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Sep 19, 2007 - Figure 3 Combustion-based fuel consumption in Sweden during the last 100 years showing oil products (gray bar, top axis) (53), coal and...
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Environ. Sci. Technol. 2007, 41, 6926-6932

A 700 Year Sediment Record of Black Carbon and Polycyclic Aromatic Hydrocarbons near the EMEP Air Monitoring Station in Aspvreten, Sweden MARIE ELMQUIST, ZDENEK ZENCAK, AND O ¨ RJAN GUSTAFSSON* Department of Applied Environmental Science (ITM), Stockholm University, 106 91 Stockholm, Sweden

In view of poor constraints on historical combustion emissions, past environmental loadings of black carbon (BC) and polycyclic aromatic hydrocarbon (PAH) were reconstructed from dated lake sediment cores collected 70 km south of Stockholm, Sweden. Compared to several dramatic variations over the recent 150 years, the preindustrial loading were steady within (50% through the entire medieval with BC fluxes of 0.071 g m-2 yr-1 and PAH fluxes of 6 µg m-2 yr-1. In the wood-burning dominated century leading up to the industrial revolution around 1850, increasing BC fluxes were leading PAH fluxes. BC fluxes reached their millennial-scale maximum around 1920, whereas PAH fluxes increased exponentially to its record maximum around 1960, 50-fold above preindustrial values. For 19201950, BC fluxes consistently decreased as PAH fluxes kept increasing. Coal and coke represented >50% of the Swedish energy market in the 1930s. Combined with sharply decreasing (1,7-)/(1,7-+2,6-dimethylphenanthrene), indicative of diminishing wood combustion, and decreasing methylphenanthrenes/phenanthrene, indicative of highertemperature combustion (coal instead of wood), the sediment archive suggests that the relative BC/PAH emission factors thus are lower for coal than for wood combustion. For the first time, both BC and PAH fluxes decreased after 1960. This trend break is a testament to the positive effects of decreasing reliance on petroleum fuels and a number of legislative actions aimed at curbing emissions and by 1990, the loading of BC was back at preindustrial levels, whereas that of PAH were the lowest since the 1910s. However, for the most recent period (1990-2004) the BC and PAH fluxes are no longer decreasing, putatively reflecting a slight increase in diesel consumption and a doubling of softwood-pellet burners for home heating.

Introduction Black carbon (BC) particles and polycyclic aromatic hydrocarbons (PAH) are emitted from incomplete combustion of carbonaceous fuels and vegetation fires. Both BC and PAHs have been shown to negatively affect human health with BC causing respiratory diseases such as bronchitis (1) even * Corresponding author phone: +46-8-6747317; fax: +46-86747638; email: [email protected]. 6926

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leading to premature deaths, and PAHs in air are carcinogenic (2) and constitute over half of the total mutagenic potency of air particulate matter (3). Time-trend-analysis of PAHs and BC in recent sedimentary records affords the possibility to evaluate the effect of growing population and changing combustion practices (49), including the accumulated effectiveness of past-century political decisions on changing fuel strategy and other mitigation efforts. Further, such decadal-millennial records may also be useful for validation of atmospheric hind cast modeling. For instance, the temporal trends of BC flux in New York City lakes do not support model-predicted historical reconstructions based on fuel consumption and uncertain emission factors (9). This first high resolution and long-term record from northern Europe is also motivated by recent studies suggesting that PAH fluxes in North America, after several decades of decrease, again are increasing during the latest decade (6, 7). Here, a high-resolution record of the environmental PAH load (about 4 yr resolution over the recent 50 years and then lower with increasing age) was produced to detail the deposition of combustion products in the Northern European background environment over a large portion of the past millennium with emphasis on the period since the industrial revolution. In addition to a longer record, we take one step further than recent work in the Northeastern U.S. (7) to also include BC since it is a very important entity for climate forcing, respiratory quality, and fate and transport of BCassociated environmental pollutants. The fluxes of BC and PAHs to the dated sediment core were interpreted using statistical data on the Swedish society’s historical changes in fuel usage. Furthermore, diagnostic PAH ratios were also used to deduce their main sources.

Experimental Section Study Area and Sediment Sampling. The Aspvreten air monitoring station, situated about 70 km south of the Stockholm metropolitan area, is part of the European Monitoring and Evaluation Programme (EMEP) (10). Sediment cores were collected in February 2005 from the nearby lake Stora Frillingen (58°51′ N, 17°21′ S; 1200 m × 250 m; 3.5 m depth; approximately 4.5 km from Aspvreten) with a specially constructed freeze corer (11). The Stora Frillingen Lake is situated in a rural area of Sweden, it has a small drainage basin, and it is surrounded by forest. This small lake is not highly productive but rich in allochtonous humic matter from nearby soil/peat sources. Freeze coring facilitated collection of these high-porosity sediments while minimizing distortion and mixing. The anoxic lake sediment was dark brown and a few leaves and grass straws were visible lying horizontally within the frozen sediment. The in situ frozen sediment cores were, upon retrieval, immediately wrapped in aluminum foil, placed in coolers, and transported to the laboratory where they were stored at -20 °C. The sediment was sectioned (∼1 cm) with a miter box saw in a cold room (+4 °C). The sediment slices were placed in prewashed highdensity polyethylene (HDPE) containers and kept at -20 °C until further analysis. Thinner sediment sections (4 mm) in the uppermost part of the core were instead obtained using a file. Because of demands in sample size and substantial risk of low-level PAH contamination during gamma counting handling for the preindustrial sections, the sedimentation rate and the BC concentration were determined in one sediment core (ASP12), whereas PAH was analyzed in a parallel core (ASP8). The two cores had similar water content 10.1021/es070546m CCC: $37.00

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and loss on ignition (LOI) depth trends. The radiochronologically constrained sedimentation rates and excess 210Pb inventories were also very similar for the parallel cores (ASP12 and ASP9) with 1.7 ( 0.1 and 2.0 ( 0.2 mm yr-1 as well as 50 and 49 dpm cm-2, respectively. Sediment Dating. The radiochronology in the Aspvreten sediment was constrained using 226Ra/210Pb, 137Cs, and 14C with well-established dating methods detailed in the Supporting Information (Text S1, Figure S1, Figure S2, Figure S3, Table S1). Quantification of BC. Black carbon (BC) was analyzed with the chemothermal oxidation (CTO) method, detailed elsewhere (4, 12, 13). Briefly, dry and thoroughly ground sediment was weighed into Ag capsules (5 × 9 mm, Sa¨ntis Analytical AB, Uppsala, Sweden) and placed in a home-built aluminum boat. The boat was held in a tube furnace (375 °C 18 h) under active airflow. After microscale acidification, in situ in the Ag capsules, the residual carbon content was determined as BC with an isotope ratio mass spectrometer (Europa Hydra 20/20, Stable Isotope Facility, UC Davis). The limit of quantification (LOQ; defined as the mean blank value plus 10 times the standard deviation) in this study was 3.3 µg BC per silver capsule. Quantification of PAHs. PAH was analyzed by standard methods commonly used in our laboratory (14, 15) and detailed in the Supporting Information (Text S2). After toluene Soxhlet extraction, purification with silica gel and dimethylformamide-pentane partitioning, the recoveries of the five deuterated internal standards were 81% for lighter PAH (m/z ) 178-206) to 95% for heavier PAH (m/z ) 252-278) and the LOQ in this study was 4.0 ng gdw-1 for phenanthrene. For all the other PAHs, the LOQs were much lower. To calculate the ∑PAH, the following PAHs were used: phenanthrene, anthracene, fluoranthene, pyrene, benzo(a)anthracene, chrysene/triphenylene, benzo(b+j+k)fluoranthene, benzo(e)pyrene, benzo(a)pyrene, indeno(1,2,3-cd)pyrene, dibenzo(a,h)anthracene, benzo(ghi)perylene, and coronene (Table S2).

yielded a sedimentation rate of 0.4 mm yr-1. The CIC model ages were coupled with radiocarbon ages (from 14C-TOC) at 30 cm, giving a preaged carbon signal of about 600 years (Figure S3; this preaging phenomenon is elaborated in ref 5).

Results and Discussion

The Preindustrial Fluxes of BC and PAHs. BC have previously been found in large concentrations in metropolitan sediments (e.g., refs 4, 9, 27-32) but also reported for remote and ancient sediments (e.g., refs 33-40). PAH are also ubiquitously dispersed in sediments (e.g., refs 5, 7, 26, 37, 39, 41-44). Since concentrations of any minor components may be strongly affected by varying input and thus dilution of the major detrital matter, it is advantageous to instead focus on fluxes. The BC flux in the Aspvreten sediment dated to the beginning of the 11th century a.d. was 0.071 g m-2 yr-1. The BC fluxes stayed invariant through the medieval followed by a 3-fold increase from the late 18th century to reach 0.29 g m-2 yr-1 in mid-1850s (Figure 1A, Text S3). Pre-industrial BC fluxes of 0.25-0.75 g m-2 yr-1 have been determined in rural mountain lake cores in central Europe (37). The ∑PAH flux increased relatively less from 6 µg m-2 yr-1 in the sediment dated to the late 14th to 15th centuries to 9 µg m-2 yr-1 by mid-1850s (Figure 1B, Text S3). Lima and co-workers (7) measured a preindustrial (1822-1842) ∑PAH flux of 2.3 ( 1.7 µg m-2 yr-1 in NE U.S.. It should be noted that 12 different PAH congeners were used in this study compared to 15 in Lima et al. (7). Given the poor constraints of the atmospheric 210Pb fluxes with increasing latitude (lake is at 58°N), we provide the actually obtained PAH and BC flux estimates, but the interested reader may use the estimated sediment focusing factor 1.6 to normalize these fluxes. The 3-4 times lower fluxes in NE U.S. probably reflects differences in population densities at this time between NE U.S. and our northern European site, which also receives long-range input of PAHs from more densely populated European continent and England (31).

Radionuclide Constraints on Sediment Chronology. The sedimentation rate was determined with 210Pb (sediments 150 years). The 210Pb-based sedimentation rate in the upper part of the core (0-30 cm) was determined with the constant initial concentration (CIC) model (16, 17) since the CICcalculated ages were more consistent with the depth of the 137Cs peak (Figure S2, Figure S3) than if the constant rate of supply (CRS) model (18, 19) was used. The 210Pbxs activity decreased exponentially downcore, yielding a sedimentation rate of 2.0 ( 0.2 mm yr-1 (0-20 cm interval; n ) 12, r2 ) 0.91 in the ASP12 core), in line with a peak in 137Cs activity in the 3.5-4.0 cm sediment section, indicating the 1986 Chernobyl accident (Figure S3). A second sediment core (ASP9) yielded similar 210Pb results (sedimentation rate 1.7 ( 0.1 mm yr-1; n ) 15, r2 ) 0.94) and a 137Cs peak in the 3.5-5.0 cm section. An average sedimentation rate of 1.85 ( 0.2 mm yr-1 was thus assumed for the parallel ASP8 core. The depth-integrated 210Pb xs inventories were similar for the two dated cores (49 dpm cm-2 and 50 dpm cm-2) showing homogeneity in sediment accumulation rate in the flat central area of this small lake. The sediment inventories were compared with a literature estimate of atmospheric 210Pb deposition flux (20) from this latitude to estimate the sediment focusing factor for 210Pb. This suggested a focusing factor of about 1.6, which is similar to what was found in the NE U.S. study of Lima and co-workers (7). For the older (>150 years) and more compacted sediments, a regression of the 14C in total organic carbon (14CTOC) activity with depth (r2 ) 0.993, Table S1, Figure S3)

Coupled PM and PAH Emission Factors. To aid interpretation of the long-term trends in fluxes of combustion products, we have compiled data on emission factors (EF) for both particulate matter (PM) and PAHs from a wide variety of combustion processes, technologies, and fuel material for cases where both are presented (Table S4). We note that there is not a constant relationship between PM and BC from these combustion processes, but both metrics are reflecting emitted carbonaceous aerosols. The PM emissions range between 7.0 mg kg-1 (domestic heating with coal briquette) to 170 000 mg kg-1 (open burning of crude oil). The industrial heating and motor vehicle groups emit between 200 and 1600 mg kg-1 of PM. Interestingly, the emissions from the domestic heating group varies by over a factor of 1000, from 7 to 40 000 mg kg-1. For the PAHs, eight dominant PAHs were combined to a ∑PAH8 metric to get an overview of the total PAH emissions. The ∑PAH8 EFs are relatively constant within the motor vehicle and the open burning groups, but vary greatly for industrial (factor of 580) and domestic (factor of 30) heating. The general trend of PM/∑PAH8 was calculated from data shown in Table S4 to the following: domestic heating < gasoline vehicles < open burning < heavy-duty diesel trucks < industrial heating. Source Diagnostic PAH Ratios. The PAH fingerprint reflects the sources (21-24). For instance, the methylphenanthrenes to phenanthrene ratio (MP/P) differentiates PAH originating from combustion (i.e., pyrogenic MP/P 0.4-0.7) or from oil spills/seepages (e.g., petrogenic MP/P > 5) (2124). Another commonly used source-diagnostic PAH ratio is the 1,7-dimethylphenanthrene (1,7-DMP) to the sum of 1,7and 2,6-dimethylphenanthrene (2,6-DMP) (4, 23, 25, 26). This ratio is used to distinguish PAH from combustion of fossil fuels (0.90) (25).

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FIGURE 1. Downcore fluxes of BC (Figure 1A) and ∑PAH (Figure 1B) for the Aspvreten sediment. The dominant combustion-based energy sources (i.e., wood, coal, and coke, or oil) for different time periods are displayed on a separate axis. The ∑PAH includes the following compounds: phenanthrene, anthracene, fluoranthene, pyrene, benzo(a)anthracene, chrysene/triphenylene, benzo(b + j + k)fluoranthene, benzo(e)pyrene, benzo(a)pyrene, indeno(1,2,3-cd)pyrene, dibenzo(a,h)anthracene, benzo(ghi)perylene, and coronene.

FIGURE 2. Source diagnostic PAH ratio of methylphenanthrene to phenanthrene (MP/P) for the dated Aspvreten sediment core (A); the partly filled areas display the end-member ratios where the PAH source is most likely pyrogenic or petrogenic (21, 24). The relative concentration of 1,7-dimethylphenanthrene to the sum of 1,7- and 2,6-dimethylphenanthrene (B) trace vehicle emissions (∼0.45) coal and coke combustion (0.65-0.68), and softwood combustion (0.90) (25). Dominant combustion-based energy sources for different time periods are displayed on a separate axis. The Aspvreten sediment MP/P ratio indicated a strong pyrogenic contribution throughout the core (Figure 2A, Table S2). The slightly higher MP/P in preindustrial sediments is likely reflecting that past wood burning practices occurred at lower temperatures compared with modern combustion processes; thus resulting in slightly more alkylation (24). The preindustrial DMP ratio was 0.72-0.79, which would suggest a mixed input from coal and softwood combustion (Figure 2B, Table S2). However, there was no coal used in Sweden during this period as wood was used for manufacturing processes, domestic heating, and cooking. It is plausible that the DMP ratio reflects a significant contribution at this period of long-range transport of PAHs from the European continent and England. However, the accuracy of DMP-based source apportionment was also earlier shown to be inconsistent with parallel compound-specific radiocarbon analysis (15). 6928

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The historical variation of combustion products in the sediment record is related to human population growth and changes in energy sources with associated emission control strategies. Census data of the Swedish population from 1750 onward is readily available (45). It shows that the Swedish population grew from 1.8 million in 1750 to about 3 million in the 1850. The almost doubling of inhabitants during this 100-year period is commensurate with an increase of the same scale in BC and PAH fluxes in the Aspvreten core (Figure 1). Whereas more fuel had to be used to cover the higher energy needs of the growing population, it thus seems that the per capita fuel consumption stayed rather invariant. Alternatively, techniques were adopted to increase the combustion efficiency. The BC and PAH Fluxes from the Industrial Revolution to the Mid-1920s. This time period recognizes the highest

BC fluxes and an exponential increase in PAH flux, commensurate with the industrial revolution in the mid-1800s. The BC flux increased steadily to reach a maximum of 0.40 g m-2 yr-1 in 1922-1928, which is the highest throughout the whole millennial-scale sediment record (Figure 1A). The ∑PAH flux increased tenfold over 70 years to 97 µg m-2 yr-1 by 1919-1929. This contrasts with a factor 13 times (Table S4). The BC and PAH Fluxes from 1990 Until the Early 2000s. The decreasing environmental loadings from the previous sections were not sustained in the most recent period. The decreasing trend seen for the BC and ∑PAH fluxes during the previous time period has halted in the most recent layer (2002-2005) (Figure 1). Since 1990, the BC flux is about 0.05 g m-2 yr-1 and the ∑PAH flux about 40 µg m-2 yr-1. In comparison, this recent BC flux 70 km outside metropolitan Stockholm is more than 10 times lower than BC in sediment records from both offshore Gulf of Maine off NE U.S. and in the Slovenian Alps in Europe (27, 37). The constant ∑PAH flux for the most recent Swedish sediments is in contrast to Pettaquamscutt River in the Northeast United States (7), where a 42% mean increase in the total sedimentary PAH fluxes was found for the time period between 1996 and 1999. That large increase in PAH fluxes was explained by higher usage of gasoline and diesel on the American market. Notably, the 50% decrease in Swedish oil consumption during the 1980s leveled off around 1990 (Figure 3). The amount of diesel sold on the Swedish market during recent time has increased from 3.1 to 3.7 million m3 (1996 to 2002) and was rising even further in 2004 (53). The PM10 (particulate matter with aerodynamic diameter less than 10 µm) has been continuously recorded at the Aspvreten monitoring station by Swedish EPA since 1980s. The time-series of PAHs recorded in air started in 1996. The sedimentary BC flux decreased 2.5 times between 1978 and 1993 and thereafter the BC flux halted. It should be noted that the resolution of the sedimentary BC flux data after 1993 is very low with only 2 data-points. For comparison, the PM10 concentration in air decreased steadily from 19 µg m-3 in 1980 to about 10 µg m-3 2004, which is a similar decrease as was observed in the sediment record. The trend break away from decreasing BC and PAH fluxes, may in part stem from the growing replacement of oil-burners with softwood pellet-burners as an alternative domestic heating fuel. However, the low DMP ratio during this time suggests fossil fuel burning as a major PAH source (Figure 2B). The consumption of softwood pellet increased from 0.5 to 1.1 million tons between 1997 and 2003 (Figure 3). The 6930

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∑PAH8 emission factors for softwood pellet burners are a factor of 9 higher than for heavy-duty diesel trucks and a factor of 67 higher than for gasoline-driven vehicles with catalytic converter (Table S4). Further, the ∑PAH8 EFs is predicted to increase by a factor of 11 when switching from oil to pellet burner (Table S4). A significant contemporary contribution from biomass combustion is further supported by receptor-based source apportionment using natural abundance radiocarbon analysis of PAHs pooled from extracts of air samples collected in the Aspvreten area between 1996 and 2001, which demonstrated that 50% of the atmospheric PAHs indeed were coming from biomass burning (54). Limited knowledge of historical energy consumption and poorly constrained emission factors for past combustion processes and fuels hinder traditional emission inventory modeling. This study demonstrates the possibility to instead use detailed sediment archives to reconstruct past environmental loadings of key combustion products such as BC and PAH. This approach established that past decisions in society on switching to cleaner-burning fuels and legislation on emission control technologies has had such an encouragingly strong effect on the environmental loadings of these important climate and health-afflicting substances that in the contemporary northern European background environment the levels are approaching those experienced before the industrial revolution.

Acknowledgments Ingemar Renberg is graciously acknowledged for lending us his freeze corer equipment. We also thank Zofia Kukulska, Hanna Gustavsson, and Martin Kruså for technical assistance. Constructive comments from four anonymous reviewers helped to improve this manuscript. The Swedish Foundation for Strategic Environmental Research (MISTRA Idesto¨d contract no. 2002-057) and the Swedish Research Council for Environment, Agricultural Sciences and Spatial planning (FORMAS contract no. 214-2006-527) financially supported this work. O ¨ .G. also acknowledges a Senior Researcher grant from the Swedish Research Council (VR contract no. 6292002-2309).

Supporting Information Available One text segment (Text S1) describes the sediment dating, Text S2 describes the PAH analysis and one text segment (Text S3) describes the flux calculations. One figure (Figure S1) shows the linear regression of ln 210Pbxs vs depth, Figure S2 compare the results from two dating models (CIC and CRS) with the 137Cs peak concentration, one figure (Figure S3) shows the downcore 137Cs activity, excess 210Pb, and 14C data and the model-derived chronology. One table (Table S1) presents the radionuclide data, Table S2 presents the concentrations of PAHs, one table (Table S3) presents the concentrations of BC and other geochemical variables and Table S4 presents the emission factors of PM and PAHs for modern combustion processes and fuel materials. This material is available free of charge via the Internet at http:// pubs.acs.org.

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Received for review March 5, 2007. Revised manuscript received May 30, 2007. Accepted August 8, 2007. ES070546M