Passive air samplers (PASs) for gaseous mercury (Hg) were deployed for time periods between 1 month and 1 year at 20 sites across the globe with continuous atmospheric Hg monitoring using active Tekran instruments. The purpose was to evaluate the accuracy of the PAS vis-à-vis the industry standard active instruments and to determine a sampling rate (SR; the volume of air stripped of gaseous Hg per unit of time) that is applicable across a wide range of conditions. The sites spanned a wide range of latitudes, altitudes, meteorological conditions, and gaseous Hg concentrations. Precision, based on 378 replicated deployments performed by numerous personnel at multiple sites, is 3.₆ ± 3.₀ % ¹ , confirming the PAS's excellent reproducibility and ease of use. Using a SR previously determined at a single site, gaseous Hg concentrations derived from the globally distributed PASs deviate from Tekran-based concentrations by 14.₂ ± 10 %. A recalibration using the entire new data set yields a slightly higher SR of 0.135₄ ± 0.016 m³ day⁻¹. When concentrations are derived from the PAS using this revised SR the difference between concentrations from active and passive sampling is reduced to 8.₈ ± 7.₅ %. At the mean gaseous Hg concentration across the study sites of 1.54 ng m⁻³, this represents an ability to resolve concentrations to within 0.13 ng m⁻³. Adjusting the sampling rate to deployment specific temperatures and wind speeds does not decrease the difference in active–passive concentration further (8.₇ ± 5.₇ %), but reduces its variability by leading to better agreement in Hg concentrations measured at sites with very high and very low temperatures and very high wind speeds. This value (8.₇ ± 5.₇ %) represents a conservative assessment of the overall uncertainty of the PAS due to inherent uncertainties of the Tekran instruments. Going forward, the recalibrated SR adjusted for temperature and wind speed should be used, especially if conditions are highly variable or deviate considerably from the average of the deployments in this study (9.8₉ ^∘C, 3.4₁ m s⁻¹). Overall, the study demonstrates that the sampler is capable of recording background gaseous Hg concentrations across a wide range of environmental conditions with accuracy similar to that of industry standard active sampling instruments. Results at sites with active speciation units were inconclusive on whether the PASs take up total gaseous Hg or solely gaseous elemental Hg primarily because gaseous oxidized Hg concentrations were in a similar range as the uncertainty of the PAS.

Trees predominantly take up mercury (Hg) from the atmosphere via stomatal assimilation of gaseous elemental Hg (GEM). Hg is oxidised in leaves/needles and transported to other tree anatomy including bole wood, where it can be stored long-term. Using Hg associated with growth rings facilitates archiving of historical GEM concentrations. Nonetheless, there are significant knowledge gaps on the cycling of Hg within trees. We investigate Hg archived in tree rings, internal tree Hg cycling, and differences in Hg uptake mechanisms in Norway spruce and European larch sampled within 1 km of a HgCl₂-contaminated site using total Hg (THg) and Hg stable isotope analyses. Tree ring samples are indicative of significant increases in THg concentrations (up to 521 µg kg⁻¹) from the background period (BGP; facility closed; 1992–present) to secondary industrial period (2ndIP; no HgCl₂ wood treatment; 1962–1992) to primary industrial period (1stIP; active HgCl₂ wood treatment; ≈ 1900–1962). Mass-dependent fractionation (MDF) Hg stable isotope data are shifted negative during industrial periods (δ²⁰²Hg of 1stIP: −4.32 ± 0.15 ‰, 2ndIP: −4.04 ± 0.32 ‰, BGP: −2.83 ± 0.74 ‰; 1 SD). Even accounting for a ≈ −2.6 ‰ MDF shift associated with stomatal uptake, these data are indicative of emissions derived from industrial activity being enriched in lighter isotopes associated with HgCl₂ reduction and Hg⁰ volatilisation. Similar MDF (δ²⁰²Hg: −3.90 ± 0.30 ‰; 1 SD) in bark Hg (137 ± 105 µg kg⁻¹) suggests that stomatal assimilation and downward transport is also the dominant uptake mechanism for bark Hg (reflective of negative stomatal-uptake MDF shift) rather than deposition to bark. THg was enriched in sapwood of all sampled trees across both tree species. This may indicate long-term storage of a fraction of Hg in sapwood or xylem solution. We also observed a small range of odd-isotope mass-independent fractionation (MIF). Differences in Δ¹⁹⁹Hg between periods of different industrial activities were significant (Δ¹⁹⁹Hg of 1stIP: 0.00 ± 0.03 ‰, 2ndIP: −0.06 ± 0.04 ‰, BGP: −0.13 ± 0.03 ‰; 1 SD), and we suggest MIF signatures are conserved during stomatal assimilation (reflect source MIF signatures). These data advance our understanding of the physiological processing of Hg within trees and provide critical direction to future research into the use of trees as archives for historical atmospheric Hg.

Because gaseous elemental mercury (GEM) is distributed globally through the atmosphere, reliable means of measuring its concentrations in air are important. Passive air samplers (PASs), designed to be cheap, simple to operate, and to work without electricity, could provide an alternative to established active sampling techniques in applications such as (1) long-term monitoring of atmospheric GEM levels in remote regions and in developing countries, (2) atmospheric mercury source identification and characterization through finely resolved spatial mapping, and (3) the recording of personal exposure to GEM. An effective GEM PAS requires a tightly constrained sampling rate, a large and stable uptake capacity, and a sensitive analytical technique. None of the GEM PASs developed to date achieve levels of accuracy and precision sufficient for the reliable determination of background concentrations over extended deployments. This is due to (1) sampling rates that vary due to meteorological factors and manufacturing inconsistencies, and/or (2) an often low, irreproducible and/or unstable uptake capacity of the employed sorbents. While we identify shortcomings of existing GEM PAS, we also reveal potential routes to overcome those difficulties. Activated carbon and nanostructured metal surfaces hold promise as effective sorbents. Sampler designs incorporating diffusive barriers should be able to notably reduce the influence of wind on sampling rates.

Passive air sampling of gaseous mercury (Hg) requires a high level of accuracy to discriminate small differences in atmospheric concentrations. Meteorological parameters have the potential to decrease this accuracy by impacting the sampling rate (SR), i.e., the volume of air that is effectively stripped of gaseous mercury per unit of time. We measured the SR of a recently calibrated passive air sampler for gaseous Hg in the laboratory under varying wind speeds (wind still to 6 m s⁻¹), temperatures (−15 to +35 °C), and relative humidities (44 to 80 %). While relative humidity has no impact on SR, SR increases slightly with both wind speed (0.003 m³ day⁻¹ increase in SR or 2.5 % of the previously calibrated SR for every m s⁻¹ increase for wind speeds > 1 m s⁻¹, typical of outdoor deployments) and temperature (0.001 m³ day⁻¹ increase in SR or 0.7 % for every 1 °C increase). The temperature dependence can be fully explained by the effect of temperature on the molecular diffusivity of gaseous mercury in air. Although these effects are relatively small, accuracy can be improved by adjusting SRs using measured or estimated temperature and wind speed data at or near sampling sites. We also assessed the possibility of reusing Radiello^® diffusive barriers previously used in the passive air samplers. The mean rate of gaseous Hg uptake was not significantly different between new and previously used diffusive barriers in both lab and outdoor deployments, irrespective of the applied cleaning procedure. No memory effect from Radiellos^® previously deployed in a high Hg atmosphere was observed. However, a loss in replicate precision for the dirtiest Radiellos^® in the indoor experiment suggests that cleaning is advisable prior to reuse.

Emissions from biomass burning are an important source of mercury (Hg) to the atmosphere and an integral component of the global Hg biogeochemical cycle. In 2018, measurements of gaseous elemental Hg (GEM) were taken on board a research aircraft along with a series of co-emitted contaminants in the emissions plume of an 88 km² boreal forest wildfire on the Garson Lake Plain (GLP) in NW Saskatchewan, Canada. A series of four flight tracks were made perpendicular to the plume at increasing distances from the fire, each with three to five passes at different altitudes at each downwind location. The maximum GEM concentration measured on the flight was 2.88 ng m⁻³, which is ≈ 2.4× background concentration. GEM concentrations were significantly correlated with the co-emitted carbon species (CO, CO₂, and CH₄). Emissions ratios (ERs) were calculated from measured GEM and carbon co-contaminant data. Using the most correlated (least uncertain) of these ratios (GEM:CO), GEM concentrations were estimated at the higher 0.5 Hz time resolution of the CO measurements, resulting in maximum GEM concentrations and enhancements of 6.76 ng m⁻³ and ≈ 5.6×, respectively. Extrapolating the estimated maximum 0.5 Hz GEM concentration data from each downwind location back to source, 1 km and 1 m (from fire) concentrations were predicted to be 12.9 and 30.0 ng m⁻³, or enhancements of ≈ 11× and ≈ 25×, respectively. ERs and emissions factors (EFs) derived from the measured data and literature values were also used to calculate Hg emissions estimates on three spatial scales: (i) the GLP fires themselves, (ii) all boreal forest biomass burning, and (iii) global biomass burning. The most robust estimate was of the GLP fires (21 ± 10 kg of Hg) using calculated EFs that used minimal literature-derived data. Using the Top-down Emission Rate Retrieval Algorithm (TERRA), we were able to determine a similar emission estimate of 22 ± 7 kg of Hg. The elevated uncertainties of the other estimates and high variability between the different methods used in the calculations highlight concerns with some of the assumptions that have been used in calculating Hg biomass burning in the literature. Among these problematic assumptions are variable ERs of contaminants based on vegetation type and fire intensity, differing atmospheric lifetimes of emitted contaminants, the use of only one co-contaminant in emissions estimate calculations, and the paucity of atmospheric Hg species concentration measurements in biomass burning plumes.

The Multi-Compartment Hg (mercury) Modeling and Analysis Project (MCHgMAP) is an international multimodel research initiative intended to simulate and analyze the geospatial distributions and temporal trends of environmental Hg to inform effectiveness evaluations of two multilateral environmental agreements (MEAs): the Minamata Convention on Mercury (MC) and the Convention on Long-Range Transboundary Air Pollution (LRTAP). This MCHgMAP overview paper presents its science objectives, background, and rationale; experimental design (multimodel ensemble (MME) architecture, inputs and evaluation data, simulations, and reporting framework); and methodologies for the evaluation and analysis of simulated environmental Hg levels. The primary goals of the project are to facilitate detection and attribution of recent (observed) and future (projected) spatial patterns and temporal trends of global environmental Hg levels and identification of key knowledge gaps in Hg science and modeling to improve future effectiveness evaluation cycles of the MEAs. The current advances and challenges of Hg models, emission inventories, and observational data are examined, and an optimized multimodel experimental design is introduced to address the key policy questions of the MEAs. A common set of emissions, environmental conditions, and observation datasets is proposed (where possible) to enhance the MME comparability. A novel harmonized simulation approach between atmospheric, land, oceanic, and multimedia models is proposed to account for the short- and long-term changes in secondary Hg exchanges and to achieve mechanistic consistency of Hg levels across environmental matrices. A comprehensive set of model experiments is proposed and prioritized to ensure systematic analysis and participation of a variety of models from the scientific community.