Envlron. Sci. Technol. 1985, 19, 721-725
Direct Measurement of Dry Deposition to a Polyethylene Bucket and Various Surrogate Surfaces Jean Muhibaler Dasch
Environmental Science Department, General Motors Research Laboratories, Warren, Michigan 48090
The polyethylene bucket commonly used as a dry deposition collector was evaluated and compared to a variety of other surrogate surfaces. Since SO2, NOz, and HN03 were not collected by a dry bucket, it most likely underestimates gaseous dry deposition to many natural surfaces. Deposition was similar to buckets with high walls compared to buckets with l-cm walls, indicating a minor influence of the walls on particle deposition. At the suburban site used for this study, dry deposition to a bucket accounted for a significant fraction of total deposition: 24% of S042-,29% of NO3-, and over half of Ca2+,Mg2+, Na+, and K+ total deposition. The bucket was compared to seven other surrogate surfaces. It collected more dry deposited material than Teflon, foil, or coated foil surfaces. However, it generally collected less than a nylon filter, quartz-fiber filter, a glass-fiber filter, or a water surface. Deposition appeared to be strongly influenced by the affinity of the surface for gases and the retention characteristics of the surface for particles.
Introduction Many collectors have been used to directly measure dry deposition, including both natural and artificial surfaces. The historical collector used is a jar or bucket that acts as a dust fall collector. This method has been criticized on the basis that the bucket cannot simulate a natural surface and that the high container walls restrict the entry of all but the largest particles that deposit by gravitational settling (I). However, due to its simplicity, the method remains in common usage and is presently employed at the sites in the National Atmospheric Deposition Program (NADP). Studies have been conducted recently to compare collection by a bucket to collection by other artificial or surrogate surfaces. Higher deposition of SO-: was found to a bucket than to Teflon plates or polycarbonate Petri dishes in an intercomparison experiment conducted in Illinois (2). On the basis of these experiments, Davidson et al. have suggested that collection efficiency may be related to either the rim height or the surface characteristics of the collector (3). Sickles et al. also measured higher S042-and NO3- fluxes to a bucket than to a Petri dish (4). However, a cellulose filter in a Petri dish collected much more S042- and NO3- than the bucket, suggesting that the height of the rim is less important than the surface characteristics. In this study, several tests were made with a polyethylene bucket to determine the importance of gaseous collection and the importance of the rim and to compare it to other surrogate surfaces. Finally, bulk deposition was compared to the sum of wet-plus-dry deposition, and the relative importance of wet and dry deposition to buckets was determined.
Materials and Methods Two sites were used for dry deposition measurements. The comparison of various surfaces was made in an open 0013-936X/85/0919-0721$01.50/0
field in suburban Warren, MI. The site is located 6 km north of the Detroit city line. Some measurements were also made in Lapeer, MI, a rural site 60 km north of the Warren site. Wet, dry, and bulk deposition measurements were made at each site for a period of 1year beginning in the summer of 1981. Wet and dry collections were made in polyethylene buckets in an Aerochem-Metrics wet/dry collector. A third polyethylene bucket for bulk collections was mounted beside the wet collector at the same height. The bulk and dry buckets were collected on a weekly basis from Monday to Monday. The wet deposition was collected on an event basis. The dry bucket was extracted overnight in 250 mL of deionized water. The walls of the bucket were not rinsed. All samples were filtered through 0.4-pm Nuclepore filters and then refrigerated until the time of analysis. The filters were not analyzed so the data represents only the soluble species. A variety of other surrogate surfaces were used as dry deposition collectors. The surfaces, including several flat surfaces mounted on a Lucite board in groups of four, a plastic pan containing 1000 mL of deionized water, and a polyethylene bucket with a l-cm rim, were mounted in a Wong collector, a 43 X 38 X 48 cm metal box open at the top and bottom. During precipitation events, a lid covered the sampler. The flat surfaces included Millipore Mitex Teflon, aluminum foil, aluminum foil coated with Apiezon grease, Ghia nylon, Gelman quartz fiber, and Gelman A-E glass-fiber filters, each 12 X 10 cm in area. During some weeks, filters were also exposed in a downward-facing position as well as an upward-facing position in the Wong collector. The downward filters were located 16 cm below the upper set. The horizontal positioning of the side-byside filters in the collector had no effect on collection efficiency (5). All filters were extracted in 50 mL of deionized water overnight, and all dry collectors were exposed for l-week periods. Samples were analyzed for the major anions and cations. Anions were analyzed by using ion chromatography. The cations, Ca2+,M2', Na+, and K+, were analyzed by atomic absorption. NH4+ was analyzed on a Technicon AutoAnalyzer using the indophenol blue method. Ambient particles were collected at this site from Aug 1981to June 1982. These concentrations and the collection methods have been reported elsewhere (6) and are listed here merely to allow a comparison with the dry flux measurements below. The average ambient particle concentrations (in pg/m3) were 4.4 f 2.8 for S04z-, 3.4 f 1.8 for NO3-, 0.29 f 0.29 for C1-, 0.93 f 0.76 for Caz+,0.11 f 0.051 for Mgz+,0.43 f 0.32 for Na+, and 0.15 f 0.060 for K+.
Results Collection of Gases by Bucket. Buckets were fumigated with SO2,NOz, or HN03 to determine the collection potential for these gases. Concentrations of premixed 10 ppm of SOz in nitrogen, premixed 10 ppm of NOz in nitrogen, or 10 ppm of HN03 (injected slowly from a syringe into a nitrogen stream) were used as test gases. The gases
0 1985 American Chemlcal Society
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Table I. Comparison of Deposited Species on the Sides and Bottom of a Bucket (pequiv/L) species ClNOC
so>NH4+ Ca2+ Mg2+ Nat conductance"
bottom
walls
wall/bottom ratio
51 f 56 49 f 21 76 f 45 21 f 20 114 f 50 29 f 12 70 f 108 31 f 13
8.2 f 11.4 10.7 f 6.4 5.6 f 5.2 0.77 f 3.2 23 f 18 8.5 f 9.2 8.8 f 13.4 3.4 f 2.4
0.15 f 0.19 0.25 f 0.17 0.097 f 0.095 0.069 f 0.41 0.20 f 0.15 0.28 f 0.29 0.14 f 0.40 0.13 f 0.11
'Units: pmhos/cm.
Table 11. Effect of Bucket Height on Deposition (pequiv/L) species
c1so>NOC
Ca2+ Mgz+ Nat conductance'
rim = 25 cm
rim = 1 cm
1 cm/25 cm ratio
78 f 64 48 f 18 92 f 51 137 f 50 34 f 13 118 f 129 33 f 12
84 f 69 41 f 15 108 f 54 164 f 67 34 f 13 132 f 161 38 f 17
1.30 f 0.65 1.00 f 0.65 1.34 f 0.68 1.22 f 0.27 1.02 f 0.26 1.23 f 0.74 1.19 f 0.39
'Units: umhos/cm.
flowed at a rate of 0.12 L/min through a Teflon tube anchored at the bottom of a loosely capped bucket for several hours. The bucket was extracted overnight in 250 mL of water. No SO-: or NO3- was detected following fumigation by SOz or NOz, respectively. A minor NO3peak resulted after a 24-h fumigation with HNO,, but this peak corresponded to less than 4% of the total HN03used for fumigation. These results indicate that gaseous collection of SO,, NOz, or HNO, by a dry bucket is of negligible importance. Next, buckets containing 250 mL of deionized water were fumigated with the same gases. The fumigation tube was mounted about 1 cm above the level of the water. Approximately one-third of the SO, and one-third of the HN03 were collected in the water, but negligible amounts of NO, were collected. Therefore, the presence of water or dew in the bucket will result in the collection of SO, and HNO, gases. The relative importance of gaseous-to-particle collection will depend on a variety of factors including the ambient gas and particle concentrations, meteorological conditions, the pH of the moisture, and the time period that the bucket is wetted. Wall Effects. The walls of the bucket have been speculated to lead to both an increase and a decrease in collection efficiency (1, 3). The importance of the walls was tested by two methods: in the first method, the walls and bottom were each washed separately, and the material collected on the walls was compared with the material
collected on the bottom; the second method was to compare deposition to a bucket with a 1-cm rim to deposition to a normal bucket. In the first test, 250 mL of water was placed in the bucket overnight, filtered, and analyzed. A second 250 mL of water was then used to wash down the walls of the bucket. The average concentration in each portion is shown in Table I based on 14 tests. The ratio is the average wall-to-bottom ratio calculated from each test. The material on the walls averaged 7-28% of the material on the bottom, although from the scatter in the ratios it is obvious that the wall component was occasionally significant. In addition, since the wall surface area is actually 4 times larger than the bottom surface area, the average deposition rate to the walls would be only 2-7% of the deposition rate to the bottom. The second test consisted of exposing two buckets: one had a normal 25-cm rim, and the other was cut off to a depth of 1cm. The high rimmed bucket was located in an Aerochem collector, and the cutoff bucket was in the Wong collector. Both were exposed simultaneously and then extracted in 250 mL of water. The results of eight tests are shown in Table 11. The average ratio of the material in the cutoff bucket to the material in the bucket varied from 1.0 to 1.34 (although once again with large standard deviations). The differences in deposition are not significant at a 0.10 level based on a paired t test. There is no indication that deposition increases with rim depth as suggested previously by the results of the Illinois intercomparison comparison (3). Comparison of Bucket to Surrogate Collectors. Deposition to a bucket was compared to deposition to seven other surfaces. Data will only be given for the chemical species, S042-, NO3-, and Ca2+. Data for other species (H+,NH4+,C1-, Mg2+,Na+, and K') are compiled elsewhere (5). The average of the weekly ratios of each surface-to-bucket deposition is shown in Table 111. Only weeks of concurrent sampling are included in each average, and the time of year of sampling is also shown. The Teflon and foil surfaces collected less material than the bucket. The Teflon surface, in particular, collected only one-fifth of the material collected by the bucket. The foils collected about half of the material collected by the bucket, and there was no significant difference between a coated and uncoated foil. During the dry deposition intercomparison experiment colat the Illinois State Water Survey (2), higher SO-: lection was found to a bucket than to Teflon or polycarbonate surfaces. In fact, the average ratio of deposition to their Teflon surfaces compared to their buckets was 0.25 f 0.17, very similar to the 0.21 f 0.13 measured in this study. However, in our experiment, higher collection was found to several flat surfaces than to a bucket, notably the nylon, quartz-fiber, and glass-fiber filters. This is another indication that the surface type is more important than
Table 111. Average Ratio of Collection by Surrogate Surface Compared to Polyethylene Bucket surface Teflon foil" foil bucket nylon quartz water glass
no. of tests 16 7 20 23 34 36 7
'Foil coated ___ with Apiezon grease. 722
sampling period Sept-Feb July-Aug July-Fe b July-Feb July-Feb July-Feb Dec-Nov Nov-Feb
Ca2+ 0.19 f 0.15 0.51 f 0.23 0.59 f 0.34 1.0 1.2 f 0.41 1.4 f 0.50 2.6 f 1.6 NDb
so>-. 0.21 f 0.13 0.39 f 0.30 0.51 f 0.46 1.0 3.3 f 2.0 4.0 f 2.4 8.2 f 5.5 14.9 f 6.7
ND, not determined.
Envlron. Sci. Technol., Voi. 19, No. 8, 1985
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NO< 0.16 f 0.12 0.33 f 0.13 0.34 f 0.28 1.0 1.2 f 0.49 1.6 f 0.74 2.5 f 1.5 2.3 f 0.43
Table IV. Average Percentage of Material on Downward-Facing Surfaces Compared with Upward-Facing Surfaces surface weeks foil nylon quartz elass
j1.1pm (9). The S042-and NO3- dry deposition rates are most likely low estimates due to the nonadsorption of gases by a dry bucket. If the dry deposition was based on measurements by the glass-fiber filter, SO?- and NO< dry deposition would be greater than the wet deposited fraction.
S u m m a r y and Conclusions
Dry deposition was measured to a bucket and several other surrogate surfaces over a period of 1 year. The deposition flux to the walls of the bucket was 2-7% of the deposition flux to the bottom. The high rim of the bucket had little influence compared to a “rimless” bucket. Measurements of bulk deposition to a bucket were in close agreement with the sum of the material collected by wet and dry polyethylene buckets. A comparison of surrogate surfaces indicated that each surface will collect different amounts of material. Teflon and foils collected less material than the bucket, whereas fibrous surfaces and water collected more material than the bucket. Approximately 90% of the Ca2+and 50-75% of the NO3- and Sod2-appeared to deposit by sedimentation based on a comparison of downward- and upwardfacing surfaces. Although only 10% of the S042-was associated with particles >1pm, sedimentation was still an important removal mechanism. These values may vary depending on the size distribution in a particular location. The deposition of gases appeared to be controlled by the chemical characteristics of the surface. The polyethylene bucket did not adsorb SO2, NO2, or HN03 unless wetted. However, other surfaces, particularly the alkaline glassfiber filter, obviously collected gaseous S and N species. No one surrogate surface can simulate dry deposition to all natural surfaces. However, it should be possible to “imitate” a particular natural surface by simulating its physical and chemical characteristics with a surrogate surface. In addition, surrogate surfaces provide a simple means of obtaining information on temporal and geographical trends in dry deposition. Acknowledgments
I thank Steve Cadle for helpful discussions and Ken Kennedy and Alice Ricci for assistance with sampling and analysis. Robert Kohn of the Analytical Chemistry Department provided the NH4+ analyses. Registry NO.NH4+,1479803-9;HzO, 7732-18-5; Al, 7429-90-5; H+,12408-02-5; Ca, 7440-70-2; Mg, 7439-95-4;Na, 7440-23-5; K, 7440-09-7; Teflon, 9002-84-0; quartz, 14808-60-7; polyethylene, 9002-88-4.
Literature Cited (1) Hicks, B. B.; Wesely, M. L.; Durham, J. L. ”Critique of Methods to Measure Dry Deposition”. 1980,EPA Report EPA-60019-80-050. (2) Dolske, D. A.; Gatz, D. F. In “The Meteorology of Acid Deposition”;Samson, P., ed.; Air Pollution Control Association: Pittsburgh, PA, 1984; pp 214-224. (3) Davidson, C. I.; Lindberg, S. E.; Schmidt,J. A.; Cartwright, L. G.; Landis, L. R. J . Geophys. Res. 1985,90,2123-2130. (4) Sickles,J. E.; Bach, W. D.; Spiller, L. L. In “Precipitation
Scavenging, Dry Deposition, and Resuspension”; Pruppacher, H. R.; Semonin, R. G.; Slinn,W. G. N., Eds.; Elsevier Science Publishing Co.: New York, 1983;pp 979-990. (5) Dasch, J. M.In “Precipitation Scavenging, Dry Deposition, and Resuspension”; Pruppacher, H. R.; Semonin, R. G.; Slinn, W. G. N., Eds.; Elsevier Science Publishing Co.: New York, 1983;pp 883-902. (6) Dasch, J. M.; Cadle, S. H. Atmos. Environ. 1984, 18, 1009-1015. (7) Appel, B. R.; Tokiwa, Y.; Haik, M.; Kothny, E. L. Atmos. Environ. 1984,18,409-416. (8) Granat, L.; Johansson, C. Atmos. Environ. 1983, 17, 191-192. (9) Dasch, J. M.; Cadle, S. H.; Wolff, G. T. Water Air Soil Pollut. 1984,21, 51-69. (10) Sehmel, G. A. Atmos. Environ. 1980,14,983-1011. (11) Cadle, S.H. Atmos. Environ. 1985,19,181-188. (12) Garland, J. A., In “Precipitation Scavenging,Dry Depos-
ition, and Resuspension”;Pruppacher, H. R.; Semonin,R. G.; Slinn, W. G. N., Eds.; Elsevier Science Publishing Co.: New York, 1983;pp 849-858. (13) Davidson, C. I. Powder Technol. 1977,18,117-126. (14) Spicer, C. W.; Schumacher, P. M. Atmos. Enuiron. 1979, 13,543-552. (15) Galloway, J. N.; Likens, G. E. USDA For. Serv. Gen. Tech. Rep. NE 1976,NE-23. (16) Volchok, H. L. In “Polluted Rain”; Toribara, T. Y., Ed.; Plenum Press: New York, 1980. (17) Johannes, A. H.; Altwicker, E. R.; Clesceri, N. L.; Ierardi,
M. E.; Jenness, S. “Relationshipsbetween Wet Deposition, Dry Deposition and Throughfall Chemistry”presented at the 75th Annual Meeting of the Air Pollution Control Association, 1982. Received for review August 22,1984.Revised manuscript received January 23, 1985. Accepted February 22, 1985.
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