Environ. Sci. Technol. 1994, 28, 939-943
Vegetation-Atmosphere Partitioning of Polycyclic Aromatic Hydrocarbons Stacl L. Slmonlch and Ronald A. Hltes' School of Public and Environmental Affairs and Department of Chemistry, Indiana University, Bloomington, Indiana 47405
The partitioning of polycyclic aromatic hydrocarbons (PAH) betweenvegetation and the atmosphere was studied throughout the growing season and under natural conditions. A vegetation-atmosphere partition coefficient (K,) was derived by analogy to partition coefficients for gasparticle partitioning and fish-water partitioning. K , is temperature dependent, and from this functional relationship we calculated PAH-vegetation binding energies. These energies were highly correlated with heats of vaporization. The PAW-vegetation partitioning process is primarily dependent upon the atmospheric gas-phase PAH concentration and ambient temperature. At low ambient temperatures (spring and fall) gas-phase PAH partition into vegetation, and at high ambient temperatures (summer), some PAH volatilize and return to the atmosphere. This study provides further evidence that atmospheric semivolatile organic compounds undergo an annual partitioning cycle with the surface of the earth.
Introduction Semivolatile organic compounds (SOCs) partition between the atmosphere and the surface of the earth, a process which is dependent on ambient temperature (13). SOCs preferentially move to the earth's surface at lower ambient temperatures and to the atmosphere at higher ambient temperatures. For this reason, higher atmospheric SOC concentrations are observed in the summer (1, 4, 5). Considering that 80% of the earth's land surface is covered with vegetation and that the vegetation generally has 6-14 times more surface area than the land on which it is growing, vegetation probably plays a significant role in annual SOC cycling. In addition, most plant surfaces exposed to the air are covered with a wax or lipid layer to prevent excessive evapotranspiration (6, 7). This combination of high surface areaand the presence of waxes and lipids suggests that the partitioning of SOCs between vegetation and the atmosphere may explain a large portion of the annual cycling of SOCs between the atmosphere and the earth's surface. Recent studies on vegetation-atmosphere partitioning have included: (a) attempts to model the interactions either through empirical models (8-12) or through controlled exposure experiments (13-17); (b) the use of vegetation to determine spatial trends in organochlorine contamination both on a regional scale (18-22) and on a global scale (23); and (c) studies of photodegradation of 2,3,7,8-TCDDon plant surfaces (13). Several studies have shown that partitioning from the soil into vegetation does not occur to a great extent for lipophilic SOCs (24-27). Evapotranspiration in plants requires the transport of water from the roots to the outer extremities of the plant and is not a likely mechanism for distribution of hydrophobic SOCs within the plant. Contaminated soils, however, may contribute to high plant concentrations ~
* E-mail:
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hitesr @ indiana.edu.
0013-938X/94/0928-0939$04.50/0
0 1994 American Chemical Society
through volatilization of compounds into the surrounding air (26, 28). We have investigated the partitioning of polycyclic aromatic hydrocarbons (PAH) between vegetation and the atmosphere, throughout the growing season, under natural conditions. We studied PAH because of their wide range of vapor pressures (some atmospheric PAH exist almost exclusively in the gas phase and others in the particle phase) and lipophilicities. By taking air samples during the collection of vegetation samples, we were able to examine the influence of ambient temperature on the partitioning of PAH between vegetation and the atmosphere.
Experimental Section Sample Collection. Tree bark, leaf, needle, seed, and air samples were collected from the Monroe County Courthouse grounds in downtown Bloomington, Indiana. Bloomington has a population of approximately 70 000, and this site is in its center with moderately heavy automobile traffic surrounding it on four sides at all times of the year. Sugar maple (Acersaccarum)leaves and seeds were collected to represent broad-flat plant surfaces; white pine (Pinus strobus) needles (both new growth and the previous year's needles) were collected to represent narrowround plant surfaces; and sugar maple and white pine bark were collected to represent rough plant surfaces. These different plant tissue types also represent varying degrees of lipid content; white pine bark had the highest average lipid content (170 mg of lipid/g of bark dry wt), followed by sugar maple bark (33 mg of lipid/g of bark dry wt), white pine needles (23 mg of lipid/g of needle dry wt), sugar maple leaves (16 mg of lipid/g of leaf dry wt), and sugar maple seeds (6 mg of lipid/g of seed dry wt). Collection of leaf, seed, and needle samples began on April 23, 1992 (after the leaves and seeds had been developing for approximately 2 weeks); samples were collected approximately every 20 days throughout their growing season. The last seed samples were collected on August 27, 1992, at the end of their cycle, while the last leaf and needle samples were collected October 29,1992. Leaf, needle, and seed samples were cut from the tree with a clean pair of scissors and stored in a clean glass jar at -4 "C until extraction. Bark from white pine and sugar maple trees was collected in July 1991 and February 1992. Samples were collected in a manner similar to that used by Hermanson and Hites (19)and Meredith and Hites (22),except that the oxidized bark layer was not removed. Bark was taken from spots with little or no moss or lichens. The outer bark (outer 1cm) was removed with a solvent washed chisel from an area 5 cm wide by 20 cm long and stored in a clean glass jar at -4 "C until extraction. Air samples were collected using a high-volume air sampler (Sierra-Misco, Berkeley, CA) from the roof of the Monroe County Courthouse (elevation about 13 m). Air samples were collected 24 h prior to the collection of all vegetation samples except for the tree bark samples. Envlron. Scl. Technol., Vol. 28, No. 5, 1994 039
Atmospheric PAH adsorbed to particles greater than 0.1 pm in diameter were collected on a 20.3 cm X 25.4 cm glass fiber filter (GFF) (Gelman Science, Ann Arbor, MI) followed by an additional GFF and a 9.5 cm X 10.0 cm diameter polyurethane foam (PUF) plug to adsorb gasphase PAH. The PUF plugs were cleaned prior to use by Soxhlet extraction for 24 h with dichloromethane, acetone, and petroleum ether; the GFFs were cleaned by heating them to 450 "C for 24 h. The second GFF was used to correct for adsorption of gas-phase molecules on the particle-laden GFF (the first GFF) (29-35). This slightly underestimated the actual adsorption of gas-phase compounds to the particle-laden GFF, but it was more representative of the true gas-particle partitioning than the uncorrected particle-laden GFF alone (29). The gasphase PAH concentration was calculated as the amount of PAH on the PUF plus two times the amount on the second GFF, divided by the volume of air sampled; the particle-phase PAH concentration was calculated as the amount on the first GFF minus the amount on the second GFF, divided by the volume of air sampled. A totalvolume of about 800 m3 of air was sampled in each 24-h period. Samples were stored at -4 "C until extraction. Sample Extraction and Cleanup. All leaf, needle, seed, bark, and air samples were directly spiked with the appropriate amount of five perdeuterated PAH internal standards (&-phenanthrene, dlo-pyrene,d12-chrysene,d12perylene, and dlz-benzo[ghilperylene) just prior to extraction. Leaf, needle, and seed samples were extracted in their original state by sonication for 1h in enough CH2C12 to cover the samples. After the initial hour, the solvent was decanted, collected, and replenished; the sample was then sonicated for an additional hour. The solvent fractions were combined. All extractions were done in duplicate with an average relative standard deviation of 8 % ,and reagent blanks (no vegetation) were done with every set of samples. All blank samples were found to be free of PAH. Recoveries of known amounts of PAH were between 89 and 97 5%. Bark samples were ground using a solvent-cleaned Udy Cyclone sample mill (Fort Collins, CO). The resulting particles were less than 1mm in diameter. The PAH were extracted from the ground bark with supercritical CHClF2. Ground tree bark is a good matrix for supercritical fluid extraction (SFE)because of its fine particle size, low water content (10-15 % by weight), and relative inertness. Approximately 0.5 g of 60/80 mesh silanized glass beads (Alltech; Deerfield, MI) were packed in the bottom of a 250 mm X 9.4 mm, 17.3-mL supercritical fluid extraction cell (Keystone Scientific; Bellefonte, PA) in order to prevent clogging of the outlet frit. Ground bark (7-8 g) was packed into the cell, and the internal standards were spiked onto the bark at the top of the cell. Eighty milliliters of liquid CHClF2 [Halocarbon-22; 99.9 5% purity; Scott Specialty Gases (Plumstedville, PA)] was delivered to the cell at a flow rate of 4 mL/min and at 325 atm and 60 "C using an Isco Model 260D syringe pump (Lincoln, NE) and a 20 cm long, 250 pm i.d. stainless steel restrictor (Keystone Scientific; Bellefonte, PA). The extract was collected in a clean 250-mL round-bottom flask containing approximately 150 mL of CH2C12. The cell temperature was constant during the extraction, and the round-bottom flask was placed in a large beaker of warm water to prevent plugging of the restrictor. Recoveries of known amounts of PAH were between 88 and 96%. All samples were 940
Environ. Sci. Technol., Vol. 28, No. 5, 1994
extracted in duplicate with an average relative standard deviation of 10%. Reagent blanks (no bark) were performed every five samples and were free of PAH. After being spiked with internal standard, the two GFFs were Soxhlet extracted separately with CHzClz for 24 h, and the PUF plug was Soxhlet extracted with petroleum ether for 24 h. The PUF plugs used to collect gas-phase PAH on warm days were cut in half, and the halves were spiked and extracted separately to determine if there was breakthrough of PAH. Breakthrough of phenanthrene and anthracene occurred on warm days (above 20 "C). In order to correct for these losses,two air samples were taken over a 24 h period on September 15, 1992 (22 "C) to determine the representative gas-phase ratio of phenanthrene and anthracene to fluoranthene and pyrene on warm days. By sampling for a shorter period of time, breakthrough was eliminated. The gas-phase concentrations for phenanthrene and anthracene on days where breakthrough occurred were corrected using these ratios. PUF plugs and GFFs, which had been cleaned but not used for sampling, were extracted as blanks with each set of GFFs or PUF plugs that were cleaned. These blanks were free of PAH. All extracts of tree bark, leaves, needles, and seeds were solvent exchangedto hexane and cleaned up before analysis using Burdick and Jackson, 2 g, silica inert solid-phase extraction (SPE) columns (Muskegon, MI). The SPE columns were cleaned with CH2C12 and activated in an oven at 170 "C for 24 h, and the SPE manifold was cleaned with CHzClz before each use. The PAH were eluted from the SPE column with a 10-mL solution of 10% CHzClz in hexane and a 10 mL solution of 15% CHZC12 in hexane. These fractions were combined for analysis. Recoveries of known amounts of PAH from the silica columns were 94-98%. All air sample extracts were sufficiently clean so that no further cleanup was necessary. Sample Analysis. All samples were analyzed on a Hewlett Packard 5989 gas chromatographic mass spectrometer equipped with a 30 m X 250 pm i.d. (0.25-pm film thickness) J&W Scientific (Folsom, CA), DB-5 fused silica capillary column. One microliter of the sample was injected on-column after being concentrated to 250 pL under a gentle stream of N2. GC temperature program conditions were 40 "C, ramped at 30 "C/min to 170 "C, then at 3 "C/min to 285 "C and held for 10 min. The mass spectrometer was operated in the electron impact ionization mode with an electron energy of 70 eV, and the ion source temperature was 250 "C. PAH were quantified by selected ion monitoring with an internal standard (see above). Phenanthrene, anthracene, fluoranthene, pyrene, benz[a]anthracene, chrysene, triphenylene, benzo[elpyrene, benzo[alpyrene, indeno[l,2,3-cdlpyrene, and benzo[ghi]perylene were quantitated. Chrysene and triphenylene were quantitated together because they are not gas chromatographically resolved. The instrumental detection limit, atmospheric detection limit, and vegetation detection limit were approximately 50 pg, 1pg/m3 of air, and 130 pg/g of dry wt vegetation, respectively for the individual PAH. Moisture and Lipid Determination. The moisture and lipid content of the vegetation was determined so that the concentrations could be given on a moistureadjusted basis (ng/g of dry wt) or a lipid-adjusted basis (ng/mg of lipid). An aliquot of each vegetation sample was weighed, dried at 95 "C for 24 h, and weighed again;
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amounts of lipids. Figure 1B shows similar fluctuations in the total PAH concentrations among the different tissue types; with initially high PAH concentrations in the spring, decreased concentrations in the summer, and higher concentrations again in the fall. Clearly, PAH concentrations should be normalized to lipid concentrations for maximum data intercomparability. In the atmosphere, the partitioning of SOCs between the gas phase and the particle phase is given by a partition coefficient
A
1400
8 1200 > u 1000 800
2
600 400 I-
k C
200 n "
100
125
150
175
200
225
250
Day Number
April
275 300 October
I"
100
125
150
175
April
200
225
Day Number
250
275
300 October
Figure 1. Total PAH concentration over the growing season on (A) gram dry weight vegetation basis and (B) lipid-adjusted basis. The error bars represent the standard deviation of four measurements.
the difference gave the percent moisture. A 5050 solution of hexane and acetone was added to the dried vegetation, and the sample was sonicated for 1 h. The solvent was decanted off into a preweighed drying pan and allowed to evaporate for 24 h, and the process was repeated. The pans were reweighed, and the percent lipids by weight was calculated. Experiments were done to ensure that none of the lipid-like material was lost during the drying step. The moisture and lipid content of the vegetation samples varied slightly throughout the growing season. The moisture content ranged from approximately 25 % to 60 % by wet weight, and the lipid content ranged from approximately 0.5% ' to 1.5 7% by dry weight. The vegetation showed decreasing moisture content and increasing lipid content as the growing season progressed.
Results and Discussion Upon inspection of the data, we noted that plant lipid concentration had a large effect on the plant's PAH concentration. Figure 1illustrates the effect of vegetation lipid content on the sorption of PAH to vegetation. Figure 1A shows the fluctuations in total PAH (sum of all individual PAH) concentration per gram of dry weight of vegetation for needles, seeds, and leaves over the growing season. Vegetation with a high lipid content (such as needles) generally had a high PAH concentration. When these concentrations are normalized to the vegetation lipid content (Figure lB), the variation is not as great among the different plant tissue types. This suggests that most of the variation in PAH concentration among the different plant tissues from the same site can be attributed to varying
Kp = (F/TSP)/A
(1)
where F is the equilibrium concentration of the SOC associated with atmospheric particles (ng/m3);TSP is the total atmospheric particle concentration (pg/m3), which is a measure of the total capacity for gas-phase sorption to particles, and A is the equilibrium gas-phase concentration of the SOC (ng/m3) (33,34). Kphas been defined in this manner so that as Kpincreases, partitioning to the particles increases. Logarithms of Kp values versus logarithms of subcooled liquid vapor pressures are highly correlated, indicating that, as the vapor pressure of the compound decreases, partitioning to atmospheric particles increases (34, 35). As expected from the ClausiusClapeyron relationship,Kpis highly dependent on ambient temperature, and the energy of SOC particle binding can be calculated from the slope of a plot of In Kp versus 1 / T (34). These plots give a positive slope; as the ambient temperature decreases, partitioning to atmospheric particles increases. Partitioning of SOCs between water and fish is given by a similar partition coefficient
Kb = (fish/lipid)/water
(2)
where fish is the SOC concentration in the fish (ng/g), lipid is the lipid content of the fish (mg/g), and water is the dissolved-phase concentration of the SOC in the water (ng/L) (36,37). The lipid term is analogous to the TSP term in eq 1; it is a measure of total capacity for accumulation of lipophilic SOCs in fish. Logarithms of Kb versus logarithms of the octanol-water partition coefficient (Kow)are highly correlated; as KO, increases, partitioning to fish increases (36, 38). There is some evidence that Kb is a function of water temperature (37), but the relationship is not as clear as it is for Kp' By analogy to these examples, we define a vegetationatmosphere partition coefficient
K, = (veg/lipid)/(gas)
(3)
where veg is the concentration of an SOC in vegetation (ng/g of dry wt), lipid is the lipid content of the vegetation, which is a measure of the total capacity for sorption of lipophilic SOCs to vegetation (mg/g of dry wt), and gas is the atmospheric gas-phase SOC concentration prior to the collection of vegetation (ng/m3). The units for K , are m3 of air per mg of lipid. A dimensionless partition constant can be obtained, if necessary, using an air density of 1.19 X lo6mg/m3 (at 25 "C). We calculated vegetationatmosphere partition coefficientsfor the 10PAH measured in each of our 75 samples (atotal of 750 data points); these samples included sugar maple leaves and seeds, pine Envlron. Sci. Technol., Vol. 28, No. 5, 1994
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