A review of passive sampling systems for ambient air mercury measurements

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

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

Comparison of active measurements, lichen biomonitoring, and passive sampling for atmospheric mercury monitoring

The number of atmospheric mercury (Hg) monitoring stations is growing globally. However, there are still many regions and locations where Hg monitoring is limited or non-existent. Expansion of the atmospheric Hg monitoring network could be facilitated by the use of cost-effective monitoring methods. As such, biomonitoring and passive monitoring offer a unique alternative to well-established monitoring by active measurements, since they do not require a power supply and require minimal workload to operate. The use of biomonitoring (lichens and mosses) and passive air samplers (PASs) (various designs with synthetic materials) has been reported in the literature, and comparisons with active measurement methods have also been made. However, these studies compared either biomonitoring or PASs (not both) to only one type of active measurement. In our work, we used transplanted (7 sampling sites) and in situ lichens (8 sampling sites) for biomonitoring, two PASs from different producers (3 sampling sites), and two different active measurement types (continuous and discontinuous active measurements, 1 and 8 sampling sites, respectively) to evaluate their effectiveness as monitoring methods. In the 9-month sampling campaign, 3 sampling locations with different characteristics (unpolluted, vicinity of a cement plant, and vicinity of a former Hg mine) were used. The results obtained with lichens and PASs clearly distinguished between sampling locations with different Hg concentrations; using both PASs and lichens together increased the confidence of our observations. The present work shows that biomonitoring and passive sampling can be effectively used to identify areas with elevated atmospheric Hg concentrations. The same can be said for discontinuous active measurements; however, the discrepancy between atmospheric Hg concentrations derived from PASs and discontinuous active measurements should be further investigated in the future.

Optimization and application of passive air sampling method for gaseous elemental mercury in Ulsan, South Korea

We compared uptake rates and concentrations of gaseous elemental mercury (GEM) by passive sampling conditions and investigated the spatial distribution of GEM in Ulsan, the largest industrial city in South Korea. For the optimization of sampling conditions, two outer sampling containers (cylindrical polyethylene terephthalate and two stainless steel bowls), two different sulfur contents of the sorbent (16.3% and 26.3%), and three sampling periods (1, 2, and 3 months) were considered. The uptake rates of GEM were not statistically different by the sampling container, but they were increased with the sulfur contents of activated carbon. A sampling condition using two stainless bowls and lower sulfur contents of activated carbon for 2-3 months was preferred with the highest precision of GEM concentrations. With the same method, passive air samples were collected for 3 months in duplicate from 10 sites in Ulsan. The concentrations of GEM ranged from 3.13 to 11.2 ng/m³ (mean 4.65 ng/m³), and the highest concentration was measured at a non-ferrous industrial complex. A zinc smelter in the non-ferrous industrial complex was identified as a major mercury source in Ulsan. This study is the first passive air sampling study investigating the spatial distributions of GEM in different types of industrial areas as well as residential areas of Ulsan.

Screen-Printed Gold Electrodes as Passive Samplers and Voltammetric Platforms for the Determination of Gaseous Elemental Mercury

We present a methodology for the determination of gaseous elemental mercury (GEM). It is based on passive sampling of Hg on screen-printed gold electrodes (SPGEs), followed by the measurement of amalgamated mercury by square wave anodic stripping voltammetry. We have explored in detail the behavior of the SPGE electrode surface during the sampling process (by time-of-flight secondary ion mass spectrometry), the stability of the voltammetric signals, and the inter-electrode reproducibility, and obtained acceptable results. Adsorption of mercury onto the SPGE follows a nearly linear behavior until the sorbent becomes saturated (equilibrium phase) for different mercury concentrations, allowing to select a sampling time of 30 min for calibration. The theoretical behavior of the sampling system was modeled, considering the changes in the diffusive path length between the porous diffusive barrier and the adsorbed surface, L. Finally, we have tested two GEM calibration protocols. The first one is based on the measurement of the mercury stripping peak area, A_Hg, and the second one is based on the measurement of the mass of mercury, m_Hg, by standard additions. We found good correlation coefficients between the GEM concentration for both A_Hg (R² = 0.9591) and m_Hg (R² = 9615) in the range of 5.82 to 59.29 ng dm^–3 GEM. Detection limits were 5.32 and 5.22 ng dm^–3 for A_Hg and m_Hg, respectively. Our results open a new line of electroanalytical strategies for the determination of GEM in atmospheric samples.

Direct Measurement of Mercury Deposition at Rural and Suburban Sites in Washington State, USA

Because of mercury’s (Hg) capacity for long-range transport in the atmosphere, and its tendency to bioaccumulate in aquatic biota, there is a critical need to measure spatial and temporal patterns of Hg atmospheric deposition. Dry deposition of Hg is commonly calculated as the product of a measured atmospheric concentration and an assumed deposition velocity. An alternative is to directly assess Hg deposition via accumulation on surrogate surfaces. Using a direct measurement approach, this study quantified Hg deposition at a rural site (Pullman) and suburban site (Puyallup) in Washington State using simple, low-cost equipment. Dry deposition was measured using an aerodynamic “wet sampler” consisting of a Teflon plate, 35 cm in diameter, holding a thin layer (2.5 mm) of recirculating acidic aqueous receiving solution. In addition, wet Hg deposition was measured using a borosilicate glass funnel with a 20-cm-diameter opening and a 1 L Teflon sampling bottle. Hg deposition was estimated based on changes in total Hg in the aqueous phase of the samplers. Dry Hg deposition was 2.4 ± 1.4 ng/m2·h (average plus/minus standard deviation; n = 4) in Pullman and 1.3 ± 0.3 ng/m2·h (n = 6) in Puyallup. Wet Hg deposition was 7.0 ± 4.8 ng/m2·h (n = 4) in Pullman and 1.1 ± 0.2 ng/m2·h (n = 3) in Puyallup. Relatively high rates of Hg deposition in Pullman were attributed to regional agricultural activities that enhance mercury re-emission and deposition including agricultural harvesting and field burning. Hg concentration in precipitation negatively correlated with precipitation depth, indicating that Hg was scavenged from the atmosphere during the beginning of storm events. Because of their relative simplicity and robustness, direct measurement approaches such as those described in this study are useful in assessing Hg deposition, and for comparing results to less direct estimates and model estimates of Hg deposition.

Characteristics and Performances of a Nanostructured Material for Passive Samplers of Gaseous Hg

Passive air samplers (PASs) have been used for mapping gaseous mercury concentration in extensive areas. In this work, an easy-to-use and -prepare gold nanoparticle (NP)-based PAS has been investigated. The PAS is constituted of a microfibrous quartz disk filter impregnated of gold NP photo-growth on TiO2 NPs (Au@TiO2) and used as gaseous mercury adsorbing material. The disk was housed in a cylinder glass container and subjected to an axial diffusive sampling. The adsorbed mercury was measured by thermal desorption using a Tekran® instrument. Different amounts of Au@TiO2 (ranging between 4.0 and 4.0 × 10-3 mg) were deposited by drop-casting onto the fibrous substrate and assessed for about 1 year of deployment in outdoor environment with a mercury concentration mean of about 1.24 ± 0.32 ng/m3 in order to optimize the adsorbing layer. PASs showed a linear relation of the adsorbed mercury as a function of time with a rate of 18.5 ± 0.4 pg/day (≈1.5% of the gaseous concentration per day). However, only the PAS with 4 mg of Au@TiO2, provided with a surface density of about 3.26 × 10-2 mg/mm2 and 50 μm thick inside the fibrous quartz, kept stability in working, with a constant sampling rate (SR) (0.0138 ± 0.0005 m3/day) over an outdoor monitoring experimental campaign of about 1 year. On the other hand, higher sampling rates have been found when PASs were deployed for a few days, making these tools also effective for one-day monitoring. Furthermore, these PASs were used and re-used after each thermal desorption to confirm the chance to reuse such structured layers within their samplers, thus supporting the purpose to design inexpensive, compact and portable air pollutant sampling devices, ideal for assessing both personal and environmental exposures. During the whole deployment, PASs were aided by simultaneous Tekran® measurements.

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.

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.

Application of the Passive Sampler Developed for Atmospheric Mercury and Its Limitation

In this study, a passive sampler for gaseous elemental mercury (GEM) was developed and applied to field monitoring. Three Radiello® diffusive bodies with gold-coated beads as Hg adsorbent were installed in an acrylic external shield. Hg uptake mass linearly increased as the deployment time increased until 8 weeks with an average gaseous Hg concentration of 2 ng m−3. The average of the experimental sampling rate (SR) was 0.083 m3 day−1 and showed a good correlation with theoretical SRs, indicating that a major adsorption mechanism was molecular diffusion. Nonetheless, the experimental SR was approximately 33% lower than the modeled SR, which could be associated with inefficient uptake of GEM in the sampler or uncertainty in constraining model parameters. It was shown that the experimental SR was statistically affected by temperature and wind speed but the calibration equation for the SR by meteorological variables should be obtained with a wider range of variables in further investigation. When the uptake rates were compared to the active Hg measurements, the correlation was not significant because the passive sampler was not sufficiently adept at detecting a small difference in the GEM concentration of from 1.8 to 2.0 ng m−3. However, the results for spatial Hg concentrations measured near cement plants in Korea suggest a possible application in field monitoring. Future research is needed to fully employ the developed passive sampler in quantitative assessment of Hg concentrations.

Stable Isotope Evidence Shows Re-emission of Elemental Mercury Vapor Occurring after Reductive Loss from Foliage

The mechanism of elemental mercury (Hg⁰) re-emission from vegetation to the atmosphere is currently poorly understood. In this study, we investigated branch-level Hg⁰ atmosphere-foliage exchange in a pristine evergreen forest by systematically combining Hg isotopic composition, air concentration and flux measurements to unravel process information. It is found that the foliage represents a diurnally changing sink for atmospheric Hg⁰ and its Hg content increases with leaf age and mass. Atmospheric Hg⁰ is the dominant source of foliar Hg and the involvement of Hg^II is not supported by isotopic evidence. Upon Hg⁰ uptake, maturing foliage becomes progressively enriched in lighter Hg isotopes and depleted in odd mass isotopes. The measured isotopic composition of foliage Hg and isotopic shift caused by Hg⁰ evasion from foliage supports that Hg⁰ emitted from foliage is derived from Hg previously metabolized and bound in the leaf interior then subsequently recycled after reduction, and not merely a retroflux of recently deposited Hg⁰ on foliar surface. An isotopic differential mass balance model indicates that the proportion of foliar Hg⁰ efflux to uptake gradually increase from emergence to senescence with an average flux ratio of 30%.

Use of Gold Nanoparticles as Substrate for Diffusive Monitoring of Gaseous Mercury

In the present work, the study and the performances of an adsorbent material for gaseous mercury employed in different diffusive bodies geometries is presented. The material is based on gold nanoparticles (AuNPs) deposited on quartz fibres filters, suitable for bonding the gaseous mercury through an amalgamation process. Following thermal desorption and analysis, the behavior of different diffusive samplers prototypes was compared. Both indoor and outdoor exposures were carried out in order to evaluate the advantages and shortcomings of the geometries in study at different sites. From the outdoor long-term exposures, a constant uptake rate (U_r), with a low influence coming from the environmental conditions, was observed for the axial geometry, reporting a high coefficient of determination (R² 0.97). Indoor exposures showed a higher reproducibility, along with a higher coefficient of determination (R² 0.99). The presented results allowed us to observe different behaviors coming from two kinds of diffusive samplers designs, showing different adsorption rates and data dispersion. This allowed us to focalize our attention on the most suitable design from these two tested prototypes, for this kind of adsorbent material.

Passive Sampling of Gaseous Elemental Mercury Based on a Composite TiO₂NP/AuNP Layer

Passive sampling systems (PASs) are a low cost strategy to quantify Hg levels in air over both different environmental locations and time periods of few hours to weeks/months. For this reason, novel nanostructured materials have been designed and developed. They consist of an adsorbent layer made of titania nanoparticles (TiO₂NPs, ≤25 nm diameter) finely decorated with gold nanoparticles. The TiO₂NPs functionalization occurred for the photocatalytic properties of titania-anatase when UV-irradiated in an aqueous solution containing HAuCl₄. The resulting nanostructured suspension was deposited by drop-casting on a thin quartz slices, dried and then incorporated into a common axial sampler to be investigated as a potential PAS device. The morphological characteristics of the sample were studied by High-Resolution Transmission Electron Microscopy, Atomic Force Microscopy, and Optical Microscopy. UV-Vis spectra showed a blue shift of the membrane when exposed to Hg⁰ vapors. The adsorbed mercury was thermally desorbed for a few minutes, and then quantified by a mercury vapor analyzer. Such a sampling system reported an efficiency of adsorption that was equal to ≈95%. Temperature and relative humidity only mildly affected the membrane performances. These structures seem to be promising candidates for mercury samplers, due to both the strong affinity of gold with Hg, and the wide adsorbing surface.

Proof of concept for a passive sampler for monitoring of gaseous elemental mercury in artisanal gold mining

Mercury emissions from artisanal gold mining operations occurring in roughly 80 developing countries are a major workplace health hazard for millions of people as well as the largest contributor to global mercury pollution. There are no portable, cheap, and rapid methods able to inform workers or health practitioners of mercury exposure on site in remote locations. In this work, a proof of concept for a miniaturized mercury sampler, prepared by the direct reduction of gold into the porous nanostructures of Vycor glass (PVG), is introduced. Mercury retention on the PVG/Au sampler induces significant color changes, due to the formation of Au-Hg amalgam that affects the surface plasmon resonance characteristics of the material. The color change can potentially be quantified by the analysis of pictures obtained with a cell phone camera rapidly and onsite. Laboratory experiments showed the viability of using PVG/Au as passive sampler for monitoring of Hg°. PVG/Au samplers were then deployed in an artisanal and small-scale gold mining (ASGM) operations in Burkina Faso and it was able to indicate personal mercury exposures. The amount of mercury quantified in the samplers for all miners was higher than the current personal exposure limit set by the US Occupational Safety & Health Administration (OSHA).

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.