scholarly journals Behavior of KCl sorbent traps and KCl trapping solutions used for atmospheric mercury speciation: stability and specificity

2021 ◽  
Vol 14 (10) ◽  
pp. 6619-6631
Author(s):  
Jan Gačnik ◽  
Igor Živković ◽  
Sergio Ribeiro Guevara ◽  
Radojko Jaćimović ◽  
Jože Kotnik ◽  
...  
Keyword(s):  
Elemental Mercury ◽  
Ambient Air ◽  
Atmospheric Mercury ◽  
Mercury Speciation ◽  
Atomic Fluorescence Spectrometry ◽  
Atomic Fluorescence ◽  
Gaseous Elemental Mercury ◽  
Specificity Test ◽  
Sorbent Materials ◽  
The Stability

Abstract. Atmospheric mercury speciation is of paramount importance for understanding the behavior of mercury once it is emitted into the atmosphere as gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM) and particulate-bound mercury (PBM). GOM and PBM can also be formed in the atmosphere; their sampling is the most problematic step in the atmospheric mercury speciation. GOM sampling with speciation traps composed of KCl sorbent materials and KCl trapping solutions are commonly used sampling methods, although the research conducted with them at ambient air concentrations is limited. The results of the specificity test demonstrated that the KCl sorbent traps are highly specific when using new traps, while their specificity drops dramatically when they are reused. The results of the stability test indicated that the highest Hg2+ losses (up to 5.5 % of Hg2+ loss) occur when low amounts of Hg2+ (< 1 ng) are loaded, due to a reduction of Hg2+ to Hg0. KCl trapping solutions have also been considered as a selective trapping media for GOM in atmospheric samples. A dimensionless Henry law constant was experimentally derived and was used to calculate the solubility of elemental Hg in KCl solution. The degree of GEM oxidation was established by purging elemental Hg calibration gas into a KCl solution and determining the GOM trapped using aqueous-phase propylation liquid–liquid extraction and gas chromatography–atomic fluorescence spectrometry (GC-AFS) measurement. A positive GOM bias was observed due to the solubility and oxidation of GEM in KCl trapping solutions, strongly suggesting that this approach is unsuitable for atmospheric mercury speciation measurements.

scholarly journals Behaviour of KCl sorbent traps and KCl trapping solutions used for atmospheric mercury speciation: stability and specificity

2021 ◽  
Author(s):  
Jan Gačnik ◽  
Igor Živković ◽  
Sergio Ribeiro Guevara ◽  
Radojko Jaćimović ◽  
Jože Kotnik ◽  
...  
Keyword(s):  
Analytical Procedure ◽  
Sampling Methods ◽  
Ambient Air ◽  
Atmospheric Mercury ◽  
Mercury Speciation ◽  
Liquid Liquid Extraction ◽  
Specificity Test ◽  
Sorbent Materials ◽  
The Stability ◽  
Work Done

Abstract. Atmospheric mercury speciation is of paramount importance for understanding the behavior of mercury once it is emitted into the atmosphere as gaseous elemental (GEM), gaseous oxidized (GOM) and particulate-bound (PBM) mercury. GOM and PBM sampling are the most problematic steps in the analytical procedure. GOM sampling with speciation traps composed of KCl sorbent materials and KCl trapping solutions are commonly used sampling methods, although the work done at ambient air concentrations is limited. The results of the specificity test showed that the KCl sorbent traps are very specific when using new traps, while their specificity drops dramatically when they are reused. The results of the stability test showed that the highest Hg2+ losses (up to 5.5 % of Hg2+ loss) occur when low amounts of Hg2+ (< 1 ng) are loaded, due to a reduction of Hg2+ to Hg0. GOM losses should be taken into account when using KCl sorbent traps for atmospheric Hg speciation, especially at low ambient GOM concentrations. KCl trapping solutions have also been considered as a selective trapping media for GOM in atmospheric samples. A dimensionless Henry’s law constant was experimentally derived and was used to calculate the solubility of elemental Hg in KCl solution. The degree of GEM oxidation was established by purging elemental Hg calibration gas into a KCl solution and determining the GOM trapped using aqueous phase propylation liquid-liquid extraction GC-AFS. A positive GOM bias was observed due to the solubility and oxidation of GEM in KCl trapping solutions strongly suggesting that this approach is unsuitable for atmospheric mercury speciation measurements.

scholarly journals Evaluation of cation exchange membrane performance under exposure to high Hg<sup>0</sup> and HgBr<sub>2</sub> concentrations

2019 ◽  
Vol 12 (2) ◽  
pp. 1207-1217 ◽  
Author(s):  
Matthieu B. Miller ◽  
Sarrah M. Dunham-Cheatham ◽  
Mae Sexauer Gustin ◽  
Grant C. Edwards
Keyword(s):  
Cation Exchange ◽  
High Efficiency ◽  
Absolute Humidity ◽  
Elemental Mercury ◽  
Mercury Vapor ◽  
Ambient Air ◽  
Atmospheric Mercury ◽  
Promising Alternative ◽  
Gaseous Elemental Mercury ◽  
High Concentrations

Abstract. Reactive mercury (RM), the sum of both gaseous oxidized Hg and particulate bound Hg, is an important component of the global atmospheric mercury cycle, but measurement currently depends on uncalibrated operationally defined methods with large uncertainty and demonstrated interferences and artifacts. Cation exchange membranes (CEMs) provide a promising alternative methodology for quantification of RM, but method validation and improvements are ongoing. For the CEM material to be reliable, uptake of gaseous elemental mercury (GEM) must be negligible under all conditions and RM compounds must be captured and retained with high efficiency. In this study, the performance of CEM material under exposure to high concentrations of GEM (1.43×106 to 1.85×106 pg m−3) and reactive gaseous mercury bromide (HgBr2 ∼5000 pg m−3) was explored using a custom-built mercury vapor permeation system. Quantification of total permeated Hg was measured via pyrolysis at 600 ∘C and detection using a Tekran® 2537A. Permeation tests were conducted over 24 to 72 h in clean laboratory air, with absolute humidity levels ranging from 0.1 to 10 g m−3 water vapor. GEM uptake by the CEM material averaged no more than 0.004 % of total exposure for all test conditions, which equates to a non-detectable GEM artifact for typical ambient air sample concentrations. Recovery of HgBr2 on CEM filters was on average 127 % compared to calculated total permeated HgBr2 based on the downstream Tekran® 2537A data. The low HgBr2 breakthrough on the downstream CEMs (< 1 %) suggests that the elevated recoveries are more likely related to suboptimal pyrolyzer conditions or inefficient collection on the Tekran® 2537A gold traps.

Performance of a new diffusive sampler for Hg0 determination in the troposphere

2007 ◽  
Vol 4 (2) ◽  
pp. 75 ◽  
Author(s):  
Henrik Skov ◽  
Britt T. Sørensen ◽  
Matthew S. Landis ◽  
Matthew S. Johnson ◽  
Paolo Sacco ◽  
...  
Keyword(s):  
Detection Limit ◽  
Elemental Mercury ◽  
Ambient Air ◽  
Atmospheric Mercury ◽  
Point Sources ◽  
Gaseous Elemental Mercury ◽  
Marine Predators ◽  
Diffusive Sampler ◽  
Ambient Concentrations ◽  
Areas Of Interest

Environmental context. Mercury is of concern to both the public and to the scientific community because it is found at high levels in some marine predators, prompting the US EPA and others to make guidelines restricting the consumption of some species. Most mercury in the environment is emitted to the atmosphere, but it is not known how it is transferred from the atmosphere to the marine environment. Therefore, it is important to study the connection between emission of mercury, its transport and removal from the atmosphere. We have developed a new sampler that is inexpensive, easy to use and with a sufficiently high detection limit that it can be used to measure the low mercury concentrations in the atmosphere at a reasonable time resolution. Abstract. Mercury behaves uniquely in the atmosphere due to its volatility and long lifetime. The existing methods for measuring atmospheric mercury are either expensive or labour intensive. The present paper presents a new measurement technique, the diffusive sampler, that is portable, inexpensive, easy to use, and does not need a power supply. The sampler is sufficiently sensitive that it can measure mercury at low ambient levels with an exposure time of 1 to 3 days. The sampler is based on the Radiello diffusive sampler, which was used to collect volatile organic compounds. In the present paper, the method is validated under controlled laboratory conditions. The uptake rate of the Radiello diffusive sampler is determined using known concentrations of gaseous elemental mercury, and is measured as a function of wind speed, relative humidity and temperature. The Radiello sampler has a detection limit of 0.14 ng m–3 for 1 day of exposure and thus can be used to measure mercury concentrations at the low levels found in ambient air. The Radiello sampler is therefore useful for mapping concentrations close to sources and sinks, in addition to ambient concentrations. For example, the sampler can be used to describe the geographical extent of Arctic mercury depletion episodes where gaseous elemental mercury is removed and stays close to 0 ng m–3 for days, and it can be a powerful tool for mapping gradients around point sources and other areas of interest.

scholarly journals Improved methods for signal processing in measurements of mercury by Tekran<sup>®</sup> 2537A and 2537B instruments

2017 ◽  
Vol 10 (12) ◽  
pp. 5063-5073 ◽  
Author(s):  
Jesse L. Ambrose
Keyword(s):  
Signal Processing ◽  
Thermal Desorption ◽  
Elemental Mercury ◽  
Operating Conditions ◽  
Atomic Fluorescence Spectrometry ◽  
Atomic Fluorescence ◽  
Gaseous Elemental Mercury ◽  
New Methods ◽  
Processing Bias ◽  
Improved Methods

Abstract. Atmospheric Hg measurements are commonly carried out using Tekran® Instruments Corporation's model 2537 Hg vapor analyzers, which employ gold amalgamation preconcentration sampling and detection by thermal desorption (TD) and atomic fluorescence spectrometry (AFS). A generally overlooked and poorly characterized source of analytical uncertainty in those measurements is the method by which the raw Hg atomic fluorescence (AF) signal is processed. Here I describe new software-based methods for processing the raw signal from the Tekran® 2537 instruments, and I evaluate the performances of those methods together with the standard Tekran® internal signal processing method. For test datasets from two Tekran® instruments (one 2537A and one 2537B), I estimate that signal processing uncertainties in Hg loadings determined with the Tekran® method are within ±[1 % +  1.2 pg] and ±[6 % + 0.21 pg], respectively. I demonstrate that the Tekran® method can produce significant low biases (≥  5 %) not only at low Hg sample loadings (<  5 pg) but also at tropospheric background concentrations of gaseous elemental mercury (GEM) and total mercury (THg) (∼  1 to 2 ng m−3) under typical operating conditions (sample loadings of 5–10 pg). Signal processing uncertainties associated with the Tekran® method can therefore represent a significant unaccounted for addition to the overall  ∼  10 to 15 % uncertainty previously estimated for Tekran®-based GEM and THg measurements. Signal processing bias can also add significantly to uncertainties in Tekran®-based gaseous oxidized mercury (GOM) and particle-bound mercury (PBM) measurements, which often derive from Hg sample loadings < 5 pg. In comparison, estimated signal processing uncertainties associated with the new methods described herein are low, ranging from within ±0.053 pg, when the Hg thermal desorption peaks are defined manually, to within ±[2 % + 0.080 pg] when peak definition is automated. Mercury limits of detection (LODs) decrease by 31 to 88 % when the new methods are used in place of the Tekran® method. I recommend that signal processing uncertainties be quantified in future applications of the Tekran® 2537 instruments.

scholarly journals Evaluation of cation exchange membrane performance under exposure to high Hg<sup>0</sup> and HgBr<sub>2</sub> concentrations

2018 ◽  
Author(s):  
Matthieu B. Miller ◽  
Mae S. Gustin ◽  
Sarrah M. Dunham-Cheatham ◽  
Grant C. Edwards
Keyword(s):  
Cation Exchange ◽  
High Efficiency ◽  
Collection Efficiency ◽  
Absolute Humidity ◽  
Elemental Mercury ◽  
Ambient Air ◽  
Atmospheric Mercury ◽  
Promising Alternative ◽  
Gaseous Elemental Mercury ◽  
Mercury Uptake

Abstract. Reactive mercury (RM) is an important component of the global atmospheric mercury cycle, but measurement currently depends on un-calibrated, operationally defined methods with large uncertainty and demonstrated interferences and artifacts. Cation exchange membranes (CEM) provide a promising alternative methodology for quantification of RM, but method validation and improvement are ongoing. For the CEM material to be reliable, uptake of gaseous elemental mercury (GEM) must be negligible for all conditions, and RM compounds must be captured and retained with high efficiency. In this study the performance of CEM material under exposure to high concentrations of GEM (1.43 × 106–1.85 × 106 pg m−3) and reactive gaseous mercury bromide (HgBr2 ~ 5000 pg m−3) was explored, using a custom-built mercury vapor permeation system, with quantification of total permeated Hg accomplished via pyrolysis at 600 °C and detection using a Tekran® 2537A. Permeation tests were conducted for 24 to 72 hours in clean laboratory air, with absolute humidity levels ranging from 0.1–10 g m−3 water vapor. Gaseous elemental mercury uptake by the CEM material averaged no more than 0.004 % of total exposure for all test conditions, which equates to a non-detectable GEM artifact for typical ambient air sample concentrations. Recovery of HgBr2 on CEM filters was > 100 % compared to calculated total permeated HgBr2, suggesting incomplete thermal decomposition at the pyrolyzer, as the CEM material collected HgBr2 with less than 1 % downstream breakthrough on average, implying a high collection efficiency.

scholarly journals Understanding atmospheric mercury speciation and mercury in snow over time at Alert, Canada

2013 ◽  
Vol 13 (6) ◽  
pp. 17021-17052 ◽  
Author(s):  
A. Steffen ◽  
J. Bottenheim ◽  
A. Cole ◽  
R. Ebinghaus ◽  
G. Lawson ◽  
...  
Keyword(s):  
Air Temperature ◽  
Low Temperatures ◽  
Elemental Mercury ◽  
Atmospheric Mercury ◽  
Mercury Speciation ◽  
Sea Salts ◽  
Gaseous Elemental Mercury ◽  
Spring Period ◽  
Arctic Haze ◽  
Very High

Abstract. Ten years of atmospheric mercury speciation data and 14 yr of mercury in snow data from Alert, Nunavut, Canada are examined. The speciation data, collected from 2002 to 2011, includes gaseous elemental mercury (GEM), particulate mercury (PHg) and reactive gaseous mercury (RGM). During the winter-spring period of atmospheric mercury depletion events (AMDEs), when GEM is close to being completely depleted from the air, the concentrations of PHg and RGM rise significantly. During this period, the median concentrations for PHg is 28.2 pg m-3 and RGM is 23.9 pg m-3 from March to June in comparison to the annual median concentrations of 11.3 and 3.2 -3 for PHg and RGM, respectively. In each of the ten years of sampling, PHg increases steadily from January through March and is higher than RGM. This pattern begins to change in April with very high levels of PHg and increasing RGM. In May, RGM transitions to be significantly higher than PHg and continues into June whereas PHg sharply drops down. The transition is thought to be driven by a combination of air temperature and particle availability. Firstly, the ratio of PHg to RGM is favoured by low temperatures suggesting that oxidized mercury may partition to available particles to form PHg. Prior to the transition, the median air temperature is −24.8 °C and after the transition the median air temperature is −5.8 °C. Secondly, high aerosol levels in the spring are a strong driver for the high PHg concentrations. In February through April, partitioning of oxidized mercury to produce PHg was favoured by increased concentrations of particles that are principally the result of Arctic Haze and some sea salts. In the snow, the concentrations of mercury peak in May for all years. The highest deposition of mercury to the snow in the spring at Alert is during and after the transition of PHg to RGM in the atmosphere.

scholarly journals A field intercomparison of three passive air samplers for gaseous mercury in ambient air

2021 ◽  
Vol 14 (5) ◽  
pp. 3657-3672
Author(s):  
Attilio Naccarato ◽  
Antonella Tassone ◽  
Maria Martino ◽  
Sacha Moretti ◽  
Antonella Macagnano ◽  
...  
Keyword(s):  
Activated Carbon ◽  
National Research Council ◽  
Elemental Mercury ◽  
Weather Conditions ◽  
Ambient Air ◽  
Atmospheric Mercury ◽  
Ease Of Use ◽  
Environmental Research ◽  
Gaseous Elemental Mercury ◽  
Gaseous Mercury

Abstract. Passive air samplers (PASs), which provide time-averaged concentrations of gaseous mercury over the timescale of weeks to months, are promising for filling a gap in the monitoring of atmospheric mercury worldwide. Their usefulness will depend on their ease of use and robustness under field conditions, their availability and affordability, and most notably, their ability to provide results of acceptable precision and accuracy. Here we describe a comparative evaluation of three PASs with respect to their ability to precisely and accurately record atmospheric background mercury concentrations at sites in both southern Italy and southern Ontario, Canada. The study includes the CNR-PAS with gold nanoparticles as a sorbent, developed by the Italian National Research Council, the IVL-PAS using an activated carbon-coated disk, developed by the Swedish Environmental Research Institute, and the MerPAS® using a sulfur-impregnated activated carbon sorbent, developed at the University of Toronto and commercialized by Tekran. Detection limits are deduced from the variability in the amount of mercury quantified in more than 20 field blank samples for each PAS. Analytical and sampling precision is quantified through 22 triplicate deployments for each PAS, ranging in duration from 2 to 12 weeks. Accuracy and bias are assessed through comparison with gaseous elemental mercury concentrations recorded by Tekran 2537 automated mercury analyzers operating alongside the PASs at both locations. The performance of the PASs was significantly better in Italy, with all of them providing concentrations that are not significantly different from the average concentrations of the Tekran 2537 instruments. In Canada, where weather conditions were much harsher and more variable during the February through April deployment period, there are differences amongst the PASs. At both sites, the MerPAS® is currently the most sensitive, precise, and accurate among the three PASs. A key reason for this is the larger size and the radial configuration of the MerPAS®, which results in lower blank levels relative to the sequestered amounts of mercury when compared to the other two PASs, which rely on axial diffusion geometries. Since blank correction becomes relatively smaller with longer deployments, performance tends to be closer amongst the PASs during deployments of 8 and 12 weeks.

scholarly journals A field intercomparison of three passive air samplers for gaseous mercury in ambient air

2020 ◽  
Author(s):  
Attilio Naccarato ◽  
Antonella Tassone ◽  
Maria Martino ◽  
Sacha Moretti ◽  
Antonella Macagnano ◽  
...  
Keyword(s):  
Activated Carbon ◽  
National Research Council ◽  
Elemental Mercury ◽  
Weather Conditions ◽  
Ambient Air ◽  
Atmospheric Mercury ◽  
Ease Of Use ◽  
Environmental Research ◽  
Gaseous Elemental Mercury ◽  
Gaseous Mercury

Abstract. Passive air samplers (PASs), providing time-averaged concentration of gaseous mercury over the time scale of weeks to months, are promising to fill a gap in the monitoring of atmospheric mercury worldwide. Their usefulness will depend on their ease-of-use and robustness under field conditions, their availability and affordability, and most notably, their ability to provide results of acceptable precision and accuracy. Here we describe a comparative evaluation of three PASs with respect to their ability to record precisely and accurately atmospheric background concentrations at sites in both southern Italy and southern Ontario. The study includes the CNR-PAS with gold nanoparticles as a sorbent, developed by the Italian National Research Council, the IVL-PAS using an activated carbon-coated disk, developed by the Swedish Environmental Research Institute, and the MerPAS® using a sulfur-impregnated activated carbon sorbent, developed at the University of Toronto and commercialized by Tekran. Detection limits are deduced from the variability in the amount of mercury quantified in more than 20 field blank samples for each PAS. Analytical and sampling precision is quantified through 22 triplicated deployments for each PAS ranging in length from two to twelve weeks. Accuracy and bias are assessed through comparison with gaseous elemental mercury concentrations recorded by Tekran 2537 automated mercury analyzers operating alongside the PASs at both locations. The performance of the PASs was significantly better in Italy, with all of them providing concentrations that are not statistically significantly different from the average of the active sampling results. In Canada, where weather conditions were much harsher and more variable during the February through April deployment period, differences were observed amongst PASs. At both sites, the MerPAS® is currently the most sensitive, precise and accurate among the three PASs. A key reason for this is the larger size and the radial configuration of the MerPAS®, which results in blank levels that are lower relative to the sequestered amounts of mercury than in the other two PASs, which rely on axial diffusion geometries. Because the blank-correction becomes relatively smaller with longer deployment, sampler performance tends to be better during deployments of 8 and 12 weeks.

scholarly journals Enhanced production of oxidised mercury over the tropical Pacific Ocean: a key missing oxidation pathway

2014 ◽  
Vol 14 (3) ◽  
pp. 1323-1335 ◽  
Author(s):  
F. Wang ◽  
A. Saiz-Lopez ◽  
A. S. Mahajan ◽  
J. C. Gómez Martín ◽  
D. Armstrong ◽  
...  
Keyword(s):  
Marine Boundary Layer ◽  
Oxidation Mechanism ◽  
Elemental Mercury ◽  
Atmospheric Mercury ◽  
Mercury Oxidation ◽  
Gaseous Elemental Mercury ◽  
Enhanced Production ◽  
Modelling Studies ◽  
Aquatic Food ◽  
The Stability

Abstract. Mercury is a contaminant of global concern. It is transported in the atmosphere primarily as gaseous elemental mercury, but its reactivity and deposition to the surface environment, through which it enters the aquatic food chain, is greatly enhanced following oxidation. Measurements and modelling studies of oxidised mercury in the polar to sub-tropical marine boundary layer (MBL) have suggested that photolytically produced bromine atoms are the primary oxidant of mercury. We report year-round measurements of elemental and oxidised mercury, along with ozone, halogen oxides (IO and BrO) and nitrogen oxides (NO2), in the MBL over the Galápagos Islands in the equatorial Pacific. Elemental mercury concentration remained low throughout the year, while higher than expected levels of oxidised mercury occurred around midday. Our results show that the production of oxidised mercury in the tropical MBL cannot be accounted for by bromine oxidation only, or by the inclusion of ozone and hydroxyl. As a two-step oxidation mechanism, where the HgBr intermediate is further oxidised to Hg(II), depends critically on the stability of HgBr, an additional oxidant is needed to react with HgBr to explain more than 50% of the observed oxidised mercury. Based on best available thermodynamic data, we show that atomic iodine, NO2, or HO2 could all play the potential role of the missing oxidant, though their relative importance cannot be determined explicitly at this time due to the uncertainties associated with mercury oxidation kinetics. We conclude that the key pathway that significantly enhances atmospheric mercury oxidation and deposition to the tropical oceans is missing from the current understanding of atmospheric mercury oxidation.

scholarly journals Air Concentrations of Gaseous Elemental Mercury and Vegetation–Air Fluxes within Saltmarshes of the Tagus Estuary, Portugal

Atmosphere ◽  
2021 ◽  
Vol 12 (2) ◽  
pp. 228
Author(s):  
Rute Cesário ◽  
Nelson J. O’Driscoll ◽  
Sara Justino ◽  
Claire E. Wilson ◽  
Carlos E. Monteiro ◽  
...  
Keyword(s):  
Leaf Area ◽  
Elemental Mercury ◽  
Ambient Air ◽  
Tagus Estuary ◽  
Contaminated Site ◽  
Gaseous Elemental Mercury ◽  
Mercury Flux ◽  
Sarcocornia Fruticosa ◽  
Air Concentrations

In situ air concentrations of gaseous elemental mercury (Hg(0)) and vegetation–atmosphere fluxes were quantified in both high (Cala Norte, CN) and low-to-moderate (Alcochete, ALC) Hg-contaminated saltmarsh areas of the Tagus estuary colonized by plant species Halimione portulacoides (Hp) and Sarcocornia fruticosa (Sf). Atmospheric Hg(0) ranged between 1.08–18.15 ng m−3 in CN and 1.18–3.53 ng m−3 in ALC. In CN, most of the high Hg(0) levels occurred during nighttime, while the opposite was observed at ALC, suggesting that photoreduction was not driving the air Hg(0) concentrations at the contaminated site. Vegetation–air Hg(0) fluxes were low in ALC and ranged from −0.76 to 1.52 ng m−2 (leaf area) h−1 for Hp and from −0.40 to 1.28 ng m−2 (leaf area) h−1 for Sf. In CN, higher Hg fluxes were observed for both plants, ranging from −9.90 to 15.45 ng m−2 (leaf area) h−1 for Hp and from −8.93 to 12.58 ng m−2 (leaf area) h−1 for Sf. Mercury flux results at CN were considered less reliable due to large and fast variations in the ambient air concentrations of Hg(0), which may have been influenced by emissions from the nearby chlor-alkali plant, or historical contamination. Improved experimental setup, the influence of high local Hg concentrations and the seasonal activity of the plants must be considered when assessing vegetation–air Hg(0) fluxes in Hg-contaminated areas.

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