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doi:10.1016/j.at

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Atmospheric Environment 42 (2008) 2877–2884

www.elsevier.com/locate/atmosenv

Antarctic polar plateau snow surface conversion of depositedoxidized mercury to gaseous elemental mercury with fractional

long-term burial

Steven Brooksa,�, Richard Arimotob, Steven Lindbergc, George Southworthd

aNOAA Atmospheric Turbulence and Diffusion Division, Oak Ridge, USAbNew Mexico State University, USA

cOak Ridge National Laboratory, Environmental Sciences Division, Emeritus, USAdOak Ridge National Laboratory, Environmental Sciences Division, USA

Received 22 September 2006; received in revised form 6 May 2007; accepted 9 May 2007

Abstract

The role of the vast Antarctic polar plateau in the global mercury cycle was previously relatively unknown. Here, for the

first time, mercury concentrations in snow and air, combined with vertical flux measurements at the South Pole

(November–December 2003 and November 2005) have provided considerable insight into the cycling of this element

through the Antarctic environment. These insights include observations showing atmospheric oxidized mercury depositing

to the snow pack, subsequent photoreduction, and emissions of gaseous elemental mercury from the snow pack. Oxidized

mercury (e.g., reactive gaseous mercury and fine particulate mercury) showed high concentrations (100–1000 pgm�3) in the

near-surface air, with these concentrations strongly correlated with vertical mixing rates and showing rapid surface

deposition. This suggests that the troposphere over Antarctica is enhanced in oxidized mercury, with mercury cycling

between elemental and oxidized states, and between the atmosphere and snow pack. Based on these limited measurements

at South Pole, we estimate that the Antarctic polar plateau could sequester as much as 60 metric tons of Hg annually.

These data also suggest that there could be a seasonal cycling of atmospheric mercury oxidation, deposition, and re-

emission via photoreduction of 490 metric tons annually. This cycling is restricted to the annual sunlit period and peaks

3–4 weeks after solar maximum. To our knowledge, these provisional values represent the first estimates of the mercury

balance and cycling for the extensive Antarctic polar plateau.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Antarctica; Mercury; Snow

1. Introduction

The vast Antarctic polar plateau covers an area of5 million km2 around the South Pole with an

e front matter r 2007 Elsevier Ltd. All rights reserved

mosenv.2007.05.029

ing author.

ess: Steve.brooks@noaa.gov (S. Brooks).

average elevation of �2800m. The potential role forthis area in the global mercury cycle has not beenfully understood. Significant filterable mercury atSouth Pole Station was reported by Arimoto et al.(2004) and low levels (1–3 ng(Hg) l�1) of totalmercury in pre-industrial ice cores from Dome Cwere reported by Vandal et al. (1993).

.

ARTICLE IN PRESSS. Brooks et al. / Atmospheric Environment 42 (2008) 2877–28842878

To further our understanding, from 2003 to 2006,as a component of the Antarctic TroposphericChemistry Investigation, mercury concentrationsand fluxes were measured in snow and air withinthe ‘‘Clean Air Sector’’ of South Pole Stationadjacent to the Atmospheric Research Observatorybuilding. The Atmospheric Research Observatory isa zero-emissions building and all vehicles areprohibited from approaching the building or else-where in the Clean Air Sector. More details of thebuilding, environment, and overall study objectivescan be found in the overview paper (Eisele et al.,this issue).

Our mercury measurements consisted of high-volume filter samples for total filterable mercury,gradient fluxes of gaseous elemental mercury,continuous speciated mercury concentration mea-surements (gaseous elemental mercury, GEM,reactive gaseous mercury, RGM, and fine particu-late-bound mercury, FPM), and, total mercuryanalyses of the snow pack (to a depth of 1m),blowing snow, and station ‘‘well’’ water supply.

2. Measurement techniques

Continuous mercury speciation measurementswere conducted from early November to lateDecember 2003 with a sensor suite consisting ofTekran models 2537a, 1130, and 1135, for thedetermination of GEM, RGM, and FPM, respec-tively. The front-end units (1130 and 1135) weremounted on the roof-top deck of the AtmosphericResearch Observatory on the predominate winddirection side (towards the centroid of the Clean AirSector). The special instrumentation set-ups for thishigh-altitude site included adjusting the mass flowrates to correct the volume flows through the front-end denuder and particulate modules, and airdensity corrections (see Banic et al., 1997). Weutilized 5min pre-concentration sampling times forGEM, and 2 h pre-concentration sampling times forRGM and FPM. A summary of this instrumenta-tion and quality control procedures can be found inBrooks et al. (2007, this issue).

Daily (intensive) or weekly (routine) 24-h high-volume filter samples were collected adjacent to theTekran mercury front-end units atop the roof-topdeck. On removal, the individual filters were gentlytilted to slough-off excessive snow. The filters withthe remaining snow were then packaged for ship-ping and later analysis for total filterable mercury.

GEM vertical gradient measurements, for theestimation of flux rates, were conducted on 15 and16 November 2005, 15m directly upwind from theAtmospheric Research Observatory building, withinthe Clean Air Sector, and just outside the daily‘‘shadow’’ of the building. A maximum samplingheight of 3m was utilized to stay within the lowest�10% of the mixing layer height, which is often nomore than 30m deep.

During the gradient sampling period, air tem-perature ranged from �30 to �35 1C, skies wereclear, precipitation was absent, and winds were5–8m s�1 from 355 to 81 (longitude grid; well withinthe Clean Air Sector). The Clean Air Sector is anarea of dominant wind direction between longitudegrid 3401 and 1101. Access to the Clean Air Sectorby vehicles and even foot is strictly limited topreserve the integrity of the site. Aircraft flight pathstraversing the sector are discouraged. In addition, a100m vehicle exclusion zone surrounds the Atmo-spheric Research Observatory building.

Gradient sampling equipment consisted of arecently factory-serviced Tekran 2537a inside thebuilding connected to an exterior 20m Teflon inletline. At 5min intervals, in precise timing with theTekran 2537a 5min per-channel (A/B) sequentialsampling period, the inlet and thermocouple weremoved up/down a vertical mast or into/out-of thesnow pack by an operator approaching and operat-ing the system from downwind. The inlet wasrepositioned 8 s prior to the next sample to allowfor dead space within the sampling line. To accountfor the possible A/B channel bias of the Tekran2537a, the positioning order was repeated withalternating channels. The above-surface gradientswere repeated for 3 h, followed by 1 h of below-surface measurements, this entire process repeatedfor 16 straight hours. The gradient heights/depthswith the hourly channels and the concentrations arelisted in Table 1.

The gradient measurements within the snow packutilized a rigid Teflon inlet (marked at various depths)pushed vertically into the surface. No attempt wasmade to seal or cover the snow surface as any air-tightcovering would have altered the wind stress andsunlight conditions. Therefore, all the measurementswithin the snow pack are a combination of interstitialair at a range of depths and some surface air at theshallow measurement depths. For example, the high-est measured GEM concentrations reported here, at3 cm depth, are significant underestimates of theactual interstitial air GEM concentrations at 3 cm.

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Table 1

Sampling heights and depths

Height/

depth (cm)

Channels

per hour

GEM average

(ngm�3)

GEM standard

deviation

300 B, A, B, A 0.878 0.029

100 A, B, A 0.963 0.039

10 A, B 1.028 0.033

2 A, B, B 1.041 0.025

�1 A, B, A, B 2.393 0.043

�3 A, B 3.254 0.065

�20 B, A 2.476 0.064

�30 A, B 1.807 0.058

�50 A, B 1.323 0.022

The performed cycle was 3 h of above-surface measurements

followed by 1 h of below-surface measurements, then the cycle

was repeated. The values in columns 3 and 4 represent the values

obtained from a single 3-h above-surface measurement, and the

two bracketing 1-h below-surface measurements. The channels

are measured sequentially with one channel being analyzed while

the other channel was sampling.

0100200300400500600700800

320 330 340 350 360

Julian day

pg (

Hg)

m-3

Filterable Hg RGM Tekran FPM Tekran

Fig. 1. Mercury concentrations measured in the near-surface air

at the Geographic South Pole, November and December 2003.

Filterable Hg was the total Hg captured by the daily high-volume

filters (total oxidized mercury). Reactive gaseous mercury

(RGM), and fine particulate-bound mercury (FPM) were

measured with the Tekran sensor suite. Total Hg filterable

tracked most closely with RGM; however, the Hg-filterable was

always less than the sum of Tekran RGM+FPM. We believe this

was due to a portion of the RGM+FPM remaining within the

thin snow layer atop the filters. This layer was partially lost

during filter removal and packaging.

S. Brooks et al. / Atmospheric Environment 42 (2008) 2877–2884 2879

The vertical GEM fluxes were derived from theabove-surface gradients by calculating a verticaltransport coefficient based on the inlet positiontemperatures and the continuous sensible heat fluxmeasured via an ultra-sonic anemometer mountedat 175 cm. The GEM fluxes were then computed bycombining the observed vertical gradients of GEMwith the corresponding vertical transport coeffi-cients from the sensible heat flux using the modifiedBowen-ratio flux method. The RGM depositionrate was estimated from the average measuredRGM concentrations, and deposition velocitiesderived from previous relaxed eddy accumulationflux studies conducted in 2001–2004 at Barrow,Alaska (Skov et al., 2006). As the RGM fluxes atSouth Pole were not directly measured, our estimateof RGM deposition rate reported here should beconsidered as a crude first estimate.

Ancillary mercury measurements included: snowpit sampling, blowing snow sampling, and untreatedstation ‘‘well’’ water sampling. Three snow pits wereexcavated with Teflon surfaces within the Clean AirSector, upwind, and at least 50m from theobservatory building. A sun shade was erected toreduce photoreduction during the digging andsampling. Snow was collected at the different levelsin pre-cleaned I-Chem bottles with clean techniques.The blowing snow samples were collected byplacing, on the snow surface, opened 2 l, pre-clean,wide-mouth Teflon bottles just within the observa-tory building shadow, with the openings facingupwind for a duration of approximately 1 h.

The ‘‘untreated’’ well water was obtained from thenormal pump system used to supply the stationwater from melted ice at approximately 125mdepth.

Applicable ancillary meteorology measurementswere conducted by William Neff’s group (NOAA,Boulder), the on-site NOAA Climatic Monitoringand Diagnostics Laboratory staff (now known asthe Global Monitoring Division), and other Ant-arctic Tropospheric Chemistry Investigationgroups. These included SODAR-derived mixinglayer depth measurements, and, vertical tempera-ture and wind speed gradients.

3. Results

Fig. 1 shows the measured oxidized mercuryconcentrations (RGM, FPM, and total filterable) inthe near-surface air at South Pole Station, November–December 2003. The mercury captured on high-volume filters (total filterable Hg) was always lessthan the sum of RGM+FPM measured with theTekran speciation sensors. While the filters shouldefficiently capture RGM and FPM, we believe theresult (filterable HgoRGM+FPM) was due to aportion of the total collected mercury remainingwithin the snow layer atop the filters. This layer ispartially lost during filter removal and packaging.Averages and standard deviations are listed inTable 2. On average, at the height of the rooftop

ARTICLE IN PRESS

Table 2

Near-surface air mercury concentrations November–December

2003

Tekran

1130

RGM

Tekran

1135

FPM

Tekran

2537a

GEM

Filterable

Hg 24 h

filters

Average

(pg(Hg)m�3)

344 224 539 203

S.D. 151 119 189 106

0

100

200

300

400

500

600

322 332 342

julian day

RG

M o

r F

PM

(p

g m

-3)

0

50

100

150

200

Mix

ing

la

ye

r d

ep

th (

m)

RGM

FPM

Mixing layer depth

Fig. 2. Measured concentrations of reactive gaseous mercury

(RGM) and fine particulate-bound mercury (FPM) in the near-

surface air, and mixing layer depth measured with SODAR. In

general, high RGM and FPM concentrations occur during deeper

mixing layer depths.

0

100

200

300

400

500

600

-1 -0.5 0 0.5 1 1.5 2

Atmosphere Richardson #

RG

M o

r F

PM

(p

g m

-3)

RGM

FPM

Turbulent Stable

Fig. 3. Measured concentrations of reactive gaseous mercury

(RGM) and fine particulate-bound mercury (FPM) in the near-

surface air, and the calculated Richardson numbers. The

Richardson numbers indicate the vertical stability of the near-

surface air. Higher RGM and FPM concentrations occur in

unstable conditions with enhanced vertical mixing.

-10-8-6-4-202468

10

0.5 1 1.5 2 2.5 3 3.5

Gaseous elemental mercury, GEM (ng m-3)

He

igh

t/D

ep

th f

rom

sn

ow

su

rfa

ce

(c

m)

Fig. 4. Average gaseous elemental mercury (GEM) concentra-

tion profile near the snow surface. GEM concentrations peaked

at a snow depth of 3 cm, indicating that the upper snow layer is

producing GEM via photoreduction and emitting GEM to the

atmosphere. The GEM average flux was +8.1 ng(Hg)m�2 h�1

(positive is out of the snow surface), the oxidized mercury average

flux into snow pack is �10.8 ng(Hg)m�2 h�1 (negative is into the

snow surface). This oxidized mercury flux is a crude estimate

based on concentrations and deposition velocities from Skov

et al. (2006). Total mercury in the surface snow averaged

198ng(Hg) l�1.

S. Brooks et al. / Atmospheric Environment 42 (2008) 2877–28842880

deck (8m above the surface), GEM comprisedroughly 50% of the total atmospheric mercury.

Fig. 2 shows the measured concentrations ofRGM and FPM in the near-surface air and themixing layer depths measured with SODAR. Ingeneral, the higher RGM and FPM concentrationsoccurred during periods of higher wind speeds anddeeper mixing layers (more vertical mixing andentrainment from the free troposphere). In environ-ments like the polar plateau, turbulence is generatedby surface wind shear. The stronger the shear, thegreater the turbulence generated (enhanced mixing)and the deeper the mixing layer. Conversely, thedeeper the mixing layer, the stronger the turbulentmixing and entrainment. This suggests (assuminghomogeneous conditions upwind of the Clean AirSector) that FPM and RGM are entrained in themixing layer from above, and deposited at the snowsurface. RGM and FPM are known to readilydeposit to snow surfaces (Lindberg et al., 2002).

Fig. 3 shows the measured concentrations ofRGM and FPM in the near-surface air and thecalculated Richardson numbers. The Richardsonnumber is the ratio of atmospheric suppression ofturbulence to its shear generation at the surface, andindicates the vertical stability of the near-surface air.The higher RGM and FPM concentrations occuronly in unstable conditions with enhanced verticalmixing. This further indicates that RGM and FPMare entrained downward into the mixing layer anddeposited to the snow surface (i.e., the source ofRGM and FPM is the air above the mixing layer).

Fig. 4 shows the average GEM concentrationprofile near the snow–air interface from a 5-h sub-sample, during the 16 h gradient sampling period,15 November 2005. GEM concentrations peaked ata depth of 3 cm, indicating that the upper snowlayer is producing GEM. As has been observed inthe Arctic and sub-Arctic, sunlight (actinic flux) inthe first few centimetres of the snow pack will inducephotoreduction of deposited oxidized mercury

ARTICLE IN PRESSS. Brooks et al. / Atmospheric Environment 42 (2008) 2877–2884 2881

(Dommergue et al., 2003). For the study period, theaverage measured GEM surface flux to the nearsurface air was +8.1 ng(Hg)m�2 h�1, while theaverage RGM deposition estimated flux was�10.8 ng(Hg)m�2 h�1. Combining the average ofthese two fluxes with the total Hg load within thesurface snow layer (top 15 cm), we can calculate anestimated lifetime for mercury in the upper snowlayer. The results from this calculation suggest avalue on the order of 16 days.

GEM concentrations in the near-surface air were�0.9 ngm�3, and decreased with height over themeasured 3m gradient. Extrapolating this gradient,and taking into account the approximate depths ofthe mixing layer, we estimate that GEM concentra-tions are quite low (o0.2 ngm�3) above the mixinglayer. This implies a sink, or oxidation, mechanismfor GEM in the lower free troposphere.

Interstitial GEM increased with depth in thesnow pack, peaking at a concentration �3 ngm�3 at�3 cm, and then decreased with depth to�1.3 ngm�3 at our lowest measured snow depthof �50 cm. This is consistent with active mercuryphotoreduction within the top few centimetres. Noattempt was made to cover the snow surface duringsampling. All these measurements include intersti-tial air over a range of depths and some near-surfaceair. Therefore, the measurements at �1 and �3 cmdepths significantly underestimate the actual inter-stitial GEM concentrations, and the strong photo-reduction of deposited oxidized mercury at theseshallow depths.

Fig. 5 shows total mercury in the snow at SouthPole from three snow pits dug on 16 November 2005,just after the completion of the flux measurements.

-120

-100

-80

-60

-40

-20

0

20

0 50 100 150 200

Total Hg ng(Hg)/l

Sam

ple

d D

ep

th (

cm

)

Blowing snow 10cm above surface

Station well 125m depth 5.0 ng(Hg)/liter

Fig. 5. Profile of total mercury in the snow pack at South Pole

station. On 16 November 2005, three snow pits were excavated,

and three blowing snow samples were collected at 10 cm above

the snow pack. The error margin on the collection depths is

72 cm. Untreated ‘‘well’’ water at South Pole Station from a

depth of 125m was found to contain 5.0 ng l�1 total mercury.

The surface snow averaged 198 ng l�1, consistentwith high oxidized mercury deposition. In compar-ison, surface snow within the northern UnitedStates or the European Alps will generally be inthe 1–10 ng(Hg) l�1 range. For examples, the snowpacks of interior Alaska was measured at1–2 ng(Hg) l�1 (Douglas and Sturm, 2004), and thesnow packs of southeast Idaho were measured at1–16 ng(Hg) l�1 (Susong et al., 2003).

Total mercury in the snow pack peaked at thesurface and decreased rapidly with depth within thesunlit zone (e.g., first 0–15 cm). Below �25 cmdepth, the total mercury was less variable, withconcentrations of �10 ng l�1. This enhancement oftotal mercury in the top 15 cm of snow packextrapolates to �35 metric tons of Hg over theentire polar plateau.

Water collected from the untreated ‘‘well’’ waterof South Pole Station (depth of 125m) had anaverage total mercury content of 5 ng l�1. This waterwas sampled as close to the source as possible(before any filtering or treatment), but should beconsidered as an upper limit, as mercury contami-nation from the piping and pumps cannot bediscounted.

As a side item, wind speeds increased very late on16 November and three blowing snow samples werecollected. The total Hg in these samples averaged84 ng l�1. This horizontal transport of mercury-richsnow could be a significant sequestering term (dueto drifting and burial below the sunlight zone) inother areas and the periphery of the polar plateau.For example, Hg-rich snow blown into deepcrevasses will escape photoreduction and thismercury will likely be sequestered long term.

Fig. 6 shows the weekly averages of filterable Hgand the solar elevation angles at South Pole Stationfrom 2003 to 2006. Most notably, total filterable Hgis virtually absent during the dark fall and winter,

0

5

10

15

20

25

1-Jan 20-Feb 11-Apr 31-May 20-Jul 8-Sep 28-Oct 17-Dec

So

lar

Ele

vati

on

(d

eg

)

0

200

400

600

800

1000

1200

1400

1600

1800

Filte

rab

le H

g (

pg

m-3

)

Fig. 6. Weekly averages of filterable Hg (discrete points) and the

solar elevation angle at South Pole Station from 2003 to 2006.

The peak annual filterable Hg lags the solar maximum by 3–4

weeks.

ARTICLE IN PRESSS. Brooks et al. / Atmospheric Environment 42 (2008) 2877–28842882

while the sunlit period peak lags the solar maximumby 3–4 weeks. Filter sampling was concentratedduring the intensive study periods of November–December 2003 and November–December 2005,other weekly values are averages of only one to three24-h samples. This peak Hg period (November–February) roughly corresponds to the annual photo-chemical peak cycles of nitrogen oxides, hydroxylradicals (OH; Doug Davies, unpublished data), andpresumably reactive halogens (Hara et al., 2004),suggesting a strong photochemical link.

If one extrapolates these mercury results from theSouth Pole to the entire Antarctic Polar Plateau(5 million km2), the numbers (with assumedsnow/ice accretion rates) are surprisingly large.They would suggest values of �35 metric tons ofHg enhancement in the upper 15 cm of the snowpack from November to February, �60 metric tonsof Hg sequestered yearly at depth below thephotoreduction zone, and an annual snow packconversion (assuming the sunlit-period filterable Hgcycle magnitudes shown in Fig. 6) of �490 metrictons of deposited oxidized atmospheric mercury toelemental mercury. This huge conversion of oxi-dized mercury to elemental mercury implies amirror oxidation mechanism for GEM in the lowertroposphere (i.e., a mercury recycling).

The yearly sequestered Hg estimated here for theplateau (�60 metric tons) corresponds to �3% ofglobal anthropogenic mercury emissions (Pacynaand Pacyna, 2002), or an amount roughly equal tothe mercury emissions from all the coal-fired powerplants in North America. The annual GEM emis-sions from the plateau snow surface (�490 metrictons) is on par with the estimates of total globalGEM emissions from all land and ocean surfaces,estimated by Lamborg et al. (2002) to be 1000 and800 metric tons year�1, respectively. However, asnoted in the above text, these plateau estimates,extrapolated from these South Pole station mea-surements, are based on limited observations, andmay not hold on the peripheries of the polarplateau.

4. Discussion

The validity of our extrapolations to the entirepolar plateau hinge on the representativeness of theSouth Pole Clean Air Sector, and, the 15/16November sampling period of the flux measure-ments to the vast plateau in the sunlit seasons.Meteorological conditions for 15 and 16 November

were typical, and could be representative, in termsof temperatures (�30 to �35 1C), winds (5–8m s�1)and solar elevation (18.51) to typical conditions inNovember and late January/early February, brack-eting the solar maximum (elevation 23.51). Eiseleet al. (this issue) presents evidence that the CleanAir Sector, with respect to air and snow chemistry,is representativeness of vast areas of the polarplateau.

Overall, the oxidized mercury concentrations atSouth Pole Station are two orders of magnitudehigher than typical measurements in central NorthAmerica or Europe. On average, at South Polestation GEM comprised not more than 50% of thetotal atmospheric mercury within the mixing layer.This is surprising different from measurements intropical or temperate areas where GEM commonlyaccounts for 98–99% of total atmospheric mercury.

Near-surface concentrations of RGM and FPMwere strongly correlated to the height of the mixinglayer and the stability (Richardson #) of the near-surface air, indicating entrainment from the lowerfree troposphere and continuous removal of thesespecies via surface deposition. In order to estimatethe depositional flux of RGM, we used a depositionvelocity of 1.2 cm s�1 (Skov et al., 2006).

Photoreduction and emissions of GEM, similar tothose studied in the Arctic (Lindberg et al., 2002;Brooks et al., 2006), were found to occur predomi-nantly in the first few centimetres of the snow packwhere GEM concentrations in interstitial air peakedabove 3 ngm�3 at a snow depth of �3 cm.

There must exist a recurring cycle in which GEMis oxidized (via halogen chemistry?) in the lowertroposphere, becomes entrained in the shallowmixing layer, deposits to the snow surface, then isphotoreduced within the snow pack, and finally re-emitted again as GEM. Superimposed on this cycleare long-term oxidized mercury burial below thephotoreduction zone, and the transport of GEMand oxidized mercury species in and out of thepolar plateau region. Further, the chemical species(halogens?) that oxidized mercury must also berecycled from the snow pack back to the atmosphere.

The ‘‘GEM oxidation in the lower troposphere’’and ‘‘halogen recycling’’ parts of this proposed cyclehave not been directly observed, and we must resortto speculation to define probable mechanisms. Inother remote environments (such as the coastalArctic), atmospheric GEM oxidation by brominechemistry has been observed (Brooks et al., 2006).Suzuki et al. (2002) determined that sea salt aerosols

ARTICLE IN PRESSS. Brooks et al. / Atmospheric Environment 42 (2008) 2877–2884 2883

are transported from the coastal region toward theinland region of the Antarctic continent, and, thatafter initially decreasing with inland distance,concentrations of Cl begin increasing with distancebeyond 750 km inland. This indicates a recycling orsource of Cl other than sea salts exists within thepolar plateau (Suzuki et al., 2002). There is evidencethat this result also extends to Br. On the polarplateau at Dome Fuji (well within the polarplateau), Hara et al. (2004) observed Cl and Brpreferentially liberated from sea-salt particles. Dur-ing the summer, concentrations of gaseous chlorinespecies (mostly HCl) and bromine species rangedfrom 0.2 to 5.3 nmolm�3, and, below detection limit(BDL) to 1.5 nmolm�3, respectively.

After mercury oxidation, deposition, and snowphotoreduction, the recycling of the involvedhalogens could be facilitated by hydroxyl radicals(OH) which then react with the remaining halideions in the snow pack, recycling and releasing thesehalogens (Domine and Shepson, 2002).

It seems fitting to speculate here on the originand cycling of mercury species to the air abovethe Antarctic polar plateau. The majority of theoxidized mercury species (RGM and FPM) in thetroposphere are removed by cloud processes andrain-out. However, these processes are virtuallyabsent over the Antarctic continent, allowing both abuild-up and transport of RGM and FPM abovethe mixing layer.

Originally, the atmospheric mercury must havearrived over the polar plateau as GEM from the S.Hemisphere background tropospheric concentra-tions or, to a lesser extent, as oxidized Hg via upperair transport followed by wintertime subsidencein the vicinity of the central plateau (where thepredominate air flow is a slow settling of cold dryair from the upper troposphere). The existence ofoxidized mercury enhancements in the tropopauseregion was first reported by Murphy et al. (1998,also see Murphy et al., 2003) based on ER-2 aircraftaerosol sampling. Via any transport pathway,southern hemispheric mercury emissions are thesole originating source. While northern hemispheremercury emissions have been decreasing over thelast couple decades, southern hemisphere mercuryemissions increased from 1990 to 1995 and havestayed roughly constant since 1995. From 1990 to1995, Africa emissions increased from 200 to 400,Australia from 50 to 100, and South America from55 to 80 tons year�1 (Pacyna et al., 2005; Lindberget al., 2007).

Is this mercury cycle in Antarctica a recentphenomenon? Pre-industrial mercury concentra-tions in the range of 0.2–3 ng(Hg) l�1 were obtainedfrom Antarctica Dome C ice cores from samplesspanning the past 34,000 years (Vandal et al., 1993).Our mercury concentrations of �10 ng l�1 at 1mdepth (below the sunlight layer), and the �5 ng l�1

concentration from the South Pole Station ‘‘well’’ at125m depth, indicate that current mercury seques-tration maybe several times these pre-industriallevels.

5. Conclusions

We are proposing a photochemically driven cycleon the Antarctic polar plateau in which mercurycycles between atmospheric and snow pack reser-voirs. We have provided a single snapshot of theflux rates, and an annual collection of weeklyaverages of total filterable mercury to elucidate theintensity and seasonality of this mercury cycle.

We conclude that the air over the vast Antarcticpolar plateau is highly enriched in oxidized mercury(RGM and FPM). When mixed down to thesurface, this oxidized Hg deposits to the snowsurface. Under sunlight conditions, the majority ofthis mercury is photoreduced and emitted as GEMafter a snow pack lifetime of roughly 16 days.However, a small fraction of the deposited mercuryin the snow pack is buried long-term (sequestered)by blowing or falling snow as indicated by the�10 ng l�1 total Hg concentrations well below thesunlit (actinic flux) depth.

Even so, a great many questions remain. What isthe interannual variability? Certainly, the original(wintertime?) airmass transport of mercury to thepolar plateau should be altered by El Nino-typeoscillations. Does significant oxidized mercury exitthe polar plateau environment to deposit to thecoastal ecosystems? In this case, impacts to biotamight be occurring. What chemical mechanism(s)oxidize GEM over the polar plateau? Bromine and/or chlorine oxidation mechanisms are likely. Whydoes the photochemical cycle of mercury lag the solarcycle? Possibly the mercury oxidation–deposition–photoreduction cycle takes a couple of weeksafter sunrise to fully develop. What are theconcentrations of mercury species above the sum-mertime mixing layer? Likely GEM concentra-tions are quite low (o0.2 ngm�3) and oxidizedmercury species concentrations are quite high(RGM+FPM ¼ �1.0 ngm�3).

ARTICLE IN PRESSS. Brooks et al. / Atmospheric Environment 42 (2008) 2877–28842884

These new findings show Antarctica not as aninert surface to mercury, but as a dynamic surfacewith high mercury concentrations and bi-directionalfluxes greatly exceeding those typically observed atlower latitudes. As with any dynamic balance, aslight change of conditions has the potential toameliorate or exacerbate the global mercury cycle.

Acknowledgements

The Antarctic Tropospheric Chemistry Investigation(ANTCI) was funded by the National ScienceFoundation. We wish to thank the staff of the NOAACMDL (now the Global Monitoring Division) atSouth Pole station for their assistance, and the otherANTCI investigators (particularly Fred Eisele andDoug Davis) for their lively discussions.

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