Environ. Sci. Technol. 2005, 39, 3911-3917
Source Apportionment of Organic Pollutants of a Highway-Traffic-Influenced Urban Area in Bayreuth (Germany) Using Biomarker and Stable Carbon Isotope Signatures BRUNO GLASER,* ANNEKATRIN DREYER, MICHAEL BOCK, STEFAN FIEDLER, MARION MEHRING, AND TOBIAS HEITMANN Institute of Soil Science and Soil Geography, University of Bayreuth, D-95440 Bayreuth, Germany
Traffic- and urban-influenced areas are prone to enhanced pollution with products of incomplete combustion of fossil fuels and biomass such as black carbon or polycyclic aromatic hydrocarbons (PAHs). Black carbon is composed of aromatic and graphitic structures and may act as a carrier for pollutants such as PAHs and heavy metals. However, little is known about possible contributions of trafficderived black carbon to the black carbon inventory in soils. Similar uncertainties exist regarding the contribution of different pollutant sources to total PAH and black carbon contents. Therefore, the objective of this study was to quantify the importance of traffic pollution to black carbon and PAH inventories in soils. PAH contamination of soils adjacent to a major German highway in the urban area of Bayreuth with about 50 000 vehicles per day was in the same order of magnitude compared to highwayclose soils reported in other studies. Using molecular (black carbon and PAHs) and compound-specific stable carbon isotope evidence (PAHs) it was demonstrated that this contamination originated not only from automobile exhausts, here primarily diesel, but also from tire abrasion and tailpipe soot which significantly contributed to the trafficcaused black carbon and PAH contamination. Low molecular weight PAHs were more widely transported than their heavy molecular counterparts (local distillation), whereas highway-traffic-caused black carbon contamination was distributed to at least 30 m from the highway. On the other hand, urban fire exhausts were distributed more homogeneously among the urban area.
Introduction It is widely accepted that traffic- and urban-influenced areas are prone to enhanced pollution with products of incomplete combustion of fossil fuels and biomass such as polycyclic aromatic hydrocarbons (PAHs), which belong to an important group of organic contaminants in the environment. However, little is known about the co-deposition of black carbon, a physically and chemically similar compound. The term black carbon is used to describe a complex chemical continuum of partly charred plant material, coal, charcoal, soot, and * Corresponding author e-mail:
[email protected]. 10.1021/es050002p CCC: $30.25 Published on Web 04/27/2005
2005 American Chemical Society
graphite particles, predominantly consisting of aromatic and graphitic moieties (1-3). Black carbon is formed by incomplete combustion of organic matter such as vegetation or fossil fuel. Global emissions have been estimated to 50-270 Tg per year with 80-90% remaining as residues in the terrestrial environment (4). Furthermore, black carbon is a long-term carbon sink and can be found ubiquitously in the environment (3). Moreover, it can act as a carrier-phase for pollutants, e.g., PAHs and heavy metals. Like black carbon, PAHs are mainly formed by incomplete combustion of organic matter (fossil fuels, wood, industrial wastes), but they are also formed by diagenesis of sediments and (micro)bial synthesis (5). Therefore, their origin can be natural as well as anthropogenic. Because of their low water solubility, PAHs have a high affinity to hydrophobic surfaces and organic matter, which makes them accumulate in soils and sediments. Because of their origin, PAHs are found in high concentrations mainly in urban- and traffic-influenced soils. Usually, PAH concentrations increase with industry, heating, and traffic (6-10). PAH contribution from natural combustion processes range from 1 to 10 µg kg-1 soil, while total PAH concentrations in temperate soils are about 10 times higher (5) and those close to a road or highway are several hundred times higher (11). Possible PAH sources in soils adjacent to highways are fossil fuel burning, tar, and road and tire abrasion. Particularly benzo(g,h,i)perylene, indeno(1,2,3-c,d)pyrene, and coronene are assumed to be associated with automobile exhausts, traffic, and street dust (12-15). Normally, PAH concentrations decline exponentially with increasing distance to a highway as to any other point source (5, 11). Transport distance generally depends on degradation (radiation, reaction with free radicals) and the compound’s molecular weight (the higher the molecular weight the shorter the transport distance). The highest concentrations of PAHs in soils were found next to highways (mean distance 4 m) but at 70 m distance traffic influences were still noticed (5). Bryselbout et al. (7) found an increasing ratio of low molecular weight PAHs (LMPAHs) to high molecular weight PAHs (HMPAHs) with increasing slope of soils adjacent to a highway. This effect is known as local distillation and it occurs when heavier compounds are discriminated against lighter compounds with respect to their transport behavior. With respect to PAHs, low molecular weight compounds are deposited at a greater distance compared to their high molecular counterparts (7). This distillation effect is also found in adjacent soils (11). In the past decade, the stable C isotope signature of individual PAHs has been used to determine the contribution of different sources to the PAH content of soils and sediments (16-28). The δ13C value of pyrolysis-derived PAHs resembles the C isotope signature of the source material, depending on combustion conditions (28-29). Using 13C evidence and 14C dating Lichtfouse et al. (30) showed that PAHs in modern soils are partly derived from ancient sources that is from fossil fuels. The objectives of this study were (i) to estimate the contribution of different black carbon and PAH sources to soils in a highway-influenced urban area, and (ii) to look for links between traffic-derived black carbon and traffic-derived PAHs using their molecular and compound-specific 13C signatures.
Experimental Section Sampling. Soil samples were taken in December 2002 next to a major German highway (Bundesautobahn 9, MunichVOL. 39, NO. 11, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Berlin, near Bayreuth at kilometer 306, about 50 000 vehicles per day), which is surrounded by the urban area of the city of Bayreuth, Bavaria, Germany (about 85 000 inhabitants). A 5-10% slope next to the highway characterizes the sampled area, approximately 5 m in height, followed by slightly rising fields used for agriculture. Three transects perpendicular to the highway and around 100 m apart from each other were sampled. Surface soil samples were taken at 0-10 cm depth with stainless steel cylinders at 1, 5, 15, 30, and 50 m distances to the highway. The background sample was taken about 500 m away in the urban area of Bayreuth. It was assumed that at this distance the contamination is not caused by highway traffic anymore but instead by settlement sources. Additionally, two tires (summer tire Semperit Toplife SR 175/ 75 R 13 T (2000) and winter tire Avon Winter 4 × 4 (1986)) as well as house chimney soot of the urban area of Bayreuth were analyzed to find out about their potential contribution to the black carbon and PAH levels of highway-adjacent soils. Soil samples were dried in aluminum boxes for 24 h at 35 °C and stored for further analysis. Tires were ground by a ball mill under liquid nitrogen cooling. Black Carbon Analysis. Black carbon analyses were carried out by the method of Glaser et al. (31) as modified by Brodowski et al. (32). About 0.5 g of ground soil sample and about 30 mg of ground tire sample were digested with 4 mL of 4 M trifluoroacetic acid (TFA) in 20-mL glass hydrolysis flasks for 4 h at 105 °C to release polyvalent cations. The residues were collected on glass fiber filters (Whatman GF/F), rinsed several times with deionized water, and dried at 40 °C. Then, each residue was transferred to a quartz digestion tube and heated with 2 mL of 65% HNO3 for 8 h at 170 °C in a high-pressure digestion apparatus. After the samples cooled to room temperature, the solutions were filtered. The residues were rinsed several times with deionized water. Finally, each combined filtrate was filled up to a total volume of 10 mL. A 2-mL sample of the digestion solution was diluted to 6 mL with deionized water, followed by the addition of 100 µL of citric acid (1 mg mL-1) as internal standard. For further purification of the samples, columns filled with a strongly acid cation-exchange resin (Dowex 8W50X, 200-400 mesh) were used. The resin was conditioned with 2 M NaOH, deionized water, 2 M HCl, and deionized water. After sample addition, the resin was eluted with 50 mL of deionized water in portions of 10 mL. The combined eluates were freeze-dried and transferred into 5-mL ReactiVials using methanol. Bisphenyl-2,2′-dicarboxylic acid (100 µL) in methanol was added (1 mg mL-1) as second internal standard (recovery standard). Each sample was evaporated to dryness under a gentle stream of nitrogen. The dried samples were derivatized with 100 µL of pyridine and 100 µL of N,O-bis(trimethylsilyl)-trifluoroacetamide for 120 min at 80 °C. The derivatives were measured with a Hewlett-Packard 6890 gas chromatograph (HP-5 capillary column 30 m × 0.25 mm × 0.25 µm; injector and detector temperatures, 300 °C; temperature program, 100 °C held for 2 min, 20 °C min-1 to 240 °C held for 7 min) equipped with a flame ionization detector (FID). The black carbon content was calculated as the sum of the formed benzenepolycarboxylic acids and then converted by the factor 2.27 into charcoal equivalents (31). Analytical recoveries being within 100 ( 10% were calculated by the recovery of internal standard 1 added at the beginning of the procedure against internal standard 2 added prior to derivatization. Because various methods of black carbon quantification differ by a factor of up to 500, all measurements in this study are interpreted relatively, that means only on yields at one site relative to another. Therefore, our conclusions are unlikely to be affected by methodological controversies. 3912
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PAH Analysis. A 30-g portion of each dried soil sample (not ground) and 100 mg of each ground tire sample were extracted with hexane/acetone 2:1 in an accelerated solvent extractor (ASE, Dionex 2000, Sunnyvale, CA, USA) adding a deuterated internal standard mixture (naphthalene-D8, acenaphthene-D10, fluorene-D10, anthracene-D10, pyreneD10, chrysene-D12, perylene-D12, benzo(g,h,i)perylene-D12; 4 µg mL-1 each) prior to extraction. During extraction, cells were filled with solvent, pressurized to 14 MPa, and heated to 120 °C for 6 min. Pressure and heat were held for 5 min (static extraction) followed by rinsing with cold solvent (60% of the cell volume) and purging with Ar for 90 s. This extraction cycle was repeated once. After the extracts were combined, samples were evaporated with a rotary evaporator at 35 °C and 550-280 mbar using toluene as a keeper, dried with Na2SO4, rinsed with 30-40 mL of hexane, and evaporated again. For purification of the extracts the following cleanup step was used. A column filled with 2 g of aluminum oxide (5% deactivated) upon 2 g of silica (5% deactivated) was used and eluted with 15 mL of hexane, 5 mL of hexane/ dichloromethane 9:1, and 20 mL of hexane/dichloromethane 4:1 subsequently. The combined elutions were evaporated to about 1 mL at 35 °C and 350-280 mbar. Because of precipitation in the evaporated samples, an additional cleanup step had to be used. For this purpose, columns were filled with 2 g of the adsorption resin HR-P and eluted with 20 mL of toluene. The eluents were evaporated at 65 °C and 170-150 mbar. Fluoranthene-D10 was added to the extract prior to the injection as a recovery standard. PAHs were measured using a Hewlett-Packard 5890 Series II gas chromatograph equipped with an Hewlett-Packard 5971A mass selective detector. Concentrations of the following PAHs were determined in selected ion monitoring mode: naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, benzo[a]anthracene, chrysene/trichlorophenylene, benzo[a]pyrene, benzo[e]pyrene, benzo(b+j+k)fluoranthene, perylene, indeno(1,2,3-c,d)pyrene, dibenzo(a,h)anthracene, benzo(g,h,i)perylene, and coronene. The individual PAH recovery averaged 43(15%, probably due to the high organic matrix content of the investigated soil samples. However, the low recovery is not critical for correct PAH concentrations as losses during sample-cleanup are automatically compensated for by the internal standards. Compound-Specific δ13C Analysis of Individual PAHs. A 10-g aliquot of Na2SO4 was added to each 35 g of dried soil sample, 20 g of domestic soot and about 1 g of ground tire sample, and Soxhlet extracted with hexane/acetone 2:1 for 24 h according to Stark et al. (17). Further purification was carried out as outlined above with the exception that 3 g of each purification resin was used and that no internal standards were added prior to sample cleanup because of bad separation of PAHs and their deuterated counterparts during C isotope measurements. Instead, 11:0 fatty acid methyl ester (FAME) was added as internal standard prior to measurements by gas chromatography-combustionisotope ratio mass spectrometry (GC-C-IRMS), which consisted of a Trace GC 2000 gas chromatograph coupled with a Delta-plus isotope ratio mass spectrometer via a Combustion III interface (Thermo Finnigan MAT, Bremen, Germany). Peak purity was confirmed by recording simultaneous full scan spectra (m/z ) 50-650) by splitting the GC column effluent: 10% entering a Thermo Finnigan MAT GCQ ion trap mass spectrometer and 90% going through the combustion interface into the IRMS. Separation of PAHs was carried out on a BPX5 fusedsilica capillary column (60 m × 0.25 mm × 0.25 µm) with He 99.996% as carrier gas (constant flow mode at 2.5 mL min-1) and an injection of 1 µL volume in splitless mode (2 min splitless time). The GC temperature program was optimized
TABLE 1. Mean δ13C Values of Individual Polycyclic Aromatic Hydrocarbon (PAH) Standards Measured by Elemental Analysis-Isotope Ratio Mass Spectrometry (EA-IRMS, n ) 2) and Measured by Gas Chromatography-CombustionIsotope Ratio Mass Spectrometry (GC-C-IMRS, n ) 5) PAH acenaphthene fluorene phenanthrene/ anthracene
δ13C (EA-IRMS)
δ13C (GC-C-IRMS)
difference
-24.14 ( 0.00 -26.99 ( 0.05 -24.16 ( 0.02
-24.10 ( 0.46 -26.75 ( 0.67 -23.60 ( 0.20
-0.04 -0.24 -0.57
to achieve a complete separation of interesting PAHs, although this could not be achieved for all PAHs, especially in the higher molecular mass range. The temperature program started at 80 °C, held for 4 min, increased to 150 °C at 15 °C min-1, heated to 300 °C at 1 °C min-1, and held for 7 min. Injector temperature was 300 °C. Peak integration and δ13C value evaluation was achieved using Isodat NT 2.0 with Service pack 2.38 (Thermo Finnigan MAT, Bremen, Germany). For baseline drift correction during individual GC runs, pulses of CO2 99.7% (Riessner Gase, Lichtenfels, Germany) were added and δ13C value evaluation of sample and standard chromatograms was accomplished by referring to the added 11:0 FAME (20 µg per vial) which was calibrated against sucrose (CH-6, IAEA, Vienna, Austria) and CaCO3 (NBS 19, NIST, Gaithersburg, MD). To determine detection limits and to evaluate accuracy and precision of GC-C-IRMS measurements, five point calibration functions were established and compared to δ13C values obtained by EA-IRMS measurements of the same pure PAH standards following the procedure suggested by Glaser and Amelung and Schmitt et al. (33-34). This time, however, none of the three measurable PAHs, acenaphthene, fluorene, and phenanthrene/anthracene, showed significant amount dependence, thus no correction for amount dependence was necessary from all measured standards. Additionally, the accuracy of the GC-C-IRMS measurements was within (0.6 ‰ compared to the EA-IRMS measurements (Table 1).
Results and Discussion Black Carbon. Soil Black Carbon Content. Black carbon concentrations decreased with increasing distance to the highway (Figure 1). Highest concentrations (74 mg black carbon-C g-1 TOC) were measured in soils next to the highway, being more than twice the black carbon level of the background soil at 500 m distance. Thus, traffic is a major source for black carbon in traffic-influenced soils. The similar black carbon concentrations between 5 and 30 m distance from the highway might be due to the annual plowing of the agricultural field, thus homogenizing black carbon contents over a greater area and depth. At the border of the agricultural field at 50 m distance from the highway, background concentrations of the urban area of Bayreuth are reached (31 mg black carbon-C g-1 TOC, Figure 1). The magnitude of the background black carbon levels of urban soils in Bayreuth is lower than black carbon levels for American prairie soils ranging between 40 and 180 mg black carbon-C g-1 TOC obtained with the same analytical method (35). Source Apportionment. Source apportionment generally refers to the quantitative assignment to a combination of distinct sources of a particular group of compounds put into a system such as the highway-influenced urban area in this study. The decreasing soil black carbon contents with increasing distance from the highway gives a first hint that automobile traffic significantly contributed to the total black carbon level of the investigated soils, at least adjacent to the highway. This assumption was corroborated by black carbon
FIGURE 1. Black carbon concentrations (( standard error of the mean, n ) 3) of urban soils next to a major German highway. analysis of tires, resulting in black carbon levels between 73 mg black carbon-C g-1 TOC (summer tire) and 286 mg black carbon-C g-1 TOC (winter tire). From these results it is evident that tire abrasion significantly contributed to the black carbon level of highway adjacent soils. However, with these analyses, it is not possible to discriminate between tire abrasion and tailpipe exhaust, both of which contain significant amounts of highly aromatic soot. For further source apportionment, benzenepolycarboxylic acid profiles were established; that is the portion of a single benzenepolycarboxylic acid to the sum of all determined benzenepolycarboxylic acids (Figure 2). Apart from the background soil, all samples including tire samples exhibited a dominance of mellitic acid, followed by benzenepentacarboxylic acid, benzenetetracarboxylic acids (pyromellitic, mellophanic, and prehnitic acid) and benzenetricarboxylic acids (hemimellitic, trimellitic, and trimesic acid). While black carbon of the tire samples is composed of 80-90% of mellitic acid, indicating a high degree of aromatic condensation, soil samples contained a maximum of 55% at 1 m distance from the highway decreasing to about 33% at 500 m distance. It is known that tires consist of 20-30% of carbon black in the size range from 10 to 500 nm (36), physically and chemically similar to soot. Thus, the high degree of condensation of black carbon in the tire samples certainly can be explained by this type of material. Therefore, from black carbon concentrations and benzenepolycarboxylic acid pattern alone the contribution of tire abrasion and vehicle soot exhaust cannot be assigned quantitatively. However, from the benzenepolycarboxlic acid pattern it is obvious that the degree of aromatic condensation of black carbon decreased with increasing distance from the highway, indicating that soot from vehicle exhaust and tire abrasion decreased from about 65% directly beside the highway with increasing distance to the highway to about 33% at 500 m distance (Figure 2). Compound-specific δ13C determination of benzenepolycarboxylic acids did not give evidence for further black carbon sources (data not shown). Polycyclic Aromatic Hydrocarbons. Concentrations. Concentrations of single PAHs and the sum of PAHs varied over about 1 order of magnitude (Table 2). Soil PAH contents at the investigated highway are similar to values of highwayinfluenced soils determined in other studies. Eusterbrock (13) found about 200 µg kg-1 soil for the sum of 10 PAHs in 3 m distance to the highway at an adjacent site, while Thua´cˇkova et al. (11) determined around 3000 µg kg-1 soil for the sum of 10 PAHs. The average PAH concentration of samples located next to the highway was above the common background concentration of 1000 µg kg-1, while the intervention value of 40 000 µg kg-1 was not exceeded. As expected, the significantly (P < 0.05) highest PAH contents were measured in soils adjacent to the highway, sharply declining to the soil samples at 5 m distance from VOL. 39, NO. 11, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Profiles of individual benzenepolycarboxylic acids for soils at different distances to the highway, winter tires (wt), and summer tires (st).
TABLE 2. Average Polycyclic Aromatic Hydrocarbon Contents [µg kg-1] in Soil Samples at Different Distances to the Highway ( Standard Error of the Mean (n ) 3, except for Background and Tire Samples where Only One Replicate Was Available) distance to highway (m) 1
5
15
30
naphthalened 52 ( 8.9 24 ( 5.4 23 ( 3.1 27 ( 6.4 17 ( 3.0 4 ( 1.6 3 ( 0.9 2 ( 0.5 acenaphthylenec acenaphtheneb,c,d 12 ( 1.8 3 ( 2.1 1 ( 0.1 1 ( 0.3 fluorene 6 ( 1.6 2 ( 1.6 4 ( 0.2 3 ( 0.3 b,c phenanthrene 234 ( 28.5 71 ( 33.1 70 ( 3.5 70 ( 7.7 37 ( 5.0 10 ( 4.0 6 ( 0.6 6 ( 2.9 anthraceneb,c fluorantheneb,c 419 ( 51.9 120 ( 48.8 105 ( 16.7 88 ( 16.4 pyreneb,c 338 ( 41.9 96 ( 41.9 81 ( 15.2 68 ( 14.8 benzo[a]anthracene b,c,d 196 ( 19.6 60 ( 25.1 56 ( 9.9 41 ( 8.9 90 ( 33.3 86 ( 14.4 72 ( 13.3 chrysene/trichlorophenyleneb,c,d 311 ( 25.1 benzo(b+j+k)fluoranthenea,b,c,d 1077 ( 136.6 171 ( 61.7 208 ( 33.3 173 ( 19.7 benzo[a]pyrene a,b,c,d 346 ( 39.4 47 ( 18.8 45 ( 12.0 39 ( 8.2 benzo[e]pyrene a,b,c,d 513 ( 60.9 66( 23.6 84 ( 14.6 69 ( 8.1 a,b,c,d perylene 90 ( 11.3 9 ( 3.9 10 ( 3.0 9 ( 2.2 256 (8 5.0 62 ( 23.4 74 (13.2 62 ( 8.9 indeno(1,2,3-c,d)pyrene a,b,c,d dibenzo(a,h)anthracene a,b,c,d 70 ( 4.4 13 ( 4.8 17 (2.6 13 ( 1.7 benzo(g,h,i)perylene a,b,c,d 529 ( 1.0 61 ( 22.3 67 ( 13.9 57 ( 9.4 coronenea,b,c,d,e 108 ( 3.7 16 ( 5.8 18 ( 3.3 13 ( 1.6 a,b,c 4613 ( 413.6 926 ( 357.8 958 ( 156.5 814 ( 125.9 sum
50
500 (background)
12 ( 1.8 4 ( 1.9 3 ( 2.5 3 ( 2.7 145 ( 118.9 21 ( 17.1 115 ( 71.5 84 ( 50.3 44 ( 21.3 57( 21.5 156 ( 65.4 48( 22.4 59( 22.1 11( 5.7 61( 25.7 12 ( 4.3 55 ( 22.0 11 ( 4.0 900 ( 479.4
17 3 1 0 52 4 95 74 44 58 185 44 71 11 65 13 59 12 808
summer tire
winter tire
5102 1146 3047 661 52 110 754 1342 8060 7504 842 560 17087 16927 62303 46230 3301 2185 14501 24164 10480 8461 4601 5922 13959 21366 3401 2072 2470 3611 515 1388 14908 22441 5504 1885 170888 167975
a-e Superscript letters indicate significant differences (P < 0.05) of individual PAH concentrations between 0 and 5 m (a), 0 and 15 m (b), 0 and 30 m (c), 0 and 50 m (d), and 15 and 50 m (e). Note also that because of the small numbers of samples the β-mistake could be rather high, which limits the power of the T-test. However, nonparametric tests were not sensitive enough to detect significant differences.
FIGURE 3. Mean sum of (a) and traffic-associated individual (b) polycyclic aromatic hydrocarbon concentrations in soil samples ( standard error (500 ) background value). the highway, being more or less constant up to the background soil (Figure 3a). The highest decrease was found to occur from 1 to 5 m distance, which gives strong evidence for highway traffic being the cause for the elevated soil contamination. Probably the slope next to the highway could be responsible for the described sharp decrease within the first 5 m. This result also agrees with other studies reporting 3914
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that the mean transportation distance of highway-born PAHs is limited to 3 to 8 m (5, 11). The behavior of single PAHs follows principally the same pattern. In Figure 3b, the concentrations of the trafficassociated PAHs benzo(g,h,i)perylene, indeno(1,2,3-c,d)pyrene, and coronene are presented along the transect. However, the most prominent PAH was benzo(b+j+k)-
FIGURE 4. Ratios of low molecular weight (LMPAHs) to high molecular weight (HMPAHs) PAHs: (a) naphthalene to indeno(1,2,3-c,d)pyrene, pyrene, and fluoranthene, and (b) sum of naphthalene, acenaphthylene, acenaphthene, and fluorene to the sum of indeno(1,2,3-c,d)pyrene, dibenzo(a,h)anthracene, benzo(g,h,i)perylene, and coronene.
TABLE 3. Source-Specific Ratios of Single Polycyclic Aromatic Hydrocarbons brown coal hard coal fluoranthene/benzo[e]pyrene pyrene/benzo[e]pyrene chrysene,trichlorophenylene/benzo[e]pyrene benzo[b+j+k]fluoranthene/benzo[e]pyrene benzo[a]pyrene/benzo[e]pyrene phenanthrene/anthracene pyrene/benzo[a]pyrene benzo[a]anthracene/benzo[a]pyrene a
Diesel-powered trucks.
b
7.7 5.4 3.0 4.0 1.5
7.1 6.8 2.4 3.2 1.0
diesel
gasoline heating oil mineral oil auto exhaust ref
7.0 8.0 1.5 2.6 0.4
20 38 3 1.4 1
85 27 32 3.8 0.2
>50
37 37 37 37 37 14 39 39
4 7-24 1-2
