Sulfate dry deposition fluxes and overall deposition velocities measured with a surrogate surface

The Science of the Total Environment 297 (2002) 193–201

Sulfate dry deposition fluxes and overall deposition velocities measured with a surrogate surface Mustafa Odabasi*, H. Ozgur Bagiroz Dokuz Eylul University, Faculty of Engineering, Department of Environmental Engineering, Kaynaklar Campus, 35160 Buca, Izmir, Turkey Received 8 November 2001; accepted 27 February 2002

Abstract Previous studies have shown that the dry deposition of particulate sulfate can account for a significant fraction of its total (wetqdry) deposition. However, there is no generally accepted method to directly measure dry deposition. In this study, the particulate sulfate dry deposition was measured using a smooth surrogate surface between September 2000 and June 2001 in Izmir, Turkey. Concurrently, ambient air samples were collected. Average particulate sulfate fluxes and ambient concentrations were 49.3"24.3 mg my2 dy1 and 11.2"6.6 mg my3 , respectively. The measured particulate dry deposition fluxes were higher than those measured in other urban and rural areas. The contribution of marine and terrestrial sources was also estimated and found to be insignificant. The particulate phase overall dry deposition velocities calculated using the dry deposition fluxes and corresponding ambient concentrations averaged 6.3"3.9 cm sy1. This value is higher than values typically used to estimate particulate sulfate deposition; however, it is in good agreement with values determined using similar surrogate surface techniques. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Sulfate; Dry deposition; Deposition velocity

1. Introduction Considerable research has been conducted to investigate the dry deposition of air pollutants. Sulfate is one of the major species that contributes to the acidic deposition to natural surfaces. This deposition has a deteriorating effect on buildings and vegetation, causing pH modification of ecosystems (Gorham, 1998). A recent study in the *Corresponding author. Tel.: q90-232-453-1008, ext. 1122; fax: q90-232-453-0922. E-mail address: [email protected] (M. Odabasi).

Izmir area indicated that dry deposition accounts for 70% of the total atmospheric deposition (wetq dry) of sulfate (Al-Momani et al., 1995). Despite its importance, there is no generally accepted method to directly measure or estimate dry deposition. The removal rate of atmospheric particles by dry deposition is a function of the physical (particle size, density, shape) and chemical properties of the aerosol, meteorological conditions (temperature, wind speed, atmospheric stability) and surface characteristics (terrain, vegetation, roughness). The understanding of how these fac-

0048-9697/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 8 - 9 6 9 7 Ž 0 2 . 0 0 1 2 7 - 4

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tors influence the dry deposition of particles is far from complete because of the complex interactions between these parameters and deposition (Zhang et al., 2001; Seinfeld and Pandis, 1998). The use of surrogate surfaces is one approach that has been used to directly measure dry deposition (Bidleman, 1988). Recently, a greased, smooth surrogate surface was successfully used to measure particulate fluxes of organic and inorganic air pollutants (Yi et al., 1997, 2001; Shahin et al., 1999, 2000; Odabasi et al., 1999; Cakan, 1999; Franz et al., 1998; Tasdemir, 1997). Since this surrogate surface does not significantly disturb airflow, it gives estimates of the lower limits of dry deposition to rougher, natural surfaces. Current dry deposition estimation methods often use measured air concentrations and modeled dry deposition velocities. These models assume the dry deposition flux of particles (Fp) can be estimated by using an overall particle dry deposition velocity (Vp) and particle phase air concentration (Cp): FpsVpØCp

(1)

To date there has been no consensus on the appropriate dry deposition velocity to use in these types of models. Estimated (Morales et al., 1998; Zeller et al., 1997a; Pratt et al., 1996) and experimental (Yi et al., 2001; Odabasi et al., 1999; Cakan, 1999; Franz et al., 1998; Tasdemir, 1997; Wyers and Duyzer, 1997; Holsen et al., 1991) dry deposition velocities range over an order of magnitude. One of the reasons for the discrepancy between the estimated and experimental dry deposition fluxes is that deposition velocity is a function of particle size. Gravitational settling has a significant effect on the deposition of coarse particles while Brownian motion dominates the deposition of very fine particles (-0.1 mm) (Seinfeld and Pandis, 1998). As the particle diameter increases above approximately 1 mm, the deposition velocities increase significantly (Zhang et al., 2001). For this reason, a multi-step modeling technique, which divides the fine and coarse particle distributions into a number of intervals and assigns an appropriate deposition velocity to each interval, gives a better estimate of dry deposition than the approach

shown in Eq. (1) (Holsen and Noll, 1992). Using this multi-step model, and other techniques, it has been found that coarse particles ()2.5 mm) and compounds associated with them are responsible for the majority of dry deposition of polycyclic aromatic hydrocarbons (PAHs), other semi-volatile organic compounds, some elements and sulfate (Yi et al., 2001; Yang et al., 1999; Kaupp and McLahlan, 1999; Lipiatou et al., 1997; Noll et al., 1997; Lestari, 1996; Holsen and Noll, 1992; Holsen et al., 1991). Recently, comparisons between many of the size dependent deposition velocity models found that they differ from each other significantly and the largest uncertainty is for the 0.1–1.0-mm particle size range, for which the deposition velocities can vary by 2–3 orders of magnitude (Zhang et al., 2001). Most of these models suggest that particles in the range of 0.1–1.0 mm diameter have deposition velocities smaller than 0.01 cm sy1. This value is not comparable to the significantly higher values obtained from field studies investigating some trace species (i.e. sulfate) (Zhang et al., 2001). The objectives of this work were: (1) to measure ambient particulate sulfate concentrations and dry deposition in Izmir, Turkey; and (2) to determine overall dry deposition velocity for particulate sulfate. 2. Experimental section 2.1. Sample collection Twenty concurrent dry deposition and ambient air samples were collected between September 2000 and June 2001 on the roof of a four-story building located on the Kaynaklar campus of the Dokuz Eylul University, Izmir, Turkey. The sampling site is located approximately 10 km southeast of Izmir’s center and surrounded by a growing coniferous forest. There are also residential areas located approximately 2 km southwest and a highway 0.5 km south of the sampling site. The flue gases from the stack of the fuel-oil fired boiler located on the sampling site interfered with the sampling during the weekdays in winter. Therefore, between 25 November, 2000 and 7 April, 2001,

M. Odabasi, H. Ozgur Bagiroz / The Science of the Total Environment 297 (2002) 193–201

samples were collected only on weekends when the boiler was not in operation. The particulate phase dry deposition flux was measured using a smooth, deposition plate (22.2=7.5 cm) with a sharp leading edge (-108) which is pointed into the wind by a wind vane. The dimensions of each greased strip placed on the deposition plate were 12.8=5.9 cm. Five greased strips, mounted on deposition plates with a total collection area of 378 cm2, were used for each deposition sample. Particulate sulfate in air was collected on 20.3=25.4 cm glass fiber filters using a highvolume sampler. All samples were collected during the daytime when there was no rain. Average sampling time was 8 h and the average sampling volume for air samples was 230 m3. 2.2. Sample preparation and analysis Glass fiber filters were rinsed with dilute nitric acid and deionized (DI) water, wrapped loosely with aluminum foil, and dried in an oven at 105 8C for several hours. Then they were allowed to cool to room temperature in a desiccator (Bagiroz, 2002; Odabasi et al., 1999). Mylar was cut into strips (12.8=7.5 cm) and the area to be greased (12.8=5.9 cm) was marked with a mechanical pen. Then the strips were rinsed with methanol and DI water. Cleaned Mylar strips were coated with Apiezon type L grease. Strips were mounted on dry deposition plates and ungreased areas were protected with PVC covers to prevent exposure to deposited material during field sampling (Odabasi et al., 1999; Cakan, 1999; Tasdemir, 1997; Yi et al., 1997). Cleaned glass fiber filters and dry deposition plates were transported to the field in containers without exposure to ambient air. After sampling filters and plates were placed back into their containers. The samples were brought back to the laboratory and stored in the dark until they were analyzed. Filter and plate samples were placed into polyethylene bottles containing 150 ml distilled water. Samples were extracted for 24 h in a water shaker at 60 8C and 270 rev.ymin. Then, sample extracts

195

were filtered through 0.45-mm membrane filters (Yi et al., 1997). Samples were analyzed for sulfate using the gravimetric method with drying of residue by Standard Methods for the Examination of Water and Wastewater (American Public Health Association, 1992). An analytical balance (Mettler AT 20) capable of weighing 0.002 mg was used to improve the detection limit of the method. The sulfate content of the soil samples (ns4) collected around the sampling site was also determined to investigate the contribution of the terrain sources to the measured sulfate fluxes. 2.3. Quality control Blank filters and dry deposition plates were routinely taken to the field to determine if there was any contamination during sample handling and preparation for analysis. The average amounts found in the blanks were 0.13 and 0.09 mg for filters and dry deposition plates, respectively. The limit of detection (LOD, mg) was defined as the mean blank mass plus three standard deviations, 3 S.D. (Cotham and Bidleman, 1995; Falconer et al., 1995; Halsall et al., 1994). LODs were 0.43 and 0.23 mg for filters and dry deposition plates, respectively. In general, sulfate amounts in the samples were substantially higher than blanks. Average sample amount (mg) to blank amount (mg) ratios were 18.8 and 7.3 for filters and plates, respectively. Sample quantities exceeding the LOD were quantified and blank-corrected by subtracting the mean blank amount from the sample amount. The analytical method was tested analyzing three artificial samples that were prepared by adding known amounts of sulfate to DI water. Average recovery of spiked amounts was 90%. 3. Results and discussion 3.1. Air concentrations In this study, particulate sulfate concentrations ranged from 2.1 to 25.9 mg my3 (average"S.D., 11.2"6.6 mg my3) (Fig. 1). These concentrations were similar to those measured at other urban sites

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Fig. 1. Ambient sulfate concentrations and dry deposition fluxes.

and higher than those measured at rural areas (Table 1). The flue gases from the stack of the fuel-oil fired boiler located on the sampling site interfered with the sampling during the weekdays in winter. Therefore, between 25 November, 2000 and 7 April, 2001 samples were collected only on weekends when the boiler was not in operation. It is possible that the air pollutant concentrations during weekends and weekdays can be different due to the changes in emissions. This possibility was investigated using the sulfur dioxide concentrations measured at two different stations in Izmir during the 2000–2001 winter period. No statistically significant difference (at the 95% confidence level) was found between the concentrations measured on weekends and weekdays. Therefore, the winter samples collected on weekends can be compared with the samples from other seasons collected on weekdays. Sulfate concentrations increased during the winter (November through March) (Fig. 1). On average, ambient sulfate concentrations measured in winter were approximately two times higher than

the concentrations measured during other seasons (spring, summer and fall). Average concentrations for winter and other seasons (15.3 and 8.0 mg my3, respectively) were statistically different (two-tailed t-test, 95% confidence level). Previous studies indicated that sulfur dioxide emissions, and as a result, ambient concentrations, increase during winter in the study area due to the increasing fossil fuel consumption for residential heating (Elbir et al., 2000). Therefore, the increase in sulfate concentrations during winter may be attributed to increased sulfur dioxide emissions. 3.2. Sulfate fluxes The range of particulate sulfate fluxes measured with the dry deposition plates was 12.9–93.0 mg my2 dy1 (average"S.D., 49.3"24.3 mg my2 dy1) (Fig. 1). These fluxes were generally higher than those previously reported for urban and rural areas (Table 2). For example, the average particulate sulfate flux for Chicago urban area was reported as 9.1 mg my2 dy1 by Yi et al. (1997). Al-Momani et al. (1995) measured an average

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197

Table 1 Particulate sulfate concentrations measured at various locations Concentration (mg my3)

Location

Area

Period

Reference

1.0–7.0a 0.3–1.0b

Chicago, IL, USA

Urban

Spring, Summer and Fall, 1994–1995

Noll et al. (1997)

4.7c

Maracaibo Lake Basin, Venezuela

Rural

Annual, 1988–1989

Morales et al. (1998)

0.32a 0.50b

Kowloon, China

Residential– commercial

April 1999

Tanner et al. (2001)

Agra, India

Suburban

November 1990–

Saxena et al. (1997)

Mumbai, India

Urban

January 1992

Venkataraman et al. (2001)

Minneapolis, USA

Urban

Annual, 1993

Pratt et al. (1996)

Kosan, Korea

Rural

September 1997

Lee et al. (2001)

Bulgaria

Rural

May 1992– October 1994

Zeller et al. (1997a)

Brdy Mountains, Czech Republic

Rural

July 1993– September 1995

Zeller et al. (1997b)

Izmir, Turkey

Suburban

September 2000– June 2001

This study

6.2b 11.0"3.9 3.6

d

c

5.9"0.8a 2.3"0.6

d

3.9"1.6c 11.2"6.6c a

Fine particles (dp-2.5 mm). Coarse particles (dp)2.5 mm). c TSP. d PM10 (dp-10 mm). b

sulfate flux of 4.5 mg my2 dy1 at a rural site in Izmir, Turkey. Similar to the air concentrations, particulate sulfate fluxes increased during the winter (November through March) (Fig. 1). On average, ambient sulfate fluxes measured in winter were 1.6 times higher than the fluxes measured during other seasons (spring, summer and fall). Average sulfate fluxes for winter and other seasons (62.9 and 38.2 mg my2 dy1, respectively) were statistically different (two-tailed t-test, 95% confidence level). The sampling site is 10 km away from the seashore. Therefore, the measured fluxes might be affected by sea-salt sulfate. The contribution of terrestrial sources to the atmospheric sulfate may be significant if the sulfate content of the local soil is high. Parmar et al. (2001) recently reported that 32% of the atmospheric sulfate measured in Agra, India was contributed from soil. Sodium and calcium are good markers for marine and terrestrial sources, respectively. In this study, the contribution

of marine and local terrestrial sources was estimated using the Na and Ca amounts from a concurrent study at the same site that investigated the elemental composition of the dry deposited material on the plates (Bozlaker, 2002). The following equations were used to calculate the amount (mg) of non-sea-salt sulfate (nss-SO2y 4 ) and the amount (mg) of non-soil sulfate (nsSO2y 4 ) (Watanabe et al., 2001; Parmar et al., 2001): 2y nss-SO2y 4 s(SO4 )sample y(SO2y 4 yNa)seawaterØ(Na)sample 2y 4

ns-SO

2y 4 sample

s(SO ) y(SO Ø(Ca)sample

2y 4

(2)

yCa)soil (3)

where (SO2y 4 )sample is the sulfate amount in the sample (mg), (SO2y 4 yNa)seawater is the concentration ratio of SO2y to Na in seawater (0.25) 4 (Parmar et al., 2001; Al-Momani et al., 1995), (Na)sample is the sodium amount in the sample

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Table 2 Sulfate dry deposition fluxes reported previously Sulfate flux (mg my2 dy1)

Method

Location

Period

Reference

Gradient

Coniferous forest, Netherlands

April–November 1993

Wyers and Duyzer (1997)

0.89

Ambient concentration and modeled deposition velocity

Brdy mountains, Czech Republic

July 1993– September 1995

Zeller et al. (1997b)

0.42

Ambient concentration and modeled deposition velocity

Coniferous forest, Bulgaria

May 1992– October 1994

Zeller et al. (1997a)

0.33

Ambient concentration and modeled deposition velocity

Urban, Minneapolis, USA

Annual, 1993

Pratt et al. (1996)

1.8–4.6

Polypropylene trays

Suburban, Agra, India

November 1990– January 1992

Saxena et al. (1997)

1.3

Polystyrene petri Dishes

Residential– commercial, Kowloon, China

April 1999

Tanner et al. (2001)

1.4

Plexiglass funnel

Rural, Maracaibo Lake Basin, Venezuela

Annual, 1988– 1989

Morales et al. (1998)

2.9–16.4

Dry deposition plates

Urban, Chicago, USA

July–October 1994

Yi et al. (1997)

1.0–4.0

Dry deposition plates

Urban, Chicago, USA

Spring, Summer and Fall 1994–1995

Noll et al. (1997)

4.5

Polyethylene Bucket

Rural, Izmir, Turkey

Annual, 1993

Al-Momani et al. (1995)

Dry deposition plates

Suburban, Izmir, Turkey

September 2000– June 2001

This study

5.0

49.3"24.3

(mg), (SO2y 4 yCa)soil is the concentration ratio of SO2y to Ca in local soil (0.013), and (Ca)sample is 4 the calcium amount in the sample (mg). It was assumed that all Na and Ca in the samples were contributed from marine and terrestrial sources, respectively. The contribution of sea-salt sulfate to the measured sulfate flux ranged between 0.2 and 15.5% (average"S.D., 2.2"3.4%) while the contribution of soil-sulfate flux ranged between 0.3 and 6.0% (average"S.D., 1.6"1.5%). These

results indicated that the influence of marine and local terrain sources on observed sulfate fluxes was not substantial. Therefore, the measured sulfate fluxes at the sampling site can be attributed to the anthropogenic sources. 3.3. Overall dry deposition velocities The overall dry deposition velocities for sulfate calculated by dividing the particulate fluxes meas-

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199

Fig. 2. Overall dry deposition velocities for sulfate.

ured with the surrogate surfaces by ambient particulate associated concentrations wEq. (1)x ranged from 2.0 to 16 cm sy1 with an overall average of 6.3"3.9 cm sy1 (Fig. 2). The particulate sulfate fluxes were correlated with the ambient particulate phase concentrations (statistically significant at the 90% confidence level) (Fig. 1). Reported values for the particle phase dry deposition velocities of sulfate and other compounds are summarized in Table 3. The ratio between the dry deposition velocities calculated in this study and previously reported values varied between 0.1 and 300. However, some of these values are not directly comparable to the results of this study because of differences in experimental procedures, estimation techniques used and physical properties of the particles (i.e. diameter, density). The agreement between the calculated dry deposition velocities in this study and the reported values using similar techniques is good (Yi et al., 2001; Odabasi et al., 1999; Cakan, 1999; Franz et al., 1998; Tasdemir, 1997; Holsen et al., 1991).

The calculated dry deposition velocities were also comparable to the modeled ones reported by Zhang et al. (2001). Recent studies indicated that a significant fraction of sulfate (up to 60%) is found on coarse particles (Parmar et al., 2001; Venkataraman et al., 2001; Tanner et al., 2001; Noll et al., 1997; Lestari, 1996). The results of the study by Noll et al. (1997) indicated that greater than 99% of the measured sulfate flux measured in Chicago, IL was due to coarse particles ()5 mm). Therefore, the discrepancy between the experimental and estimated dry deposition velocities may be due to coarse particles that were not taken into consideration in the dry deposition velocity estimates by Morales et al. (1998) and Zeller et al. (1997a). In summary, the measured sulfate concentrations and fluxes in this study were generally higher than the ones measured in other urban and rural areas. On average, ambient sulfate concentrations and fluxes measured in winter were approximately two times higher than those measured during other

200

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Table 3 Dry deposition velocities for sulfate and other compounds associated with the particles Speciesa

Vp (cm sy1)

Method

Reference

Sulfate Sulfate Sulfate Sulfate PM

0–3.0 0–4.0 0.10 0.10–0.30 0.02–0.4

Zhang et al. (2001) Wyers and Duyzer (1997) Morales et al. (1998) Zeller et al. (1997a) Gallagher et al. (1997)

DE

28.0–58.4

Cu and Zn Mg OC PCB PCB PCB PAH PAH PAH Sulfate

2.0 12.0 5.0"2.0 5.0 6.5"5.0 4.4–7.2 0.4–3.7 6.7"2.8 4.5"3.1 6.3"3.9

Modeled (3.8 mm particles) Gradient Modeled Modeled Eddy correlation (0.10–0.18 mm particles) Dry deposition plates (10– 100 mm particles) Dry deposition plates Dry deposition plates Dry deposition plates Dry deposition plates Dry deposition plates Dry deposition plates Dry deposition plates Dry deposition plates Dry deposition plates Dry deposition plates

Kim et al. (2000) Yi et al. (2001) Yi et al. (2001) Cakan (1999) Holsen et al. (1991) Tasdemir (1997) Franz et al. (1998) Franz et al. (1998) Odabasi et al. (1999) Vardar et al. (2002) This study

a Particulate matter (PM), diatomaceous earth (DE), polychlorinated biphenyls (PCB), organochlorine pesticides (OC), polycyclic aromatic hydrocarbons (PAH).

seasons, probably due to increased sulfur dioxide emissions from residential heating in winter. The contribution of marine and terrestrial sources was not significant. Therefore, the observed sulfate concentrations and fluxes can be attributed to the anthropogenic sources. The experimental particulate phase overall dry deposition velocities were higher than the values typically used to estimate particulate sulfate deposition. However, their agreement with the values determined using similar surrogate surface techniques was good. Acknowledgments The assistance of Ayse Bozlaker during the field sampling is greatly appreciated. References Al-Momani IF, Ataman OY, Anwari MA, Tuncel S, Kose C, Tuncel G. Chemical composition of precipitation near an industrial area at Izmir, Turkey. Atmos Environ 1995;29:1131 –1143. American Public Health Association, American Water Works Association, Water Environment Federation. Standard methods for the examination of water and wastewater (18th ed.). Washington, DC, USA, 1992.

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