2.1 Study site

The study was conducted at Toolik Field Station (68^∘38^′ N, 149^∘36^′ W) on the Arctic Coastal Plain in northern Alaska, USA, 180 km inland from the Arctic Ocean coast. All measurements were conducted on an acidic tussock tundra, on Aquiturbels soils with an active layer of 60–100 cm (Obrist et al., 2017). The climate of Toolik Field Station is characterised by low mean annual temperatures of −8.5 ^∘C and a mean annual precipitation of 312 mm a⁻¹ (Cherry et al., 2014). During the 1-year Hg isotope campaign from October 2015 to September 2016, the tundra was snow-covered for a total of 248 d (Agnan et al., 2018), leading to a relatively short snow-free growing season.

2.2 Hg stable isotope sampling and measurement

Hg(0) was continuously sampled in the atmosphere (0.3 and 2 m above ground), in interstitial air of surface snow (0 and 0.1 m above ground), and in soil pore air (0.4 m below ground) at low flow rates of 0.2 L min⁻¹. Interstitial snow air for Hg(0) stable isotope measurements was sampled from a dedicated snow tower adapted from Seok et al. (2009), consisting of an aluminium construction with three gas inlets on horizontal bars at 0, 0.1, and 0.3 m above ground (Fig. S1a in the Supplement). Two soil wells as described in Obrist et al. (2017) were dedicated to Hg(0) stable isotope sampling of soil air. Each soil well had two gas inlets, positioned 1.5 m apart at 0.4 m depth, each inlet consisting of a 47 mm single stage filter assembly (Savillex, Eden Prairie, USA) with a Teflon^® filter membrane (Fig. S1b).

Hg(0) was trapped on iodated activated carbon (IAC) traps (Brooks Rand, 0.1 g in custom made 12 cm long glass tubes with an inner diameter of 4 mm, Fu et al., 2014) and samples were changed manually during site visits every 6 to 8 weeks. During site visits in March and June 2016, higher temporal resolution (2–4 d) sampling was conducted with higher flow rates of 1.5–2 L min⁻¹. Overall, total volumes of sampled air per sample ranged from 5.7 to 17.7 m³ (Table S1 in the Supplement). During the growing season (June to September 2016), diel variation of atmospheric Hg(0) was assessed using two parallel sampling lines operated with a time switch. Daytime samples were collected from 06:00 to 22:00 AKST (Alaska standard time – used for all times referenced throughout the paper), and night-time samples were collected from 22:00 to 06:00. Of the 14 soil pore air samples taken during the 1-year Hg isotope campaign, only three samples contained sufficient Hg (>2.5 ng) for isotopic analysis. The reason for this was that the Hg(0) in soil pore air was largely depleted (below the detection limit of ≈0.1 ng m⁻³) (Obrist et al., 2017; Agnan et al., 2018), in particular during winter months, making isotopic characterisation of the soil pore air Hg(0) pool impossible during this time period.

The protocol for Hg stable isotope measurements of Hg(0) was adapted from Fu et al. (2014). We used lower amounts of IAC trap material (0.1 g) to reduce possible matrix effects during cold-vapour generation. Breakthrough was tested in the lab and under field conditions by connecting a Tekran 2537 after the IAC trap, and Hg(0) measurements were always below the detection limit (<0.1 ng m⁻³). IAC traps were combusted in a two-stage oven system and Hg was recovered in a 4.2 N HNO₃, 1.2 N HCl oxidising acid trap. Hg stable isotope ratios of trap solutions were measured by cold-vapour separation multi-collector inductively coupled plasma mass spectrometry (CV MC-ICP/MS) at the Observatory Midi-Pyrénées, Toulouse, France (Jiskra et al., 2019; Sun et al., 2013). Six process blanks and one field blank were measured during the sample processing and were 0.25 ng Hg/trap (max 0.85 ng Hg/trap) on average, representing 1 %–2 % of the typical Hg amounts collected during sampling periods. The amounts of Hg collected on IAC-traps sampling atmospheric Hg(0) were compared to Hg(0) concentration measurements using a Tekran 2537 and revealed sample yields of 107±23 % (mean ± 1 SD, n=22).

Mass-dependent fractionation (MDF) of Hg stable isotopes is reported in small delta notation (δ) in per mille (‰) deviation from to the reference NIST 3133 Hg standard:

where “xxx” refers to measured isotope masses: 199, 200, 201, 202, and 204. Mass-independent fractionation (MIF) is reported in capital delta notation (Δ), which is defined as the difference between the measured δ¹⁹⁹Hg, δ²⁰⁰Hg, δ²⁰¹Hg, and δ²⁰⁴Hg values and those predicted for MDF relative to δ²⁰²Hg using the kinetic MDF law:

where SF^xxx is the mass-dependent scaling factor of 0.252 for ¹⁹⁹Hg, 0.502 for ²⁰⁰Hg, 0.752 for ²⁰¹Hg, and 1.493 for ²⁰⁴Hg (Blum and Bergquist, 2007). Hg isotope enrichment factors associated with two pools (ε^xxxHg_{pool 1−pool 2} and E^xxxHg_{pool 1−pool 2}) were calculated from the difference in the MDF and MIF signatures between two pools (pool 1 and pool 2) as follows:

The MDF enrichment factors of a reaction (ε^xxxHg_reaction) were determined by fitting a linear regression model (lm function of R) to the observational data following Mariotti et al. (1981):

where δ^xxxHg_residual corresponds to the Hg isotope signature of the residual Hg(0), δ^xxxHg₀ corresponds to the initial Hg(0) isotope signature, and f corresponds to the fraction of Hg(0) remaining in the gas phase. Note that for high f>0.4 the systematic error of this simplified approach is minimal. The MIF enrichment factor (E^xxxHg_reaction) was calculated as follows:

The long-term precision was assessed through repeated analysis of the ETH-Fluka Hg standard, which yielded values of $-\mathrm{1.44}\pm \mathrm{0.19}$ ‰, 0.08±0.1 ‰, 0.02±0.1 ‰, 0.02±0.09 ‰, and $-\mathrm{0.03}\pm \mathrm{0.2}$ ‰ (2σ, n=73) for δ²⁰²Hg, Δ¹⁹⁹Hg, Δ²⁰⁰Hg, Δ²⁰¹Hg, and Δ²⁰⁴Hg, respectively, which is in agreement with published values (Jiskra et al., 2015; Smith et al., 2015). The UM-Almaden standard, now available as NIST-8610, was measured less frequently and results were $-\mathrm{0.58}\pm \mathrm{0.15}$ ‰, $-\mathrm{0.02}\pm \mathrm{0.09}$ ‰, 0.00±0.1 ‰, $-\mathrm{0.06}\pm \mathrm{0.12}$ ‰, and $-\mathrm{0.04}\pm \mathrm{0.23}$ ‰ (2 SD, n=21) for δ²⁰²Hg, Δ¹⁹⁹Hg, Δ²⁰⁰Hg, Δ²⁰¹Hg, and Δ²⁰⁴Hg, which is in agreement with previously reported values (Demers et al., 2013; Jiskra et al., 2015; Enrico et al., 2016).

2.3 Hg(0) flux measurements

Micrometeorological flux measurements to quantify Hg(0) exchange at the ecosystem level were conducted using the aerodynamic gradient flux method. Surface–atmosphere flux was calculated by measurement of concentration gradients in the atmosphere above the tundra in conjunction with atmospheric turbulence parameters as follows:

where k denotes the von Kármán constant (0.4), u_∗ is the friction velocity, z is the measurement height, ϕ_h is the Monin–Obukhov scaling coefficient (dimensionless) (Monson and Baldocchi, 2014), and $\partial c\left(\mathrm{Hg}\right(\mathrm{0}\left)\right)/\partial z$ is the vertical Hg(0) gas concentration gradient. Hg(0) concentrations at heights of 61 and 363 cm above the soil surface were measured through 0.2 µm Teflon^® inlet filters connected to ∼35 m of perfluoroalkoxy polymer (PFA) lines. A valve control system with three-way solenoid valves (NResearch, West Caldwell, NJ, USA) allowed switching between the gradient inlets to take place every 10 min. A set of trace gas analysers with a total sampling flow of 1.5 L min⁻¹ was connected to the gradient inlets by solenoid valves. The trace analysers included an air mercury analyser (Model 2537A, Tekran Inc. Toronto, Canada) and a cavity ring-down (CRD) greenhouse gas analyser to measure CO₂, H₂O, and CH₄ (Los Gatos Research, San Jose, USA). Fluxes were only calculated during periods of appropriate turbulence following Edwards et al. (2005) and as described in Obrist et al. (2017). Gradient data recorded during very stable (ζ>2) and very unstable (ζ>2) periods were removed from the dataset (4 %). The overall data coverage was 79 %. Only flux data with wind directions from the tundra (40–300^∘ N) were considered (73 %). No gap filling was performed. For quality control, sampling line blanks and line intercomparisons, where the two gradient lines were put on the same height, were performed approximately every 6 to 8 weeks (Obrist et al., 2017). The planetary boundary layer (PBL) stability was assessed via the stability index (ζ, dimensionless), defined as

In the above-mentioned equation, z represents the height of the Metek USA-1 sonic anemometer (Metek GmbH, Elmshorn, Germany), i.e. 236 cm above ground, and L represents the Monin–Obukhov length. The PBL was considered stable when ζ>0.1, unstable when $\mathit{\zeta }<-\mathrm{0.1}$, and neutral when $-\mathrm{0.1}<\mathit{\zeta }<\mathrm{0.1}$ (Peichl et al., 2013). During the sampling period, auxiliary variables showed the following daily average values: air temperature of −7.4 ^∘C (from −40.6 to 20.4 ^∘C), relative humidity of 74 % (from 37 % to 98 %), and a wind speed of 2.36 m s⁻¹ (from 0 to 7.82 m s⁻¹).

2.4 Backward trajectory modelling

Backward trajectories of air masses were modelled using the HYSPLIT transport and dispersion model (Stein et al., 2015) from the NOAA Air Resources Laboratory, which is accessible via the READY website (Rolph et al., 2017).

3.1 Winter

Over the winter period, atmospheric Hg(0) concentrations and CO₂ mixing ratios were relatively constant and there was little diel variation (Figs. 1a, S2). Low solar radiation led to relatively stable PBL conditions throughout the day (Fig. 1b). Hg(0) flux measurements revealed a small deposition ($-\mathrm{0.34}\pm \mathrm{5.8}$ ng m⁻² h⁻¹, mean ±1 SD, median = −0.2 ng m⁻² h⁻¹) (Obrist et al., 2017). The Hg(0) net deposition flux is supported by observed depletions of atmospheric Hg(0) in interstitial snow air (0.69±0.22 ng m⁻³, mean ±1 SD, 0 and 0.1 m sampling height) relative to atmospheric levels (1.07±0.04 ng m⁻³ – mean ±1 SD) (p=0.02, two-sided t test), implying a net sink of atmospheric Hg(0) in the ecosystem (Fig. 2c). A wintertime Hg(0) sink can either occur due to (i) Hg(0) deposition to Arctic snow, (ii) uptake by soil or litter, or (iii) assimilation by vegetation still active under the snowpack. Depletion of atmospheric Hg(0) in interstitial snow air was associated with an increase in δ²⁰²Hg (1.08 ‰ ± 0.20 ‰ versus 0.77 ‰ ± 0.16 ‰ in ambient Hg(0), mean ± 1 SD, p=0.02, two-sided t test) (Fig. 2a) and a decrease in Δ¹⁹⁹Hg (−0.31 ‰ ± 0.05 ‰ versus −0.23 ‰ ± 0.06 ‰ in ambient Hg(0), mean ± 1 SD, p=0.04, two-sided t test) (Fig. 2b). Both Hg(0) dry deposition to surface snow (Douglas and Blum, 2019) and by vegetation uptake (Demers et al., 2013; Enrico et al., 2016; Obrist et al., 2017) have been reported to discriminate heavier Hg(0) isotopes, consistent with these observations in the interstitial snow air. From a mass balance perspective, however, Hg(0) dry deposition to snow is considered to only play a minor role in the interior Arctic tundra. For example, using snow data in Agnan et al. (2018), we calculated a total seasonal snow Hg pool of only 50 ng m⁻² at Toolik Field Station. Assuming that all this Hg in the snow was originating from the dry deposition of Hg(0), this would account for <10 % of the Hg(0) deposition during the snow-covered period (total of 2.4 µg m⁻¹ a⁻¹; Obrist et al., 2017). In contrast, Douglas and Blum (2019) recently suggested that Hg(0) dry deposition to snow was the major source of Hg in meltwater collected on the coast of the Arctic Ocean close to Utqiagvik (former Barrow), ∼400 km north-west of Toolik Field Station. In snow on the coast of the Arctic Ocean, concentrations of halogens, which are considered to mediate reactive Hg(0) uptake, are elevated compared with inland sites (Douglas and Sturm, 2004; Agnan et al., 2018; Douglas et al., 2017), leading to much higher snow Hg pools (>2000 ng m⁻²) in coastal snowpacks (Douglas et al., 2017).

A major wintertime Hg(0) deposition pathway to soils would be inconsistent with the observed enrichment in heavier Hg(0) isotopes in the interstitial snow air (Fig. 2a), as Hg(0) oxidation by humic acids in soils would lead to more negative δ²⁰²Hg values (Zheng et al., 2018) (see discussion below in Sect. 3.3.3). Hg(0) re-emission from snow was insignificant in winter due to the absence of sunlight, as indicated by lower Hg(0) concentrations in interstitial snow air than in the atmosphere. By excluding snow and soil uptake as dominant deposition pathway, we infer that the uptake of Hg(0) by ground vegetation and/or litter on the soil surface may be driving Hg(0) deposition during the winter months. MDF and MIF signatures observed in interstitial snow air Hg(0) would be consistent with Hg(0) uptake by lichen. Hg isotope signatures of lichen measured at the same site (δ²⁰²Hg = −0.80 ‰ ± 0.20 ‰, Δ¹⁹⁹Hg = 0.20 ‰ ± 0.21 ‰, mean ± 1 SD, n=12) were in the opposite direction of interstitial snow air Hg(0) (Olson et al., 2019). Lichen have been reported to actively exchange CO₂ for photosynthesis under snow cover (Kappen, 1993), and could possibly also take up atmospheric Hg(0) during winter months.

Figure 1Diel variation during winter (a, b, c), spring (d, e, f), and summer (g, h, i): (a, d, g) average Hg(0) concentration (in red) and the CO₂ mixing ratio (in blue) in the atmosphere (2 m height). (b, e, h) The median planetary boundary layer stability parameter (ζ, grey), where positive values represent stable conditions, and solar radiation is shown in yellow. (c, f, i) The average Hg(0) flux (in red) and the average CO₂ flux (in blue). The shaded areas in (a) and (c) represent the mean ± 1 SD interval of concentration and flux measurements, respectively.

Download

Figure 2Hg(0) measurements in interstitial snow air profiles. (a) Mass-dependent fractionation stable isotope signature of Hg(0) (δ²⁰²Hg), (b) mass-independent fractionation stable isotope signature of Hg(0) (Δ¹⁹⁹Hg), and (c) mean Hg(0) concentration. Note that the Hg(0) concentration marked with an asterisk (*) was calculated from Hg recovered on the IAC traps, whereas other Hg(0) concentration profiles were measured by an automated trace gas system deployed in the snowpack (Agnan et al., 2018). The dashed horizontal lines represent the average snow height during the respective period.

Download

3.2 Spring

During spring 2016, two major AMDEs occurred (events 1 and 3; Fig. 3). During both events air masses were transported from the Arctic coast or the Arctic Ocean to Toolik Field Station, as inferred from HYSPLIT backward trajectory modelling. During the 19 March–20 March 2016 period (event 1 in Fig. 3), atmospheric Hg(0) concentrations dropped below the detection limit (<0.1 ng m⁻³), while atmospheric Hg(II) concentrations remained low (<0.1 ng m⁻³), with the exception of individual spikes of up to 0.4 ng m⁻³. During the 1–3 April 2016 period (event 3 in Fig. 3), atmospheric Hg(0) concentrations decreased below the detection limit at times, while Hg(II) concentrations remained high (around 0.4 ng m⁻³) for 2 days. During the similar AMDEs  1 and 3, O₃ mixing ratios dropped below 10 ppb. Van Dam et al. (2013) previously reported that during AMDEs ozone depletion events (ODEs) also occurred at Toolik Field Station, concurrent AMDEs and ODEs were also observed on the coast at Utqiagvik ∼400 km to the north-west. Van Dam et al. (2013) suggested that AMDEs and ODEs were driven by bromine emissions from the Arctic Ocean and transported to different extents to the interior tundra. During event 1, no elevated Hg(II) concentrations were observed; thus, we assume that air depleted in Hg(0) was transported to Toolik Field Station, while the deposition of Hg(II) likely occurred closer to the coast. During the 26–29 March 2016 period (event 2 in Fig. 3), when air masses were coming from the south, atmospheric Hg(0) concentrations temporally decreased to 0.75 ng m⁻³, O₃ remained high (>30 ppb), and Hg(II) concentrations remained enhanced around 0.2 ng m⁻³ for 2 days while turbulent PBL conditions prevailed. The source of these enhanced Hg(II) levels transported from the Brooks Range mountains remains unknown.

Figure 3Time series during the springtime AMDE period (18 March–4 April 2016) for (a) the atmospheric Hg(0) concentration (red), the atmospheric Hg(II) concentration (green), and the O₃ mixing ratio (blue); (b) planetary boundary layer stability (ζ), where the shaded areas in green represent stable conditions (ζ>0.1) and shaded areas in red represent turbulent conditions ($\mathit{\zeta }<-\mathrm{0.1}$); (c) solar radiation and air temperature; and (d) Hg(0) flux, where Hg(0) deposition is shown using green and Hg(0) re-emission is shown using red. Midnight is indicated by dashed lines. Strong AMDEs when Hg(0) concentrations dropped (1, 3) or Hg(II) concentrations increased (2) are marked using numbers.

Download

During the AMDE period (17 March–5 April 2016) an overall net Hg(0) re-emission (1.6±11 ng m⁻² h⁻¹, mean ± 1 SD, median = 0.28 ng m⁻² h⁻¹) was measured which peaked after the Hg(II) deposition, making the AMDE period the only time of the year when net Hg(0) re-emission occurred (Obrist et al., 2017). Strong Hg(0) re-emission from the snowpack has been reported during and after AMDEs due to the fast reduction of Hg(II) deposition (Johnson et al., 2008; Douglas et al., 2012).

Snowmelt occurred in May in 2016, during which the snow height declined rapidly between 7 May (24 cm) and 13 May (0 cm) and a small Hg(0) deposition possibly occurred ($-\mathrm{0.24}\pm \mathrm{3.9}$ ng m⁻² h⁻¹, mean ± 1 SD, median: 0.08 ng m⁻² h⁻¹). In contrast to wintertime patterns, the interstitial snow air Hg(0) during snowmelt showed low Δ¹⁹⁹Hg values of −0.62 ‰ and −0.44 ‰ (n=2) versus −0.23 ‰ ± 0.06 ‰ in ambient Hg(0) (Fig. S3b). The negative Δ¹⁹⁹Hg values in Hg(0) suggest a substantial contribution of Hg(0) re-emission after photoreduction of Hg(II) in snow, which exhibited negative Δ¹⁹⁹Hg values with a minimum of −1.37 ‰ (Obrist et al., 2017). This observation is consistent with previous chamber experiments, where a negative Δ¹⁹⁹Hg value of −2.08 ‰ was reported for Hg(0) re-emission from snow (Sherman et al., 2010).

Even-MIF (Δ²⁰⁰Hg) is considered to be unaffected by post-deposition processes such as re-emission (Sherman et al., 2010; Enrico et al., 2016), providing a conservative tracer for the pathway of atmospheric Hg deposition. Δ²⁰⁰Hg values measured in snow impacted by AMDEs at Toolik Field Station and other sites in Alaska (Obrist et al., 2017; Sherman et al., 2010, 2012a) are similar to the Δ²⁰⁰Hg values of atmospheric Hg(0) (Fig. 4) (−0.06 ‰ ± 0.06 ‰ versus −0.05 ‰ ± 0.04 ‰, mean ± 1 SD). This similarity can be explained by a quantitative oxidation of atmospheric Hg(0) to Hg(II) (e.g. event 1 in Fig. 3) that is deposited to snow. Thus, Hg(II) in snow inherits the isotopic composition of the source Hg(0) due to conservation of mass, irrespective of the isotopic fractionation factor associated with Hg(0) oxidation. Several samples exhibited Δ²⁰⁰Hg values between that of atmospheric Hg(0) and Hg(II) in precipitation measured in temperate regions (Fig. 4). This intermediate Δ²⁰⁰Hg signature can be explained by either AMDEs with non-quantitative oxidation due to limited Br oxidant availability (e.g. event 2 in Fig. 3), or a mixing of AMDE-derived Hg(II) with Hg(II) present in the overlying Arctic free troposphere. Even-MIF (Δ²⁰⁰Hg) has been suggested as a promising tracer to distinguish between atmospheric deposition of Hg(II) in precipitation, which exhibits positive Δ²⁰⁰Hg anomalies, and direct Hg(0) deposition (e.g. uptake by vegetation), which exhibits small negative Δ²⁰⁰Hg (Enrico et al., 2016; Sun et al., 2019). We caution that the presence of AMDEs complicates the use of Δ²⁰⁰Hg for mixing model-based Hg deposition calculations in the Arctic (Obrist et al., 2017).

Figure 4Mass-dependent fractionation (δ²⁰²Hg) versus even mass-independent fractionation (Δ²⁰⁰Hg) of atmospheric Hg(0) (red circles), Hg(II) in wet deposition sampled at locations or during seasons when no AMDEs occurred (blue triangles), and Hg(II) in Arctic show sampled during the springtime AMDE season (green diamonds) for the Arctic tundra at Toolik Field Station (filled symbols), and reported from elsewhere (empty symbols). Atmospheric Hg(0) data are from Gratz et al. (2010), Demers et al. (2013, 2015), Fu et al. (2016a), Enrico et al. (2016), and Obrist et al. (2017); Hg(II) in wet deposition are from Demers et al. (2013), Sherman et al. (2015), Chen et al. (2012), and Gratz et al. (2010); and Hg(II) in snow from AMDEs are from Obrist et al. (2017) and Sherman et al. (2010, 2012a). Error bars represent the analytical precision determined by the 2 SD from multiple measurements of an in-house standard.

3.3 Summer

3.3.1 Drivers of diel cycling in atmospheric Hg(0)

Figure 5 represents a time series of the atmospheric Hg(0) concentration, the CO₂ mixing ratio, the PBL stability, and Hg(0) fluxes during midsummer (15–28 July 2016). Atmospheric Hg(0) concentrations generally declined during each night, and the strongest Hg(0) depletions (Hg(0) <1 ng m⁻³) were observed when the PBL was stable (ζ>0.1, events 2 and 3 and green bars in Fig. 5b). These Hg(0) night-time minima coincided with maxima of atmospheric CO₂ mixing ratios of 410 to 420 ppm. These patterns are consistent with measured Hg(0) deposition fluxes during nights (daily minima at 00:00, $-\mathrm{2.56}\pm \mathrm{0.22}$ ng m⁻² h⁻¹ (mean ± 1 SD), Fig. 1i), under corresponding CO₂ accumulation in the PBL driven by night-time CO₂ soil respiration (Wofsy et al., 1993; Schlesinger and Andrews, 2000; Grant and Omonode, 2018). On nights with unstable PBL conditions (e.g. event 1 in Fig. 5), diel Hg(0) and CO₂ variations were lower or absent due to increased mixing with free tropospheric air containing background levels of Hg(0) and CO₂. During late summer (Fig. S4, 20–31 August 2016), the longer duration of stable nocturnal PBL conditions led to even more pronounced night-time depletions in Hg(0). During daytime under strong solar radiation, flux measurements showed a net Hg(0) emission around noon (daily maxima at 11:00, 2.9±0.92 ng m⁻² h⁻¹, mean ± 1 SD, Fig. 1i). Daytime Hg(0) re-emission, however, did not lead to a build-up of atmospheric Hg(0) above the surface due to prevailing turbulent conditions, allowing for the efficient mixing with background free tropospheric air. These patterns demonstrate how atmospheric Hg(0) and CO₂ are both controlled by the magnitude and direction of the net ecosystem exchange fluxes in conjunction with the PBL stability. Overall, flux measurements showed the tundra ecosystem to be a net sink of atmospheric Hg(0) over the duration of the growing season ($-\mathrm{0.76}\pm \mathrm{6.3}$ ng m⁻² h⁻¹ – mean ±1 SD, median: −1.2 ng m⁻² h⁻¹).

Figure 5Time series during a midsummer period (15–28 July 2016) of the (a) atmospheric Hg(0) concentration and CO₂ mixing ratio; (b) median planetary boundary layer stability (ζ), where the shaded areas in green represents stable conditions (ζ>0.1) and shaded areas in red represent turbulent conditions ($\mathit{\zeta }<-\mathrm{0.1}$); (c) solar radiation and air temperature; and (d) Hg(0) flux, with Hg(0) deposition in green and Hg(0) re-emission in red. Midnight is indicated by dashed lines.

Download

3.3.2 Hg isotope fractionation during foliar uptake of atmospheric Hg(0)

Figure 6 shows a scatterplot of atmospheric Hg(0) concentrations and the δ²⁰²Hg values sampled during different times of the day in summer 2016. Hg(0) sampled during the night was characterised by lower Hg(0) concentrations (1.06±0.13 ng m⁻³), higher δ²⁰²Hg (1.31 ‰ ± 0.15 ‰), and similar Δ¹⁹⁹Hg values (−0.28 ‰ ± 0.08 ‰) (22:00–06:00, mean ± 1 SD, n=4), compared with Hg(0) sampled during the day (Hg(0) = 1.16±0.11 ng m⁻³, p=0.001; δ²⁰²Hg = 0.95 ‰ ± 0.22 ‰, p=0.01; Δ¹⁹⁹Hg = −0.26 ‰ ± 0.05 ‰, p=0.70, mean ± 1 SD, 06:00–22:00, n=2; 24 h, n=4; p values based on two-sided t tests). An enrichment of heavy atmospheric Hg(0) isotopes during the night is consistent with the preferential uptake of light Hg(0) isotopes by vegetation (Demers et al., 2013; Enrico et al., 2016; Obrist et al., 2017; Olson et al., 2019; Yuan et al., 2018; Yu et al., 2016), which is the dominant Hg(0) deposition pathway at the study site (Obrist et al., 2017). Attributing the diurnal concentration and δ²⁰²Hg variation of Hg(0) to vegetation uptake, we calculated an enrichment factor of vegetation uptake, ε²⁰²Hg_{vegetation uptake}, of −4.22 ‰ ± 1.01 ‰ (mean ± 1 SE, R²=0.68, p=0.003) (Fig. S5) using a Rayleigh model. Enrico et al. (2016) estimated an ε²⁰²Hg_plant−air of −2.6 ‰ for foliar uptake by sphagnum moss, using a Rayleigh model to fit the atmospheric Hg(0) concentration and δ²⁰²Hg measured at two locations, a mountain site unaffected by local terrestrial–atmosphere exchange and a peat bog where Hg(0) in air was depleted by foliar uptake. Similarly, observations of the difference in δ²⁰²Hg between plants and atmospheric Hg(0) suggested a ε²⁰²Hg_plant−air value between −1 and −3 ‰ (Demers et al., 2013; Enrico et al., 2016; Obrist et al., 2017; Olson et al., 2019; Yuan et al., 2018; Yu et al., 2016).

Figure 6Mass-dependent Hg isotope signature (δ²⁰²Hg) of atmospheric Hg(0) versus Hg(0) concentration during the snow-free growing period (11 June–10 September 2016). The dashed line represents a non-linear Rayleigh fit and the shaded area is the 95 % confidence interval (see main text). Error bars represent the analytical precision determined by the 2 SD from multiple measurements of an in-house standard.

Download

At Toolik Field Station, the difference between δ²⁰²Hg in vegetation relative to atmospheric Hg(0) was also considerably lower (range of −1.29 ‰ to −2.09 ‰, depending on vegetation species; Olson et al., 2019) than the fractionation factor derived from the atmospheric pattern. This discrepancy can be explained by the fact that δ²⁰²Hg signatures measured in vegetation not only reflect the isotopic fractionation during foliar uptake, but also contain a re-emission component. Hg(II) reduction is expected to lead to more positive δ²⁰²Hg values in the residual, foliar Hg(II) fraction, irrespective of the reduction mechanism (Bergquist and Blum, 2007; Zheng and Hintelmann, 2010; Kritee et al., 2007; Jiskra et al., 2015). Re-emission of foliar Hg is supported by the observed negative shifts in odd-mass isotope MIF (Δ¹⁹⁹Hg) in vegetation relative to Δ¹⁹⁹Hg of atmospheric Hg(0), which have been observed at Toolik Field Station (Olson et al., 2019) and elsewhere (Enrico et al., 2016; Demers et al., 2013). Positive Δ¹⁹⁹Hg in atmospheric Hg(0) re-emitted from foliage has recently been constrained by flux bag experiments (Yuan et al., 2018). Therefore, it is expected that the fractionation factor of foliar uptake is larger than just the difference between δ²⁰²Hg in foliage and Hg(0) in the atmosphere. Instead, the difference reflects a net fractionation consisting of the isotopic fractionation during foliar uptake, as well as during foliar reduction and re-emission. Our observation that the fractionation factor derived from δ²⁰²Hg of atmospheric Hg(0) (Fig. 6) is larger than the difference of δ²⁰²Hg between vegetation and the atmosphere could also be associated with the diel variation of Hg(0) fluxes and PBL dynamics. During daytime, atmospheric turbulence is higher and, therefore, local signals of terrestrial re-emission are expected to be diluted by mixing with background Hg(0). At night, when the PBL is stable, foliar uptake of lighter Hg(0) isotopes is imprinted on the residual atmospheric Hg(0).

3.3.3 Sink of Hg(0) in soil inferred from Hg stable isotopes

At our study site, soil pore air Hg(0) concentrations were below ambient levels measured in the atmosphere all year (Obrist et al., 2017), with an average concentration of 0.54±0.14 ng m⁻³, indicating a consistent sink of Hg(0) in soils. Hg(0) in soil pore air showed a lower δ²⁰²Hg (−0.01 ‰ ± 0.39 ‰) and a higher Δ¹⁹⁹Hg (−0.18 ‰ ± 0.07 ‰) (mean ± 1 SD, n=3) compared with ambient atmospheric Hg(0) (Hg(0) = 1.1±0.09 ng m⁻³, p=0.001, δ²⁰²Hg = 0.81 ‰ ± 0.18 ‰, p=0.057, Δ¹⁹⁹Hg = −0.25 ‰ ± 0.04 ‰, p=0.27; 24 h, mean ± 1 SD, p value of two-sided t test, n=6) during the summer and fall periods, when we were able to quantify soil pore air isotope patterns. Fitting the Hg stable isotope fractionation trajectory for MDF and MIF of three data points of soil pore air samples and the atmospheric Hg(0) samples resulted in enrichment factors of ε²⁰²Hg${}_{\mathrm{soil}\mathrm{air}-\mathrm{atmosphere}}=-\mathrm{1.00}$ ‰ ± 0.25 ‰ (mean ± 1 SE, R²=0.69, p=0.005; Fig. 7a) and E¹⁹⁹Hg${}_{\mathrm{soil}\mathrm{air}-\mathrm{atmosphere}}=\mathrm{0.07}$ ‰ ± 0.04 (mean ± 1 SE, R²=0.32, p=0.11; Fig. 7b). Recently, (Zheng et al., 2018) investigated Hg stable isotope fractionation during the oxidation of dissolved Hg(0) by low molecular weight thiol compounds and natural humic acids (HAs). For oxidation by HAs, they reported an enrichment of light Hg(0) isotopes (ε²⁰²Hg${}_{\mathrm{Hg}\left(\mathrm{0}\right)-\mathrm{Hg}\left(\mathrm{II}\right)}=-\mathrm{1.54}$ ‰ ± 0.05 ‰, mean ± 1 SE) and a positive odd-mass Hg MIF (E¹⁹⁹Hg${}_{\mathrm{Hg}\left(\mathrm{0}\right)-\mathrm{Hg}\left(\mathrm{II}\right)}=-\mathrm{0.18}$ ‰ ± 0.03 ‰, mean ± 1 SE) in the residual Hg(0) fraction (Zheng et al., 2018). Our limited number of soil air measurements (n=3) agree with the fractionation trajectory for HA oxidation (red straight lines in Fig. 7), suggesting abiotic oxidation in soils as a cause of the observed soil Hg(0) uptake. In spite of a consistent soil Hg(0) sink in soils, Obrist et al. (2014) estimated that the soil Hg(0) sink results in small Hg(0) fluxes ($<-\mathrm{0.03}$ ng m⁻² h⁻¹) due to low diffusivity. Comparing the soil Hg(0) uptake flux reported by Obrist et al. (2014) to the net ecosystem flux measured at Toolik Field Station, we estimate that such a soil Hg(0) sink would only account for <5 % of the total Hg(0) deposition. Soil air δ²⁰²Hg is enriched in light isotopes relative to wintertime atmospheric Hg(0), whereas interstitial snow air Hg(0) is enriched in heavier isotopes, in the opposite direction of soil air (Fig. 8), suggesting that soil uptake of Hg(0) has a minor effect on interstitial Hg(0) patterns in the snowpack above.

Figure 7Hg stable isotope systematics of Hg(0) in the atmosphere during summer/fall and in soil pore air measured during the same time period. (a) Mass-dependent Hg isotope signature (δ²⁰²Hg) against Hg(0) concentration. (b) Mass-independent Hg isotope signature (Δ¹⁹⁹Hg) against mass-dependent Hg isotope signature (δ²⁰²Hg). The dashed green lines represent the regression of a Rayleigh model (a) and a linear model (b). The straight red lines represent the trajectories for abiotic dark oxidation of Hg(0) by natural humic acids (HAs) from Zheng et al. (2018). For comparison with observations, the intercept of the linear regressions was adjusted to fit through the average of atmospheric Hg(0). The shaded areas represent the 95 % confidence interval. Error bars represent the analytical precision determined by the 2 SD from multiple measurements of an in-house standard.

Download

Figure 8Overview of the mass-independent Hg isotope signature (Δ¹⁹⁹Hg) against the mass-dependent Hg isotope signature (δ²⁰²Hg) of atmospheric Hg(0) (red) and Hg(0) in the pore air of snow (blue) and soil (green) during winter (circles), the AMDE season (squares), and summer/fall (triangles). Bulk vegetation measurements (green diamonds) are reproduced from Obrist et al. (2017). The straight arrow represents the fractionation during Hg(0) uptake by vegetation, and the dashed arrow represents the expected development of the corresponding residual Hg(0) pool. Error bars represent the analytical precision determined by the 2 SD from multiple measurements of an in-house standard.

Download