Arctic Ocean: Is It a Sink or a Source of Atmospheric Mercury

Article pubs.acs.org/est

Arctic Ocean: Is It a Sink or a Source of Atmospheric Mercury? Ashu P. Dastoor*,† and Dorothy A. Durnford‡ †

Air Quality Research Division, Environment Canada, 2121 TransCanada Highway, Dorval, Quebec H9P 1J3, Canada Meteorological Service of Canada, Environment Canada, 2121 TransCanada Highway, Dorval, Quebec H9P 1J3, Canada

‡

S Supporting Information *

ABSTRACT: High levels of mercury in marine mammals threaten the health of Arctic inhabitants. Whether the Arctic Ocean (AO) is a sink or a source of atmospheric mercury is unknown. Given the paucity of observations in the Arctic, models are useful in addressing this question. GEOS-Chem and GRAHM, two complex numerical mercury models, present contrasting pictures of atmospheric mercury input to AO at 45 and 108 Mg yr−1, respectively, and ocean evasion at 90 and 33 Mg yr−1, respectively. We provide a comprehensive evaluation of GRAHM simulated atmospheric mercury input to AO using mercury observations in air, precipitation and snowpacks, and an analysis of the discrepancy between the two modeling estimates using observations. We discover two peaks in high-latitude summertime concentrations of atmospheric mercury. We show that the first is caused mainly by snowmelt revolatilization and the second by AO evasion of mercury. Riverine mercury export to AO is estimated at 50 Mg yr−1 based on measured DOC export and at 15.5−31 Mg yr−1 based on simulated mercury in meltwater. The range of simulated mercury fluxes to and from AO reflects uncertainties in modeling mercury in the Arctic; comprehensive observations in all compartments of the Arctic ecosystem are needed to close the gap.

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INTRODUCTION Anthropogenic mercury, primarily emitted as long-lived (6−12 months) gaseous elemental mercury (Hg0), enters the Arctic environment from lower latitudes by long-range transport in the atmosphere and the oceans. Mercury is deposited to Arctic ecosystems through the oxidation of Hg0 to short-lived (1−2 weeks) divalent mercury species (Hg2+). The sources and sinks of mercury to the Arctic Ocean (AO) are described by the following pathways:1,2 atmospheric deposition, oceanic evasion, transport by ocean currents, river inflows, coastal erosion, seaice drift and sedimentation. AO receives mercury from the atmosphere through direct deposition of anthropogenic and natural mercury to the open water and by the melting of snow on sea ice. Riverine input to AO results from weathering and erosion of natural mercury from rocks and soils, river contamination and atmospheric deposition in the river basin. The mass balance of mercury in AO is an issue of active debate. Outridge et al.,2 using a combination of observations and model, concluded that atmospheric deposition (98 Mg yr−1) and coastal erosion (47 Mg yr−1) are the dominant sources of mercury, and sedimentation is the largest sink of mercury in AO. Rivers were estimated to deliver 12.5 Mg yr−1 of mercury to AO. They also estimated that ocean transport results in a small net loss of mercury from AO. In contrast, Fisher et al.3 using a model (GEOS-Chem), concluded that circumpolar rivers are the main source (80 Mg yr−1) of mercury to AO and that AO is a net source of mercury to the atmosphere (45 Mg yr−1). Fisher et al.3 estimated that the atmosphere deposits 25 Mg yr−1 of mercury directly to open AO and 20 Mg yr−1 via melting of snow on sea ice, whereas ocean mercury evasion is 90 Mg yr−1. Durnford et al.4 © 2013 American Chemical Society

implemented a multilayer scheme of air/snow/meltwater exchange of mercury over land and sea-ice in the mercury model GRAHM and investigated atmosphere/surface fluxes of mercury in the Arctic (Figure 1). They estimated that the

Figure 1. Air−surface fluxes of mercury (Mg yr−1) in the Arctic polewards of 66.5°N and the riverine source of mercury to AO. All fluxes represent the annual average of a 5-year (2005−2009) simulation from Durnford et al.4 The riverine source was calculated in this study. “Model” refers to the model simulated meltwater based estimate. “Measurement” refers to the measured riverine Hg to dissolved organic carbon export ratio based estimate. Received: Revised: Accepted: Published: 1707

January 15, 2013 December 11, 2013 December 13, 2013 December 13, 2013 dx.doi.org/10.1021/es404473e | Environ. Sci. Technol. 2014, 48, 1707−1717

Environmental Science & Technology

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the leaf area index and incoming direct solar radiation.18 Mercury evasion from oceans is parametrized using fixed surface ocean Hg0 concentrations that vary spatially and seasonally, a temperature-dependent Henry’s law constant, and gas exchange velocities.20 The spatial distribution of Hg0 concentrations in the surface ocean is applied using distributions of mercury deposition to oceans from GRAHM and primary productivity data from satellite observations. Total oceanic evasion in GRAHM is constrained by global mercury budgets from Mason.19 Durnford et al.4 developed and implemented a dynamic multilayer snowpack/meltwater model for mercury in GRAHM. The depth of the top snowpack layer ranges from 2 to 30 cm, according to the snowpack’s total depth. For thicker snowpacks, a second snowpack layer extends from 30 cm to a maximum of 150 cm below the pack’s surface. If required, a third snowpack layer occupies the portion of the pack that is deeper than 150 cm. Elemental and oxidized mercury species are represented in all three snowpack layers. The net photoreduction of oxidized mercury is represented in the top layer by a rate of reaction determined by the availability of UV radiation estimated using the Bouguer−Lambert law21 and by the presence of mercury-retaining oxidants within the snowpack.22 Snowpacks over first-year sea ice and under canopies are assumed to contain higher levels of oxidants.23,24 Mercury is transferred from the top snowpack layer to the atmosphere via molecular and turbulent diffusions, and between the top two snowpack layers via molecular diffusion.25 Mercury is buried in a third snowpack layer through snow accumulation. The effective rate of turbulent diffusion is based on atmospheric surface-level turbulent kinetic energy. This rate is reduced under momentum-absorbing coniferous and mixed coniferous/deciduous canopies.26,27 At snowmelt, the generation of the ionic pulse is represented by transferring the snowpack’s oxidized mercury to the meltwater at a rate exceeding the melt rate of the snow.28 Photoreduction and evasion of mercury from meltwater and deposition of mercury to the meltwater are also represented. Durnford et al.4 found that stronger oceanic evasion of mercury is required to support the observed elevated concentrations of Hg0 in the Arctic surface air in summer. They added latitudinally increasing evasion of mercury from AO during May through August in GRAHM to reproduce the summertime maximum in concentrations of surface-level atmospheric Hg0.4,29 Ocean evasion of Hg0 is activated in GRAHM only if there is open water30,31 and the temperature at the air−sea interface is −4 °C or greater.32 Consequently, evasion of mercury from AO in the model starts later in the season at higher latitudes compared to lower latitudes during the warm season. The multiyear (2005−2009) simulation of GRAHM performed at a 1° × 1° horizontal resolution by Durnford et al.4 is used in this study.

atmosphere is a net source of mercury to AO delivering 75 Mg yr−1 of mercury (58 Mg yr−1 direct deposition, 50 Mg yr−1 deposition via snowmelt and 33 Mg yr−1 evasion). The degree of disagreement between the exchange of mercury fluxes between atmosphere and AO between the two modeling estimates indicates substantial sources of uncertainty. The problem is that models so far have been mainly evaluated for the surface air concentration of Hg0 in the Arctic; this does not fully constrain the models. Detailed model evaluation in all compartments of the Arctic ecosystem is required to address the uncertainties. This is difficult given the lack of observed data and the models’ lack or inadequate representation of all sources and sinks of mercury to AO. However, it is possible to constrain the input of atmospheric mercury to AO by extending the evaluation to include concentrations of speciated mercury in air, concentrations of mercury in precipitation and snowpacks, and air-snow fluxes of mercury. Here, we provide the most comprehensive evaluation of model (GRAHM) simulated atmospheric mercury in the Arctic based on observations from many locations and multiple years. We also present two new estimates of the riverine source of mercury to AO, one based on measured data and another using GRAHM. Finally, we provide an analysis of the discrepancies between the two modeling (GRAHM, GEOS-Chem) estimates of mercury in the Arctic.

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METHODS This study analyzes simulations produced by Environment Canada’s atmospheric mercury model, GRAHM (Global and Regional Atmospheric Heavy Metals model).4−8 GRAHM is an extension of Environment Canada’s operational weather forecast model, Global Environmental Multiscale − Global Deterministic Prediction System (GEM-GDPS). In GRAHM, both meteorological and mercury processes are simulated; the simulation of mercury processes uses information predicted by the meteorological component of the model, including transport, boundary layer turbulent mixing, the formation of cloudwater/ice, and precipitation. In GRAHM, chemical and physical transformations and transport of three mercury species, Hg0, Hg2+ (gas phase) and Hg2+ (particle phase) are simulated in the global atmosphere. The temperature-dependent gaseous oxidation of mercury by ozone occurs throughout the atmosphere.9 In the polar regions and marine boundary layer, the gaseous oxidation of mercury by halogens occurs.10,11 Mercury is reduced in the aqueous phase photochemically and by the sulfite anion.12 Dry deposition of Hg0 and Hg2+ (gas and particle phases) is based on the resistance approach.13,14 The partitioning of mercury between cloud droplets and air uses a temperaturedependent Henry’s law constant. Cloud droplets, raindrops and solid hydrometeors scavenge mercury. The simulation of AMDEs involves representing “bromine explosions”,15 halogen oxidation, atmospheric deposition, revolatilization from snowpacks and the transport of mercury-depleted air masses. The global anthropogenic mercury emissions used were produced by AMAP for 2005.16 Nonanthropogenic terrestrial and oceanic mercury emissions follow Gbor et al.,17 Shetty et al.18 and Mason.19 These terrestrial emissions consist of direct natural emissions, which are distributed according to the natural geological enrichment of mercury, and emissions of previously deposited mercury, which are allocated according to the distribution of deposition of mercury for historic years. The seasonal and diurnal cycles of terrestrial emissions are based on

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MULTIMEDIA EVALUATION OF GRAHM Mercury cycling in the Arctic is most dynamic in spring and summer. At polar sunrise, significant amounts of reactive Br are released to the atmosphere. The Hg2+ produced by rapid Br/ Hg0 reactions (known as AMDEs) is then deposited to terrestrial and aquatic surfaces. Hg0 concentrations can locally decrease below instrumental detection limits whereas concentrations of Hg2+ (gaseous or on particles) can increase by one to 2 orders of magnitude within a few hours. A significant part 1708

dx.doi.org/10.1021/es404473e | Environ. Sci. Technol. 2014, 48, 1707−1717

1709

latitude (° N)

longitude (° E)

species

months

2005 2005 2008

2005 2007−2008 2006 Mar 4−Apr 4 Apr 4−Jun 23 Feb 29−Jun 15

Apr 7−Jun 9 Apr 20−May 19 Mar 23−May 31

2005 Mar 1−May 31 2005−2009 Mar 1−May 31 2005−2009 Jan 1−Dec 31 2005 Mar 1−May 31 2005−2009 Mar 1−May 31 2005−2009 Jan 1−Dec 31 2007−2008 Mar 1−May 31 2007−2008 Jan 1−Dec 31 2008 Mar 1−May 31 2008 Jan 1−Dec 31 2005−2006 Mar 1−May 31 2005−2006 Jan 1−Dec 31 2008 Mar 1−May 31 2005 Mar 23−May 26 Arctic Ocean watershed south of 66.5° N 2005 Mar 2−Jun 3 2005 Mar 29−May 29 2009 Jan−Dec 2007 Jan−Nov

years

128 (128) 324 (324) 432 (432)

0.0 (−0.1) 4.9 (2.7) [0.2 (−0.2)]c [0.0 (−0.2)]d

6.0 [6.3−7.0]c [3.0−4.1]d 8.8

8.4 37 3.8 3.8

10h 22h 12 11 256 120 280

1.22 1.22−1.36 1.44−1.50 0.10 0.10−0.13 0.01−0.02 1.06−1.14c 1.44−1.46c 0.13c [0.06]d 0.02c [0.01]d 1.34 1.43−1.48 1.00 0.19

median

4416 4416 17520−17568 4416 4416 17520−17568 4416 17520−17568 4416 17568 4416 17520 4416 3120

n

GRAHM

0.4 3.6 0.5e

4.9/6.0j 0.6−13.5 6.9

5/5j 2−3 6 600 1896 7704

7.5i 47i 2.1 3.9

1.11 1.11−1.54 1.36−1.60 0.17 0.03−0.22 0.01−0.06 1.51−1.79 1.61−1.71 0.01e 0.01e 1.24−1.57 1.24−1.56 1.17 0.14g

median

observed

10h 22 12 11

2093 1750−2125 6383−8118 499 416−588 1288−2162 2083-2185 7453−8032 641 1473 1851−1944 6144−7614 2058 685

n

a

39 39 70

39 68 69

39 65 66 67

65

f

62 62 62 62 62 62 63 63 63 63 64 64

source

per year. bHg2+ is the sum of reactive gaseous mercury (RGM) and mercury associated with particles. cAt Ny-Ålesund: includes marine influences. dValues from the nearest landlocked grid point. eValues within the method detection limit from zero are included as is. fProvided by Steve Brooks, National Oceanic and Atmospheric Administration. gRGM only. hNumber of precipitation events. iFrom snow collected on tables. jSite1/Site2.

a

location

concentrations in air (ng m−3): Arctic (66.5° N - 90° N) Alert 82.5 −62.3 Hg0 Alert 82.5 −62.3 Hg0 Alert 82.5 −62.3 Hg0 Alert 82.5 −62.3 Hg2+b Alert 82.5 −62.3 Hg2+b Alert 82.5 −62.3 Hg2+b Ny-Ålesund 78.9 11.9 Hg0 Ny-Ålesund 78.9 11.9 Hg0 Ny-Ålesund 78.9 11.9 Hg2+b Ny-Ålesund 78.9 11.9 Hg2+b Amderma 69.8 61.7 Hg0 Amderma 69.8 61.7 Hg0 Barrow 71.3 −156.8 Hg0 Barrow 71.3 −156.8 Hg2+b concentrations in precipitation (ng L−1): Arctic (66.5° N - 90° N) and the Alert 82.5 −62.3 THg Barrow 71.3 −156.8 THg Bettles 66.9 −151.7 THg Fort Vermillion 58.4 −116.0 THg concentrations in seasonal snowpacks (ng L−1): Arctic (66.5° N - 90° N) Alert 82.5 −62.3 THg Ny-Ålesund 78.9 11.9 THg Barrow 71.3 −156.8 THg snowpack/atmosphere net flux (ng m‑2 hr−1): Arctic (66.5° N - 90° N) Alert 82.5 −62.3 Hg0 (THg) Alert 82.5 −62.3 Hg0 (THg) Ny-Ålesund 78.9 11.9 Hg0 (THg)

a

Table 1. Mercury Concentrations in Air, Precipitation and Snowpacks, And Snowpack/Atmosphere Fluxes

Environmental Science & Technology Article



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Figure 2. Shown, at four Arctic sites for 2007 and 2008, are observed (red) and simulated (blue) concentrations of atmospheric Hg0, simulated revolatilization of Hg0 from snowpacks (magenta), and simulated ocean evasion (green) of Hg0 from adjacent open ocean waters. Summertime maxima in atmospheric Hg0 concentrations are indicated by gray shading. A six-hour running mean centered on the stated date/time was applied to Hg0 concentrations. Evasion values are daily totals. Revolatilization values are scaled (divided by four) daily totals. The same data is provided for additional years in Figure S1 (Supporting Information).

of the Hg2+ that is deposited to snowpacks is subsequently reduced and revolatilized to the atmosphere as Hg0.33 Atmospheric Hg0 concentrations peak in the Arctic during summer,34 possibly as a result of revolatilization from snowpacks/meltwater and/or oceanic evasion.3,4 GRAHM has been extensively evaluated on the global scale.4−8 Table 1 compares simulated concentrations of Hg0 and Hg2+ in air, total mercury (THg) concentrations in precipitation and snow, and snow/air Hg0 fluxes with available observed data at Alert (Canada), Ny-Ålesund (Norway), Amderma (Russia), and Barrow (USA) for 2005−2009. Because observations in each medium considered are available at Alert for 2005, they are presented separately. At Alert, Hg0 springtime concentrations are ∼0.2 ng m−3 lower than the yearly median as a result of AMDEs. Because the partitioning between gas and particle phase Hg2+ is poorly understood, we compare observed and simulated concentrations of total Hg2+. Measurements of Hg2+ carry large uncertainties and the species are not yet identified;35−37 measured concentrations can be low by a factor of 2 to 3 particularly at lower temperatures, among elevated concentrations of atmospheric ozone, and when using Teflon tubing. Observed and simulated springtime median Hg2+ concentrations are an order of magnitude higher than

during the entire year. The model median concentrations of Hg0 and Hg2+ are within the range of observed medians; however, the simulated interannual variation is lower because of limitations imposed by model resolution. At Barrow, only gaseous Hg2+ concentrations were measured from March 23 to May 26, 2008. Although the majority of Hg2+ is thought to be in gaseous form at Barrow38 the higher median concentration of total Hg2+ simulated by the model (0.19 ng/m3) compared to the measured gaseous Hg2+ concentration (0.14 ng/m3) indicates the possible presence of particulate Hg2+. Significant AMDEs are simulated offshore near Ny-Ålesund. The Hg0depleted air is then transported to Ny-Ålesund. The observed median Hg0 concentrations at Ny-Ålesund are higher; the influence at Ny-Ålesund of evasion from the Gulf Stream may be stronger than is represented in the model. Mercury concentrations in snow collected on tables during precipitation events were measured at Alert and Barrow. Median mercury concentrations in precipitation are significantly higher at Barrow than at Alert. This is well simulated by the model. The volume of precipitation was not always reported at the two Arctic sites for a comparison of total deposition. Bettles, Alaska, and Fort Vermillion, Canada are in the AO watershed; deposition at these sites contributes to the 1710



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snowpack with the short-lived (few days) ionic pulse;22 some of the highest Hg0 concentrations are observed (and modeled) around this time. The second maximum in Arctic Hg0 air concentrations (mid June to mid July at Amderma and Barrow, mid July to early August at Alert and Ny-Ålesund) is exclusively supported by oceanic evasion in GRAHM (Figures 2 and S1 (Supporting Information)) and in Fisher et al.3 The two separate peaks are most evident at Alert and Barrow; Hg0 air concentrations vary more between spring and summer at Alert and Barrow than at Amderma and Ny-Ålesund. At Amderma, the distinction between the two maxima is less evident, particularly in 2007; the snowmelt season and the periods of active AO evasion of mercury overlap at Amderma in 2007 and 2008. This evaluation of GRAHM’s simulated mercury concentrations in both air and snow and its simulated fluxes in spring and summer reasonably constrains the atmospheric input of mercury to the Arctic in the model during the period when mercury cycling in the region is most dynamic.

riverine source of mercury for AO. The model simulates the median mercury concentration in precipitation at Fort Vermillion well but overestimates it at Bettles. Precipitation in the Bettles region is highly variable spatially, making the comparison between the simulated and observed concentrations difficult due to insufficient model resolution. However, the mean model (4.8 ng L−1) and observed (5.6 ng L−1) mercury concentrations in precipitation at Bettles are closer than the medians. The comparison of simulated precipitation amounts with measured precipitation rates from meteorological networks (not shown here) shows that modeled precipitation amounts are within the range of measurements, therefore simulated total deposition fluxes are also expected to be well constrained. Simulated median concentrations of mercury in seasonal snowpacks at Alert, Barrow, and Ny-Ålesund are also within the measured range. Some of the discrepancies between measured and modeled concentrations of mercury in snowpacks are caused by the effect of blowing snow on measured concentrations. Blowing snow is not represented in the model. Observed snowpack/atmosphere fluxes are deduced by observing concentrations of atmospheric Hg0 at two levels close to the surface.39 The fluxes are assumed to be the net flux of Hg0. However, the magnitude of the negative (depositional) net fluxes of Hg0 at Alert (max = 1080 ng m−2 hr−1 39) suggests that differences in concentrations of Hg0 at the two heights cannot be interpreted as a flux of Hg0 alone; Hg0 is likely being oxidized at low levels to Hg2+, which is readily deposited. Therefore, adding the deposition of Hg2+ that results from the oxidation of Hg0 in the surface layer to the simulated Hg0 flux (THg flux) provides an upper limit to the fluxes of Hg0 inferred from observations; we expect the magnitude of the observed net flux to fall within the range of the simulated net fluxes of Hg0 and THg. At Alert, simulated AMDEs start in mid-March. Observed AMDEs in 2005−2009 start at the beginning of March. Consequently, the median simulated revolatilization during March is considerably lower than observed. The production of Br species in the Arctic is parametrized in the model using satellite-derived BrO data from years previous to the years evaluated in this study. The onset of AMDEs has been found to be occurring earlier in the spring in more recent years possibly due to changes in temperature and their impact on the onset of bromine release in the Arctic.40 This discrepancy in the model may lead to a small underprediction of AMDE-related net mercury deposition in the Arctic. It has been suggested that the July peak in surface air concentrations of Hg0 in the Arctic34 is not driven by revolatilization from snowpacks/meltwater, which is enhanced at the onset of snowmelt (May−June) but by evasion from AO.3,4 We show here that, in fact, surface air concentrations of Hg0 in the Arctic tend to be characterized by two distinct summertime maxima (Figures 2 and S1 (Supporting Information)) with the timing of the peaks varying with location and year. The first maximum (mid to end of May at Amderma and Barrow, end of May to mid June at Alert, and June at Ny-Ålesund) is supported primarily by revolatilization from snowpacks/meltwater (Figures 2 and S1 (Supporting Information)). Periodic Hg0 maxima are also observed in March/April at all four sites; these are caused by the rapid revolatilization of Hg0 deposited onto snowpacks during AMDEs. The AMDE season has ended by the onset of snowmelt. At the onset of snowmelt, concentrations of atmospheric Hg0 are observed to surge as Hg2+ exits the

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RIVERINE SOURCE OF MERCURY TO THE ARCTIC OCEAN Fisher et al.3 inferred that circumpolar rivers and coastal erosion deliver 80 Mg yr−1 and 15 Mg yr−1 of mercury, respectively, to AO. Only a single water sample from each of the three major Russian rivers (Yenisei, Ob, and Lena) has been analyzed for mercury.41 Moreover, the samples were collected in autumn alone, in 1991 or 1993. Using these and seasonally sampled mercury concentrations for the Mackenzie River in North America over 2003−2005,42 Outridge et al.2 estimated the riverine source of mercury to AO at 12.5 Mg yr−1 with an upper limit of 39 Mg yr−1. Concentrations of dissolved organic carbon (DOC) and dissolved mercury (HgD) in Arctic rivers are positively and often significantly correlated.42−47 Kirk et al.1 used estimates of the riverine export of DOC48 with HgD/DOC and HgD/THg relationships from the Yukon River44 to obtain a riverine source of THg to arctic and subarctic waters of 108 Mg yr−1. In the North American watershed, the Mackenzie and Yukon Rivers export 1.9 to 4.31,38,42,43 and 4.444 Mg yr−1 of THg to AO, respectively, and 0.3 to 0.542,49 and 0.444 Mg yr−1 of HgD, respectively. The rate of THg export is relatively much higher (∼1.6-fold) from the Yukon River (discharge = 203 km3 yr−1;44) than from the Mackenzie River (discharge = 315 km3 yr−1 44). The Klondike gold rush of the 1890s was located in Dawson City on the Yukon River. Furthermore, the basin’s melting glaciers and the attendant release of their trapped mercury support an unusually strong summertime mercury export from the Yukon River.42−46,50 Because the riverine estimates of Kirk et al.1 are based on mercury measurements from the Yukon river alone, which is likely more contaminated compared to Mackenzie, we recalculated the riverine source of mercury to AO using multiple estimates of the DOC export to AO44,48,51−53 (Table S1, Supporting Information) and DOC/ HgD/THg statistics (Table S2, Supporting Information) from both the Mackenzie42,49 and Yukon44 Rivers (average HgD/ DOC = 0.28 Mg Tg1−; average HgD/THg = 0.12 Mg Mg1−; Table S2, Supporting Information). Ratios averaged over a pristine (Mackenzie) and a contaminated (Yukon) River are more likely to be representative of the Arctic riverine watersheds. Multiple studies provide estimates of DOC export from all rivers/regions except for the Arctic Archipelago, Barents Sea, and Chukchi Sea (Table S1, Supporting 1711



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Information).44,48,51−53 DOC export from Arctic Archipelago, Barents Sea, and Chukchi Sea is only available from Manizza et al.48 The DOC export estimate of Manizza et al.48 for all rivers/ regions, upon which the calculation by Kirk et al.1 is based, exceeds on average those of the other sources by 1.90-fold (range = 1.28−3.38 fold; Table S1, Supporting Information). Consequently, for all regions except Arctic Archipelago, Barents Sea, and Chukchi Sea, we use the average DOC export from all sources other than Manizza et al.48 For the three regions where the sole estimate is provided by Manizza et al.,48 we reduced their export by 1.90-fold. Using the revised DOC export to AO and the revised DOC/HgD/HgT statistics (Table S2, Supporting Information), we estimate HgD exports from the North American, Russian, and all AO watersheds at 1.0, 5.0, and 6.0 Mg yr−1, respectively, and the THg exports at 8.6, 41.6, and 50.2 Mg yr−1, respectively (Table S3, Supporting Information). The export of mercury by Arctic rivers tends to be strongest in spring42−46,50 when river water primarily represents melted snow.42,50,54 Atmospheric deposition of mercury through snowfall during winter/spring was found to be the main pathway of mercury released by Kobbefjord River upon snowmelt in west Greenland.50 We provide another estimate of the riverine export of mercury to AO using GRAHM’s simulated meltwater mercury concentrations. GRAHM’s estimate of the annual average transfer of THg from the snowpack to its meltwater is presented in Figure 3. The

transferred to the meltwater in the North American and Russian watersheds, respectively. In GRAHM, ∼70% and ∼65% of the mercury deposited onto snowpacks in the North American and Russian watersheds is rapidly revolatilized.4 Besides evasion, subcatchment sedimentation and biological uptake, and sedimentation in mainstem rivers reduce the meltwater runoff mercury export to AO.49 In addition, mercury concentrations decrease as rivers cross the freshwater−saltwater transition zone into AO.43 We incorporate subwatershed losses of mercury due to sedimentation and biological uptake by increasing the initial loss rate to 85% in North America following Carrie et al.49 Given the weaker simulated revolatilization in the Russian watershed, this initial loss rate is set at 80%. We apply a further loss of 29% to represent mercury loss during mainstem riverine transport.49 Thus, we estimate the North American, Russian, and all AO watersheds’ riverine mercury export to AO during spring freshet at 2.8, 12.7, and 15.5 Mg yr−1, respectively. Measured exports of mercury from the Mackenzie River during the open water period (June− August) for 2004 and 2007−2009 ranged from 1.2 to 3.5 Mg yr−1;1,43 and these values are comparable to the reported annual exports (1.2−2.9 Mg yr−1 for 2003−200542 and 4.3 Mg yr−1;49). Fifty percent of the annual mercury export from Yukon River and Mackenzie River was found to occur during the spring freshet.42,44 On the basis of these observations, and assuming that the riverine export of mercury to AO during the spring freshet primarily represents atmospheric deposition, we estimate an annual export of riverine mercury from North American, Russian, and all Arctic watersheds to AO in the range of 2.8− 5.6, 12.7−25.4, and 15.5−31.0 Mg yr−1, respectively. These model based estimates of the riverine export of mercury to AO are in good agreement with the measurement based estimates by Outridge et al.2 (12.5−39 Mg yr−1) but lower than the measurement based estimates provided in this study (50 Mg yr−1). It should be noted that the measurement based estimates presented in this study represent all sources of mercury to the rivers in the Arctic including atmospheric deposition, weathering of soils, and river contamination from historic mining activities; whereas model (GRAHM) estimates are based on the assumption that the spring freshet mainly contains atmospheric mercury. Therefore, our modeling based estimate of riverine mercury to AO is likely underestimated. All three estimates (from Outridge et al.2 and measurement and modeling estimates from this study) are lower than the inferred riverine source of mercury to AO in Fisher et al.3 (80 Mg yr−1). Measurements reveal that the riverine export of mercury to AO is dominantly particulate-bound (Mackenzie River: 75% and Yukon River: 90%);42,44 therefore sedimentation further reduces the riverine mercury input to the AO surface water.

Figure 3. Shown is the average amount of mercury that is transferred from the snowpack to its meltwater (μg m−2 yr−1) polewards of 40° N as estimated from a 5-year (2005−2009) simulation. The areas in North America and Russia that drain into AO are outlined in magenta and red, respectively.

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ANALYSIS OF SIMULATED ARCTIC OCEAN MERCURY FLUXES Following are the main differences between GEOS-Chem3 and GRAHM4 (see the Methods section) that impact the representation of mercury cycling in the Arctic. GEOS-Chem uses meteorological variables as inputs to the model for simulating mercury processes. GRAHM is an integrated model, where both meteorological and mercury processes are simulated within the model. GEOS-Chem is dynamically coupled with oceanic mercury processes. GRAHM represents oceans as a boundary with parametrized evasion of mercury from oceans. In both models, atmospheric mercury is oxidized

elevated simulated values along Canada’s east coast, in the Hudson Bay area, the Chukchi and Kara Seas, and in the High Arctic are supported by observations of elevated concentrations of marine mercury3,30 and/or deduced source regions of elevated observed concentrations of atmospheric mercury.29 In the regions of North America and Russia that drain into AO55 (Figure 3), the model’s estimated transfer of mercury to the snowpack meltwater is ∼1 to 3 μg m−2 yr−1 for 2005−2009. Summing each watershed’s entire meltwater mercury, we calculate that on average, 8.0 and 31.2 Mg yr−1 of mercury is 1712



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75%), the direct deposition of mercury to ocean waters (25/58 Mg yr−1), and the magnitude of ocean evasion (90/33 Mg yr−1). We have used observations of mercury in air, precipitation, and snowpacks to constrain the net atmospheric deposition of mercury in GRAHM (Table 1); GRAHM does not appear to overpredict mercury deposition in the Arctic. Observations indicate that 20 to 51% (average = 39%) of mercury in surface snow is lost, primarily through revolatilization, within 24 h.22,56 At a given location, forests, snowpack oxidants, and fresh snowfalls inhibit revolatilization.22 GRAHM represents the spatial and temporal variation of these factors (see the Methods section). The comparison of modeled and measured six-hourly mean concentrations of Hg0 shows that the AMDE-associated depletion and subsequent revolatilization cycles of Hg0 are well simulated by GRAHM (Figure 2). Given the high observed daily loss rate of mercury in snow and the reproduction of the depletion/revolatilization cycles of Hg0 in GRAHM, the seasonal revolatilization rate simulated by GRAHM (75%) seems reasonable. Total mercury concentrations measured at the sea surface under the ice were found to be low in the Canadian Arctic Archipelago whereas concentrations of Hg0 or dissolved gaseous mercury concentrations (DGM) were much higher under the ice compared to open water.30,57−59 These observations suggest reduction processes and build-up of Hg0 concentrations under sea ice during the ice-covered season.1 The enhanced concentrations of mercury in air that were measured during the ship’s passage through ice-covered regions indicate high mercury evasion.30,58 Elevated DGM concentrations measured in surface waters near the Mackenzie River Delta, north of Alaska and close to the North Pole suggest the influence of riverine export of mercury to AO.30 In addition to the atmospheric input of mercury via meltwater over sea ice and deposition to open water, the riverine export of mercury and the accumulation of Hg0 under sea ice provide important sources of Hg0 evasion from AO during summer. Very few observations of mercury evasion from AO are available for constraining the models. Summer and fall Hg0 evasion in the Canadian Arctic Archipelago is measured to be higher compared to the north Atlantic, Mediterranean and Baltic waters.1 Wide-ranging air−sea fluxes of Hg0 including both deposition (−38.4 ng m−2 day−1: along west coast of Greenland) and very high evasion (2352 ng m−2 day−1: along the coast of Alaska) were observed in AO30 from mid-July through September. Hg0 evasion at two sites in the Canadian Arctic Archipelago was estimated from Hg0 concentrations under the sea ice at 129 ± 2858 ng m−2 day−1 in early May. Because sea ice forms a barrier to evasion,30,31 such a strong flux is expected to be short-lived. At 10 sites in the Archipelago, the flux during August−September ranged from 2.6 to 30557 ng m−2 day−1 (median = 36 ng m−2 day−1). Measurements indicate significant spatial and seasonal variability in evasion of Hg0 from AO. Sea ice extent and wind speed are seen as critical factors in influencing the evasion of Hg0 from AO1. Given the wide range of fluxes of Hg0 from AO, it is not possible to derive the annual evasion rate of Hg0 from AO using these limited observations. GEOS-Chem estimates a mean mercury evasion in the Arctic (70°-90° N) during August/September at 44 ng m−2 day−1 (90 Mg yr−1 from AO). Limited observations suggest that this is reasonable. Riverine and erosional sources of mercury are added uniformly to AO in GEOS-Chem3. Because 75% to 92% of the riverine and 100% of the erosional mercury is

by bromine species at high latitudes. However, there are differences in reaction rates and the bromine activation process in the Arctic reflecting current uncertainties. GEOS-Chem used a 4°×5° horizontal resolution in their simulation and GRAHM used a 1°×1° horizontal resolution. In GEOS-Chem, the ocean is represented by two layers: the ocean mixed layer and the deep ocean layer. Mercury concentrations are simulated in the mixed layer by parametrizing the photochemical and biological reduction of Hg2+, evasion of Hg0, and the uptake of deposited Hg2+ onto particles. Mercury reduction is parametrized using solar radiation and net primary productivity data. Ocean evasion of Hg0 is parametrized using the temperature-dependent Henry’s law constant and the gas exchange velocity.20 Loss of particulate mercury to the deep ocean layer is modeled based on fixed carbon flux estimates. Mercury species in the mixed layer are exchanged vertically with the deep ocean layer using fixed mercury concentrations in the deep ocean. The horizontal transport or mixing of mercury species is not represented; therefore, the inflow/outflow of mercury to and from AO though ocean circulation is neglected. Riverine and erosion sources of mercury to AO are inferred from the mass balance of mercury in AO. In GRAHM, the concentrations of mercury species in oceans including AO are not modeled. The oceanic evasion from AO is parametrized using spatially and seasonally varying fixed surface ocean Hg0 concentrations and model simulated temperature-dependent Henry’s law constant and gas exchange velocity20 (see the Methods section). Currently, exchange of air−sea fluxes of mercury are the only processes represented in AO in GRAHM. GRAHM contains a detailed multilayer snowpack and meltwater model that represents physical and chemical processes of mercury species in snowpacks and meltwater, and mercury fluxes between air/snowpack/meltwater (methods). In GEOS-Chem, 60% of mercury deposited onto snowpacks is assumed to be reducible. Parametrized revolatilization of Hg0 from the reducible pool is proportional to solar radiation. At snowmelt, the accumulated nonreducible and the remaining reducible pool of mercury are transferred to the underlying ocean or land via meltwater. Both GEOS-Chem and GRAHM require augmented AO evasion during summer to support observations of elevated summertime Arctic atmospheric Hg0 concentrations.3,4 In this study, we have shown that there are two peaks of Hg0, one supported mainly by revolatilization of Hg0 from snowpacks/ meltwater and the second supported by evasion of Hg0 from AO. In both models, warm-season evasion from AO is calculated as a remainder and depends on the sea ice cover, sea surface temperature, and the gas exchange velocity.20 To increase the summertime evasion of mercury from AO in GEOS-Chem, an inferred riverine mercury input is added uniformly to AO surface waters following the temporal profile of observed riverine export, which peaks in early summer. In GRAHM, the summertime evasion of mercury from AO is increased by adding latitudinally and temporally varying evasion of mercury from AO from May−August (see the Methods section). The crucial differences in the air−surface fluxes of mercury in the Arctic from GEOS-Chem (70° N to 90° N3) and GRAHM (66.5° N to 90° N4; Figure 1) are the gross (61/263 Mg yr−1 for GEOS-Chem/GRAHM) and net (30/66 Mg yr−1) depositions of mercury to snowpacks, the average fraction of deposited mercury that is revolatilized from snowpacks (51%/ 1713



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particulate,1,42,44,49 much of this mercury settles gravitationally in coastal areas.1,60,61 GRAHM simulates a weaker mean mercury evasion over the entire AO than does GEOS-Chem but stronger evasion over the central AO (Table S4, Supporting Information). GRAHM’s mean evasion of mercury in the low Arctic is lower than measured evasion rates in the Canadian Arctic.57 Sensitivity studies with GRAHM (not shown) indicate that a mean evasion stronger than ∼4 ng m−2 day−1 during July/August from 66.5° N to 80° N yields overly high concentrations of atmospheric Hg0 at Amderma (69.7° N) and Barrow (71.3° N). In contrast, a considerably stronger mean evasion (∼60 ng m−2 day−1) during July/August from 80° N to 90° N is found necessary to support atmospheric Hg0 concentrations at both Ny-Ålesund (78.9° N) and Alert (82.5° N) in the model (Figures 2 and S1 (Supporting Information)). Strong evasion polewards of 80° N is supported by an observation-based study.29 Temporally, the progression of the maximal inferred evasion in GRAHM from June/July at Amderma and Barrow to July/August at Alert and Ny-Ålesund (Figures 2 and S1 (Supporting Information)) conforms to the hypothesis that sea ice forms a barrier to evasion;31 because sea ice melts earlier at lower latitudes, the onset of evasion is earlier. This gradient in mercury evasion from AO is consistent with the shift in summer peak surface air mercury concentrations from June/July at Amderma and Barrow to July/August at Alert and Ny-Ålesund (Figures 2 and S1 (Supporting Information)). GRAHM’s results suggest that representing the temporal and spatial variability of AO evasion in the models is important. The total input of mercury to AO from the atmosphere and rivers estimated by the two models is very close (125/124−139 Mg yr−1 for GEOS-Chem/GRAHM); however, the relative importance of these sources to AO is different (riverine being the dominant source in GEOS-Chem and atmosphere being the dominant source in GRAHM). The simulated net annual gain of mercury from these two sources (and erosion in GEOSChem) in AO is also different between the two models (50/ 91−106 Mg yr−1 for GEOS-Chem/GRAHM) because of the different evasion rates (90/33 Mg yr−1 for GEOS-Chem/ GRAHM). Nonetheless, despite their notable differences, both models agree that ocean evasion supports elevated concentrations of atmospheric mercury during summer in the Arctic. GRAHM also simulates the previously undiscussed atmospheric mercury peak that coincides with the snowmelt season. The limited observations available currently do not refute either model’s estimate of mercury fluxes in the Arctic as being unrealistic. Climate change may impact the various pathways by which mercury enters AO in significantly different ways. An improved understanding of the relative importance of the sources of mercury to AO is critical in assessing the overall impact of climate change on mercury levels in AO. In this study, we have provided a comprehensive evaluation of GRAHM’s simulation of the atmospheric mercury input to the Arctic. Comprehensive modeling of mercury processes in oceans and the evaluation of all sources of mercury to AO are essential for improving the understanding of the mass balance of mercury in AO. An improved knowledge of mercury biogeochemistry in all components of the Arctic ecosystem is needed to develop model parametrizations. Measurements indicate an important role of sea-ice dynamics on both mercury processes under sea ice and on air−sea fluxes of mercury; an improved representation of sea ice in AO and its impact on mercury

processes in models will aid in reducing the uncertainties related to evasion of mercury from AO. A better knowledge of the partitioning of mercury in the dissolved and solid phases in AO is also important. Seasonally varying measurements of concentrations of mercury species in seawater at the inflow and outflow and across the AO basin, in circumpolar river inflows and in snowpack and meltwater would be useful for constraining the models. Additionally, measurements of mercury fluxes between air and snowpack/meltwater/soils/seawater and wet depositional fluxes are needed.

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ASSOCIATED CONTENT

S Supporting Information *

Expanded version of Figure 2, derivation of DOC-based estimate of riverine mercury export to AO, and GRAHM’s simulated evasion. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*A. P. Dastoor. Phone: (514)421-4766. Fax: (514)421-2106. Email: [email protected]. Author Contributions

The paper was written through equal contributions of both authors. Both authors have given approval to the final version of the paper. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Observations of mercury concentrations at Alert62 and Amderma64 were provided by Alexandra Steffen and Amanda Cole of Environment Canada. Steve Brooks, National Oceanic and Atmospheric Administration, provided the mercury observations from Barrow. Thomas Douglas (Cold Regions Research & Engineering Laboratory, Fort Wainwright, Alaska and the Department of Chemistry and Geophysical Institute, University of Alaska Fairbanks) provided the concentrations of atmospheric reactive gaseous mercury (RGM) and mercury in snow from Douglas et al.65 For Ny-Ålesund, Anne Steen (Norwegian Institute for Water Research) and Katrine AspmoPfaffhuber (Norwegian Institute for Air Research) provided observed mercury concentrations and flux data from Steen et al. 63,70 whereas Aurelien Dommergue (Laboratoire de Glaciologie et Géophysique de l’Environnement, Université Joseph FourierGrenoble) provided the emission data discussed in Dommergue et al.68 We are deeply grateful to the above authors for providing us with these various data. Finally, we are thankful to Maria Andersson (University of Connecticut, Groton, Connecticut and ESSIQ AB, Sweden) for providing many invaluable insights into mercury evasion from AO.

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ABBREVIATIONS

AO, Arctic Ocean; Hg0, gaseous elemental mercury; Hg2+, divalent mercury species; RGM, reactive gaseous mercury; THg, total mercury; HgD, dissolved mercury; DOC, dissolved organic carbon 1714



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Arctic Ocean: Is It a Sink or a Source of Atmospheric Mercury

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