Atmospheric Mercury Depletion Events Research Articles

Climate/ocean dynamics and possible atmospheric mercury depletion events during the Late Sturtian deglaciation

Late Sturtian deglaciation, resulting in development of oxic conditions and deposition of banded iron formations (BIFs), was a pivotal period in evolution of global ocean chemistry. The driving forces behind these Neoproterozoic BIFs and climatic/oceanic changes remain controversial, however. Here we use mercury (Hg) to elucidate the cause(s) of these dramatic changes to earth biogeochemical cycles. In the Guandong section, South China, we observe anomalously low Hg concentrations and Hg to total organic carbon (Hg/TOC) ratios in the Late Sturtian BIFs, suggesting a period of limited continental volcanism during the Late Sturtian deglaciation. We hypothesize that instead of a major LIP event, long-lasting and subdued submarine volcanism slowly increased atmospheric CO2 to trigger the greenhouse climate that caused deglaciation. Negative shifts of Δ199Hg in the studied section suggest that deglaciation was accompanied by atmospheric mercury depletion events (AMDEs). Extremely low TOC levels in the studied section may suggest low oceanic productivity, and this may support a non-biological cause of oceanic oxygenation during the Late Sturtian glaciation, through the release of photochemically produced H2O2 in melting glaciers.

Assessment of Mercury Concentrations and Fluxes Deposited from the Atmosphere on the Territory of the Yamal-Nenets Autonomous Area

The problem of mercury input and its further distribution in the Arctic environment is actively debated, especially in recent times, due to the observed processes of permafrost thawing causing the enhanced release of mercury into the Arctic atmosphere and further distribution in the terrestrial and aquatic ecosystem. The atmospheric mercury deposition occurs via dry deposition and wet scavenging by precipitation events. Here we present a study of Hg in wet precipitation on the remote territory of the Russian Arctic; the data were obtained at the monitoring stations Nadym and Salekhard in 2016–2018. Mercury pollution of the Salekhard atmosphere in cold time is mainly determined by regional and local sources, while in Nadym, long-range transport of mercury and local fuel combustion are the main sources of pollutants in the cold season, while internal regional sources have a greater impact on the warm season. Total mercury concentrations in wet precipitation in Nadym varied from <0.5 to 63.3 ng/L. The highest Hg concentrations in the springtime were most likely attributed to atmospheric mercury depletion events (AMDE). The contributions of wet atmospheric precipitation during the AMDE period to the annual Hg deposition were 16.7% and 9.8% in 2016/2017 and 2017/2018, respectively. The average annual volume-weighted Hg concentration (VWC) in the atmospheric precipitation in Nadym is notably higher than the values reported for the remote regions in the Arctic and comparable with the values obtained for the other urbanized regions of the world. Annual Hg fluxes in Nadym are nevertheless close to the average annual fluxes for remote territories of the Arctic zone and significantly lower than the annual fluxes reported for unpolluted sites of continental-scale monitoring networks of the different parts of the world (USA, Europe, and China). The increase of Hg deposition flux with wet precipitation in Nadym in 2018 might be caused by regional emissions of gas and oil combustion, wildfires, and Hg re-emission from soils due to the rising air temperature. The 37 cm increase of the seasonally thawed layer (STL) in 2018 compared to the 10-year average reflects that the climatic changes in the Nadym region might increase Hg(0) evasion, considering a great pool of Hg is contained in permafrost.

Dynamics of gaseous oxidized mercury at Villum Research Station during the High Arctic summer

Abstract. While much research has been devoted to the subject of gaseous elemental mercury (GEM) and gaseous oxidized mercury (GOM) in the Arctic spring during atmospheric mercury depletion events, few studies have examined the behavior of GOM in the High Arctic summer. GOM, once deposited and incorporated into the ecosystem, can pose a threat to human and wildlife health, though there remain large uncertainties regarding the transformation, deposition, and assimilation of mercury into the food web. Therefore, to further our understanding of the dynamics of GOM in the High Arctic during the late summer, we performed measurements of GEM and GOM, along with meteorological parameters and atmospheric constituents, and utilized modeled air mass history during two summer campaigns in 2019 and 2020 at Villum Research Station (Villum) in northeastern Greenland. Seven events of enhanced GOM concentrations were identified and investigated in greater detail. In general, the common factors associated with event periods at ground level were higher levels of radiation and lower H2O mixing ratios, accumulated precipitation, and relative humidity (RH), although none were connected with cold temperatures. Non-event periods at ground level each displayed a different pattern in one or more parameters when compared to event periods. Generally, air masses during event periods for both campaigns were colder and drier, arrived from higher altitudes, and spent more time above the mixed layer and less time in a cloud compared to non-events, although some events deviated from this general pattern. Non-event air masses displayed a different pattern in one or more parameters when compared to event periods, although they were generally warmer and wetter and arrived from lower altitudes with little radiation. Coarse-mode aerosols were hypothesized to provide the heterogenous surface for halogen propagation during some of the events, while for others the source is unknown. While these general patterns were observed for event and non-event periods, analysis of individual events showed more specific origins. Five of the seven events were associated with air masses that experienced similar conditions: transported from the cold, dry, and sunlit free troposphere. However, two events experienced contrasting conditions, with air masses being warm and wet with surface layer contact under little radiation. Two episodes of extremely high levels of NCoarse and BC, which appear to originate from flaring emissions in Russia, did not contribute to enhanced GOM levels. This work aims to provide a better understanding of the dynamics of GOM during the High Arctic summer.

Variability in gaseous elemental mercury at Villum Research Station, Station Nord, in North Greenland from 1999 to 2017

Abstract. Mercury is ubiquitous in the atmosphere, and atmospheric transport is an important source for this element in the Arctic. Measurements of gaseous elemental mercury (GEM) have been carried out at Villum Research Station (Villum) at Station Nord, situated in northern Greenland. The measurements cover the period 1999–2017, with a gap in the data for the period 2003–2008 (for a total of 11 years). The measurements were compared with model results from the Danish Eulerian Hemispheric Model (DEHM) that describes the contribution from direct anthropogenic transport, marine emissions and general background concentration. The percentage of time spent over different surfaces was calculated by back-trajectory analysis, and the reaction kinetics were determined by a comparison with ozone. The GEM measurements were analysed for trends, both seasonal and annual. The only significant trends found were negative ones for the winter and autumn months. Comparison of the measurements to simulations using the Danish Eulerian Hemispheric Model (DEHM) indicated that direct transport of anthropogenic emissions of mercury accounts for between 14 % and 17 % of the measured mercury. Analysis of the kinetics of the observed atmospheric mercury depletion events (AMDEs) confirms the results of a previous study at Villum of the competing reactions of GEM and ozone with Br, which suggests that the lifetime of GEM is about a month. However, a GEM lifetime of 12 months gave the best agreement between the model and measurements. The chemical lifetime is shorter, and thus, the apparent lifetime appears to be the result of deposition followed by reduction and re-emission; for this reason, the term “relaxation time” is preferred to “lifetime” for GEM. The relaxation time for GEM causes a delay between emission reductions and the effect on actual concentrations. No significant annual trend was found for the measured concentrations of GEM over the measurement period, despite emission reductions. This is interesting, and together with low direct transport of GEM to Villum as found by the DEHM model, it shows that the dynamics of GEM are very complex. Therefore, in the coming years, intensive measurement networks are needed to describe the global distribution of mercury in the environment as the use of models to predict future levels will still be highly uncertain. The situation is increasingly complex due to global changes that most likely will change the transport patterns of mercury, not only in the atmosphere but also between matrixes.

Mercury Isotopes Reveal Atmospheric Gaseous Mercury Deposition Directly to the Arctic Coastal Snowpack

Springtime atmospheric mercury depletion events (AMDEs) lead to snow with elevated mercury concentrations (>200 ng Hg/L) in the Arctic and Antarctic. During AMDEs gaseous elemental mercury (GEM) is...

Thawing of snow and ice caused extraordinary high and fast mercury fluxes to lake sediments in Antarctica

Abstract Due to its biomagnification mercury (Hg) is an element of concern especially in the remote pristine ecosystems such as Antarctica. Despite the sensitivity of the region, there is still a limited knowledge about the influence of natural processes, i.e. volcanism, climate, atmospheric mercury depletion events or freeze-thawing cycles, on the Hg cycle in Antarctica. We analysed total Hg and solid phase Hg species (Hg-thermo–desorption-AAS) in 1600 years old sediments from Limnopolar Lake (Byers Peninsula, South Shetland Islands). Some sections of the core show unusual high Hg concentrations (1141–11286 ng g−1) and accumulation rates (310–4902 µg m−2 yr−1). Hg-thermo-desorption analyses indicate that Hg is bound in the sediment by organic matter of different degree of degradation. We propose a combination of different natural processes to explain the high enrichment of Hg in the sediments. The nearby (30 km) Deception Island volcano appears to be the main source of Hg, while climatic conditions have controlled freezing-thawing of the ice-cover and the enrichment of Hg in the snowpack on the frozen lake (and its catchment). The release of Hg from the snowpack during thawing events appears to cause the extreme Hg enrichment in the sediments. The global atmospheric pollution is a minority signal in our record. The ∼2.2 fold increase in Hg concentrations since AD ∼ 1860 points industrial activities as the unique pollution cause. Our findings indicate that thawing of Antarctic ice might release large quantities of Hg to freshwater and marine systems and related food chains.

Mercury in Active‐Layer Tundra Soils of Alaska: Concentrations, Pools, Origins, and Spatial Distribution

AbstractTundra soils serve as major sources of mercury (Hg) input to the Arctic Ocean via river runoff and coastal erosion; yet little information is available on tundra soil Hg concentrations, pool sizes, origins, and dynamics. We present a detailed investigation of Hg in the active layer (upper ~100 cm subject to seasonal thaw) of tundra soils across 11 sites in Alaska. Soil Hg concentrations in organic horizons (151 ± 7 μg/kg) were in the upper range of temperate soil organic horizons, and concentrations in mineral horizons (98 ± 6 μg/kg) were much higher than in temperate soils. Soil Hg concentrations declined from inland to coastal sites, in contrast to a hypothesized northward increase expected because of proximity to coastal atmospheric mercury depletion events. Principle component analyses and elemental ratios results show that exogenic sources dominated over geogenic sources—in A‐horizons (66 ± 4%) and mineral B‐horizons (51 ± 1%). 14C age‐dating suggested recent origins of Hg in surface soils but showed that mineral soils (more than 7,300 years old) must have accumulated atmospheric inputs across millennia leading to high soil concentrations and pools. We estimated a total Northern Hemisphere active‐layer tundra soil Hg pool of 184 Gg (range of 136 to 274 Gg), suggesting a globally important Hg storage pool. Tundra soils are subject to seasonal thaw and freeze dynamics, thereby providing large inputs to rivers, lakes, and the Arctic Ocean. Understanding processes that mobilize Hg from tundra soils will be critical to understanding future Arctic wildlife and human Hg exposures.

Fluxes of gaseous elemental mercury (GEM) in the High Arctic during atmospheric mercury depletion events (AMDEs)

Abstract. Measurements of gaseous elemental mercury (GEM) fluxes over snow surfaces using a relaxed eddy accumulation (REA) system are carried out at the High Arctic site at the Villum Research Station, Station Nord, in North Greenland. Simultaneously, CO2 fluxes are determined using the eddy covariance (EC) technique. The REA system with dual inlets and dual analyzers is used to measure fluxes directly over the snow. The measurements were carried out from 23 April to 12 May during spring 2016, where atmospheric mercury depletion events (AMDEs) took place. The measurements showed a net emission of 8.9 ng m−2 min−1, with only a few minor episodes of net depositional fluxes, from a maximum deposition of 8.1 ng m−2 min−1 to a maximum emission of 179.2 ng m−2 min−1. The data support the theory that gaseous oxidized mercury (GOM) is deposited during AMDEs followed by formation of GEM on surface snow and is re-emitted as GEM shortly after the AMDEs. Furthermore, observation of the relation between GEM fluxes and atmospheric temperature suggests that GEM emission partly could be affected by surface heating. However, it is also clear that the GEM emissions are affected by many parameters.

Mercury fluxes over an Australian alpine grassland and observation of nocturnal atmospheric mercury depletion events

Abstract. Aerodynamic gradient measurements of the air–surface exchange of gaseous elemental mercury (GEM) were undertaken over a 40 ha alpine grassland in Australia's Snowy Mountains region across a 3-week period during the late austral summer. Bi-directional GEM fluxes were observed throughout the study, with overall mean value of 0.2 ± 14.5 ng m−2 h−1 and mean nocturnal fluxes of −1.5 ± 7.8 ng m−2 h−1 compared to diurnal fluxes of 1.8 ± 18.6 ng m−2 h−1. Deposition velocities ranged from −2.2 to 2.9 cm s−1, whilst ambient GEM concentrations throughout the study were 0.59 ± 0.10 ng m−3. Cumulative GEM fluxes correlated well with 24 h running mean soil temperatures, and one precipitation event was shown to have a positive impact on diurnal emission fluxes. The underlying vegetation had largely senesced and showed little stomatal control on fluxes. Nocturnal atmospheric mercury depletion events (NAMDEs) were observed concomitant with O3 depletion and dew formation under shallow, stable nocturnal boundary layers. A mass balance box model was able to reproduce ambient GEM concentration patterns during NAMDE and non-NAMDE nights without invoking chemical oxidation of GEM throughout the column, indicating a significant role of surface processes controlling deposition in these events. Surface deposition was enhanced under NAMDE nights, though uptake to dew likely represents less than one-fifth of this enhanced deposition. Instead, enhancement of the surface GEM gradient as a result of oxidation at the surface in the presence of dew is hypothesised to be responsible for a large portion of GEM depletion during these particular events. GEM emission pulses following nights with significant deposition provide evidence for the prompt recycling of 17 % of deposited mercury, with the remaining portion retained in surface sinks. The long-term impacts of any sinks are however likely to be minimal, as cumulative GEM flux across the study period was close to zero.

Structures, Vibrational Frequencies, and Bond Energies of the BrHgOX and BrHgXO Species Formed in Atmospheric Mercury Depletion Events.

Photochemistry during the polar spring leads to atmospheric mercury depletion events (AMDEs): Hg(0), which typically lives for months in the atmosphere, and can experience losses of more than 90% in less than a day. These dramatic losses are known to be initiated largely by Br + Hg + M → BrHg• + M, but the fate of BrHg• is a matter of guesswork. It is believed that BrHg• largely reacts with halogen oxides XO (X = Cl, Br, and I) to form BrHgOX compounds, but these species have never been studied experimentally. Here, we use quantum chemistry to characterize the structures, vibrational frequencies, and thermodynamics of these BrHgOX species and their BrHgXO isomers. The BrHgXO isomers have never previously been studied in experiments or computations. We find the BrHgOX species are 24-28 kcal/mol more stable than their BrHgXO isomers. When formed during polar AMDEs, BrHgBrO and BrHgIO appear sufficiently stable in that they will not dissociate before undergoing deposition, but BrHgClO is probably not that stable.

A Pulse of Mercury and Major Ions in Snowmelt Runoff from a Small Arctic Alaska Watershed.

Atmospheric mercury (Hg) is deposited to Polar Regions during springtime atmospheric mercury depletion events (AMDEs) that require halogens and snow or ice surfaces. The fate of this Hg during and following snowmelt is largely unknown. We measured Hg, major ions, and stable water isotopes from the snowpack through the entire spring melt runoff period for two years. Our small (2.5 ha) watershed is near Barrow (now Utqiaġvik), Alaska. We measured discharge, made 10 000 snow depths, and collected over 100 samples of snow and meltwater for chemical analysis in 2008 and 2009 from the watershed snowpack and ephemeral stream channel. Results show an "ionic pulse" of mercury and major ions in runoff during both snowmelt seasons, but major ion and Hg runoff concentrations were roughly 50% higher in 2008 than in 2009. Though total discharge as a percent of total watershed snowpack water equivalent prior to the melt was similar in both years (36% in 2008 melt runoff and 34% in 2009), it is possible that record low precipitation in the summer of 2007 led to the higher major ion and Hg concentrations in 2008 melt runoff. Total dissolved Hg meltwater runoff of 14.3 (± 0.7) mg/ha in 2008 and 8.1 (± 0.4) mg/ha in 2009 is five to seven times higher than that reported from other arctic watersheds. We calculate 78% of snowpack Hg was exported with snowmelt runoff in 2008 and 41% in 2009. Our results suggest AMDE Hg complexed with Cl- or Br- may be less likely to be photochemically reduced and re-emitted to the atmosphere prior to snowmelt, and we estimate that roughly 25% of the Hg in snowmelt is attributable to AMDEs. Projected Arctic warming, with more open sea ice leads providing halogen sources that promote AMDEs, may provide enhanced Hg deposition, reduced Hg emission and, ultimately, an increase in snowpack and snowmelt runoff Hg concentrations.

Tundra uptake of atmospheric elemental mercury drives Arctic mercury pollution.

Anthropogenic activities have led to large-scale mercury (Hg) pollution in the Arctic. It has been suggested that sea-salt-induced chemical cycling of Hg (through 'atmospheric mercury depletion events', or AMDEs) and wet deposition via precipitation are sources of Hg to the Arctic in its oxidized form (Hg(ii)). However, there is little evidence for the occurrence of AMDEs outside of coastal regions, and their importance to net Hg deposition has been questioned. Furthermore, wet-deposition measurements in the Arctic showed some of the lowest levels of Hg deposition via precipitation worldwide, raising questions as to the sources of high Arctic Hg loading. Here we present a comprehensive Hg-deposition mass-balance study, and show that most of the Hg (about 70%) in the interior Arctic tundra is derived from gaseous elemental Hg (Hg(0)) deposition, with only minor contributions from the deposition of Hg(ii) via precipitation or AMDEs. We find that deposition of Hg(0)-the form ubiquitously present in the global atmosphere-occurs throughout the year, and that it is enhanced in summer through the uptake of Hg(0) by vegetation. Tundra uptake of gaseous Hg(0) leads to high soil Hg concentrations, with Hg masses greatly exceeding the levels found in temperate soils. Our concurrent Hg stable isotope measurements in the atmosphere, snowpack, vegetation and soils support our finding that Hg(0) dominates as a source to the tundra. Hg concentration and stable isotope data from an inland-to-coastal transect show high soil Hg concentrations consistently derived from Hg(0), suggesting that the Arctic tundra might be a globally important Hg sink. We suggest that the high tundra soil Hg concentrations might also explain why Arctic rivers annually transport large amounts of Hg to the Arctic Ocean.

Speciation of mercury in the waters of the Weddell, Amundsen and Ross Seas (Southern Ocean)

Despite the distance from large anthropogenic emission sources, toxic mercury is transported via the atmosphere and oceans to the Southern Ocean. Seawater samples were collected at selected stations and were analysed for total mercury (HgT) (8 stations), dissolved gaseous mercury (DGM) (62 stations) and methylmercury (12 stations) during winter (Weddell Sea), spring (Weddell Sea) and summer (Amundsen and Ross Seas) in the Southern Ocean. The HgT distribution in water columns was found to not vary significantly with depth. In the Weddell Sea the average column concentration was higher in spring (2.6 +/- 1.3 pM, 2 stations) than in winter (2.0 +/- 1.0 pM, 6 stations). We hypothesize that the seasonal HgT increase is due to atmospheric deposition of particulate Hg(II) formed during atmospheric mercury depletion events (AMDEs), as well as the addition of inorganic mercury species from melting sea ice and snow. Furthermore, HgT concentrations found in this study were significantly higher than previously measured in the Southern Ocean (Cossa et al., 2011), which was hypothesized to be due to seasonal variations in atmospheric deposition. The average water column DGM concentration in the Weddell Sea was 454 +/- 254 fM in winter and 384 +/- 239 fM in spring. The lowest average DGM concentration was found in summer in the Amundsen and Ross Seas (299 +/- 137 fM). The highest observed concentration in winter was hypothesized to be caused by the larger sea ice coverage, which is known to reduce the evasion of Hg(0) from the sea surface. The average monomethylmercury (MMHg) concentration in the Weddell Sea was 60 +/- 30 fM in winter (6 stations) and 95 +/- 85 fM in spring (2 stations), showing no significant seasonal difference. In the Amundsen and Ross Seas the summer average concentration of MeHg (MMHg and dimethylmercury; DMHg) was 135 +/- 189 fM (4 stations). The highest MeHg concentration was found in modified circumpolar deep water, which is known to have high primary production.

Sources and cycling of mercury in the paleo Arctic Ocean from Hg stable isotope variations in Eocene and Quaternary sediments

Mercury stable isotopic compositions were determined for marine sediments from eight locations in the Arctic Ocean Basin. Mass dependent fractionation (MDF) and mass independent fractionation (MIF) of Hg stable isotopes were recorded across a variety of depositional environments, water depths, and stratigraphic ages. δ202Hg (MDF) ranges from −2.34‰ to −0.78‰; Δ199Hg (MIF) from −0.18‰ to +0.12‰; and Δ201Hg (MIF) from −0.29‰ to +0.05‰ for the complete data set (n=33). Holocene sediments from the Chukchi Sea and Morris Jesup Rise record the most negative Δ199Hg values, while Pleistocene sediments from the Central Arctic Ocean record the most positive Δ199Hg values. The most negative δ202Hg values are recorded in Pleistocene sediments. Eocene sediments (Lomonosov Ridge) show some overlap in their Hg isotopic compositions with Quaternary sediments, with a sample of the Arctic Ocean PETM (56Ma) most closely matching the average Hg isotopic composition of Holocene Arctic marine sediments. Collectively, these data support a terrestrially-dominated Hg source input for Arctic Ocean sediment through time, although other sources, as well as influences of sea ice, atmospheric mercury depletion events (AMDEs), and anthropogenic Hg (in core top samples) on Hg isotopic signatures must also be considered.

Antarctic winter mercury and ozone depletion events over sea ice

During atmospheric mercury and ozone depletion events in the springtime in polar regions gaseous elemental mercury and ozone undergo rapid declines. Mercury is quicldy transformed into oxidation products, which are subsequently removed by deposition. Here we show that such events also occur during Antarctic winter over sea ice areas, leading to additional deposition of mercury. Over four months in the Weddell Sea we measured gaseous elemental, oxidized, and particulate-bound mercury, as well as ozone in the troposphere and total and elemental mercury concentrations in snow, demonstrating a series of depletion and deposition events between July and September. The winter depletions in July were characterized by stronger correlations between mercury and ozone and larger formation of particulate-bound mercury in air compared to later spring events. It appears that light at large solar zenith angles is sufficient to initiate the photolytic formation of halogen radicals. We also propose a dark mechanism that could explain observed events in air masses coming from dark regions. Br-2 that could be the main actor in dark conditions was possibly formed in high concentrations in the marine boundary layer in the dark. These high concentrations may also have caused the formation of high concentrations of CHBr3 and CH2I2 in the top layers of the Antarctic sea ice observed during winter. These new findings show that the extent of depletion events is larger than previously believed and that winter depletions result in additional deposition of mercury that could be transferred to marine and terrestrial ecosystems.

Air–snowpack exchange of bromine, ozone and mercury in the springtime Arctic simulated by the 1-D model PHANTAS – Part 2: Mercury and its speciation

Abstract. Atmospheric mercury depletion events (AMDEs) refer to a recurring depletion of mercury occurring in the springtime Arctic (and Antarctic) boundary layer, in general, concurrently with ozone depletion events (ODEs). To close some of the knowledge gaps in the physical and chemical mechanisms of AMDEs and ODEs, we have developed a one-dimensional model that simulates multiphase chemistry and transport of trace constituents throughout porous snowpack and in the overlying atmospheric boundary layer (ABL). This paper constitutes Part 2 of the study, describing the mercury component of the model and its application to the simulation of AMDEs. Building on model components reported in Part 1 ("In-snow bromine activation and its impact on ozone"), we have developed a chemical mechanism for the redox reactions of mercury in the gas and aqueous phases with temperature dependent reaction rates and equilibrium constants accounted for wherever possible. Thus the model allows us to study the chemical and physical processes taking place during ODEs and AMDEs within a single framework where two-way interactions between the snowpack and the atmosphere are simulated in a detailed, process-oriented manner. Model runs are conducted for meteorological and chemical conditions that represent the springtime Arctic ABL characterized by the presence of "haze" (sulfate aerosols) and the saline snowpack on sea ice. The oxidation of gaseous elemental mercury (GEM) is initiated via reaction with Br-atom to form HgBr, followed by competitions between its thermal decomposition and further reactions to give thermally stable Hg(II) products. To shed light on uncertain kinetics and mechanisms of this multi-step oxidation process, we have tested different combinations of their rate constants based on published laboratory and quantum mechanical studies. For some combinations of the rate constants, the model simulates roughly linear relationships between the gaseous mercury and ozone concentrations as observed during AMDEs/ODEs by including the reaction HgBr + BrO and assuming its rate constant to be the same as for the reaction HgBr + Br, while for other combinations the results are more realistic by neglecting the reaction HgBr + BrO. Speciation of gaseous oxidized mercury (GOM) changes significantly depending on whether or not BrO is assumed to react with HgBr to form Hg(OBr)Br. Similarly to ozone (reported in Part 1), GEM is depleted via bromine radical chemistry more vigorously in the snowpack interstitial air than in the ambient air. However, the impact of such in-snow sink of GEM is found to be often masked by the re-emissions of GEM from the snow following the photo-reduction of Hg(II) deposited from the atmosphere. GOM formed in the ambient air is found to undergo fast "dry deposition" to the snowpack by being trapped on the snow grains in the top ~1 mm layer. We hypothesize that liquid-like layers on the surface of snow grains are connected to create a network throughout the snowpack, thereby facilitating the vertical diffusion of trace constituents trapped on the snow grains at much greater rates than one would expect inside solid ice crystals. Nonetheless, on the timescale of a week simulated in this study, the signal of atmospheric deposition does not extend notably below the top 1 cm of the snowpack. We propose and show that particulate-bound mercury (PBM) is produced mainly as HgBr42− by taking up GOM into bromide-enriched aerosols after ozone is significantly depleted in the air mass. In the Arctic, "haze" aerosols may thus retain PBM in ozone-depleted air masses, allowing the airborne transport of oxidized mercury from the area of its production farther than in the form of GOM. Temperature dependence of thermodynamic constants calculated in this study for Henry's law and aqueous-phase halide complex formation of Hg(II) species is a critical factor for this proposition, calling for experimental verification. The proposed mechanism may explain observed changes in the GOM–PBM partitioning with seasons, air temperature and the concurrent progress of ozone depletion in the high Arctic. The net deposition of mercury to the surface snow is shown to increase with the thickness of the turbulent ABL and to correspond well with the column amount of BrO in the atmosphere.

Air–snowpack exchange of bromine, ozone and mercury in the springtime Arctic simulated by the 1-D model PHANTAS – Part 1: In-snow bromine activation and its impact on ozone

Abstract. To provide a theoretical framework towards a better understanding of ozone depletion events (ODEs) and atmospheric mercury depletion events (AMDEs) in the polar boundary layer, we have developed a one-dimensional model that simulates multiphase chemistry and transport of trace constituents from porous snowpack and through the atmospheric boundary layer (ABL) as a unified system. This paper constitutes Part 1 of the study, describing a general configuration of the model and the results of simulations related to reactive bromine release from the snowpack and ODEs during the Arctic spring. A common set of aqueous-phase reactions describes chemistry both within the liquid-like layer (LLL) on the grain surface of the snowpack and within deliquesced "haze" aerosols mainly composed of sulfate in the atmosphere. Gas-phase reactions are also represented by the same mechanism in the atmosphere and in the snowpack interstitial air (SIA). Consequently, the model attains the capacity of simulating interactions between chemistry and mass transfer that become particularly intricate near the interface between the atmosphere and the snowpack. In the SIA, reactive uptake on LLL-coated snow grains and vertical mass transfer act simultaneously on gaseous HOBr, a fraction of which enters from the atmosphere while another fraction is formed via gas-phase chemistry in the SIA itself. A "bromine explosion", by which HOBr formed in the ambient air is deposited and then converted heterogeneously to Br2, is found to be a dominant process of reactive bromine formation in the top 1 mm layer of the snowpack. Deeper in the snowpack, HOBr formed within the SIA leads to an in-snow bromine explosion, but a significant fraction of Br2 is also produced via aqueous radical chemistry in the LLL on the surface of the snow grains. These top- and deeper-layer productions of Br2 both contribute to the release of Br2 to the atmosphere, but the deeper-layer production is found to be more important for the net outflux of reactive bromine. Although ozone is removed via bromine chemistry, it is also among the key species that control both the conventional and in-snow bromine explosions. On the other hand, aqueous-phase radical chemistry initiated by photolytic OH formation in the LLL is also a significant contributor to the in-snow source of Br2 and can operate without ozone, whereas the delivery of Br2 to the atmosphere becomes much smaller after ozone is depleted. Catalytic ozone loss via bromine radical chemistry occurs more rapidly in the SIA than in the ambient air, giving rise to apparent dry deposition velocities for ozone from the air to the snow on the order of 10−3 cm s−1 during daytime. Overall, however, the depletion of ozone in the system is caused predominantly by ozone loss in the ambient air. Increasing depth of the turbulent ABL under windy conditions will delay the buildup of reactive bromine and the resultant loss of ozone, while leading to the higher column amount of BrO in the atmosphere. During the Arctic spring, if moderately saline and acidic snowpack is as prevalent as assumed in our model runs on sea ice, the shallow, stable ABL under calm weather conditions may undergo persistent ODEs without substantial contributions from blowing/drifting snow and wind-pumping mechanisms, whereas the column densities of BrO in the ABL will likely remain too low in the course of such events to be detected unambiguously by satellite nadir measurements.

Atmospheric mercury speciation and mercury in snow over time at Alert, Canada

Abstract. Ten years of atmospheric mercury speciation data and 14 years 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 concentration of both PHg and RGM rise significantly. During this period, the median concentrations for PHg is 28.2 pgm−3 and RGM is 23.9 pgm−3, from March to June, in comparison to the annual median concentrations of 11.3 and 3.2 pgm−3 for PHg and RGM, respectively. In each of the ten years of sampling, the concentration of PHg increases steadily from January through March and is higher than the concentration of RGM. This pattern begins to change in April when the levels of PHg peak and RGM begin to increase. In May, the high PHg and low RGM concentration regime observed in the early spring undergoes a transition to a regime with higher RGM and much lower PHg concentrations. The higher RGM concentration continues into June. The transition is driven by the atmospheric conditions of air temperature and particle availability. Firstly, a high ratio of the concentrations of PHg to RGM is reported at low temperatures which suggests that oxidized gaseous mercury partitions 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, the high PHg concentrations occur in the spring when high particle concentrations are present. The high particle concentrations are principally due to Arctic haze and sea salts. In the snow, the concentrations of mercury peak in May for all years. Springtime deposition of total mercury to the snow at Alert peaks in May when atmospheric conditions favour higher levels of RGM. Therefore, the conditions in the atmosphere directly impact when the highest amount of mercury will be deposited to the snow during the Arctic spring.

Convective forcing of mercury and ozone in the Arctic boundary layer induced by leads in sea ice

The ongoing regime shift of Arctic sea ice from perennial to seasonal ice is associated with more dynamic patterns of opening and closing sea-ice leads (large transient channels of open water in the ice), which may affect atmospheric and biogeochemical cycles in the Arctic. Mercury and ozone are rapidly removed from the atmospheric boundary layer during depletion events in the Arctic, caused by destruction of ozone along with oxidation of gaseous elemental mercury (Hg(0)) to oxidized mercury (Hg(II)) in the atmosphere and its subsequent deposition to snow and ice. Ozone depletion events can change the oxidative capacity of the air by affecting atmospheric hydroxyl radical chemistry, whereas atmospheric mercury depletion events can increase the deposition of mercury to the Arctic, some of which can enter ecosystems during snowmelt. Here we present near-surface measurements of atmospheric mercury and ozone from two Arctic field campaigns near Barrow, Alaska. We find that coastal depletion events are directly linked to sea-ice dynamics. A consolidated ice cover facilitates the depletion of Hg(0) and ozone, but these immediately recover to near-background concentrations in the upwind presence of open sea-ice leads. We attribute the rapid recoveries of Hg(0) and ozone to lead-initiated shallow convection in the stable Arctic boundary layer, which mixes Hg(0) and ozone from undepleted air masses aloft. This convective forcing provides additional Hg(0) to the surface layer at a time of active depletion chemistry, where it is subject to renewed oxidation. Future work will need to establish the degree to which large-scale changes in sea-ice dynamics across the Arctic alter ozone chemistry and mercury deposition in fragile Arctic ecosystems.

Mercury uptake within an ice algal community during the spring bloom in first‐year Arctic sea ice

In this study, we examine mercury bioaccumulation by a first‐year sea‐ice (FYI) algal community in the western Canadian Arctic during the spring of 2008. Total mercury concentration in bottom sea‐ice particulate ([PHg]T) ranged 0.004–0.022 µg/g dw and was limited by the amount of mercury available for uptake when the spring bloom commenced. Mercury in ice algae originated from a combination of brine and seawater as sources, while atmospheric mercury depletion events did not appear to significantly contribute as a source in a coupled manner. We show that the bottom sea‐ice brine presents a chemically and biologically unique niche from which inorganic Hg makes its way into the food web. Once incorporated into algae, mercury can be transported spatially and trophically within the ecosystem by a range of processes including grazing, resuspension, remineralization, and sedimentation. Ice algae contribute 10–60% of the annual primary production in the Arctic and are thought to become even more productive and abundant under a mild climate change scenario. Replacement of multiyear ice with FYI in the Beaufort Sea alone could result in an additional influx of ∼48 kg/yr of particle bound Hg. Further studies are thus warranted to elucidate mechanisms by which mercury transformation processes and transfer into the Arctic marine food web are impacted by the interaction between sea ice, brine, and seawater.