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Atmospheric Environment 43 (2009) 4070–4077

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Atmospheric Environment

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Atmospheric mercury cycles in northern Wisconsin

C.J. Watras a,b,*, K.A. Morrison a,b, J.L. Rubsam a, B. Rodger a

a Wisconsin Department of Natural Resources, Madison, WI 54507, USAb University of Wisconsin–Madison, Center for Limnology, Trout Lake Research Station, Boulder Junction, WI 54512, USA

a r t i c l e i n f o

Article history:Received 12 February 2009Received in revised form22 April 2009Accepted 28 April 2009

Keywords:Atmospheric mercuryMercury cycleMercury depositionOzone

* Corresponding author at: Wisconsin Departmentson, WI 54507, USA. Tel.: þ1 715 356 9494; fax: þ1 7

E-mail address: cjwatras@wisc.edu (C.J. Watras).

1352-2310/$ – see front matter Published by Elsevierdoi:10.1016/j.atmosenv.2009.04.051

a b s t r a c t

Total gaseous mercury (TGM) in the lower atmosphere of northern Wisconsin exhibits strong annual anddiurnal cycles similar to those previously reported for other rural monitoring sites across mid-latitudeNorth America. Annually, TGM was highest in late winter and then gradually declined until late summer.During 2002–04, the average TGM concentration was 1.4 � 0.2 (SD) ng m�3, and the amplitude of theannual cycle was 0.4 ng m�3 (w30% of the long-term mean). The diurnal cycle was characterized byincreasing TGM concentrations during the morning followed by decreases during the afternoon andnight. The diurnal amplitude was variable but it was largest in spring and summer, when daily TGMoscillations of 20–40% were not uncommon. Notably, we also observed a diurnal cycle for TGM indoors ina room ventilated through an open window. Even though TGM concentrations were an order ofmagnitude higher indoors, (presumably due to historical practices within the building: e.g. latex paint,fluorescent lamps, thermometers), the diurnal cycle was remarkably similar to that observed outdoors.The indoor cycle was not directly attributable to human activity, the metabolic activity of vegetation ordiurnal atmospheric dynamics; but it was related to changes in temperature and oxidants in outdoor airthat infiltrated the room. Although there was an obvious difference in the proximal source of indoor andoutdoor TGM, similarities in behavior suggest that common TGM cycles may be driven largely byadsorption/desorption reactions involving solid surfaces, such as leaves, snow, dust and walls. Suchbehavior would imply a short residence time for Hg in the lower atmosphere and intense recycling –consistent with the ‘‘ping-pong ball’’ or ‘‘multi-hop’’ conceptual models proposed by others.

Published by Elsevier Ltd.

1. Introduction a ping-pong ball bouncing on a stone floor with occasional patches of

Early measurements of TGM in oceanic and remote continentalregions indicated that gas-phase Hg concentrations were low andrelatively uniform across the northern hemisphere (Fitzgerald,1989). These observations implied complete hemispheric mixingand, consequently, an atmospheric residence time of about oneyear. Early mass balances were consistent with this residence timein that global estimates of annual atmospheric Hg deposition wereroughly equal to the tropospheric Hg pool (ca 10 mg Hg m�2).Subsequent global budgets have suggested a somewhat shorteratmospheric Hg lifetime (0.75 years) due to recycling at the earth’ssurface (Mason and Sheu, 2002).

However, observations across smaller spatial and temporal scaleshave suggested that the atmospheric Hg cycle is more dynamic thanglobal budgets would imply. Based on Scandinavian studies of Hgdeposition near emission sources and Hg evasion from lake waters,Jernelov (1996, 2000) likened the behavior of atmospheric Hg to

of Natural Resources, Madi-15 356 6866.

Ltd.

soft carpet. Similarly, Hedgecock and Pirrone (2004) conceiveda ‘‘multi-hop’’ model to describe the behavior of atmospheric Hg inthe marine boundary layer. Reviewing numerous studies over thelast decade, Gustin et al. (2008) have concluded that the atmo-spheric residence time of individual Hg atoms may be only a fewhours. Rapid exchange across the boundary between earth andatmosphere could explain the apparent residence time of w1 year.

In this paper, we describe the cyclical behavior of Hg in the air ofrural northern Wisconsin, based on data collected above the forestcanopy, in open-field precipitation and in forest throughfall duringw2 years of continuous monitoring. We also compare TGM cyclesin outdoor and indoor air, observations which tend to support thehypothesis that gaseous Hg oscillates between earth and air overshort time scales.

2. Methods

2.1. Gas-phase mercury

TGM measurements were made using a Tekran 2537A mercuryvapor analyzer. The primary sampling location was US NADP-MDN

Month (2002-04)

Ozo

ne (

ppm

)

0.00

0.01

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0.06B

Jan Mar May Jul Sep Nov Jan Mar May Jul Sep Nov Jan Mar

Jan Mar May Jul Sep Nov Jan Mar May Jul Sep Nov Jan Mar

TG

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3 )

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Fig. 1. Annual cycles of total gaseous mercury (TGM) and ozone at US NADP/MDN siteWI36 (2002–04). Data are monthly means � SD for hourly integrated samples. Brokenlines indicate long-term means.

C.J. Watras et al. / Atmospheric Environment 43 (2009) 4070–4077 4071

atmospheric monitoring site WI36 in northern Wisconsin(46�000N, 89�370W). From October 2002 through January 2004, airwas sampled continuously from a 20 m tower above a stand of RedPine (Pinus resinosa) at the edge of the site. The air was drawnthrough a Teflon manifold at w10 L min�1 and sampled continu-ously using the Tekran protocol for sequential trapping at 5 minintervals on twin gold cartridges. Prior to trapping, the air passedthrough a 0.2 mm Teflon filter to remove particles. The analyzer washoused in a temperature-controlled (w26 � 2 �C) lab trailer at thebase of the tower.

Indoor air was monitored at the nearby UW-Madison Trout Lakelaboratory (TLL, roughly 5 km south of site WI36) for comparisonwith outdoor TGM. Air inside the general laboratory was sampledcontinuously for a nine-day period in late summer. Becauseventilation was not regulated mechanically, the air turnover timewas variable but estimated to be on the order of a few hours due toopen windows and exhaust fans in various parts of the lab. TheTekran analyzer was located in a lab room (w28 m3) where thedoor and window were always open. Indoor air was drawncontinuously into the analyzer through a 1 m length of acid-washedTeflon tubing. We also sampled air inside the Hg-clean laboratory atTLL for 20 h using the same protocol. The clean lab was positivelypressurized with filtered outdoor air with an estimated turnovertime of w10 min. Otherwise, the clean lab was similar in size,construction and historical use to the general lab room. We antic-ipated that TGM concentrations would be stable and low in theclean lab (w2 ng m�3), but high and erratic in the general lab.

2.2. Atmospheric mercury deposition

Bulk atmospheric Hg deposition was monitored continuously inthe open field at WI36 using an IVL-type bulk collector withweekly-integrated samples (Jenson and Iverfeldt, 1994). Samplingand analysis protocols followed Morrison et al. (1995). An identicalbulk collector was located in a nearby forest to measure Hg incanopy throughfall. Canopy coverage (red pine and oak) above thethroughfall collector was estimated to be w4 m2 m�2 duringsummer (leaf area/ground area). From December through April,snowpack samples also were collected weekly under the canopy forcomparison with bulk deposition.

Bulk deposition and snowpack samples were analyzed for totalHg (HgT) using purge-trap/CVAFS as in US EPA Method 1631.Method detection limits at TLL were 0.05 ng Hg L�1 (pooled vari-ance of reagent blanks). On-going precision (mean � SD) was99.7% � 6.2%, n ¼ 735.

2.3. Atmospheric ozone monitoring

Ozone was monitored concurrently with TGM at WI36 usinga Teledyne API 400 ozone analyzer that drew air from the sameTeflon manifold. Hourly samples were collected following the Stateof Wisconsin’s ozone quality assurance plan (WDNR, 2003). Due totechnical problems with the analyzer, ozone samples were notcollected between November 2003 and January 2004.

Ozone is one of several oxidants in the troposphere that cantransform gaseous elemental Hg to the more reactive Hg(II) species(cf. *OH, HO2, RO2, H2O2, NO3 and halogens). As in prior studies, weutilize ozone concentrations as a proxy for the oxidation potentialof the lower atmosphere (e.g. Stamenkovic et al., 2007).

2.4. Quality control and quality assurance

The Tekran 2537A was calibrated daily using an internal Hg0

permeation tube. Daily calibrations were verified using periodicmanual injections (w140 pg injection�1). Performance of the

Teledyne 400 was verified bi-weekly with one-point precisionchecks (0.080–0.100 ppm O3 standard), and annually with five-point precision checks using standards that ranged from 0.00 to0.450 ppm. Tolerance was �10%.

3. Results and discussion

3.1. Annual cycle of TGM at WI36

The annual cycle of TGM at WI36 was similar to that reported forseveral CAMNET sites spanning mid-continental North America(Blanchard et al., 2002; Kellerhals et al., 2003; Poissant et al., 2005).The winter maximum was followed by a gradual decline until latesummer (Fig. 1A). Over the full study period (2002–04), the averageTGM concentration above the forest canopy was 1.4 � 0.2(SD) ng m�3, and the amplitude of the annual cycle was w0.4 ng m�3.These values seem to be typical for remote, mid-latitude regions inthe northern hemisphere (Kim et al., 2005; Temme et al., 2007).

Several mechanisms have been proposed to explain annual TGMcycles like the one in Wisconsin. Purported mechanisms involveseasonal changes in Hg sources, Hg sinks and weather (e.g. Slemr,1996; Slemr and Scheel, 1998; Ebinghaus et al., 2002). Wintermaxima have been attributed to weather modulation (decreasedmixing height, increased wind velocity, more frequent inversions)and source modulation (increased fuel combustion for heating).Summer minima have been attributed largely to sink modulation

Hg

Con

cent

rati

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t. a

ve.,n

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)

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Throughfall

Open field

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Jun Dec Jun Dec Jun

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g/m

2 /wk)

0.0

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A

B

C

Fig. 2. Annual cycles of bulk atmospheric Hg deposition in open-field precipitationand canopy throughfall near WI36. Data are seasonally-averaged running means (13week) of weekly-integrated samples collected continuously using IVL-type bulkdeposition collectors.

C.J. Watras et al. / Atmospheric Environment 43 (2009) 4070–40774072

due to increases in atmospheric oxidants, sunlight, temperatureand precipitation – all of which can affect TGM speciation andremoval from the atmosphere. However, some observations atremote sites are not consistent with these mechanisms. Forexample, recent observations in the southern hemisphere showthat TGM cycles are in phase with those in the northern hemi-sphere despite the reversal of seasons (Slemr et al., 2008). Otherdata indicate that maxima for oxidized and reduced atmosphericHg species (GEM, RGM and HgP) co-occur despite seasonal changesin oxidation potential (Poissant et al., 2005).

3.2. Annual cycle of ozone at WI36

The annual cycle of ozone at WI36 was not in phase with theTGM cycle. Ozone concentrations were high in spring and lowduring autumn and winter (Fig. 1B); and they were within therange considered tropospheric ‘‘background’’ for mid-latitudeNorth America (e.g. Parrish et al., 1999). Similar annual O3 cycleswith amplitudes of 15–20 ppb have been reported for other rural,mid-latitude sites across the northern hemisphere (e.g. Talbot et al.,2005; Derwent et al., 2007). A previous study of O3 at WI36 (1994–99) showed similarly strong seasonal oscillations as well asevidence of a gradually increasing trend (Watras et al., 2000), asobserved at Mace Head in a time-series from 1987 to 98 (Derwentet al., 2007).

3.3. Annual cycle of atmospheric mercury deposition

Unlike TGM, bulk atmospheric Hg deposition was highest insummer and lowest in winter (Fig. 2). Asynchrony between the twoannual cycles is generally consistent with the sink modulationhypothesis for TGM (i.e. higher rates of deposition depress TGMconcentrations). The difference between open-field and throughfalldeposition suggests that vegetative surface area may be animportant factor.

Despite substantial rainfall interception by the summer canopy(Fig. 2B), total Hg deposition and concentration were highestbeneath the trees (Fig. 2A and C). Even though the amount ofprecipitation was 1.4-fold higher in the open field, Hg depositionand concentration were substantially higher in throughfall (2-foldand 1.5-fold higher, respectively). This canopy effect has beenwidely observed in temperate/boreal forests, and it is generallyattributed to dry deposition on vegetative surfaces followed bywash-off during rainfall events (e.g. Iverfeldt et al., 1996; Lindberg,1996; St. Louis et al., 2001). In northern Wisconsin, the canopyeffect was minor during winter, which suggests that the summercanopy was a stronger sink for Hg – perhaps, in part, due to thelarger leaf area. The data also imply that some of the Hg sorbed ontoleaf surfaces was weakly bound and easily washed-off by rainfall.

Although atmospheric HgP may accumulate on leaf surfaces,data for WI36 indicated that there was sufficient TGM in the air toaccount for the excess Hg in throughfall. Given a summer TGMconcentration of w1.2 ng m�3, one could account for a typicalthroughfall load (0.4 mg Hg m�2) by adsorbing and washing-offw30% of the Hg pool in the lower atmosphere (w1 km). Notably,this hypothetical adsorption rate is consistent with the daily TGMoscillations of 20–40% observed in summer air above the forestcanopy (Section 3.5).

During winter, atmospheric Hg deposition was low in throughfalland in the open field. Weekly sampling from December throughApril indicated that Hg concentrations in forest snowpack were alsolow and relatively stable. The mean snowpack concentration atduplicate forested sites sampled over 15 weeks was 3.7 � 1.2 ng L�1

(SD). The mean of nine sites sampled on one date was 4.1�0.9 ng L�1

(SD). Although the Hg concentration in snowpack was stable and

similar to the Hg concentration in bulk winter deposition (cf. Fig. 2A),cycling at the air–snow interface cannot be ruled out.

3.4. Diurnal cycles of TGM and O3 at WI36

In all seasons, the diurnal cycle of TGM was characterized byincreasing concentrations during the morning followed by decreasesduring the afternoon and night (Fig. 3). The daily amplitude washighly variable but on average it was greater in summer than inwinter (Fig. 4). Averaged over all months, the diurnal amplitude wasw0.2 ng m�3. In summer, it rose to w0.3 ng m�3; and in winter it fellto w0.1 ng m�3. As shown on Fig. 3, the diurnal cycle occasionallybroke down, presumably due to the vagaries of weather and/or theincursion of air masses from differing regions. Nonetheless, dailyoscillations of 20–40% were not uncommon in summer.

Similar diurnal TGM cycles have been reported for seven CAM-NET sites, with the daily amplitude ranging from 3% to 13%(Kellerhals et al., 2003). The daily CAMNET cycle has been attributedto night-time TGM depletion from a shallow nocturnal boundarylayer, followed by day-time recharge when the night-time inversionlayer breaks down. Hypothetically, the day-time TGM recharge resultsfrom the downward mixing of undepleted air and the stimulation ofTGM surface emissions by solar radiation. Increased TGM surfaceemissions during daylight hours have also been attributed to diurnal

Time of day (hours)0 6 12 18 24

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NovemberAugust

Fig. 4. Hourly-averaged TGM data for a late summer and early winter month, usingdata for each day from Fig. 3. As compiled, the daily average � sd and (range) was1.5 � 0.03 ng m�3 (0.1) in November and 1.2 � 0.07 ng m�3 (0.2) in August. Up anddown arrows indicate times of sunrise and sunset, respectively.

01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 01

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zone (ppm)

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Fig. 3. Diurnal cycles of TGM (unfilled circles) and ozone (solid line) at WI36 during early winter and late summer months. For TGM, the mean amplitude (ng m�3� SE) of the diurnalcycle was 0.14� 0.014 in November and 0.33� 0.04 in August. For ozone, the mean amplitude (ppm� SE) of the diurnal cycle was 0.01�0.001 in November and 0.03� 0.003 in August.

C.J. Watras et al. / Atmospheric Environment 43 (2009) 4070–4077 4073

changes in plant physiology and stomatal exchange (e.g. Lindberget al., 1998, 2002, 2005; Marsik et al., 2005; Gbor et al., 2006).

However, other mechanisms could potentially explain bothphases of the daily TGM cycle and their seasonality at WI36 withoutdirectly invoking atmospheric dynamics or biogenic emissions. Onesuch mechanism would be a diurnal adsorption/desorption cycleinvolving reactions with solid surfaces in the forest. The adsorptionphase might be related to diurnal cycles in atmospheric oxidantsand temperature. At WI36, there were mid-day peaks in O3 andtemperature regardless of season; and, during summer, the diurnalamplitude of O3 oscillations increased by roughly 2-fold. Similarobservations have been reported for rural New Hampshire byTalbot et al. (2005), who suggested that the larger O3 amplitudesduring summer might be due to higher rates of O3 deposition to theleafy summer canopy at night. Although O3 is only one of severaloxidants affecting Hg speciation in the atmosphere, a redox-relatedmechanism might explain both the daily and seasonal TGM cycles,as well as the summer increase in throughfall Hg.

For example, the reaction of Hg0 with O3 is known to cause rapidloss to the walls of reaction vessels at a rate that depends ontemperature and the relative amount of available surface (surfacearea/container volume) (Hall, 1995). The sorbed Hg species hasbeen identified as HgO (Pal and Ariya, 2004); and a reactionpathway has been proposed whereby gaseous elemental Hg and O3

form a metastable HgO3 complex that reacts with solid surfacesyielding HgO(s) and O2 (Calvert and Lindberg, 2005). At night, thesubsequent reaction of HgO(s) with condensed H2O due to night-time humidity (dew) could hypothetically yield a photo-labile

Ozone (ppmv)0.00 0.01 0.02 0.03 0.04 0.05

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Fig. 5. Diurnal cycle of TGM in indoor air at the Trout Lake Laboratory. (A). TGM concentration in general lab air measured at 5 min intervals (white circles), hourly air temperature(solid line) and hourly ozone (dash-dot line) in late summer 2002. (B). Relationship between indoor TGM and air temperature (hourly averages) with regression equation (solid line,R2 ¼ 0.84, P < 0.0001). Dashed line shows known relationship between temperature and mercury vapor density (Hg0) at saturation, for comparison. (C). Relationship betweenindoor TGM and ozone (R2 ¼ 0.46, P < 0.001).

C.J. Watras et al. / Atmospheric Environment 43 (2009) 4070–40774074

species, such as Hg(OH)2, and off-gassing that begins at sunrise.During summer, an increase in the amplitude of the diurnal TGMcycle is consistent with seasonal increases in air temperature,night-time humidity, oxidant concentrations and leaf area in theforest canopy. During winter, Hg0 off-gassing due to reactions inliquid water is unlikely.

Since O3 is only one of several oxidants that vary diurnally andseasonally in the lower troposphere, and since the principle reac-tion pathway governing the exchange of Hg between air and solidsurfaces is unknown, our field observations do not imply anyparticular sorption/desorption mechanism. Nonetheless, severalstudies strongly suggest that terrestrial vegetation plays animportant role in the Hg cycle. Deposition monitoring has shown

that litterfall is the major source of Hg to the forest floor (Grigal,2002), and experimental studies have shown that most of the Hg inthe forest canopy is accumulated directly from the atmosphere byleaves (Ericksen et al., 2003). Recent mesocosm experiments indi-cate that the dominant pathway for Hg uptake by foliage is passivesorption rather than active stomatal transport (Stamenkovic andGustin, 2009).

3.5. Diurnal cycle of TGM in indoor air

Unexpectedly, there was a strong diurnal TGM cycle in indoor airat TLL similar to that observed outdoors (Fig. 5A). Since indoor TGMconcentrations were 10-fold to 20-fold higher than those outdoors,

9/24/2002 9/26/2002 9/28/2002 9/30/2002 10/2/2002

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Fig. 6. (A). Observed (white circles) and modeled (black circles) TGM in general lab air at TLL. The modeled TGM was calculated at hourly time-steps by mass balance as:DTGM ¼ F � E, where F is the net surface flux (Foff � Fon) and E is the exhaust flux due to air exchange with the outdoors. Foff ¼ k1(aþ bT þ cT2 þ dT3), where T is Celsius temperature;Fon ¼ k2[Hgo][O3], where k2 ¼ me�EA/RT, where T is Kelvin temperature (cf. Hall, 1995) E ¼ v[Hgoinside � Hgooutside], with v in turnovers/h. (B). TGM in clean-room air at TLL, spikesshow inflow of air from general laboratory when the clean-room door was briefly left wide open.

C.J. Watras et al. / Atmospheric Environment 43 (2009) 4070–4077 4075

the indoor cycle did not simply reflect the incursion of outdoorTGM into the building. Likewise, it did not directly reflect themetabolism of forest foliage or atmospheric dynamics. Instead, itpresumably was due to oscillations in small amounts of fugitive Hgwithin the building. Hg is a common constituent of indoor dust dueto historical uses, such as pre-1990 latex paint, broken mercurythermometers and fluorescent lamps (e.g. Rasmussen et al., 2001).In a study of 21 Seattle homes, indoor TGM, ranging from 2 to40 ng m�3, was positively correlated with Hg in household dust(range w1–15 ppm) (Bloom and Hutchings, unpubl. data). Theseindoor TGM levels are well below the US EPA human healththreshold of 300 ng m�3, indicating a modest degree of unspecifiedhistorical contamination.

Although elevated indoor TGM is not unusual per se, there is noa priori reason to expect a strong diurnal cycle. Since air flowthrough the lab was not controlled (i.e. no mechanical heating or

air-conditioning cycle), and since the observation period includeda weekend when the lab was sparsely occupied, the cycle cannot beattributed easily to a human activity within the building. A moreplausible explanation would be surface exchange reactionsgoverned, perhaps, by co-factors that vary diurnally, such as light,temperature, humidity, and oxidant concentrations (cf. Engle et al.,2005). The incursion of co-factors generated outdoors would befacilitated by the free flow of air through windows that remainedopen day and night in the general lab.

As shown on Fig. 5B and C, indoor TGM was positively correlatedwith outdoor air temperature and ozone concentration. Whena simple, heuristic model was used to predict TGM as a function ofoutdoor air temperature, ozone concentration and the air exchangerate in the lab, the essential features of the indoor cycle could bereproduced reasonably well (Fig. 6A). The model assumed that themobilization of TGM into room was proportional to the Hg0 vapor

C.J. Watras et al. / Atmospheric Environment 43 (2009) 4070–40774076

saturation density (a known function of temperature, Fig. 5B). It alsoassumed that the net removal of TGM from room air occurred viaventilation (replacement with outdoor air at 1.2 ng m3) and by anunspecified redox reaction proportional to the ozone concentration.In this exercise, we scaled rate constants to provide the best fit. Forthe data shown on Fig. 6A, the room ventilation rate was 0.35 turn-overs per hour (which seems reasonable for a 28 m3 room), and thenet redox reaction rate constant was 2.0 � 10�16 cm3 molec�1 s�1 at20 �C (w3 orders of magnitude higher than literature values for thegas phase reaction of Hg0 with ozone). A randomly varying ventila-tion rate (ranging from 0.1 to 0.6 turnovers per hour) produceda more spiky but clearly discernable diurnal cycle (not shown).

Air exchange with the outdoors is necessary to keep TGM fromcontinually building up within the room and, perhaps, to providediurnally cycling oxidant. The effect of an abnormally high airexchange rate can be seen in TGM data from the clean lab at TLL(Fig. 6B). This laboratory is flushed continually with filteredoutdoor air at a very high rate that overwhelms any internal cycling(w6 turnovers per hour). Consequently, TGM concentrations weresimilar to those in outdoor air (w2 ng m�3) and they showed thegradual night-time decline and morning rise that characterizes thediurnal TGM cycle outdoors. When the door connecting the cleanlab to the general laboratory was left open for a few minutes, TGMincreased sharply due to the turbulent advection of general lab air.After the door was closed, general lab air was rapidly flushed fromthe clean lab and TGM quickly declined.

Although our modeling exercise was simplistic and not intendedto identify any particular reaction pathway, the occurrence of strongTGM cycles indoors and outdoors suggests that they are driven, inpart, by a similar set of surface exchange reactions. Stated anotherway, the indoor and outdoor TGM cycles together imply that diurnalchanges in atmospheric chemistry mediate a bi-directional surfaceflux that varies with time of day. The governing reactions remainunknown, but biogenic emissions and atmospheric dynamics arenot necessary or sufficient to directly explain our observations. Thenature and amount of reactive surface may be an important co-factor, however, perhaps partly explaining the greater amplitude ofthe diurnal TGM cycle above the summer forest.

4. Conclusions

The adsorption/desorption hypothesis implies that the atmo-spheric residence time of individual Hg atoms is short but their life-cycle is complex, involving multiple hops or bounces as firstproposed by Jernelov (1996) and more recently by Hedgecock andPirrone (2004) and Gustin et al. (2008). The spatial distribution ofHg in forested environments suggests that organic soils affordstrong sequestration (Grigal, 2003). Along with organic lacustrinesediments, forest soils may be the ‘‘soft spots’’ where Hg concen-trations tend to build-up over time (Jernelov, 2000). This concep-tual model implies that the atmospheric Hg cycle has importantlocal and regional dimensions (cf. Weiss-Penzias et al., 2003).

Although air mass effects are evident in the data for northernWisconsin, exchange reactions with solid surfaces appear to be moreimportant drivers of temporal variability in TGM. This may reflect thelack of down-wind Hg-emission sources in the immediate area. Thelarge summer amplitude of the diurnal TGM cycle is presumablyrelated to the higher density of exchange surfaces in the canopy(leaves) and the forest floor (herbaceous plants, exposed plant litterand organic soil), along with factors governing reaction rates (light,temperature, humidity, oxidant concentrations).

The annual cycle of TGM mirrored the annual cycle of atmo-spheric Hg deposition, suggesting that higher rates of precipitationduring summer resulted in higher rates of removal to the forestfloor where Hg can be strongly sequestered by organic matter.

During winter, snowpack-covered soil and TGM cycling at the air–snow interface may contribute to higher average TGM concentra-tions. The role of specific oxidants and particular reaction pathwaysremain unclear at all temporal scales, but the diurnal and annualcycles of O3 suggest that oscillations in oxidation potential are animportant factor. The governing reactions are likely complex, sinceatmospheric oxidants like O3 can drive Hg in two directions, i.e.both to and from solid substrates (e.g. Engle et al., 2005). Explicitmechanistic explanations likely await a better understanding of thecapacity of solid surfaces to reversibly sorb metallic gases like Hg.

Acknowledgements

We thank the Forest County Potawatomi Tribal Community(FCPC) for use of their Tekran 2537A analyzer. Dr. Eric Prestbo ofTekran Instruments Corporation and two anonymous reviewersprovided insightful comments on the draft manuscript. Researchwas supported by joint funding from the WDNR and the FCPC. Thisis a contribution from the Trout Lake Station, Center for Limnology,University of Wisconsin–Madison.

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