Contribution of Biomass Burning toAtmospheric Polycyclic AromaticHydrocarbons at Three EuropeanBackground SitesM A N O L I S M A N D A L A K I S , †

O R J A N G U S T A F S S O N , * , †

T O M A S A L S B E R G , †

A N N A - L E N A E G E B A C K , †

C H R I S T O P H E R M . R E D D Y , ‡ L I X U , ‡

J A N A K L A N O V A , § I V A N H O L O U B E K , § A N DE U R I P I D E S G . S T E P H A N O U |

Department of Applied Environmental Science (ITM),Stockholm University, 10691 Stockholm, Sweden,Department of Marine Chemistry and Geochemistry, WoodsHole Oceanographic Institution, Woods Hole, Massachusetts02543, RECETOX-TOCOEN & Associates, Kamenice 126/3,625 00 Brno, Czech Republic, and Environmental ChemicalProcesses Laboratory (ECPL), Department of Chemistry,University of Crete, GR-71409, Heraklion, Greece

Radiocarbon analysis of atmospheric polycyclic aromatichydrocarbons (PAHs) from three background areas in Sweden,Croatia, and Greece was performed to apportion theirorigin between fossil and biomass combustion. Diagnosticratios of PAHs implied that wood and coal combustionwas relatively more important in the northern European site,while combustion of fossil fuels was the dominantsource of PAHs to the two central-southern Europeanbackground sites. The radiocarbon content (∆14C) ofatmospheric PAHs in Sweden ranged between -388‰and -381‰, while more depleted values were observedfor Greece (-914‰) and Croatia (-888‰). Using a 14C isotopicmass balance model it was calculated that biomassburning contributes nearly 10% of the total PAH burden inthe studied southern European atmosphere with fossilfuel combustion making up the 90% balance. In contrast,biomass burning contributes about 50% of total PAHs in theatmosphere at the Swedish site. Our results suggest thatthe relative contributions of biomass burning and fossil fuelsto atmospheric PAHs may differ considerably betweencountries, and therefore, different national control strategiesmight be needed if a further reduction of these pollutantsis to be achieved on a continental-global scale.

IntroductionAir pollution remains of great public health concern (1-3)as, for instance, epidemiological studies demonstrate thatparticle-borne air pollution contributes significantly tohuman morbidity (1) and mortality (1, 2). Despite theimprovement of air quality over the past decades, it is

distressing that about 30-40% of Europeans live in citieswhere the pollution level is equal or above the guidelines ofthe European Union (4). In Austria, France, and Switzerlandthis human exposure causes 6% of total mortality, corre-sponding 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 thetotal mutagenic activity of ambient aerosols (7). For thisreason the European Commission recently suggested adirective enforcing activities to reduce air pollution by PAHs(8). The development of effective control and mitigationstrategies toward the reduction of atmospheric PAHs requiresa reliable identification and apportionment of the variousrelease sources.

Atmospheric PAHs are generally formed by incompletecombustion of any carbon-based fuel. Vehicle exhausts,power and heat generation plants, residential heating,incinerators, and several industrial processes (e.g., cokeproduction, aluminum smelting) are all significant sourcesof PAHs (5, 9, 10). While combustion of fossil fuels is currentlythe main source of PAHs (5, 10), the utilization of biomassfuels is becoming increasingly important (11, 12) due to theglobal momentum to restrict the emissions of greenhousegases (13) and also due to the rising cost of oil. Assessmentof the current (and predicted future) contribution to theambient PAH load from biomass fuels with traditionalemission inventory approaches is challenged by variationsin reported biomass emission, spanning over several ordersof magnitude (e.g., 1-370 µg of PAHs per kilogram of wood)(5). This dilemma of highly variable emission factors suggeststhat we should also try to develop alternative strategies forassessing the contribution of various sources to the PAHspresent in the ambient environment.

As a complement and alternative to traditional emissioninventory modeling approaches, several methods are con-tinuously developed with the aim of diagnostically assessingthe sources of PAHs based on their inherent molecularproperties. For instance, the ratios of specific PAHs (e.g.,phenanthrene-to-anthracene and fluoranthene-to-pyreneratios) are frequently used as indicators of specific combus-tion processes (6, 14, 15). Sulfur-containing heterocycliccompounds (e.g., dibenzothiophenes and benzonaph-thothiophenes) may be used as tracers for coal combustion,petroleum products, and diesel exhaust. Further, retene (16)and 1,7-dimethylphenanthrene (17) have been suggested asmolecular tracers for tracking and assessing emissions fromburning of specific biomass fuels. However, a recent studycombining these latter molecular markers with PAH-specificradiocarbon measurements called their utility as sourcetracers into question for PAHs in anoxic sediments (18). Incertain situations stable carbon isotopic composition (δ13C)of individual PAHs may provide insight about the contributionof specific sources (19, 20), but overlapping end memberδ13C values for several PAH sources may limit the broad utilityof this approach (18).

Radiocarbon analysis of specific compounds and com-pound classes has recently been established as a unique andpowerful tool for quantitatively assessing the relative con-tributions of contemporary biomass versus fossil fuel com-bustion sources of PAHs (18, 21, 22). The underlying principleof the radiocarbon approach is that fossil fuels, and thecompounds emitted from their combustion, contain nearlyno 14C because the geologic age of these fuels is much greaterthan the half-life of radiocarbon (5730 year). In contrast, allsubstances derived from biomass burning would exhibit a

* Corresponding author phone: +46-8-6747317; fax: +46-8-6747638; e-mail: orjan.gustafsson@itm.su.se.

† Stockholm University.‡ Woods Hole Oceanographic Institution.§ RECETOX-TOCOEN & Associates.| University of Crete.

Environ. Sci. Technol. 2005, 39, 2976-2982

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14C/12C ratio of the atmosphere and current living matter(about 1.2 × 10-12).

Nevertheless, radiocarbon analysis of specific organiccompounds or compound classes is exceedingly rare becauseof the demanding analytical requirements. First, even withmodern microscale preparation techniques for acceleratormass spectrometry (AMS) (23) the amount of carbon requiredfor each 14C measurement (at least 20 µg of carbon) is quitehigh compared with the abundance of most organic chemi-cals in the atmosphere. Further, the compound-specificisotope analysis requires not only extensive air sampling butalso a significant effort in order to isolate individualcompounds in these large quantities from the total complexorganic material of the aerosols. To the best of our knowledge,radiocarbon measurements of atmospheric PAHs have todate only been performed for one large urban aerosol sample(21-22). In the current study PAHs were harvested fromleftover extracts of previous long-term PAH air-monitoringprojects and large air samples collected from backgroundareas of Europe. The radiocarbon content of several isolated,harvested, and pooled PAHs was measured, and the con-tribution of biomass burning was subsequently estimated.

Materials and MethodsSampling and Extraction Protocols. Twelve 24-h air sampleswere collected during July 2003 at the marine backgroundsampling station of Finokalia (35°20′N, 25°40′E), a coastalsite 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 localmeteorology are described elsewhere (24). Air sampling wasperformed using a high-volume air sampler consisting of aglass fiber filter and a polyurethane foam plug arranged inseries to collect both particulate and gaseous PAHs. Bothcollection media were extracted, as described below, andthe extracts from all 12 samples were pooled together beforetheir cleanup and isolation of PAHs. Preceding sampling thefilters were heated at 450 °C for 5 h. Also prior to usepolyurethane foam plugs were boiled in water, rinsed withacetone, and Soxhlet extracted twice for 24 h with di-chloromethane. Finally, they were dried in a vacuumdesiccator and sealed in glass jars until sampling.

PAH extracts of monthly aerosol and vapor samples thathad been collected from the background air monitoringstation of Aspvreten, Sweden (58°48′N, 17°23′E) between 1995and 2001 were obtained. Aspvreten is a forested and semiruralsite situated about 80 km southwest of Stockholm and about2 km from the coast of the Baltic Sea, and its furthercharacteristics are detailed elsewhere (25). The leftover PAHextracts from this long-term monitoring campaign werepooled into two samples for radiocarbon determination ofthe PAHs corresponding to the time periods 1995-1997 and1998-2001, respectively.

Very similar procedures were followed for Croatian airsamples collected and pooled from four locations near Zadar(Adriatic coast; 49°06′N, 15°14′E) and one location in theVelebit Mountains (44°49′N, 14°58′E) during spring 2003.Extracts of 50 24-h air samples (10 days of sampling at fivesites) were pooled together to obtain one sample for 14Canalysis of PAHs. Air sampling in Croatia and Sweden wasconducted using similar high-volume sampler systems asdescribed above for Finokalia, Greece. Both the particulateand gaseous phases had been extracted and were includedin the leftover extracts. Thus, all PAHs isolated in the currentstudy for 14C analysis corresponded to PAHs from both theparticulate and gaseous phases in the ambient atmosphere.

For each air sample collected from Finokalia the poly-urethane foam plug and the corresponding glass fiber filterwere placed in a Soxhlet apparatus, and they were extractedwith dichloromethane for 24 h. The extract was concentrated

to 4 mL by rotary evaporation and then to 1 mL under agentle nitrogen stream at ambient temperature. The extractwas subsequently applied onto a column of deactivated silicagel (SiO2-10% H2O, 63-200 µm particle size, height 10 cm,i.d. 1 cm) topped with sodium sulfate and eluted with 60 mLof n-hexane. All leftover PAH extracts from Zadar & Velebitwere also treated with similar SiO2 column chromatographicmethods. Acetone was used for Soxhlet extraction of theAspvreten samples. Following a reduction in extract volumeby rotary evaporation and subsequent addition of water, theAspvreten PAHs were transferred to hexane by liquid-liquidextraction prior to cleanup using Isolute solid-phase extrac-tion (SPE) Florisil cartridges (International Sorbent Technol-ogy, Mid Glamorgan, U.K.).

The hexane fractions from all the pooled aerosol sampleswere further treated by a dimethylformamide (DMF)-pentane cleanup procedure (26) in the laboratory at Stock-holm University. In brief, the solvent was evaporated andexchanged to n-pentane (2 mL), and the extract waspartitioned twice with DMF-5% H2O (2 mL). The DMF layerswere pooled in the same glass tube, mixed with 4 mL ofwater, and partitioned twice with n-hexane (4 mL). Then-hexane fractions were collected in the same flask andevaporated to about 0.5 mL. Finally, the extracts were elutedthrough a miniaturized SiO2 column (height 1 cm, i.d. 0.5cm; elution with 8 mL of n-hexane) to remove any residuesof DMF or water.

Preparative Capillary Gas Chromatography and Ac-celerator Mass Spectroscopy. After DMF cleanup all the finalPAH extracts were evaporated to about 200 µL, and each onewas repeatedly injected (about 40 serial injections of 5 µLeach) onto a preparative capillary gas chromatograph (PCGC)programmed to trap selected PAHs in the StockholmUniversity laboratory. The Gerstel PCGC system and opti-mization of operational parameters for the isolation of PAHshave both been described elsewhere (18, 27). Since theabundance of target PAH compounds present in these largeair sample extracts still was quite low relative to therequirements of the state-of-the-art 14C measurement tech-nique (23), compound-class-specific radiocarbon analysis(CCSRA) of several isolated and pooled PAHs was the aim.From each extract the most abundant PAH members (nineindividual compounds plus seven coeluting pairs) wereharvested together in the same trap of the PCGC preparativefraction collector to obtain a sufficient amount for oneradiocarbon measurement (Supporting Information, TableS1). Phenanthrene/anthracene and chrysene/benzo[a]an-thracene were not harvested from the extracts of Aspvretensince these samples contained deuterated phenanthrene andchrysene, which eluted so closely to the native compoundsthat isolation of the latter during PCGC was not feasible.This is unlikely to make a significant difference as phenan-threne makes up a similar fraction of the total PAH fromboth biomass and fossil fuel combustion (28).

The isolated PAHs were rinsed from the glass traps (5times with 200 µL of dichloromethane) and further cleanedon separate SiO2 columns (height 4 cm, i.d. 0.5 cm; elutionwith 25 mL of hexane/dichloromethane (1:1)). A small portionof the final extract was saved to test for purity and yield ofthe analytical procedure, while the remaining material wasshipped to Woods Hole Oceanographic Institution (WHOI)for further treatment and final isotopic analysis. There, thePCGC isolates were again passed through a Si column toremove any residual column bleed and then transferred toprecombusted quartz tubes. The solvent was evaporatedunder a stream of nitrogen, and ∼100 mg of copper oxidewas added. The tubes were evacuated, sealed, and combustedat 850 °C for 5 h, and the resulting carbon dioxide was isolatedthrough a series of cold traps and quantified by manometry.About 10% of the carbon dioxide was reserved for δ13C analysis

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by isotope ratio mass spectrometry, and the remainingamount was reduced to graphite (23). Targets of the graphitewere pressed and mounted on target wheels for 14C analysisby accelerator mass spectrometry (AMS) at the NationalOcean Sciences AMS (NOSAMS) facility at WHOI. All 14Cmeasurements are expressed as the per mil (‰) deviationfrom SRM 4990B (∆14C) (29).

Results and DiscussionDistribution and Diagnostic Ratios of PAHs. The concen-trations and diagnostic ratios of atmospheric PAHs are oftenused to evaluate the impact and contribution of varioussources to a specific region. Atmospheric concentrations ofPAHs were available for Finokalia (30), Aspvreten, and Zadar& Velebit (unpublished data) and are briefly discussed here.Long-term measurements in Aspvreten between 1995 and2001 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). Thisdecline of atmospheric PAHs likely reflects the strict legisla-tion applied in Sweden during this period for the mitigationof air pollution. The more recent year-round concentrationsof PAHs in Aspvreten are substantially lower than thoseobserved at Finokalia and the Zadar & Velebit sites, althougha certain seasonal influence cannot be excluded for theCroatian site since it has not been sampled year round. Theconcentration of ΣPAH measured at the five Croatian sitesduring spring 2003 ranged from 0.6 to 10.2 ng m-3, providingan average value of 4.8 ng m-3, while the average year-roundconcentration of ΣPAH at Finokalia during 2000-2002 was10.8 ng m-3 (Supporting Information, Table S3). In general,the levels of PAHs in the studied background areas werelower than those previously reported for several urban centersof Europe (9, 31, 32) but considerably higher than thosepresented for remote areas of the Arctic (33). The lowerconcentrations of atmospheric PAHs in Sweden could beexplained by the less strict regulation of emissions in southernEuropean countries, a seasonal effect, and/or the moreefficient scavenging of atmospheric PAHs in higher latitudesthrough both temperature-driven dry deposition/condensa-tion and precipitation-borne wet deposition.

In contrast to ΣPAH concentrations, the fluoranthene tofluoranthene-plus-pyrene ratio (Fl/(Fl+Py)) was higher atthe 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 combustionsources over petrogenic ones (14, 15). More specifically,previous studies have shown that Fl/(Fl+Py) ratios below0.40 imply the prominence of unburned petroleum (petro-genic sources), ratios from 0.40 to 0.50 suggest the combustionof liquid fossil fuels (vehicle and crude oil), whereas ratioslarger than 0.50 are characteristic for grass, wood, or coalcombustion (15). According to these values the increasingFl/(Fl+Py) ratio is consistent with the combustion of woodand coal being relatively more important in northern thansouthern European countries, while the opposite appears tohold for the combustion of liquid fossil fuels.

The prominence of combustion over petrogenic sourceswas also supported by the relative concentrations of indeno-[1,2,3-cd]pyrene (IP) and benzo[ghi]perylene (BgP). It hasbeen suggested that IP/(IP+BgP) ratios lower than 0.20 likelyimply petroleum, ratios between 0.20 and 0.50 liquid fossilfuel (vehicle and crude oil) combustion, and ratios largerthan 0.50 grass, wood, and coal combustion (15). The averageIP/(IP+BgP) ratio at Finokalia, Zadar & Velebit, and Aspvretenwas 0.51, 0.31, and 0.55, respectively, largely corroboratingthe above results for the Greek and Swedish sites. Overall,both Fl/(Fl+Py) and IP/(IP+BgP) diagnostic ratios demon-strated that combustion processes, rather than unburned

fossil fuels, were the origin of atmospheric PAHs, but thesemolecular markers exhibit a partial disagreement regardingthe dominant combustion sources in each area.

The relative abundance of 1,7-dimethylphenanthrene (1,7-DMP) and 2,6-dimethylphenanthrene (2,6-DMP) has alsobeen suggested to be a useful tool for distinguishing betweensoft 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 woodcombustion, while ratios lower than ∼0.45 are indicative ofvehicle emissions (15). The concentrations of dimeth-ylphenanthrenes 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 andgasoline-fueled vehicles and, for the largely deforested islandof Crete, a 37% contribution from wood combustion. Thissuggested large biomass contribution is not supported bythe radiocarbon data (below), and this study thus adds toother recent findings (18) to question the utility of this DMPratio as a reliable source tracer. Unfortunately, data on theconcentrations of dimethylphenanthrenes were not availablefor the other two sampling sites.

Carbon Isotopic Composition of Atmospheric PAHs. Thetreatment of the atmospheric samples by SiO2 columnchromatography and dimethylformamide provided ap-propriately clean extracts for the successful isolation of PAHsby PCGC. The gas chromatograms of the Aspvreten airextracts, before and after PCGC trapping of PAHs, demon-strate the efficiency of this isolation/harvesting procedure(Figure 1). The total amount of pooled target PAHs harvestedfrom the four samples ranged between 14 and 68 µg, whilethe purity of the isolates varied from 89% to 96% (Table 1).

The stable carbon isotopic composition (δ13C) of atmo-spheric PAHs has been explored as a measure to apportionthe emission sources in several previous studies (19, 20, 34-

FIGURE 1. High-resolution gas chromatograms of an aerosol extractfrom Aspvreten (a) before and (b) after the isolation of PAHs byPCGC. The peaks corresponding to dibenzothiophene (1), phen-anthrene/anthracene (2), 3,2-methylphenanthrene (3), 4,1-meth-ylphenanthrene (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[1,2,3-cd]pyrene (15), benzo[ghi]perylene (16) are indicated in eachchromatogram (the samples were run with different GC oventemperature programs).

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36). The δ13C values of the currently isolated PAHs exhibitedlow variability between different sampling sites, relative toprevious studies, and ranged from -29.2‰ to -27.7‰ (Table1). Overall, these values were in the same range as thosepreviously observed in rural aerosols of Canada (-29‰ to-23‰) (37) but more negative than those measured in urbanaerosols 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 usedas a rough indicator for the origin of PAHs, a quantitativeassignment of the relative importance of different sources isdifficult. This is due to the overlap of the end member δ13Cvalues 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 airextracts indicated that wood, gasoline, and coal combustionmay all be significant sources of PAHs, while combustion ofdiesel should be of minor importance for these sites.

In contrast, the radiocarbon signal (∆14C) of PAHs washighly variable between the different sites. The ∆14C of PAHsin air samples of Finokalia and Zadar & Velebit was -914‰and -888‰, respectively, while much less depleted valueswere observed for the two Aspvreten samples (-388‰ and-381‰) (Table 1). All chemical species originating from fossilsources have a ∆14C of -1000‰, while the radiocarbonabundance of the compounds emitted from modern biomassshould be in the vicinity of +225‰ (40). The latter value isapproximately equivalent to the 14C signal previously mea-sured in soft wood (41), while a ∆14C value of +235‰ hasbeen attributed to plant waxes (42). In this context, the ∆14Cvalues of atmospheric PAHs indicate that the combustion offossil fuels is the main source of PAHs in the two backgroundareas of Greece and Croatia, while there is a substantialcontribution of biomass burning to the ambient PAHs inAspvreten, Sweden.

Although radiocarbon measurements have been per-formed for several different organic compounds isolated froma range of environmental matrixes such as sediments (18,21, 42-44), soils (45), and marine animals (46), 14C data forPAHs or other substances present in atmospheric aerosolsare very rare. This is mainly due to the much lowerconcentrations of the target compounds in ambient aircompared to the other matrixes previously investigated. Tothe best of our knowledge, 14C measurements of atmosphericPAHs have been performed only for the NIST atmosphericreference material SRM 1649, which is ambient particulatematter collected from a parking lot in urban Washington,D.C., during 1976-1977 (21, 22). The ∆14C values of individualPAHs in SRM 1649 ranged from -963‰ to -914‰ andrevealed that this sample predominantly contained fossilPAHs. Radiocarbon results have also recently been reportedfor several PAHs isolated from four surface sediments in andaround Stockholm, Sweden, collected in 2002 (ranging from-934‰ and -550 ‰) (18) and for the two sedimentaryreference 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 inresidential fireplaces contained PAHs with substantiallyhigher radiocarbon abundances (-274‰ to +74‰) (40), andthis was consistent with the contemporary levels of 14C inwood. Fatty acids are the only other compound class whoseradiocarbon abundance has been measured in semiurbanaerosols (47, 48). With the exception of C24 and C26 mono-carboxylic fatty acids, the ∆14C of the fatty acids from C16 toC34 ranged between +407‰ and -90‰, suggesting that thesecompounds were mainly emitted from living higher plantsand possibly from marine organisms (47). In several otherstudies atmospheric measurements of radiocarbon have beenconducted on the mixture of non-methane volatile organiccompounds (49), the total carbon (50), and the bulk organiccarbon fraction (51, 52) of aerosols with variable results.

On the basis of the ∆14C data the relative contribution ofbiomass burning and fossil fuel combustion as sources tothe PAH burden of these European background aerosols wasfurther estimated with a simple isotopic mass balanceapproach

where ∆14CPAH is the measured 14C content of PAH, ∆14CBiomass

and ∆14CFossil are the characteristic radiocarbon abundanceof biomass (+225‰) and fossil material (-1000‰), respec-tively, Fbiomass and (1- Fbiomass) are the fraction of the PAHmember derived from biomass burning and fossil fuelcombustion, respectively. This method has been describedelsewhere (18, 40), and it has already been applied toquantitatively apportion the modern biomass componentof PAHs in household soot (40) and surface sediments (18).By following eq 1 the fraction of biomass-derived PAHscalculated for the aerosols of Finokalia and Zadar & Velebitwas 7% and 9%, respectively (Figure 2), indicating a clearpredominance of fossil fuel sources (around 90%) for bothareas. The relative composition of 1,7-DMP to 2,6-DMP inaerosols of Finokalia (30) also supported the dominance ofmotor vehicles (fossil fuel powered) emissions, but thecontribution of wood burning deduced from this moleculartracer (37%) was not supported by the radiocarbon-basedestimate of biomass burning. Benner et al. (17) noticed thatsource apportionment using dimethylphenanthrenes agreedbetter with the 14C content of the bulk particulate organicmatter than with the 14C abundance of the PAH fraction.Therefore, the apportionment of biomass-derived PAHsthrough their radiocarbon composition is considered to bemore reliable.

In contrast to the Croatian and Greek samples, biomassburning was found to be an important source of atmosphericPAHs at the forested semirural region of Aspvreten, Sweden.The fraction of biomass-derived PAHs for the periods 1995-1997 and 1998-2001 was 50% and 51%, respectively, implying

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) 14.1 95.4 OS-41865 -381 51Aspvreten, Sweden (1998-2001) 35.7 96.1 OS-41862 -27.7 -388 50Zadar & Velebit, Croatia (2003) 68.0 89.2 OS-43143 -29.2 -888 9Finokalia, Greece (2003) 36.1 93.5 OS-43150 -29.0 -914 7

a The numbers in parentheses indicate the time period of aerosol sampling. b This purity was assessed prior to shipment from StockholmUniversity, and the subsequent additional cleanup at WHOI may have further increased these purities. c AMS accession numbers for each 14Canalysis. d Standard deviation for all δ13C measurements is (0.1‰, based on replicate analysis of standards. e The relative standard error for these14C data is 1-3%. f FBiomass is the percentage contribution of biomass burning to atmospheric PAHs calculated by an isotopic mass balance approach(18, 40).

∆14CPAH ) (∆14CBiomass)(Fbiomass) +(∆14CFossil)(1 - Fbiomass) (1)

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an equal contribution from biomass burning and fossil fuelcombustion. Although the concentrations of PAHs in thisarea decreased by a factor of 5 during 1995-2001, the relativecontributions from the two source groups apparently haveremained constant. These results suggest similar reductionsin the emissions from fossil fuel and biomass combustion.The introduction of lower emission wood-burning technologyfor household fireplaces and woodstoves in combination withthe use of catalytic converters in vehicles and the continuousrenewal of the vehicle fleet during the last 15 years may havecontributed to the decreasing emissions from both sourceclasses.

A previous study proposed that wood burning for resi-dential heating is the major source of PAHs in Sweden andsuggested that it contributes around 60% of the totalatmospheric emissions of PAHs (9). The field-based ap-portionment of biomass burning through the ∆14C-PAHmeasurements of the current study, while only from onesingle background location, confirms that estimation. Thehigh biomass contribution to ambient PAHs in Swedencompared to the other two background sites in central andsouthern Europe may be explained by the relatively higherconsumption of wood for residential heating in forested areassuch as Sweden (9, 11, 12). A high biomass contribution toambient aerosols may also be expected for other northerncountries (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-basedfuels and their contribution to PAH emissions may furtherincrease soon due to the rising cost of oil. In Sweden a politicalmove to close down nuclear power in favor of renewableenergy sources may accelerate this process. Here, the use ofsoftwood pellets increases steadily by about 30% annually,and about 10 000 pellet burners have been installed inresidential boilers during 2000-2002 (11).

Our results suggest that the contribution of fossil fuelcombustion and biomass burning to atmospheric PAHs maydiffer considerably between countries and regions acrossEurope. Consequently, different national policies and control

strategies might be needed if a further unilateral reductionof PAH levels is to be achieved on a continental to globalscale. Radiocarbon analysis of atmospheric PAHs constitutesa sharp and novel tool to assess the relative contributionsfrom fossil versus biomass combustion sources as well as to“ground truth” such estimates from traditional emissioninventory models through measurements directly on theambient PAHs. Time series monitoring of the ambient ∆14C-PAH signal will be useful to assess the success of enforcedsource mitigation strategies. Radiocarbon analysis of specificcompounds in atmospheric samples is still an expensive andlaborious technique, but it may become more of a routinetechnique soon since research toward the on-line connectionof AMS with gas chromatography systems is now in progress(54).

AcknowledgmentsWe gratefully acknowledge financial support from theSwedish Foundation for Strategic Environmental Research(MISTRA Idestod, contract no. 2002-057), the SwedishResearch Council (VR contract no. 629-2002-2309), theSwedish Environmental Protection Agency (the NationalMonitoring Program), the U.S. National Science Foundation(NSF, contract no. CHE-0089172), and the Commission ofthe European Union (project APOPSBAL, contract no. ICA2-CT-2002-10007). Skillful technical assistance in the fieldsampling by Hans Karlsson, Torbjorn Alesand, and HansAreskoug and in the laboratory analyses by Ann-Sofie Karsrudand Michael Strandell is appreciated. We are also indebtedto Manolis Tsapakis for graciously sharing prepublicationconcentration data for PAHs from Finokalia.

Supporting Information AvailableThree tables presenting the amounts of PAHs isolated fromeach sample extract by PCGC (Table S1), the annual averageconcentrations of PAHs in the atmosphere of Aspvreten,Sweden, between 1995 and 2001 (Table S2), and the average

FIGURE 2. Map showing the location of the sampling sites and the corresponding concentrations of ΣPAH (summed concentrations of12 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.

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atmospheric concentrations of PAHs in Finokalia, Greece,and in Zadar & Velebit, Croatia (Table S3).

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Received for review November 18, 2004. Revised manuscriptreceived February 3, 2005. Accepted February 9, 2005.

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