Science of the Total Environment 509–510 (2015) 3–15

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Review

Atmospheric mercury in the Canadian Arctic. Part I: A review of recentfield measurements

Alexandra Steffen a,⁎, Igor Lehnherr b, Amanda Cole a, Parisa Ariya c,d, Ashu Dastoor e, Dorothy Durnford e,Jane Kirk f, Martin Pilote g

a Environment Canada, Air Quality Processes Research, Toronto M3H 5T4, Ontario, Canadab University of Waterloo, Department of Earth and Environmental Sciences, Waterloo N2L 3G1, Ontario, Canadac McGill University, Department of Chemistry, 801 Sherbrooke St. W., Montreal H3A 2K6, Quebec, Canadad McGill University, Department of Atmospheric and Oceanic Sciences, 801 Sherbrooke St. W., Montreal H3A 2K6, Quebec, Canadae Environment Canada, National Prediction Development Division, Dorval H9P 1J3, Quebec, Canadaf Environment Canada, Aquatic Contaminants Research Division, Burlington L7R 4A6, Ontario, Canadag Environment Canada, Aquatic Contaminants Research Division, Montreal H2Y 2E7, Quebec, Canada

H I G H L I G H T S

• This paper reviews progress made in the study of the transport, transformation, deposition and reemission of atmospheric Hg in the Canadian Arctic, focusing onfield measurements.

• Redox processes control the speciation of atmospheric Hg and bromine radicals are the primary oxidant of atmospheric Hg depletion in the spring• It is expected that a smaller fraction of deposited Hg will be reemitted from coastal snowpacks.• Atmospheric gaseous Hg concentrations have decreased in some parts of the Arctic but at a rate that was less than that at lower latitudes• Remaining knowledge gaps are: the identification of oxidized Hg species in the air, total deposition amounts, physical-chemical processes over sea ice and im-pacts of Arctic climate change

⁎ Corresponding author. Tel.: +1 416 739 4116; fax: +E-mail address: Alexandra.Steffen@ec.gc.ca (A. Steffen

http://dx.doi.org/10.1016/j.scitotenv.2014.10.1090048-9697/Crown Copyright © 2014 Published by Elsevie

a b s t r a c t

a r t i c l e i n f o

Article history:Received 17 June 2014Received in revised form 27 October 2014Accepted 31 October 2014

Keywords:MercuryArcticAtmosphereRedoxSnowTemporal trends

Long-range atmospheric transport and deposition are important sources of mercury (Hg) to Arctic aquatic andterrestrial ecosystems.We reviewhere recent progressmade in the study of the transport, transformation, depo-sition and reemission of atmospheric Hg in the Canadian Arctic, focusing on field measurements (see Dastooret al., this issue for a review of modeling studies on the same topics). Redox processes control the speciation ofatmospheric Hg, and thus impart an important influence on Hg deposition, particularly during atmosphericmercury depletion events (AMDEs). Bromine radicals were identified as the primary oxidant of atmosphericHg during AMDEs. Since the start of monitoring at Alert (NU) in 1995, the timing of peak AMDE occurrencehas shifted to earlier times in the spring (from May to April) in recent years, and while AMDE frequency andGEM concentrations are correlated with local meteorological conditions, the reasons for this timing-shift arenot understood. Mercury is subject to various post-depositional processes in snowpacks and a large portion ofdeposited oxidized Hg can be reemitted following photoreduction; how much Hg is deposited and reemitteddepends on geographical location, meteorological, vegetative and sea-ice conditions, as well as snow chemistry.Halide anions in the snow can stabilize Hg, therefore it is expected that a smaller fraction of deposited Hg will bereemitted from coastal snowpacks. Atmospheric gaseous Hg concentrations have decreased in some parts of theArctic (e.g., Alert) from 2000 to 2009 but at a rate that was less than that at lower latitudes. Despite numerousrecent advances, a number of knowledge gaps remain, including uncertainties in the identification of oxidizedHg species in the air (and how this relates to dry vs. wet deposition), physical–chemical processes in air, snowand water—especially over sea ice—and the relationship between these processes and climate change.

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1 416 739 4179.).

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4 A. Steffen et al. / Science of the Total Environment 509–510 (2015) 3–15

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42. Atmospheric processes of Hg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.1. Overview of atmospheric Hg depletion events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.2. Chemical reactions of Hg in the Arctic atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.3. Temporal trends of atmospheric Hg depletion events at Alert . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.4. Distribution of atmospheric Hg species in the Arctic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3. Deposition of atmospheric Hg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84. Summary of Canadian atmospheric measurement data and spatial patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4.1. Gaseous elemental Hg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114.2. Reactive gaseous Hg and airborne particulate Hg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5. Temporal trends of gaseous elemental Hg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

1. Introduction

The atmosphere plays a fundamental role in the mercury (Hg) cyclein the Canadian Arctic, facilitating both the transport and deposition ofthis toxic contaminant. Due to its long (6–24 months; Pacyna et al.,2006) atmospheric residence time (lifetime in the air), Hg can undergolong-range transport from distant point sources to remote regions suchas the Arctic. Various wet- and dry-depositional processes lead to theinput of atmospheric Hg to Arctic terrestrial, freshwater, and marineecosystems, where it may eventually bioaccumulate and biomagnifyin local foodwebs following its conversion to methylmercury (MeHg)(Lehnherr, 2014). Therefore, the transport, deposition and fate ofatmospheric Hg play an important role in determining Hg exposure inhumans and wildlife. Efforts to characterize the atmospheric transportand delivery of Hg to the Arctic have been challenging because Hgemissions to the atmosphere occur from both natural sources andanthropogenic activities, and complex exchanges of Hg occur betweenthe atmosphere and its interfaces with soil, water, and the cryosphere(Durnford and Dastoor, 2011). Furthermore, the cycle of atmosphericHg has a number of features that are unique to the Arctic and PolarRegions, compared to lower-latitude regions, due to processes such asatmospheric mercury depletion events (Steffen et al., 2008a) (AMDEs,Section 2.1).

Measurements of atmospheric Hg in the Canadian Arctic were initi-ated in the early 1990s, leading to the influential discovery of AMDEs in1995 (Schroeder et al., 1998). During springtime AMDEs, gaseous ele-mental Hg is rapidly depleted from the lower atmosphere through oxi-dative processes and deposited on the ground or bound to aerosolsurfaces (Steffen et al., 2014). This process has sparked interest in atmo-spheric Hg and the role of AMDEs in the Arctic Hg cycle. Phase II of theNorthern Contaminants Program(NCP) (1998–2003) resulted in the es-tablishment of automated, long-term measurements of gaseous ele-mental Hg (GEM) at Alert, revealing seasonal patterns resulting fromAMDEs such as the rapid removal of GEM from the air within aperiod of hours after polar sunrise. AMDEs were also observed duringspringtime at subarctic latitudes, in Kuujjuarapik (Québec) andChurchill (Manitoba) for example, where incoming solar radiation re-mains present during the entire winter, suggesting that other processesmay also occur (Poissant and Pilote, 2003; Kirk et al., 2006). Sites whereAMDEs predominately occur and the chemistry responsible for theseevents were better characterized. The first measurements of ionic ornon-elemental atmospheric Hg species, namely reactive gas phase Hg(RGM) and particulate Hg (PHg), were conducted at Alert (Nunavut)(Cole et al., 2013; Steffen et al., 2014) and Kuujjuarapik. Collectivelythis research provided a new understanding of Hg processes in theArctic, and was highlighted as a key finding in the previous CanadianArctic Contaminants Assessment Report (CACAR II) (Bidleman et al.,

2003). However, the report also concluded that the significance ofAMDEs to the Arctic environment was yet to be determined.

During Phase III of the NCP further progress has beenmade on eluci-dating the chemical reactions resulting in oxidation of atmospheric Hgin the Arctic and the critical role of halogens as oxidants. Field measure-ments on Hg speciation, including both GEM and its oxidized counter-parts RGM and PHg, were collected in the Canadian Arctic (Kirk et al.,2006a, 2012; Steffen et al., 2014). Automated measurements of air Hgconcentrations at Alert (High Arctic) and Kuujjuarapik (sub-Arctic)over the last decade provide new insights into temporal trends at differ-ent time scales. The geographic coverage of air measurements has beenexpanded with continuous data now available for one new site in theYukon. Extensivemodeling of atmospheric processes has been conduct-ed (discussed in Dastoor et al., this issue). This has resulted in a new un-derstanding of long-range atmospheric transport of Hg from varioussource regions around the world as well as refined estimates of atmo-spheric Hg deposition in the Arctic (Dastoor et al., 2008; Dastoor andDurnford, 2014; Durnford et al., 2010). Atmospheric deposition is animportant source of Hg to Arctic aquatic and terrestrial ecosystemsand the objective of this review is to highlight recent findings, identifyknowledge gaps, and recommend directions for future research on theatmospheric deposition of Hg in the Canadian Arctic.

This is Part 1 of a two part review paper describing thework that hasbeen undertaken on atmospheric Hg in the Canadian Arctic. Part 1 de-scribes the measurements undertaken to describe Hg from its presencein the Arctic air; to the reactions it undergoes in the atmosphere; to howit is distributed; and to how this distribution affects deposition andemission of Hg to the Arctic surfaces, including a model describing theuptake or emission of Hg in the snow. Part 2 (Dastoor et al., this issue)describes themodeling of Hg from its transport, transformation, deposi-tion and emission which has made use of the results described in thisPart 1 of the atmospheric Hg paper.

2. Atmospheric processes of Hg

Three forms of Hg are considered in atmospheric Hg processes,including GEM or Hg(0), and two operationally defined forms:1) RGM, which consists of some oxidized gaseous inorganic Hg(II) andHg(I) species; and 2) PHg, which consists of oxidized Hg associatedwith particles. Each of these three categories has a different residencetime in the atmosphere. GEM can remain in the atmosphere for long pe-riods of time and, thus can be distributed globally as a result of long-range transport from point sources. In contrast, the more reactive oxi-dized forms of Hg are generally scavenged or deposited more locallynear emission sources either directly or following atmospheric chemicalconversions. Hence, it is highly likely that GEM is the dominant form ofHg that is transported to the Arctic by the atmosphere (Durnford et al.,

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2010). The conversion of GEM to PHg and RGM in theArctic atmosphereis a key step leading to the deposition of Hg to Arctic ecosystems.Atmospherically-deposited Hg can be subsequently methylated toMeHg in aquatic environments, and this organic toxic form of Hg canthen bioaccumulate in biota andhumans (Lehnherr, 2014) (see Chételatet al., this issue; Braune et al., this issue).

It should benoted that RGMandPHg as discussed in this paper are op-erationally defined as that which stick to a KCl coated denuder and mea-sured on particles less than 2.5 μm, using a Tekran model 2537/1130/1135 instrument, respectively. Measuring RGM and PHg at the pg/m3

level is very challenging. Recently, some research has shown that theremay be some bias in the RGM and PHg measurements using this instru-mentation (Gustin et al., 2013; Kos et al., 2013; Lyman et al., 2010); how-ever, for the purposes of this review these measurements are consideredoperationally defined and accepted as is. It should also be noted that thedata presented in the following have all been treatedwith the same qual-ity assurance protocols that are described by Steffen et al. (2012).

2.1. Overview of atmospheric Hg depletion events

AMDEs are characterized as a series of photochemically-initiated re-actions in the atmosphere which result in the conversion of GEM toRGM, which can also associate with aerosols to form PHg. Both RGMand PHg have short atmospheric lifetimes and are rapidly depositednear sources to terrestrial and aquatic surfaces, which are still snowand ice-covered during polar spring when AMDEs take place. Extensiveresearch has been conducted on AMDEs and the findings have beenreviewed (Steffen et al., 2008), showing that the initiation of AMDEsrequires Hg in the atmosphere, cold temperatures, a stable inversionlayer, sunlight, and reactive halogens (Dommergue et al., 2009;Nguyen et al., 2009; Poissant et al., 2008). Sea salt in ice, snow or frostflowers may provide a large source of halogen species that, under cer-tain conditions, can react to form radicals such as bromine (Br), brominemonoxide (BrO), chlorine (Cl), and chlorine monoxide (ClO) to initiatethese photochemical reactions (Simpson et al., 2007). Radicals can beproduced directly upon photolysis of their precursor or, through sec-ondary reactions; the original photochemically initiated radicals caninteract with a suite of molecules, generating new radicals in atmo-sphere or atmospheric interfaces (Subir et al., 2012). The fate of Hgdeposited during AMDEs is less clear, since both or either RGM andPHg are deposited to surfaces depending on the reaction processes.Studies have concluded that some of the deposited RGM is reduced toGEM and re-emitted into the atmosphere while some remains or is re-oxidized at the surface of, or within, the snowpack (see Durnford andDastoor, 2011). Studies of the photochemistry of Hg within the snow-pack have shown that reactions occur within the first fewcentimeters of the snow surface, and flux measurements indicate thatthere is net deposition of Hg to the snow in early spring followed bya net emission of Hg from the snowpack in the summer (Constantet al., 2007; Dommergue et al., 2003, 2007; Ferrari et al., 2005;Lalonde et al., 2002; Poulain et al., 2004). Global and regional modelsnow incorporate AMDEs into their chemical mechanisms although

Table 1A summary of selected experimental and theoretical reaction rate coefficients for the oxidationoxidant concentrations (Sander and Bottenheim, 2012) The calculated lifetime corresponds to

Oxidant Estimatedconcentrationin polar spring(molecules cm−3)

Br 107–108

Cl 104

BrO b5 · 108

OH 105–106

O3 b1012

there is still an incomplete understanding of the chemical processesthat drive deposition and re-emission of Hg in this environment.

2.2. Chemical reactions of Hg in the Arctic atmosphere

Until recently, only indirect evidence was available for the oxidationof long-livedHg(0) by halogen atoms— particularly bromine— to easilydeposited RGM species including Hg(I) and Hg(II). This evidence wasbased on a strong correlation between AMDEs and ozone depletionevents, which were believed to be caused by catalytic loss of ozonethrough reaction with halogen atoms and showed a negative correla-tion with concentrations of bromine oxide—one of the components ofthe catalytic ozone loss cycle—in the boundary layer and total air col-umn (Simpson et al., 2007).

Direct evidence for the oxidation of Hg in the gas phase has morerecently been gathered through laboratory measurements (Ariya et al.,2002; Donohoue et al., 2006; Pal and Ariya, 2004; Raofie and Ariya,2003) and theoretical calculations (Goodsite et al., 2004; Khalizovet al., 2003; Maron et al., 2008; Shepler et al., 2005) of reaction ratesand reaction products. These findings are reviewed in more detailelsewhere (Ariya et al., 2008, 2009) but a summary of selected oxidationreaction rate coefficients is presented in Table 1 along with approxi-mate oxidant concentrations ((Sander and Bottenheim, 2012) andreferences therein). This table illustrates how the importance of eachreaction depends on both the oxidant concentration and the rate coeffi-cient for a given reaction, though these estimates do not incorporatetemperature- and pressure-dependence of the reactions. For example,the reaction of Hg(0) with Br is slower than the reaction with Cl by afactor of 2–25 but since the concentration of Br during bromineexplosions can be approximately 100–2,000 times higher than that ofCl (Cavender et al., 2008), the reaction of Hg(0) with Br is consideredmore important. In fact, models of atmospheric Hg chemistry suggestthat oxidation by bromine radicals alone can account for the rapidoxidation of GEM that occurs during AMDEs when GEM concentrationscan sometimes drop by a factor of ten in a matter of hours (Ariya et al.,2004; Goodsite et al., 2004; Holmes et al., 2006, 2010; Skov et al., 2004)and may also be a significant oxidant of GEM in other seasons (Holmeset al., 2010). However, there is evidence to suggest that Cl concen-trations may be significantly higher during some AMDEs and couldsignificantly contribute to observed GEM oxidation (Stephens et al.,2012). It is also clear fromTable 1 that there is a great deal of uncertaintyassociated with many of the rate coefficients. Some discrepancy be-tween different experiments has been attributed to reactions on thewalls of reaction chambers, suggesting not only that the gas phase reac-tion rates may be inaccurate but also that additional reactions on thesurface of particles, snow, ice, and ocean water may contribute to GEMoxidation in the polar atmosphere (Sabir et al., 2011). Additionally,there are limited data for the temperature dependence of these reac-tions (Donohoue et al., 2005, 2006; Goodsite et al., 2004). Regardlessof their source, these large uncertainties in reaction rates must benarrowed in order to accurately model Hg oxidation during AMDEsand throughout the year.

of GEM (Ariya et al., 2008, 2009)., along with estimated GEM lifetime based on estimatedthe time required to oxidize 63% of initial Hg(0) with each oxidant.

Rate coefficient forreaction with GEM(cm3 molecules−1 s−1)

GEM chemicallifetime withthis oxidant

4 · 10−13–3 · 10−12 b1 h to 3 days5 · 10−13–1 · 10−11 115 days to 5.9 years10−15–10−13 N5 h toN23 days9 · 10−14–3 · 10−13 36 days to 3.6 years10−20–10−18 11 days to 4 years

March

May

June

April

1996–2000, 2002

March

February

May

June

April

2003–2009

February

a b

Fig. 1.Average distribution of AMDEs in each of the springmonths at Alert (Nunavut), for the years (a) 1996 to 2000 and 2002, and (b) 2003 to 2009. Correcting data for different samplingrates resulted in changes of b1% (Cole and Steffen, 2011).

6 A. Steffen et al. / Science of the Total Environment 509–510 (2015) 3–15

The reduction of Hg(I) andHg(II) back toHg(0) is another importantcomponent of the atmospheric Hg cycle in the Arctic. Reduction isgenerally assumed to take place in the aqueous phase, since Hg(I) andHg(II) species are soluble, and reduction results in volatile Hg(0) thatis easily released to the atmosphere. This process also occurs in lakesand oceans (Chételat et al., this issue; Braune et al., this issue). However,Hg reduction also occurs in snowpack and potentially in cloud and raindroplets, and in particles with significant water content. Emission ofGEM from snow has been observed following AMDEs (Section 3)and appears to depend on exposure to ultraviolet (UV-B) radiation(Lalonde et al., 2003; Poulain et al., 2004). Little is known about the

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Fig. 2. Frequency of AMDEs (bars) andmeanGEMconcentrations (lines) at Alert for springmonths binned by local temperature and wind direction.Source: Cole and Steffen (2010).

reactants involved in reduction reactions or their rates. For example,rate constants for the reduction of Hg(II) ions by hydroperoxyl (HO2)radicals and for the decomposition of the mercuric sulphite (HgSO3)complex to Hg(0)—both in water—have been measured but are stillhighly uncertain (Sabir et al., 2011, and references therein). Similarly,the photoreduction of Hg(II) species in the presence of organic matterhas been suggested (Si and Ariya, 2008, 2011), however the implica-tions and relevance of this process have yet to be evaluated in situ.Photoreduction of Hg on surfaces, such as aerosols rather than withinthe water itself, has not been studied (Sabir et al., 2012). It isa difficult research question to investigate because oxidation ofHg(0) back toHg(I) or Hg(II) can occur at the same time andmay be en-hanced by the presence of water molecules or clusters (Shepler et al.,2007) and Cl or Br ions in the water (Sheu and Mason, 2004).

Given the uncertainty in both reduction and oxidation chemistryon environmental surfaces, heterogeneous (i.e. air-surface) reactionsof Hg are a major knowledge gap that currently limits our under-standing of factors that drive Hg fluxes in the Arctic environment.Finally, although mercuric chloride (HgCl2) and mercuric bromide(HgBr2) were identified as products of the Hg + Cl and Hg + Brreactions in laboratory experiments (Ariya et al., 2002), the actual prod-ucts of Hg reactions in the ambient environment have not yet beencharacterized.

2.3. Temporal trends of atmospheric Hg depletion events at Alert

Patterns in Hg speciation data collected over several years provideinsight into the atmospheric processes that occur during AMDEs.Datasets reporting the AMDE phenomenon have been gatheredfor varying periods of time at several Arctic stations, includingAlert (Nunavut), Barrow (Alaska), Amderma (Russia), Station Nord(Greenland), and Ny-Ålesund (Svalbard) (for a review see Steffenet al., 2008) and in the sub-Arctic at Kuujjuarapik (Quebec) (Gauchardet al., 2005; Poissant and Pilote, 2003; Steffen et al., 2005). Analysis ofthe Alert dataset from 1995 to 2007 revealed no significant time trendin the overall combined frequency and strength of depletion eventsover the AMDE season (Cole and Steffen, 2010). However, it did revealsignificant changes with respect to when these AMDEs occur. Overtime, the month of maximum AMDE activity shifted from May toApril, with increased AMDE activity in March as well. This observationis illustrated in Fig. 1, which shows the average distribution of AMDEoccurrence in each of the spring months for 1995 to 2002 and for2003 to 2009. The reason for the shift in timing of depletion events isnot yet understood, though it has been found that the frequency ofdepletion events and overall GEM concentrations exhibit complex rela-tionships with local meteorology (Fig. 2). The top panel of Fig. 2 showsthat the integrated frequency of depletion events—a quantity thatcombines the length, magnitude, and frequency of depletion events—is lower at higher temperatures within each month. This pattern is

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consistent with previous findings that the halogen chemistry initiatingAMDEs and ozone depletion events may be temperature-dependent(Adams et al., 2002; Koop et al., 2000; Sander et al., 2006; Tarasickand Bottenheim, 2002). The frequency of AMDEs was not related tolocal wind speed but was found to have a relationship with winddirection (Fig. 2, bottom panel). The fact that air masses originatingfrom over the Arctic Ocean in the spring are depleted of GEMmore fre-quently than those originating elsewhere suggests that AMDEs occurover the ocean. This conclusionwas also reached by researchers analyz-ing GEM data at Ny-Ålesund using particle dispersion modeling(Hirdman et al., 2009). Finally, it was found that March GEM concentra-tions at Alert correlated with the Polar/Eurasian Teleconnection andNorth Atlantic Oscillation indices (Climate Prediction Center, 2009;Sander and Bottenheim, 2012). These two indices are not independent,and suggest that March AMDEs are more frequent and/or intense inyears when the circumpolar vortex is strong (shown by a positivePolar/Eurasia Teleconnectiion Pattern (PET) and negative North AtlanticOscillation Index (NAO) phase). However, there was no temporal trendin those indices, nor did the correlation extend into April and May (theprimary AMDE season).

Table 2Speciation of atmospheric Hg between GEM, RGM and PHg in the Arctic during both the AMDEcomparison.

Site GEM RGM

Alert, NU (Polar night) 95% 0.6%Alert (AMDE season) 88.6% 5.7%Churchill, MB (non-AMDE season) 99.2% 0.5%Churchill, MB (AMDE-season) 71.3% 10.3%Low- and mid-latitude rural areas 95–98% 0.2–1%

Urban areas 79–99% 0.4–13%

2.4. Distribution of atmospheric Hg species in the Arctic

The distribution of atmospheric Hg species differs in the Arctic fromother regions, as shown by a time series of GEM, RGM, and PHg from2002 to 2011 at Alert (Fig. 3) and a time series of GEM, RGM, and PHgmeasurements from several months in 2004 at Churchill in sub-ArcticManitoba (Fig. 3 inset). While there is a significant difference in thelength of these time series, the results from Churchill show that the dis-tribution in the springtime reflects the same distribution seen in eachspring in Alert. The seasonal cycles of GEM, RGM, and PHg can also beseen in Fig. 3 which clearly demonstrate the periodicity of AMDEs inspringtime and the significant changes in Hg speciation that occur dur-ing these events.

Background atmospheric Hg speciation in low- and mid-latituderural areas is dominated by GEM which represents about 95–98% oftotal Hg (THg), with approximately 0.2–1% RGM and 0.2–2% PHg mak-ing up the balance (Peterson et al., 2009; Rutter et al., 2009; Schroederand Munthe, 1998). The distribution of Hg species in urban areas isreported to be 79–99% GEM, 0.4–13% RGM, and 0.4–8% PHg (Rutteret al., 2009). Cobbett et al. (2007b) reported that during the 2005

and non-AMDE seasons; data from low- andmid-latitude rural and urban areas shown for

PHg Source

4.4% Cobbett et al. (2007)5.7% Steffen et al. (2014)0.3% Kirk et al. (2006)

18.3% Kirk et al. (2006)0.2–2% Schroeder and Munthe (1998);

Petersen et al. (2009); Rutter et al. (2009)0.4–8% Rutter et al. (2009)

8 A. Steffen et al. / Science of the Total Environment 509–510 (2015) 3–15

polar night (mid-winter) at Alert, the distribution of Hg species wassimilar to that of southern rural latitudes (i.e. 95% GEM, 0.63% RGM,and 4.4% PHg; Table 2). In contrast, during AMDEs, the Arctic springtimeatmosphere changed to 88.6%, 5.7%, and 5.7% of GEM, RGM, and PHg,respectively. At Churchill in 2004, the change in distribution was evenmore dramatic, with GEM, RGM, and PHg distributed on average overthree non-AMDE months as 99.2%, 0.5%, and 0.3%, respectively. How-ever, during the AMDE period of March to May, it changed to 71.3%,10.3%, and 18.3% on average (based on 1 season of data). Although thedistribution of atmospheric species during AMDEs is similar to that re-ported in an urban environment, it should be noted that the high springconcentrations of RGM and PHg in the Arctic are due to oxidation ofGEM in the atmosphere, while high concentration levels in urbanareas reflect direct industrial emissions of RGM and PHg. Furthermore,since RGM and PHg have a higher deposition velocity than GEM, thespringtime shift to high concentrations of short-lived Hg species war-rants investigation to determine the magnitude of net springtime atmo-spheric deposition ofHg to theArctic environment. Thiswas investigatedby Steffen et al. (2014) using the Alert speciation data from 2002 to2011; however, quantifying net deposition to the entire Arctic region re-quires a coupled surface-atmosphere model (Dastoor et al., this issue).

The distribution of GEM, RGM, and PHg during AMDEs differsamong Arctic measurement sites and the time in spring. For example,at Barrow in 2001, RGM was the dominant species during AMDEs(Lindberg et al., 2002). In Ny-Ålesund in 2003, the distribution was re-ported to vary throughout the study period but showed that RGM wasthe predominant oxidation product during AMDEs (Gauchard et al.,2005; Sprovieri et al., 2002), whereas during a more recent study atNy-Ålesund, the distribution also varied but showed a predominanceof PHg (Steen et al., 2009). Some researchers have suggested that thepredominance of either PHg or RGM as the product of GEM oxidationcan be an indication of the origin of the air mass (Gauchard et al.,2005; Lindberg et al., 2002) but perhaps the timing in which the sam-pling occurredmore accurately reflects the distribution of these species.At Churchill and Alert, Kirk et al. (2006b) and Cobbett et al. (2007b), re-spectively, showed a predominance of PHg during AMDEs at the begin-ning of spring and a higher predominance of RGM in AMDEs occurringtoward the end of spring (Fig. 3). This pattern has been shown to repeateach year at Alert (Steffen et al., 2014). It has been suggested that a re-lationship exists between the local meteorology and the distribution ofHg species during AMDEs, and a significant correlation exists between

Julian days (Jan 3

40 60 80 100

PH

g a

nd

RG

M (

pg

m-3

)

0

100

200

300

400February March April

PHgRGMMercury in Snow

Fig. 4. Atmospheric Hg speciation data (PHg in green, RGM in pink) at Alert (Nunavut)Modified from Steffen et al. (2014).

increases in PHg and lower air temperatures and humidity (Cobbettet al., 2007).

3. Deposition of atmospheric Hg

The atmospheric processes involved in the deposition of Hg duringAMDEs have been the topic of a number of studies. Steffen et al.(2002) reported that not all Hg is lost from the air during AMDEs.Although GEM may be depleted in the air, depending on the windconditions, there remains some of the other Hg species in the air.These species can be available for transport to another location. This im-plies that not all GEM oxidized during an AMDE is deposited onto thesnowpack at the site of GEM depletion and oxidation. More recentwork at Alert shows that the deposition of Hg from the air duringAMDEs is dependent on the conditions of the atmosphere such as tem-perature, relative humidity and aerosol contribution (Cobbett et al.,2007a; Steffen et al., 2014). The authors found that higher temperaturesand lower aerosol concentrations were the conditions under which thehighest concentration of Hg in the snowwas reported. Thus, in order toaccurately understand the wet and dry deposition processes of Hg fromair to snow that result from AMDEs, prevailing atmospheric conditionsmust be fully characterized.Wet deposition is defined as the scavengingof gaseous and particulate Hg species into atmospheric precipitationwhile dry deposition is defined as direct deposition to the surface. Cur-rently, deposition of HgduringAMDEs is typically inferred from concen-trations of THg in the snowpack and not primarily through directmeasurements of dry or wet deposition. Direct measurements of RGMand PHg fluxes in the Arctic are scarce and very limited. During a 2004study at Kuujjuarapik (March 9 to 16), the maximum fluxes of drydeposition directly quantified during AMDEs were −14.0 (GEM),−2.3 (PHg) and −1.2 ng/m2/h (RGM). Maximum GEM, PHg and RGMdeposition velocity were calculated to be 0.3, 1.2 and 1.6 cm/s, respec-tively. At Alert, snow/ice crystal samples have been collected on a Teflonsnow table on an event basis during springtime since 2002 and give areasonable estimate of Hg that is removed from the atmosphere andscavenged by snow (Steffen et al., 2014). Data from this monitoringshow that depletion events are often accompanied by an increase inTHg concentration in the fallen snow. Additionally, the time whenhigher concentrations of THg are found in the snow repeats from yearto year. It has been observed in the snow long-term dataset that themost significant increase of THg in the snow generally occurs when

1 to June 30)

120 140 160 180

Hg

in S

no

w (

pg

g-1

)

0

50

100

150

200

250

300500

600May June July

from February to June, 2002 to 2011 and THg concentrations in snow (blue bars).

9A. Steffen et al. / Science of the Total Environment 509–510 (2015) 3–15

there is a switch from PHg predominance to RGM predominance in theatmosphere (Fig. 4). These results show that the highest deposition ofHg through snowfall does not occur throughout the springtime periodbut rather when the atmosphere is characterized by an increase in rela-tive humidity, air temperature greater than −6 °C (on average), and asignificant decrease in particle concentration (Steffen et al., 2014). Inaddition, sea ice dynamics can further influence the overlying atmo-spheric chemistry, including AMDEs (Moore et al., 2014). These authorsshowed that the creation of sea ice leads aids in the replenishment ofGEM from aloft and the refreezing of the sea ice creates the atmosphericchemistry conditions to initiate depletion events. These findings aboutspecific atmospheric conditions leading to deposition may help explaina report from Ny-Ålesund which found that there were times when noincrease in the concentration of THg in the snowpack was observed fol-lowing AMDEs (Ferrari et al., 2005). Other direct wet deposition fluxmeasurements were reported by Sanei et al. (2010), who employed amodified Mercury Deposition Network (MDN) precipitation collectorin Churchill for over 27 months between 2006 and 2008 (Sanei et al.,2010). Contrary to other observations at Churchill (Kirk et al., 2006)and elsewhere in the Arctic, Sanei et al. (2010) reported that there is lit-tle evidence of higher deposition of Hg in the springtime during AMDEs.However, atmospheric models predict that there should be depositionof Hg during the springtime period at this location. This suggests thatHg deposition during AMDEs occurs as dry deposition and thereforemight not be recorded during wet deposition monitoring in the Arctic.However, further investigation is warranted to reconcile wet depositionmeasurements with measurements of increased Hg concentrations insnow during and immediately following AMDEs.

The deposition and fate of Hg during and after depletion events havebeen examined bymeasuring the concentration of THg in the snowpack(Poissant et al., 2008; Steffen et al., 2008b). Several researchers havereported that when the concentration of THg in the snowpack iselevated following anAMDE, there can be a 20–50% loss of that Hgwith-in 24 h (Durnford andDastoor, 2011) and 50–90% losswithin a fewdays(Constant et al., 2007; Kirk et al., 2006b). This loss is likely a result ofphotochemical reduction of Hg(II) to GEM and emission to the atmo-sphere, and is temperature dependent (Constant et al., 2007; Lalondeet al., 2003; Outridge et al., 2008; Poulain et al., 2004; Sherman et al.,2010). An alternate approach that has also been employed is tomeasurethe atmospheric fluxes of GEM above the Arctic snowpack to quantifythe deposition and emission of GEM around AMDEs. In 2000, verticalgradients of GEM concentrations were measured at Alert above thesnow surface during springtime. These measurements showed higherconcentrations of GEM immediately above the snowpack, thereby sug-gesting an emission from the surface (Steffen et al., 2002). This evasionwas observed for several days and throughout an extended AMDE dur-ingwhich GEM appeared to be emitted from the snow surface and thenunderwent photooxidation, which depleted it from the air again. In2005 at Alert, Cobbett et al. (2007b) reported a net depositional fluxof GEM to the snow during polar night between January and Marchwhen no AMDEs occur; however, during the spring there was no netGEM deposition to the snow surface and no evidence of net emissionof GEM following AMDEs (Cobbett et al., 2007b). Significant cycling ofHgwas observed between the surface and the atmosphere but an over-all net flux of zero was reported throughout the entire season in 2005(Cobbett et al., 2007). In March 2004 at Kuujjuarapik, net depositionof GEM was observed during AMDEs with no reemission thereafter;however, reemission of PHg and RGM was observed following one ofthe two AMDEs observed during this period of measurement. A fluxchamber study in Barrow reported a total emission of GEM from the sur-face of 4–7%of the THg content in the snowwithin a 24-hour period andthe authors suggested that this was an upper limit to the photoreduc-tion of easily reduced Hg after AMDEs (Johnson et al., 2008). Similarlyto the findings of Cobbett et al. (2007), Norwegian researchers inNy-Ålesund reported net deposition of GEM during polar night; how-ever they measured a net emission of GEM in the spring (Steen et al.,

2009). It should be noted that these measurements of GEM flux do notaccount for fluxes of RGMor PHg, which are the fractions thought to de-posit more readily to the surface during AMDEs. Deposition of RGM andPHg species, which are less susceptible to reemission from the surface,would increase the overall net deposition of Hg to the surface duringthe AMDE season.

A number of factors such as meteorological conditions, sea-ice con-ditions and snow chemistry parameters also play a key role in the depo-sition and fate of Hg in Arctic environments. For example, consolidatedice cover promotes depletion of GEMand AMDEs; however, the upwindpresence of open sea-ice leads can result in convective forcing andmixing of non-depleted overlying air masses into the boundary layer(Moore et al., 2014). This mechanism provides an increased supply ofGEM for oxidation during AMDEs and could result in increased deposi-tion of RGM/PHg (Moore et al., 2014). This is consistent with model re-sults, which show that deposition of atmospheric Hg is higher in yearswith higher wind speeds as a result of increased turbulent mixing ofHg-enriched air masses and thus higher deposition during AMDEs(Fisher et al., 2013). Conversely, the model showed that high air tem-perature, low sea-ice fraction, low cloudiness, and a shallow surfaceocean mixed layer lead to lower deposition of atmospheric Hg andhigher reemission through photoreduction (Fisher et al., 2013). Similar-ly, a negative correlation has been observed between air temperatureand the occurrence of AMDEs (Berg et al., 2013). The meltwater fluxof Hg from the cryosphere (a significant portion ofwhich is atmosphericin origin) to the surface ocean is enhanced by high solar radiation, lead-ing to increasedmelting, and a large air temperature difference betweenspring, when cold conditions favor deposition of atmospheric Hg, andsummer, when warmer conditions leads to increased meltwater inputsfrom sea ice and rivers (Fisher et al., 2013). Climate change, which isprojected to result in increased cloudiness and warming in spring,may therefore lead to decreased Hg inputs to the Arctic Ocean (Fisheret al., 2013), althoughhow itwill impact inputs from rivers is still uncer-tain (Fisher et al., 2012; Dastoor and Durnford, 2014).

Durnford et al. (2012a) performed a statistical study relating obser-vations of the 24-h fractional loss of mercury from surface snow, andmercury in surface snow, seasonal snowpacks, the snowpackmeltwater's ionic pulse, and long-term snowpack-related records tosimulated values of Hg deposition and meteorological variables. Theseauthors found that oxidized Hg deposited through wet processes wasretained preferentially by the snowpack-related media over oxidizedHg deposited through dry processes. This may be caused either by theburying of deposited mercury by fresh snowfalls (Witherow andLyons, 2008; Dommergue et al., 2010) and/or by the more central loca-tion within a snowpack's snow grain of mercury deposited throughwetversus dry processes (Seigneur et al., 1998; Douglas et al., 2008). Thispreferential retention within snowpacks of mercury deposited throughwet processes has important implications for climate change. If, in achanging climate, precipitation patterns change, the spatial distributionof wet deposition will also change. Consequently, the spatial distribu-tions of mercury concentrations in snowpacks and their meltwater,and also, therefore, of the transfer of mercury to the underlying surfacefrom the meltwater, will change.

The presence of halogen anions such as Br and Cl in snow can stabi-lize deposited Hg(II) by slowing the reduction of this Hg(II) back toHg(0) and/or enhancing the oxidation of Hg(0) by halides (Poulainet al., 2004; St. Louis et al., 2007; Lalonde et al., 2003), resulting inlowered reemission of GEM over sea ice compared to tundra (Steffenet al. 2013). Therefore, it is likely that both deposition and retention ofRGMand PHg in coastal snowpacks are enhanced compared to terrestri-al snowpacks, and these are reflected in higher snow THg concentra-tions measured at coastal vs. inland sites (St. Louis et al., 2007; Poulainet al., 2007a). Furthermore, THg concentrations measured in snowunder tree canopies are higher than those in adjacent open areas(Poulain et al., 2007b). Canopies can reduce reduction and volatilizationsignificantly by attenuating incident solar radiation, thereby slowing the

dominates in daylightoxidation

UV-B; 305-320 nm diffusioncontinuous ubiquitous

slow

GEM

RGM

ventilationepisodic localised

rapid

snowmeltionic pulse

Wet and dry deposition, canopy throughfallGEM PHg RGM

snowmeltincreased reemission

top ~2 cm

stabilizers:B

r -, C

l-

top ~50 to 100

cmReduction

Fig. 5. A schematic of the physical and chemical processes that govern the behavior of cryospheric Hg modified from Durnford and Dastoor, 2011. Pink represents processes relatedto snow melt, blue represents processed related to vertical transport; reducers (red) are H2O2, HO2•, humic acids, sulphite-based compounds, and oxalic acid; oxidants (green) areH2O2, Br•, Br2, O3, OH•, alkenes, alkyl and nitrates.

10 A. Steffen et al. / Science of the Total Environment 509–510 (2015) 3–15

rate of photoreduction of RGM in snow, increasing the fraction of Hgde-posited as PHg and the concentration of oxidants in the snowpack, andfinally, dampening wind ventilation of snowpack that promotes GEMemission. Furthermore, snow and ice crystal type and formation canplay an important role in determining Hg concentrations in thecryosphere (Douglas et al., 2008).

To understand and quantify key physical and chemical processesthat influence Hg deposition and fate in Arctic snowpacks, researcherscreated a snow exchange model that includes atmospheric deposition(Fig. 5) (Durnford and Dastoor, 2011). To develop a model parameteri-zation for the fate of deposited Hg in the Arctic, Durnford and Dastoor(2011) analyzed the basic mechanisms of the air-snow exchange ofHg using measurements in snow from around the world. Based onfield and laboratory data andmodel results, they presented a conceptualmechanism of the physical and chemical processes governing the fate ofdeposited Hg to the snow as illustrated in Fig. 5. It is hypothesized thatall GEM deposited onto snow-covered surfaces is likely revolatilizedimmediately—as indicated in the top left-hand corner of Fig. 5. How-ever, the snowpack likely retains most, if not all, of the deposited PHg.The fate of RGM deposited onto the snowpack is far more complicatedbecause it may undergo a series of chemical reactions within the snow-pack, as described above and in Section 2.2. For instance, RGM can be re-duced to GEM, which may then be oxidized back to RGM again.Synthesis of existing data suggests that oxidation of GEM tends todominate over reduction of RGM in daylight. Furthermore, hydrogenperoxide acts as both a reductant (in pH neutral snow) and an oxidant(in acidic snow), and the presence of halides in the snowpack is foundto stabilize the oxidized Hg in the snowpack. Emitted GEM is sourcedmore from surface snow than deeper snowpack layers, and GEMemission to the atmosphere increases significantly at the onset ofsnowmelt (Durnford and Dastoor, 2011). Finally, at the onset of snow-melt, Hg(0) emission to the atmosphere increases (Dommergue et al.,2003; Dommergue et al., 2010; Sommar et al., 2007; Brooks et al.,2008) and a considerable fraction of the snowpack's Hg(II) burdenexits the snowpack in the melt water ionic pulse (Bales et al., 1990;Kuhn, 2001). Fig. 5 indicates that PHg deposited onto the snowpackexits during snowmelt (narrower pink arrow labeled “snowmelt”)whereas RGM within the snowpack exits during the ionic pulse

(wider pink arrow labeled “ionic pulse”). It is believed that Hg concen-trations are higher during the ionic pulse period than during the re-mainder of the snowmelt period.

According to a statistical study, the concentration of THg in seasonalsnowpacks is strongly affected by the burial of deposited Hg with freshsnow—THg concentrations increase with increasing snow precipitationand its frequency (Durnford et al., 2012a). In contrast, deeper snow-packs apparently dilute the concentration of THg in the ionic pulse ofmeltwater from the snowpack. The concentration of THg in long-termcryospheric records varies by latitude possibly due to mid-latitudeanthropogenic sources of Hg. Long-term cryospheric mercury con-centrations also appear to decrease with an increasing occurrence ofsunny, dry conditions; such conditions promote photoreduction andsubsequent revolatilization, and reduce the likelihood of wet depositionand the burial of mercury by fresh snowfalls (Durnford et al., 2012a).

Carignan and Sonke (2010) examined Hg deposition along the coastof Hudson Bay. They found that, not only is the snow surface subject todeposition of Hg from AMDEs, but the lichens on trees around HudsonBay also accumulate this atmospherically deposited Hg. Although thisvegetation is known to be a good indicator of atmospheric depositionof Hg (Bargagli and Barghigiani, 1991; Horvat et al., 2000; Riget et al.,2000), Hg retention by lichens has only been investigated at a few loca-tions in the Canadian Arctic (Gamberg et al., this issue). In the studyby Carignan and Sonke, the THg concentration in lichens decreasedwith distance from the coast of Hudson Bay and showed the same pat-tern as snow THg concentrations in the area (Constant et al., 2007).These findings, along with those of other researchers across the Arctic(Douglas and Sturm, 2004; St. Louis et al., 2007; Poulain et al., 2007a)show that the concentration of THg in the snowdecreaseswith distanceinland from the ocean and indicates that Hg deposition and retentionare greater closer to ocean water (see also Section 4.1).

While numerous studies have concentrated on the exchange of Hgbetween air and snow on land in coastal areas, there have been fewinvestigations into the cycling of Hg over sea ice. Elevated levels ofTHg in frost flowers, hoar frost, sea snow, and seawater over the ArcticOcean have been reported (Douglas and Sturm, 2004; Douglas et al.,2005). Furthermore, frost flowers may enhance, at least temporarily,the retention of atmospheric Hg deposited during AMDEs by providing

11A. Steffen et al. / Science of the Total Environment 509–510 (2015) 3–15

a surface onwhich GEM that is reemitted from the snowpack can be ox-idized oncemore and adsorbed on to the crystal surface (Sherman et al.,2012). Future climate and sea-ice conditions are predicted to favor thegrowth of frost flowers and may therefore result in increased flux of at-mospheric Hg to the Arctic Ocean (Sherman et al., 2012). A continueddecrease in perennial ice may also favor the occurrence of bromine ex-plosions, which play an important role in the deposition of atmosphericHg to the Arctic (Nghiem et al., 2012). A recent publication of measure-ments over the Beaufort Sea showed a comparison of two snow-coveredice cores and suggested that elevated THg levels in surface snow, pre-sumably due to atmospheric deposition, do not extend down to thesea ice until melting of the snow has begun (Chaulk et al., 2011).

Modeling results by Hirdman et al. (2009) provide additionalsupport for the hypothesis that AMDEs result in the net deposition ofHg from the atmosphere to the Arctic Ocean. The authors performed astatistical analysis on the results from a Lagrangian particle dispersionmodel (FLEXPART) and GEM concentrations measured at Ny-Ålesundto identify source regions of high- and low-Hg air masses. It wasfound that, in the spring, air masses depleted in Hg had undergonelow-level transport over the ocean. However, in the summer, air masseswith strong surface contact over the Arctic Ocean showed a high Hgconcentration. This suggests that the Arctic Ocean is a net sink for Hgin the springtime but a net source in the summer, possibly due to emis-sions from the snow surface and evasion from the ocean (Hirdman et al.,2009). Durnford et al. (2012b), using an atmospheric model, found netdeposition in the Arctic in both spring and summer, but with that ofspring (63 Mg) 1.5-fold greater than that of summer (43 Mg). Usingan ocean-atmosphere model, Fisher et al. (2012; 2013) demonstratethat the summer-timepeak inGEMconcentration observed in theArcticatmosphere is a result of evasion from the Arctic Ocean, likely as a resultof elevated riverineHg inputs at this time of year. The atmosphericmer-cury model used by Dastoor and Durnford (2014) simulated an initialwarm-season peak in atmospheric GEM concentrations in the Arcticdriven by revolatilization from snowpacks and a second, later peak driv-en by ocean evasion. Hirdman et al. (2009) also found that in thewinter,low Hg concentrations were associated with air masses from the freetroposphere. This latter model analysis has not been replicated withdata from the Canadian Arctic but the findings are in agreement withdata reported from Alert for both Hg and ozone (Bottenheim andChan, 2006; Cole and Steffen, 2010).

The overall impact of atmospheric Hg deposition on the Arctic re-mains unclear, not only in Canada but across the circumpolar Arctic. Amass balance budget was produced for THg in the Arctic Ocean, and itwas concluded that the contribution of the atmosphere to the Hg poolin the Arctic Ocean is less than what is derived for other oceans but isstill significant overall (Outridge et al., 2008). Using data collected at

Table 3Multi-annual medians and seasonal averages of GEM (ng m−3), RGM (pg m−3) and PHg (pg mData are from Cole et al. (2013), Kirk et al. (2006) and unpublished data.

Site (measurement period) Days of data/days in period

Multi-annualmedian GEM

SpringGEM mean(SD)

SuGE(S

Alert (1995 to 2009)82.5° N, 62.5° W

4629/5479 1.56 1.24 (0.53) 1.8

Barrow (2007 to 2009)71.3° N, 156.6° W

780/1017 1.13 1.06 (0.69) 1.4

Churchill (Mar–Aug 2004)58.6° N, 94.2° W

138/150 N/A 1.26 (0.61) 1.8

Kuujjuarapik (Aug 1999 to 2009)55.3° N, 77.7° W

3479/3778 1.63 1. 72 (0.64) 1.8

Little Fox Lake (Jun 2007 to 2009)61.3° N, 135.6° W

710/942 1.31 1.43 (0.11) 1.2

Whistler (Aug 2008 to 2009)50.1° N, 122.9°W

332/491 1.14 1.14 (0.09) 1.1

Amundsen Gulf (Mar–May 2008)71.1° N, 124.8° W

54/92 N/A 0.94 (0.58) N/

a Kuujjuarapik January–March 2008.

Barrow (Alaska), a mass balance budget was developed for the Arcticthat showed a net surface gain in Hg in the springtime (Brooks et al.,2006). Earlymodeling estimates of net deposition of Hg to theArctic, in-cluding deposition from AMDEs, range from 50 to 300 tonnes (t) of Hgper year (Ariya et al., 2004; Banic et al., 2003; Lu et al., 2001; Skov et al.,2004). However, more recent models which incorporate snowpackphotoreduction and GEM reemission tend to yield lower depositionalvalues ranging from 60 t yr−1 (Holmes et al., 2010), to 143 t yr−1

(Dastoor et al., this issue). Further, Durnford et al. (2012b) estimatedthat, in the Arctic, gross deposition during spring, when AMDEs are ac-tive, represents 59% of the annual gross deposition. Results from theseinvestigations and comparisonwith fieldmeasurements of depositionalfluxes are discussed inmore detail in Dastoor et al. (this issue). It is clearfrom the findings described here that the cycling of Hg at the snow-airinterface and within the snowpack and melt water is very dynamic,but the processes governing this cycling are not well understood.Some of these issues for marine snow are addressed in more depth inSection 4 of Braune et al. (this issue).

4. Summary of Canadian atmospheric measurement data andspatial patterns

Atmospheric GEM concentrations have been monitored in Canadaat approximately ten different sites since 1995. Of the ten sites, GEMhas been continuously measured at only three sites between 1995and the present. In the CanadianArctic and sub-Arctic, the spatial cover-age is even sparser. Alert, Kuujjuarapik, and Little Fox Lake (Yukon) arethe only sub-arctic and Arctic sites where continuous atmospheric GEMmeasurements are made at present—measurements at Little Fox Lakebegan in 2007. Supplementary data from short campaigns (b1 year)are available for Resolute (Nunavut) (Lahoutifard et al., 2005), Churchill(Kirk et al., 2006b), and the Amundsen Gulf (Northwest Territories).Continuous monitoring data for speciated Hg (i.e. GEM, RGM, and PHgconcurrently) are even more limited. Alert is the only site wherespeciated measurements have been collected over multiple years.Short periods of speciated Hg monitoring have been done atKuujjuarapik (Steffen et al., 2003), Churchill (Kirk et al., 2006b), onHudson Bay (Poissant et al., 2006) and around the Amundsen Gulf dur-ing the IPY (2007–2008).

4.1. Gaseous elemental Hg

Geographic variationwas observed in seasonal mean concentrationsof GEM from Alert and the Amundsen Gulf in the High Arctic; Little FoxLake, Churchill, and Kuujjuarapik in the sub-Arctic; and, for comparison,at the Alaskan Arctic site of Barrow and the high altitude temperate site

−3) concentrations across Canada and Alaska.

mmerM meanD)

Fall GEMmean(SD)

WinterGEM mean(SD)

SpringRGM mean(SD)

SpringPHg mean(SD)

GEM trend(ng m−3 y−1)

0 (0.35) 1.49 (0.11) 1.59 (0.17) 50 (72) 70 (65) -0.009

0 (0.27) 1.06 (0.12) 1.03 (0.26) N/A

1 (0.19) N/A N/A 213 (199) 342 (495) N/A

6 (0.58) 1.58 (0.46) 1.56 (0.43) 18 (41)a 141 (75)a -0.038

7 (0.12) 1.17 (0.14) 1.35 (0.14) N/A

7 (0.17) 1.20 (0.25) 1.21 (0.21) N/A

A N/A N/A N/A

12 A. Steffen et al. / Science of the Total Environment 509–510 (2015) 3–15

ofWhistler (British Columbia) (Table 3). This table reflects the availabledata of atmospheric Hg monitoring in the Canadian Arctic and, as well,shows the lack of existing spatial coverage. The data presented was col-lected and analyzed using the same protocols as described in Steffenet al (2012). The heights of the inlets vary between sampling locationand are described elsewhere (Cole et al., 2013). Mercury is emittedfrom surfaces such as snow packs and sea ice and thus inlet heightsmay impact the comparability of data from several locations. Meanvalues for each season (winter: December to February, spring: MarchtoMay, summer: June to August, and autumn: September toNovember)and for the entire dataset were calculated over different time periods.These measurement data reveal that annual concentrations of GEMare lower at the western sites of Barrow, Little Fox Lake, and Whistler—overall median concentrations of 1.13, 1.31 and 1.14 ng m−3, respec-tively—compared to eastern sites at Alert and Kuujjuarapik—1.56 and1.63 ng m−3, respectively (Table 3).

Geographic differences were also observed in the seasonal pat-terns of GEM among these sites. At Alert, Kuujjuarapik, and Barrow,there were several days in the spring where the GEM concentrationdropped rapidly to below 1 ng m−3, typically used as the definitionof an AMDE. These AMDEs occurred at Alert and Barrow primarilyfrom March to May, at Kuujjuarapik from February to April, andwere not observed at Little Fox Lake or Whistler. Data from onespring and summer at Churchill also showed AMDEs (Fig. 3 inset).Those sites with AMDE activity are Arctic and sub-Arctic sites thatare in relatively close proximity to sea ice. The sub-Arctic site at LittleFox Lake does not experience AMDEs, which supports the hypothesisthat these events occur over or near a maritime source of sea salt(Douglas and Sturm, 2004; Douglas et al., 2005). Continuous GEMmeasurements at Mingan (50°16′N; 64°14′W), north of the Gulf ofSt. Lawrence, has also revealed the occurrence of AMDEs pointingto an influence of sea salt from this marine environment (Poissant,2000).

Sites with AMDEs also experienced rapid increases in GEM con-centrations in the late spring and summer that reached averagedaily concentrations of N5 ng m−3 at Kuujjuarapik (Cole et al.,2013). As a result, GEM concentrations peaked in summer at thesefour sites as shown in Table 3. It should be noted that the data fromChurchill are for spring and summer only and cannot be comparedwith fall averages. In addition, while AMDEs at Alert depressed thespringtime average GEM concentration to the lowest seasonalvalue, the high GEM peaks in late spring at Barrow andKuujjuarapik—which are not prevalent until June at the higher-latitude site of Alert—brought the springtime average concentrationsup to values that are similar to or higher than fall and winter aver-ages (Table 3). Finally, AMDEs and accompanying rapid increases inGEM resulted in high variability of both spring and summer GEMconcentrations at sites where AMDEs occur—shown by the high stan-dard deviation of seasonal measurements in Table 3. In contrast, atLittle Fox Lake and Whistler, where AMDEs were not observed inthe time series, the mean GEM concentrations were consistent be-tween seasons and much less variable within each season.

4.2. Reactive gaseous Hg and airborne particulate Hg

Long-term measurements of atmospheric Hg speciation are cur-rently limited to Alert. Measurements of RGM and PHg from 2002to 2011 are shown with GEM measurements in Figs. 3 and 4. Sixmonths of GEM, RGM, and PHg data were also collected at Churchill(Fig. 3, inset) (Kirk et al., 2006b). Both sites reveal that RGM and/orPHg values were high in the springtime during periods when GEMis unusually low (Table 3), consistent with the conversion of GEMto RGM and/or PHg during AMDEs. However, it should be notedthat currently these measurements are only operationally definedbecause there are no existing calibration standards and the exactchemical compounds that comprise RGM and PHg are unknown.

Therefore, data from these two sites have not been quantitativelycompared. Identification of these chemical species remains an im-portant knowledge gap.

5. Temporal trends of gaseous elemental Hg

As global anthropogenic emissions of Hg continue to change, and asthe Arctic undergoes dramatic environmental changes, such as shrink-ing permanent sea ice, the continuous monitoring of atmospheric Hgprovides important information about long-term changes in the trans-port, chemistry, and deposition of this pollutant in the Arctic environ-ment. Worldwide atmospheric measurements of GEM up to the early2000s suggest that concentrations of atmospheric Hg increased fromthe 1970s to a peak in the 1980s and then decreased to a plateau around1996 to 2001 (Slemr et al., 2003). Similarly, a recent reconstruction ofGEM levels in firn air (trapped gases in porous ice similar to compactedsnow that is less dense than ice) fromGreenland indicated that GEM in-creased from the 1940s to the 1970s and reached a plateau around themid-1990s (Fäin et al., 2009). Another study of long-term trends at Res-olute Bay examined measurements from 1974 to 2000 of filterable Hg.That is, manual samples of Hg collected by passing air through particlefilters and is likely PHg and RGM combined (Li et al., 2009). These au-thors reported a decrease of approximately 3% per year in totalfilterableHg in summer and fall, which is similar to the world-wide decrease inHg emissions from anthropogenic activities between 1983 and 1995.Considerable variability was found in the data during the winterand early spring months suggesting some influence of AMDEs in thesamples.

Continuous measurements of GEM have been made since 1995 atAlert, and for 11 years at Kuujjuarapik (1999–2009). Temporal trendsin these datasets have been assessed using statistical techniques to ac-count for large seasonal variability. The techniques included seasonaldecomposition (Temme et al., 2004) and the seasonal Kendall test fortrend (Cole and Steffen, 2010). While analysis of the Alert data to theend of 2004 found no significant trend in GEM air concentrations(Temme et al., 2007), subsequent analysis using additional years ofdata revealed that GEM decreased at a rate of -0.6% per year from1995 to 2007 (Cole and Steffen, 2010).

The seasonal Kendall test for trend and related Sen's estimator ofslope were also used to calculate ten-year trends from 2000 to 2009 atAlert, Kuujjuarapik, Ny-Alesund, and three Canadian mid-latitude sites(Egbert, Ontario; Kejimkujik, Nova Scotia; St. Anicet, Quebec) (Coleet al., 2013). The time period was chosen to compare trends betweenmultiple sites over the same time interval. Over this period, GEM con-centrations at Kuujjuarapik and at the three mid-latitude sites declinedat a rate of approximately−2% per year, while Alert concentrations de-clined by−0.9% per year (Cole et al., 2013) and no decreasewas seen atNy-Alesund (Berg et al., 2013). Trends at the four non-Arctic sitesagreed well with the reported decrease in background GEM concentra-tion at Mace Head (Ireland) of −1.8 ± 0.1% per year over the period1996 to 2009 (Ebinghaus et al., 2011). The long-term trends at theArcticsites were also more variable from month to month, partially due toAMDE chemistry and likely also due to effects from changes in sea icecover. Trends in RGM and PHg at Alert were also reported for a seven-year period (2002–2009); both have increased in the spring whentheir concentrations are significant, but high interannual variabilitylimits the reliability of these trends (Cole et al., 2013).

At this time, it is not clear why atmospheric GEM levels have de-creased more rapidly at sub-Arctic and temperate locations than atpolar sites. It is important to determine the mechanism responsible forthemore positive GEM trends in the Arctic, because the effect on the de-position of mercury to the Arctic ecosystem could be very different. Forexample, decreased oxidation chemistry would lead to increased springGEM concentrations but decreased deposition to the surface, while in-creased transport of mercury from lower latitudes would increaseboth Arctic GEM concentrations and total mercury deposition. It is also

13A. Steffen et al. / Science of the Total Environment 509–510 (2015) 3–15

possible that Alert is more strongly influenced by source regions whereHg emissions are increasing (such as Asia) comparedwith Kuujjuarapik,Saint-Anicet, Mace Head, or Cape Point (see also Dastoor et al., thisissue), or that large climate-related variability is impacting the trends(Cole and Steffen, 2010). Decreasing GEM trends mid-latitude sitessuch as at St-Anicet also likely reflect a small contribution from de-creased Hg emissions as a result of improved emission control in indus-tries such as coal-fired power plants in the north-eastern USAand Canada, and/or decreased emissions from the North Atlantic(Soerensen et al., 2012). Detailed modeling studies and better knowl-edge of natural emission trends would help resolve the reason(s) forlatitude-dependent trends and also extrapolate the impact on mercurydeposition in the Arctic. Also, continued observations in the Antarctic(Pfaffhuber et al., 2012; Brooks et al., 2008), where AMDEs occur butemission sources aremore distant, and at other remote Northern Hemi-sphere locations like Little Fox Lake that do not experience AMDEs, mayhelp separate the effects of chemistry, emissions, and transport.

6. Conclusions

Significant advances have been made toward understanding theunique processes governing atmospheric Hg transport and fate in theArctic and sources of Hg to this region. Several of these advances relateto the chemical processes of AMDEs—a phenomenon characterized byrapid depletion of GEM from the lower atmosphere through oxidativeprocesses and subsequent deposition of this Hg on the ground or tothe surfaces of particles. It was verified that Br atoms, thought to origi-nate from marine waters, are the most significant oxidants of Hg inthe Arctic atmosphere. The speciation of atmospheric Hg changes dur-ing springtime AMDEs in the Arctic as GEM is oxidized, leading tolarge increases in RGM and PHg concentrations. Long-term measure-ments of Hg species at the High Arctic monitoring station at Alertrevealed that the timing of AMDEs has shifted to earlier in the yearalthough the frequency of AMDEs has not changed. The timing ofAMDEs was correlated with local temperature, time of year, and winddirection, but these factors alone did not completely account for ob-served trends and variability of AMDEs. Further study of the factors af-fecting AMDE chemistry is needed in order to predict their futurecontributions to Hg deposition in the Arctic.

Field studies on Hg fluxes from the atmosphere to snow indicatethat, while a fraction of the Hg deposited during AMDEs is retained inthe snowpack, a significant portion is quickly reduced and emitted tothe atmosphere. Field and modeling studies suggest that halogen com-pounds from themarine environment and frost flowers facilitate oxida-tive and stabilizing reactions with Hg that increase its retention in thesnow. Snowpacks rich in halogen species are found near coastal regions,and reactions betweenHg andhalogens in the snowmay in part explainthe observation that the extent of Hg re-emission increases with dis-tance from open ocean water. The concentration of THg in seasonalsnowpacks is also affected by the burial of Hg with fresh snow. There-fore, snow accumulation of Hg may be higher in regions with higherand frequent snow precipitation. The outcome of these findings is thatHg retention and cycling in the Arctic seem to differ depending on thetime of year, proximity to the ocean, and meteorology.

Model derived estimates suggest that net deposition of Hg above theArctic Circle (north of 66.5°) occurs at a rate of 143 t yr−1 (Dastoor et al.,this issue). However, both model simulations and estimates derivedfrom lake sediments (Muir et al., 2009) show that the deposition of at-mospheric Hg generally declines with increasing latitude, and deposi-tion of atmospheric Hg in Arctic and subarctic lakes is significantlylower than in mid-latitude North American lakes.

The geographic coverage of air measurements in the Canadian Arcticis still sparse but now includes additional data for Little Fox Lake(Yukon), Churchill (Manitoba) and the Amundsen Gulf (NorthwestTerritories) to complement the long-term monitoring sites at Alert inthe High Arctic and Kuujjuarapik in sub-Arctic Quebec. Annual

concentrations of GEM were lower at western Arctic sites of Little FoxLake, and Barrow (Alaska) and similar to a western temperate site atWhistler (British Columbia). Rapid drops in atmospheric GEM concen-tration, associated with AMDEs, were observed for the coastal sites atAlert, Barrow, Churchill, and Kuujjuarapik but not for the inland site atLittle Fox Lake, which supports the hypothesis that these events occurnear marine sources of halogens.

Air concentrations, measured as GEM, declined from 2000 to 2009 atAlert and Kuujjuarapik. The annual rate of decline at Kuujjuarapik of−2.0±0.9% (95% confidence limits)was comparable to non-Arcticmon-itoring sites in North America and elsewhere, while the trend for Alertwas lower (−0.9± 0.6%). A slower rate of decline at Alertmay be relatedto the environmental conditions occurring in the High Arctic or exposureto different pathways of long-range atmospheric transport of Hg. Accord-ing to modeling analyses in mid-latitude locations of North America, thedecline in regional Hg emissions between 1990 and 2005 was wellreflected by a decline in air concentrations and deposition. However, inthe High Arctic, the model results (Dastoor et al., this issue) suggestedthat the Hg trends were mostly related to changes in meteorology andglobal changes in anthropogenic emissions. Results from modeled andmeasured GEM concentrations illustrate that Hg trends observed at tem-perate locations cannot be extrapolated to the Arctic.

Although tremendous progress has been made toward understand-ing long-range atmospheric transport of Hg and its deposition in theArctic, many knowledge gaps still impede our capacity to quantify thesignificance of this cycling for bioaccumulation of methylmercury inbiota and humans. These gaps include uncertainties in the identificationof oxidized Hg species in the air, physical-chemical processes in air,snow and water—especially over the sea ice—and the relationship be-tween these processes and climate change. Greater spatial coverage oflong term atmospheric speciated mercury needs to be initiated so thatlong term changes at locations other than Alert can be assessed. A fullerunderstanding of the impacts of air exchange and sea ice dynamics onthe deposition of mercury needs to be addressed. The Arctic is coveredby significant permafrost under the snow pack and howmuchmercuryresides in the permafrost is unknown aswell as howmuchwill be emit-ted to the atmosphere as melting increases. Moving forward, these un-knowns should be investigated.

The Arctic spans a significant portion of the Canadian landscape. Thisregion is undergoing drastic changes that affect the complex biogeo-chemical cycling of pollutants such as Hg, as described in this specialissue. The changes are dynamic and can have a cascading effect on thetransport, transformation, deposition and uptake of Hg in the Arctic. Itis vital that future changes in anthropogenic activities, such as landuse changes, resource development and transportation in the Arctic re-gion be considered as both new sources of Hg to the region and for theirimpacts on the chemistry dictating the cycling of Hg in the area.

Acknowledgments

We would like to acknowledge the support of the NorthernContaminants Program. We also thank Heather Morrison who contrib-uted to and edited an earlier version of this manuscript. Finally, wethank Melissa Antoniadis and Julie Narayan for their assistance withgraphics design and editing.

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