Environ. Sci. Technol. 1997, 31, 272-278
Comparison of Dry Deposition Predicted from Models and Measured with a Water Surface Sampler SEUNG-MUK YI,† THOMAS M. HOLSEN,* AND KENNETH E. NOLL Department of Chemical and Environmental Engineering, Illinois Institute of Technology, 10W 33rd Street, Chicago, Illinois 60616
Atmospheric deposition, which is commonly classified as either dry or wet, has received a great deal of study over the past decade due to concerns about the effect of deposited material on the environment. Atmospheric deposition is an important mechanism controlling the fate of airborne toxics and their transfer from the atmosphere to natural surfaces. In this study, a circular water surface sampler was developed to measure the dry deposition flux of atmospheric gases and particles. The sampler consists of a sharp-edged, acrylic plate, filled with water (37 cm diameter and 0.5 cm deep) that is continuously replenished from a reservoir by a pump that maintains a constant water depth. To evaluate the water surface sampler, the sulfate flux was measured in Chicago, IL. Sulfate was selected as the test compound because it is deposited to water as both a particulate (SO42-) and a gas (SO2) (SO2 is quickly hydrolyzed and oxidized to SO42- in water). The SO2 fluxes measured directly with a water surface sampler were found to agree well with those predicted with two models. One model was an empirical model that accounts for both natural and forced evaporation, and the other model was a resistance model developed by analogy to electrical or heat flow through a series of resistances.
Introduction Atmospheric deposition, which is commonly classified as either dry or wet, has received a great deal of study over the past decade due to concerns about the effect of deposited material on the environment. Atmospheric deposition is an important mechanism controlling the fate of airborne toxics and their transfer from the atmosphere to natural surfaces. For example, approximately 95 and 58% of Lake Michigan’s current lead and PCB loading is from atmospheric deposition (dry + wet) (1). In spite of the fact that there is increasing awareness of the importance of dry deposition in the fate of airborne pollutants, many uncertainties exist in the methods used to measure and calculate dry deposition (2-5). The quantification of dry deposition is difficult due to large spatial and temporal variations and because interactions between the surface and the atmosphere can have large effects on the amount of deposited material. The use of a surrogate * To whom correspondence should be addressed. Telephone: 312567-3559; e-mail:
[email protected]. † Present address: Department of Environmental Engineering, Ewha Womans University, 11-1 Daehyon-Dong, Seodaemun Gu Seoul 120-750, Korea.
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surface to measure dry deposition is an increasingly important technique that can be used to directly assess deposited material and allows comparisons to be made between measured and modeled data (5). Previously, solid surfaces such as Teflon plates, various types of filters, polyethylene buckets, and Petri dishes have been used as surrogate surfaces (3, 6, 7). These studies have shown that the collector geometry has a large impact on the amount of material collected. Dry deposition studies by Noll et al. (8) showed that a greased strip on the top of a smooth knife-edge plate can be used as a surrogate surface for particle dry deposition measurements. The grease prevents particle bounce, and the surface geometry allows the application of flat plate boundary layer models developed from wind tunnel studies. Comparisons between dry deposition measured with this type of surface and dry deposition modeled from simultaneously measured complete size distributions using a multi-step model have been in good agreement (5). There are a number of problems associated with the use of greased surrogate surfaces for collection of dry deposition for compounds that exist in both the gas and the particulate phases. Greased surfaces are not very amenable to routine sampling of organic compounds, which are analyzed by gas chromatography due to difficulties in separating the analyte from the grease. In addition, there are uncertainties about the interaction between the grease used to collect particles and the vapor phase of the contaminant. In comparison to greased, solid surfaces, water surfaces exhibit some unique characteristics. Deposition of gaseous pollutants to water is controlled by a combination of atmospheric and surface resistances. Nonvolatile species such as trace metals deposit irreversibly. Dry deposition to a water surfaces is one of the key mechanisms that determines the direction and magnitude of pollutant movement in the ecosystem. In spite of all the advantages and importance of water surfaces, they have been scarcely used as a collection surface for dry deposition (9, 10). The goal of this study is to develop a water surface that can be used to measure the dry deposition of compounds that exist in both the gas and the particulate phases (Figures 1 and 2), to characterize the collection properties of the water surface, and to compare its properties with a greased knifeedge surrogate surface. To meet these goals sulfate, calcium, and lead fluxes were measured. Calcium and lead are deposited to water as a particulate while sulfate is deposited as both a gas (SO2) and a particulate (SO42-). SO2 mass transfer coefficients calculated from an empirical equation developed to account for both natural and forced water evaporation were also compared to values determined from direct measurements with the water surface. Two-Film Gas Exchange. To calculate the overall mass transfer coefficients, the two-film gas exchange model was used. This model assumes that transport of ambient SO2 across the air-water interface is dominated by molecular diffusion across thin air and water films. The following twofilm equation was used to calculate the overall SO2 mass transfer coefficient (11, 12):
SO2 flux ) 2KA1 (Ca - Ca*)
(1)
1 1 H tanh γ ) + KA1 2kA1 1kA2RT γ
(2)
where
2
Ca* )
S0013-936X(96)00410-5 CCC: $14.00
CwH RT
(4)
1996 American Chemical Society
xkDA2 k ) 1 DA2 k
x
γ ) yL
(3)
A2
2
where KA1 (cm/s) is the overall SO2 air-phase mass transfer coefficient across the air-water interface; 1kA2 (cm/s) is the individual SO2 water-phase mass transfer coefficient across the air-water interface; 2kA1 (cm/s) is the individual SO2 airphase mass transfer coefficient across the air-water interface; yL is the water film thickness; k is the reaction constant for a pseudo-first-order reaction (SO2(g) + H2O f H+ + HSO3-); DA2 is the diffusivity of SO2 in water; Cw (µg/m3) is the SO2 concentration in water; H (atm L-1 mol-1) is the Henry’s law constant that defines the equilibrium distribution of the chemical in air and water; R (0.08208 atm L-1 mol-1 K-1) is the gas law constant; T (K) is the ambient air temperature. Henry’s law constant was corrected for ambient temperature using the method of Maahs (13):
log
1 1376.1 ) - 4.521 H T
Equation 2 indicates that the overall resistance of SO2 transfer across the air-water interface is the sum of the resistance in the air phase (1/2kA1) and water phase
(
1
)
H tanh γ kA2RT γ
The water-phase resistance of SO2 is characterized by the individual SO2 water-phase mass transfer coefficient (1kA2) and is controlled by the reaction of SO2 in the water phase. The hydrolysis and oxidation of SO2 in water are extremely rapid (i.e., k and γ in eqs 3 and 4 are large) so the water-phase resistance for SO2 is negligible compared with the air-phase resistance (14-18). Under these conditions, eq 2 reduces to
1 1 ) 2 KA1 2kA1
kA1 ) 2kA1N + 2kA1F ) (0.14GrB11/3 + 0.664ReLam0.5)DA1ScA11/3 (6) LLam
for laminar flow, and 2
area weighted average 2kA1 ) 9
kA1 ) 2kA1N + 2kA1F ) (0.14GrB11/3 + 0.036(Retotal0.8 - ReLam0.8))DA1ScA11/3 (7) Ltotal - LLam
for turbulent flow where GrB1 ≡ [gζB1L3(yBi - yB)]/v2, ScA1 ≡ v/DA1, Retotal ≡ (v10Ltotal)/v, and ReLam ≡ (v10LLam)/v. Ltotal is the total length of a deposition surface; LLam is the total length of the deposition surface in the laminar flow regime; g is gravitational acceleration; ζB1 is the concentration coefficient of volume expansion; yB is the water vapor concentration (i, interface); v is the kinematic viscosity of air; DA1 is the diffusion coefficient in the air; and v10 is the wind speed at 10 m above water surface. The overall SO2 mass transfer coefficient (2KA1) can also be calculated using eq 1 by dividing measured SO2 flux (water surface minus greased surface flux) by the ambient SO2
9
∑( k 2
A1)i,Lam(area)i,Lam
i)1
∑( k 2
+
A1)i,Turb(area)i,Turb
i)1
(8)
9
∑(area)
i
i)1
(5)
Thibodeaux (11) provided an empirical equation to calculate 2kA1 based on measurements of the evaporation of water. The empirical equation involves both natural and forced evaporation and can be used to calculate the H2O mass transfer coefficient. The individual SO2 air-phase mass transfer coefficient through the stagnant air film is correlated with the H2O mass transfer coefficient as follows: 2
concentration (assuming Ca* is equal to 0). This overall coefficient should be equal to the individual SO2 mass transfer coefficients (2kA1) calculated from eqs 6 and 7 using meteorological data and the geometry of the water surface sampler. Calculation of Critical Reynolds Number for the Water Surface. For flow past a flat plate, the critical Reynolds number for the transition from laminar to turbulent flow varies with experimental conditions. Thibodeaux (11), Crawford (19), Friedlander (20), and Reist (21) have all reported critical Reynolds numbers for a flat plate in the range of 104106. In this study, 105 was used for critical Reynolds number as recommended by Thibodeaux (11) because eqs 6 and 7 are based on this critical Reynolds number. In order to calculate 2kA1, the location where the flow regime becomes turbulent must be determined for use in eq 6 or 7. To determine this location, the water surface plate was divided into seven segments 5 cm wide parallel to the wind direction and two segments 2 cm wide. After determining the location calculated from the critical Reynolds number (LLam ) 105 × v/v10) in each segment, the appropriate theoretical equation for laminar flow (Re < 105) and turbulent flow (Re > 105) was used to calculate individual SO2 air-phase mass transfer coefficients of each segment using eqs 4 and 5. After calculating the individual SO2 air-phase mass transfer coefficient for each segment, the appropriate area for each flow regime was multiplied by the corresponding mass transfer coefficient. The area-weighted average individual SO2 airphase mass transfer coefficients for the water surface sampler was then calculated as shown below:
where i is the segment, (area)i is the area of i segment (width of i segment × length of i segment), (area)i,Lam is the laminar flow regime area of i segment, and (area)i,Turb is the turbulent flow regime area of i segment.
Experimental Section Sampling Program. Ambient dry deposition samples were collected with both the water and knife-edge surrogate surfaces. The samplers were exposed during periods of no rain or threat of rain at the IIT sampling site located near downtown Chicago. The water surface measured the dry deposition of both the gas and the particulate phases (SO2 + SO42-), while the greased surface measured the dry deposition of only the particulate phase (SO42-) (Figure 1). Ambient calcium and lead fluxes were also measured simultaneously with both surfaces to determine if the two surfaces measured the same amount of particulate dry deposition. SO2 fluxes to the water surface and greased surface were also measured in a laboratory chamber to compare the flux of SO2 gas to the two surfaces and to determine if the greased surface collected significant amounts of SO2. Meteorological data were obtained concurrently at the IIT sampling site, and ambient SO2 measurements were obtained from a sampling site at the University of Chicago located approximately 3 km south of the IIT sampling site. Sampling information is summarized in Table 1. Description of Sampling Sites. Illinois Institute of Technology (IIT), Chicago, IL (Urban Site, Southwest Shore of Lake Michigan). The water and greased surfaces were exposed to the atmosphere on a 1.4 m high platform on the roof of a four-story building (11.6 m height) located in a mixed commercial and residential area on the south side of Chicago. The site is located on the campus of IIT, which is located 5
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FIGURE 1. Comparison of dry deposition to a greased strip on a knife-edge surrogate surface and the water surface sampler.
TABLE 1. Summary of Sample Information and Data Calculationsa
av wind direction
sampling date
sampling time (min)
land
7/20/94 7/21/94 7/22/94 7/31/94 9/4/94 9/12/94 9/13/94 9/14/94 10/6/94 10/27-10/28/94
260 390 540 635 450 600 505 620 540 1120
5/9/94 5/10/94 5/17/94 5/18/94 5/20/94 8/29/94 9/2/94 9/9/94 9/29/94 9/30-10/6/94 10/11-10/15/94 10/20-10/22/94
525 520 570 455 565 615 470 340 625 2495 2605 1465
av lake
av
water surface strip SO2 overall sulfate flux sulfate flux SO2 fluxb av SO2 mass transfer av av wind coeff (2KA1)c temp av RH speed (mg m-2 (mg m-2 concn (mg m-2 (µg/m3) day-1) day-1) day-1) (cm/s) (°C) (%) (m/s) 23.16 17.84 25.42 21.18 55.38 51.87 30.48 36.86 38.13 23.84 32.42 17.73 9.29 10.73 17.22 25.08 12.35 30.62 19.31 9.87 13.40 31.56 10.03 17.27
16.40 12.79 7.56 12.48 6.99 10.04 12.68 4.85 2.94 4.21 9.09 6.41 4.12 8.32 10.44 11.69 5.54 12.64 4.42 8.55 1.00 4.26 1.92 6.61
4.51 3.37 11.91 5.80 32.26 27.89 11.87 21.34 23.46 13.09 15.55 7.55 3.45 1.61 4.52 8.93 4.54 11.99 9.93 0.88 8.27 18.20 5.41 7.11
3.59 2.23 10.12 7.30 26.75 38.07 13.74 12.83 18.53 8.01 14.12 4.74 5.73 1.38 2.74 12.46 7.69 20.20 20.14 4.19 13.51 24.20 12.15 10.76
1.45 1.75 1.36 0.92 1.40 0.85 1.00 1.93 1.47 1.89 1.40 1.84 0.70 1.35 1.91 0.83 0.68 0.69 0.57 0.24 0.71 0.87 0.52 0.91
31.3 27.2 25.5 28.0 18.5 28.6 29.8 30.9 21.6 14.3 25.6 19.7 14.4 10.1 11.7 21.1 22.6 18.6 25.6 16.7 18.1 15.6 18.3 17.7
78.0 72.9 76.6 60.3 68.3 55.3 54.4 63.3 56.2 49.6 63.5 43.5 47.6 68.8 64.7 47.6 60.6 70.0 75.7 65.9 67.0 79.6 57.7 62.4
4.62 3.63 4.38 3.76 2.83 3.37 4.92 3.86 4.43 5.05 4.08 4.10 2.75 4.15 4.82 2.42 2.60 1.74 2.46 2.48 3.14 2.65 2.62 2.99
av wind direction land land land land land land land land land land lake lake lake lake lake lake lake lake lake land, lake lake land, lake
a All samples were collected between 08:00 and 19:00. b SO flux ) (water surface sulfate flux - strip sulfate flux) × (64/96). (64/96) is the unit 2 conversion factor (sulfate to SO2). c SO2 overall mass transfer coefficient (2KA1) (cm/s) ) SO2 dry deposition velocity ) SO2 flux/av SO2 concn × (105/86400). (105/86400) is the unit conversion factor (m/day to cm/s).
km south of Chicago’s center and 1.6 km west of Lake Michigan. Surrounding the site are campus buildings, open grassy areas, and parking lots. This sampling site has been used in the Lake Michigan Urban Air Toxics Study (LMUATS) and the Lake Michigan Mass Budget/Mass Balance Study. University of Chicago, Chicago, IL (Urban Site, Southwest Shore of Lake Michigan). Ambient SO2 concentrations were taken on a 2 m high platform on the roof of 13 m high building located in a residential area on the south side of Chicago. The site is located on the University of Chicago campus on the Geosciences building, 8 km south of Chicago’s center and 2.5 km west of Lake Michigan. Traffic in the immediate vicinity is local, providing access to the campus. The site is on one of the tallest buildings in the area.
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Sampling Instruments. Description of the Water Surface Sampler. Figure 2 is a diagram of the water surface sampler and identifies the various components that are described below. Water Surface Holder. The shape of the water surface holder was selected to provide minimum air flow disruption and to allow comparison with other smooth surface samplers (8, 22). The water surface holder is 50 cm in diameter and has an airfoil shape with a leading edge angle of attack of less than 10° to minimize airflow disruptions caused by the collector geometry. Davidson et al. (7) have characterized a symmetrical low-speed airfoil (frisbee-shaped) for wind speeds below 15 m/s and angles of attack of the wind less than (10°. This geometry provides minimum air flow
leading edge (2.5 µm) while lead is associated primarily with fine particles (
