Micrometeorological measurements of mercury vapor fluxes over background forest soils in eastern Tennessee

Pergamon

Atmospheric Environment Vol. 29, No. 2, pp. 267-282, 1995 Elsevier Science Lid Printed in Great Britain

1352-2310(94)00198-7 MICROMETEOROLOGICAL MEASUREMENTS OF MERCURY VAPOR FLUXES OVER BACKGROUND FOREST SOILS IN EASTERN TENNESSEE KI-HYUN KIM and STEVEN E. LINDBERG Environmental Sciences Division, Oak Ridge National Laboratory*, P.O. Box 2008, Oak Ridge, TN 37831, U.S.A.

and TILDEN P. MEYERS Atmospheric Turbulence Diffusion Division, NOAA/ERL/Air Resources Laboratory, P.O. Box 2456, Oak Ridge, TN 37831, U.S.A. (First received 22 April 1994 and in final form 18 June 1994) Al~ract--We used the modified Bowen ratio method to estimate the fluxes of vapor-phase elemental Hg (Hg °) over background forest soils during the summer and fall of 1993. Fluxes were derived from the concentration gradients of total gaseous Hg between sampling heights of 25 and 165 cm and the concurrently determined turbulent diffusion coefficients of reference trace gases (i.e. H20 or CO2). The concentration and gradient data of Hg° measured during the campaigns generally fell in relatively narrow ranges of 1.5;!-3.68 and -0.16 to 0.32ngm -3 (over 140cm), respectively: means ( + 1 S.D.) for the corresponding: emission and deposition fluxes were found to be 7.5 + 7.0 (n = 30) and - 2.2 + 2.4 ng m- 2 h- 1 (n=9), respeo.ively. From the data collected during a series of sequential measurements, reproducible patterns of diurnal exchange emerged: (1) small bidirectional fluxes of Hg ° in the morning, (2) peak emissions near midafternoon, and (3) generally insignificantexchange during the nighttime. The fluxes of Hg over soil surfaces appear to be driven by a combined effect of several meteorological factors, including wind speed, vertical mixing, and soil temperature. Comparison of environmental conditions for both emission and deposition events showed that the direction of fluxes may be strongly influenced by the stability conditions of the boundary layer. The overall results of our emission and dry-deposition measurements in concern-with recent studies of wet-deposition rates in the forest ecosystem suggest that source strengths of this forest soil system may be of the same order of magnitude as sink strengths. Key word index: Air-surface exchange, emission, dry deposition, wet deposition, cycling, trace metals, and air toxics

INTRODUCTION Mercury, like m a n y other natural and anthropogenic pollutant chemicals, is found to be dispersed extensively throughout the earth's atmosphere. The widespread occurrence of atmospheric Hg suggests that there is a significant process leading to the airborne transport of Hg from the earth's surface. The atmospheric burden of Hg is known to consist of both natural and anthropogenic sources. Previous estimates of the global Hg budget indicate that these sources are almost equally important contributors to

* Research sponsored by the U.S. Department of Energy and the Electric Power Research Institute under contract with ORNL. ORNL is managed by Martin Marietta Energy Systems, Inc., for the U.S. Department of Energy under contract DE-AC05-84OR21400.

the atmospheric Hg burden (Lindqvist and Rodhe, 1985; Nriagu and Pacyna, 1988). In fact, the global patterns of slow, but steady, increase in background atmospheric Hg concentration over 13 year periods (i.e. from 1.76+0.36 to 2.02+0.39 n g H g m -3 in the northern hemisphere during the periods of 1977-1990) suggest that anthropogenic emissions may be a more prominent source than natural emissions (Slemr and Langer, 1992). Although some advances have been made in our understanding of the global Hg budget, much effort is still required to construct accurate models of, and to address properly the fundamental features of global Hg cycling. Central to such efforts are the processes that govern the interaction of Hg between the earth's surface and the atmosphere. A n u m b e r of attempts have been made over the past several years to provide quantitative analysis of the environmental mobility of

267

268

K.-H. KIM et al.

atmospheric Hg. The oceanic emissions of Hg have been estimated through application of a stagnant film model (Liss, 1983) to an extensive database taken from various marine sites (Kim and Fitzgerald, 1986; Gill and Fitzgerald, 1987; Fitzgerald, 1989). These authors conclude that the evasional flux of Hg from surface seawater is strongly influenced by biological productivity and that the oceanic sources of Hg comprise the dominant portion of the natural Hg budget. In comparison, information concerning the role of terrestrial ecosystems in global Hg cycling is extremely limited. Only in recent years have a couple of attempts been reported to measure fluxes of Hg from terrestrial environments directly. Employing dynamic flux chamber techniques, Schroeder et al. (1989) and Xiao et al. (1988, 1991) were able to quantify exchange rates of rig over boreal forest soils in Sweden. The following salient conclusions can be drawn from their studies: (1) the background soil environment exhibits bidirectional fluxes of Hg, ranging over - 1 0 t o + 3 n g m - 2 h - l [( + ) as emission and ( - ) as deposition]; (2) as a general rule, natural waters (e.g. lake waters) tend to exhibit enhanced emission rates of Hg relative to soil surfaces; and (3) temperature is, in many cases, found to have a pronounced influence on air-surface exchange rates. Despite such characterizations of air-surface exchange processes, our knowledge of the behavior of Hg in terrestrial systems is still limited because of the extremely sparse database and the absence of reliable methodologies with which to measure Hg flux. As a part of an overall program in our laboratory aimed at accurate quantification of atmospheric Hg fluxes, we have been extensively involved in development and application of the modified Bowen ratio (MBR) method, a type of a micrometeorological method. Application of micrometeorological techniques for trace gas flux measurements has been highly recommended over traditional techniques (e.g. chamber or cuvette method) with a number of proven advantages (Huebert and Robert, 1985). The MBR method allows quantification of Hg fluxes using vertical gradients of vapor-phase Hg concentrations and simultaneously measured turbulent-mixing parameters. Intensive tests of the MBR method on Hg flux measurements have been conducted over Hg-contaminated forest soils of East Fork Poplar Creek in Oak Ridge, TN, to develop confidence in its application to Hg flux measurements at background sites. The results of these studies have been published elsewhere (Kim et al., 1993; Lindberg et al., submitted). In this study, we applied the MBR method to flux measurements of Hg over background forest soils at Walker Branch Watershed in Oak Ridge. Here, we present the results of our flux measurements, obtained from intensive sampling in the summer and fall of 1993. In the course of this study, we collected the gradient and flux data for Hg and the associated meteorological parameters from a series of measurements during the daytime and over diurnal cycles.

Using these data, we attempted to elucidate the factors that regulate the transfer of Hg across the soil-air interface as well as to characterize the temporal trends in Hg exchange in the background environment. To derive more temporally representative flux estimates of Hg at the study site as well as to evaluate meaningful budgets of Hg in such ecosystems, we are continuing our efforts to establish a comprehensive database over a longer time scale. The results of our extended measurements of Hg exchange rates will be published subsequently. EXPERIMENTAL

Site characteristics

The measurement site was located within Walker Branch Watershed in Oak Ridge, TN (lat. 35° 58'N, long 84 ° 17' W), which is a 100 ha forest catchment in moderately complex terrain with elevations of 265-365 m, The soil at the site is classified as a cherty silt loam (Fullerton series Typic Paleudult formed in Knox Group residuum) derived from dolomitic limestone and is characteristically infertile and acidic (Hutchison and Baldocchi, 1989). The soils at the study site are well drained because of their physical location near the top of a ridge and their moderate water-retention capacity. The litter in the O1 horizon consisted of decaying leaves (8.7 Mgha- 1), twigs (1.8 Mgha- 1), and wood (3.8 Mgha- i) (Edwards et al., 1989). The litter of the Oi and 0 2 horizons, which generally extend to a depth of 5 cm, yields a total mass of 32 Mgha- '. Our analysis of the total Hg contents in soil from the study site showed that the Hg concentrations from surficial mineral (0.50 + 0.15/~g g - 1, n = 5) and upper organic layers (0.68 +0.14 vgg- t, n = 5) were comparable to or moderately higher than the levels typically found from other background sites, including several in this region; for example, 0.1-0.3 pgg- ~ (e.g. Nilsson et al., 1989; Lindqvist et al., 1991; DOE, 1993). The study site is forested with an uneven-aged stand of oak (Quercas spp.), hickory (Carya spp.), and loblolly pine (Pinus spp.) with a mean canopy height of 23 m. Major codominant species in the understory vegetation consist of seedlings and saplings of these species along with suppressed Cornusflorida L., Nyssa sylvatica Marsh., and Oxydendrum arboreum (L.) DC. The leaf area index (LAD and plant area index (PAl) of the forest are 4.9 and 5.5, respectively, during years of sufficient precipitation (Hutchison et al., 1986). Air sampling and mercury analysis

The procedures used in our sampling and analysis of Hg vapor closely follow those used in our previous studies (Kim et al., 1993; Kim and Lindberg, 1994; Lindberg et al., submitted): (1) preconcentration of vapor-phase Hg on goldcoated sand amalgamation traps and (2) detection of the thermally desorbed Hg by cold-vapor atomic fluorescence spectrophotometry (CVAFS). To obtain highly precise quantifications of Hg concentrations and resulting gradients, we developed a multiple replicate sampling system, which is a six-port manifold connected to six separate mass flow controllers (MFCs) (Kim and Lindberg, 1994). This configuration of the flow controller system, which requires one small vacuum pump to control vacuum levels of all MFCs, facilitates simultaneous collection of six replicate samples from each level of the gradient measurements. For each field experiment, the air stream was drawn into the replicate Hgcollection traps at flow rates of approximately 400cm a min - ~ for sampling periods of 1-3 h. The total volume of sampled air was then calculated by using the flow rate correction procedures of Kim and Lindberg (1994). Most of our daily gradient measurements were made by using all six

Mercury vapor fluxes replicates at each of two sampling heights. However, when intensive sequential experiments were conducted over half to full diurnal cycles, three to four replicates were collected from each of two sampling heights. The total amount of gaseous Hg was determined using the two-stage Au amalgaraation gas-flow technique (Fitzgerald and Gill, 1979) which requires an "analytical" amalgamation trap in addition to the field-sampling amalgamation trap. The wide use of this technique has been attributed to its effectiveness in separating interfering substances from Hg before its release to the detection system. Because our field amalgamation trap collects total gaseous Hg, which exists predominantly (e.g. /> 95%) in its elemental form (Schroeder and Jackson, 1987), the Hg collected will be referred hereafter as Hg °. Laboratory analysis of the Hg ° contained in each field amalgamation t::ap was performed in the following sequence: (1) thermal desorption of Hg ° from the field trap (at 450°C), (2) retrapping of Hg ° by an analytical trap, (3) thermal desorption of Hg ° from the analytical trap, and (4) detection of Hg ° by the CVAFS system. The analytical system was standardi2ed by measuring known volumes of a Hg-saturated atmosphere via an airtight gas syringe (Dumarey et al., 1985). The precision of our CVAFS system in analyzing replicate air samples (expressed in terms of relative standard error (R.SE.), where R.S.E.=standard error (S.E.)/mean) was tested in two types of experiments: (1) calibration from multiple injections of Hg°-saturated standard air samples, and (2) analysis of replicate field-trap samples. The level of 3recision from the latter (mean R.S.E. = 1.3%~ range =0.5--2.2) is moderately higher than, but sometimes approaches, the former (mean R.S.E.=0.4%; range=0.2-1.2) (Kim and Lindberg, 1994). The working detection limit for the system is determined to be approximately l pg of Hg ° (calc,Jlated as 3 standard deviation (S.D.) of ypical gold trap blanks).

Application of the MF~'R technique Application of the eddy correlation method to measure real-time fluxes of chemical species requires that fastresponse sensors be employed to measure the contribution of all eddies to the flut. The MBR method, developed to overcome this limitation, is based on the assumption that atmospheric turbulence transfers scalar quantities indiscriminately (Businger, 1986). If the flux, Fc,, and gradient, Ac,, of a reference constituent is known, the flux of an unknown constituent, Fc~, can be determined by measuring its gradient, Ac2, over the same height interval:

Fc~

Fc~ = ~ A C

269

where co and X are the instantaneous vertical velocity and the scalar of the interest (H20 or CO2 in this case), respectively, and the bracketed quantities denote a running mean subtracted from the instantaneous values to determine the fluctuating quantities. A three-dimensional sonic anemometer (model K-probe; Applied Tech., Boulder, CO), mounted at a height of 165 cm, was used to measure the components of the wind vector. A microbead thermistor (model E45A401C; Victory Engineering Corp., Springfield, N J) was used to monitor sensible heat flux. The basic meteorological parameters, which include wind speed, wind direction, air temperature, and relative humidity, were measured continuously at 165 cm above the surface. Platinum resistance thermistors installed at 2 and 6 cm beneath the surface were used to measure soil temperature. The sensor signals were sampled at a frequency of 10 Hz, and the turbulent eddy fluxes were computed on a 30 min interval. (Occasionally, a 15 min sampling interval was also used.) At the end of each 30 min period, a coordinate rotation was performed to obtain a zero mean and transverse velocity (co= v = 0). [This is the normal practice to determine the turbulent flux normal to the mean streamline (McMillen, 1988).] The MBR technique to date has mostly been used above crop and forest canopies to derive air-surface exchange rates of trace species. Only in recent years has the feasibility of its application been carefully examined for the forest soil surface which is known to be an important source/sink for some trace species (Baldocchi and Meyers, 1991; Meyers and Baldocchi, 1993) and may be considered as a potential location for application of the MBR technique (Lee and Black, 1993). To test the feasibility of using the MBR technique over forest floor, Baldocchi and Meyers (1991) previously examined the surface energy balance at the present field study site. Although not serving as an absolute measure, their data showed that the energy balance residual for all components was not significantly greater than zero and proved that the forest floor at this site is suitable for the application of micrometeorological trace gas flux measurements. Subsequently, these authors used these methods to quantify deposition rates of SO2 and 03 to these soils (Meyers and Baldocchi, 1993). To check the feasibility of our micrometeorological approach to mercury flux measurements over the background soils, we designed and performed two types of quality assurance (QA) experiments during our sampling periods. In

I~

2 = K AC 2

where F and A represent the fluxes and concentration gradients of chemical species, respectively, and K is the turbulent transfer coefficient for these species. For the application of this technique in our experiment, the concentration gradients of Hg ° (AC2) were measured at two heights (25 and 165 era) above the soil by following the Hg ° sampling and analysis procedure described in the preceding subsection. (Because sampling heights of 25 and 165 cm were - used consistently for all of our measurements, we will express the gradients between two heights throughout this paper as concentration differences.) The Hg ° gradients were mostly quantified over periods of 1-3 h but extended up to 10 h on two occasions (nighttime measurements). The vertical gradients of reference trace gases (AC I :H20 and CO 2 in our study) were quantified from the same height interval by using an open-path fast-response infrared absorption gas analyzer (Aublc and Meycrs, 1992). The fluxes of the reference trace gases, F~, measured by using the eddy correlation system, were quantified from the following relationship:

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Hg ° (rig m"3) Fig. 1. The results of multi-height gradient measurements (on 22 June 1993) illustrate a log-linear relationship between the heights of sampling and Hg ° concentrations measured. The mean and I.S.E of Hg ° concentrations at each height are 1.97_+0.03 (25 cm), 1.92+0.01 (60 cm), 1.90 +_0.01 (95 cm), and 1.88+0.02 (165 cm). For the environmental conditions during this measurement, refer Table 2.

270

K.-H. KIM et al.

the first experiment, the expected existence of a log-linear relationship between the concentration of trace species and sampling heights above ground was tested by collecting additional samples between the regular sampling heights on two separate occasions. The data acquired from our fourlevel gradient experiment on 22 June 1993 (Fig. 1), for example, demonstrate this relationship for Hg ° vapor. This well-behaved log profile for Hg °, which is in accord with surface layer similarity theory (e.g. Panofsky and Dutton, 1984), supports the application of gradient theory to quantify ;~ air-surface exchange rates of Hg ° over these soils. In the -- .B second QA experiment, air samples were collected simultano~~ eously on three occasions from two adjacent, replicate era~: dient sampling systems 10 m apart. The results of the inde- ~ pendently determined concentrations and the resulting gra- ._~ dients from each pair of measurements are provided in " "~ Table 1. A remarkable similarity in the measured concentra- ~ tions and resulting gradients is apparent from each paired experiment. In fact, differences in measured concentrations ~ fell in a relatively narrow range of about +8%, while the ~ differences in the resulting gradients were within the range of ~ +0-30%. This second type of QA experiment demonstrates ore-~ the existence of a homogeneous turbulence field within which ~ t~ t . . ~¢ Hg levels at a given height are integrated over distance. Both o to types of QA experiments thus provide confidence in our ~ .~ applications of micormeteorological flux measurements of o k3 Hg vapor over background soil sites. We have also per~,~ formed extensive proof-of-principal tests of the method over o mercury-contaminated soils (Lindberg et al., s u b m i t t e d ) . . o = ~',. The fluxes of Hg ° were computed using the concurrently ~ determined parameters, which include the turbulent diffusion "~ l,I coefficients, K, and the concentration gradients of Hg ° [cf. _8 ~ equation (1)]. Because samples for meteorological para~ o~ meters and Hg ° gradients were measured over different ,~ o~ o sampling intervals of 15-30 min (meteorological data) and t--. -- -, ~ 2 h (He ° data), we averaged turbulent diffusion coefficients -0 '~ at intervals similar to those of Hg ° gradient measurements. ~" ~ ~,~, We computed K values using the measured gradient and flux data of H20 vapor. On some occasions when we failed to :~ quantify K values directly from the HxO vapor data, K -~ ~ to values were derived from the CO 2 data. Comparison of the ~ .0 two independently determined K values were made occa- .~ "~ ;~ sionally during the sampling periods. Figure 2a shows a .~ comparison of the two K values acquired from 25 to 31 ~ ~,', October 1993. Data points in Fig. 2a represent sampling "~ g events at 30 min intervals over each daily cycle. The corn~ parison showed considerably good agreement between the g two values and justified the use of substitution procedures (Meyers et al., in prep.)• In addition, when our gradient/flux "~ measurement systems for the two reference gases (H,O and ~ "" CO2) were damaged by a lightning event (during the month of June) and mechanical failure (during the late November), ~o _~g K values were estimated from other strongly correlated ~ parameters such as fluctuations in vertical velocities, a,,, or ~ the reciprocal of aerodynamic resistance, I/R,. Good correl- -~ [~ ations between K values and these meteorological para~ meters have also been verified (Fig. 2b and c). "~ "~

RESULTS

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hange rates of Hg ° were measperimental c a m p a i g n s in 1993. ;red the period of 21 M a y to 31 ere as s u m m e r campaign), a n d ae period of 25 O c t o b e r to 24 ~apaign). We c o n d u c t e d a total ~ u r e m e n t s d u r i n g these camperimental schemes were used •(1) a single run per day d u r i n g

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Mercury vapor fluxes 0.04

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Fig. 2. (a) Illustrates comparison of turbulent diffusion coefficients, K, derived using both H,O vapor and CO z. (b) and (e) show relationships between K values and two concurrently determined parameters, fluctuations in vertical winds (ol,) and reciprocal of aerodynamic resistance, R°-', during 25-31 Oct 1993.

midmorning to early afternoon (a total n u m b e r of 10 measurements in 10 days); (2) 2-3 sequential measurements per day during midmorning to late afternoon (a total n u m b e r of 13 sequential measurements over 6 daily cycles); and [3) 6 - 9 sequential measurements covering a half to full diurnal cycle (a total n u m b e r of 21 sequential measurements over 5 daily cycles). The first type of experiments were performed to monitor the day-to-day variations in the Hg ° fluxes. The second and third type of experiments were performed to describe the continuous patterns of Hg ° exchange over a short-term scale. Combining all three types of experiments, we farther investigated the seasonal trends in the air-surface exchange of Hg ° between summer and fall. The data from the individual Hg °

0

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200 250 Day of The Year

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50 40 30 20 10 0 10 20 30 40 50 No. of Occurrences Fig. 3. Meteorological conditions over Walker Branch Watershed for the periods of May through November, 1993: (a) soil temperature at 2 cm depth, (b) wind speed, (c) precipitation, and (d) relative frequencies in wind directions for each of 12 sectors.

1300 1230 1030 930 1130 1030 1245 1130 830 1100 1400 1730 2000 530 830 1130 1400 830 I100 1400 1700 1930 530 730 1000 1200 1300 1400 1500 1600 1700 1800 1045 1300 I000 1230 1500 1200 1400 1300 1400 1400 1300 1430

1530 1500 1400 1500 1430 1330 1515 1445 1100 1400 1730 2000 530 800 I100 1400 1700 1i00 1400 1700 1930 530 830 1000 1200 1300 1400 1500 1600 1700 1800 1900 1300 1515 1230 1500 1630 1400 1600 1400 1600 1600 1400 1630

1.80 2.14 2.12 1.92 1.78 2.38 3.13 1.97 2.44 1.74 3.51 1.77 2.33 1.83 1.96 1.78 2.08 3.26 1.64 1.52 2.85 26.50 74.31 5.04 2.59 2.58 1.88 1.79 2.54 2.11 2.55 2.61 2.08 1.86 2.19 1.70 1.85 2.14 2.23 3.71 2.53 2.20 2.47 2.09

1.78 2.10 2.01 1.86 1.72 2.23 2.94 1.88 2.47 1.75 3.45 1.54 2.26 1.93 1.98 1.77 1.92 3.35 1.67 1.52 2.04 27.31 75.82 5.23 2.54 2.19 1.80 1.77 2.22 2.05 2.71 2.40 1.92 1.77 2.16 1.61 1.78 2.15 2.08 3.68 2.53 2.06 2.45 2.07 -

-

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0.02 0.04 0.I1 0.06 0.06 0.15 0.19 0.09 0.03 0.01 0.06 0.23 0.07 0.10 0.02 0.01 0.16 0.09 0.03 0.00 0.81 0.81 1.51 0.19 0.05 0.39 0.08 0.02 0.32 0.06 0.16 0.21 0.16 0.09 0.03 0.09 0.07 0.01 0.15 0.03 0.01 0.14 0.02 0.02

0.I1 0.07 0.05 0.02 0.05 0.15 0.09 0.08 0.28 0.07 0.03 0.09 0.05 0.05 0.07 0.04 0.05 0.08 0.06 0.06 0.04 0.56 1.82 0.12 0.I0 0.07 0.07 0.17 0.57 0.28 0.33 0.15 0.04 0.08 0.07 0.02 0.03 0.09 0.07 0.20 0.09 0.11 0.14 0.04

[Hg °] [Hg °] Experiment at 0.25 m* at 1.65 m* Hg ° gradient 90% C l t end (ngm -3) (ngm -'~) (ngm -3) (ngm -3) ns 0.10 0.05 0.05 0.05 0.05 0.05 0.05 ns ns 0.05 0.05 0.05 0.05 ns ns 0.05 0.05 0.10 ns 0.05 0.05 0.05 0.10 ns 0.05 0.05 ns ns ns ns 0.05 0.05 0.05 ns 0.05 0.05 ns 0.05 ns ns 0.05 ns 0.10

Sig(P
7.7 6.7 5.1 1.9 4.7 7.2 9.5 8.3 18.0 8.30 1.7 2.0 0.7 0.30 2.0 2.5 3.2 4.3 4.0 4.0 1.4 12.7 81.9 2.9 5.2 4.5 5.2 11.9 51.3 32.8 15.4 3.8 4.6 11.5 1.8 1.1 3.7 9.5 6.8 20.1 4.2 15.4 23.4 3.7

Hg ° flux 90% CI:I: (ngm 2h -I) ( n g m - 2 h -I) 0.0196 0.0266 0.0282 0.0259 0.0260 0.0134 0.0293 0.0288 0.0179 0.0331 0.0153 0.0061 0.0038 0.0018 0.0080 0.0173 0.0180 0.0150 0.0183 0.0187 0.0098 0.0063 0.0125 0.0066 0.0145 0.0180 0.0205 0.0195 0.0250 0.0325 0.0130 0.0070 0.0318 0.0400 0.0071 0.0151 0.0345 0.0294 0.0271 0.0279 0.0130 0.0390 0.0464 0.0254

K, (ms -1) 0.26 0.99 0.38 2.51 0.34 0.19 1.72 1.31 0.44 0.45 0.27 0.12 0.09 0.08 0.26 0.35 0.29 0.12 0.31 0.41 0.24 0.I 7 0.13 0.31 0.26 0.24 0.33 0.46 0.28 0.40 0.19 0.13 1.48 1.47 0.19 0.22 0.73 0.60 0.38 0.21 0.03 0.70 0.34 0.30

Wind speed (ms -I)

13.0 14.3 13.6 14.2 6.9 11.9 12.6

11.9 25.2 21.3 21.5 30.0 21.9 29.0 26.7 23.1 27.5 26.6 24.9 21.7 20.4 24.3 28.0 29.9 26.8 31.7 33.8 31.6 24.7 22.0 25.7 29.9 32.8 34.1 30.7 27.3 26.2 26.5 25.3 30.0 32.0 17.3 20.9

Air temp (°C)

25.8 23.9 21.8 21.1 23.6 26.6 27.6 25.9 31.7 32.6 29.1 24.5 22.9 25.4 28.7 31.6 36.2 31.0 27.6 26.6 26.2 25.4 26.6 28.6 16.8 17.6 14.0 12.1 12.6 13.3 13.7 12.1 12.3 12.6

27.2

13.7 22.6 20.2 20.9 26.9 20.8 25.2 24.1 23.1

Soil temp. (2 cm depth) (°C)

53 44 54 50

94 52 58 61 62 94 57 63 89 77 71 82 92 94 90 82 72 79 48 34 45 77 93 87 75 60 52 59 66 70 73 75 53 40 65 45

RH (%)

0.084 0.081 0.158 0.138 0.128 0.132 0.072 0.176 0.205 0.121

0.079 0.093 0.095 0.059 0.045 0.069 0.046 0.078 0.092 0.101 0.097 0.121 0.150 0.072 0.047

0.328 0.136 0.121

a,,, (ms - t )

* Mean concentrations were derived from 6 replicate samples for most cases. For sequential measurements over diurnal cycles however, 3 to 4 replicate samples were collected from each sampling height between 25 and 165 cm. t 90% confidence interval (CI) for the concentration gradients were estimated using a t-test on the replicate samples collected from each height. ~:90% CI for fluxes were derived using turbulent diffusion coefficients (K) and 90% CI of gradients.

11/24/93

11/24/93

5/21/93 5/24/93 5/26/93 6/2/93 6/10/93 6/15/93 6/17/94 6/22/94 6/28/93 6/28/93 6/28/93 6/28/93 6/29/93 6/29/93 6/30/93 6/30/93 6/30/93 7/7/93 7/7/93 7/7/93 7/7/93 7/8/93 7/8/93 7/9/93 7/9/93 7/9/93 7/9/93 7/9/93 7/9/93 7/9/93 7/9/93 7/9/93 8/31/93 8/31/93 10/25/93 10/25/93 11/5/93 11/10/93 11/10/93 11/12/93 11/12/93 11/19/93

Experiment start

Table 2. Statistical summary of the individual gradient/flux experiments. For this presentation, the meteorological data measured at short intervals of 30 (or 15) min were averaged for the corresponding duration of the actual gradient/flux measurements (1-3h)

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Mercury vapor fluxes gradient/flux experiments as well as the mean meteorological parameters measured during all campaign periods are presented in Table 2. A relatively wide range of meteorological conditions was monitore,:l during these campaign periods (Fig. 3). Each data point in Figs 3a and b represents 15-30 min average values from continuous measurements. The soil temperature data taken at a depth of 2 cm exhibit variations over fairly large amplitudes during late May through late November (range = 12.1-37.2°C). Air temperatures, which are generally higher than soil temperature by 1-2°C, exhibited similar trends. Wind speeds measured during these campaign periods were in most cases below 0.5 m s - ' but occasionally exceeded 1 m s-1. Figure 3c shows the daily rainfall amounts measured from a forest clearing near the study site. The total precipitation during the summer months shows a gradual increase across June (5.4cm), July (7.7cm), and August (8.7cm). These trends were also seen in the fall months: October (5.9 cm) and November (9.1 cm). The plot of relative frequencies in the wind direction (Fig. 3d) indicates that the winds were rising predominantly from southerly and southwesterly directions. Relative humidity ranged from 31 to 98%. Although a detailed record of scil moisture during these periods is not available, the data (measured in volume percent) indicate relatively strong seasonal variability at the study area (P. J. Hanson, personal comm.): 14-21% (late May to early June), 7 - 1 2 % (July to August), and 11-20% (October to November). Concentration, gradient, and f l u x data

The mean concentrations of Hg ° for each level of gradient experiments were established through a statistical analysis to eliminate outliers from each dataset (Skoog et al., 1992). Outliers were occasionally observed (31 out of 447 analyses) but were of moderate magnitude (within + 15% of the means). When the

273

precision of our measurement technique to collect and analyze replicate air samples was expressed in terms of R.S.E., the mean and 1 S.D. of R.S.E.s were 1.4+0.3% (n = 88). The mean and 1 S.D. of the Hg ° concentrations measured at 165cm were 2 . 1 5 + 0 . 5 1 n g m -3 throughout the campaign periods (range = 1.52-3.68, n = 39). This does not include five experimental data sets collected under non-steady-state conditions which were induced by the plume incursion and by advection (two emission events: 1700-1930 on July 7 and 1200-1300 on July 9; and three deposition events: from 1930 on July 8 to 1000 on July 9; environmental conditions during these periods are detailed below). When grouped on a seasonal basis, the Hg ° concentrations in the summer (May-August: mean = 2.11 ng m - 3; 1 S.D. = 0.49; and n = 29) were slightly lower and less variable than the fall results (October-November. m e a n = 2.26 ng m - 3; 1 S.D. =0.57; and n=10). These concentration data are analogous to those typically observed from the background environment (e.g. Xiao et al., 1991). The existence of very small gradients must be meaningfully interpreted in light of the basic premises of the micrometeorologicai gradient approach. For this reason, we established sampling and analytical techniques by which errors associated with quantification of gradients can be reduced to an insignificant level (Kim and Lindberg, 1994). As a result, about 60% of the concentration gradients measured at ambient Hg ° levels were quantified at a confidence interval of 95% (i.e. P < 0.05), despite the generally low Hg concentrations over the background soils. Statistical analyses on all measured Hg ° concentration gradient data and the corresponding flux data from each individual experiment are summarized in Table 3. The results indicate that concentration gradients of as low as 0.06 ng m - 3 can be quantified at an excellent confidence interval (P<0.05). Although these small gradients are significant, they are dose to

Table 3. A statistical analysis of the resulting Hg ° gradient and flux data. Five experimental results (two emission and thr~e deposition events) acquired during three diurnal experiments have been excluded for this analysis (refer to the text) Hg° gradient (ng m- a) Mean + 1 S.D.

Range

Hg° flux (ng m- 2 h- t) Mean 4- 1 S.D.

No. of events

Range

All data

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0.10 ± 0.07 - 0.05 ± 0.05

-

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K.-H. KIM et al.

274

the bias we have measured between our sampling systems ( ~ 0 . 0 1 - 0 . 0 3 n g m -3, Kim and Lindberg, 1994). However, from a total of 20 bias tests, we were unable to identify a single bias that was significant at the P < 0.05 level. As a result, we have not corrected our field gradients for these small differences between sampling systems. From a total of 44 experiments, 39 experimental results were used for the statistical analysis of the computed Hg ° fluxes. (We excluded the above-mentioned five experiments performed under adverse conditions.) Out of 39 experiments, 23 cases were identified as statistically significant: 20 cases at P < 0.05 and 3 cases at P < 0.10 (see Table 3). These 23 experiments comprise 20 cases of positive (emission) and 3 cases of negative (deposition) fluxes. The remaining 16 experiments, although not statistically significant (P>0.10), are made up of 10 emission events and 6 deposition events. Frequency distribution of the computed Hg ° fluxes is illustrated in Fig. 4. These data clearly show that emission events outnumbered deposition events and occurred at generally higher rates during the sampled periods. It is also indicated that small fluxes of < + 5 ng m - 2 h - ~ constitute about 50% of all measured flux values and that more than half of such cases are statistically insignificant at a 90% confidence interval.

Temporal trends in air-surface exchange of mercury The short-term variations in the computed Hg ° fluxes over the background forest soils were investigated using the results of our daily sequential measurements. Over six daily cycles, two to three sequential runs were conducted for total durations of 4 - 8 h. Figure 5 illustrates the data from the 30 June and 10 November experiments. These sequential measurements during daytime indicate that (l) both upward and downward fluxes of Hg can occur in the morning, but emissions tend to become more pronounced over

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the course of the day; and (2) the daily emission maxima may occur during midafternoon (e.g.

1300-1500). The results of three diurnal cycle experiments (on 28-29 June, 7-8 July, and 9 July) present more extended features of Hg ° exchange over daily cycles (Figs 6-8). The results of two full diurnal cycle measurements on 28-29 June (Fig. 6) and 7-8 July (Fig. 7) show some similarities in relative patterns of Hg ° exchange, although the resulting fluxes were quite different between runs. O n 28-29 June, small downward fluxes were observed in the early morning, reached peak emission rates during the afternoon, and then decreased to much lower levels at night (often near zero). The earlier part of the 7-8 July experiment bears some general similarities to trends seen in other runs, with small deposition rates in the morning followed by an emission peak during the afternoon. However, the latter part of this experiment becomes more complicated because of a local plume event. Compared with the two full diurnal experiments, the 9 July experimental schemes consisted of: more runs (i.e. up to nine) at shorter time intervals ( ~ 1 h) over a half diurnal cycle ( ~ 11 h). The results from these experiments (Fig. 8a) show slight deposition in the morning (10O0), no significant exchange through noon, peak emission in the early afternoon (130O), a

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notable reduction ir, the net emission rates, and the suggestion of a second peak in the flux (1500-1600), which was only marginally significant (P <0.15). The data from our second diurnal experiments on 7-8 July are particularly interesting and may illustrate an important limita~:ion of the gradient method. We observed the highest dry deposition rates during our entire study ( - 18 ar.d - 6 8 n g m -2 h - t) as the measurements progressed from early evening to early morning. As winds began to shift from a generally southerly direction in the late afternoon (on 7 July) to a northerly flow at about 1900 (Fig. 7c), the concentration of Hg ° at the 165 cm height increased from less than 2 n g m -3 to over 75 n g m - 3 . This event was apparently a result of an incursion of a plume of contaminated air from the U.S. Department of Energy's Oak Ridge Y12 plant ~ 3 km to the north of our sampling site, where Hg ° levels in this range are known to occur (Turner et al., 1991). It is possible that both of these depo,;ition peaks were influenced by non-steady-state conditions during advection of the plume into this sit,.', as reflected in the dramatic increase in the concentration of Hg °. The signal of this plume was apparently still detectable 30 h after initiation of the event because we found a large downward gradient of - 0.19 ng m - 3 in concert with still elevated Hg ° levels of ~ 5 n g m -3 on the following day (0730-1000 on 9 July).

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The MBR technique assumes that concentration gradients are the result of turbulent exchange leading to a steady-state condition at the point of measurement. Significant advection of plume Hg to the site would violate this assumption, and the 40-fold increase in Hg ° over ~ 15 h could have created artifact gradients as a result of inadequate mixing near the ground. (The 160% coefficient of variation in Hg ° during this period was 4 to 5 times higher than during our intensive runs.) Although this situation raises some doubt about the validity of the fluxes during this period, we would expect deposition to occur during a plume event because the chemical gradient then favors soil uptake over emission. In fact, the dry-deposition velocities computed from the "plume event" data are

276

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in the range of those computed from non-plume data, suggesting that the elevated fluxes may not be artifacts. Hence, based on all of the data, it appears that very low to insignificant air-surface exchange generally prevails under normal conditions at night (as seen from the first diurnal experiment on 28-29 June) but that a plume event could result in significant dry deposition. The peak emission flux of ~ 29 ng m - 2 h - t (at 1700-1930 on 7-8 July) was measured just before the drastic increase in Hg ° (Fig. 7c). This flux was the result of an exceptionally large gradient (0.81 ng m-a) measured under relatively high soil temperature and low wind speed (see Table 2). This large emission flux may have been influenced by non-steady-state conditions induced by the plume event, as the wind shift that delivered the plume began during this experiment ( ~ 1830). We also noted that the environmental conditions observed during this emission event are analogous to those of a first peak emission on 9 July from 1200 to 1300 (Fig. 8a). It is likely that a similar type of phenomenon influenced this peak flux. Beating in mind that peak emissions of Hg ° tend to follow those of soil temperature and K data (e.g. Figs 6 and 7), comparison of the patterns shown in Figs 8a and b also suggests that the occurrences of the first emission peak may be associated with some artifacts resulting from advection. Because of the eccentric flux patterns and the evidence of non-steady-state conditions in-

The computed air-surface exchange rates of Hg ° are the result of an interplay between the rate of release and uptake across the soil-air boundary and rate of vertical transport in the atmosphere. In order to describe quantitatively such processes, the possible relationships between Hg ° fluxes and the measured meteorological parameters need to be elucidated. To this end, regression analysis was performed using the statistically significant Hg ° flux data (i.e. P ~ 0.1) and environmental parameters averaged over the corresponding time periods. Although we commonly noted good correlations between emissions fluxes and environmental parameters, we were unable to define such relationships from the deposition data set because of the limited database: only three out of nine deposition events were quantified as statistically significant at P ~< 0.1. When regression analyses were performed against the emission flux data, the summer data set alone generally exhibited best correlation results for most of the environmental parameters. The most significant linear regression results include those of wind speed (r=0.50, P<0.10, n=15), trco (r=0.57, P<0.20, n=9), and relative humidity (RH: r=0.50, P<0.10, n = 15). It is suspected that the correlation between flux and RH may probably be an artifact of the relationship between RH and temperature which tends to be inversely correlated. (For example, from data sets collected during summer campaigns, linear regression analysis between RH and soil temperature showed a significant inverse correlation: r=0.44, P<0.001, n = 124.) Previous studies of mercury fluxes over soils have documented that a relatively strong exponential relationship exists between flux and soil temperature (Lindl:~rg et al., 1979, submitted; Siegel and Siegel, 1988). These phenomena have often been accounted for by physicochemical characteristics of elemental mercury (high vapor pressure and low water solubility) and temperature dependence of soil microbial activities in the production/consumption of Hg °. Our initial analysis of the relationship between the flux and soil temperature data taken at a depth of 2 cm suggest that virtually no correlation exists between the two parameters (r = 0.01, n = 20). However, the scatter plot indicates that the bulk of the data tends toward an exponential relationship. Four data points do not exhibit this tendency: (1) three relatively high fluxes of ~ 9 - 2 0 ngm -2 h - t measured under low soil temperature during the fall and (2) one relatively low flux of

Mercury vapor fluxes ~ 5 ng m - 2 h - 1 measured under elevated soil temperature during the summer. By disregarding these four data points, a significant exponential correlation was seen between Hg ° flux and soil temperature at 2 cm (r =0.48, P<0.10, n = 16; see Fig. 9), as expected based on studies of mercury-rich soils (Lindberg et al., 1979, submitted; Siegel and Siegel, 1988). The data represented by these :!our occasions suggest that the influence of soil temperature can be less important than other environmental parameters under certain conditions and/or that an exponential correlation between the Hg ° flux and temperature data from the studied background soil site may not be as well defined as it is in other environmental surfaces (such as Hg-rich soils). Wl:en the relationship between flux and temperature was further examined using other temperature data sets (i.e. air temperature and soil temperature at 6 cm depth), we found that the best correlation with flux was from air temperature data (r =0.62, P<0.01, n = 26), which tend to exhibit similar trends with the soil temperature measured at 2 em. However, relatively ]poor regression results were obtained from the soil temperature data measured at 6 c m (r=0.24, P>0.1[0, n = 10). The temperature dependence of soil emission processes has often been described in terms of the Arrhenius equation (Xiao et al., 1991; for details, refer to Lindberg et al., submitted). Previous estimates of activation energy, acqaired through such application, show that the values from various environmental surfaces generally fall in a relatively narrow range: 25.8 +2.6 from contaminated soils (Lindberg et al., submitted), 28.0+5.7 from mercuriferous volcanic soils (Siegel and Siegel, 1988), and 29.6+ 1.0 kcal m o l - i from lake surfaces in Sweden (Xiao et al., 1991). When our soil temperature data were fit into the Arrhenius equation [r = 0.48; P<0.10, n = 10 for I / T , ~ vs In (Hg ° flux)I, we obtained an activation energy of 17.3 + 7.7 kcal mol-1 w[ich is lower than the previous

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results. This comparison suggests that Hg ° emissions over environmental surfaces may not be accounted for solely in terms of direct vaporization of Hg °. (The enthalpy of rig ° vaporization is ~ 14 kcal m o l - 1.) The data from our diurnal experiments generally indicated a slight time lag in the daily maxima between the influential parameters (such as soil temperature or turbulent mixing coefficients) and Hg ° fluxes (Figs 6-8). Several factors involved in the production/release of Hg ° should be considered to account for this phenomenon. Taking into account the high volatility of Hg °, changes in soil temperature or K values will immediately influence the total amount of Hg ° emitted from surficial soil layers. Changes in these parameters can also influence the rate of Hg ° diffusion from lower to surficial soil layers or that of microbial mediation by which elemental Hg is produced from different Hg precursors. However, these processes would respond at much slower rates than those influencing surficial volatilization. As in the case of Hg °, abundant evidence indicates that temperature exerts important controls over the soil fluxes of trace gases which are predominantly of biological origin [e.g. reduced sulfur species: Goldan et al. (1987); NOx species: Anderson and Levine (1987); and CH4: Shurpali et aL (1993)]. However, quantitative assessments of such relationships have often been constrained because the effects of other environmental parameters tend to integrate with those of temperature. For reduced sulfur gases, Goldan et al. (1987) have shown that soil type was an important factor in determining the proportions of emissions among various sulfur species. In the case of NOx gases, soil moisture (wet or dry conditions) and nutrient content were also identified to be controlling factors of soil emissions (Davidson, 1992; Hutchinson et al., 1993). Interpretation of relationships between Hg ° fluxes and such parameters from our studies is, however, limited because our measurements were conducted over the same sampling site and the data for soil moisture content were not collected concurrently with each flux measurement at the same site. Generalization of mercury air-surface exchange

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Fig. 9. A log regression analysis between flux and soil temperature data taken at 2 cm depth. For this analysis, experimental results with statistically significant gradients (P ~<0.10) were used without three flux values acquired during fall.campaigns (on 5, 10, and 19 Nov.) and one value (on 9 July) during summer campaigns.

The temporal pattern of the air-surface exchange of Hg °, derived from our gradient/flux measurements, clearly indicate that emission is more dominant than deposition over daily as well as seasonal cycles. To generalize temporal trends of exchange over diurnal cycles, we computed hourly mean exchange rates of Hg ° over a 24 h cycle (Fig. 10). Because the selection of time bands for our gradient measurements varied among experiments, the original flux data (which were of up to 10h duration) were divided into hourly intervals. The mean flux values representative of each hourly time band were then derived by averaging these data sets. The results show a smoother pattern than is seen from individual measurements, as expected. These results confirm the fact that generally stronger levels of emission tend to occur for the periods

278

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covering late morning through early evening, while weaker levels of deposition are commonly observed over short-term periods in the late morning. The nighttime data in Fig. 10a, which rely on our first diurnal measurements (28-29 June), indicate that emission, although small in absolute magnitude, can be more prominent than deposition even during the night. In fact, the generally insignificant exchange of Hg ° over the nocturnal cycles and its extension to early morning periods are in line with the expectation that, under stably stratified conditions, the exchange of trace gases in the atmospheric boundary layer is mainly controlled by a slow molecular diffusion protess rather than by rapid dynamic mixing caused by turbulence. Figure 11 illustrates the seasonal trends in the measured concentration, gradient, and flux data for Hg °. When these Hg ° flux data were grouped into summer (May-August) and fall (October-November) seasons, they displayed a very weak seasonal trend: the results summarized in Table 4 indicate that emission fluxes measured during the summer campaign periods are marginally larger ( ~ 10%) than the results from the

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fall campaign periods. The limited number of data points from the fall-term measurements make it more difficult to establish seasonal trends in deposition fluxes. However, comparison of relative strengths and frequencies in deposition fluxes between summer and fall seasons (Table 4) suggests the possibility that deposition of Hg ° to the forest soil environment may be more dominant in the summer than in the fall. It seems difficult to explain this trend without further data, but we suspect it to be related to such factors as seasonal differences in soil moisture or forest floor plant physiology. Despite our findings of the lower rates and frequency of dry deposition in the forest soil environment, it is still important to characterize some general

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279

aspects of this process to comprehend better the basic features of Hg ° exchange across the soil-atmosphere interface. There have been only limited studies of dry deposition of Hg ° to plant-soil systems. On the basis of field gradient measurements, Barton et al. (1981) derived a deposition velocity (Vd = flux/concentration) of Hg ° to a grassland in the range of 0.06-0.1 crn s - i. More recently, Lindberg et al. (1992) estimated from model-based predictions that weekly mean Vd values of Hg ° for a deciduous forest canopy range between 0.006 (winter) and 0.12 cms - t (summer). The most relevant comparison of Vd values with our work can be made from the studies of Xiao et al. (1991). Using their data set, we derived the mean deposition velocities of 0.010 for the warm season (n=2) and 0.012 +0.006cms -1 for the cold season (n=5). In comparison, estimates of our Vd values (not including the plume data) display a trend with an enhanced deposition velocity Hg ° during summer compared with that for fall: the mean and 1 S.D. for summer and fall seasons are 0.030+0.024 (n = 7) and 0.009 cm s-~ (n = 2), respectively. Despite the limited data, the general ranges of deposition velocities estimated from our study are quite analogous to those estimated from the work of Xiao et al. It is interesting that the Va values of 0.02+0.01 c m s - I computed from the questionable plume-impacted gradients measured on 7-8 July exhibit remarkable similarity to the values we computed from the non-plume periods. This may indicate that these gradients were a true reflection of enhanced deposition rates under elevated levels of Hg °, that is, we would expect that artificially high fluxes resulting from artifact gradients would have resulted in unusually high values of lid. It is interesting to understand why deposition occurs on some occasions, and emission on others. In an attempt to verify the existence of environmental factors and conditions that may exert important controls over the direction of fluxes, we analyzed the emission/deposition data sets that were collected during the time band when bidirectional fluxes most commonly occurred (i.e. 0900-1600, see Fig. 9). The results of this analysis show no significant differences in the mean values of most of the parameters that might influence the magnitude and direction of air-surface exchange for Hg °, such as air or soil temperature [e.g. 22.3 + 5.6 (n= 142) for emission vs 23.5 + 7.0 (n = 32) for deposition events]. However, a pronounced difference was noted in the wind speed data: the mean and 1 S.D. for the wind speed data during emission and deposition flux episodes are 1.01 + 0.96 (n = 142) and 0.33 + 0.19 m s- 1 (n = 32), respectively. Although not as clear as the wind speed data, this trend was also consistently seen from the ao, data, which parametrize the fluctuations in vertical wind speeds I-about 40% difference in mean values: 0.14 ___0.17 (n = 55) for emission and 0.10+0.03 (n = 18) for deposition events]. These findings support the idea that emission may occur under more turbulent conditions than depos-

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ition does. Although we are uncertain why emission would be favored by more turbulent conditions than would deposition, one possible explanation may be sought in terms of the effect of fluctuations in static pressure, which can exhibit a strong correlation with turbulence related parameters such as tr,o or wind speed. Baldocchi and Meyers (1991) noticed markedly increased levels of CO2 emission rates from the same forest floor at Walker Branch Watershed with increasing levels of static pressure fluctuations. It has been postulated by Kimball and Lemon (1971) that fluctuations in static pressure can aid the diffusion process of gases through coarse soils and litter. Hence, net emission may be favored over deposition under these conditions. However, further data are needed to evaluate the role of static pressure fluctuations in governing the exchange process of mercury from soils. In addition, among various factors and parameters involved in the air-surface exchange of trace species, concentration has often been noted as an important factor influencing exchange rates for a number of chemical species [e.g. NO (Slemr and Seiler, 1991)]. In this respect, the steady-state mixing ratio at which the emission rate of Hg ° across the soil surfaces is balanced by its deposition rate can be referred to as a compensation point. Although our data are still too limited to define such a relationship, comparison of the Hg ° concentration data measured during bidirectional flux periods does show that concentrations during deposition episodes (2.3 + 0.6 ngm-3; n = 7) were slightly enhanced relative to emission episodes (2.1 + 0.5 ngm-3; n = 26). Comparison of different approaches to quantification of exchange rate

Table 4 summarizes our micrometeorologically derived Hg ° fluxes and previously reported soil fluxes from different measurement techniques. It should be pointed out that all of the previous field flux data were acquired through an application of soil-enclosure techniques in the boreal forest environments (Xiao et al., 1991; Schroeder et al., 1989). Although the effect of all the contributing factors to the computed fluxes cannot directly be compared among studies, some insights into the temporal/spatial variabilities in exchange process as well as the validity of measurement techniques can be offered by comparing both absolute and relative patterns of source/sink strengths among various studies and approaches. Despite some similarities between our and other reported deposition fluxes, the emission fluxes from the current study are larger by about an order of magnitude than those reported from the field enclosure techniques over the boreal forest environment in Sweden. It should also be emphasized that, like our observations, the field chamber studies of Schroeder et al. (1989) are also indicative of emission-dominant trends over time. However, Xiao etal. (1991), while employing the technique essentially identical to that of Schroeder et al. (1991), found more temperature-dependent patterns in which

emission occurs dominantly during the warm season and deposition during the cold season. It is also interesting to note that emission fluxes determined by our micrometeorological methods are generally smaller than those estimated from our more limited laboratory chamber studies (Table 4, see also Kim et al., 1993). In fact, high emission fluxes in concert with the absence of deposition signals from laboratory chamber studies are quite reasonable in light of the fact that all of the measurements were conducted on disturbed soil cores under relatively favorable conditions for Hg emissions: well-mixed conditions under relatively high soil temperature (range = 29.6-32.1 °C). A comparative study to evaluate various field measurement techniques (mainly the micrometeorological and field-chamber methods) is currently under way in our laboratory (Kim and Lindberg, in prep.). Preliminary field-chamber measurements were performed at the same field site over the Walker Branch Watershed forest soils. The initial results from this chamber study seem quite promising as our chamber blank levels are consistently quite small (e.g.+ 1 ng m - 2 h - 1 ) . In addition, our field-chamber fluxes are very similar to the data acquired by using our micrometeorological approach (e.g. + 30%). It is our opinion that the generally weak emission signals or deposition-dominant cold season patterns, seen from the previous enclosure studies of Xiao et al. can in part be accounted for by the relatively large blank values that often exceeded the sample fluxes reported by these authors (Xiao et al., 1988). The level of difference between our and the other studies is, however, still explainable taking into consideration: (1) environmental conditions encountered at each study site (e.g. temperate vs boreal forest and consequent differences in soil temperature regimes) and (2) the inherent differences between the flux measuring techniques.

SUMMARY AND I M P L I C A T I O N S

The significance of forest soil environments as natural sources/sinks of Hg vapor has been examined through an application of micrometeorological methods over a deciduous forest in eastern Tennessee, U.S.A. The data from our gradient measurements show that, despite the generally low Hg concentrations (ambient Hg levels of 1.52-3.68 ngm-3) and the associated small gradients, approximately 60% of the measured concentration gradients are statistically significant (at P<0.05 and occasionally at P<0.1). The remaining 40% of the measurements in most cases represented fluxes that were too small in magnitude to be statistically significant from fluxes of zero. The resulting flux estimates indicate that emission episodes (mean = 7.5 + 7,0 ng m - 2 h - 1, n = 30) are more dominant in strengths and frequencies than dry-deposition episodes (mean = - 2.2 + 2,4 ng m - 2 h - 1, n = 9). Our flux measurements during daytime display a distinctive trend in which daily emission maxima tend

Mercury vapor fluxes to develop during the afternoon, and the occurrences of deposition tend to center in the morning. A few nighttime measurements conducted during the summer campaign suggest that net exchange during typicad nocturnal periods may be insignificant. However, we measured the highest dry deposition rates during a nighttime plume event. The results of our study suggest that micrometeorological technique can be used as an incisive tool to derive reliable fluxes of Hg ° in various environmental settings. Despite many proven advantages, application of micrometeorological methods should, however, be limited unl,~s the fundamental premises on which the method is based are satisfied. For this reason, the data a,,'quired during non-steady-state conditions, which may have prevailed during a plume event, have been excluded from data generalization. Although the validity of such data set may be in question, the resulting very high dry-deposition rates suggest that anthropogenic sources can significantly alter the basic pictures of natural Hg cycling. In analogy to previous field-flux chamber studies in a boreal forest, our micrometeorological flux data show distinctive temporal trends of air-surface exchange in which emission is consistently more dominant in both strength and frequency than dry deposition. Observations of generally small emission fluxes from previous studies can be explained in part by severe blank problems of earlier enclosure techniques. We examined factors governing the direction of fluxes by comparing the respective environmental conditions of emission and deposition. The results indicate that the direction of Hg ° fluxes may be strongly influenced by the stability of the atmospheric boundary layer, as evidenced by notabh" differences in the wind speed and a~, data, with a tendency for emission to occur under more turbulent cond.itions than deposition does. Our regression analysis l~¢tween fluxes and environmental parameters also showed that emission of Hg ° were significantly correlated with several environmental parameters including soil temperature, wind speed, fluctuations invertical wind (o~), and RH. Preliminary studies of deposition processes in the study area have shown that the mean annual flux of Hg ° in throughfall plus litterfall to these forest soils is of the order of - 4 to - 5 ng m - 2 h - * (Lindberg et al., in press). Although our emission data are too limited to estimate annual :~luxes, comparison with the wetdeposition fluxes indicates that the strength of net source mechanisms (soil emission) could be approximately the same as the strength of total sink mechanisms (dry plus wet deposition). This finding has significant implications for mercury cycling between the troposphere and biosphere. If emission is as important as suggested by these new data, then the total sink capacity of terrestrial ecosystems for airborne mercury may have to be reassessed. Acknowledgements--We thank Jim Owens for his assistance during the field sampling and laboratory analysis and Paul

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Hanson and Suzanna Greco for providing gravimetric soil moisture data. We are also grateful to Dis Dennis Baldocchi, Paul Hanson, and BillScbroeder for their critical and careful reviews on the manuscript. This research was sponsored by the U.S. Department of Energy and the Electric Power Research Institute (RP 3218-02)under contract with ORNL. ORNL is managed by Martin Marietta Energy Systems,Inc., for the U.S. Department of Energy under contract DE-AC0584OR21400. Publication No. 4291, Environmental Sciences Division, ORNL.

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