Environ. Sci. Technol. 2005, 39, 2976-2982
Contribution of Biomass Burning to Atmospheric Polycyclic Aromatic Hydrocarbons at Three European Background Sites MANOLIS MANDALAKIS,† O ¨ R J A N G U S T A F S S O N , * ,† TOMAS ALSBERG,† A N N A - L E N A E G E B A¨ C K , † CHRISTOPHER M. REDDY,‡ LI XU,‡ JANA KLANOVA,§ IVAN HOLOUBEK,§ AND EURIPIDES G. STEPHANOU| Department of Applied Environmental Science (ITM), Stockholm University, 10691 Stockholm, Sweden, Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, RECETOX-TOCOEN & Associates, Kamenice 126/3, 625 00 Brno, Czech Republic, and Environmental Chemical Processes Laboratory (ECPL), Department of Chemistry, University of Crete, GR-71409, Heraklion, Greece
Radiocarbon analysis of atmospheric polycyclic aromatic hydrocarbons (PAHs) from three background areas in Sweden, Croatia, and Greece was performed to apportion their origin between fossil and biomass combustion. Diagnostic ratios of PAHs implied that wood and coal combustion was relatively more important in the northern European site, while combustion of fossil fuels was the dominant source of PAHs to the two central-southern European background sites. The radiocarbon content (∆14C) of atmospheric PAHs in Sweden ranged between -388‰ and -381‰, while more depleted values were observed for Greece (-914‰) and Croatia (-888‰). Using a 14C isotopic mass balance model it was calculated that biomass burning contributes nearly 10% of the total PAH burden in the studied southern European atmosphere with fossil fuel combustion making up the 90% balance. In contrast, biomass burning contributes about 50% of total PAHs in the atmosphere at the Swedish site. Our results suggest that the relative contributions of biomass burning and fossil fuels to atmospheric PAHs may differ considerably between countries, and therefore, different national control strategies might be needed if a further reduction of these pollutants is to be achieved on a continental-global scale.
Introduction Air pollution remains of great public health concern (1-3) as, for instance, epidemiological studies demonstrate that particle-borne air pollution contributes significantly to human morbidity (1) and mortality (1, 2). Despite the improvement of air quality over the past decades, it is * Corresponding author phone: +46-8-6747317; fax: +46-86747638; e-mail:
[email protected]. † Stockholm University. ‡ Woods Hole Oceanographic Institution. § RECETOX-TOCOEN & Associates. | University of Crete. 2976
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distressing that about 30-40% of Europeans live in cities where the pollution level is equal or above the guidelines of the European Union (4). In Austria, France, and Switzerland this human exposure causes 6% of total mortality, corresponding to more than 40 000 premature deaths annually (1). The ubiquitous polycyclic aromatic hydrocarbons (PAHs) are excellent molecular tracers of combustion particles (5, 6) and in themselves account for most (35-82%) of the total mutagenic activity of ambient aerosols (7). For this reason the European Commission recently suggested a directive enforcing activities to reduce air pollution by PAHs (8). The development of effective control and mitigation strategies toward the reduction of atmospheric PAHs requires a reliable identification and apportionment of the various release sources. Atmospheric PAHs are generally formed by incomplete combustion of any carbon-based fuel. Vehicle exhausts, power and heat generation plants, residential heating, incinerators, and several industrial processes (e.g., coke production, aluminum smelting) are all significant sources of PAHs (5, 9, 10). While combustion of fossil fuels is currently the main source of PAHs (5, 10), the utilization of biomass fuels is becoming increasingly important (11, 12) due to the global momentum to restrict the emissions of greenhouse gases (13) and also due to the rising cost of oil. Assessment of the current (and predicted future) contribution to the ambient PAH load from biomass fuels with traditional emission inventory approaches is challenged by variations in reported biomass emission, spanning over several orders of magnitude (e.g., 1-370 µg of PAHs per kilogram of wood) (5). This dilemma of highly variable emission factors suggests that we should also try to develop alternative strategies for assessing the contribution of various sources to the PAHs present in the ambient environment. As a complement and alternative to traditional emission inventory modeling approaches, several methods are continuously developed with the aim of diagnostically assessing the sources of PAHs based on their inherent molecular properties. For instance, the ratios of specific PAHs (e.g., phenanthrene-to-anthracene and fluoranthene-to-pyrene ratios) are frequently used as indicators of specific combustion processes (6, 14, 15). Sulfur-containing heterocyclic compounds (e.g., dibenzothiophenes and benzonaphthothiophenes) may be used as tracers for coal combustion, petroleum products, and diesel exhaust. Further, retene (16) and 1,7-dimethylphenanthrene (17) have been suggested as molecular tracers for tracking and assessing emissions from burning of specific biomass fuels. However, a recent study combining these latter molecular markers with PAH-specific radiocarbon measurements called their utility as source tracers into question for PAHs in anoxic sediments (18). In certain situations stable carbon isotopic composition (δ13C) of individual PAHs may provide insight about the contribution of specific sources (19, 20), but overlapping end member δ13C values for several PAH sources may limit the broad utility of this approach (18). Radiocarbon analysis of specific compounds and compound classes has recently been established as a unique and powerful tool for quantitatively assessing the relative contributions of contemporary biomass versus fossil fuel combustion sources of PAHs (18, 21, 22). The underlying principle of the radiocarbon approach is that fossil fuels, and the compounds emitted from their combustion, contain nearly no 14C because the geologic age of these fuels is much greater than the half-life of radiocarbon (5730 year). In contrast, all substances derived from biomass burning would exhibit a 10.1021/es048184v CCC: $30.25
2005 American Chemical Society Published on Web 03/12/2005
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C/12C ratio of the atmosphere and current living matter (about 1.2 × 10-12). Nevertheless, radiocarbon analysis of specific organic compounds or compound classes is exceedingly rare because of the demanding analytical requirements. First, even with modern microscale preparation techniques for accelerator mass spectrometry (AMS) (23) the amount of carbon required for each 14C measurement (at least 20 µg of carbon) is quite high compared with the abundance of most organic chemicals in the atmosphere. Further, the compound-specific isotope analysis requires not only extensive air sampling but also a significant effort in order to isolate individual compounds in these large quantities from the total complex organic material of the aerosols. To the best of our knowledge, radiocarbon measurements of atmospheric PAHs have to date only been performed for one large urban aerosol sample (21-22). In the current study PAHs were harvested from leftover extracts of previous long-term PAH air-monitoring projects and large air samples collected from background areas of Europe. The radiocarbon content of several isolated, harvested, and pooled PAHs was measured, and the contribution of biomass burning was subsequently estimated.
Materials and Methods Sampling and Extraction Protocols. Twelve 24-h air samples were collected during July 2003 at the marine background sampling station of Finokalia (35°20′N, 25°40′E), a coastal site 70 km eastward of Heraklion on the island of Crete, Greece. The sampling tower is on the top of a hilly elevation (130 m above the sea level), and the site and its local meteorology are described elsewhere (24). Air sampling was performed using a high-volume air sampler consisting of a glass fiber filter and a polyurethane foam plug arranged in series to collect both particulate and gaseous PAHs. Both collection media were extracted, as described below, and the extracts from all 12 samples were pooled together before their cleanup and isolation of PAHs. Preceding sampling the filters were heated at 450 °C for 5 h. Also prior to use polyurethane foam plugs were boiled in water, rinsed with acetone, and Soxhlet extracted twice for 24 h with dichloromethane. Finally, they were dried in a vacuum desiccator and sealed in glass jars until sampling. PAH extracts of monthly aerosol and vapor samples that had been collected from the background air monitoring station of Aspvreten, Sweden (58°48′N, 17°23′E) between 1995 and 2001 were obtained. Aspvreten is a forested and semirural site situated about 80 km southwest of Stockholm and about 2 km from the coast of the Baltic Sea, and its further characteristics are detailed elsewhere (25). The leftover PAH extracts from this long-term monitoring campaign were pooled into two samples for radiocarbon determination of the PAHs corresponding to the time periods 1995-1997 and 1998-2001, respectively. Very similar procedures were followed for Croatian air samples collected and pooled from four locations near Zadar (Adriatic coast; 49°06′N, 15°14′E) and one location in the Velebit Mountains (44°49′N, 14°58′E) during spring 2003. Extracts of 50 24-h air samples (10 days of sampling at five sites) were pooled together to obtain one sample for 14C analysis of PAHs. Air sampling in Croatia and Sweden was conducted using similar high-volume sampler systems as described above for Finokalia, Greece. Both the particulate and gaseous phases had been extracted and were included in the leftover extracts. Thus, all PAHs isolated in the current study for 14C analysis corresponded to PAHs from both the particulate and gaseous phases in the ambient atmosphere. For each air sample collected from Finokalia the polyurethane foam plug and the corresponding glass fiber filter were placed in a Soxhlet apparatus, and they were extracted with dichloromethane for 24 h. The extract was concentrated
to 4 mL by rotary evaporation and then to 1 mL under a gentle nitrogen stream at ambient temperature. The extract was subsequently applied onto a column of deactivated silica gel (SiO2-10% H2O, 63-200 µm particle size, height 10 cm, i.d. 1 cm) topped with sodium sulfate and eluted with 60 mL of n-hexane. All leftover PAH extracts from Zadar & Velebit were also treated with similar SiO2 column chromatographic methods. Acetone was used for Soxhlet extraction of the Aspvreten samples. Following a reduction in extract volume by rotary evaporation and subsequent addition of water, the Aspvreten PAHs were transferred to hexane by liquid-liquid extraction prior to cleanup using Isolute solid-phase extraction (SPE) Florisil cartridges (International Sorbent Technology, Mid Glamorgan, U.K.). The hexane fractions from all the pooled aerosol samples were further treated by a dimethylformamide (DMF)pentane cleanup procedure (26) in the laboratory at Stockholm University. In brief, the solvent was evaporated and exchanged to n-pentane (2 mL), and the extract was partitioned twice with DMF-5% H2O (2 mL). The DMF layers were pooled in the same glass tube, mixed with 4 mL of water, and partitioned twice with n-hexane (4 mL). The n-hexane fractions were collected in the same flask and evaporated to about 0.5 mL. Finally, the extracts were eluted through a miniaturized SiO2 column (height 1 cm, i.d. 0.5 cm; elution with 8 mL of n-hexane) to remove any residues of DMF or water. Preparative Capillary Gas Chromatography and Accelerator Mass Spectroscopy. After DMF cleanup all the final PAH extracts were evaporated to about 200 µL, and each one was repeatedly injected (about 40 serial injections of 5 µL each) onto a preparative capillary gas chromatograph (PCGC) programmed to trap selected PAHs in the Stockholm University laboratory. The Gerstel PCGC system and optimization of operational parameters for the isolation of PAHs have both been described elsewhere (18, 27). Since the abundance of target PAH compounds present in these large air sample extracts still was quite low relative to the requirements of the state-of-the-art 14C measurement technique (23), compound-class-specific radiocarbon analysis (CCSRA) of several isolated and pooled PAHs was the aim. From each extract the most abundant PAH members (nine individual compounds plus seven coeluting pairs) were harvested together in the same trap of the PCGC preparative fraction collector to obtain a sufficient amount for one radiocarbon measurement (Supporting Information, Table S1). Phenanthrene/anthracene and chrysene/benzo[a]anthracene were not harvested from the extracts of Aspvreten since these samples contained deuterated phenanthrene and chrysene, which eluted so closely to the native compounds that isolation of the latter during PCGC was not feasible. This is unlikely to make a significant difference as phenanthrene makes up a similar fraction of the total PAH from both biomass and fossil fuel combustion (28). The isolated PAHs were rinsed from the glass traps (5 times with 200 µL of dichloromethane) and further cleaned on separate SiO2 columns (height 4 cm, i.d. 0.5 cm; elution with 25 mL of hexane/dichloromethane (1:1)). A small portion of the final extract was saved to test for purity and yield of the analytical procedure, while the remaining material was shipped to Woods Hole Oceanographic Institution (WHOI) for further treatment and final isotopic analysis. There, the PCGC isolates were again passed through a Si column to remove any residual column bleed and then transferred to precombusted quartz tubes. The solvent was evaporated under a stream of nitrogen, and ∼100 mg of copper oxide was added. The tubes were evacuated, sealed, and combusted at 850 °C for 5 h, and the resulting carbon dioxide was isolated through a series of cold traps and quantified by manometry. About 10% of the carbon dioxide was reserved for δ13C analysis VOL. 39, NO. 9, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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by isotope ratio mass spectrometry, and the remaining amount was reduced to graphite (23). Targets of the graphite were pressed and mounted on target wheels for 14C analysis by accelerator mass spectrometry (AMS) at the National Ocean Sciences AMS (NOSAMS) facility at WHOI. All 14C measurements are expressed as the per mil (‰) deviation from SRM 4990B (∆14C) (29).
Results and Discussion Distribution and Diagnostic Ratios of PAHs. The concentrations and diagnostic ratios of atmospheric PAHs are often used to evaluate the impact and contribution of various sources to a specific region. Atmospheric concentrations of PAHs were available for Finokalia (30), Aspvreten, and Zadar & Velebit (unpublished data) and are briefly discussed here. Long-term measurements in Aspvreten between 1995 and 2001 indicated a gradual decline of annual-average ΣPAH (summed concentrations of 12 parent PAHs) from 9.0 (1995; extrapolated from only 6 months of sampling) to 5.5 (1996) to 1.6 ng m-3 (2001) (Supporting Information, Table S2). This decline of atmospheric PAHs likely reflects the strict legislation applied in Sweden during this period for the mitigation of air pollution. The more recent year-round concentrations of PAHs in Aspvreten are substantially lower than those observed at Finokalia and the Zadar & Velebit sites, although a certain seasonal influence cannot be excluded for the Croatian site since it has not been sampled year round. The concentration of ΣPAH measured at the five Croatian sites during spring 2003 ranged from 0.6 to 10.2 ng m-3, providing an average value of 4.8 ng m-3, while the average year-round concentration of ΣPAH at Finokalia during 2000-2002 was 10.8 ng m-3 (Supporting Information, Table S3). In general, the levels of PAHs in the studied background areas were lower than those previously reported for several urban centers of Europe (9, 31, 32) but considerably higher than those presented for remote areas of the Arctic (33). The lower concentrations of atmospheric PAHs in Sweden could be explained by the less strict regulation of emissions in southern European countries, a seasonal effect, and/or the more efficient scavenging of atmospheric PAHs in higher latitudes through both temperature-driven dry deposition/condensation and precipitation-borne wet deposition. In contrast to ΣPAH concentrations, the fluoranthene to fluoranthene-plus-pyrene ratio (Fl/(Fl+Py)) was higher at the northern site. The average Fl/(Fl+Py) at Finokalia, Zadar & Velebit, and Aspvreten was 0.49, 0.57, and 0.68, respectively, which in all cases indicated the predominance of combustion sources over petrogenic ones (14, 15). More specifically, previous studies have shown that Fl/(Fl+Py) ratios below 0.40 imply the prominence of unburned petroleum (petrogenic sources), ratios from 0.40 to 0.50 suggest the combustion of liquid fossil fuels (vehicle and crude oil), whereas ratios larger than 0.50 are characteristic for grass, wood, or coal combustion (15). According to these values the increasing Fl/(Fl+Py) ratio is consistent with the combustion of wood and coal being relatively more important in northern than southern European countries, while the opposite appears to hold for the combustion of liquid fossil fuels. The prominence of combustion over petrogenic sources was also supported by the relative concentrations of indeno[1,2,3-cd]pyrene (IP) and benzo[ghi]perylene (BgP). It has been suggested that IP/(IP+BgP) ratios lower than 0.20 likely imply petroleum, ratios between 0.20 and 0.50 liquid fossil fuel (vehicle and crude oil) combustion, and ratios larger than 0.50 grass, wood, and coal combustion (15). The average IP/(IP+BgP) ratio at Finokalia, Zadar & Velebit, and Aspvreten was 0.51, 0.31, and 0.55, respectively, largely corroborating the above results for the Greek and Swedish sites. Overall, both Fl/(Fl+Py) and IP/(IP+BgP) diagnostic ratios demonstrated that combustion processes, rather than unburned 2978
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FIGURE 1. High-resolution gas chromatograms of an aerosol extract from Aspvreten (a) before and (b) after the isolation of PAHs by PCGC. The peaks corresponding to dibenzothiophene (1), phenanthrene/anthracene (2), 3,2-methylphenanthrene (3), 4,1-methylphenanthrene (4), phenylnaphthalene (5), dimethylphenanthrenes (6), fluoranthene (7), pyrene (8), methylpyrene (9), benzo[ghi]fluoranthene (10), chrysene/benzo[a]anthracene (11), benzo[b+k]fluoranthene (12), benzo[a+e]pyrene (13), perylene (14), indeno[1,2,3cd]pyrene (15), benzo[ghi]perylene (16) are indicated in each chromatogram (the samples were run with different GC oven temperature programs). fossil fuels, were the origin of atmospheric PAHs, but these molecular markers exhibit a partial disagreement regarding the dominant combustion sources in each area. The relative abundance of 1,7-dimethylphenanthrene (1,7DMP) and 2,6-dimethylphenanthrene (2,6-DMP) has also been suggested to be a useful tool for distinguishing between soft wood combustion and motor vehicle emissions (15, 17). In general, air samples with a 1,7-DMP/(1,7-DMP+2,6-DMP) ratio between 0.70 and 0.90 supposedly indicate wood combustion, while ratios lower than ∼0.45 are indicative of vehicle emissions (15). The concentrations of dimethylphenanthrenes were measured in aerosols of Finokalia (30), and the average value of the 1,7-DMP/(1,7-DMP+2,6-DMP) ratio was 0.54, implying mixed emissions from diesel and gasoline-fueled vehicles and, for the largely deforested island of Crete, a 37% contribution from wood combustion. This suggested large biomass contribution is not supported by the radiocarbon data (below), and this study thus adds to other recent findings (18) to question the utility of this DMP ratio as a reliable source tracer. Unfortunately, data on the concentrations of dimethylphenanthrenes were not available for the other two sampling sites. Carbon Isotopic Composition of Atmospheric PAHs. The treatment of the atmospheric samples by SiO2 column chromatography and dimethylformamide provided appropriately clean extracts for the successful isolation of PAHs by PCGC. The gas chromatograms of the Aspvreten air extracts, before and after PCGC trapping of PAHs, demonstrate the efficiency of this isolation/harvesting procedure (Figure 1). The total amount of pooled target PAHs harvested from the four samples ranged between 14 and 68 µg, while the purity of the isolates varied from 89% to 96% (Table 1). The stable carbon isotopic composition (δ13C) of atmospheric PAHs has been explored as a measure to apportion the emission sources in several previous studies (19, 20, 34-
TABLE 1. Yield, Purity, and Isotopic Composition of PAHs Isolated from Four Aerosol Samples sampling sitea
yield (µg)
purity (%)b
NOSAMS accession no.c
δ13C (‰)d
∆14C (‰)e
FBiomass(%)f
Aspvreten, Sweden (1995-1997) Aspvreten, Sweden (1998-2001) Zadar & Velebit, Croatia (2003) Finokalia, Greece (2003)
14.1 35.7 68.0 36.1
95.4 96.1 89.2 93.5
OS-41865 OS-41862 OS-43143 OS-43150
-27.7 -29.2 -29.0
-381 -388 -888 -914
51 50 9 7
a The numbers in parentheses indicate the time period of aerosol sampling. b This purity was assessed prior to shipment from Stockholm University, and the subsequent additional cleanup at WHOI may have further increased these purities. c AMS accession numbers for each 14C analysis. d Standard deviation for all δ13C measurements is (0.1‰, based on replicate analysis of standards. e The relative standard error for these 14C data is 1-3%. f F Biomass is the percentage contribution of biomass burning to atmospheric PAHs calculated by an isotopic mass balance approach (18, 40).
36). The δ13C values of the currently isolated PAHs exhibited low variability between different sampling sites, relative to previous studies, and ranged from -29.2‰ to -27.7‰ (Table 1). Overall, these values were in the same range as those previously observed in rural aerosols of Canada (-29‰ to -23‰) (37) but more negative than those measured in urban aerosols of Kuala Lumpur, Malaysia (-27.9‰ to -17.7‰) (35), and in aerosols collected from an urban area of Canada (-26‰ to -23‰) (37). Although, the δ13C signal can be used as a rough indicator for the origin of PAHs, a quantitative assignment of the relative importance of different sources is difficult. This is due to the overlap of the end member δ13C values of different PAH sources, such as diesel (-24‰ to -23‰) (35), coal (-31‰ to -25‰) (36, 38), gasoline (-30‰ to -19‰) (35, 38), and wood-burning smoke (-31.6‰ to -26.8‰) (35, 39). Nevertheless, the δ13C of PAHs in our air extracts indicated that wood, gasoline, and coal combustion may all be significant sources of PAHs, while combustion of diesel should be of minor importance for these sites. In contrast, the radiocarbon signal (∆14C) of PAHs was highly variable between the different sites. The ∆14C of PAHs in air samples of Finokalia and Zadar & Velebit was -914‰ and -888‰, respectively, while much less depleted values were observed for the two Aspvreten samples (-388‰ and -381‰) (Table 1). All chemical species originating from fossil sources have a ∆14C of -1000‰, while the radiocarbon abundance of the compounds emitted from modern biomass should be in the vicinity of +225‰ (40). The latter value is approximately equivalent to the 14C signal previously measured in soft wood (41), while a ∆14C value of +235‰ has been attributed to plant waxes (42). In this context, the ∆14C values of atmospheric PAHs indicate that the combustion of fossil fuels is the main source of PAHs in the two background areas of Greece and Croatia, while there is a substantial contribution of biomass burning to the ambient PAHs in Aspvreten, Sweden. Although radiocarbon measurements have been performed for several different organic compounds isolated from a range of environmental matrixes such as sediments (18, 21, 42-44), soils (45), and marine animals (46), 14C data for PAHs or other substances present in atmospheric aerosols are very rare. This is mainly due to the much lower concentrations of the target compounds in ambient air compared to the other matrixes previously investigated. To the best of our knowledge, 14C measurements of atmospheric PAHs have been performed only for the NIST atmospheric reference material SRM 1649, which is ambient particulate matter collected from a parking lot in urban Washington, D.C., during 1976-1977 (21, 22). The ∆14C values of individual PAHs in SRM 1649 ranged from -963‰ to -914‰ and revealed that this sample predominantly contained fossil PAHs. Radiocarbon results have also recently been reported for several PAHs isolated from four surface sediments in and around Stockholm, Sweden, collected in 2002 (ranging from -934‰ and -550 ‰) (18) and for the two sedimentary reference materials SRM 1941a (Baltimore, MD, collected in
1991) and SRM 1944 (New York/New Jersey Waterways, collected in 1994) (-986‰ to -711‰) (21). On the contrary, household soot from creosote-impregnated wood used in residential fireplaces contained PAHs with substantially higher radiocarbon abundances (-274‰ to +74‰) (40), and this was consistent with the contemporary levels of 14C in wood. Fatty acids are the only other compound class whose radiocarbon abundance has been measured in semiurban aerosols (47, 48). With the exception of C24 and C26 monocarboxylic fatty acids, the ∆14C of the fatty acids from C16 to C34 ranged between +407‰ and -90‰, suggesting that these compounds were mainly emitted from living higher plants and possibly from marine organisms (47). In several other studies atmospheric measurements of radiocarbon have been conducted on the mixture of non-methane volatile organic compounds (49), the total carbon (50), and the bulk organic carbon fraction (51, 52) of aerosols with variable results. On the basis of the ∆14C data the relative contribution of biomass burning and fossil fuel combustion as sources to the PAH burden of these European background aerosols was further estimated with a simple isotopic mass balance approach
∆14CPAH ) (∆14CBiomass)(Fbiomass) + (∆14CFossil)(1 - Fbiomass) (1) where ∆14CPAH is the measured 14C content of PAH, ∆14CBiomass and ∆14CFossil are the characteristic radiocarbon abundance of biomass (+225‰) and fossil material (-1000‰), respectively, Fbiomass and (1- Fbiomass) are the fraction of the PAH member derived from biomass burning and fossil fuel combustion, respectively. This method has been described elsewhere (18, 40), and it has already been applied to quantitatively apportion the modern biomass component of PAHs in household soot (40) and surface sediments (18). By following eq 1 the fraction of biomass-derived PAHs calculated for the aerosols of Finokalia and Zadar & Velebit was 7% and 9%, respectively (Figure 2), indicating a clear predominance of fossil fuel sources (around 90%) for both areas. The relative composition of 1,7-DMP to 2,6-DMP in aerosols of Finokalia (30) also supported the dominance of motor vehicles (fossil fuel powered) emissions, but the contribution of wood burning deduced from this molecular tracer (37%) was not supported by the radiocarbon-based estimate of biomass burning. Benner et al. (17) noticed that source apportionment using dimethylphenanthrenes agreed better with the 14C content of the bulk particulate organic matter than with the 14C abundance of the PAH fraction. Therefore, the apportionment of biomass-derived PAHs through their radiocarbon composition is considered to be more reliable. In contrast to the Croatian and Greek samples, biomass burning was found to be an important source of atmospheric PAHs at the forested semirural region of Aspvreten, Sweden. The fraction of biomass-derived PAHs for the periods 19951997 and 1998-2001 was 50% and 51%, respectively, implying VOL. 39, NO. 9, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Map showing the location of the sampling sites and the corresponding concentrations of ΣPAH (summed concentrations of 12 parent PAHs) in ambient air samples. The calculated percentage contribution of biomass burning (white bars) and fossil fuel combustion (black bars) to the atmospheric PAH burdens are also shown. an equal contribution from biomass burning and fossil fuel combustion. Although the concentrations of PAHs in this area decreased by a factor of 5 during 1995-2001, the relative contributions from the two source groups apparently have remained constant. These results suggest similar reductions in the emissions from fossil fuel and biomass combustion. The introduction of lower emission wood-burning technology for household fireplaces and woodstoves in combination with the use of catalytic converters in vehicles and the continuous renewal of the vehicle fleet during the last 15 years may have contributed to the decreasing emissions from both source classes. A previous study proposed that wood burning for residential heating is the major source of PAHs in Sweden and suggested that it contributes around 60% of the total atmospheric emissions of PAHs (9). The field-based apportionment of biomass burning through the ∆14C-PAH measurements of the current study, while only from one single background location, confirms that estimation. The high biomass contribution to ambient PAHs in Sweden compared to the other two background sites in central and southern Europe may be explained by the relatively higher consumption of wood for residential heating in forested areas such as Sweden (9, 11, 12). A high biomass contribution to ambient aerosols may also be expected for other northern countries (e.g., Finland, Canada, northern United States) where wood fuels are also extensively used domestically (53). In all of these countries the consumption of wood-based fuels and their contribution to PAH emissions may further increase soon due to the rising cost of oil. In Sweden a political move to close down nuclear power in favor of renewable energy sources may accelerate this process. Here, the use of softwood pellets increases steadily by about 30% annually, and about 10 000 pellet burners have been installed in residential boilers during 2000-2002 (11). Our results suggest that the contribution of fossil fuel combustion and biomass burning to atmospheric PAHs may differ considerably between countries and regions across Europe. Consequently, different national policies and control 2980
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strategies might be needed if a further unilateral reduction of PAH levels is to be achieved on a continental to global scale. Radiocarbon analysis of atmospheric PAHs constitutes a sharp and novel tool to assess the relative contributions from fossil versus biomass combustion sources as well as to “ground truth” such estimates from traditional emission inventory models through measurements directly on the ambient PAHs. Time series monitoring of the ambient ∆14CPAH signal will be useful to assess the success of enforced source mitigation strategies. Radiocarbon analysis of specific compounds in atmospheric samples is still an expensive and laborious technique, but it may become more of a routine technique soon since research toward the on-line connection of AMS with gas chromatography systems is now in progress (54).
Acknowledgments We gratefully acknowledge financial support from the Swedish Foundation for Strategic Environmental Research (MISTRA Idesto¨d, contract no. 2002-057), the Swedish Research Council (VR contract no. 629-2002-2309), the Swedish Environmental Protection Agency (the National Monitoring Program), the U.S. National Science Foundation (NSF, contract no. CHE-0089172), and the Commission of the European Union (project APOPSBAL, contract no. ICA2CT-2002-10007). Skillful technical assistance in the field sampling by Hans Karlsson, Torbjo¨rn Alesand, and Hans Areskoug and in the laboratory analyses by Ann-Sofie Ka¨rsrud and Michael Strandell is appreciated. We are also indebted to Manolis Tsapakis for graciously sharing prepublication concentration data for PAHs from Finokalia.
Supporting Information Available Three tables presenting the amounts of PAHs isolated from each sample extract by PCGC (Table S1), the annual average concentrations of PAHs in the atmosphere of Aspvreten, Sweden, between 1995 and 2001 (Table S2), and the average
atmospheric concentrations of PAHs in Finokalia, Greece, and in Zadar & Velebit, Croatia (Table S3).
(21)
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Received for review November 18, 2004. Revised manuscript received February 3, 2005. Accepted February 9, 2005. ES048184V
