Environ. Sci. Technol. 1997, 31, 279-282
Polycyclic Aromatic Hydrocarbon Accumulation in Urban, Suburban, and Rural Vegetation DIANE M. WAGROWSKI AND RONALD A. HITES* School of Public and Environmental Affairs and Department of Chemistry, Indiana University, Bloomington, Indiana 47405
Polycyclic aromatic hydrocarbons (PAH) partition from the atmosphere to vegetation, soil, water, and sediment. In an effort to quantitate vegetation’s ability to scavenge PAH, we collected vegetation samples from urban, suburban, and rural locations during the summer and fall of 1995 and determined PAH burdens (ng/cm2) on a mass per total leaf surface area basis. The total PAH burdens in the rural vegetation samples were, on average, 10 times lower than in the urban samples, confirming that atmospheric PAH burdens are higher near presumed source regions. We also compared the individual PAH burden profiles for urban, suburban, and rural samples. All samples had similar profiles for fluorene, phenanthrene, fluoranthene, and pyrene, while the rural samples were depleted of anthracene, benz[a]anthracene, and several higher molecular weight PAH. In order to calculate a PAH to vegetation mass balance for the northeastern region of the United States, we divided this land area into rural, suburban, and urban classifications. We then identified the types of vegetation in each area and estimated the total leaf surface area. Using our PAH burdens, we calculated that about 160 t total PAH/yr flow from the atmosphere to vegetation. Vegetation in this region scavenges about 4% of the total amount of PAH emitted in this area.
Introduction Polycyclic aromatic hydrocarbons (PAH) are byproducts from the incomplete combustion or pyrolysis of organic material (1). Residential heating, coke production, incineration, and internal combustion engines are all major sources of PAH. In 1983, the total PAH atmospheric emission rate in the United States was approximately 7.5 × 106 kg/yr (2). Several of these compounds are carcinogenic and/or mutagenic; therefore, they may pose a human health threat (3). In addition, these compounds are lipophilic and may accumulate in vegetation, an environmental fate that could indirectly cause human exposure through food consumption. Clearly, a knowledge of the mechanisms and rates of PAH accumulation in vegetation is important. Recent research in our laboratory (4, 5) has indicated that vegetation plays a significant role in removing PAH from the atmosphere. Based on measurements of PAH burdens in vegetation from Bloomington, IN, we estimated that about 45% of the PAH emitted in the northeastern region of the United States is removed by vegetation. This model was based on data obtained from samples taken exclusively in Bloomington and extrapolated to the entire northeastern region of the United States (an area bounded by North and South * E-mail address:
[email protected].
S0013-936X(96)00419-1 CCC: $14.00
1996 American Chemical Society
Dakota, Nebraska, Kansas, Missouri, Kentucky, and Virginia). In order to test the validity of this assumption, we have now studied several vegetation types from more widespread locations. In this study, we have once again focused on the northeastern United States (see above), and we have continued to use maple leaves and white pine needles to represent broadleaf and needleleaf vegetation. We have also added corn leaves to our study. Corn covers approximately 11% of this land area (6, 7) and is a major crop in this region of the United States. Our sampling sites also spanned a range of rural, suburban, and urban locations.
Experimental Section Sample Collection. Corn (Zea mays L.) and sugar maple (Acer saccarum) leaves and white pine (Pinus strobus) needles were collected on various dates over the summer and fall of 1995 from the locations indicated in Figure 1. Maple leaves were collected from the outer edge, bottom, and within each tree canopy. These leaves were placed together into a jar or bag. Pine needles (new growth and previous year’s growth) were collected in a similar manner. The samples were placed in glass jars that had been previously cleaned and heated at 450 °C or into clean, re-sealable plastic bags. Samples were stored at -20 °C until extraction. Moisture and Lipid Determination. The moisture and lipid contents of all leaves and needles were determined as in Simonich and Hites (11). Between 2 and 5 g of intact leaves from each plant was weighed into a small beaker, dried at 95 °C for 24 h, and re-weighed to obtain the percent moisture. A 1:1 mixture of hexane and acetone (EM Science, Gibbstown, NJ) was added to cover the vegetation, and the samples were sonicated for 2 h. The solvent was filtered, decanted into a pre-weighed drying pan, and allowed to evaporate. The pans were re-weighed, and the percent lipids by dry weight was calculated. Sample Extraction and Cleanup. Each broadleaf sample was carefully removed from the container and placed on clean aluminum foil. A ruler was placed next to the leaf; the leaf was visually divided into geometric shapes; the dimensions were recorded; the areas were calculated, summed, and doubled to give the total leaf surface area. From each needleleaf sample, a representative cluster was selected. The number of needles was counted, the length and diameter were recorded, the surface area (dπl, where d is the needle diameter and l is the length of the needle) was calculated, and the needles were weighed to determine a surface area per unit fresh weight, which was then used to determine the surface areas of the actual extracted samples. Leaves were placed into 250-mL Erlenmeyer flasks, directly spiked with an internal standard solution consisting of anthracene-d10, pyrene-d10, benz[a]anthracene-d12, and perylene-d12, and then the flasks were sealed with a glass stopper. (All PAH standards were obtained from Ultra Scientific, North Kingstown, RI.) Each sample was sonicated for 1 h in 200 mL of dichloromethane (EM Science, Gibbstown, NJ). The solution was then filtered into round-bottomed flasks. The samples were again sonicated for 1 h in 200 mL of dichloromethane and filtered into the same flasks. All sample extracts were rotary evaporated to 25 mL, solvent exchanged three times with 50-mL volumes of hexane (EM Science, Gibbstown, NJ), rotary evaporated to 5 mL, and further concentrated to 0.5 mL under a gentle stream of nitrogen. Procedural blanks (no sample) were run with every set of extractions, and they were free of all PAH. Recoveries of known amounts of PAH were 49-124% for the entire procedure.
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FIGURE 1. Sample locations and types of vegetation collected. BCSP is Brown County State Park. Symbols: ([) corn leaves; (b) maple leaves only; (4) maple leaves and pine needles.
TABLE 1. Total PAH Burdens with Standard Errors sample
no. of samples
PAH (ng/g dry weight)
PAH (ng/mg of lipids)
PAH (ng/cm2)
corn leaves rural maple leaves rural pine needles suburban maple leaves urban maple leaves
30 12 3 10 11
27 ( 2.2 220 ( 52 370 ( 110 510 ( 100 1600 ( 210
1.8 ( 0.19 9.5 ( 1.8 24 ( 8.3 47 ( 12 93 ( 12
0.06 ( 0.01 0.43 ( 0.12 0.76 ( 0.11 1.96 ( 0.35 4.28 ( 0.70
Silica solid-phase extraction (SPE) columns (2 g, Burdick and Jackson, Muskegon, MI) were cleaned with 10 mL of dichloromethane and heated for 24 h at 165 °C to activate the adsorbant. Ten milliliter aliquots each of 100% hexane, 10% dichloromethane in hexane, 15% dichloromethane in hexane, and 100% dichloromethane were used to elute each sample through the SPE column. The last three fractions were combined and rotary evaporated to 5 mL. Sample Analysis. Samples were analyzed on a Hewlett Packard 5989A gas chromatographic mass spectrometer equipped with a 30 m × 250 µm i.d. DB-5MS capillary column with a 0.25-µm film thickness (J&W Scientific, Folsom, CA). After being concentrated to 250 µL under a gentle stream of nitrogen, 1 µL of the sample was injected in the splitless mode. The GC temperature program was held at 40 °C for 1 min, ramped at 25 °C/min to 140 °C, ramped at 4 °C/min to 240 °C, ramped at 2 °C/min to 290 °C, and held for 18 min. The mass spectrometer was operated in the electron impact ionization mode with the ion source temperature at 250 °C and an electron energy of 70 eV. PAH were quantified by selected ion monitoring with an internal standard as indicated above. Acenaphthylene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, retene, benz[a]anthracene, chrysene and triphenylene (unresolvable by GC), benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[e]pyrene, benzo[a]pyrene, indeno[1,2,3-cd]pyrene, benzo[ghi]perylene, and coronene were quantitated.
Results and Discussion Burdens. PAH accumulation in vegetation depends on the physical properties of the compounds, such as their molecular
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weights, aqueous solubilities, and vapor pressures (8-10), the atmospheric temperature (11), and the plant species and structure (11-14). To compare various types of vegetation, PAH burdens have been normalized to leaf surface area and to the lipid content (11, 13). We have elected to normalize the PAH amounts on the leaves to the surface area of the leaf; this results in an area based burden in units of ng/cm2. The first step in our data analysis was a correlation analysis between individual PAH burdens and the total PAH burdens for all 66 samples. This correlation analysis indicated that, as the total PAH burden increased, each individual PAH burden increased correspondingly. At the 99.5 % confidence level, the critical value for Pearson’s product-moment correlation coefficient (r) is 0.31. Our values ranged from 0.64 to 0.99, indicating that the burdens of all compounds were very significantly correlated with one another and with the total PAH burden. Thus, to simplify the following discussion, we will discuss only total PAH burdens. Individual PAH burdens and total PAH burdens based on leaf area, wet weight, dry weight, and the lipid content are available as Supporting Information. Table 1 shows that the average total PAH burden in corn leaves is significantly lower than all other samples. We believe the low burdens for the corn are not dependent on the corn plants themselves but on their rural growing locations. For example, Cotham and Bidleman (15) reported that PAH concentrations in air at a rural site (Green Bay, WI) were nearly 20 times lower than at an urban site (Chicago, IL). Therefore, there is a lack of atmospheric PAH for deposition to the leaves.
TABLE 2. Total Amount of PAH Scavenged (with Standard Errors) in the Northeast Region of the United States vegetation type
total land area (km2)
PAH flow (kg/yr)
% total PAH scavenged
crops rural pine (needleleaf) rural maple (broadleaf) suburban urban
1.1 × 0.22 × 106 1.2 × 106 0.29 × 106 0.015 × 106
(0.66 ( 0.31) × (2.3 ( 1.3) × 104 (5.2 ( 2.7) × 104 (6.8 ( 4.1) × 104 (0.77 ( 0.46) × 104
0.2 ( 0.1 0.6 ( 0.3 1.3 ( 0.7 1.7 ( 1.1 0.2 ( 0.1
106
total
The maple and pine samples were collected from a range of rural, suburban, and urban locations; see Figure 1. The Michigan, Brown County (Indiana), West Virginia, and New York sites are rural; the Park Ridge, IL, and Bloomington, IN, sites are suburban; and the Chicago site is urban. We hypothesized that vegetation burdens would steadily increase from rural to urban areas due to a corresponding increase in atmospheric concentrations. In general, the burdens in Table 1 show an increase in PAH burdens from rural to urban locations. We found that the samples from Brown County State Park had approximately 5 times higher PAH burdens than those from the other rural sites. Although this park is located in a rural area, 2 million people visit it each year (16), many in automobiles, trailers, and buses. In addition, campfires and barbecues contribute to PAH emissions. We compared the individual PAH profiles for the samples from urban, suburban, and rural locations; see Figure 2. All non-detects were set to zero in order to compare the detected relative burdens for each location. These profiles were calculated by normalizing the burdens of the individual compounds to a total of 100% and plotting the data on a common logarithmic scale. Similar relative abundances for fluorene, phenanthrene, fluoranthene, and pyrene were found at all locations. Anthracene, benz[a]anthracene, and several high molecular weight PAH were depleted in the corn and rural samples. The lack of these PAH suggests that some PAH may be degraded in the atmosphere or on vegetation or deposited into different environmental sinks. Mass Balance. Since the total surface area of vegetation in the northeastern region of the United States can be estimated, a total loading rate of PAH to vegetation can be calculated. The green leaf area of plants is expressed as a dimensionless quantity, called the leaf area index (LAI), which is the total one-sided foliage area per unit soil surface area (17). Since our PAH burdens are expressed as a mass per two-sided leaf surface area, we must double all LAI values in the following discussion. Literature values for the LAI of corn, soybeans, and wheat are between 4 and 6 (18-20). Doubling the LAI of 5 ( 1 for crops and using 1.1 × 106 km2 (6) for the total rural cropland area in the northeastern United States, the total plant foliage area is (1.1 ( 0.5) × 107 km2. Using 0.06 ( 0.01 ng/cm2 as the average total PAH burden for crops (see Table 1) and assuming a 1-yr residence time for PAH in vegetation (5), we calculate that (6.6 ( 3.1) × 103 kg/yr of PAH moves from the atmosphere to crops. The remaining rural land area is classified as pasture, range, forest, and federal land (21). Federal land includes national parks, national forests, and military installations (22, 23). This remaining total rural land area is 1.4 × 106 km2, and it can further be divided into needleleaf (pine) and broadleaf (maple) areas of 0.22 × 106 and 1.2 × 106 km2, respectively (24, 25). We doubled the average LAI of 7 ( 2 (26-29) for the needleleaf areas, resulting in a total needle foliage area of (3.1 ( 1.7) × 106 km2, and doubled the LAI of 5 ( 1 (17, 28) for the broadleaf areas, resulting in a total broadleaf foliage area of (1.2 ( 0.5) × 107 km2. Using 0.76 ( 0.11 ng/cm2 as the total PAH burden to pine needles (see Table 1), we calculate (2.3 ( 1.3) × 104 kg/yr of PAH moves from the atmosphere to needleleaves. Using 0.43 ( 0.12 ng/cm2 as the total PAH burden for maple leaves (see Table 1), we calculate (5.2 ( 2.7) × 104 kg/yr of
104
(16 ( 5) × 104
4.0 ( 1.3
FIGURE 2. Average relative burdens (with standard errors) of PAH in vegetation. PAH are listed in order of increasing molecular weight. Abbreviations: Acenaphthylene (AL), acenaphthene (AE), fluorene (FLE), phenanthrene (PH), anthracene (AN), fluoranthene (FLA), pyrene (PY), retene (RE), benz[a]anthracene (BaA), chrysene and triphenylene (CY), benzo[b]fluoranthene (BbF), benzo[k]-fluoranthene (BkF), benzo[e]pyrene (BeP), benzo[a]pyrene (BaP), indeno[1,2,3,cd]pyrene (IcdP), benzo[ghi]perylene (BghiP), and coronene (CR). PAH partition from the atmosphere to broadleaf trees. In both cases, we assumed a residence time of 1 yr for PAH in vegetation (5). The remaining land is classified as suburban and urban [cities with 100 000 or more inhabitants (30)]. Urban land area is 1.5 × 104 km2, and suburban land area is 2.9 × 105 km2 (31). We averaged the LAI values for broad and needleleaves. Doubling this average LAI of 6 ( 2 (5, 17, 26-29), the total leaf area in urban sites in the northeastern United States is (1.8 ( 1.0) × 105 km2. Assuming a 1-yr residence time and using the urban PAH burden from Table 1 [(4.28 ( 0.70) ng/ cm2], we calculate that (7.7 ( 4.6) × 103 kg/yr of PAH move from the atmosphere to urban vegetation. Similarly for the suburban area, the total leaf area is (3.5 ( 2.0) × 106 km2. Using the suburban PAH burden from Table 1 [(1.96 ( 0.35) ng/cm2], we calculate that (6.8 ( 4.1) × 104 kg/yr of PAH move from the atmosphere to suburban vegetation. These calculated values and their associated standard errors are all
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summarized in Table 2. The total flow of PAH from the atmosphere to vegetation in this region is (16 ( 5) × 104 kg/yr or about 160 t/yr. We previously estimated that 3.9 × 106 kg/yr of PAH are deposited from the atmosphere into the northeastern United States (5). Using the flows from Table 2, crops, rural needleleaf, rural broadleaf, suburban, and urban vegetation scavenge 0.2 ( 0.1%, 0.6 ( 0.3%, 1.3 ( 0.7%, 1.7 ( 1.1%, and 0.2 ( 0.1% of these PAH, respectively. All vegetation in this region of the United States scavenges a total of about 4% of all PAH emitted. Why is our revised calculation about 10 times lower than our initial estimate of about 45%? This previous estimate was based on relatively few samples, which were all collected in Bloomington, IN, a suburban site. To expand the basis of this estimate, we have now taken samples from a broader geographical region and from various plant types (including crop plants). In addition, because we have observed that the PAH burdens vary according to land usage, we divided the land area of the northeastern United States into rural, suburban, and urban classifications, and we identified the vegetation types in each area. Based on these refinements, we believe that our revised mass balance is more accurate than our initial estimates (5). This research indicates that PAH burdens are dependent on location. Since vegetation in rural areas is not exposed to the same amount of PAH emissions as urban vegetation, rural vegetation will not have as high a burden of these compounds. Luckily, most of our food is grown in rural locations.
Acknowledgments We wish to thank Susan and Joseph Glassmeyer; Matt O’Dell; and George, Helen, and Michael Wagrowski for their help in collecting samples. We also thank Michele Fang for the Brown County State Park data.
Supporting Information Available A table with the individual and total PAH burdens for each sample and total PAH burdens on a wet weight, dry weight, and lipid basis (6 pages) will appear following these pages in the microfilm edition of this volume of the journal. Photocopies of the Supporting Information from this paper or microfiche (105 × 148 mm, 24× reduction, negatives) may be obtained from Microforms Office, American Chemical Society, 1155 16th St. NW, Washington, DC 20036. Full bibliographic citation (journal, title of article, names of authors, inclusive pagination, volume number, and issue number) and prepayment, check or money order for $16.50 for photocopy ($17.50 foreign) or $12.00 for microfiche ($13.00 foreign), are required. Canadian residents should add 7% GST. Supporting Information is also available to subscribers electronically via the Internet at http://pubs.acs.org (WWW) and pubs.acs.org (Gopher).
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Haroz, R. K., Eds.; Reidel: Dordrecht, 1983; pp 277-298. (3) Evaluation and Estimation of Potential Carcinogenic Risks of Polynuclear Aromatic Hydrocarbons; U.S. Environmental Protection Agency, Office of Research and Development, U.S. Government Printing Office: Washington, DC, 1985. (4) Simonich, S. L.; Hites, R. A. Environ. Sci. Technol. 1995, 29, 29052914. (5) Simonich, S. L.; Hites, R. A. Nature 1994, 370, 49-51. (6) Womack, L. M.; Traub, L. G. U.S.sState Agricultural Data: United States Department of Agriculture, Economic Research Service: Agriculture Information Bulletin 512; Washington, DC, 1987. (7) Major World Crop Areas and Climatic Profiles; Agriculture Handbook 664; World Agriculture Outlook Board, U.S. Department of Agriculture: Washington, DC, 1987. (8) Briggs, G. G.; Bromilow, R. H.; Evans, A. A.; Williams, M. Pestic. Sci. 1983, 14, 492-500. (9) Bacci, E.; Calamari, D.; Gaggi, C.; Vighi, M. Environ. Sci. Technol. 1990, 24, 885-889. (10) Trapp, S.; Matthies, M.; Scheunert, I.; Topp, E. Environ. Sci. Technol. 1990, 24, 1246-1252. (11) Simonich, S. L.; Hites, R. A. Environ. Sci. Technol. 1994, 28, 939943. (12) Paterson, S.; Mackay, D.; McFarlane, C. Environ. Sci. Technol. 1994, 28, 2259-2266. (13) McCrady, J. K. Chemosphere 1994, 28, 207-216. (14) Keymeulen, R.; Schamp, N.; Van Langenhove, H. Atmos. Environ. 1993, 27A, 175-180. (15) Cotham, W. E.; Bidleman, T. F. Environ. Sci. Technol. 1995, 29, 2782-2789. (16) Indiana State Parks Systems Plan; Indiana Department of Natural Resources: 1984. (17) Dufrene, E.; Breda, N. Oecologia 1995, 104, 156-162. (18) Flesch, T. K.; Dale, R. F. Agron. J. 1987, 79, 1008-1014. (19) Savoy, B. R.; Cothren, J. T.; Shumway, C. R. Agron. J. 1992, 84, 956-959. (20) Redelfs, M. S.; Stone, L. R.; Kanemasu, E. T.; Kirkham, M. B. Agron. J. 1987, 79, 254-259. (21) Statistical Abstract of the United States, 114th ed.; U.S. Bureau of the Census, U.S. Government Printing Office: Washington, DC, 1994. (22) Summary Report of Real Property Owned by the United States Throughout the World as of September 30, 1993; U.S. General Services Administration: Washington, DC, July 1995. (23) Summary Report of Real Property Leased by the United States Throughout the World as of September 30, 1993; U.S. General Services Administration: Washington, DC, July 1995. (24) United States Department of Agriculture, Eastwide Table Generator, http://www.srsfia.usfs.msstate.edu/scripts/ ew.htm#SectionVI, January 1996. (25) Kuchler, A. W. National Atlas of the United States of America: Potential Natural Vegetation; Department of the Interior, U.S. Geological Survey; Reston, VA, 1985. (26) Jarvis, P. G. Physiological Processes Limiting Plant Productivity; Johnson, C. B., Ed.; Butterworths; London, 1981; pp 81-107. (27) Spanner, M. A.; Pierce, L. L.; Peterson, D. L.; Running, S. W. Int. J. Remote Sensing 1990, 11, 95-111. (28) Gower, S. T.; Norman, J. M. Ecology 1991, 72, 1896-1900. (29) Gong, P.; Pu, R.; Miller, J. R. Photogramm. Eng. Remote Sens. 1995, 61, 1107-1117. (30) United States Bureau of the Census, Statistical Abstract, http: //www.census.gov/ftp/pub/statab/freq/95s0046.txt, January 1996.
Received for review May 13, 1996. Revised manuscript received August 26, 1996. Accepted August 29, 1996.X ES960419I
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Abstract published in Advance ACS Abstracts, November 1, 1996.
