Environ. Sci. Technol. 1996, 30, 3570-3577
Spatial Distribution of PAHs in the U.K. Atmosphere Using Pine Needles PAOLO TREMOLADA,† VICTORIA BURNETT,‡ D A V I D E C A L A M A R I , * ,§ A N D KEVIN C. JONES‡ Ecotoxicology Group, Institute of Agricultural Entomology, University of Milan, Via Celoria 2, 20133 Milan, Italy, Institute of Environmental and Biological Sciences, Lancaster University, Lancaster LA1 4YQ, U.K., and Third Faculty of Sciences, Environmental Research Group, University of Milan, Via Ravasi 2, 21100 Varese, Italy
A study of the spatial distribution and mixture of polycyclic aromatic hydrocarbons (PAHs) in pine needles sampled across the U.K. in the summer of 1994 is presented. PAHs reach pine needles via atmospheric transport and deposition processes. Phenanthrene was distributed irregularly across the U.K., while the other PAHs generally decreased on a northward gradient from the southern England to northern Scotland by a factor of ∼7. A relationship was found between the mean PAH concentrations of each area sampled and the population density. Fingerprint technique enabled differences in the PAH composition among the different areas to be highlighted. A southern, central, and northern fingerprint were determined over a more general uniform contamination pattern. Calculated air concentrations, through bioconcentration factors (BCF) based on octanol-air partition coefficients (Koa), were compared with measured data from the literature. The underestimation of the calculated values were related to the Koa of each compound, indicating that for log Koa values >8-9, Koa-based BCFs do not correctly predict mean air concentrations.
Introduction Polycyclic aromatic hydrocarbons (PAHs) are a group of ubiquitous environmental contaminants, some of which are of environmental concern because of their mutagenic and carcinogenic properties. They are formed during incomplete combustion from natural and anthropogenic sources. Many human activities result in the formation of PAHs: notably vehicle emissions, the use of fuel for residential heating, industrial processes, electric power production, and waste incineration (1). Evidence indicates * Author to whom correspondence should be addressed; e-mail address:
[email protected]. † Institute of Agricultural Entomology, University of Milan. ‡ Lancaster University. § Environmental Research Group, University of Milan.
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that anthropogenic sources have greatly increased the environmental burden of these compounds since the mid1800s (2). In the present study, we wished to assess the current regional distribution of PAHs in air across the U.K. For reasons of cost and convenience, we were unable to make direct measurements of PAHs in air nationwide, although some urban and rural PAH monitoring stations do exist in the U.K. (3, 4). Analysis of soils would have reflected a mixture of past and recent inputs. We therefore decided to use analysis of vegetation as a surrogate for direct air sampling. Vegetation has been used previously as an indicator of persistent organic pollutants in the troposphere (5, 6), notably for large-scale distribution studies (7, 8). This approach is based on the premise that the leaf contents can provide a ‘time integral’ of the airborne concentration. It is acknowledged that many limitations may exist and need to be borne in mind in a study of this type. These include kinetic constraints over the rate of uptake of compounds from the air; analytical complications in dealing with waxy, lipid-rich plant tissues; and variability in the uptake characteristics of different plant species and compounds. However, given our objective of obtaining a regional/nationwide indication of the distribution of PAHs throughout the U.K. this approach was adopted. Hydrophobic, persistent organic contaminants reach plant leaves primarily from the atmosphere (9-11), with root uptake and translocation being of very limited importance. However, persistent organic contaminants may exist in the atmosphere in either the gas or the particulate phase or both. Consequently, the mechanism of transfer to the leaf may be via gaseous dry deposition, particulate dry deposition, or wet deposition. Gaseous dry deposition has been shown to be the key process for many organic contaminants (12, 13). Compounds with a vapor phase component in air are envisaged as being subject to an air-leaf exchange process, moving toward equilibrium over time. Whether the leaf attains equilibrium with the air therefore presumably depends on the compound, the plant species, and the period of exposure. Bacci et al. (14) have shown the dependence of the bioconcentration of organic chemical vapors in azalea leaves on physicochemical properties, and the octanol-air partition coefficient has been defined as a key partitioning descriptor (14-16). A recent validation of BCFs calculated from octanol-air partition coefficients (Koa) together with field and laboratory measured BCFs was published by Morosini et al. (17) for a number of chlorinated hydrocarbons. Several models of the partitioning of SOC between air and vegetation have been studied, using Koa-based BCFs to predict leaf-air partitioning (13, 16, 18-20). The U.K. provides an interesting study area. The south, midlands, and parts of the north of England are the most densely populated, while Wales and Scotland are much less populated. Areas of the far north of Scotland could be regarded as remote. The mean annual temperatures in the south of England (10 °C) are warmer than in the far north of Scotland (3-4 °C). Although limitations may be derived from representing all the U.K. by a limited number of samples (28 samples), from the sampling modalities followed by this work and
S0013-936X(96)00269-6 CCC: $12.00
1996 American Chemical Society
FIGURE 1. Sample location on the U.K. map. Codes are those of Table 1.
from the above-mentioned factors by which accumulation in vegetation may depend on, the diffuse PAH contamination levels in the U.K. atmosphere has been analyzed using pine needles as passive samplers. Pine (Pinus sylvestris) needle samples were taken across the U.K. on a generally north-south gradient away from local point sources. Pine needles have been used in studies of this type before (7, 21-24). In addition, pine is widely distributed across the country.
Materials and Methods Sampling. A series of latitudinal transects across the U.K. were sampled in two periods in 1994, May-June and August. Nine broadly west-east transects were sampled to give a total of 28 samples nationwide. The transect sampling approach consisted of the collection of a variable number of samples at a fixed distance (in tens of kilometers) across an area. Each sampling station (one pine needle sample) is represented by a pool of needles collected under several trees near each other. The samples were collected in sites selected to be representative of a given region; samples were taken as far as possible away from main cities, principal roads, and industrial plants in order to reflect general/ diffuse regional differences in atmospheric PAH concentrations. Figure 1 shows the sample location of the nine transects. Further details on the sample sites and their locations on the U.K. grid system are available from the authors.
Even if many alternatives are possible in the collection of pine needles, e.g., collection of fresh needles of different years from the tree or collection of freshly shed pine needles from the ground, and considering that every method has advantages and disadvantages, we decided for the collection of “freshly” shed pine needles as sampling modality. The collection of the needles after they were shed by the tree was performed in order to give the longest time for them to have approached equilibrium with the air (16). Pine needle samples (about 10 g each) were wrapped in aluminium foil and closed in glass jars. The foil and glass jars had both been rinsed twice previously in n-hexane. Samples were stored at -20 °C until analysis was performed. Chemical Analysis. Sample Pretreatment. Samples were homogenized, and the water content was measured on subsamples by weighing after 12 h in the oven at 105 °C. Extraction. A pre-extraction was performed with nhexane to clean the six-place Soxhtec extraction apparatus and the thimbles (2 h hot extraction and 2 h rinsing). Samples (∼3 g) were then weighed into the extraction thimbles, and the extraction was performed using n-hexane (2 h hot extraction and 2 h rinsing). One blank extraction was run in each batch of five samples. Cleanup. Several chromatographic columns were prepared to purify the extracts using alumina (Aluminium Oxide Active, BDH, Poole, England), florisil (Florisil about 60-100 U.S. mesh, BDH, Poole, England), and silica (silica gel 60, particle size 0.063-0.200 mm, Merck, Darmstadt, Germany). Sample extracts were concentrated to a few milliliters under a stream of nitrogen, loaded onto an alumina column (3 g of alumina deactivated 4% + 0.2 g of anhydrous sodium sulfate on the top in 7 mm i.d. glass column). The sample was eluted with 6 mL of DCM, and this step was repeated after reduction of the sample to about 1 mL by a nitrogen stream. Florisil column chromatography (1 g of Florisil in pasteur pipet) was repeated twice, loading the sample concentrated to about 1 mL and eluting it with 10 mL of DCM. The sample was gently dried under a nitrogen stream, resuspended with n-hexane, and loaded on a silica gel column (2.5 g of silica gel activated at 130 °C for 3 h + 0.2 g of anhydrous sodium sulfate on the top in 7 mm i.d. glass column). The PAHs were eluted in a second fraction, after a 34-mL n-hexane fraction, using 10 mL of n-hexane:DCM (50:50). The sample was gently dried under a nitrogen stream, resuspended with 1 mL of acetonitrile, and filtered ready for analysis by HPLC. Analysis. HPLC analyses were performed using a PerkinElmer LC Model 250 with a Perkin-Elmer LS40 fluorescence detector and a Spherisorb S5 ODS2 column. The run program was as follows: flux, 1.5 mL/min; 20 min (equilibration time), 40% water and 60% acetonitrile; 15 min, 40% water and 60% acetonitrile; 10 min, a gradient to 100% acetonitrile; 20 min, 100% acetonitrile; and 5 min to return to 40% water and 60% acetonitrile. The wavelength program was as follow: 10 min, 280 nm excitation, 340 nm emission; 6 min, 240 nm excitation, 400 nm emission; 44 min, 305 nm excitation, 430 nm emission. The following compounds were determined (abbreviations are given in parentheses): acenaphthene (ACE), fluorene (FLU), phenanthrene (PHE), anthracene (ANTH), fluoranthene (FLUO), 1-methylphenanthrene (MPHE), pyrene (PYR), benzanthracene (BENZANTH), chrysene (CHRY), benzo[b]fluoranthene (B[b]F), dibenz[ac]anthracene (D[ac]A), benzo[k]fluoranthene (B[k]F), benzo[a]pyrene
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TABLE 1
Concentration of 16 Individual PAHs and ∑PAH in 28 Pine Needle Samples from the U.K.a sites
FLU
PHE
