Environ. Sci. Technol. 1999, 33, 2113-2117
Dry Deposition of Reduced and Reactive Nitrogen: A Surrogate Surfaces Approach USAMA M. SHAHIN,† XIANG ZHU,‡ AND T H O M A S M . H O L S E N * ,† Department of Civil and Environmental Engineering, Clarkson University, Potsdam, New York 13699, and Department of Chemical and Environmental Engineering, Illinois Institute of Technology, Chicago, Illinois 60616
Nitrogen dry deposition causes pH modification of ecosystems, promotes eutrophication in some water bodies, interferes with the nutrient geochemical cycle on land, and has a deteriorating effect on buildings. In this study, a water surface sampler (WSS) and knife-leadingedge surrogate surface (KSS) covered with both a Nylasorb filter and a greased disk were used to directly measure nitrate dry deposition in Chicago between May and October 1997. Concurrently, the KSS covered with both a citric acidcoated paper filter and a greased disk and the WSS were used to measure ammonia dry deposition. The average measured dry deposition flux for HNO3 was 3.78 ( 1.24 mg m-2 day-1; for particulate nitrate, it was 1.46 ( 0.3 mg m-2 day-1; and for ammonia gas, it was 2.64 (1.15 mg m-2 day-1. Nitrate fluxes to the WSS and Nylasorb filter on the KSS were statistically equal, as were the total ammonia fluxes to the WSS and the citric acid-impregnated filter on the KSS. The experimental measurements indicated that HNO3 and particulate nitrate were the major species responsible for the nitrate flux to the WSS and that ammonia gas was the major source of deposited ammonia. The average mass transfer coefficients (MTCs) of HNO3 and NH3 to the WSS were 1.5 ( 0.22 and 2.46 ( 1 cm/s, respectively. SO2 and HNO3 MTCs were statistically the same. After adjusting for the differences in molecular weights, the HNO3 and NH3 mass transfer coefficients were statistically equal to the SO2 MTC.
Introduction A great deal of research has been conducted to investigate dry deposition to natural surfaces. One area in particular that has received a great deal of study recently is acidic and basic deposition. This deposition has a deteriorating effect on buildings, causes pH modification of ecosystems, promotes eutrophication in some water bodies, and interferes with the nutrient geochemical cycle on land. Sulfates and nitrates are the major species that contribute to the acidic deposition to natural surfaces, while ammonia is the most prevalent alkaline gas in the troposphere (1-3). Several approaches had been used in an attempt to quantify dry deposition including direct measurements using surrogate surfaces, micrometeorological, eddy accumulation, * Corresponding author phone: (315)268-3851; fax: (315)268-7636; e-mail:
[email protected]. † Clarkson University. ‡ Illinois Institute of Technology. 10.1021/es981299c CCC: $18.00 Published on Web 05/06/1999
1999 American Chemical Society
and gradient methods (4, 5). To date, there is no accepted technique that can be used to evaluate these approaches, which has led to large uncertainties in dry deposition estimates. For example, the largest uncertainties associated with estimating the relative role of atmospheric deposition of nitrogen to inland and estuarine waters are associated with (i) accurately quantifying the dry deposition flux and (ii) determining the indirect atmospheric loading via watershed runoff (6). The use of a surrogate surface to measure dry deposition is an increasingly important technique that can be used to directly assess deposited material and allow comparisons to be made between measured and modeled data (1). Previously, solid surfaces such as Teflon plates, various types of filters, polyethylene buckets, and Petri dishes have been used as surrogate surfaces (5, 7, 8). These studies have shown that both the collector geometry and the surface characteristics have a large impact on the amount of material collected. For example, aerodynamically designed surfaces such as those used in this study collect less deposition than rougher surfaces. Greased surrogate surfaces prevent particle rebound and resuspension, which may be important for some natural surface. The greased surface used in this study is hydrophobic, preventing the interaction of polar gaseous species such as SO2, HNO3, and NH3 with the collection surface. In comparison to greased, solid surfaces, a water surface acts as an infinite sink for both particles and polar gaseous species. The regional deposition models of Levy and Moxim (9), Logan (10), and Sirois and Barrie (11) indicate that dry deposition accounts for 46-63% of the total atmospheric deposition of nitrates. These models agree well with the measurements made using a vegetative throughfall approach, which found the dry deposition of NO3- to be 60% of the total atmospheric input (15). For NH3 and NH4+, dry deposition has been reported to comprise 30-63% of the total deposition (13, 14). On the basis of the watershed mass balance approach, Fisher and Oppenheimer (15) and Hinga et al. (16) estimated that dry deposition accounts for 4061% of the total atmospheric nitrogen deposition to the Chesapeake Bay. The relative contribution of dry deposition reported for 1984-1987 by the National Dry Deposition Network (30-45%) was similar to the amounts cited above. Due to the uncertainties that exist in both modeling and measuring the deposition of nitrogen species, this study was undertaken to develop new surrogate surface measurement techniques. This project had the following specific objectives: (i) measuring the dry deposition of nitrogen-containing compounds using surrogate surfaces techniques; (ii) determining the relative importance of the different nitrogen sources to the measured deposition; (iii) quantifying the mass transfer coefficient (MTC) of nitric acid and ammonia gas to the water surface sampler; and (iv) comparing the MTCs of sulfur dioxide, ammonia, and nitric acid to the surrogate surfaces.
Materials and Methods Ambient Concentration Measurement. To measure ambient concentrations of gaseous HNO3, SO2, NH3, sulfates, nitrates, and ammonium salts in the fine particles, the U.S. EPA recommended annular denuder system (ADS) (17) was used. It consisted of a cyclone, four denuder tubes, and a backup filter pack. The first denuder tube was coated with NaCl (0.1% NaCl +1% methanol solution), the second and the third tubes were coated with Na2CO3 (1% Na2CO3 +1% glycerol + 50% methanol solution), and the fourth tube was coated with 1% citric acid solution in methanol as a solvent. Ambient air was VOL. 33, NO. 12, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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drawn through a Teflon-coated aluminum cyclone, which removed particles larger than 2.5 µm in size. The denuder tubes followed the cyclone where gaseous HNO3, SO2, and NH3 were removed from the airstream. The fine particles were removed on the backup filter. After sampling, the denuder was taken to the laboratory where it was disassembled in a particle-free hood. Each denuder tube was extracted with 15 mL of double-distilled deionized water (DDW). The extracted solution of the second and third denuder tubes was treated with 15 µL of 3% H2O2 solution, which oxidizes SO32- to SO42-. The extraction solution was filtered and stored in 60-mL polyethylene bottles until analysis. Complete denuder cleaning, field use, extraction, and atmospheric concentration calculation procedures are available elsewhere (17). Deposition Flux Measurement. Nitrate, sulfate, and ammonia dry deposition were measured as a part of this research. Different samplers were developed to measure the deposition flux of the different phases of these species. The water surface sampler (WSS) was used to measure particulateand gas-phase fluxes while greased Mylar disks on the KSS were used to measure particulate fluxes. Nylasorb filters on the KSS were used to measure nitric acid and particulate nitrate fluxes. Citric acid-impregnated paper filters on the KSS were used to measure ammonia gas and particulate ammonia fluxes. The WSS and KSS had sharp leading edges to minimize the disruption of airflow and consequently minimize boundary layer resistance. For this reason, they collect less deposition than would rougher surfaces. Water Surface Sampler (WSS). A brief description of the water surface sampler is given here. It is described in more detail in Yi et al. (18). The WSS had four major parts: a water surface holder, circular water surface plate, a pump, and a reservoir system. The holder was 40 cm diameter, 10 cm deep bowl that had a 100 cm diameter circular flat plate fit to its top. A 37 cm diameter acrylic plate with overflow weir was held inside the holder bowl so that there was a 1 cm open area between the holder and the weir. When the WSS was in use, the flat plate was horizontal, and the surface of the water (1 cm deep) was at the same height as the top of the sharp leading edge. The sampling water contained 2 g/L sodium azide to inhibit biological activity. Sampling began by pumping water to the center of the plate and allowing the sampling water to fill the 450 mL volume between the center of the plate and the weir. The sampling water flowed over the weir into the bowl and then into a 4-L reservoir by gravity. The sampling water was continuously circulated between the water surface plate with a pump at a rate of 500 mL/min. The WSS was allowed to run for 20 min to clean the plate and bowl before sampling commenced. Blanks were taken from the sampling water, and the bottle was weighed before sampling. At the end of the sampling period, the pump was shut off, the sampling water was weighed again, and samples were taken. The WSS samples and blanks were filtered using a 30-mL syringe and a 0.2-µm ion chromatography filter and stored in clean polyethylene bottles. The WSS was used to measure the flux of particulate- and gas-phase sulfate, nitrate, ammonia, and nitrogen oxides. Greased Mylar Disks. Particulate dry deposition was measured using a knife-leading-edge surrogate surface (KSS) covered with Mylar disks (Graphic Arts Systems, Cleveland, OH) 47 mm in diameter coated with about 1.5 mg of Apezion L grease. The grease is nonpolar and has very little interaction with polar gas-phase compounds like NH3, SO2, and HNO3 (19). A Mylar cover with a hole 45 mm in diameter was placed over the ungreased edge of the collection disk to help hold the greased disk to the KSS. The Mylar disk and covers were cut to the correct size and cleaned with double distilled methanol and rinsed twice with deionized water. The disks 2114
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were then placed into a laminar flow particle-free airstream where they were allowed to dry. After being dried, the disks were greased and stored in a dust-free box until they were used. Nylon Filters. Nylon (Gelman Nylasorb 47 mm diameter, 1 µm pore size) on the KSS was used to collect nitric acid and particulate nitrate deposition. Nylon is a perfect sink for HNO3 (20). It has very limited interaction with NO and NO2 (21, 22). Nylasorb filters were attached to the KSS with a 45 mm diameter Mylar cover placed around the edges of the Nylon filter. Citric Acid-Impregnated Fiber Glass Filters. A paper filter impregnated with citric acid on the KSS was used to collect ammonia gas and particulate ammonia deposition. Citric acid is an organic acid that reacts rapidly with ammonia gas to form ammonium citrate. Citric acid is used as a coating acid in the denuder due to the stability of ammonium citrates formed by the reaction of ammonia gas and citric acid. The paper filters used were Whatman 47 mm diameter, 1 µm pore size. The paper filters were cleaned with DDW and methanol. The cleaned filters were soaked in a 1% citric acid solution in methanol as a carrier solvent. Paper filters were fixed to the KSS in the same manner as the Nylasorb filters and Mylar disks. Sampling Site. Samples were collected at the Illinois Institute of Technology (IIT) in Chicago. The sampling equipment was mounted on a 1.4 m high platform on the roof of a four-story building (11.6 m height). The site is located 5 km south of Chicago’s center and 1.6 km west of Lake Michigan in a mixed commercial and residential area on the southside of Chicago. Surrounding the site are campus buildings, open grassy areas, and parking lots. The top of the KSS and WSS were 1.5 m above the platform and out of the calculated roof wake boundary (23, 24). This site is the urban research and monitoring site of the Integrated Atmospheric Deposition Network (IADN). Sampling Program. Samples were collected from May to October 1997. During this sampling period, 52 nitrate and sulfate samples were collected (32 daytime and 20 nighttime). Nitrate deposition was collected using dry deposition plates containing greased disks, and Nylasorb filters, and the water surface sampler (WSS). Sulfate deposition was collected using the WSS and greased Mylar disk attached to the KSS. Concurrently, HNO3 and SO2 concentrations were measured using the ADS. Concentrations of nitrate- and sulfatecontaining particles smaller than 2.5 µm were measured using the backup filter attached to the ADS for the first 29 samples. There were a total of 23 ammonia samples divided into 16 daytime and 7 nighttime. Ammonia deposition was collected using the KSS containing both greased disks and citric acid-impregnated paper filters and the WSS. The ambient NH3 concentration was measured using the ADS. Typically, daytime sampling started at 9 AM and concluded at 6 PM. Nighttime sampling began at 9 PM and concluded at 6 AM. The average sampling time for all samples was 9 h. Analytical Methods. Anion Extraction. After sampling, greased disks and Nylasorb filters were placed in 60-mL polyethylene bottles, and the bottles were filled with 15 mL of carbonate buffer (1.8 mM sodium carbonate and 1.7 mM sodium bicarbonate). Next, the samples were extracted in an ultrasonic bath at 60 °C for 4 h, after which time the bottles were allowed to cool to room temperature. The samples were filtered using a 30-mL syringe and a 0.2-µm ion chromatography filter and stored in clean polyethylene bottles. Cation Extraction. The cation samples (greased disk and paper filters) were extracted following the same procedures as the anion samples except the extraction solution was 10 mM methanesulfonic acid.
TABLE 1. Summary of the Average Field Blank for the Different Samples % of the average measured concentration sampler
SO2
HNO3
NH3
denuder greased disk Teflon filter Nylon filter (flux) nylon filter (concn) paper filter (flux) paper filter (concn)
4.3 4 1.2 NAa NA NA NA
4.7 2.4 10 15 4 NA NA
8.7 0 2.3 NA NA 4.5 4.3
a
NA, not applicable.
Ion Concentration Determinations. The nitrate and sulfate concentrations in the water, on the greased disks, on the Nylasorb membranes, and in ADS samples were measured with a Dionex DX-100 ion chromatograph with AS4A-SC (4 mm) column. NH4+ concentrations were determined using Hach calorimetric technique kits (Hach Company, Loveland CO). The color change was monitored by using a DR2000 Hach spectrophotometer. Quality Assurance/Quality Control. Greased disks dry deposition sample preparation and collection followed the procedures detailed in the Lake Michigan Mass Balance Study (LMMS) Methods Compendium (25). SO42- and NO3- concentrations were measured following the procedures detailed in ref 17. Calibration of the ion chromatograph (IC) was performed for each sample batch or every 500 min by measuring the concentration of a serial dilution of known concentration. Calibration was acceptable only if r 2 > 98%. Field blanks were taken at a rate of 10% of the field samples. They were handled in the same manner as regular samples without performing actual sampling. All samples were blank corrected (Table 1). All the measured concentrations and fluxes in this study were significantly higher than instrumental detection limits.
Results and Discussion To examine the relationship between wind direction and dry deposition, the samples were divided into “land” and “lake” sectors. The urban and industrialized area of Chicago borders the sampling site on the south through the northwest while the northeastern to southeastern boundary is Lake Michigan. The winds were classified based on the average wind direction taken from the north at the sampling station. Winds with an average wind direction of 0-140° were classified as lake, otherwise the samples were classified as land. Total Flux Measurements. Nitrate Flux. The average total nitrate flux to the WSS was 5.24 ( 1.24 mg m-2 day-1, which was significantly higher than the particulate flux (1.46 ( 0.30 mg m-2 day-1) measured with the greased disk on the KSS (mean ( 95% confidence interval). Previous studies indicated that the dry deposition flux of lead, a primarily anthropogenic primarily found in the submicron particles size range, was statistically the same to the greased disk and the WSS. Similarly, the flux of calcium, a primarily crustal element found in the coarse particle size range, to both samplers was statistically the same (18). This finding indicates that both these surrogate surfaces have similar characteristics for atmospheric particle collection. The difference between these two nitrate fluxes is therefore due to the deposition of nitratecontaining gases or gases that are precursors of nitrate to the WSS. The gas-phase nitrate deposition contributes from 50 to 86% of the total nitrate dry deposition with an average of 73% for both the land and lake samples (Figure 1).
FIGURE 1. Comparison between gas-phase and particulate-phase nitrate flux to the WSS. The gas-phase nitrate contributed from 14 to 50% of the total nitrate deposited.
FIGURE 2. Total ammonia flux to the WSS and citric acid-coated paper filter. Statistically, these fluxes were found to be equal. The land sector samples had statistically the same average total and particulate nitrate fluxes as the lake samples (5.18 (1.62 vs 5.3 ( 1.93 mg m-2 day-1 and 1.56 ( 0.48 vs 1.37 ( 0.38 mg m-2 day-1, respectively). This similarity is probably due to the complex chemistry of nitrogen coupled with varying wind direction during sampling. Both the average total and particulate nitrate fluxes in the daytime were statistically higher than at nighttime (6.69 ( 1.72 vs 2.82 ( 0.99 mg m-2 day-1 and 1.55 ( 0.42 vs 1.31 ( 0.43 mg m-2 day-1, respectively). Higher daytime fluxes are due to the dependence of nitrate deposition on nitric acid ambient concentration. Nitric acid is formed in the atmosphere by photochemical oxidation of NO2 at the daytime and decomposition of NO3 at the early afternoon. Recent estimates of nitrogen loading to Chesapeake Bay and Delaware Bay were 3.85 and 4.45 mg m-2 day-1, respectively (6), which agrees well with the results of this study (3.22 mg m-2 day-1). The agreement with the East Coast data may indicate that particulate NO3- and HNO3 concentrations are similar in both regions. Ammonia Flux. Total ammonia flux was measured by the WSS while the particulate ammonia flux was measured with the greased disk on the KSS. The ammonia flux to the WSS was always significantly higher than the detection limit. However, particulate ammonium fluxes to the greased disk on the KSS were always less than the detection limit of 0.13 mg m-2 day-1. This finding is reasonable since the average particulate ammonium flux was estimated to be 0.055 mg m-2 day-1 (this estimate is the product of the average NH4+ concentration of 6.39E-10 mg/cm3 and a NH4+ deposition velocity of 0.10 cm/s) (26). As will be discussed later, the WSS and the citric acid-coated paper filter on the KSS had statistically equal fluxes (Figure 2). This finding suggests that loss of particulate ammonium from the KSS was not significant. The mean dry deposition flux of ammonia to the WSS when the wind was blowing from the land was statistically higher than the mean flux when the wind was blowing from VOL. 33, NO. 12, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Comparison between nitrogen deposition to the WSS, nylon surfaces, and citric acid-impregnated paper filter on knifeedge surrogate surfaces. the lake (3.21 ( 0.89 vs 1.37 ( 0.59 mg m-2 day-1) (Figure 2). Ammonia is a primary air pollutant that is emitted to the atmosphere primarily from land sources. Land surfaces have a low reactivity with ammonia gas as compared to water surfaces. Natural water bodies act as sinks for ammonia, which results in the lowering of its atmospheric concentrations during transport over water bodies. This fact may in part explain the lower concentrations measured when the wind was from the lake sector. The average total ammonia flux for the samples collected in the daytime was statistically the same as the average total ammonia collected in the nighttime (2.26 ( 0.72 vs 3.02 ( 1.58 mg m-2 day-1). Comparison between Fluxes Measured with Different Surfaces. Water is a universal solvent that has the potential to collect atmospheric NO, NO2, peroxyacetyl nitrate (PAN), HNO2, HNO3, NH3, and particulate ammonium and nitrate. These compounds may undergo several reactions in the aqueous environment including hydrolysis, oxidation, or reduction. For example, previous studies of PAN interaction with the aqueous phase have been conducted in alkaline solutions where PAN hydrolyzed rapidly to yield nitrite ion (27). A study by Garland and Penkett (28) indicated that the uptake of PAN by deionized water and seawater is very slow. Investigation of PAN interaction with water at low pH indicated that PAN is soluble in water with nitrate being the major anionic product (29). Nitric oxide (NO) and nitrogen dioxide (NO2) are believed to be sparingly soluble in water, producing nitrous and nitric acids (30). In this study, several different types of surfaces were used to measure the deposition of nitrogen. The comparison of fluxes to these different surfaces should indicate which species are responsible for the deposition. The nitrate dry deposition to the WSS is strongly correlated to the dry deposition to the nylon filters on the KSS (r 2 ) 0.82) (Figure 3). The linear relationship between nitrate flux to WSS and nylon surfaces were statistically significant at the 95% confidence level. The slope of the best-fit model was estimated to be 1.03 ( 0.16 (which is equal to one at the 95% confidence level). This slope coupled with the strong linear relationship supports the conclusion that the nitrate flux to the WSS was statistically the same as that to the nylon filter on the KSS. Similarly, ammonia dry deposition to the WSS is correlated to the dry deposition to the citric acid-coated paper filter (r 2 ) 0.38). The linear relationship between ammonia flux to WSS and citric acid-impregnated paper filter surfaces was statistically significant at the 95% confidence level. The slope of the best-fit model was determined to be 0.88 ( 0.52 (which equals one at the 95% confidence level). This finding supports the hypothesis that the dry deposition of ammonia to the WSS was statistically the same as ammonia dry deposition to the citric acid-coated filter on the KSS. 2116
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FIGURE 4. Relationship between HNO3 concentration and flux. The slope of the regression line is the best-fit average MTC. These results were confirmed with a paired t-test for two sample means between the measured mean ammonia and nitrate fluxes to the WSS and the citric acid-coated paper filter and nylon filter on the KSS, respectively. The null hypothesis was that the difference in the nitrate and ammonia mean fluxes to the WSS and the Nylon and paper filters were zero, respectively. This hypothesis could not be rejected at the 95% confidence level. The finding that the nitrate flux to the WSS is statistically the same as the flux to the nylon surface on the KSS indicates that during these sampling periods (i) the WSS and nylon surface on the KSS had the same collection efficiency for measuring the deposition of nitrates, (ii) nitric acid and particulate nitrates were the major sources for nitrate deposition to the WSS; NO, NO2, PAN, and HNO2 were minor contributors to the nitrate flux measured with the WSS, and (iii) the water-side resistance to HNO3 deposition was negligible. The ammonia flux to the WSS was also statistically the same as the flux to the citric acid-impregnated paper filter on the KSS. The ammonium flux to the Mylar film on the KSS was below detection limits. These findings indicate that during these sampling periods (i) the WSS and the citric acidimpregnated paper filter on the KSS had the same collection efficiency for measuring the deposition of ammonia, (ii) ammonia gas was the major source for ammonia deposition, and (iii) water-side resistance to ammonia deposition was negligible. Mass Transfer Coefficients. Nitric acid is highly soluble in water (H ) 2.1 × 105 M atm-1) and strongly acidic (pKa ) 0). It dissociates rapidly in aqueous solution (on a time scale of 10-5 s) yielding nitrate and hydrogen ions, precluding its build up to a substantial concentration in bulk water surfaces (31). For this reason, air-side resistance controls deposition of HNO3 to water at near-neutral pH values. Ammonia gas is moderately soluble in water (H ) 62 M atm-1) and strongly basic (pKa ) 9.3). It dissociates rapidly in aqueous solution yielding ammonium and hydroxyl ions on a time scale of 10-8 s, which prevents its buildup to a substantial concentration in bulk water surface (32). Therefore, air-side resistance also controls ammonia deposition to water surfaces at near-neutral pH values. The mass transfer coefficient (MTC) of gaseous species controlled by air-side resistance can be found by dividing the species flux to the WSS by its concentration in the air or MTC ) F/C where MTC is the mass transfer coefficient (cm/ s), F is the mass flux of nitric acid or ammonia (mg cm-2 s-1), and C is the air concentration of nitric acid or ammonia (mg/cm3). The gas-phase flux to the WSS can be found by subtracting the greased disk (particulate) flux from the flux to the WSS (particulate + gas). The dry deposition flux of ammonia and nitric acid is plotted against their ambient concentrations in Figure 4. The linear relationship between nitrate and ammonia fluxes
to the WSS and ambient concentrations were statistically significant at the 95% confidence level. The slopes of the best-fit model are the average dry deposition velocity of nitric acid and ammonia gases, which are 1.5 ( 0.22 and 2.5 ( 0.5 cm/s, respectively. Comparison to Other Compounds. The measured MTCs of nitric acid and ammonia were compared to that of SO2. SO2 was chosen for this purpose because (i) its deposition has been successfully measured with the WSS (18, 33) and (ii) its MTC can be easily related to nitric acid and ammonia gases. SO2 is a slightly soluble (H ) 1.24 M atm-1), moderately acidic gas that dissociates rapidly (K ) 4 × 10-4 s-1) to yield HSO3- (31). Sulfite is oxidized rapidly in aqueous solutions, making the water a permanent sink for SO2. Because of this fast uptake of SO2 and oxidation of sulfite ion, its deposition at near-neutral pH is controlled by air-side resistance (34). The MTC of SO2 was calculated following the same procedure used by Yi et al. in which the gas-phase SO2 flux to the WSS was divided by the SO2 gas-phase concentration (18, 33). Using the two-film theory and empirical correlations for the diffusion coefficient, it is possible to correlate the mass transfer coefficients of two different compounds as
( )
MTC1 MW2 ) MTC2 MW1
1/2
This equation can be used to correlate the air-side MTC of different chemicals that are simultaneously diffusing through the air film to their molecular weight. The equality of the MTC of NH3 and SO2 was examined using a paired t-test. First, the nitric acid MTC was compared to the SO2 MTC. Second, the ammonia MTC was compared to the MTC of SO2. In both cases, the test results indicated that the corrected MTCs of SO2, HNO3, and NH3 were statistically the same. In summary, results indicated that for these samples the nitrate flux was dependent on the sampling time and independent of wind direction, while ammonia flux was independent of the sampling time but dependent on the wind direction. In these experiments, nitric acid and particulate nitrate were the major sources of nitrate deposition. Similarly, ammonia gas was the major source for ammonia deposition to the samplers used. Particulate nitrate did not contribute significantly to the ammonia flux. As expected, air-side resistance controlled both ammonia and nitric acid air-water exchange. The average mass transfer coefficients of HNO3 and NH3 to the WSS were 1.5 ( 0.22 and 2.46 ( 1 cm/s, respectively. Comparison between the measured mass transfer coefficients of SO2 and HNO3 shows that they were statistically the same at the 95% confidence level. After adjusting for the differences in molecular weights, the NH3 mass transfer coefficients were statistically equal to the SO2 MTC at the 95% confidence level.
Acknowledgments The authors gratefully acknowledge the support and advice of Kenneth Noll, Krishna Pagilla, and Demetrios Moschandreas for their help during this project. This work was funded, in part, by the U.S. EPA Cooperative Agreement CR82205401-1 (Gary Evans, Project Officer) and U.S. EPA NCERQA Grant R 826647-01-0 (Dr. Bala Krishnan, Project Officer).
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Received for review December 14, 1998. Revised manuscript received March 23, 1999. Accepted March 29, 1999. ES981299C
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