Novel Dynamic Flux Chamber for Measuring Air–Surface Exchange of

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Novel Dynamic Flux Chamber for Measuring Air−Surface Exchange of Hgo from Soils Che-Jen Lin,*,†,‡,§,¶ Wei Zhu,†,∥,¶ Xianchang Li,⊥ Xinbin Feng,*,† Jonas Sommar,† and Lihai Shang† †

State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China ‡ Department of Civil Engineering, Lamar University, Beaumont, Texas 77710, United States § College of Environmental Science and Engineering, South China University of Technology, Guangzhou 510006, China ∥ Graduate University of Chinese Academy of Sciences, Beijing 100049, China ⊥ Department of Mechanical Engineering, Lamar University, Beaumont, Texas 77710, United States S Supporting Information *

ABSTRACT: Quantifying the air-surface exchange of Hgo from soils is critical to understanding the cycling of mercury in different environmental compartments. Dynamic flux chambers (DFCs) have been widely employed for Hgo flux measurement over soils. However, DFCs of different sizes, shapes, and sampling flow rates yield distinct measured fluxes for a soil substrate under identical environmental conditions. In this study, we performed an integrated modeling, laboratory and field study to design a DFC capable of producing a steady and uniform air flow over a flat surface. The new DFC was fabricated using polycarbonate sheets. The internal velocity field was experimentally verified against model predictions using both theoretical and computational fluid dynamics techniques, suggesting fully developed flow with velocity profiles in excellent agreement with model results. Laboratory flux measurements demonstrated that the new design improves data reproducibility as compared to a conventional DFC, and reproduces the model-predicted flux trend with increasing sampling flow. A mathematical relationship between the sampling flow rate and surface friction velocity, a variable commonly parametrized in atmospheric models, was developed for field application. For the first time, the internal shear property of a DFC can be precisely controlled using the sampling flow rate, and the flux under atmospheric condition can be inferred from the measured flux and surface shear property. The demonstrated methodology potentially bridges the gap in measured fluxes obtained by the DFC method and the micrometeorological methods.

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INTRODUCTION Mercury (Hg) is a potent neurotoxin of human concern1,2 because of the exposure of methylmercury caused by fish consumption and other pathways including rice consumption at contaminated sites.3,4 Methylmercury is formed through the methylation of relatively nontoxic inorganic Hg from atmospheric deposition.5,6 Because of the long atmospheric lifetime in its elemental form (∼1 year), Hg is subject to longrange transport and considered a global pollutant.7−10 The emissions, transport, transformation, and deposition continuously cycles the toxic metal in different environmental compartments.11 Human activities perturb global Hg cycling by altering atmospheric Hg budget through emissions of gaseous elemental mercury [GEM or Hgo], gaseous oxidized mercury [GOM] or reactive gaseous mercury [RGM], and particulate bound mercury [PHg]. In contrast, natural emissions release predominantly Hgo.12,13 These sources include (1) geogenic sources such as volcanoes, (2) weathering processes of Earth’s crust and forest fires, and (3) the recycling of deposited Hg from the oceans and terrestrial systems (so© 2012 American Chemical Society

called re-emissions) that are analytically indistinguishable from primary natural sources. According to a recent estimate, natural emissions account for nearly two-thirds of global Hg input into the atmosphere.14 However, these estimates remain highly uncertain because of the limitations in the measurement approaches and scale-up calculations.15−17 To better understand the biogeochemical cycling of mercury, it is important to quantify the air-surface exchange of Hgo.18 This can be achieved by a number of field techniques including enclosure methods, optical long-path spectroscopic methods, micrometeorological methods and bulk methods.19 The dynamic flux chamber (DFC) method is the most widely utilized enclosure method because of its intuitive nature, field portability and versatility over flat surfaces. Over the years, DFCs of various sizes and shapes have been applied to measure Received: Revised: Accepted: Published: 8910

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Figure 1. Two-dimensional flow velocity profile under development and when fully developed. H is the channel height, x indicates the flow direction and z indicates the vertical dimension.

Hgo exchange fluxes over soil and water surfaces.20−28 However, due to a lack of a standard design and operating protocols, earlier measurements were made using different sampling flow rates and the results are not directly comparable. It has been recognized that DFCs of different shapes and internal flow conditions can yield distinct measured fluxes over the same surface.29 The main reason is that the mass transfer characteristics inside a DFC can be significantly altered by the flow field. Unfortunately, although earlier efforts have been made,30−32 the flow structure inside DFCs remains poorly understood. For many DFCs in use, the shear stress over the measurement surface is potentially nonuniform. Therefore, its dependence on flushing flow rate is nonlinear and unpredictable. This creates two difficulties in the interpretation of flux data measured by DFCs. First, the reported flux could be biased due to a lack of understanding on the internal flow mechanics. Second, using DFC-measured flux for scale-up estimate could produce significant uncertainty because there is no linkage between the shear property inside a DFC and in the atmosphere. In our view, the internal flow mechanics of a DFC should meet a number of criteria (Supporting Information, Note 1). First, the flow over the measurement area is steady and uniform. Second, the shear parameters over the measurement area can be related to those used in atmospheric modeling for scale-up calculation. Third, the internal flow condition is predictable and controllable by the sampling flow rates. In this study, we examined the flow and mass transfer characteristics of two conventional DFCs using computational fluid dynamics (CFD) techniques, proposed a DFC design that achieves the above-mentioned criteria, and developed mathematical relationships that relate the internal shear properties of the DFC to the parameters used in atmospheric modeling. In addition, we implemented the new design, experimentally investigated the internal flow structure at various flushing flow rates, tested the new DFC for Hgo flux measurements under laboratory conditions, and compared its performance to that of a conventional DFC.33−35 For field application, we proposed a methodology that uses the flux measured by the DFC to infer the flux under atmospheric condition for scale-up calculations.

regime, the velocity profile can be characterized by a parabolic function ⎡ ⎛ 2z − H ⎞2 ⎤ u ⎟ ⎥ = 1.5⎢1 − ⎜ ⎝ H ⎠⎦ uave ⎣

(1) −1

where uave is the average velocity (m s ), H is the channel height (m), and z is the distance from the bottom wall (m). The maximum velocity (umax) is 1.5 times the mean velocity (uave). The shear stress (τw, N m−2) is the product of fluid viscosity and velocity gradient at the wall surface ⎛ du ⎞ τw = μ⎜ ⎟ ⎝ dz ⎠

z=0

⎛u ⎞ = 6μ⎜ ave ⎟ ⎝ H ⎠

(2)

where μ is the dynamic viscosity (N s m−2). The friction velocity, the flow velocity near the wall, can be defined as the square root of the ratio of shear stress to fluid density ⎛ 12 ⎞1/2 ⎛ τ ⎞1/2 u 0 = ⎜ w ⎟ = uave⎜ ⎟ * ⎝ρ⎠ ⎝ Re 2H ⎠

(3) −3

where ρ is the air density (kg m ). Re2H is the Reynolds number with a characteristic length of 2H, which is the hydraulic radius defined as four times the ratio of crosssectional area to wall boundary perimeter. To ensure the flow is laminar, Re2H should be less than ∼2200. Note that the friction velocity defined by the wall shear stress (u*0) is somewhat different from the friction velocity defined by atmospheric air velocity profile, u*, and will be discussed later. In the atmospheric boundary layer, the vertical velocity profile (u) is expressed as

u ⎛z⎞ u = * ln⎜ ⎟ k ⎝ z0 ⎠

(4) 36

where k is the von Karman constant (∼0.451), z is the height from the surface (m), and z0 is the roughness height (m) that depends on the land use type. u* is termed friction velocity and is defined as

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u = *

MATERIALS AND METHODS Design Concepts. The design aims to create a steady, uniform and predictable flow inside the DFC over a wide range of flushing flow rates. One of the most effective ways to create a stable uniform flow is using two parallel plates, as shown in Figure 1. After an entrance region, the uniform flow can be fully developed and the flow velocity profile does not change with respect to the position in the flow direction. Therefore, the shear stress and mass transfer remains constant. In the laminar

|τ | ρ

(5)

where

τ = −ρu′ν′

(6)

which represents the turbulence level of the atmosphere. u′ and v′ are the fluctuation of velocity in horizontal directions (m s−1). The velocity profile is only valid for z > z0 because the velocity becomes negative when z < z0. The zero velocity occurs at z = z0, which is equivalent to the nonslip solid wall. 8911

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Figure 2. Schematics of the physical dimensions of the dynamic flux chamber: (a) top view, (b) side view, and (c) the corresponding locations in the measurement zone for velocity measurement during flow verification experiments. Air enters from the left and goes through an entrance zone (300 mm), a measurement zone (300 mm), and an exit zone (100 mm). The shaded area is the measurement zone.

workers30−32 also used this approach. The fundamental differences between the two designs are discussed in the Supporting Information, Note 2. Figure 2 shows the DFC design dimensions. The flow channel has a height of 0.03 m (or 30 mm) and a width of 0.3 m. The main DFC body is 0.7 m in length (X axis) and divided into three zones: an entrance zone (0.3 m) for flow development, a measurement zone (0.3 m) and an exit zone (0.1 m). The DFC design has a 3-cm extension on the side wall of measurement zone to allow the chamber to sit stably over most soil surfaces. On the basis of the sampling flow reported in earlier work,29 the flow rate typically ranges from 5 to 15 L min−1 (8.3 × 10−5 to 2.5 × 10−4 m3 s−1), which yields an mean flow velocity of 0.00925−0.0278 m s−1. This corresponds to a Reynolds number, Re, from 36 to 108, indicating a laminar flow. The length of entrance region (le) for a laminar flow to fully develop can be estimated by

Physically, friction velocity can be interpreted as the average velocity near the surface boundary layer. To relate the flow condition in the DFC to the atmospheric velocity profile, it is proposed that the shear stress at zero velocity is equal to each other. It should be noted that the internal DFC flow does not simulate atmospheric turbulence. However, the mass transfer rate (i.e., flux) is closely related to shear stress, which is used for representing the mass transfer condition in the atmospheric boundary layer. From eq 4 ⎛ du ⎞ τw = μ⎜ ⎟ ⎝ dz ⎠

u =μ * kz 0

(7)

z Q z0 u = 6kuave 0 = 6k * H Ac H

(8)

z=z0

With eqs 2 and 7,

le/H = 0.06Re

where Q is the volumetric flow rate (m3 s−1) and Ac is the cross sectional area perpendicular to the flow direction (m2). Equation 8 establishes the relationship between the mean velocity (or flushing flow rate) inside the DFC and the corresponding friction velocity in the atmospheric boundary layer. The atmospheric friction velocity can therefore be matched in the DFC by a given mean flow velocity (uave) and the height of the flux chamber (H). Design of DFC Body. A shallower channel attains fully developed flow conditions more quickly (eq 9) and therefore a thin chamber design is proposed. The design of Gao and co-

(9)

Therefore, a theoretical length of 0.19 m is needed at 15 L min−1. The entrance zone length of 0.3 m was chosen for additional flow rate flexibility. The entrance of the chamber is open to the ambient air. After the flow is fully developed, the flow reaches the measurement zone and then leaves the chamber. The exit zone allows additional shear buffer before channel reduction and connection to a suction pump for flow rate control. Computational Simulations of Flow and Hgo Mass Transfer. To understand how changes of the flushing flow rate 8912

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(4) the fabrication is reasonably achievable for the design dimensions. Three materials were evaluated, including FEP/ PFA Teflon, polycarbonate and quartz. Polycarbonate was selected as the chamber material because of its excellent mechanical properties, reasonable light transmittance and relative ease of fabrication, which represents a balanced compromise to meet the above-mentioned criteria (see the Supporting Information, Note 3, for additional discussion regarding material selection). The DFC was fabricated using precision cuts of 3-mm thick polycarbonate plates. The polycarbonate plates were glued airtight to the design dimensions. The fabrication error for each dimension was smaller than 0.5 mm. A sieve plate with 518 equally distributed circular holes (2 mm in diameter) was mounted at the air entrance to avoid potential flow disturbance caused by external winds, since the inlet channel is directly open to the ambient air. The sieve can also further shorten the distance required to reach the fully developed flow and improve the laminar behavior at the expense of slightly higher pressure drop. Measurement of Velocity Field inside the DFC. The three-dimensional velocity distribution in the measurement zone (Figure 2a and b) was measured using a thermal anemometer (Testo 405-v1, Testo Instruments Co. Ltd., Germany, accuracy ±0.015 m s−1). A polycarbonate plate with 40 circular sampling ports (6 mm in diameter, Figure 2c) was placed under the measurement zone with only one port open each time for inserting the anemometer at different depths (5, 15, and 25 mm from the bottom). The other ports were sealed with thin tapes. The velocity measurements were made at four flushing flow rates (10, 20, 30, and 50 L min−1) controlled by a vacuum pump and a flow meter. The measured velocities at different locations were interpolated and visualized using a script coded in MATLAB (Release 2009a). Measurement of Hgo Flux. The DFC method for measuring Hgo flux has been described in details elsewhere.27,29,44 A description of the measurements conducted in this study, including data quality assurance, is given in the Suppporting Information, Note 4. The measured Hgo flux is calculated as31

affect the mass transfer within a DFC, Computation Fluid Dynamic (CFD) simulations were performed using Fluent (version 6.3.26). The three-dimensional, time-averaged, steadystate Navier−Stokes equations and the equations for mass conservation and Hgo transport were solved, which resolved the velocity fields and concentration gradients in the DFC. The governing equations of mass conservation, momentum, and diffusion transport are given as: ∂ (ρui) = 0 ∂xi

(10)

∂ ∂P ∂ ⎛ ∂uj ⎞ + (ρuiuj) = ρgj⃗ − ⎜μ ⎟ ∂xi ∂xi ∂xi ⎝ ∂xi ⎠

(11)

∂ ∂ ⎛ ∂Cj ⎞ (ρuiCj) = ⎜ρDj ⎟ ∂xi ∂xi ⎝ ∂xi ⎠ −3

(12) −1

where ρ is air density (kg m ), u is flow velocity (m s ), μ is dynamic viscosity of air (N s m−2), C is the concentration expressed as mass fraction, g is gravitational constant (m s−2), P is the pressure driving the air flow (Pa), D is the diffusivity of Hgo (1.194 × 10−5 m2 s−1),37 x is the position variable. The subscripts denote either the dimensional components (i) or species (j). Hgo evasion is caused by the release of desorbed Hgo, either physically or chemically, from the soil surface.38−40 In this assessment, it is assumed that the underlying soil substrate is a constant diffusion source of Hgo. The GEM concentration in the ambient air from the inlet is assumed to be 1.0 ng m−3, and the substrate surface has a constant concentration of 38 ng m−3 (both are arbitrary values for evaluating the trend of mass transfer). After assigning the appropriate boundary conditions, iterations were made until mathematical convergence was reached, indicating steady-state solutions. Theoretical Calculation of Hgo Mass Transfer. For verifying the results of CFD simulations (i.e., eqs 10−12), a theoretical approach was also applied to calculate the mass transfer rate under the laminar flow. The overall Hgo flux is calculated as F = k mass(Cgs − Cair)

F=

(13)

where kmass is overall mass transfer coefficient (m s−1) estimated from boundary layer theory using a dimensionless analysis:41 Sh =

k massH 0.03(H /L)ReSc = 4.86 + D 1 + 0.016[(H /L)ReSc]2/3

(Co − C i) × Q A

(15) −2

−1

where F is the estimated Hg flux (ng m h ), Q is the flushing flow rate (m3 h−1), A is the substrate surface area exposed to the air in the DFC (m 2), and (C o − Ci ) is the Hg o concentration difference between outlet and inlet air (ΔC) of the DFC. Effect of the Flushing Flow Rate. Verification of the effect of flushing flow rate on the measured Hgo flux was performed under laboratory conditions. Throughout the course of experiments, the ambient Hgo concentration was relatively constant (10.2 ± 0.8 ng m−3). The temperature was kept at 23 ± 0.5 °C and no temperature difference inside and outside the DFC was observed. The light intensity was constant under indoor fluorescent light (6.5 W m−2). Two DFCs were used in the flux verification experiments. One was the new DFC and the other was of semicylindrical shape made of quartz with a measurement area over soils of 0.06 m2.35 The potential Hgo release from the quartz wall in the presence of O3 45 was negligible in the Hg concentration range under measurement. Both ΔC and calculated fluxes were recorded at flushing flow rates of 3−32 L min−1. The two tested

(14)

where Sh is the Sherwood number defined as the ratio of convective transport to diffusive transport, Sc is the Schmitt number defined as the ratio of momentum (air) diffusivity (kinematic viscosity, m2 s−1) to mass (Hgo) diffusivity (m2 s−1), and L is the distance measured from the starting point of the measurement zone (m). Using eq 14, the overall mass transfer constant, kmass, can be estimated from the DFC dimensions and flow parameters. DFC Fabrication. The selection of the DFC material was based on four criteria: (1) the material allows adequate transmission of both visible and UV light because light is an important factor influencing Hg flux,42,43 (2) the material is mechanically strong and sufficiently rigid to avoid structural deformation that changes the internal velocity field,31 (3) the material is chemically inert for minimizing the DFC blank, and 8913

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Figure 3. CFD-predicted velocity profiles at the entrance and exit of the measurement zone along the center planes in horizontal and vertical directions.

dome shape (the PC-2 DFC in Figure 1 of,29 4.1 L), are shown in Supporting Information Figure S2. For the rectangular chamber, most of the air streamlines extend from the entrance directly toward the exit. Due to the gradual flow expansion, a fraction of the air flow hits the exit wall and creates back flow near the four side walls (Supporting Information Figure S2b), leading to the nonuniform Hgo vapor distribution (Supporting Information Figure S2c). For the dome-shape chamber, most of the streamlines quickly route through the chamber body toward the chamber exit at the top. There is also a backward circulation created near the side wall corner of the dome (Supporting Information Figure S2b). Since the surface velocity along the radial axis is nonuniform, the concentration gradient of Hgo is not uniform (Supporting Information Figure S2c). It was suggested that the internal flow pattern and concentration distribution can be significantly modified by changing the flushing flow rate.29 This increases data uncertainty when applying the measured fluxes in atmospheric models for estimating the magnitude of air-surface exchange, because the shear property of the soil surface and its relation to the measured fluxes is unknown. Flow Mechanics inside the New DFC. The CFDpredicted velocity profile inside the new DFC at a flushing flow rate of 10 L min−1 was analyzed. Supporting Information Figure S3 shows the velocity vectors near the entrance on the symmetric plane (i.e., the vertical plane at the center line of top view). As expected, air enters the DFC smoothly and the flow is quickly developed due to shallow depth of the DFC channel. The theoretical estimate indicates that 0.13 m is required for the flow to fully develop (eq 9). The CFD result shows that it takes only 0.07 m because the side walls of the chamber provides additional shear to the flow. The maximum velocity is identical to the theoretical value (0.028 m s−1), which is 1.5 times uave (0.0185 m s−1). Special attention was paid on the velocity profiles at the entrance and exit of the measurement zone. Figure 3 shows the velocity distributions at the entrance and exit along the center planes in both horizontal (top view) and vertical (side view) directions. The flow is mostly uniform along the Y axis except at near the sidewall (i.e., the boundary layer). The maximum velocity is 0.028 m s−1 at 10 L min−1. In the vertical plane, the

soils had Hg contents that differ by about an order of magnitude. One was a polluted agricultural soil (AS, THg = 37.1 mg kg−1), the other was an Hg-enriched soil (MS, THg = 448.6 mg kg−1). The THg was determined using an F-732 spectrophotometer (Shanghai Huaguang, China) after waterbathed digestion of soil samples in a mixture of concentrated HNO3 and HCl (v/v = 1:3) at 95 °C. Each dry soil was mixed thoroughly, sieved (≤2 mm), and placed in a wooden salver (40 × 40 × 5 cm) lined with polyvinyl film. Both DFCs were cleaned in a 10% HNO3 (v/v) bath for 24 h and then washed repeatedly by 18.2 MΩ Milli-Q water. DFC blanks were determined before each experiment by placing the DFC over a clean polycarbonate plate sealed to the DFC bottom. The blanks were reasonably low and considered negligible compared to the observed flux (new DFC: 0.6 ± 4.1 ng m−2 h−1, n = 67; quartz DFC: 0.3 ± 1.6 ng m−3 h−1, n = 53). At each flushing flow rate, the flux was measured five times and the standard deviation of the 5 replicates was shown as the error bar. Deployment of the New DFC for Field Measurement. The new DFC was deployed for field observations of Hgo flux at a landfill site (Gaoyan landfill, Guiyang, China) in September 2011. The site characteristics had been described.46 The soil roughness height there was estimated to be 10−2 m. The flushing flow rate was set at 15 L min−1. The field DFC blank was found to be 0.5 ± 2.7 ng m−2 h−1 (n = 46). This was about 2 orders of magnitude smaller than the mean measured flux and considered negligible. The meteorological parameters (solar irradiation, wind speed, temperature, etc.) were observed at 20min intervals using a portable HOBO weather station (HOBO U-30, Onset Corp., U.S.A.) installed near the DFC at 2 m height. The friction velocity was inferred from the wind velocity and the roughness height using eq 4. Additional discussion regarding the field deployment is provided in the Supporting Information, Note 5.

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RESULTS AND DISCUSSION

Flow and Hgo Concentration in Two Conventional DFCs. The internal flow and concentration fields obtained by CFD simulations for two conventional DFCs, one rectangular shape (L × W × H = 25 × 12.5 × 12.5 cm, 3.9 L) and the other 8914

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Effect of the Flushing Flow Rate on Model Results. Simulations using flow rates at 1−15 L min−1 were performed to understand the effect of the flushing flow rate on the flow structure and mass transfer. Supporting Information Figure S4 compares the spatial distribution of shear stress over the bottom surface at 5 and 15 L min−1. In both cases, the shear stresses are uniform and fully developed flow is achieved well before the measurement zone, indicating a wide operational flow range. The sidewall effect becomes smaller at 15 L min−1 because of the higher shear velocity, suggesting that a higher flushing flow rate improves flow consistency as long as the flow regime remains laminar. On the other hand, a fully developed flow can be reached in a shorter distance from the chamber entrance at the expense of greater sidewall effect. To validate the flow conditions, the CFD model results (eqs 10−12) are compared with those estimated by theoretical calculations (eqs 1−9 and 13−14). The shear stress is linearly proportional to the flushing flow rate and the values predicted by both approaches are practically identical (Figure 6). This is

profile is a perfect parabola as expected from a fully developed laminar flow. Furthermore, no difference exists between the velocity profile at the entrance and exit of the measurement zone, suggesting an excellent uniform flow. The velocity distributions are also plotted at the flow rates of 5 and 15 L min−1. The basic phenomena remain the same. The distribution of the shear stress over the bottom surface at 10 L min−1 flow rate is very uniform (Figure 4). The sidewall boundary layer

Figure 4. Distribution of the shear stress on the bottom surface of the DFC at 10 L min−1 flushing flow rate.

does affect the shear stress slightly but it is unavoidable and acceptable. CFD produces an average of 6.40 × 10−5 Pa, while the theoretical value (eq 2) is 6.37 × 10−5 Pa. Theoretically, Hgo release is directly related to the shear stress because it represents the convective driving force that removes evaded Hgo vapor from the substrate surface, which subsequently promotes Hgo diffusion through the boundary layer. A uniform shear stress distribution leads to a uniform mass transfer rate over the measurement surface. Figure 5 Figure 6. Effect of flushing flow rates of on shear stress and Hgo evasion flux. The theoretical values are based on the prediction by eqs 1−9 and eqs 13−14. The numerical averages are obtained from CFD simulations (eqs 10−12).

important since the shear property of the new DFC can be controlled accurately. Similarly, the Hgo fluxes estimated by both approaches agree with each other excellently. Under the assumption of constant Hgo at soil surface, the measured flux increases with the flushing flow rate and gradually levels off until it reaches a flux value that represents the mass transfer limit (Figure 6). Under this condition, diffusion in the boundary layer is no longer a limiting factor and the mass transfer is dominated by convective transport. One of the DFC design objectives is to establish a relationship between the flushing flow rate and friction velocity commonly used in atmospheric modeling. The relationship is given by eq 8 for the new design. The equation shows that the friction velocity (u*) for a given surface roughness length (z0) can be estimated by the cross-sectional area perpendicular to the flow direction, chamber thickness and the flushing flow rate. The magnitude of u* is linearly proportional to the flushing flow rate and can be accurately controlled. The roughness length for various land uses has been reported extensively (ranging from 10−5 m for ice surface to 1 m for tree cover).36 For soil surfaces, we recommended a value of 10−2∼10−3 m. The uncertainty of the calculated shear stress and mass transfer

Figure 5. Contour of the mercury concentration on the symmetric center plane and vertical exit plane of the measurement zone at 10 L min−1 flushing flow rate.

shows the spatial distribution of Hgo concentration on two different cross-sectional planes. On the vertical center plane, Hgo starts to evade when air moves over the measurement zone and diffuses into the air as the flow goes downstream. The development of mass transfer boundary layer can be clearly seen. At the exit of measurement zone, the distribution is well laminated across the Y axis, suggesting a uniform concentration gradient. The concentration near the side walls is more elevated due to the reduced velocity near the wall boundary. 8915

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Figure 7. Visualization of the 3-D velocity distribution inside the new DFC at: 10, 20, 30, and 50 L min−1. The velocity profiles at different distances along X axis (Figure 1c) are identical and therefore only one frame is shown for each flushing flow rate.

flux should be relatively small since the results obtained by two different modeling approaches are nearly identical (Figure 6). Measured Velocity Distribution inside the Fabricated DFC. The measured velocity profiles at different distances along the flow direction (X axis) were found to be identical, suggesting a fully developed, uniform flow throughout the measurement zone. The measured velocities were interpolated using to visualize the 3-D velocity field at various flushing flow rates over the Y−Z plane. In the flow rate range (10−50 L min−1), the shape of the measured velocity profiles (Figure 7) is consistent with the CFD predictions (Figure 3). The flow velocity along the Y axis is fairly uniform except at near two sidewalls. The internal velocity distribution shows a parabolic profile, suggesting a uniform flow condition over a flushing flow rate range of 10−50 L min−1. The peak velocities measured in the DFC was compared to those calculated by models at various flushing flow rates (Figure 8). The measured peak velocities increases linearly with respect to the flushing flow rate and agrees with the model predictions. The measured velocities undershoot the model-predicted values, but they are well within the uncertainty of the micro anemometer used in the measurement. The excellent linearity demonstrates that the flow condition is accurately controlled by the applied flushing flow rate. Effect of the Flushing Flow Rates on the Measured Flux. Model results show that the Hgo flux measured by the DFC increases with the flushing flow rate until it asymptotically approaches a value limited by diffusion mass transfer. To verify this behavior, Hgo evasion fluxes from the two soils were measured at 3−32 L min−1 flushing flow under laboratory conditions. The trend of the measured Hgo flux (Figure 9) resembles the trend predicted by models (Figure 6). As the flushing flow rate (and so the shear stress) increases, ΔC gradually decreases while the flux increases until leveling off (40 ng m−2 h−1 for AS and 547 ng m−2 h−1 for MS). The trend is

Figure 8. Measured and model-predicted peak velocities at the vertical center plane over measurement area.

consistent for both soils (Figures 9a and 9b). More importantly, ΔC and the Hg flux exhibit very small data variability in the 5replicate measurements. This demonstrates the performance of the DFC design and suggests that Hgo evasion under the laboratory conditions is controlled by the physical mass-transfer process. Figure 9 also confirms that the applied flushing flow rate has a strong influence on the measured Hgo flux for the same soil under identical environmental conditions (i.e., light, soil moisture and temperature). The difference in flushing flow rates can force about a 2-fold difference to the flux. For example, AS flux changed from 23 to 40 ng m−2 h−1 and MS flux changed from 306 to 547 ng m−2 h−1 over the flow rate range in the measurements. Similar phenomenon has also been 8916

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Figure 10. Measured Hg flux and ΔC at different flushing flow rates with a conventional semicylinder chamber over: (a) polluted agricultural soil (AS) and (b) high Hg-enriched mining soil (MS).

Figure 9. Measured Hgo flux and ΔC at different flushing flow rates using the new DFC: (a) over polluted agricultural soil (AS) and (b) over high Hg-enriched mining soil (MS).

higher flow rates (≥10 L min−1). Similar observations have also been observed.27,29 Third, the effect of flushing flow rates is more prominent compared to that using the new DFC (3- to 10-fold difference versus 2-fold difference over the flow rate range). It is also interesting that the flux measured by the new DFC is significantly higher than the value obtained by the conventional DFC at low flushing flow rate (≤5 L min−1). This could be a result of direct air flow from the entrance toward the exit in the conventional DFC, leading to a reduced surface shear. At higher flow rates, the measured fluxes are more comparable, probably due to the increased eddy turbulence inside the conventional DFC. Nevertheless, it is clear that the new DFC exhibits a much more predictable trend for both ΔC and fluxes (Figure 9) and significantly reduces data uncertainty, as evidenced by the smaller standard deviation of the laboratory data. The flux behavior of the new DFC is also consistent with the trend predicted by the two-resistance exchange interface model (TREIM).32 Based on the observed trends in ΔC and Hgo fluxes, we conclude that Hgo evasion from soil is highly dependent on the shear flow over the soil surface under measurement.

noted in earlier studies.27,29,47,48 Given the large variability in the shape and size of DFCs utilized in the past, the influence of flushing flow rate could be even greater yet unquantifiable. The measured flux difference caused by changing the flushing flow rate can exceed an order of magnitude in extreme cases.29 During the Nevada STORM study (7 DFCs with various flushing flow rates), the measured Hg fluxes varied from 51 ± 40 ng m−2 h−1 to 391 ± 165 ng m−2 h−1 at the same site.26,49 This highlights the importance of having a DFC design with predictable internal flow mechanics. Performance Comparison with a Conventional DFC. The performance of the new DFC was compared to that of a conventional DFC 33 under identical measurement conditions over the same soils (Figure 10). The measured fluxes and the observed ΔC have several distinct features that deserve attention. First, the error bars for both ΔC and Hgo fluxes of the 5 replicates are substantially greater, especially for the high Hg-content soil (MS). It is likely that the flow pattern inside the conventional DFC changes significantly at different flushing flow rates, leading to a greater data uncertainty. Second, there is not a consistent trend in the observed ΔC and measured fluxes as the flushing flow rate increases, particularly unpredictable at 8917

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Deployment of the New DFC for Field Measurement. The new DFC was deployed for Hgo flux measurement at a landfill site. For the first time, the surface shear property inside a DFC becomes predictable and controllable. A methodology that utilizes the measured flux to infer the flux under atmospheric condition is proposed here. From eq 13, the Hgo flux is proportional to the overall mass transfer coefficient, kmass. Therefore, the flux in the atmospheric boundary layer can be estimated from the measured flux if the ratio of the two overall mass transfer coefficients is known: Fa = Fm

k mass(a) k mass(m)

(16)

where Fa (ng m−2 h−1) is the inferred Hgo flux under atmospheric condition, Fm (ng m−2 h−1) is flux measured by the new DFC, kmass(a) (m s−1) is the overall mass transfer coefficient under atmospheric condition, and kmass(m) (m s−1) is the overall mass transfer coefficient in the DFC that can be estimated from the applied flushing flow rate (eq 14): ⎛ 0.03(H /L)(Q /Ac )(DH /D) k mass(m) = ⎜⎜4.86 + + H /L)(Q /Ac )(DH /D)]2/3 1 0.016[( ⎝ ⎞D ⎟⎟ ⎠H

(17)

where Ac is the flow cross-sectional area (0.009 m for the DFC) and DH is the hydraulic radius (0.0545 m for the new DFC). Other parameters have been defined previously. Note that the midpoint of the measurement zone is taken as L (0.15 m for the DFC). We propose using the friction velocity inferred from wind measurement to represent the surface shear property in the atmospheric boundary layer. Therefore, the overall mass transfer coefficient under atmospheric friction velocity can be estimated using eqs 8 and 14: 2

Figure 11. Measured and recalculated (inferred) Hg flux over a landfill site using the new DFC and the associated meteorological parameters: (a) time series and (b) scattered plot.

wind on the measured flux has not been noted because an enclosure method (i.e., DFC) isolates the soil surface from natural wind. Intuitively, wind should have an impact on the airsurface exchange because of its influence on surface shear. The flux increase caused by wind has been reported for flux measured by micrometeorological approaches.47,49,54,55 The inferred flux estimated by eq 16 shows that a greater wind velocity leads to a higher scaling factor of the measured flux (Figure 11b), suggesting the inferred flux could better represent the atmospheric flux. Supporting Information Figure S5 shows the correlation between the measured flux and the wind speed at 2-m height before and after the flux was corrected for the friction velocity using eq 16. Pearson’s correlation test showed weak correlation between the measured flux and wind but significant correlation (2-tailed, p < 0.01) after the flux was corrected for the surface shear. The capability to incorporate the effect of wind in flux measurements using DFC enhances the applicability of the DFC method, and potentially bridges the gap between the measured flux values obtained by enclosure and micrometeorological methods.

k mass(a) = ⎛ ⎞ 0.03(H /L)[Hu /(6kz 0)](DH /D) * ⎜⎜4.86 + ⎟⎟ 1 + 0.016{(H /L)[Hu /(6kz 0)](DH /D)}2/3 ⎠ ⎝ * D (18) H

Using eqs 16−18, the Hgo flux in the atmospheric boundary layer can be calculated from the flux measured using the new DFC (derivation shown in the Supporting Information, Note 6). The flux measured using the new DFC, solar irradiation, inferred friction velocity at the site, and the recalculated (inferred) flux are shown in Figure 11a. The measured flux, ranging from −87 to 421 ng m−2 h−1 with a net daily emission of 3151 ng m−2 d−1, shows a diurnal variation and is generally positively correlated with solar irradiation with the second peak less enhanced by insolation. Applying eq 16, the net daily emission under atmospheric condition is estimated to be 4955 ng m−2 d−1, about 1.5 times higher than the measured emission due to the higher friction velocity at the site compared to the internal friction velocity of the DFC. It has been demonstrated that environmental parameters, including temperature, solar irradiation, soil moisture, and ambient Hgo concentration can significantly influence the flux measured by a DFC.26−28,39,42,50−53 However, the effect of

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ASSOCIATED CONTENT

S Supporting Information *

Six technical notes and five figures regarding the DFC design consideration, fabrication and field measurement data are presented in the Supporting Information document. This information is available free of charge via the Internet at http:// pubs.acs.org. 8918

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(15) Pirrone, N.; Cinnirella, S.; Feng, X.; Finkelman, R. B.; Friedli, H. R.; Leaner, J.; Mason, R.; Mukherjee, A. B.; Stracher, G. B.; Streets, D. G.; Telmer, K. Global mercury emissions to the atmosphere from anthropogenic and natural sources. Atmos. Chem. Phys. 2010, 10 (13), 5951−5964. (16) Zehner, R. E.; Gustin, M. S. Estimation of mercury vapor flux from natural substrate in Nevada. Environ. Sci. Technol. 2002, 36 (19), 4039−4045. (17) Gustin, M.; Jaffe, D. Reducing the uncertainty in measurement and understanding of mercury in the atmosphere. Environ. Sci. Technol. 2010, 44 (7), 2222−2227. (18) Gustin, M. S. Are mercury emissions from geologic sources significant? A status report. Sci. Total Environ. 2003, 304 (1−3), 153− 167. (19) Sommar, J.; Zhu, W.; Lin, C.-J.; Feng, X. Field approaches to measure mercury exchange between natural surfaces and the atmosphereA review. Crit. Rev. Environ. Sci. Technol. 2012. (20) During, A.; Rinklebe, J.; Boehme, F.; Wennrich, R.; Staerk, H.-J.; Mothes, S.; Du Laing, G.; Schulz, E.; Neue, H.-U. Mercury volatilization from three floodplain soils at the Central Elbe River, Germany. Soil Sediment Contam. 2009, 18 (4), 429−444. (21) Feng, X.; Shang, L.; Tang, S.; Yan, H.; Liu, C. Gaseous mercury exchange rate between air and water over Baihua reservoir, Guizhou, China during cold season. J. Phys. IV 2003, 107, 451−454. (22) Ferrara, R.; Mazzolai, B. A dynamic flux chamber to measure mercury emission from aquatic systems. Sci. Total Environ. 1998, 215 (1−2), 51−57. (23) Poissant, L.; Casimir, A. Water-air and soil-air exchange rate of total gaseous mercury measured at background sites. Atmos. Environ. 1998, 32 (5), 883−893. (24) Sommar, J.; Waengberg, I.; Berg, T.; Gardfelt, K.; Munthe, J.; Richter, A.; Urba, A.; Wittrock, F.; Schroeder, W. H. Circumpolar transport and air-surface exchange of atmospheric mercury at NyAlesund (79 degrees N), Svalbard, spring 2002. Atmos. Chem. Phys. 2007, 7, 151−166. (25) Zhang, H.; Lindberg, S. E.; Marsik, F. J.; Keeler, G. J. Mercury air/surface exchange kinetics of background soils of the Tahquamenon River watershed in the Michigan Upper Peninsula. Water, Air, Soil Pollut. 2001, 126 (1−2), 151−169. (26) Gustin, M. S.; Lindberg, S.; Marsik, F.; Casimir, A.; Ebinghaus, R.; Edwards, G.; Hubble-Fitzgerald, C.; Kemp, R.; Kock, H.; Leonard, T.; London, J.; Majewski, M.; Montecinos, C.; Owens, J.; Pilote, M.; Poissant, L.; Rasmussen, P.; Schaedlich, F.; Schneeberger, D.; Schroeder, W.; Sommar, J.; Turner, R.; Vette, A.; Wallschlaeger, D.; Xiao, Z.; Zhang, H. Nevada STORMS project: Measurement of mercury emissions from naturally enriched surfaces. J. Geophys. Res., [Atmos.] 1999, 104 (D17), 21831−21844. (27) Lindberg, S. E.; Zhang, H.; Vette, A. F.; Gustin, M. S.; Barnett, M. O.; Kuiken, T. Dynamic flux chamber measurement of gaseous mercury emission fluxes over soils: Part 2Effect of flushing flow rate and verification of a two-resistance exchange interface simulation model. Atmos. Environ. 2002, 36 (5), 847−859. (28) Lin, C.-J.; Gustin, M. S.; Singhasuk, P.; Eckley, C.; Miller, M. Empirical models for estimating mercury flux from soils. Environ. Sci. Technol. 2010, 44 (22), 8522−8528. (29) Eckley, C. S.; Gustin, M.; Lin, C. J.; Li, X.; Miller, M. B. The influence of dynamic chamber design and operating parameters on calculated surface-to-air mercury fluxes. Atmos. Environ. 2010, 44 (2), 194−203. (30) Gao, F.; Yates, S. R. Simulation of enclosure-based methods for measuring gas emissions from soil to the atmosphere. J. Geophys. Res., [Atmos.] 1998, 103 (D20), 26127−26136. (31) Gao, F.; Yates, S. R.; Yates, M. V.; Gan, J. Y.; Ernst, F. F. Design, fabrication, and application of a dynamic chamber for measuring gas emissions from soil. Environ. Sci. Technol. 1997, 31 (1), 148−153. (32) Zhang, H.; Lindberg, S. E.; Barnett, M. O.; Vette, A. F.; Gustin, M. S. Dynamic flux chamber measurement of gaseous mercury emission fluxes over soils. Part 1: simulation of gaseous mercury

AUTHOR INFORMATION

Corresponding Author

*Phone: +1 409 880 8761 (C.-J.L.); +86 851 5891356 (X.F.). Fax: +1 409 880 8121 (C.-J.L.); +86 851 5891609 (X.F.). Email: [email protected] (C.-J.L.); [email protected] (X.F.). Author Contributions ¶

These authors (in alphabetical order) contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported in part by Natural Science Foundation of China (41030752, 41021062) and Texas Air Research Center (050LUB0110A). The authors gratefully acknowledge the support of K. C. Wong Education Foundation, Hong Kong.

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