Environ. Sci. Technol. 2004, 38, 5344-5349
Radiocarbon Apportionment of Fossil versus Biofuel Combustion Sources of Polycyclic Aromatic Hydrocarbons in the Stockholm Metropolitan Area MANOLIS MANDALAKIS,† O ¨ R J A N G U S T A F S S O N , * ,† CHRISTOPHER M. REDDY,‡ AND LI XU‡ Institute of Applied Environmental Research (ITM), Stockholm University, 10691 Stockholm, Sweden, and Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543
Source-diagnostic markers and the isotopic composition of polycyclic aromatic hydrocarbons (PAHs) were examined in surface sediments from the greater Stockholm waterways to deduce the contribution from biomass sources to the environmental PAH load. The summed concentration of 20 PAHs ranged from 0.8 to 45.1 µg/g (dry weight) and exhibited a steep decline with increasing distance from the city center evidencing that sources within the metropolitan area of Stockholm dominate its PAH burden. Several diagnostic PAH ratios indicated an overwhelming predominance of pyrogenic sources over the petrogenic ones, while retene and 1,7-dimethylphenanthrene were unable to correctly evaluate the contribution from biomass combustion. The stable carbon isotope composition (δ13C) of individual PAHs ranged from -24.8 to -27.0‰ but also was proved inefficient to discriminate between different types of fuels due to the overlapping signals in various sources. The ∆14C values of PAHs ranged between -550.4 and -934.1‰, indicating a clear predominance of fossil fuel sources. By using an isotopic mass balance approach, we estimated that on average 17 ( 9% of PAHs derived from biomass combustion. This radiocarbon apportionment, in conjunction with detailed energy statistics for the Stockholm region, revealed that the ambient PAH burden is roughly similar, per unit energy produced, from fossil fuels and biofuels. Societies’ shifting energy policies toward a larger reliance on biofuels may thus not lead to further deterioration of air quality and respiratory ailments for the urban population.
Introduction Combustion of fossil fuels is the major source of energy for today’s global economy (1) but is also largely responsible for both the greenhouse effect (2-4) and the particulate air pollution of significant public health concern (5-7). Epidemiological studies have shown that particle-borne air pollution contributes significantly to human morbidity (5) and mortality (5, 6). Approximately 30-40% of Europeans live in cities where the pollution level is equal or above the * Corresponding author phone: +46-8-6747317; fax: +46-86747638; e-mail:
[email protected]. † Stockholm University. ‡ Woods Hole Oceanographic Institution. 5344
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 20, 2004
guidelines of the European Union (8). In just Austria, France, and Switzerland, this human exposure causes 6% of total mortality, corresponding to more than 40 000 premature deaths annually (5). The ubiquitous polycyclic aromatic hydrocarbons (PAHs) (9-11) are excellent molecular markers of combustion particles (9, 12) and in themselves account for most (35-82%) of the total mutagenic activity of ambient aerosols (13). For this reason, the European Commission recently suggested a directive enforcing activities to reduce air pollution by PAHs (14). The principal sources of PAHs and other particle-borne air pollution are traffic, power plants, residential heating, and industrial processes (12, 15, 16); each of these may be fuelled with a variety of either fossil or biomass material and operated under a wide range of combustion conditions. While combustion of fossil fuels currently dominate energy production, the utilization of biomass fuels is becoming increasingly important due to the global momentum to restrict the emissions of greenhouse gases and also due to the rising cost of oil. The replacement of fossil fuels by biofuels has been advocated by the Kyoto protocol (17), but a suspected side effect of this strategy is the increased environmental pollution by carcinogenic polycyclic aromatic hydrocarbons (PAHs). This stems from the emission factors (EFs) of PAHs and other combustion pollutants, which are generally believed to be higher for wood-based fuels than for fossil fuels (12, 15, 16). Furthermore, attempts at efficiently decreasing the current PAH emissions require knowledge of the relative importance of the different PAH sources. Unfortunately, this information is not readily attainable with traditional emission inventory strategies. The large variations in reported EFs for any given combustion method (12, 15, 16) make predictions from traditional emission inventory approaches uncertain. However, it is well-known that solid fuels are fundamentally more difficult to combust than liquid or gaseous fuels and specific EFs are generally much higher from small combustion units, like domestic boilers, as compared to larger plants for district heating and power production (18). The dilemma of highly variable PAH emission factors, particularly for biofuel combustion processes, suggest that alternative approaches should be attempted, investigating source-diagnostic properties of the load of pollutants actually found in the environment. Surface sediments provide an ideally homogenized natural archive of particle-borne pollution, integrating the input over the past several years (9, 19-21), as a result of the combined processes of deposition, runoff, water mixing, and sedimentation. In this study, large-volume surface sediments were collected throughout the greater Stockholm waterways, and compound-specific radiocarbon dating was applied for several PAHs to apportion the relative contribution of biomass versus fossil fuel combustion sources. The concentration, the molecular fingerprint, and the stable carbon isotopic composition of PAHs were also examined.
Materials and Methods Study Area. Sediment samples were collected from seven sites around the city of Stockholm (Figure 1) between October and November 2002. All samples were collected at 20-30 m water depth. The greater Stockholm area is inhabited by more than 1.5 million people. The automobiles in traffic was estimated to be comprised of ∼550 000 gasoline cars, ∼17 000 light-duty diesel cars and trucks, and ∼5000 heavy-duty diesel trucks and buses (22). There is intensive traffic on the shipping lanes to the city of Stockholm, especially during the summers. Most industries and almost all households are 10.1021/es049088x CCC: $27.50
2004 American Chemical Society Published on Web 09/17/2004
MD 800 mass spectrometer operating in selective ion monitoring mode. The samples were injected on-column, and the analytes were separated by a PTE-5 (Supelco) capillary column (5%-diphenyl-dimethylpolysiloxane, 30 m length, 0.25 mm i.d, 0.25 µm film thickness). The GC oven was temperature programmed from 70 °C (2 min hold) to 200 °C at 20 °C min-1 and to 310 °C at 5 °C min-1 (15 min hold).
FIGURE 1. Map showing the location of the sampling sites in Slussen (S1), Waldemarsudde (S2), Fja1 derholmarna (S3), Lilla Va1 rtan (S4), Ekhagen Bay (S5), Stora Va1 rtan (S6), and Lake Ma1 laren (S7), around the city of Stockholm. connected to municipal wastewater treatment plants, all equipped for mechanical, biological, and chemical treatment (22). Heating oil and wood are the dominating energy sources for the many central heating boilers and other domestic heating facilities (16, 23-25). The last 20 years, the use of softwood pellets increased rapidly in Sweden (23, 24), and about 250 000 tons of pellets are consumed every year in Stockholm’s district heating system (24). Sampling and Pretreatment. A large volume (around 40 L) of the upper 0-2 cm sediments was collected from each site by an integrating Lundgren trawl (26). Since the burial velocity of accumulating sediments in the inner Stockholm archipelago is on the order of 2-4 mm/year (27), the samples correspond to 5-10 year integrated records. At all stations, the sediments were anoxic, sulfide-rich, fluffy, and black. Although they were fine-grained without any major macroscopic heterogeneity, all sediments were passed through a 1 cm sieve to remove any leaves or gravels. Each sample (more than 90% water) was centrifuged at 9000 rpm for 10 min, and the supernatant water was discarded. The pellets were transferred to amber glass jars, and they were stored in a freezer (-15 °C) until extraction. Quantification of PAHs. Wet sediment samples (between 0.7 and 2.9 g) were placed in prewashed cellulose thimbles, spiked with deuterated PAHs (phenanthrene-d10, fluoranthene-d10, pyrene-d10, and benzo[a]pyrene-d12, benzo[g,h,i]perylene-d12) and were extracted in a Soxhlet/Dean-Stark apparatus with toluene for 24 h. The extracts were reduced in volume by rotary evaporation and treated with copper (fine powder, particle size
