Do we understand what the mercury speciation instruments are actually measuring? Results of RAMIX

Anthropogenic short-lived halogens increase human exposure to mercury contamination due to enhanced mercury oxidation over continents

Mercury (Hg) is a contaminant of global concern, and an accurate understanding of its atmospheric fate is needed to assess its risks to humans and ecosystem health. Atmospheric oxidation of Hg is key to the deposition of this toxic metal to the Earth's surface. Short-lived halogens (SLHs) can provide halogen radicals to directly oxidize Hg and perturb the budget of other Hg oxidants (e.g., OH and O3). In addition to known ocean emissions of halogens, recent observational evidence has revealed abundant anthropogenic emissions of SLHs over continental areas. However, the impacts of anthropogenic SLHs emissions on the atmospheric fate of Hg and human exposure to Hg contamination remain unknown. Here, we show that the inclusion of anthropogenic SLHs substantially increased local Hg oxidation and, consequently, deposition in/near Hg continental source regions by up to 20%, thereby decreasing Hg export from source regions to clean environments. Our modeling results indicated that the inclusion of anthropogenic SLHs can lead to higher Hg exposure in/near Hg source regions than estimated in previous assessments, e.g., with increases of 8.7% and 7.5% in China and India, respectively, consequently leading to higher Hg-related human health risks. These results highlight the urgent need for policymakers to reduce local Hg and SLHs emissions. We conclude that the substantial impacts of anthropogenic SLHs emissions should be included in model assessments of the Hg budget and associated health risks at local and global scales.

Application of traceable calibration for gaseous oxidized mercury in air

BACKGROUND

The current speciation methods for mercury (Hg) measurements are fraught with considerable uncertainty, from sample collection to calibration. High reactivity of gaseous oxidized Hg (GOM) species and their ultra-trace level presence makes them difficult to sample and calibrate. Given that improper calibration may lead to measurement biases, reliable and metrologically traceable calibration methods are required for accurately quantifying GOM in air. In the present study, we applied the recently developed calibration method based on non-thermal plasma oxidation of elemental Hg, to a commercially available Hg air speciation system for actual environmental measurements of GOM for the first time.

RESULTS

Hg species such as HgO, HgCl2, and HgBr2 were produced with trace amounts of reactant gases (oxygen and electrolytically produced chlorine and bromine) and the production was driven by plasma-assisted oxidation. The plasma oxidation efficiency of elemental Hg with oxygen was 98.5 ± 7.5 % (k = 2), while that for chlorine and bromine was 96.8 ± 6.9 % (k = 2) and 97.4 ± 9.6 % (k = 2), respectively. The calibration method was tested against the internal permeation (Hg0) source of the Tekran 2537B Hg analyzer on-field by loading HgO to different KCl-coated denuders using the plasma. GOM concentrations were measured using the Tekran speciation system. With internal calibration, concentrations were up to 9.1 % lower than those in plasma calibration, thereby emphasizing the importance of the calibration strategy. Measurement uncertainty (k = 2) further emphasizes this distinction. Internal calibration measurement uncertainty was 36.8 %, while plasma calibration boasted lower uncertainty at 13.8 %.

SIGNIFICANCE

The non-thermal plasma calibration strategy, as a unique and discrete calibration method traceable to the NIST SRM 3133 for ambient air GOM measurements, provide a higher level of confidence in the accuracy of GOM measurements with several advantages over other methods. Calibrations at extreme low concentrations (<100 pg) are possible with this method relevant to ambient air GOM concentrations.

What is the utility of measuring gaseous HgII dry deposition using Aerohead samplers?: A review

The most efficient way to quantify HgII inputs to ecosystems is to measure wet and dry deposition. Wet deposition of HgII is determined by measuring Hg concentrations and the volume of precipitation. Dry deposition of HgII is determined through direct measurement and/or determined indirectly by measuring air concentrations and using model-generated deposition velocities. Here, data collected using an Aerohead sampler holding cation exchange membranes are summarized, and the utility of this method for understanding dry deposition, and other measurements and processes is discussed. This analysis includes information from publications, and recent data collected at Guadalupe Mountains National Park, Texas, USA, and Amsterdam Island, Southern Indian Ocean. This method primarily measures gaseous HgII and little particulate-bound Hg. The Aerohead method is useful for looking at large-scale trends in deposition, verifying Hg depletion events, calculating dry deposition velocities for compounds with specific chemistry, and identification of sources of HgII. At numerous locations in the western USA, deposition rates were greater at higher elevations due to elevated concentrations associated with long-range transport of atmospheric pollution. When used in tandem with the Reactive Mercury Active System or a dual-channel system, more accurate deposition velocities - that vary as a function of GOM compound chemistry - can be calculated.

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.

Over a decade of atmospheric mercury monitoring at Amsterdam Island in the French Southern and Antarctic Lands

The Minamata Convention, a global and legally binding treaty that entered into force in 2017, aims to protect human health and the environment from harmful mercury (Hg) effects by reducing anthropogenic Hg emissions and environmental levels. The Conference of the Parties is to periodically evaluate the Convention's effectiveness, starting in 2023, using existing monitoring data and observed trends. Monitoring atmospheric Hg levels has been proposed as a key indicator. However, data gaps exist, especially in the Southern Hemisphere. Here, we present over a decade of atmospheric Hg monitoring data at Amsterdam Island (37.80°S, 77.55°E), in the remote southern Indian Ocean. Datasets include gaseous elemental and oxidised Hg species ambient air concentrations from either active/continuous or passive/discrete acquisition methods, and annual total Hg wet deposition fluxes. These datasets are made available to the community to support policy-making and further scientific advancements.

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.

Impact of forest fire on the mercury stable isotope composition in litter and soil in the Amazon

Mercury (Hg) emissions from forest fires, especially tropical forests such as the Amazonian forest, were shown to contribute significantly to the atmospheric mercury budget, but new methods are still necessary to improve the traceability and to reduce the great uncertainties related to this emission source. Recent studies have shown that the combustion process can result in Hg stable isotope fractionation that allows tracking coal combustion Hg emissions, as influenced by different factors such as combustion temperature. The main goal of the present study was, therefore, to investigate for the first time the potential of Hg stable isotopes to trace forest fire Hg emissions and pathways. More specifically, small-scale and a large scale prescribed forest fire experiments were conducted in the Brazilian Amazonian forest to study the impact of fire severity on Hg isotopic composition of litter, soil, and ash samples and associated Hg isotope fractionation pathways. In the small-scale experiment, no difference was found in the mercury isotopic composition of the samples collected before and after burning. In contrast, the larger-scale experiment resulted in significant mass dependent fractionation (MDF δ202Hg) in soils and ash suggesting that higher combustion temperature influence Hg isotopic fractionation with the emission of lighter Hg isotopes to the atmosphere and enrichment with heavier Hg in ashes. As for coal combustion, mass independent fractionation was not observed. To our knowledge, these results are the first to highlight the potential of forest fires to cause Hg isotopic fractionation, depending on the fire severity. The results also allowed to establish an isotopic fingerprint for tropical forest fire Hg emissions that corresponds to a mixture of litter and soil Hg isotopic composition (resulting atmospheric δ202Hg, Δ200Hg and Δ199Hg were -1.79 ± 0.24‰, -0.05 ± 0.04‰ and -0.45 ± 0.12‰, respectively).

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.

A Review of Dry Deposition Schemes for Speciated Atmospheric Mercury

This study reviewed the existing framework of dry deposition schemes for speciated atmospheric mercury. As the most commonly used methods for mercury dry deposition estimation, the big-leaf resistance scheme for gaseous oxidized mercury (GOM), the size distribution regarded resistance scheme for particulate bound mercury (PBM), and the bidirectional air-surface exchange scheme for gaseous elemental mercury (GEM) were introduced in detail. Sensitivity analysis were conducted to quantitatively identify the key parameters for the estimation of speciated mercury dry deposition velocities. The dry deposition velocity of GOM was found to be sensitive to the wind speed and some land use related parameters. The chemical forms of GOM could have a significant impact on the dry deposition velocity. The dry deposition velocity of PBM is sensitive to the mass fraction of PBM in coarse particles, while that of GEM is most sensitive to air temperature. Future research needs were proposed accordingly.

Dispersion of airborne mercury species emitted from the cement plant

The cement industry is the second largest source of anthropogenic mercury (Hg) emissions in Europe, accounting for 11% of global anthropogenic Hg emissions. The main objective of this study was to examine the influence of Hg emissions from the Salonit Anhovo cement plant on Hg levels measured in the ambient air at Vodarna, 1 km downwind from the flue gas chimney. The findings reveal that the plant raw mill operational status plays an important role in Hg concentrations in the flue gas emitted from the plant. Emitted total gaseous mercury was, on average, higher (49.4 μg/m³) when raw mills were in the direct mode (both raw mills-off) and lower (23.4 μg/m³) in the combined mode (both raw mills-on). The average Hg concentrations in Vodarna were 3.14 ng/m³ for gaseous elemental mercury, 53.7 pg/m³ for gaseous oxidised mercury, and 41.9 pg/m³ for particulate bound mercury for the whole measurement period. Atmospheric Hg speciation in Vodarna, coupled with plant emissions and wind data, has revealed that the total gaseous mercury emitted from the cement plant is clearly related to all Hg species measured in Vodarna. Wind blowing from the northeastern quadrant (mostly NE, ENE) is responsible for the elevated Hg levels in Vodarna, where gaseous oxidised mercury levels are highly linked to the cement plant emissions. However, elevated levels of Hg species in the absence of northeastern winds indicate potential inputs from other unknown local sources as well as inputs from regional and global transport mechanisms.

Updated trends for atmospheric mercury in the Arctic: 1995-2018

The Arctic region forms a unique environment with specific physical, chemical, and biological processes affecting mercury (Hg) cycles and limited anthropogenic Hg sources. However, historic global emissions and long range atmospheric transport has led to elevated Hg in Arctic wildlife and waterways. Continuous atmospheric Hg measurements, spanning 20 years, and increased monitoring sites has allowed a more comprehensive understanding of how Arctic atmospheric mercury is changing over time. Time-series trend analysis of TGM (Total Gaseous Mercury) in air was performed from 10 circumpolar air monitoring stations, comprising of high-Arctic, and sub-Arctic sites. GOM (gaseous oxidised mercury) and PHg (particulate bound mercury) measurements were also available at 2 high-Arctic sites. Seasonal mean TGM for sub-Arctic sites were lowest during fall ranging from 1.1 ng m^-3 Hyytiälä to 1.3 ng m^-3, Little Fox Lake. Mean TGM concentrations at high-Arctic sites showed the greatest variability, with highest daily means in spring ranging between 4.2 ng m^-3 at Amderma and 2.4 ng m^-3 at Zeppelin, largely driven by local chemistry. Annual TGM trend analysis was negative for 8 of the 10 sites. High-Arctic seasonal TGM trends saw smallest decline during summer. Fall trends ranged from -0.8% to -2.6% yr^-1. Across the sub-Arctic sites spring showed the largest significant decreases, ranging between -7.7% to -0.36% yr^-1, while fall generally had no significant trends. High-Arctic speciation of GOM and PHg at Alert and Zeppelin showed that the timing and composition of atmospheric mercury deposition events are shifting. Alert GOM trends are increasing throughout the year, while PHg trends decreased or not significant. Zeppelin saw the opposite, moving towards increasing PHg and decreasing GOM. Atmospheric mercury trends over the last 20 years indicate that Hg concentrations are decreasing across the Arctic, though not uniformly. This is potentially driven by environmental change, such as plant productivity and sea ice dynamics.

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.

Elevated Gaseous Oxidized Mercury Revealed by a Newly Developed Speciated Atmospheric Mercury Monitoring System

Gaseous oxidized mercury (Hg²⁺) monitoring is one of the largest challenges in the mercury research field, where existing methods cannot simultaneously satisfy the measurement requirements of both accuracy and time precision, especially in high-particulate environments. Here, we verified that dual-stage cation exchange membrane (CEM) sampler is incapable of gaseous elemental mercury (Hg⁰) uptake even if particulate matter is trapped on CEM, whereas the Hg²⁺ capture efficiency of the sampler is more than 90%. We then developed a Cation Exchange Membrane-Coupled Speciated Atmospheric Mercury Monitoring System (CSAMS) by coupling the dual-stage CEM sampler with the commercial Tekran 2537/1130/1135 system and configuring a new sampling and analysis procedure, so as to improve the monitoring accuracy of Hg²⁺ and ensure the simultaneous measurement of Hg⁰, Hg²⁺, and Hg_p in 2 h time resolution. We deployed the CSAMS in urban Beijing in September 2021 and observed an unprecedented elevated Hg²⁺ during the daytime with an average amplitude of 510 pg m^-3. Using a zero-dimensional box model, the elevated Hg²⁺ production rate was attributed to high atmospheric oxidant concentrations, Hg⁰ heterogeneous and interfacial oxidation processes on the surface of atmospheric particles, or potential unknown oxidants.

Calibration Approach for Gaseous Oxidized Mercury Based on Nonthermal Plasma Oxidation of Elemental Mercury

Atmospheric mercury measurements carried out in the recent decades have been a subject of bias largely due to insufficient consideration of metrological traceability and associated measurement uncertainty, which are ultimately needed for the demonstration of comparability of the measurement results. This is particularly challenging for gaseous Hg^II species, which are reactive and their ambient concentrations are very low, causing difficulties in proper sampling and calibration. Calibration for atmospheric Hg^II exists, but barriers to reliable calibration are most evident at ambient Hg^II concentration levels. We present a calibration of Hg^II species based on nonthermal plasma oxidation of Hg⁰ to Hg^II. Hg⁰ was produced by quantitative reduction of Hg^II in aqueous solution by SnCl₂ and aeration. The generated Hg⁰ in a stream of He and traces of reaction gas (O₂, Cl₂, or Br₂) was then oxidized to different Hg^II species by nonthermal plasma. A highly sensitive ¹⁹⁷Hg radiotracer was used to evaluate the oxidation efficiency. Nonthermal plasma oxidation efficiencies with corresponding expanded standard uncertainty values were 100.5 ± 4.7% (k = 2) for 100 pg of HgO, 96.8 ± 7.3% (k = 2) for 250 pg of HgCl₂, and 77.3 ± 9.4% (k = 2) for 250 pg of HgBr₂. The presence of HgO, HgCl₂, and HgBr₂ was confirmed by temperature-programmed desorption quadrupole mass spectrometry (TPD-QMS). This work demonstrates the potential for nonthermal plasma oxidation to generate reliable and repeatable amounts of Hg^II compounds for routine calibration of ambient air measurement instrumentation.

Atmospheric mercury sources in a coastal-urban environment: a case study in Boston, Massachusetts, USA

Mercury (Hg) is an environmental toxicant dangerous to human health and the environment. Its anthropogenic emissions are regulated by global, regional, and local policies. Here, we investigate Hg sources in the coastal city of Boston, the third largest metropolitan area in the Northeastern United States. With a median of 1.37 ng m^-3, atmospheric Hg concentrations measured from August 2017 to April 2019 were at the low end of the range reported in the Northern Hemisphere and in the range reported at North American rural sites. Despite relatively low ambient Hg concentrations, we estimate anthropogenic emissions to be 3-7 times higher than in current emission inventories using a measurement-model framework, suggesting an underestimation of small point and/or nonpoint emissions. We also test the hypothesis that a legacy Hg source from the ocean contributes to atmospheric Hg concentrations in the study area; legacy emissions (recycling of previously deposited Hg) account for ∼60% of Hg emitted annually worldwide (and much of this recycling takes place through the oceans). We find that elevated concentrations observed during easterly oceanic winds can be fully explained by low wind speeds and recirculating air allowing for accumulation of land-based emissions. This study suggests that the influence of nonpoint land-based emissions may be comparable in size to point sources in some regions and highlights the benefits of further top-down studies in other areas.

Biomonitoring of Hg0, Hg2 and Particulate Hg in a Mining Context Using Tree Barks. INTERNATIONAL JOURNAL OF ENVIRONMENTAL RESEARCH AND PUBLIC HEALTH 2021;18:5191. [PMID: 34068268 PMCID: PMC8153109 DOI: 10.3390/ijerph18105191]

The biomonitoring of atmospheric mercury (Hg) is an important topic in the recent scientific literature given the cost-benefit advantage of obtaining indirect measurements of gaseous Hg using biological tissues. Lichens, mosses, and trees are the most commonly used organisms, with many standardized methods for some of them used across European countries by scientists and pollution regulators. Most of the species used the uptake of gaseous Hg (plant leaves), or a mixture of gaseous and particulate Hg (mosses and lichens), but no method is capable of differentiating between main atmospheric Hg phases (particulate and gaseous), essential in a risk assessment. The purpose of this work was to evaluate different uptake patterns of biological tissues in terms of atmospheric Hg compounds. To accomplish this, the feasibility of two plant tissues from a tree commonly found in urban environments has been evaluated for the biomonitoring of gaseous Hg species in a Hg mining environment. Sampling included leaves and barks from Platanus hispanica and particulate matter from the atmosphere of the urban area around Almadén (south-central Spain), while analytical determinations included data for total Hg concentrations in biological and geological samples, Hg speciation data and total gaseous Hg (TGM). The results allowed us to identify the main Hg compounds in leaves and bark tissues and in atmospheric particulate matter, finding that leaves bioaccumulated only gaseous Hg (Hg0 and Hg2+), preferably during daylight hours, whereas the barks accumulated a combination of TGM and particulate bound Hg (PBM) during the day and at night. Subsequent merging of the atmospheric Hg speciation data obtained from leaves and barks allowed indicative maps of the main sources of TGM and PBM emissions to be obtained, thereby perfectly delimiting the main TGM and PBM sources in the urban area around Almadén. This method complements TGM biomonitoring systems already tested with other urban trees, adding the detection of PBM emission sources and, therefore, biomonitoring all Hg species present in the atmosphere. Scenarios other than mining sites should be evaluated to determine the utility of this method for Hg biospeciation in the atmosphere.

Validating an Evaporative Calibrator for Gaseous Oxidized Mercury

Understanding atmospheric mercury chemistry is the key for explaining the biogeochemical cycle of mercury and for improving the predictive capability of computational models. Increased efforts are being made to ensure comparable Hg speciation measurements in the air through establishing metrological traceability. While traceability for elemental mercury has been recently set, this is by no means the case for gaseous oxidized mercury (GOM). Since a calibration unit suitable for traceable GOM calibrations based on evaporation of HgCl₂ solution was recently developed, the purpose of our work was to extensively evaluate its performance. A highly specific and sensitive ¹⁹⁷Hg radiotracer was used for validation over a wide range of concentrations. By comparing experimental and calculated values, we obtained recoveries for the calibration unit. The average recoveries ranged from 88.5% for 1178 ng m⁻³ HgCl₂ gas concentration to 39.4% for 5.90 ng m⁻³ HgCl₂ gas concentration. The losses were due to the adsorption of oxidized Hg on the inner walls of the calibrator and tubing. An adsorption isotherm was applied to estimate adsorption enthalpy (ΔH_ads); a ΔH_ads value of −12.33 kJ mol⁻¹ was obtained, suggesting exothermal adsorption. The results of the calibrator performance evaluation suggest that a newly developed calibration unit is only suitable for concentrations of HgCl₂ higher than 1 µg m⁻³. The concentration dependence of recoveries prevents the system from being used for calibration of instruments for ambient GOM measurements. Moreover, the previously assessed uncertainty of this unit at µg m⁻³ level (2.0%, k = 2) was re-evaluated by including uncertainty related to recovery and was found to be 4.1%, k = 2. Calibrator performance was also evaluated for HgBr₂ gas calibration; the recoveries were much lower for HgBr₂ gas than for HgCl₂ gas even at a high HgBr₂ gas concentration (>1 µg m⁻³). As HgBr₂ is often used as a proxy for various atmospheric HgBr species, the suitability of the unit for such calibration must be further developed.

Development of an Understanding of Reactive Mercury in Ambient Air: A Review

This review focuses on providing the history of measurement efforts to quantify and characterize the compounds of reactive mercury (RM), and the current status of measurement methods and knowledge. RM collectively represents gaseous oxidized mercury (GOM) and that bound to particles. The presence of RM was first recognized through measurement of coal-fired power plant emissions. Once discovered, researchers focused on developing methods for measuring RM in ambient air. First, tubular KCl-coated denuders were used for stack gas measurements, followed by mist chambers and annular denuders for ambient air measurements. For ~15 years, thermal desorption of an annular KCl denuder in the Tekran® speciation system was thought to be the gold standard for ambient GOM measurements. Research over the past ~10 years has shown that the KCl denuder does not collect GOM compounds with equal efficiency, and there are interferences with collection. Using a membrane-based system and an automated system—the Detector for Oxidized mercury System (DOHGS)—concentrations measured with the KCl denuder in the Tekran speciation system underestimate GOM concentrations by 1.3 to 13 times. Using nylon membranes it has been demonstrated that GOM/RM chemistry varies across space and time, and that this depends on the oxidant chemistry of the air. Future work should focus on development of better surfaces for collecting GOM/RM compounds, analytical methods to characterize GOM/RM chemistry, and high-resolution, calibrated measurement systems.

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.

Traceable Determination of Atmospheric Mercury Using Iodinated Activated Carbon Traps

Traceable determination of atmospheric mercury (Hg) represents a major analytical problem due to low environmental concentrations. Although Hg pre-concentration on activated carbon (AC) traps is a simple method for sample collection, Hg determination is difficult due to a complex matrix that cannot be easily digested using wet chemistry. Two approaches for Hg loading on iodinated AC, the purging of elemental mercury (Hg0) and the spiking a solution of standard reference material (SRM), were used to test whether spiking SRM solution on AC can be used for the traceable determination of atmospheric mercury collected as Hg0. Mercury on AC was determined using atomic absorption spectrometry after sample combustion. The detector’s response for both loading methods was identical in a wide concentration range, indicating that the spiking of SRM on AC can, indeed, be used for the calibration of analytical systems used for the determination of atmospheric mercury. This was confirmed by the determination of Hg in a real atmospheric sample collected on an iodinated AC trap and using an SRM spiking calibration. Different ACs were compared regarding their ability to quantitatively capture Hg while having the lowest breakthrough. Use of a specific impregnating solution probably converted Hg on AC to Millon’s iodide, as estimated from the fractionation thermogram.

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.

Recent advances in understanding and measurement of Hg in the environment: Surface-atmosphere exchange of gaseous elemental mercury (Hg0)

The atmosphere is the major transport pathway for distribution of mercury (Hg) globally. Gaseous elemental mercury (GEM, hereafter Hg⁰) is the predominant form in both anthropogenic and natural emissions. Evaluation of the efficacy of reductions in emissions set by the UN's Minamata Convention (UN-MC) is critically dependent on the knowledge of the dynamics of the global Hg cycle. Of these dynamics including e.g. red-ox reactions, methylation-demethylation and dry-wet deposition, poorly constrained atmosphere-surface Hg⁰ fluxes especially limit predictability of the timescales of its global biogeochemical cycle. This review focuses on Hg⁰ flux field observational studies, namely the theory, applications, strengths, and limitations of the various experimental methodologies applied to gauge the exchange flux and decipher active sub-processes. We present an in-depth review, a comprehensive literature synthesis, and methodological and instrumentation advances for terrestrial and marine Hg⁰ flux studies in recent years. In particular, we outline the theory of a wide range of measurement techniques and detail the operational protocols. Today, the most frequently used measurement techniques to determine the net Hg⁰ flux (>95% of the published flux data) are dynamic flux chambers for small-scale and micrometeorological approaches for large-scale measurements. Furthermore, top-down approaches based on Hg⁰ concentration measurements have been applied as tools to better constrain Hg emissions as an independent way to e.g. challenge emission inventories. This review is an up-dated, thoroughly revised edition of Sommar et al. 2013 (DOI: 10.1080/10643389.2012.671733). To the tabulation of >100 cited flux studies 1988-2009 given in the former publication, we have here listed corresponding studies published during the last decade with a few exceptions (2008-2019). During that decade, Hg stable isotope ratios of samples involved in atmosphere-terrestrial interaction is at hand and provide in combination with concentration and/or flux measurements novel constraints to quantitatively and qualitatively assess the bi-directional Hg⁰ flux. Recent efforts in the development of relaxed eddy accumulation and eddy covariance Hg⁰ flux methods bear the potential to facilitate long-term, ecosystem-scale flux measurements to reduce the prevailing large uncertainties in Hg⁰ flux estimates. Standardization of methods for Hg⁰ flux measurements is crucial to investigate how land-use change and how climate warming impact ecosystem-specific Hg⁰ sink-source characteristics and to validate frequently applied model parameterizations describing the regional and global scale Hg cycle.

Recent advances in understanding and measurement of mercury in the environment: Terrestrial Hg cycling

This review documents recent advances in terrestrial mercury cycling. Terrestrial mercury (Hg) research has matured in some areas, and is developing rapidly in others. We summarize the state of the science circa 2010 as a starting point, and then present the advances during the last decade in three areas: land use, sulfate deposition, and climate change. The advances are presented in the framework of three Hg "gateways" to the terrestrial environment: inputs from the atmosphere, uptake in food, and runoff with surface water. Among the most notable advances: These and other advances reported here are of value in evaluating the effectiveness of the Minamata Convention on reducing environmental Hg exposure to humans and wildlife.

An updated review of atmospheric mercury

The atmosphere is a key component of the biogeochemical cycle of mercury, acting as a reservoir, transport mechanism, and facilitator of chemical reactions. The chemical and physical behavior of atmospheric mercury determines how, when, and where emitted mercury pollution impacts ecosystems. In this review, we provide current information about what is known and what remains uncertain regarding mercury in the atmosphere. We discuss new ambient, laboratory, and theoretical information about the chemistry of mercury in various atmospheric media. We review what is known about mercury in and on solid- and liquid-phase aerosols. We present recent findings related to wet and dry deposition and spatial and temporal trends in atmospheric mercury concentrations. We also review atmospheric measurement methods that are in wide use and those that are currently under development.

Long-Term Observations of Atmospheric Speciated Mercury at a Coastal Site in the Northern Gulf of Mexico during 2007–2018

Atmospheric mercury species (gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), and particulate-bound mercury (PBM)), trace pollutants (O3, SO2, CO, NO, NOY, and black carbon), and meteorological parameters have been continuously measured since 2007 at an Atmospheric Mercury Network (AMNet) site that is located on the northern coast of the Gulf of Mexico in Moss Point, Mississippi. For the data that were collected between 2007 and 2018, the average concentrations and standard deviations are 1.39 ± 0.22 ng m−3 for GEM, 5.1 ± 10.2 pg m−3 for GOM, 5.9 ± 13.0 pg m−3 for PBM, and 309 ± 407 ng m−2 wk−1 for mercury wet deposition, with interannual trends of −0.009 ng m−3 yr−1 for GEM, −0.36 pg m−3 yr−1 for GOM, 0.18 pg m−3 yr−1 for PBM, and 2.8 ng m−2 wk−1 yr−1 for mercury wet deposition. The diurnal variation of GEM shows lower concentrations in the early morning due to GEM depletion, likely due to plant uptake in high humidity events and slight elevation during the day, likely due to downward mixing to the surface of higher concentrations of GEM in the air aloft. The seasonal variation of GEM shows higher levels in winter and spring and lower levels in summer and fall. Diurnal variations of both GOM and PBM show broad peaks in the afternoon likely due to the photochemical oxidation of GEM. Seasonally, PBM measurements exhibit higher levels in winter and early spring and lower levels in summer with rising levels in fall, while GOM measurements show high levels in late spring/early summer and late fall and low levels in winter. The seasonal variation of mercury wet deposition shows higher values in summer and lower values in winter, due to larger rainfall amounts in summer than in winter. As expected, anticorrelation between mercury wet deposition and the sum of GOM and PBM, but positive correlation between mercury wet deposition and rainfall were observed. Correlation among GOM, ozone, and SO2 suggests possible different GOM sources: direct emissions and photochemical oxidation of GEM, with the possible influence of boundary layer dynamics and seasonal variability. This study indicates that the monitoring site experiences are impacted from local and regional mercury sources as well as large scale mercury cycling phenomena.

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.

Concentrations and gas-particle partitioning of atmospheric reactive mercury at an urban site in Beijing, China

Measurements of speciated atmospheric mercury play a key role in identifying mercury behavior in the atmosphere. In this study, we measured speciated atmospheric mercury, including gaseous elemental mercury (GEM), reactive gaseous mercury (RGM), and particulate bound mercury (PBM) (<2.5 μm), in 2015 and 2016 at an urban site in Beijing, China. The mean concentrations of GEM, RGM, and PBM were 4.70 ± 3.53 ng m^-3, 18.47 ± 22.27 pg m^-3, and 85.18 ± 95.34 pg m^-3, respectively. The concentration of PM_2.5 significantly affected the distribution of reactive mercury between the gaseous and particulate phases. With the raising of PM_2.5 levels, PBM concentrations increased, on the contrary, the concentrations of RGM decreased gradually. The mean concentration of PBM during air-pollution events was more than three times that during clear days. During days with air pollution, the relative humidity significantly affected the gas-particle partitioning of reactive mercury. The linear relationships between gas-particle partitioning coefficient and meteorological factors (air temperature and relative humidity) were obtained over the four seasons. The data also showed that the gas-particle partitioning coefficient of reactive mercury was related to particle composition (e.g., Cl^-, BC). The data present in this paper suggested the influence of anthropogenic emissions on reactive mercury in Beijing urban. And the findings will contribute to understand the gas-particle partitioning of reactive mercury and its influencing factors with complex urban pollution.

Ambient mercury source identification at a New York State urban site: Rochester, NY

Gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), and particle-bound mercury (PBM) were measured continuously in Rochester, NY (NY43) from January 2012 to December 2014. Continuous measurements of ozone (O₃), sulfur dioxide (SO₂), carbon monoxide (CO), nitrogen oxide (NO), nitrogen dioxide (NO₂), particulate matter (PM_2.5), and meteorological data were also made at this site. A principal component analysis (PCA) of the resulting 15 variables showed that the ambient mercury in Rochester was primarily produced by non-local sources in contrast to earlier studies that showed that local sources were present. Positive matrix factorization (PMF) analysis of the atmospheric mercury and other pollutant species concentrations showed that transport and atmospheric processes have become the major source of mercury in Rochester. Conditional bivariate probability function (CBPF) and potential source contribution function (PSCF) were used to identify local and distant mercury sources. The results in this study showed that the closure of a coal-fired power plant and promulgation of several fuel quality policies reduced local mercury emissions making long-distance transport the major source of mercury in Rochester.

Mercury wet deposition and speciated mercury air concentrations at rural and urban sites across New York state: Temporal patterns, sources and scavenging coefficients

Measurements of ambient speciated mercury (Hg) concentrations and Hg wet deposition were made at two urban sites (Bronx, NY and Rochester, NY) and one rural site (Potsdam, NY) in New York State in 2013 and 2014 to: 1) determine the factors influencing Hg wet deposition concentrations, 2) identify the contribution of gaseous oxidized Hg (GOM) and particulate bound Hg (PBM) scavenging to Hg wet deposition concentrations, and 3) identify potential source areas associated with high concentration events. The Bronx had the highest mean gaseous elemental Hg (GEM) and GOM concentrations, Rochester had the highest mean PBM and the lowest GOM concentrations, and Potsdam had the lowest mean GEM and PBM concentrations. The annual volume weighted mean (VWM) Hg concentrations and Hg wet deposition fluxes in the Bronx, Rochester, and Potsdam were significantly different with mean values of 10.3 ± 8.16, 10.2 ± 9.06, and 5.07 ± 1.79 ngL^-1 and 8.45 ± 0.64, 6.65 ± 0.21, and 5.25 ± 0.49 μg/m² year^-1, respectively. Hg wet deposition flux and precipitation depth were positively correlated at all three sites as were Hg concentration in precipitation and weekly GOM concentrations at the Bronx and Potsdam sites. Scavenging coefficients (SC) of 680, 630, 850 for GOM and 410, 320, and 410 for PBM at Bronx, Rochester, and Potsdam, respectively, suggest GOM is responsible for most of the scavenged Hg. Measured GOM and PBM concentrations were relatively constant before precipitation events and Hg concentrations in precipitation did not vary significantly during precipitation events implying the scavenging process mainly occurred in clouds. VWM Hg concentrations, monthly accumulated Hg flux, and SCs for GOM and PBM were higher at the urban sites and significantly different for non-snow and snow events. Local sources appeared more important at the rural site while regional sources affected high urban concentrations.

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.

First kinetic study of the atmospherically important reactions BrHg˙ + NO2 and BrHg˙ + HOO

We use computational chemistry to determine the rate constants and product yields for the reactions of BrHg˙ with the atmospherically abundant radicals NO₂ and HOO. The reactants, products, and well-defined transition states are characterized using CCSD(T) with large basis sets. The potential energy profiles for the barrierless addition of HOO and NO₂ to BrHg˙ are characterized using CASPT2 and RHF-CCSDT, and the rate constants are computed as a function of temperature and pressure using variational transition state theory and master equation simulations. The calculated rate constant for the addition of NO₂ to BrHg˙ is larger than that for the addition of HOO by a factor of up to two under atmospheric conditions. For the reaction of HOO with BrHg˙ the addition reaction entirely dominates competing HOO + BrHg˙ reaction channels. The addition of NO₂ to BrHg˙ initially produces both BrHgNO₂ and BrHgONO, but after a few seconds under atmospheric conditions the sole product is syn-BrHgONO. A previously unsuspected reaction channel for BrHg˙ + NO₂ competes with the addition to yield Hg + BrNO₂. This reaction reduces the mercury oxidation state in BrHg˙ from Hg(i) to Hg(0) and slows the atmospheric oxidation of Hg(0). While the rate constant for this reduction channel is not well-constrained by the present calculations, it may be as much as 18% as large as the oxidation channel under some atmospheric conditions. As no experimental kinetic or product yield data are available for the reactions studied here, this work will provide guidance for atmospheric modelers and experimental kineticists.

Evaluation of passive sampling of gaseous mercury using different sorbing materials

Atmospheric mercury monitoring is essential because of its potential human health and ecological impacts. Current automated monitoring systems include limitations such as high cost, complicated configuration, and electricity requirements. Passive samplers require no electric power and are more appropriate for screening applications and long-term monitoring. Sampling rate is a major factor to evaluate the performance of a passive sampler. In this study, laboratory experiments were carried out using an exposure chamber to search for high efficiency sorbents for gaseous mercury. Four types of sorbents, including sulfur-impregnated carbon (SIC), chlorine-impregnated carbon (CIC), bromine-impregnated carbon (BIC), and gold-coated sand (GCS) were evaluated under a wide range of meteorological parameters, including temperature, relative humidity, and wind speed. The results showed that the four sorbents all have a high sampling rate above 0.01 m³g^-1 day^-1, and wind speed has a positive correlation with the sampling rate. Under different temperature and relative humidity, the sampling rate of SIC keeps stable. The sampling rate of CIC and BIC shows a negative correlation with temperature, and GCS is influenced by all the three meteorological factors. Furthermore, long-term experiments were carried out to investigate the uptake capacity of GCS and SIC. Uptake curves show that the mass amount of sorbent in a passive sampler can influence uptake capacity. In the passive sampler, 0.9 g SIC or 0.9 g GCS can achieve stable uptake efficiency for at least 110 days with gaseous mercury concentration at or below 2 ng/m³. For mercury concentration at or below 21 ng/m³, 0.9 g SIC can maintain stable uptake efficiency for 70 days, and 0.9 g GCS can maintain stability for 45 days.

Gaseous Elemental Mercury and Total and Leached Mercury in Building Materials from the Former Hg-Mining Area of Abbadia San Salvatore (Central Italy)

Mercury has a strong environmental impact since both its organic and inorganic forms are toxic, and it represents a pollutant of global concern. Liquid Hg is highly volatile and can be released during natural and anthropogenic processes in the hydrosphere, biosphere and atmosphere. In this study, the distribution of Gaseous Elemental Mercury (GEM) and the total and leached mercury concentrations on paint, plaster, roof tiles, concrete, metals, dust and wood structures were determined in the main buildings and structures of the former Hg-mining area of Abbadia San Salvatore (Siena, Central Italy). The mining complex (divided into seven units) covers a surface of about 65 ha and contains mining structures and managers' and workers' buildings. Nine surveys of GEM measurements were carried out from July 2011 to August 2015 for the buildings and structures located in Units 2, 3 and 6, the latter being the area where liquid mercury was produced. Measurements were also performed in February, April, July, September and December 2016 in the edifices and mining structures of Unit 6. GEM concentrations showed a strong variability in time and space mostly depending on ambient temperature and the operational activities that were carried out in each building. The Unit 2 surveys carried out in the hotter period (from June to September) showed GEM concentrations up to 27,500 ng·m-3, while in Unit 6, they were on average much higher, and occasionally, they saturated the GEM measurement device (>50,000 ng·m-3). Concentrations of total (in mg·kg-1) and leached (in μg·L-1) mercury measured in different building materials (up to 46,580 mg·kg-1 and 4470 mg·L-1, respectively) were highly variable, being related to the edifice or mining structure from which they were collected. The results obtained in this study are of relevant interest for operational cleanings to be carried out during reclamation activities.

Tropospheric GOM at the Pic du Midi Observatory-Correcting Bias in Denuder Based Observations

Gaseous elemental mercury (GEM, Hg) emissions are transformed to divalent reactive Hg (RM) forms throughout the troposphere and stratosphere. RM is often operationally quantified as the sum of particle bound Hg (PBM) and gaseous oxidized Hg (GOM). The measurement of GOM and PBM is challenging and under mounting criticism. Here we intercompare six months of automated GOM and PBM measurements using a Tekran (TK) KCl-coated denuder and quartz regenerable particulate filter method (GOM_TK, PBM_TK, and RM_TK) with RM_CEM collected on cation exchange membranes (CEMs) at the high altitude Pic du Midi Observatory. We find that RM_TK is systematically lower by a factor of 1.3 than RM_CEM. We observe a significant relationship between GOM_TK (but not PBM_TK) and Tekran flush_TK blanks suggesting significant loss (36%) of labile GOM_TK from the denuder or inlet. Adding the flush_TK blank to RM_TK results in good agreement with RM_CEM (slope = 1.01, r² = 0.90) suggesting we can correct bias in RM_TK and GOM_TK. We provide a bias corrected (*) Pic du Midi data set for 2012-2014 that shows GOM* and RM* levels in dry free tropospheric air of 198 ± 57 and 229 ± 58 pg m^-3 which agree well with in-flight observed RM and with model based GOM and RM estimates.

Uncertainty Assessment of Gaseous Oxidized Mercury Measurements Collected by Atmospheric Mercury Network

Gaseous oxidized mercury (GOM) measurement uncertainties undoubtedly impact the understanding of mercury biogeochemical cycling; however, there is a lack of consensus on the uncertainty magnitude. The numerical method presented in this study provides an alternative means of estimating the uncertainties of previous GOM measurements. Weekly GOM in ambient air was predicted from measured weekly mercury wet deposition using a scavenging ratio approach, and compared against field measurements of 2-4 hly GOM to estimate the measurement biases of the Tekran speciation instruments at 13 Atmospheric Mercury Network (AMNet) sites. Multiyear average GOM measurements were estimated to be biased low by more than a factor of 2 at six sites, between a factor of 1.5 and 1.8 at six other sites, and below a factor of 1.3 at one site. The differences between predicted and observed were significantly larger during summer than other seasons potentially because of higher ozone concentrations that may interfere with GOM sampling. The analysis data collected over six years at multiple sites suggests a systematic bias in GOM measurements, supporting the need for further investigation of measurement technologies and identifying the chemical composition of GOM.

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

The Teledyne Advanced Pollution Instrumentation (TAPI) model 602 Beta^Plus particulate system provides nondestructive analysis of particulate matter (PM_2.5) mass concentration. This instrument was used to determine if measurements made with cation exchange membranes (CEM) were comparable to standard methods, the β attenuation method at two locations in Reno, NV and an environmental β attenuation method and gravimetric method at Great Basin National Park, NV. TAPI PM_2.5 CEM measurements were statistically similar to the other three PM_2.5 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.

Automated Calibration of Atmospheric Oxidized Mercury Measurements

The atmosphere is an important reservoir for mercury pollution, and understanding of oxidation processes is essential to elucidating the fate of atmospheric mercury. Several recent studies have shown that a low bias exists in a widely applied method for atmospheric oxidized mercury measurements. We developed an automated, permeation tube-based calibrator for elemental and oxidized mercury, and we integrated this calibrator with atmospheric mercury instrumentation (Tekran 2537/1130/1135 speciation systems) in Reno, Nevada and at Mauna Loa Observatory, Hawaii, U.S.A. While the calibrator has limitations, it was able to routinely inject stable amounts of HgCl₂ and HgBr₂ into atmospheric mercury measurement systems over periods of several months. In Reno, recovery of injected mercury compounds as gaseous oxidized mercury (as opposed to elemental mercury) decreased with increasing specific humidity, as has been shown in other studies, although this trend was not observed at Mauna Loa, likely due to differences in atmospheric chemistry at the two locations. Recovery of injected mercury compounds as oxidized mercury was greater in Mauna Loa than in Reno, and greater still for a cation-exchange membrane-based measurement system. These results show that routine calibration of atmospheric oxidized mercury measurements is both feasible and necessary.

Evidence for Different Reactive Hg Sources and Chemical Compounds at Adjacent Valley and High Elevation Locations

The spatial distribution of chemical compounds and concentration of reactive mercury (RM), defined as the sum of gaseous oxidized mercury (GOM) and <3 μm particulate bound mercury (PBM), are poorly characterized. The objective of this study was to understand the chemistry, concentration, and spatial and temporal distribution of GOM at adjacent locations (12 km apart) with a difference in elevation of ∼1000 m. Atmospheric GOM measurements were made with passive and active samplers using membranes, and at one location, a Tekran mercury measurement system was used. The chemistry of GOM varied across time and location. On the basis of data collected, chemistry at the low elevation site adjacent to a highway was primarily influenced by pollutants generated by mobile sources (GOM = nitrogen and sulfur-based compounds), and the high elevation site (GOM = halogen-based compounds) was affected by long-range transport in the free troposphere over the marine boundary layer into Nevada. Data collected at these two locations demonstrate that different GOM compounds exist depending on the oxidants present in the air. Measurements of GOM made by the KCl denuder in the Tekran instrument located at the low elevation site were lower than that measured using membranes by 1.7-13 times. Accurate measurements of atmospheric concentrations and chemistry of RM are necessary for proper assessment of environmental impacts, and field measurements are essential for atmospheric models, which in turn influence policy decisions.

Evaluating the effectiveness of the Minamata Convention on Mercury: Principles and recommendations for next steps

The Minamata Convention on Mercury is a multilateral environmental agreement that obligates Parties to reduce or control sources of mercury pollution in order to protect human health and the environment. The Convention includes provisions on providing technical assistance and capacity building, particularly for developing countries and countries with economies in transition, to promote its effective implementation. Evaluating the effectiveness of the Convention (as required by Article 22) is a crucial component to ensure that it meets this objective. We describe an approach to measure effectiveness, which includes a suite of short-, medium-, and long-term metrics related to five major mercury control Articles in the Convention, as well as metrics derived from monitoring of mercury in the environment using select bioindicators, including people. The use of existing biotic Hg data will define spatial gradients (e.g., biological mercury hotspots), baselines to develop relevant temporal trends, and an ability to assess risk to taxa and human communities of greatest concern. We also recommend the development of a technical document that describes monitoring options for the Conference of Parties, to provide science-based standardized guidelines for collecting relevant monitoring information, as guided by Article 19.

Isotopic Composition of Gaseous Elemental Mercury in the Free Troposphere of the Pic du Midi Observatory, France

Understanding the sources and transformations of mercury (Hg) in the free troposphere is a critical aspect of global Hg research. Here we present one year of observations of atmospheric Hg speciation and gaseous elemental Hg (GEM) isotopic composition at the high-altitude Pic du Midi Observatory (2860 m above sea level) in France. Biweekly integrated GEM from February 2012 to January 2013 revealed significant variations in δ(202)HgGEM (-0.04‰ to 0.52‰) but not in Δ(199)HgGEM (-0.17‰ to -0.27‰) or Δ(200)HgGEM (-0.10‰ to 0.05‰). δ(202)HgGEM was negatively correlated with CO and reflected air mass origins from Europe (high CO, low δ(202)HgGEM) and from the Atlantic Ocean (low CO, high δ(202)HgGEM). We suggest that the δ(202)HgGEM variations represent mixing of recent low δ(202)HgGEM European anthropogenic emissions with high δ(202)HgGEM northern hemispheric background GEM. In addition, Atlantic Ocean free troposphere air masses showed a positive correlation between δ(202)HgGEM and gaseous oxidized Hg (GOM) concentrations, indicative of mass-dependent Hg isotope fractionation during GEM oxidation. On the basis of atmospheric δ(202)HgGEM and speciated Hg observations, we suggest that the oceanic free troposphere is a reservoir within which GEM is readily oxidized to GOM.

Impact of Measurement Uncertainties on Receptor Modeling of Speciated Atmospheric Mercury

Gaseous oxidized mercury (GOM) and particle-bound mercury (PBM) measurement uncertainties could potentially affect the analysis and modeling of atmospheric mercury. This study investigated the impact of GOM measurement uncertainties on Principal Components Analysis (PCA), Absolute Principal Component Scores (APCS), and Concentration-Weighted Trajectory (CWT) receptor modeling results. The atmospheric mercury data input into these receptor models were modified by combining GOM and PBM into a single reactive mercury (RM) parameter and excluding low GOM measurements to improve the data quality. PCA and APCS results derived from RM or excluding low GOM measurements were similar to those in previous studies, except for a non-unique component and an additional component extracted from the RM dataset. The percent variance explained by the major components from a previous study differed slightly compared to RM and excluding low GOM measurements. CWT results were more sensitive to the input of RM than GOM excluding low measurements. Larger discrepancies were found between RM and GOM source regions than those between RM and PBM. Depending on the season, CWT source regions of RM differed by 40–61% compared to GOM from a previous study. No improvement in correlations between CWT results and anthropogenic mercury emissions were found.

Biogeochemical transformations of mercury in solid waste landfills and pathways for release

Mercury (Hg) is present in a variety of solid wastes including industrial wastes, household products, consumer electronics, and medical wastes, some of which can be disposed in conventional landfills. The presence of this neurotoxic metal in landfills is a concern due to the potential for it to leach or volatilize from the landfill and impact local ecosystems. The objective of this review is to describe general practices for the disposal of mercury-bearing solid wastes, summarize previous studies on the release of mercury from landfills, and delineate the expected transformations of Hg within landfill environments that would influence transport of Hg via landfill gas and leachate. A few studies have documented the emissions of Hg as landfill gas, primarily as gaseous elemental Hg(0) and smaller amounts as methylated Hg species. Much less is known regarding the release of Hg in leachate. Landfill conditions are unique from other subsurface environments in that they can contain water with very high conductivity and organic carbon concentration. Landfills also experience large changes in redox potential (and the associated microbial community) that greatly influence Hg speciation, transformations, and mobilization potential. Generally, Hg is not likely to persist in large quantities as dissolved species, since Hg(0) tends to evolve in the gas phase and divalent Hg(ii) sorbs strongly to particulate phases including organic carbon and sulfides. However, Hg(ii) has the potential to associate with or form colloidal particles that can be mobilized in porous media under high organic carbon conditions. Moreover, the anaerobic conditions within landfills can foster the growth of microorganisms that produced monomethyl- and dimethyl-Hg species, the forms of mercury with high potential for bioaccumulation. Much advancement has recently been made in the mercury biogeochemistry research field, and this study seeks to incorporate these findings for landfill settings.

High Mercury Wet Deposition at a "Clean Air" Site in Puerto Rico

Atmospheric mercury deposition measurements are rare in tropical latitudes. Here we report on seven years (April 2005 to April 2012, with gaps) of wet Hg deposition measurements at a tropical wet forest in the Luquillo Mountains, northeastern Puerto Rico, U.S. Despite receiving unpolluted air off the Atlantic Ocean from northeasterly trade winds, during two complete years the site averaged 27.9 μg m(-2) yr(-1) wet Hg deposition, or about 30% more than Florida and the Gulf Coast, the highest deposition areas within the U.S. These high Hg deposition rates are driven in part by high rainfall, which averaged 2855 mm yr(-1). The volume-weighted mean Hg concentration was 9.8 ng L(-1), and was highest during summer and lowest during the winter dry season. Rainout of Hg (decreasing concentration with increasing rainfall depth) was minimal. The high Hg deposition was not supported by gaseous oxidized mercury (GOM) at ground level, which remained near global background concentrations (<10 pg m(-3)). Rather, a strong positive correlation between Hg concentrations and the maximum height of rain detected within clouds (echo tops) suggests that droplets in high convective cloud tops scavenge GOM from above the mixing layer. The high wet Hg deposition at this "clean air" site suggests that other tropical areas may be hotspots for Hg deposition as well.

The Nevada Rural Ozone Initiative (NVROI): Insights to understanding air pollution in complex terrain

The Nevada Rural Ozone Initiative (NVROI) was established to better understand O3 concentrations in the Western United States (US). The major working hypothesis for development of the sampling network was that the sources of O3 to Nevada are regional and global. Within the framework of this overarching hypothesis, we specifically address two conceptual meteorological hypotheses: (1) The high elevation, complex terrain, and deep convective mixing that characterize Nevada, make this state ideally located to intercept polluted parcels of air transported into the US from the free troposphere; and (2) site specific terrain features will influence O3 concentrations observed at surface sites. Here, the impact of complex terrain and site location on observations are discussed. Data collected in Nevada at 6 sites (1385 to 2082 m above sea level (asl)) are compared with that collected at high elevation sites in Yosemite National Park and the White Mountains, California. Average daily maximum 1-hour concentrations of O3 during the first year of the NVROI ranged from 58 to 69 ppbv (spring), 53 to 62 ppbv (summer), 44 to 49 ppbv (fall), and 37 to 45 ppbv (winter). These were similar to those measured at 3 sites in Yosemite National Park (2022 to 3031 m asl), and at 4 sites in the White Mountains (1237 to 4342 m asl) (58 to 67 ppbv (summer) and 47 to 58 ppbv (fall)). Results show, that in complex terrain, collection of data should occur at high and low elevation sites to capture surface impacts, and site location with respect to topography should be considered. Additionally, concentrations measured are above the threshold reported for causing a reduction in growth and visible injury for plants (40 ppbv), and sustained exposure at high elevation locations in the Western USA may be detrimental for ecosystems.

Identifying sources of ozone to three rural locations in Nevada, USA, using ancillary gas pollutants, aerosol chemistry, and mercury

Ozone (O3) is a secondary air pollutant of long standing and increasing concern for environmental and human health, and as such, the US Environmental Protection Agency will revise the National Ambient Air Quality Standard of 75 ppbv to ≤ 70 ppbv. Long term measurements at the Great Basin National Park (GBNP) indicate that O3 in remote areas of Nevada will exceed a revised standard. As part of the Nevada Rural Ozone Initiative, measurements of O3 and other air pollutants were made at 3 remote sites between February 2012 and March 2014, GBNP, Paradise Valley (PAVA), and Echo Peak (ECHO). Exceptionally high concentrations of each air pollutant were defined relative to each site as mixing ratios that exceeded the 90th percentile of all hourly data. Case studies were analyzed for all periods during which mean daily O3 exceeded the 90th percentile concurrently with a maximum 8-h average (MDA8) O3 that was "exceptionally high" for the site (65 ppbv at PAVA, 70 ppbv at ECHO and GBNP), and of potential regulatory significance. An MDA8 ≥ 65 ppbv occurred only five times at PAVA, whereas this occurred on 49 and 65 days at GBNP and ECHO, respectively. The overall correlation between O3 and other pollutants was poor, consistent with the large distance from significant primary emission sources. Mean CO at these locations exceeded concentrations reported for background sites in 2000. Trajectory residence time calculations and air pollutant concentrations indicate that exceedances at GBNP and ECHO were promoted by air masses originating from multiple sources, including wildfires, transport of pollution from southern California and the marine boundary layer, and transport of Asian pollution plumes. Results indicate that the State of Nevada will exceed a revised O3 standard due to sources that are beyond their control.

Development of a statistical model to identify spatial and meteorological drivers of elevated O3 in Nevada and its application to other rural mountainous regions

Measurements of O3 at relatively remote monitoring sites are useful for quantifying baseline O3, and subsequently the magnitude of O3 not controllable by local regulations. As the National Ambient Air Quality Standard (NAAQS) for O3 becomes more stringent, there is an increased need to quantify baseline O3 particularly in the Western US, where regional and global sources can significantly enhance O3 measured at surface sites, yielding baseline mixing ratios approaching or exceeding the NAAQS threshold. Past work has indicated that meteorological conditions as well as site specific spatial characteristics (e.g. elevation, basin size, gradient) are significantly correlated with O3 intercepted at rural monitoring sites. Here, we use 3 years of measurements from sites throughout rural Nevada to develop a categorical tree model to identify spatial and meteorological characteristics that are associated with elevated baseline O3. Data from other sites in the Intermountain Western US are used to test the applicability of the model for sites throughout the region. Our analyses indicate that increased elevation and basin size were associated with increased frequency of elevated O3. On a daily time scale, relative humidity had the strongest association with observed MDA8 O3. Seventy-four percent of MDA8 O3 observations>60 ppbv occurred when daily minimum relative humidity was <15%. Further, we found that including ancillary pollutant data did not improve the predictive accuracy for measurements >60 ppbv whereas including upper air meteorological measurements improved the accuracy of predicting periods when O3 was >60 ppbv. These findings indicate that transport, rather than local production, influences O3 measurements in Nevada, and that high elevation sites in rural Nevada, are representative of baseline conditions in the Intermountain Western US.

Investigating the influence of long-range transport on surface O3 in Nevada, USA, using observations from multiple measurement platforms

The current United States (US) National Ambient Air Quality Standard (NAAQS) for O3 (75 ppb) is expected to be revised to between 60 and 70 ppb. As the NAAQS becomes more stringent, characterizing the extent of O3 and precursors transported into the US is increasingly important. Given the high elevation, complex terrain, and location in the Intermountain West, the State of Nevada is ideally situated to intercept air transported into the US. Until recently, measurements of O3 and associated pollutants were limited to areas in and around the cities of Las Vegas and Reno. In 2011, the Nevada Rural Ozone Initiative began and through this project 13 surface monitoring sites were established. Also in 2011, the NASA Ames Alpha Jet Atmospheric eXperiment (AJAX) began making routine aircraft measurements of O3 and other greenhouse gases in Nevada. The availability of aircraft and surface measurements in a relatively rural, remote setting in the Intermountain West presented a unique opportunity to investigate sources contributing to the O3 observed in Nevada. Our analyses indicate that stratosphere to troposphere transport, long-range transport of Asian pollution, and regional emissions from urban areas and wildfires influence surface observations. The complexity of sources identified here along with the fact that O3 frequently approaches the threshold being considered for a revised NAAQS indicate that interstate and international cooperation will be necessary to achieve compliance with a more stringent regulatory standard. Further, on a seasonal basis we found no significant difference between daily 1-h maximum O3 at surface sites, which ranged in elevation from 888 to 2307 m, and aircraft measurements of O3 <2500 m which suggests that similar processes influence daytime O3 across rural Nevada and indicates that column measurements from Railroad Valley, NV are useful in understanding these processes.

Uncertainties of Gaseous Oxidized Mercury Measurements Using KCl-Coated Denuders, Cation-Exchange Membranes, and Nylon Membranes: Humidity Influences

Quantifying the concentration of gaseous oxidized mercury (GOM) and identifying the chemical compounds in the atmosphere are important for developing accurate local, regional, and global biogeochemical cycles. The major hypothesis driving this work was that relative humidity affects collection of GOM on KCl-coated denuders and nylon membranes, both currently being applied to measure GOM. Using a laboratory manifold system and ambient air, GOM capture efficiency on 3 different collection surfaces, including KCl-coated denuders, nylon membranes, and cation-exchange membranes, was investigated at relative humidity ranging from 25 to 75%. Recovery of permeated HgBr2 on KCl-coated denuders declined by 4-60% during spikes of relative humidity (25 to 75%). When spikes were turned off GOM recoveries returned to 60 ± 19% of permeated levels. In some cases, KCl-coated denuders were gradually passivated over time after additional humidity was applied. In this study, GOM recovery on nylon membranes decreased with high humidity and ozone concentrations. However, additional humidity enhanced GOM recovery on cation-exchange membranes. In addition, reduction and oxidation of elemental mercury during experiments was observed. The findings in this study can help to explain field observations in previous studies.