Environ. Sci. Technol. 1991, 25, 332-338
Chemical and Biological Characterization of Particulate-, Semivolatile-, and Gas-Phase-Associated Compounds in Diluted Heavy-Duty Diesel Exhausts: A Comparison of Three Different Semivolatile-Phase Samplers Roger N. Westerholm," Jacob Almin, and Hang Lit Department of Analytical Chemistry, Arrhenius Laboratory, Stockholm University, S-106 9 1 Stockholm, Sweden
J. Ulf Rannug Department of Genetics, Wallenberg Laboratory, Stockholm University, S-106 9 1 Stockholm, Sweden
Karl-Erik Egeback and Kerstin Gragg Automotive Emission Research Section, The National Environmental Protection Board, S-61 1 82 Nykoping, Sweden
Exhaust emissions from a heavy-duty diesel vehicle during transient driving conditions have been characterized chemically and tested for mutagenicity. Both the particulate phase and the semivolatile phase were investigated chemically and by the Ames test. Three different sampling techniques, Le., cryocondensation and two adsorbents XAD-2 and polyurethane foam (PUF), were used for collection of the semivolatile phase together with a particulate filter situated upstream. The cryogenic technique was the least efficient type and the PUF technique gave the highest recovery of polycyclic aromatic hydrocarbons (PAH) and mutagenic activity. The amount of PAH, mainly three-ringed PAH, emitted in the semivolatile phase was approximately 3 times higher than that emitted in the particulate phase. The contribution of the semivolatile phase to the total mutagenicity was approximately 20% in strain TA100hS9, 10% in strain TA98-S9, and 37% in strain TA98+S9. The results thus show the importance of both particulate- and semivolatile-phase-associated compounds when possible health effects from diesel engine exhausts are being considered.
Introduction In an ideal combustion process in air, only carbon dioxide, nitrogen oxides, and water are formed. During real-life combustion of organic fuels, however, a large number of other compounds are formed. One group of compounds formed is polycyclic aromatic compounds (PAC), of which polycyclic aromatic hydrocarbons (PAH) form a subgroup. Several PAH, which are known to be mutagenic in short-term tests, are carcinogenic toward rodents ( I ) and, thus, potential human carcinogens. Several studies of urban air have shown that PAH are particulate-phase and gas-phase associated (2-5). The distribution of these is dependent on temperature and surfaces available for condensation/adsorption (6). A significant source of the PAC detected in the urban street environment is combustion products from vehicles. Results from chemical characterizations of automobile exhausts have shown that, in experimentally diluted exhaust, PAH are distributed between the particulate phase and the gaseous phase (7,8). In the technique used, a dilution tunnel simulates the situation where reactions between ambient air and exhausts can take place. However, the residence time prior to sampling is less than 2 s, which is probably too short for establishing a steady state in the particle-phase/gas-phase distribution. The dilution tunnel ~
t Permanent address: Department of Chemistry Anhui Normal University, Wuhu Anhui, People's Republic of China.
332 Environ. Sci. Technol., Vol. 25, No. 2, 1991
Table I. Fuel Standard Analysis According to American Standard of Testing Materials (ASTM)
cetane index density (15 "C), g/mL distillation (760 mmHg) 50% recovered, "C 90% recovered, "C final boiling point, "C sulfur content, % w / w flash point, "C aromatics, 70v / v paraffins, '3'0 v / v
method
analysis
ASTM D 976 ASTM D 4052 ASTM D 86
50.0 0.8299
ASTM ASTM ASTM ASTM
D 2622 D 93 D 1319 D 1319
250 313 350 0.24 64 22.6 74.8
technique was initially developed by Habibi (9) for lead salt measurements, but is now used mainly in order to lower the exhaust temperature and to avoid condensation of water in sampling pipes for measurements of regulated pollutants, i.e., unburned fuel hydrocarbons (HC), nitrogen oxides (NO,), carbon monoxide, and particulate emissions. Particulate sampling by means of a filter may also change the actual particle/gas distribution of compounds by blowoff effects or condensation and adsorption of gaseous compounds onto the collected particles. For a detailed review regarding the partitioning between the particulate phase and the gas phase, see Pankow ( I O ) . In the present study, diesel exhaust emissions were chemically characterized and three different samplers for the semivolatile phase were evaluated by means of chemical analysis and genotoxicity tests of the collected emission samples. By definition, semivolatile organic compounds (SOC) are compounds having vapor pressures approximately between atm at ambient temperatures (11). Due to and the fact that the majority of compounds present in the automotive exhaust are still unknown, the term semivolatile phase may not be strictly correct. However, most of the compounds, e.g., PAH trapped in the sampling device situated downstream from the filter fulfill the requirements for semivolatile compounds (7, 13, 28). Accordingly, this justifies the use of the term semivolatile phase in this connection.
Experimental Section The heavy-duty diesel truck was a Scania 143 H manufactured by Saab-Scania AB, Sijdertdje, Sweden, equipped with a turbo-charged Scania DSC 1403 diesel engine, 331 kW at rated speed, 30 rps, compression ratio 16:1, bore and stroke 127 mm X 140 mm, and a swept volume of 14.19 L. The odometer reading was approximately 200000 km. All tests carried out were hot engine
0013-936X/9 1/0925-0332$02.50/0
0 1991 American Chemical Society
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Figure 1. The bus cycle (stochasticher fahrcyclus fur stadt linien omnibusse).
tests and the intake air to the engine was conditioned to 20 "C during the tests. For this investigation, a commercial fuel was used, data on which are presented in Table I. During the tests, the fuel was kept at constant temperature, 25 "C, by means of a heat exchanger. The vehicle was operated on a chassis dynamometer (Schenk) during the tests. During the transient driving cycle, a flywheel capacity of 16 964 kg was used. A driving cycle, developed at the Technische Hochschule in Braunschweig (FRG), was used for operation of the vehicle. This cycle, termed Stochasticher Fahrcyclus fur Stadt-Linien Omnibusse (bus cycle) is displayed in Figure 1. The duration time of the driving cycle was 29 min and the driving distance was 11.0 km with a top speed of 58.2 km/h and an average speed of 22.9 km/h, simulating public transportation. Sampling and Analysis. All exhaust sampling was made on diluted exhausts in a dilution tunnel, which was designed to fulfill the specifications in the U.S. Federal Register (30). The flow of diluted exhaust was 120 m3/min maintained by a venturi, and the average dilution ratio was approximately 70. Measurements of regulated emissions was in accordance with the test procedure described in the literature (30). Regulated emissions were measured as follows: carbon monoxide with a nondispersive infrared analyzer (NDIR; Beckman 864), total unburned hydrocarbons (HC) with a flame ionization detector (FID; Beckman 402), oxides of nitrogen (NO,) with a chemiluminescence analyzer (Beckman 955), particulate emissions by the use of Teflon-coated glass fiber filters (Pallflex T60A20; Pallflex Inc.). Sampling of oxygenates and light aromatics was accomplished by means of a cryogenic sampling technique (31) in which a cryogradient is established over a sampling tube packed with a sorbent bed. The cryogradient sampling equipment was connected to the dilution tunnel with a probe made of Teflon, which was heated to 105 "C to prevent condensation of water. One sample was collected for each driving cycle. After sampling, the tubes were sealed and stored in a vessel containing dry ice and transported to the laboratory for analysis. Blank samples were taken when the vehicle was disconnected from the dilution tunnel. Analysis of oxygenates and light aromatics was achieved with a two-dimensional gas chromatograph, which is described in detail elsewhere (12). The chromatographic column used for the first separation (column I) was made of glass tubing (2 m X 1.8 mm i.d.), silanized, and packed with 5% 1,2,3-tris(2-cyanoethoxy)propane(Alltech Inc.) on deactivated Chromosorb WAW (100-120 mesh;
Chrompack). Separation of the compounds eluting from column I was performed on a fused-silica column (50 m X 0.3 mm i.d., OV-101 0.17 pm; Hewlett-Packard). Temperature programming was as follows: isothermal 25 "C for 8 min, increase 3 "C/min to 60 "C,hold 5 min, increase 10 "C/min to 170 "C, final temperature. Following separation on the second column, all compounds were quantified with a flame ionization detector (FID) except xylene and ethyl benzene, which were quantified after the first column. Particulate Phase. Filters used for particulate emission measurements (Pallflex T60A20) were weighed before and after sampling according to the specifications in the literature (30). However, before sampling, the filters were washed with ethanol, acetone, and dichloromethane and dried at 200 "C;this cleaning procedure is described elsewhere (8). After sampling, the filters were stored at -20 "C until extraction. The filters were Soxhlet extracted with dichloromethane for 18 h (170 mL, 20-min refluxing time). All solvents used were distilled in an all-glass column if not otherwise stated. Each dichloromethane extract was evaporated under reduced pressure to just dryness, diluted with acetone to a known volume, and divided into two fractions, one for chemical analysis and one for biological testing. Both fractions were then stored at -20 "C until chemical analysis/biological testing. Blank samples were performed as described above. The organic soluble fraction (OSF) of the filter samples was measured by weighing the filters before and after extraction with dichloromethane. Semivolatile Phase. Sampling of the semivolatile phase was carried out by introduction of three separate sampling tubes into the dilution tunnel, i.e., one for each semivolatile-phase sampler. Particulate emission was trapped upstream, with each semivolatile-phase sampler on an individual Pallflex T60A20 filter. The filter diameter in front of the cryogradient sampler was 15 cm and for the other two semivolatile samplers, 25 cm. Cryogenic sampling of the semivolatile phase was accomplished with two parallel cryogenic sampling devices, which have been used in exhaust emissions evaluations both with gasoline- and diesel-fueled vehicles (13,14). Two parallel sampling devices were used to increase the sampling capacity by a factor of 2; the device used here is described in detail elsewhere (7). However, during this investigation the liquid nitrogen cooled condenser was omitted. Blank samples were taken with the vehicle disconnected from the dilution tunnel and treated as below. After sampling (volumetric flow approximately 170 L / min), the condensers were extracted with acetone for 18 h. The acetone extract was then treated by the same procedure as for the particulate extract. The adsorbent sampler, filled with XAD-2 (Amberlite 0.3-0.78 mm; BDH Chemicals Ltd.), is described in detail elsewhere (13). Purification of the XAD-2 adsorbent material was made by washing 100 g with 13 X 150 mL aliquots of distilled water, followed by 4 X 150 mL aliquots of 99% ethanol, in a Buchner funnel, after which the resin was transferred to a cellulose extraction thimble and extracted with acetone (250 mL, 25-min refluxing time) for 24 h. A second extraction with acetone for 96 h was carried out, and finally, a third extraction with acetone was made, this being used as a blank for both chemical analysis and biological testing. The purified XAD-2 was dried under vacuum at room temperature and then stored in a sealed glass flask until sampling. Immediately before sampling, the sampler was filled with 70 g of the XAD-2, giving a bed height of approximately 60 mm. The XAD-2 material was Environ. Sci. Technol., Vol. 25, No. 2, 1991
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cooled to -20 "C to minimize static eletricity during packing of the sampler. After sampling (volumetric flow rate approximately 240 L/min), the resin was transferred to a sealed glass flask and stored at -20 "C until extraction. The XAD-2 samples were then extracted with acetone (250 mL) for 24 h, the acetone extract being then evaporated under reduced pressure and treated as above. A specially designed sampling device of anodized aluminum was made to contain the polyurethane foam (PUF) plugs. The density of the PUF plug material was 23 g/cm3 and the dimensions of the plugs were height 50 mm and diameter 70 mm. Before sampling, the PUF plugs were washed four a t a time with 8 X 250 mL portions of distilled water, squeezed between portions, followed by 4 X 250 mL portions of 99% ethanol, squeezed between portions, and 4 X 250 mL portions of acetone (pro analysi, Merck). In addition, the PUF plugs were extracted, two at a time, in a Soxhlet extractor (1.5 L) with toluene for 1 2 h (30-min refluxing time), acetone for 12 h (60-min refluxing time), and finally with acetone for 12 h. The latter obtained extract was used as a blank sample for both chemical analysis and biological testing. The purified PUF plugs were stored separately in sealed glass jars until sampling. After sampling (volumetric flow rate approximately 340 L/min), the PUF plug was transferred to a sealed glass jar and stored at -20 "C until extraction. The samples were Soxhlet extracted with acetone for 12 h and the extract was then evaporated under reduced pressure and treated in the same way as for the particulate extract. Each PUF plug was used only once for sampling. However, reextraction and analysis of used PUF plugs gave less than 1% PAH and no significant mutagenic activity was detected, suggesting that the PUF plugs can be reused at least once. Chemical Fractionation. The particulate- and the semivolatile-phase crude extracts were fractionated according to polarity into five fractions before biological testing. However, prior to the chemical fractionation procedure, an internal standard (2,2'-binaphthyl) was added that elutes in the PAH fraction. Five fractions were collected: fraction I, "light" aliphatic hydrocarbons; fraction 11, "heavy" aliphatic hydrocarbons, PAH; fraction 111, nitro-PAH; fraction IV, dinitro-PAH; quinones; fraction V, polar material. This procedure followed that according to Alsberg et al. (15),although the solvent elution volumes were modified to fit the column size used. Since fraction I1 from the silica gel column contained aliphatic material, it was subjected to further cleanup using high-performance liquid chromatography (HPLC) ( 1 6 , l n . The HPLC system consisted of a Waters 590 programmable solvent delivery module (PSDM) (Waters Inc.) connected to three pneumatically actuated six-port Rehodyne switching valves controlled by the PSDM. The first valve was used for back-flushing the column, which was packed with 10-pm pBondapak-NH, particles (250 mm, 7.8 mm i.d.; Waters) and used for the separation. Monitoring of the effluent was made with a Hitachi 655A-22 UV detector adjusted to 254 nm (Hitachi Inc.). Aliphatics, monoaromatics, and diaromatics were eluted before back-flushing the column, which separated primarily according to the number of rings of the analytes. In the back-flush peak, PAH with three or more aromatic rings were eluted (16, 17). With this procedure two subfractions were obtained containing aliphatic material and PAH, respectively. Analysis of the subfraction containing the PAH was made by gas chromatography-mass spectrometry (GCMS). The gas chromatograph, a Hewlett-Packard 5790A was equipped with a split/splitless injector and a fused334
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silica capillary column (15 m X 0.22 mm i.d., SE-54; Chrompack). Temperature programming was as follows: isothermal 70 "C for 1 min, increase 7 "C/min to 300 "C, isothermal 300 "C for 9 min, MS interface temperature set to 300 "C. A JEOL D300 mass spectrometer was used (JEOL Inc.), operated in the electron impact ionization mode. The ion source temperature was 200 "C, ionization voltage 70 eV, scan range 35-350 amu, and the scan time 1.0 s. The mass spectrometer was interfaced to the Finningan computer system INCOS 2000 (Finnigan Inc.). Quantitation was made by integrating the areas of the molecular ions (M+) of the internal standard (2,2'-binaphthyl) and the PAH. A standard mixture containing known amounts of internal standard and all determined PAH was used for response factor calculations and for determination of retention times. Analysis of fraction I11 originating from both particulate and semivolatile phases with regard to 1-nitropyrene was accomplished by means of a method developed by Tejada and co-workers (19),by which the sample was analyzed by a reversed-phase HPLC system, using heart-cutting and on-line reduction of 1-nitropyrene to 1-aminopyrene. This system consisted of Waters Automated Valve Station (WAVS; Waters) a programmable solvent delivery module (Model 590; Waters) and a Shimadzu fluorescence HPLC monitor (Shimadzu Inc.), which were controlled by a Gradient Master (Laboratory Data Control). After preseparation on a Spherisorb 5 C8 (Phase Sep) column (45 X 4.6 mm i.d.1, catalytic reduction was executed in a column (40 X 3.0 mm i.d.) filled with 5-pm alumina particles coated with 1.03% platinum/rhodium, 1:lO (19). The peak corresponding to the retention time of 1-nitropyrene and the internal standard (benz[a]anthracene) was switched via two six-port high-pressure valves to the analytical column, a Supelcosil LCl8-DB, 5 pm, 150 X 4.6 mm i.d. (Supelco Inc.). Here, 1-aminopyrene was separated from benz[a]anthracene and other substances coeluting from the precolumn. Methanol/water was used as mobile phase, which during the preseparation consisted of 55% methanol in water. All runs were gradient programmed, and the mobile-phase composition nominally ranged from 55%, 60%, and 75% to 85%, during the second separation step. Further, oxygen was removed from the mobile phase by a catalyst column (45 X 4.6 mm id.) situated in front of the injection valve. This column contained traces of nickel, platinum, and rhodium on a ceramic support, obtained from finely ground material from an automobile exhaust catalytic converter. The temperature of the oxygen scrubber was held a t 70 "C. Quantitation was performed by using the integrated fluorescence signal of 1-aminopyrene with that of benz[alanthracene. For this purpose, the fluorescence detector was used, a t an excitation wavelength of 284 nm and emission wavelength of 387 nm for benz[a]anthracene, and excitation wavelength of 360 nm and emission wavelength of 430 nm for 1-aminopyrene. A Trilab 2000 laboratory data system (Trivector Scientific Ltd.) was used for data processing. Mutagenicity tests were carried out using Salmonella typhimurium strains TA98 and TAlOO according to Maron and Ames (20) with a slight modification, histidine and biotin being added to the minimal medium instead of to the soft agar. A liver preparation (S9) from Arochlorpretreated male Sprague-Dawley rats was used as metabolizing system in an amount of 50-pL/plate. The particulate phase was tested as a crude extract in three concentrations per plate, corresponding to a driving distance of 0.2,0.4, and 0.8 m, respectively. The five fractions (I-V)
Table IV. Emission of Particulate-Associated Polycyclic Aromatic Compounds (pg/km)"
Table 11. Emissions of Regulated Pollutants, Fuel Consumption and Organic Soluble Fraction (g/km)"
HC
co
NO, particulate emission fuel consumption organic soluble fraction (OSF)
n
mv
SD
6 6
2.28 9.80 18.7 0.62 443 0.16
3.2 19 3.2 19 1.1 12
6 17 6 8
asample number (n), mean value (mv) f standard deviation
(SD)('70). Table 111. Emissions of Oxygenates and Light Aromatic Compounds (mg/km)a
acrolein acetone 2-propanol methacrolein 3-buten-2-one n-butanal methyl ethyl ketone 3-methylbutanal benzene toluene xylene ethylbenzene
mv
SD
33 30 2.4 5.5 12 6.9 6.4 0.95 15 11 4.3 1.3
13 4.3 19 13 5.9 10 1.0 7.3 17 28 4.7 26
" n = 3, mean value (mv) f standard deviation (SD) (%).
of the particulate extract were tested in concentrations of 0.5, 1.0, and 2.0 m per plate. A recombined sample of the five fractions was also tested in 0.25, 0.5, and 1.0 m per plate. One crude extract from the semivolatile phase (PUF) was also tested at the same concentrations as the fractions of the particulate phase. The five fractions from the three different semivolatile-phase samplers were tested at 1.0, 2.0, and 4.0 m per plate, except for the cryogenic sample, which was tested only at the two highest concentrations. All acetone extracts were evaporated to nearly dryness under nitrogen and then diluted with dimethyl sulfoxide to known volumes and tested for mutagenicity. The volumes of each sample added to the soft agar were 25,50, and 100 FL, respectively.
Results and Discussion Fuel consumption, organic soluble fraction, and regulated pollutants are presented in Table 11. Emissions of CO and HC are in good agreement with previously reported values originating from a heavy-duty diesel truck operated according to the "bus cycle" (21);however, regarding NO,, the emission is about half in this investigation. The particulate emission presented in Table I1 is -35% lower compared to published data (21)originating from a fuel with a similar sulfur content Le., 0.26% by weight. An explanation of this is that different engine designs influence the exhaust emissions. The organic soluble fraction presented in Table I1 resembles that which can be extracted from the carbon matrix from diesel engine generated particulate material, this being in the range of 10-40% by weight (22). The emissions of oxygenates are presented in Table 111. Comparing the emission factors with those previously reported (231, it can be seen that the emission factors are approximately 20-5070 lower in this study, omitting 2propanol and acetone. The 2-propanol emission is approximately the same, while the acetone emission is 50%
mv 2-methylfluorene phenanthrene anthracene 3-methylphenanthrene 2-methylanthracene 4- + 9-methylphenanthrene 1-methylphenanthrene fluoranthene pyrene benzo[a]fluorene 2-methylpyrene 1-methylpyrene benzo[ghi]fluoranthene cyclopenta [ cd] pyrene benz [a]anthracene chrysene/ triphenylene benzo[b and klfluoranthene benzo [ e ]pyrene benzo[a]pyrene perylene indeno[ 1,2,3-cd]fluoranthene indeno[ 1,2,3-cd]pyrene picene benzo[ghi] perylene coronene sum of PAH (25) 1-nitropyrene dibenzothiophene 4-methyldibenzothiophene 3-methyldibenzothiophene
