cation exchange membranes simultaneously with lead samples, collected on Teflon membranes. Lead isotopic ratios have previously identified trans-Pacific lead sources based on the 206/207 and 208/207 lead ratios. Influence from trans-Pacific air masses was higher from March to June associated with long-range transport of pollutants. Spring months are well known for increased transport across the Pacific; however, fall months were also influenced by trans-Pacific air masses in this study. Western North American background ozone concentrations have been measured and modeled at 50 to 55ppbv. Median ozone concentrations at both rural sites in Nevada were within this range. Sources leading to enhancements in ozone of 2 to 18ppbv above monthly medians in Nevada included emissions from Eurasia, regional urban centers, and global and regional wildfires, resulting in concentrations close to the USA air quality standard. At the high elevation locations, ozone was derived from pollutants being transported in the free troposphere that originate around the globe; however, Eurasia and Asia were dominant sources to the Western USA. Negative correlations between reactive mercury and percent Asian lead, Northern Eurasia and East Asia trajectories indicated reactive mercury concentrations at the two high elevation sites were produced by oxidants from local, regional, and marine boundary layer sources.

Evidence for Nonstomatal Uptake of Hg by Aspen and Translocation of Hg from Foliage to Tree Rings in Austrian Pine.

To determine whether trees are reliable biomonitors of air mercury (Hg) pollution concentrations were measured in bark, foliage, and tree rings. Data were developed using 4-year old Pinus and Populus trees grown from common genetic stock in Oregon and subsequently transferred to four air treatments differing in gaseous oxidized mercury (GOM) chemistry and total gaseous Hg (TGM) concentrations. Soil of a subset of trees was spiked with HgBr2 in solution to test for root uptake. Results indicate no significant effect of the soil spike or GOM compounds on tree tissue Hg concentrations. TGM treatment had a significant effect on Pinus and Populus foliage, and Pinus year 5 growth ring concentrations. Populus foliar Hg concentrations were highest in the exposure where 24 h TGM concentrations were highest, indicating the importance of the nonstomatal pathway for uptake. Pinus tree ring concentrations were correlated to day time TGM concentrations suggesting Hg accumulation into tree rings is by way of the stomata and subsequent translocation by way of phloem. Populus leaves and Pinus rings can be used as biomonitors for TGM concentrations over space. However, the use of trees as temporal proxies requires further investigation due to radial translocation observed in active sapwood tree rings.

Development of a Particulate Mass Measurement System for Quantification of Ambient Reactive Mercury.

The Teledyne Advanced Pollution Instrumentation (TAPI) model 602 BetaPlus particulate system provides nondestructive analysis of particulate matter (PM2.5) mass concentration. This instrument was used to determine if measurements made with cation exchange membranes (CEM) were comparable to standard methods, the ß attenuation method at two locations in Reno, NV and an environmental ß attenuation method and gravimetric method at Great Basin National Park, NV. TAPI PM2.5 CEM measurements were statistically similar to the other three PM2.5 methods. Once this was established, the second objective, a destructive method for measurement of reactive mercury (RM = gaseous oxidized and particulate bound Hg), was tested. Samples collected at 16.7 L per min (Lpm) for 24 h on CEM from the TAPI were compared to those measured by the University of Nevada, Reno-Reactive Mercury Active System (UNRRMAS, 1 Lpm) CEM and a Tekran 2537/1130/1135 system (7 Lpm). Given the use of CEM in the TAPI and UNRRMAS, we hypothesized that both should collect RM. Due to the high flow rate and different inlets, TAPI data were systematically lower than the UNRRMAS. Correlation between RM concentrations demonstrated that the TAPI may be used to estimate 24 h resolution RM concentrations in Nevada.