Contribution of Biomass Burning to the Polycyclic Aromatic

Hydrocarbon Contents in European Background Aerosols

MANOLIS MANDALAKIS,† ÖRJAN GUSTAFSSON,†,* TOMAS ALSBERG,† ANNA-LENA EGEBÄCK†,

CHRISTOPHER M. REDDY,‡ LI XU,‡ JANA KLANOVA,§ IVAN HOLOUBEK,§ AND EURIPIDES G.

STEPHANOUװ

Dept. of Applied Environmental Science (ITM), Stockholm University, 10691 Stockholm, Sweden,

Dept. of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole,

MA 02543, USA,

RECETOX - TOCOEN & Associates, Kamenice 126/3, 625 00 Brno, Czech Republic,

Environmental Chemical Processes Laboratory (ECPL), Department of Chemistry, University of Crete

GR-71409, Heraklion, Greece

† Stockholm University.

‡ Woods Hole Oceanographic Institution.

§ RECETOX - TOCOEN & Associates

University of Crete װ

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

1

Abstract

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 northern Europe, while combustion of fossil fuels was the dominant source

of PAHs to the 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 aerosols of central Sweden. 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.

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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 last decades, it is distressing that about

30 to 40% of Europeans live in cities where the pollution level is equal or above the guidelines of the

European Union (4). In just 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 to 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). Whilst 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 to 370 µg PAHs per kg of wood) (5). This dilemma of highly

variable emission factors suggests that we should try to develop also alternative strategies for assessing

the contribution of various sources to the PAHs present in the ambient environment.

3

As a complement and alternative to traditional emission inventory modeling approaches, several

methods, aimed at diagnostically assessing the sources of PAHs based on their inherent molecular

properties, are continuously developed. 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

dating 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).

More recently, radiocarbon analysis of specific compounds and compound classes has 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 yr). In contrast, all substances derived from biomass burning would exhibit a

14C/12C ratio of the atmosphere and current living matter (about 1.2 x 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 micro-

scale 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 aerosols. Further, the compound-specific isotope analysis requires not only extensive air

4

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 (22). In the current study, PAHs were harvested from left-over aerosol extracts of previous

long-term PAH monitoring projects and from large aerosol 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 aerosol samples were collected during July 2003

at the marine background sampling station of Finokalia (35o 20’N, 25o 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). Aerosol 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 twelve samples were

pooled together before their cleanup and isolation of PAHs. Preceding sampling, the filters were heated

at 450 oC for 5 hours. Also, polyurethane foam plugs were boiled in water, rinsed with acetone and

Soxhlet extracted twice for 24 hours with dichloromethane. Finally, they were dried in a vacuum

desiccator and sealed in glass jars until sampling.

PAH extracts of monthly aerosol samples that had been collected from the background air

monitoring station of Aspvreten, Sweden (58o 48’N, 17o 23’E) between 1995 and 2001 were obtained.

Aspvreten is a forested site, situated in a national park about 80 km southwest of Stockholm and about

2 km from the coast of the Baltic Sea and its further characteristics are detailed elsewhere (53). The

leftover PAH extracts from this long-term monitoring campaign were pooled into two samples for

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radiocarbon determination of the PAHs corresponding to the time periods 1995-1997 and 1998-2001,

respectively.

Very similar procedures were followed for Croatian aerosol samples collected and pooled from four

locations near Zadar (Adriatic coast; 49o 06’N, 15o 14’E) and one location in theVelebit Mountains

(44o 49’N, 14o 58’E) during spring 2003. Extracts of fifty 24-h aerosol samples (10 days of sampling at

5 sites) were pooled together to obtain one sample for 14C analysis of PAHs. Aerosol 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

left-over 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 aerosol 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 hours. 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 left-over PAH extracts from Zadar & Velebit

had also been 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 clean-up 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 (25) in the laboratory of 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,

6

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 the optimization of operational parameters for the isolation of PAHs have both been

described elsewhere (18, 26). Since the abundance of target PAH compounds present in these large

aerosol 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 (9 individual compounds plus

7 co-eluting pairs) were harvested together in the same trap of the PCGC preparative fraction collector

to obtain 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 the isolation of the latter during PCGC was not feasible.

The isolated PAHs were rinsed from the glass traps (5 times with 200 µl of dichloromethane) and

further cleaned up 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 pre-combusted quartz tubes. The solvent was evaporated under a stream of nitrogen and

7

~100 mg of copper oxide was added. The tubes were evacuated, sealed, 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 by isotope ratio mass

spectrometry, and the remaining 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) (27).

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 (28), 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 to 1.6 ng m-3 (Supporting Information, Table S2). This

decline of atmospheric PAHs is probably the result of the strict legislation applied in Sweden during

this period for the mitigation of air pollution. The more recent concentrations of PAHs in Aspvreten are

substantially lower than those observed at Finokalia and the Zadar & Velebit sites. The concentration

of ΣPAH measured at the five Croatian sites during the spring of 2003 ranged from 0.6 to 10.2 ng m-3,

providing an average value of 4.8 ng m-3, while the average 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, 29, 30), but considerably higher than those presented for remote areas of the Arctic (31).

The lower concentrations of atmospheric PAHs in Sweden could be explained by the less strict

regulation of emissions in southern European countries and/or the more efficient scavenging of

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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 in the north. 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), 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 appear 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, between 0.20 and 0.50 liquid fossil fuel

(vehicle and crude oil) combustion, and ratios larger than 0.50 imply 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. Overall, both Fl/(Fl+Py) and IP/(IP+BgP)

diagnostic ratios demonstrated that combustion processes, rather than unburned fossil fuels, was 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 (1,7-DMP) 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, aerosols 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

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indicative of vehicle emissions (15). The concentrations of dimethylphenanthrenes were measured in

aerosols of Finokalia (28) and the average value of 1,7-DMP/(1,7-DMP+2,6-DMP) ratio was 0.54,

implying mixed emissions from diesel, 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 aerosol 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 aerosol extracts,

before and after PCGC trapping of PAHs, demonstrates 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, 32-34). 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‰) (35), but more negative than those

measured in urban aerosols of Kuala Lumpur, Malaysia (-27.9 to -17.7‰) (33) and in aerosols

collected from an urban area of Canada (-26 to -23‰) (35). 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‰) (33), coal (-31 to -25‰) (34, 36), gasoline (-30 to -19‰) (33, 36) and

wood burning smoke (-31.6 to -26.8‰) (33, 37). Nevertheless, the δ13C of PAHs in our aerosol extracts

10

indicated that wood, gasoline and coal combustion may all be significant sources of PAHs, whilst

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 aerosols of Finokalia and Zadar & Velebit was -914‰ and -888‰, respectively,

while much less depleted values were observed for Aspvreten (-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‰ (38).

The latter value is approximately equivalent to the 14C signal previously measured in soft wood (39),

while a ∆14C value of +235‰ has been attributed to plant waxes (40). In this context, the ∆14C values

of atmospheric PAHs indicate that the combustion of fossil fuels is the main source of PAHs in

background areas of Greece and Croatia, while there is a substantial contribution of biomass burning to

the ambient PAHs in Aspvreten, Sweden. The ∆14C-PAH data thus suggest a greater importance of

biomass burning for northern than southern European countries, which is also in line with the results of

Fl/(Fl+Py) diagnostic ratio, espoused above.

Although radiocarbon measurements have been performed for several different organic compounds

isolated from a range of environmental matrices such as sediments (18, 21, 40-42), soils (43) and

marine animals (44), 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 matrices 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, DC during 1976-1977 (21). 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

11

sedimentary reference materials SRM 1941a (Baltimore, MD, USA 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‰) (38), 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 semi-urban aerosols (45, 46). 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 (45). In several other studies, atmospheric measurements of radiocarbon have

been conducted on the mixture of non-methane volatile organic compounds (47), the total carbon (48)

and the bulk organic carbon fraction (49, 50) of aerosols, with variable results.

Based on 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. This method has been described elsewhere (18, 38) and it has already

been applied to quantitatively apportion the modern biomass component of PAHs in household soot

(38) and surface sediments (18). By following this approach, 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 (28) 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) have noticed that source apportionment using dimethylphenanthrenes agreed better

with the 14C content of the 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.

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In contrast to the Croatian and Greek samples, biomass burning was found to be an important source

of atmospheric PAHs at the forested semi-rural region of Aspvreten, Sweden. The fraction of biomass-

derived PAHs for the periods 1995-1997 and 1998-2001 was 50% and 51%, respectively, implying 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 USA) where wood-fuels are

also extensively used domestically (51). In all of these countries, the consumption of wood-based fuels

and their contribution to PAH emissions may further increase in the near future 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.

13

Consequently, different national policies and control 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 ∆14C-PAH signal will be useful to assess the success of enforced source

mitigation strategies. Radiocarbon analysis of specific compounds in atmospheric particles is still an

expensive and laborious technique but it may become more of a routine technique in the near future

since research toward the on-line connection of AMS with gas chromatography systems is now in

progress (52).

Acknowledgements

We gratefully acknowledge financial support from the Swedish Foundation for Strategic

Environmental Research (MISTRA Idestöd, contract no. 2002-057), the Swedish Research Council

(VR contract no. 629-2002-2309), the Swedish Environmental Protection Agency (the National

Monitoring Program), the US National Science Foundation (NSF, contract no. CHE-0089172), and the

Commission of the European Union (project APOPSBAL contract no. ICA2-CT-2002-10007). Skillful

technical assistance in the laboratory by Ann-Sofie Kärsrud and Michael Strandell is appreciated. We

are also indebted to Manolis Tsapakis for graciously sharing pre-publication concentration data for

PAHs from Finokalia.

Supporting Information Available

Three tables presenting the amounts of PAHs isolated from each aerosol extract by PCGC (Table S1),

the annual average concentrations of PAHs in the atmosphere of Aspvreten, Sweden between 1995 and

14

2001 (Table S2) and the average atmospheric concentrations of PAHs in Finokalia, Greece and in

Zadar & Velebit, Croatia (Table S3).

15

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Table 1. Yield, purity and isotopic composition of PAHs isolated from four aerosol samples.

Sampling site* Yield (µg)

Purity (%)&

AMS accession no§

δ13C (‰)†

∆14C (‰)‡

FBiomass

(%)# Aspvreten, Sweden ('95-'97) 14.1 95.4 OS-41865 - -381 51 Aspvreten, Sweden ('98-'01) 35.7 96.1 OS-41862 -27.7 -388 50

Zadar & Velebit, Croatia ('03) 68.0 89.2 OS-43143 -29.2 -888 9

Finokalia, Greece ('03) 36.1 93.5 OS-43150 -29.0 -914 7

* The numbers in parenthesis indicate the time period of aerosol sampling. & this purity was assessed prior to shipment from Stockholm University and the subsequent additional clean-up at WHOI may have further increased these purities. § AMS accession numbers for each 14C analysis. † Standard deviation for all δ13C measurements is ±0.1‰, based on replicate analysis of standards. ‡ The relative standard error for these 14C data is 1-3%. #FBiomass is the percentage contribution of biomass burning to atmospheric PAHs calculated by an isotopic mass balance approach (18, 38).

23

Figure Captions

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[1,2,3-cd]pyrene (15), benzo[ghi]perylene

(16) are indicated in each chromatogram (the samples were run with a different GC oven

temperature programs).

Figure 2. Map showing the location of the sampling sites and the corresponding concentrations of

ΣPAH (summed concentrations of 12 parent PAHs) in ambient aerosols. The calculated

percentage contribution of biomass burning (white bars) and fossil fuel combustion (black bars)

to the atmospheric PAH burdens are also shown.

24

15.00 20.00 25.00 30.00 35.00 40.000

200000

400000

600000

800000

1000000

1200000

1400000

1600000

1800000

2000000

2200000

2400000

2600000

2800000

3000000

(1)

(2)

(3)

(6)

(4)(5)

(7)

(8)

(9) (10)

(11)

(12)(13) (15) (16)

(14)

a)

8.00 10.00 12.00 14.00 16.00 18.00 20.00 22.00 24.00 26.00 28.00 30.00 32.000

50000

100000

150000

200000

250000

300000

350000

400000

450000

(1)

(3)(6)

(4)(5)

(7) (8)

(9)(10) (12)

(13) (15) (16)

b)

15.00 20.00 25.00 30.00 35.00 40.000

200000

400000

600000

800000

1000000

1200000

1400000

1600000

1800000

2000000

2200000

2400000

2600000

2800000

3000000

(1)

(2)

(3)

(6)

(4)(5)

(7)

(8)

(9) (10)

(11)

(12)(13) (15) (16)

(14)

a)

15.00 20.00 25.00 30.00 35.00 40.000

200000

400000

600000

800000

1000000

1200000

1400000

1600000

1800000

2000000

2200000

2400000

2600000

2800000

3000000

(1)

(2)

(3)

(6)

(4)(5)

(7)

(8)

(9) (10)

(11)

(12)(13) (15) (16)

(14)

a)

8.00 10.00 12.00 14.00 16.00 18.00 20.00 22.00 24.00 26.00 28.00 30.00 32.000

50000

100000

150000

200000

250000

300000

350000

400000

450000

(1)

(3)(6)

(4)(5)

(7) (8)

(9)(10) (12)

(13) (15) (16)

b)

8.00 10.00 12.00 14.00 16.00 18.00 20.00 22.00 24.00 26.00 28.00 30.00 32.000

50000

100000

150000

200000

250000

300000

350000

400000

450000

(1)

(3)(6)

(4)(5)

(7) (8)

(9)(10) (12)

(13) (15) (16)

8.00 10.00 12.00 14.00 16.00 18.00 20.00 22.00 24.00 26.00 28.00 30.00 32.000

50000

100000

150000

200000

250000

300000

350000

400000

450000

(1)

(3)(6)

(4)(5)

(7) (8)

(9)(10) (12)

(13) (15) (16)

b)

Figure1_Mandalakis et al.

25

30

40

50

60

70

-10 0 10 20 30 40Longitute (East)

Latit

ude

(Nor

th)

0

20

40

60

80

100

% S

ourc

es c

ontr

ibut

ion

0 20 40 60 80

100

% S

ourc

es c

ontr

ibut

ion

0

20

40

60

80

100

% S

ourc

es c

ontr

ibut

ion

ΣPAH: 4.8±2.6 ng m-3

Aspvreten

Zadar & Velebit

Finokalia

ΣPAH: 1.6±1.6 ng m-3

ΣPAH: 10.8 ng m-3

Fossil fuel combustion

Biomass burning

Figure2_Mandalakis et al.

26