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Organic emissions profile for a light-duty diesel vehicle
Atmospheric Environment 33 (1999) 797—805
Organic emissions profile for a light-duty diesel vehicle Walter O. Siegl *, Robert H. Hammerle , Heiko M. Herrmann, Bernd W. Wenclawiak, B. Luers-Jongen Research Laboratory, Ford Motor Company, SRL-3083, P.O. Box 2053, Dearborn, MI 48121, USA Analytische Chemie I, Universitat-GH-Siegen, 57068 Siegen, Germany FEV Motorentechnik GmbH and Co. KG, Aachen, Germany Received 17 February 1998; accepted 18 May 1998
Abstract In this report we describe the speciated gas-phase hydrocarbon and carbonyl emissions as collected from a recent model automobile powered by a 2.5l indirect injection diesel engine and outfitted with a production oxidation catalyst for exhaust after-treatment. The vehicle was run on a typical low sulfur (500 ppm S) European diesel fuel and measurements were made over the European MVEG test cycle. The diluted tailpipe emissions were sampled for light hydrocarbons (C —C ) using Tedlar bags and semi-volatile hydrocarbons (C —C ) using Tenax cartridges. Both the > light and semi-volatile hydrocarbon fractions were speciated using capillary gas chromatography. Combining the two sets of speciation data provided a profile of the gas-phase hydrocarbon emissions from a light duty diesel vehicle. Of the total gas phase non-methane hydrocarbons emitted, 80% were accounted for in the light hydrocarbon fraction, and 20% in the semi-volatile fraction. The semi-volatile fraction, which extended only to about C , was composed almost entirely of unburned fuel molecules, but with enrichment of the aromatic species relative to the fuel itself. n-Alkanes (paraffins) and methylnaphthalenes accounted for approximately 37% of the semi-volatile fraction. Aldehydes and ketones represented 34% of NMOG. Formaldehyde and acetaldehyde, account for 74% of the total carbonyl emissions. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Exhaust; Automobile; Volatile organic compounds (VOC); Diesel
1. Introduction Data on the composition and photoreactivity of diesel hydrocarbon (HC) emissions is required as input for atmospheric models and for accurate comparison of the relative environmental impact of diesel emissions with those from vehicles powered with gasoline or alternative fuels. Hydrocarbon species are not all equal with respect to impact on ozone formation, therefore any comparison
*Corresponding author.
of the relative environmental impact of hydrocarbon emissions from diesel vehicles requires knowledge of individual HC emission rates. In recent years, much has been learned about the composition and atmospheric reactivity of the hydrocarbon emissions from gasoline-fueled vehicles (Cf., Auto/Oil, 1992; Auto/Oil, 1993; EPEFE, 1996), however, only limited data on exhaust emissions from diesel-fueled vehicles are available. Recently, there has been increased interest in the measurement of speciated hydrocarbons above C in both urban air and highway tunnel samples (Lowenthal et al., 1994; Zielinska and Fung, 1994; Zielinska and Fujita, 1994; Zielinska et al., 1996; Sagebiel et al., 1996). Although
1352-2310/99/$ — see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 9 8 ) 0 0 2 0 9 - X
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several reports describe the partial speciation of HC emissions from light and heavy-duty engines (Jemma et al., 1992; Lepperhoff et al., 1994; Newkirk et al., 1995; Hammerle et al., 1995; Ogawa et al., 1995; Clark et al., 1996; Gautam et al., 1996; Hublin et al., 1996; Ogawa et al., 1997), to our knowledge, no profile has been reported for the full range of gas-phase HC or organic emissions from a light duty diesel-fueled vehicle. In this report we describe the measurement of the speciated gas-phase hydrocarbon and carbonyl emissions collected from a recent model diesel-powered automobile outfitted with a production oxidation catalyst for exhaust after-treatment. The vehicle was one of a fleet of five vehicles selected to represent advanced diesel technology as available in model years 1992—1993 (Hammerle et al., 1994). The vehicle was run on a typical low sulfur European diesel fuel and measurements were made over the European MVEG test cycle. The tailpipe emissions were sampled for light hydrocarbons (C —C ), semi volatile hydrocarbons (C —C ), and aldehydes and > ketones.
2. Experimental methods 2.1. Vehicle testing A 1992 Mercedes Benz 250D sedan, with a turbocharged 5-cylinder 2.5l indirect injection diesel engine, production oxidation catalyst for exhaust after treatment, and 10 050 km accumulated mileage, was tested in a fully instrumented chassis dynamometer facility over the Motor Vehicle Emissions Group (MVEG) cycle. Additional details of the vehicle and test methodology are reported elsewhere (Hammerle et al., 1994). The fuel used was a low sulfur (500 ppm S) version of a standard European diesel fuel with a ¹ of 348°C, cetane number 53.6, density 829.8 kg m\, and aromatic content of 23.9 (%v/v). The physical properties of the fuel and a speciation analysis were reported earlier (Hammerle et al., 1995). 2.2 Sampling Exhaust gases from the vehicle were carried through a heated line to a dilution tube where they were mixed with humidity- and temperature-controlled air (Lepperhoff et al., 1994). This method of sampling reflects the natural dilution and cooling of the exhaust as it leaves the tailpipe. Various sampling ports were available on the dilution tube for sample withdrawal. The average dilution factor over the three tests was 19.3. For the light HC collection, a fixed portion of the diluted exhaust was pulled through an ambient temperature filter (Pallflex TX 40HI20WW, 35°C), to remove
particulate matter, and into a Tedlar bag. The bags were protected from light and were analyzed within 2 h of collection. A second portion of the diluted exhaust was filtered (Pallflex TX 40HI20WW, 35°C) and then pulled through a Tenax adsorbent trap for collection of the semi-volatile fraction of the exhaust. The flow through the trap was approximately 0.15l min\. Back up traps were employed, but analysis showed no meaningful concentrations of species in the range of interest. Custom-made adsorption tubes were obtained from Dra¨ger (Lu¨beck, Germany). The sorbent tubes were made of borosilicate glass, 6 mm o.d.;100 mm long, 4.7 mm i.d., packed with 31 mm of 60—80 mesh Tenax TA. Stainless-steel frits held the packing at the inlet and exit ends. For the semivolatile HC fraction, a single sample was collected for the entire MVEG drive cycle. A separate portion of the dilute exhaust was drawn through a filter (Pallflex TX 40HI20WW, 35°C) and then through a DNPH-coated silica cartridge for collection of the aldehydes and ketones. The procedure used was the same one as developed for the Auto/Oil Air Quality Improvement Research Program (AQIRP) (Swarin et al., 1992; Siegl et al., 1993). 2.3. Analysis The analytical methods used for the analysis of the light hydrocarbon and aldehyde and ketone fractions were based on those described for the AQIRP program (Swarin et al., 1992; Siegl et al., 1993). However, the analysis of the semi-volatile HC fraction required development of a new method. The semi-volatile HCs were analyzed by capillary gas chromatography after thermal desorption of the Tenax traps followed by cryo-focussing. A Hewlett Packard HP5890 Series II gas chromatograph (GC) was equipped with a cryo-focussing inlet system (described below) connected to a single split/splitless injector and two detectors. Two identical columns were connected to the injector with a ‘‘½’’ connector; one column went to a flame ionization detector (FID) and the other to a mass spectrometer (MS). This arrangement allowed the collection of both FID and MS data from a single analytical run. Each arm of the ‘‘½’’ from the injector was connected to a 2 m;0.32 mm i.d. pre-column of deactivated silica (Promochem) followed by a 50 m;0.32 mm i.d. fused silica analytical column with a 1.0 um methylsilicone coating (BP1, Firma SGE) for separation of the HC species. Connections between columns were made with press-fit connectors. Helium was used as the carrier gas with a column head pressure of 10 psi. The column flow to the FID was 1.2 ml min\ at 40°C. The FID was maintained at 300°C; helium was also used as the detector make up gas. The temperature-programmed analysis used a starting temperature of 40°C (held for 5 min)
W.O. Siegl et al. / Atmospheric Environment 33 (1999) 797—805
followed by heating at 8°C min\ to a final temperature of 300°C (held for 20 min). The mass spectrometer was a Delsi—Nermag Automass used at an ionization energy of 70 eV, with the interface between the GC and the mass spectrometer maintained at 300°C. The thermal desorption/cryo-focussing inlet system (GERSTEL Thermodesorptionssystem TDS 1 and Kaltaufgabesystem KAS 3) allowed both direct injection of liquid samples with a syringe (for fuel analysis) and thermal desorption of samples adsorbed on Tenax traps. During transfer, the cryo-focussing unit was maintained at sub-ambient temperature (!50°C) using a vapor stream of liquid nitrogen. After transfer of the sample to the cryo-focussing unit was complete, the cryo-trap was heated rapidly (10°C s\) to 310°C to transfer the sample to the head of the analytical column. During heating of the cryo-focussing unit, the GC injector was maintained in the splitless mode. The Tenax traps were thermally desorbed with a helium flow (30 ml min\) in the direction counter to the flow during the collection process. The heating rate could be programmed. Optimal results were obtained when the heating was programmed at 20°C min\ to 280°C, the maximum rate allowed with the equipment. During transfer of the sample from the Tenax trap to the cryofocussing unit, the GC injection port was maintained in the split configuration (split ratio 20 : 1). The GC was calibrated daily for retention times using a solution of the standard diesel fuel in n-hexane (21.05 mg carbon ml\). Using the n-alkane peaks in the fuel as reference peaks, the software calculated updated retention times for each peak in the library. Calibration for quantitation was also carried out using the same diesel fuel standard. The FID detector was used for quantitation. An ‘‘average’’ response factor was calculated by dividing the sum of the peak area from dodecane (n-C ) to (but not including) octadecane (n-C ) by the ng of carbon in the fuel sample in this same range (as determined by liquid injection). Response factors for individual hydrocarbons were assumed to be equivalent on a per mole of carbon basis. Zielinska et al. (1996), have recently reported that individual response factors in the semi-volatile HC fall in a narrow range. Exhaust Sample Analysis, Standard Method — The loaded Tenax tube was placed in the thermal desorption unit so that the helium flow was in the direction opposite to the flow during the sample collection process. The trap was purged of air with helium for 2 min and then the transfer needle of the thermal desorption unit was inserted in the injection port of the cryo-focussing unit. The trap was heated at the maximum rate of 20°C min\ with a helium flow of 30 ml min\. During the desorption, the cryo-focussing tube was maintained at !50°C and the GC injection port was in the split mode with a split ratio of approximately 20 : 1. After the desorption unit had been held at the maximum temperature (280°C) for
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5 min, the transfer needle of the thermal desorption unit was disengaged from the cryo-focussing unit, the helium flow through the cryo unit was initiated, and the GC injector was changed to the splitless mode. The cryofocussing unit was heated at 10°C s\ with the helium flow to the GC column at 1.2 ml min\. After transfer of the sample to the GC column, the temperature of the GC oven was programmed as indicated above. 2.4. Identification The library of species for identification of hydrocarbons in the semi-volatile range was based on the analysis of the two fuels used in a larger study, a low sulfur (500 ppm S) version of a standard European diesel fuel with a ¹ of 348°C (used in the current tests), and also a sulfur-free fuel which met the Swedish criteria for a Class I diesel fuel and had a ¹ of 280°C. The com posite list of chromatogram peaks found in the two diesel fuels contains 180 entries representing those species which were observed at the 0.01% level and above; the list of peaks with retention times was published earlier (Hammerle et al., 1995). The C —C (semi-volatile) range of the library con tains 79 species starting with dodecane. Where possible, species were identified by comparing GC retention times with those of authentic samples. The identities of other species in the library were determined by comparing mass spectra of the library entry with those contained in a NIST library. (Because of the vacuum drawn on the end of the column going to the mass spectrometer, retention times from this column were not identical to retention times for the column going to the FID.) The large number of peaks in the chromatogram, particularly in the C —C region, meant that baseline resolution was not achieved during the approximately 1 h temperature-programmed GC analysis. (Total sample analysis time, including thermal desorption, was 1.25 h.) Some overlap of peaks resulted under these conditions. Care was taken to look for the dominant species in each chromatography peak. Where specific names were not established for a peak, an attempt was made to identify the dominant class of species represented by the peak, e.g. aromatic or branched alkane. All of the n-alkanes and several aromatic species were identified. The identified species include most of the major peaks in the semi-volatile range.
3. Results and discussion Compared with current technology gasoline vehicles, the CO and HC emissions from this diesel vehicle are relatively low (Cf. Auto/Oil, 1992). The regulated emissions for the diesel vehicle are summarized in Table 1. (For reference, data are also shown for the same vehicle run on the US EPA UDDS [FTP] cycle). The values
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reported are a three-test average. The standard deviations shown reflect the reproducibility of the emissions from test to test and not the accuracy of the measurements. Similar to gasoline-fueled vehicles, exhaust HC emissions from diesel-fueled vehicles are expected to be a mixture of unburned and partially burned fuel species
Table 1 Regulated emissions from the diesel test vehicle (3-test average) Species
MVEG test Average
CO (g km\) NO (g km\) V HC (g km\) PM (g km\)
0.591 0.560 0.091 0.066
FTP test S.D
Average
S.D
0.008 0.008
0.528 0.591 0.068 0.069
0.020 0.001
Measured by heated FID.
(Kaiser et al., 1991, 1992, 1993, 1994; Siegl et al., 1992). Hydrocarbons for speciation analysis were collected using both Tedlar bags (C —C ) and Tenax traps (C ). > The light hydrocarbons (C —C ) were speciated (separ ated, quantified, and identified) according to well established methods, and the emission rates are shown in Table 2. The list is arranged in order of increasing chromatographic retention time, which corresponds approximately to increasing boiling point. Of the total light HC mass, 96% was identified. Species up to and including the xylenes represent primarily products of partial combustion, whereas species in the list after the xylenes represent primarily unburned fuel. Of the 96% of the light HC mass which can be identified, 78% is contributed by partial combustion products. Methane and ethene (ethylene) together account for 59% of total light hydrocarbons and ethene accounts for 38% of light nonmethane hydrocarbons (NMHC). Measurement of the semi-volatile gas-phase HC emissions (volatility greater than that of dodecane) was carried out using Tenax traps. The use of Tenax to trap the
Table 2 Methane and light HC fraction (C —C ) from diesel exhaust (average of 3 MVEG tests) Compound
Emission rate (mg km\)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
13.12 9.90 2.20 1.84 1.00 0.66 1.15 0.04 0.56 0.41 0.38 0.90 0.07 1.19 0.08 0.12 1.16 0.09 0.05 0.41 0.19 0.98 0.56 0.60 0.46 0.19 0.95
37.8% 8.4% 7.0% 3.8% 2.5% 4.4% 0.2% 2.2% 1.6% 1.5% 3.5% 0.3% 4.5% 0.3% 0.5% 4.4% 0.3% 0.2% 1.6% 0.7% 3.8% 2.1% 2.3% 1.8% 0.7% 3.6%
30.1% 6.7% 5.6% 3.1% 2.0% 3.5% 0.1% 1.7% 1.2% 1.2% 2.7% 0.2% 3.6% 0.2% 0.4% 3.5% 0.3% 0.1% 1.3% 0.6% 3.0% 1.7% 1.8% 1.4% 0.6% 2.9%
39.29 26.17
100.0%
79.5%
Methane Ethene Ethyne Propene Benzene Toluene 1-Octene Ethylbenzene m&p-Xylene 2,4-Dimethyloctane Unknown (¹ "35.86 min) 0 1-Methyl-3-ethylbenzene 1,2,4-Trimethylbenzene Decane 1,2,3-Trimethylbenzene Unknown (¹ "37.54 min) 0 1,4-Diethylbenzene 1-Methyl-4-n-propylbenzene 1,2-Diethylbenzene 1-Methyl-2-n-propylbenzene 1,3-Dimethyl-2-ethylbenzene Undecane Unknown (¹ "41.11 min) 0 n-Pentylbenzene Unknown (¹ "41.70 min) 0 Napthalene Dodecane
Total light HC Total light NMHC
% of light-NMHC
% of total NMHC
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semi-volatile HCs provided two benefits. First, the semivolatile HCs from 4—5l of dilute exhaust could be concentrated and loaded as a single injection on a GC column. Second, the heavy HC fraction was stabilized, and could be stored at reduced temperatures without loss of sample integrity. Tenax TA was chosen because, due to its high thermal stability, it could be thermally desorbed. This method of sample collection/recovery was preferred relative to cold-trapping or adsorption followed by solvent extraction and subsequent extract concentration. The latter methods require additional sample handling and afford greater opportunities for sample loss. To determine our ability to recover HCs from the Tenax trap without discriminatory losses, a set of experiments was carried out in which traps were spiked with solutions of various standards. A standard mixture containing the even-numbered n-alkanes, C —C , was in jected on a Tenax trap and recovery was measured by comparing the FID response relative to the response from a direct liquid injection. The results indicate that recovery was nearly quantitative, within experimental error, over the molecular weight range of greatest interest (C —C ), but that transfer losses were significant below C . (The transfer losses were due to limitations in the particular equipment available and not in the method itself.) Attempts to extend recovery to a broader range of n-alkanes by adjusting the operating parameters of the cryo-focussing unit were unsuccessful. Recovery experiments carried out with standards prepared from diesel fuel instead of the pure n-alkanes gave similar results; recovery of the C —C fraction was nearly quantitative. The GC area-counts in the C —C region observed for a liquid injection and for a trap desorption for the same standard solution of fuel agreed to within 6%. Species identification for the semi-volatile hydrocarbon analysis was based on a comparison of retention times with those in the library. Based on the chromatograms of the fuels, the C —C library contains 79 spe cies starting with dodecane. Chemical names can be associated with 17 of these species and a large number of additional species can be labeled as either aliphatic or aromatic based upon mass spectral data. Table 3 contains a summary of the speciated semivolatile HC emission rates observed for the diesel vehicle over three vehicle tests; species with emissions rates between 0.01—0.04 mg km\ have been summed and appear as a single entry. The quantitation limit was an emission level of 0.01 mg km\. The sum of the semi-volatile hydrocarbons is 6.72 mg km\ compared with 39.3 mg km\ for the C —C fraction (26.2 mg km\ for C —C ). For the semi-volatile fraction, 95% of the mass lies between dodecane (n-C ) and pentadecane (n-C ); 71% lies be tween dodecane (n-C ) and tetradecane (n-C ). For tyfour percent (44%) of the mass was identified as aliphatic and 41% as aromatic. In comparison, the aromatic content of the fuel was only 24% (Hammerle et al.,
801
1995). An average of 47 different species were detected above dodecane (n-C ); the 20 most abundant species accounted for 83% of the mass. Of these 20 species, eight could be assigned a name and another seven could be labeled as either aliphatic or aromatic. The n-C , n-C , and n-C paraffins accounted for 20% of the semi-vol atile HC mass, and the methyl- and dimethylnaphthalenes accounted for 17% of the semi-volatile HC mass. In Fig. 1 the relative amounts of individual n-alkane (paraffin) species in the fuel and in the exhaust are plotted according to carbon number. The n-alkanes as determined by the light HC and the semi-volatile HC methods are plotted in separate series. The shapes of the exhaust profile and the low end of the fuel profile are similar. Only the low-mid range of the fuel makes a significant contribution to the gas-phase HC emissions. Both the fuel and exhaust samples reach a maximum at C ; how ever, the exhaust composition drops off rapidly after C . The higher boiling components of the fuel are most likely to be found condensed on the particulate matter. This analysis was not carried out for the current study. However, Ogawa et al., (1997), have recently shown that the soluble organic fraction of the particle matter emissions contains the mid-high boiling fuel components. A comparison can be made between dodecane (n-C ) as measured by the light HC method (average: 0.90 mg km\) and dodecane as measured by the semivolatile HC method (average: 1.41 mg km\). The higher value reported by the semi-volatile HC method is consistent with our belief that Tenax traps represent a more efficient collection device than Tedlar bags for collecting the semi-volatile HCs. Similarly, the deviation of undecane (n-C ), measured from the Tedlar bag, from the trend line, suggests that it too undergoes losses in the Tedlar bag. Difficulties associated with storing the heavier HCs in Tedlar bags have been noted by others (cf. Zielinska et al., 1996). For HC species of C and below, collection in Tedlar bags represents an efficient sampling method. The poor recovery of HC below dodecane from the Tenax traps, while using the current hardware (as cited above), makes additional comparison between the collection methods not meaningful. The same semi-volatile HC peaks were observed in the GC chromatograms of the exhaust and the fuel. No additional peaks were found in significant amount in the exhaust. However, the ratios of the peaks differ between the fuel and exhaust. Fig. 2 contains histograms of the dodecane to tetradecane region (which accounts for 71% of the total semi-volatile HC exhaust emissions) for both the fuel and exhaust samples. Some differences in ratios would be expected based on the data presented in Fig. 1, which shows an effect predominantly associated with volatility. Fig. 2 indicates that other phenomena are also operational. Peaks 73, 88, and 103 represent dodecane, tridecane, and tetradecane respectively. In the exhaust sample, the three peaks decrease with increasing carbon
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Table 3 Semi-volatile HC fraction ('C ) of diesel exhaust (average of 3 MVEG tests) Species (or library entry)
Emission rate (mg km\)
% of S—V NMHC
% of total NMHC
Naphthalene Dodecane 1 C76,aliphatic 2 C78,aromatic 3 C79,aromatic 4 80,aromatic 5 C81,aliphatic#aromatic 6 C82,aliphatic 7 C83,aromatic 8 C84,aliphatic 9 C85,aliphatic 10 C86,aromatic 11 Tridecane 12 2-Methylnapthalene 13 C91,aliphatic 14 1-Methylnaphthalene 15 C95 16 C,aliphatic 17 C98,aliphatic#aromatic 18 Acenaphthene 19 C -NaphthaleneC1 20 Tetradecane 21 C104,aliphatic 22 C -NaphthaleneC2 23 C106,aromatic 24 C -NaphthaleneC3 25 C -NaphthaleneC4 26 C111,aromatic 27 C114,aliphatic#aromatic 28 C116 29 Pentadecane 30 C119,aromatic 31 C -NaphthaleneC1 Sum of 16 minor peaks
1.60 1.41 0.40 0.10 0.07 0.05 0.36 0.13 0.11 0.16 0.42 0.11 0.83 0.69 0.09 0.44 0.09 0.22 0.07 0.19 0.13 0.41 0.05 0.10 0.08 0.08 0.23 0.13 0.16 0.21 0.13 0.07 0.07 0.34
6.0% 1.5% 1.0% 0.7% 5.4% 1.9% 1.6% 2.4% 6.3% 1.6% 12.4% 10.3% 1.3% 6.5% 1.3% 3.3% 1.0% 2.8% 1.9% 6.1% 0.7% 1.5% 1.2% 1.2% 3.4% 1.9% 2.4% 3.1% 1.9% 1.0% 1.0% 5.1%
1.2% 0.3% 0.2% 0.2% 1.1% 0.4% 0.3% 0.5% 1.3% 0.3% 2.5% 2.1% 0.3% 1.3% 0.3% 0.7% 0.2% 0.6% 0.4% 1.2% 0.2% 0.3% 0.2% 0.2% 0.7% 0.4% 0.5% 0.6% 0.4% 0.2% 0.2% 1.0%
Total
6.72
100.0%
20.4%
number as expected from Fig. 1. However, several peaks, notably the starred peaks: 90, 93, and 102, have increased significantly relative to the n-alkane peaks. These peaks represent the aromatic hydrocarbons: 2-methylnaphthalene, 1-methylnaphthalene, and an unspecified C -naph thalene, respectively. The enrichment of the semi-volatile fraction of the exhaust sample with aromatic hydrocarbons could be due to a process which selectively removes saturated hydrocarbons or to a process which converts larger aromatics to smaller aromatics. Conversion of alkylbenzenes to benzene and toluene in the spark-ignition engine has been observed (Kaiser et al., 1992). Together, the emission rate data presented in Tables 2 and 3 provide a profile of the gas-phase hydrocarbon emissions from a representative light duty diesel vehicle.
Total NMHC emissions were 32.9 mg km\, 80% in the light HC fraction and 20% in the semi-volatile HC fraction. Approximately 85% of the NMHC mass can be identified and additional mass can be characterized as either aromatic or paraffinic. Ethene, the most abundant NMHC species, accounts for 30% of the total mass. If we assume that the species ethene through m-/p-xylene, as shown in Table 2, all represent partial combustion products, then partial combustion products represent 53% and unburned fuel 47% of the NMHC in the diesel exhaust. Absent from the exhaust are the small alkanes (e.g., butane, 2-methylbutane, pentane, 2-methylpentane) and small branched olefins (e.g., 3-methylpropene) which are prevalent in gasoline vehicle exhaust. Toluene, frequently one of the most abundant hydrocarbons in gaso-
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line vehicle exhaust, was only observed at the 2% level in the NMHC exhaust from this diesel vehicle. The aldehydes and ketones were also collected and speciated using well-established methods (Swarin et al., 1992; Siegl et al., 1993). The speciated aldehyde and ketone emission rates are presented in Table 4 (the mass of oxygen is included in the emission rates). The two carbonyl species which are classified as air toxics, formaldehyde and acetaldehyde, account for 74% of the total.
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Formaldehyde alone accounts for 54% of the total. The formaldehyde and acetaldehyde emissions from this diesel vehicle are higher than the emission rates recently reported for a gasoline-powered vehicle fleet (FTP test): formaldehyde (6.6 mg km\ vs. 1.0 mg km\) and acetaldehyde (2.4 mg km\ vs. 0.8 mg km\) (Auto/Oil, 1992). If NMOG (non-methane organic gases) emissions are calculated (NMOG"NMHC#carbonyl species), the average NMOG emissions are 50.2 mg km\, and the carbonyl species (aldehydes#ketones) represent 34% of the total NMOG emissions. For gasoline vehicles, the carbonyl species typically represent about 5% of the total NMOG emissions.
4. Conclusions
Fig. 1. Distribution of n-alkanes in the fuel and exhaust samples.
The gas-phase organic emissions (light HC, semi-volatile HC, and carbonyl) from a recent model, dieselfueled, light-duty vehicle have been speciated and quantified. Combining the two HC speciation data sets provides a profile of the gas-phase HC emissions from a light duty diesel vehicle. Methane accounted for 33% of total hydrocarbon emissions. Of the non-methane hydrocarbons, 80% were present in the light hydrocarbon fraction (C —C ), and 20% in the semi-volatile fraction ('C ). Approximately, 85% of the NMHC mass can be identified. Ethene accounts for 30% of the total NMHC.
Fig. 2. Composition of the exhaust and fuel profiles of the n-C to n-C region (as recovered from Tenax traps).
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Table 4 Aldehyde and ketone emissions profile (mg km\) (MVEG cycle, 3-test average) Species
Formaldehyde Acetaldehyde Acetone Propionaldehyde Crotonaldehyde Methylethyl ketone Butyraldehyde Benzaldehyde Total Total NMOG
Average
% S.D.
9.23 3.51 1.80 0.86 0.65 0.00 0.49 0.65
5.5% 0.9% 8.6% 6.1% 6.7% 0.0% 17.6% 5.3%
17.19 50.20
6.3%
Partial combustion products represent 53%, and unburned fuel 47% of the NMHC in the diesel exhaust. The semi-volatile fraction extended only to about C . n Alkanes and methylnaphthalenes were the major components, accounting for approximately 37% of the semivolatile fraction. Although the semi-volatile HC fraction contained almost entirely fuel components, the ratio of species was changed with an enrichment of aromatic species observed in the exhaust. Formaldehyde and acetaldehyde, account for 74% of the total carbonyl emissions. The NMOG emission rate was 50.2 mg km\ with aldehydes and ketones accounting for 34% of total NMOG.
References Auto/Oil Air Quality Improvement Research Program, 1992. Special Publication No. 920, ISBN 1-56091-238-3. Society of Automotive Engineers, Warrendale, PA. Auto/Oil Air Quality Improvement Research Program, 1993. Vol. II, Special Publication No. 1000, ISBN 1-56091-439-4. Society of Automotive Engineers, Warrendale, PA. Clark, N., Akinson, C. M., McKain, D.L., Nine, R.D., El-Gazzar, L., 1996. Speciation of Hydrocarbon Emissions from a Medium-Duty Diesel Engine. SAE Paper No. 960322. Society of Automotive Engineers, Warrendale, PA. European Programme on Emissions, Fuels and Engine Technologies, (EPEFE), 1996. Special Publication No. 1204. Society of Automotive Engineers, Warrendale, PA, ISBN 156091-861-6. Gautam, M., Gupta, D., El-Gazzar, L., Lyons, D.W., Popuri, S., 1996. Speciation of heavy duty diesel exhaust emissions under steady state operating conditions. SAE Paper No. 962159, Society of Automotive Engineers, Warrendale, PA. Hammerle, R.H., Ketcher, D.A., Horrocks, R.W., Lepperhoff, G., Huthwohl, G., Luers, B., 1994. Emissions from current
Perecent of carbonyls 53.7% 20.4% 10.5% 5.0% 3.8% 0.0% 2.9% 3.8% 100%
Percent of NMOG 18.4% 7.0% 3.6% 1.7% 1.3% 0.0% 1.0% 1.3% 34.2%
diesel vehicles. SAE Paper No. 942043, Society of Automotive Engineers, Warrendale, PA. Hammerle, R.H., Siegl, W.O., Herrmann, H.M., Wenclawiak, B.W., 1995. A method for the speciation of diesel fuel and the semi — volatile hydrocarbon fraction of diesel-fueled vehicle exhaust emissions. SAE Paper No. 952353, Society of Automotive Engineers, Warrendale, PA. Hublin, M., Gadd, P.G., Hall, D.E. Schindler, K.P., 1996. European programmes on emissions, fuels and engine technologies (EPEFE) — light duty diesel study. SAE Paper No. 961073, Society of Automotive Engineers, Warrendale, PA. Jemma, C.A., Lance, D.L., Shore, P.R., 1992. Speciation of hydrocarbon emissions from European vehicles. SAE Paper No. 922376, Society of Automotive Engineers, Warrendale, PA. Kaiser, E.W., Siegl, W.O., Henig, Y.I., Anderson, R.W., Trinker, F.H., 1991. Effect of fuel structure on emissions from a spark-ignited engine. Environmental Science and Technology 25, 2005-2012; corr. ibid. 26, 2618. Kaiser, E.W., Siegl, W.O., Cotton, D.F., Anderson, R.W., 1992 Effect of fuel structure on emissions from a spark-ignited engine II. Naphthene and Aromatic Fuels. Environmental Science and Technology 26, 1581—1586. Kaiser, E.W., Siegl, W.O., Cotton, D.F., Anderson, R.W., 1993. Effect of fuel structure on emissions from a spark-ignited engine III. Olefin Fuels. Environmental Science and Technology 27, 1440-1447. Kaiser, E.W., Siegl, W.O., Anderson, R.W., 1994. Fuel structure and the nature of engine-out emissions. SAE Paper No. 941960, Society of Automotive Engineers, Warrendale, PA. Lepperhoff, G., Huthwohl, G., Luers-Jongen, B., Hammerle, R.H., 1994. Methods to analyze non-regulated emissions from diesel engines. SAE Paper No. 941952, Society of Automotive Engineers, Warrendale, PA. Lowenthal, D.H., Zielinska, B., Chow, J.C., Watson, M., Gautam, M., Ferguson, D.H., Nueroth, G.R., Stevens, K.D., 1994. Characterization of heavy-duty diesel vehicle emissions. Atmospheric Environment 28, 731—743.
W.O. Siegl et al. / Atmospheric Environment 33 (1999) 797—805 Newkirk, M.S., Bass, E.A., 1995. Reactivity comparison of exhaust emissions from heavy-duty engines operating on gasoline, diesel, and alternative fuels. SAE Paper No. 952442, Society of Automotive Engineers, Warrendale, PA. Ogawa,T., Araga, T., Okada, M., Fujimoto, Y., 1995. Fuel effects on particulate emissions from D.I. engine-chemical analysis and characterization of diesel fuel. SAE Paper No. 952351, Society of Automotive Engineers, Warrendale, PA. Ogawa,T., Nakakita, K., Yamamoto, M., Okada, M., Fujimoto, Y., 1997. Fuel effects on particulate emissions from D.I. engine — relationship among diesel fuel, exhaust gas and particulates. SAE Paper No. 971605, Society of Automotive Engineers, Warrendale, PA. Sagebiel, J.C., Zielinska, B., Pierson, W.R., Gertler, A.W., 1996. Real-world emission and calculated reactivities of organic species from motor vehicles. Atmospheric Environment 30, 2287—2296. Siegl, W.O., McCabe, R.W., Chun, W., Kaiser, E.W., Perry, J., Henig, Y.I., Trinker, F.H., Anderson, R.W., 1992. Speciated hydrocarbon emissions from the combustion of single component fuels. I. Effect of fuel structure. Journal Air and Waste Management Association 42, 912—920.
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Siegl, W.O., Richert, J.F.O., Jensen, T.E., Schuetzle, D., Swarin, S.J., Loo, J.F., Prostak, A., Nagy, D., Schlenker, A.M., 1993. Improved emissions speciation methodology for phase II of the auto/oil air quality improvement research program. Hydrocarbons and oxygenates. SAE Technical Paper No. 930142, Society of Automotive Engineers, Warrendale, PA. Swarin, S.J., Loo, J.F., Chladek, E., Drouillard, M.S., Tejada, S., 1992. Advanced emission speciation methodologies for the auto/oil air quality improvement research program — II. Aldehydes, ketones, and alcohols. SAE Paper No. 920321, Society of Automotive Engineers, Warrendale, PA. Zielinska, B., Fung, K.K., 1994. The composition and concentration of hydrocarbons in the range of C2 to C18 emitted from motor vehicles. Science of the Total Environment 146 / 147, 281—288. Zielinska, B., Fujita, E., 1994. The composition and concentration of Hydrocarbons in the range C2 to C18 in downtown Los Angeles, CA. Research on Chemical Intermediates 20, 321—334. Zielinska, B., Sagebiel, J.C., Harshfield, G., Gertler. A.W., Pierson, W.R., 1996. Volatile organic compounds up to C20 emitted from motor vehicles: measurement methods. Atmospheric Environment 30, 2269—2286.
Organic emissions profile for a light-duty diesel vehicle Walter O. Siegl *, Robert H. Hammerle , Heiko M. Herrmann, Bernd W. Wenclawiak, B. Luers-Jongen Research Laboratory, Ford Motor Company, SRL-3083, P.O. Box 2053, Dearborn, MI 48121, USA Analytische Chemie I, Universitat-GH-Siegen, 57068 Siegen, Germany FEV Motorentechnik GmbH and Co. KG, Aachen, Germany Received 17 February 1998; accepted 18 May 1998
Abstract In this report we describe the speciated gas-phase hydrocarbon and carbonyl emissions as collected from a recent model automobile powered by a 2.5l indirect injection diesel engine and outfitted with a production oxidation catalyst for exhaust after-treatment. The vehicle was run on a typical low sulfur (500 ppm S) European diesel fuel and measurements were made over the European MVEG test cycle. The diluted tailpipe emissions were sampled for light hydrocarbons (C —C ) using Tedlar bags and semi-volatile hydrocarbons (C —C ) using Tenax cartridges. Both the > light and semi-volatile hydrocarbon fractions were speciated using capillary gas chromatography. Combining the two sets of speciation data provided a profile of the gas-phase hydrocarbon emissions from a light duty diesel vehicle. Of the total gas phase non-methane hydrocarbons emitted, 80% were accounted for in the light hydrocarbon fraction, and 20% in the semi-volatile fraction. The semi-volatile fraction, which extended only to about C , was composed almost entirely of unburned fuel molecules, but with enrichment of the aromatic species relative to the fuel itself. n-Alkanes (paraffins) and methylnaphthalenes accounted for approximately 37% of the semi-volatile fraction. Aldehydes and ketones represented 34% of NMOG. Formaldehyde and acetaldehyde, account for 74% of the total carbonyl emissions. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Exhaust; Automobile; Volatile organic compounds (VOC); Diesel
1. Introduction Data on the composition and photoreactivity of diesel hydrocarbon (HC) emissions is required as input for atmospheric models and for accurate comparison of the relative environmental impact of diesel emissions with those from vehicles powered with gasoline or alternative fuels. Hydrocarbon species are not all equal with respect to impact on ozone formation, therefore any comparison
*Corresponding author.
of the relative environmental impact of hydrocarbon emissions from diesel vehicles requires knowledge of individual HC emission rates. In recent years, much has been learned about the composition and atmospheric reactivity of the hydrocarbon emissions from gasoline-fueled vehicles (Cf., Auto/Oil, 1992; Auto/Oil, 1993; EPEFE, 1996), however, only limited data on exhaust emissions from diesel-fueled vehicles are available. Recently, there has been increased interest in the measurement of speciated hydrocarbons above C in both urban air and highway tunnel samples (Lowenthal et al., 1994; Zielinska and Fung, 1994; Zielinska and Fujita, 1994; Zielinska et al., 1996; Sagebiel et al., 1996). Although
1352-2310/99/$ — see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 9 8 ) 0 0 2 0 9 - X
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several reports describe the partial speciation of HC emissions from light and heavy-duty engines (Jemma et al., 1992; Lepperhoff et al., 1994; Newkirk et al., 1995; Hammerle et al., 1995; Ogawa et al., 1995; Clark et al., 1996; Gautam et al., 1996; Hublin et al., 1996; Ogawa et al., 1997), to our knowledge, no profile has been reported for the full range of gas-phase HC or organic emissions from a light duty diesel-fueled vehicle. In this report we describe the measurement of the speciated gas-phase hydrocarbon and carbonyl emissions collected from a recent model diesel-powered automobile outfitted with a production oxidation catalyst for exhaust after-treatment. The vehicle was one of a fleet of five vehicles selected to represent advanced diesel technology as available in model years 1992—1993 (Hammerle et al., 1994). The vehicle was run on a typical low sulfur European diesel fuel and measurements were made over the European MVEG test cycle. The tailpipe emissions were sampled for light hydrocarbons (C —C ), semi volatile hydrocarbons (C —C ), and aldehydes and > ketones.
2. Experimental methods 2.1. Vehicle testing A 1992 Mercedes Benz 250D sedan, with a turbocharged 5-cylinder 2.5l indirect injection diesel engine, production oxidation catalyst for exhaust after treatment, and 10 050 km accumulated mileage, was tested in a fully instrumented chassis dynamometer facility over the Motor Vehicle Emissions Group (MVEG) cycle. Additional details of the vehicle and test methodology are reported elsewhere (Hammerle et al., 1994). The fuel used was a low sulfur (500 ppm S) version of a standard European diesel fuel with a ¹ of 348°C, cetane number 53.6, density 829.8 kg m\, and aromatic content of 23.9 (%v/v). The physical properties of the fuel and a speciation analysis were reported earlier (Hammerle et al., 1995). 2.2 Sampling Exhaust gases from the vehicle were carried through a heated line to a dilution tube where they were mixed with humidity- and temperature-controlled air (Lepperhoff et al., 1994). This method of sampling reflects the natural dilution and cooling of the exhaust as it leaves the tailpipe. Various sampling ports were available on the dilution tube for sample withdrawal. The average dilution factor over the three tests was 19.3. For the light HC collection, a fixed portion of the diluted exhaust was pulled through an ambient temperature filter (Pallflex TX 40HI20WW, 35°C), to remove
particulate matter, and into a Tedlar bag. The bags were protected from light and were analyzed within 2 h of collection. A second portion of the diluted exhaust was filtered (Pallflex TX 40HI20WW, 35°C) and then pulled through a Tenax adsorbent trap for collection of the semi-volatile fraction of the exhaust. The flow through the trap was approximately 0.15l min\. Back up traps were employed, but analysis showed no meaningful concentrations of species in the range of interest. Custom-made adsorption tubes were obtained from Dra¨ger (Lu¨beck, Germany). The sorbent tubes were made of borosilicate glass, 6 mm o.d.;100 mm long, 4.7 mm i.d., packed with 31 mm of 60—80 mesh Tenax TA. Stainless-steel frits held the packing at the inlet and exit ends. For the semivolatile HC fraction, a single sample was collected for the entire MVEG drive cycle. A separate portion of the dilute exhaust was drawn through a filter (Pallflex TX 40HI20WW, 35°C) and then through a DNPH-coated silica cartridge for collection of the aldehydes and ketones. The procedure used was the same one as developed for the Auto/Oil Air Quality Improvement Research Program (AQIRP) (Swarin et al., 1992; Siegl et al., 1993). 2.3. Analysis The analytical methods used for the analysis of the light hydrocarbon and aldehyde and ketone fractions were based on those described for the AQIRP program (Swarin et al., 1992; Siegl et al., 1993). However, the analysis of the semi-volatile HC fraction required development of a new method. The semi-volatile HCs were analyzed by capillary gas chromatography after thermal desorption of the Tenax traps followed by cryo-focussing. A Hewlett Packard HP5890 Series II gas chromatograph (GC) was equipped with a cryo-focussing inlet system (described below) connected to a single split/splitless injector and two detectors. Two identical columns were connected to the injector with a ‘‘½’’ connector; one column went to a flame ionization detector (FID) and the other to a mass spectrometer (MS). This arrangement allowed the collection of both FID and MS data from a single analytical run. Each arm of the ‘‘½’’ from the injector was connected to a 2 m;0.32 mm i.d. pre-column of deactivated silica (Promochem) followed by a 50 m;0.32 mm i.d. fused silica analytical column with a 1.0 um methylsilicone coating (BP1, Firma SGE) for separation of the HC species. Connections between columns were made with press-fit connectors. Helium was used as the carrier gas with a column head pressure of 10 psi. The column flow to the FID was 1.2 ml min\ at 40°C. The FID was maintained at 300°C; helium was also used as the detector make up gas. The temperature-programmed analysis used a starting temperature of 40°C (held for 5 min)
W.O. Siegl et al. / Atmospheric Environment 33 (1999) 797—805
followed by heating at 8°C min\ to a final temperature of 300°C (held for 20 min). The mass spectrometer was a Delsi—Nermag Automass used at an ionization energy of 70 eV, with the interface between the GC and the mass spectrometer maintained at 300°C. The thermal desorption/cryo-focussing inlet system (GERSTEL Thermodesorptionssystem TDS 1 and Kaltaufgabesystem KAS 3) allowed both direct injection of liquid samples with a syringe (for fuel analysis) and thermal desorption of samples adsorbed on Tenax traps. During transfer, the cryo-focussing unit was maintained at sub-ambient temperature (!50°C) using a vapor stream of liquid nitrogen. After transfer of the sample to the cryo-focussing unit was complete, the cryo-trap was heated rapidly (10°C s\) to 310°C to transfer the sample to the head of the analytical column. During heating of the cryo-focussing unit, the GC injector was maintained in the splitless mode. The Tenax traps were thermally desorbed with a helium flow (30 ml min\) in the direction counter to the flow during the collection process. The heating rate could be programmed. Optimal results were obtained when the heating was programmed at 20°C min\ to 280°C, the maximum rate allowed with the equipment. During transfer of the sample from the Tenax trap to the cryofocussing unit, the GC injection port was maintained in the split configuration (split ratio 20 : 1). The GC was calibrated daily for retention times using a solution of the standard diesel fuel in n-hexane (21.05 mg carbon ml\). Using the n-alkane peaks in the fuel as reference peaks, the software calculated updated retention times for each peak in the library. Calibration for quantitation was also carried out using the same diesel fuel standard. The FID detector was used for quantitation. An ‘‘average’’ response factor was calculated by dividing the sum of the peak area from dodecane (n-C ) to (but not including) octadecane (n-C ) by the ng of carbon in the fuel sample in this same range (as determined by liquid injection). Response factors for individual hydrocarbons were assumed to be equivalent on a per mole of carbon basis. Zielinska et al. (1996), have recently reported that individual response factors in the semi-volatile HC fall in a narrow range. Exhaust Sample Analysis, Standard Method — The loaded Tenax tube was placed in the thermal desorption unit so that the helium flow was in the direction opposite to the flow during the sample collection process. The trap was purged of air with helium for 2 min and then the transfer needle of the thermal desorption unit was inserted in the injection port of the cryo-focussing unit. The trap was heated at the maximum rate of 20°C min\ with a helium flow of 30 ml min\. During the desorption, the cryo-focussing tube was maintained at !50°C and the GC injection port was in the split mode with a split ratio of approximately 20 : 1. After the desorption unit had been held at the maximum temperature (280°C) for
799
5 min, the transfer needle of the thermal desorption unit was disengaged from the cryo-focussing unit, the helium flow through the cryo unit was initiated, and the GC injector was changed to the splitless mode. The cryofocussing unit was heated at 10°C s\ with the helium flow to the GC column at 1.2 ml min\. After transfer of the sample to the GC column, the temperature of the GC oven was programmed as indicated above. 2.4. Identification The library of species for identification of hydrocarbons in the semi-volatile range was based on the analysis of the two fuels used in a larger study, a low sulfur (500 ppm S) version of a standard European diesel fuel with a ¹ of 348°C (used in the current tests), and also a sulfur-free fuel which met the Swedish criteria for a Class I diesel fuel and had a ¹ of 280°C. The com posite list of chromatogram peaks found in the two diesel fuels contains 180 entries representing those species which were observed at the 0.01% level and above; the list of peaks with retention times was published earlier (Hammerle et al., 1995). The C —C (semi-volatile) range of the library con tains 79 species starting with dodecane. Where possible, species were identified by comparing GC retention times with those of authentic samples. The identities of other species in the library were determined by comparing mass spectra of the library entry with those contained in a NIST library. (Because of the vacuum drawn on the end of the column going to the mass spectrometer, retention times from this column were not identical to retention times for the column going to the FID.) The large number of peaks in the chromatogram, particularly in the C —C region, meant that baseline resolution was not achieved during the approximately 1 h temperature-programmed GC analysis. (Total sample analysis time, including thermal desorption, was 1.25 h.) Some overlap of peaks resulted under these conditions. Care was taken to look for the dominant species in each chromatography peak. Where specific names were not established for a peak, an attempt was made to identify the dominant class of species represented by the peak, e.g. aromatic or branched alkane. All of the n-alkanes and several aromatic species were identified. The identified species include most of the major peaks in the semi-volatile range.
3. Results and discussion Compared with current technology gasoline vehicles, the CO and HC emissions from this diesel vehicle are relatively low (Cf. Auto/Oil, 1992). The regulated emissions for the diesel vehicle are summarized in Table 1. (For reference, data are also shown for the same vehicle run on the US EPA UDDS [FTP] cycle). The values
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reported are a three-test average. The standard deviations shown reflect the reproducibility of the emissions from test to test and not the accuracy of the measurements. Similar to gasoline-fueled vehicles, exhaust HC emissions from diesel-fueled vehicles are expected to be a mixture of unburned and partially burned fuel species
Table 1 Regulated emissions from the diesel test vehicle (3-test average) Species
MVEG test Average
CO (g km\) NO (g km\) V HC (g km\) PM (g km\)
0.591 0.560 0.091 0.066
FTP test S.D
Average
S.D
0.008 0.008
0.528 0.591 0.068 0.069
0.020 0.001
Measured by heated FID.
(Kaiser et al., 1991, 1992, 1993, 1994; Siegl et al., 1992). Hydrocarbons for speciation analysis were collected using both Tedlar bags (C —C ) and Tenax traps (C ). > The light hydrocarbons (C —C ) were speciated (separ ated, quantified, and identified) according to well established methods, and the emission rates are shown in Table 2. The list is arranged in order of increasing chromatographic retention time, which corresponds approximately to increasing boiling point. Of the total light HC mass, 96% was identified. Species up to and including the xylenes represent primarily products of partial combustion, whereas species in the list after the xylenes represent primarily unburned fuel. Of the 96% of the light HC mass which can be identified, 78% is contributed by partial combustion products. Methane and ethene (ethylene) together account for 59% of total light hydrocarbons and ethene accounts for 38% of light nonmethane hydrocarbons (NMHC). Measurement of the semi-volatile gas-phase HC emissions (volatility greater than that of dodecane) was carried out using Tenax traps. The use of Tenax to trap the
Table 2 Methane and light HC fraction (C —C ) from diesel exhaust (average of 3 MVEG tests) Compound
Emission rate (mg km\)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
13.12 9.90 2.20 1.84 1.00 0.66 1.15 0.04 0.56 0.41 0.38 0.90 0.07 1.19 0.08 0.12 1.16 0.09 0.05 0.41 0.19 0.98 0.56 0.60 0.46 0.19 0.95
37.8% 8.4% 7.0% 3.8% 2.5% 4.4% 0.2% 2.2% 1.6% 1.5% 3.5% 0.3% 4.5% 0.3% 0.5% 4.4% 0.3% 0.2% 1.6% 0.7% 3.8% 2.1% 2.3% 1.8% 0.7% 3.6%
30.1% 6.7% 5.6% 3.1% 2.0% 3.5% 0.1% 1.7% 1.2% 1.2% 2.7% 0.2% 3.6% 0.2% 0.4% 3.5% 0.3% 0.1% 1.3% 0.6% 3.0% 1.7% 1.8% 1.4% 0.6% 2.9%
39.29 26.17
100.0%
79.5%
Methane Ethene Ethyne Propene Benzene Toluene 1-Octene Ethylbenzene m&p-Xylene 2,4-Dimethyloctane Unknown (¹ "35.86 min) 0 1-Methyl-3-ethylbenzene 1,2,4-Trimethylbenzene Decane 1,2,3-Trimethylbenzene Unknown (¹ "37.54 min) 0 1,4-Diethylbenzene 1-Methyl-4-n-propylbenzene 1,2-Diethylbenzene 1-Methyl-2-n-propylbenzene 1,3-Dimethyl-2-ethylbenzene Undecane Unknown (¹ "41.11 min) 0 n-Pentylbenzene Unknown (¹ "41.70 min) 0 Napthalene Dodecane
Total light HC Total light NMHC
% of light-NMHC
% of total NMHC
W.O. Siegl et al. / Atmospheric Environment 33 (1999) 797—805
semi-volatile HCs provided two benefits. First, the semivolatile HCs from 4—5l of dilute exhaust could be concentrated and loaded as a single injection on a GC column. Second, the heavy HC fraction was stabilized, and could be stored at reduced temperatures without loss of sample integrity. Tenax TA was chosen because, due to its high thermal stability, it could be thermally desorbed. This method of sample collection/recovery was preferred relative to cold-trapping or adsorption followed by solvent extraction and subsequent extract concentration. The latter methods require additional sample handling and afford greater opportunities for sample loss. To determine our ability to recover HCs from the Tenax trap without discriminatory losses, a set of experiments was carried out in which traps were spiked with solutions of various standards. A standard mixture containing the even-numbered n-alkanes, C —C , was in jected on a Tenax trap and recovery was measured by comparing the FID response relative to the response from a direct liquid injection. The results indicate that recovery was nearly quantitative, within experimental error, over the molecular weight range of greatest interest (C —C ), but that transfer losses were significant below C . (The transfer losses were due to limitations in the particular equipment available and not in the method itself.) Attempts to extend recovery to a broader range of n-alkanes by adjusting the operating parameters of the cryo-focussing unit were unsuccessful. Recovery experiments carried out with standards prepared from diesel fuel instead of the pure n-alkanes gave similar results; recovery of the C —C fraction was nearly quantitative. The GC area-counts in the C —C region observed for a liquid injection and for a trap desorption for the same standard solution of fuel agreed to within 6%. Species identification for the semi-volatile hydrocarbon analysis was based on a comparison of retention times with those in the library. Based on the chromatograms of the fuels, the C —C library contains 79 spe cies starting with dodecane. Chemical names can be associated with 17 of these species and a large number of additional species can be labeled as either aliphatic or aromatic based upon mass spectral data. Table 3 contains a summary of the speciated semivolatile HC emission rates observed for the diesel vehicle over three vehicle tests; species with emissions rates between 0.01—0.04 mg km\ have been summed and appear as a single entry. The quantitation limit was an emission level of 0.01 mg km\. The sum of the semi-volatile hydrocarbons is 6.72 mg km\ compared with 39.3 mg km\ for the C —C fraction (26.2 mg km\ for C —C ). For the semi-volatile fraction, 95% of the mass lies between dodecane (n-C ) and pentadecane (n-C ); 71% lies be tween dodecane (n-C ) and tetradecane (n-C ). For tyfour percent (44%) of the mass was identified as aliphatic and 41% as aromatic. In comparison, the aromatic content of the fuel was only 24% (Hammerle et al.,
801
1995). An average of 47 different species were detected above dodecane (n-C ); the 20 most abundant species accounted for 83% of the mass. Of these 20 species, eight could be assigned a name and another seven could be labeled as either aliphatic or aromatic. The n-C , n-C , and n-C paraffins accounted for 20% of the semi-vol atile HC mass, and the methyl- and dimethylnaphthalenes accounted for 17% of the semi-volatile HC mass. In Fig. 1 the relative amounts of individual n-alkane (paraffin) species in the fuel and in the exhaust are plotted according to carbon number. The n-alkanes as determined by the light HC and the semi-volatile HC methods are plotted in separate series. The shapes of the exhaust profile and the low end of the fuel profile are similar. Only the low-mid range of the fuel makes a significant contribution to the gas-phase HC emissions. Both the fuel and exhaust samples reach a maximum at C ; how ever, the exhaust composition drops off rapidly after C . The higher boiling components of the fuel are most likely to be found condensed on the particulate matter. This analysis was not carried out for the current study. However, Ogawa et al., (1997), have recently shown that the soluble organic fraction of the particle matter emissions contains the mid-high boiling fuel components. A comparison can be made between dodecane (n-C ) as measured by the light HC method (average: 0.90 mg km\) and dodecane as measured by the semivolatile HC method (average: 1.41 mg km\). The higher value reported by the semi-volatile HC method is consistent with our belief that Tenax traps represent a more efficient collection device than Tedlar bags for collecting the semi-volatile HCs. Similarly, the deviation of undecane (n-C ), measured from the Tedlar bag, from the trend line, suggests that it too undergoes losses in the Tedlar bag. Difficulties associated with storing the heavier HCs in Tedlar bags have been noted by others (cf. Zielinska et al., 1996). For HC species of C and below, collection in Tedlar bags represents an efficient sampling method. The poor recovery of HC below dodecane from the Tenax traps, while using the current hardware (as cited above), makes additional comparison between the collection methods not meaningful. The same semi-volatile HC peaks were observed in the GC chromatograms of the exhaust and the fuel. No additional peaks were found in significant amount in the exhaust. However, the ratios of the peaks differ between the fuel and exhaust. Fig. 2 contains histograms of the dodecane to tetradecane region (which accounts for 71% of the total semi-volatile HC exhaust emissions) for both the fuel and exhaust samples. Some differences in ratios would be expected based on the data presented in Fig. 1, which shows an effect predominantly associated with volatility. Fig. 2 indicates that other phenomena are also operational. Peaks 73, 88, and 103 represent dodecane, tridecane, and tetradecane respectively. In the exhaust sample, the three peaks decrease with increasing carbon
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Table 3 Semi-volatile HC fraction ('C ) of diesel exhaust (average of 3 MVEG tests) Species (or library entry)
Emission rate (mg km\)
% of S—V NMHC
% of total NMHC
Naphthalene Dodecane 1 C76,aliphatic 2 C78,aromatic 3 C79,aromatic 4 80,aromatic 5 C81,aliphatic#aromatic 6 C82,aliphatic 7 C83,aromatic 8 C84,aliphatic 9 C85,aliphatic 10 C86,aromatic 11 Tridecane 12 2-Methylnapthalene 13 C91,aliphatic 14 1-Methylnaphthalene 15 C95 16 C,aliphatic 17 C98,aliphatic#aromatic 18 Acenaphthene 19 C -NaphthaleneC1 20 Tetradecane 21 C104,aliphatic 22 C -NaphthaleneC2 23 C106,aromatic 24 C -NaphthaleneC3 25 C -NaphthaleneC4 26 C111,aromatic 27 C114,aliphatic#aromatic 28 C116 29 Pentadecane 30 C119,aromatic 31 C -NaphthaleneC1 Sum of 16 minor peaks
1.60 1.41 0.40 0.10 0.07 0.05 0.36 0.13 0.11 0.16 0.42 0.11 0.83 0.69 0.09 0.44 0.09 0.22 0.07 0.19 0.13 0.41 0.05 0.10 0.08 0.08 0.23 0.13 0.16 0.21 0.13 0.07 0.07 0.34
6.0% 1.5% 1.0% 0.7% 5.4% 1.9% 1.6% 2.4% 6.3% 1.6% 12.4% 10.3% 1.3% 6.5% 1.3% 3.3% 1.0% 2.8% 1.9% 6.1% 0.7% 1.5% 1.2% 1.2% 3.4% 1.9% 2.4% 3.1% 1.9% 1.0% 1.0% 5.1%
1.2% 0.3% 0.2% 0.2% 1.1% 0.4% 0.3% 0.5% 1.3% 0.3% 2.5% 2.1% 0.3% 1.3% 0.3% 0.7% 0.2% 0.6% 0.4% 1.2% 0.2% 0.3% 0.2% 0.2% 0.7% 0.4% 0.5% 0.6% 0.4% 0.2% 0.2% 1.0%
Total
6.72
100.0%
20.4%
number as expected from Fig. 1. However, several peaks, notably the starred peaks: 90, 93, and 102, have increased significantly relative to the n-alkane peaks. These peaks represent the aromatic hydrocarbons: 2-methylnaphthalene, 1-methylnaphthalene, and an unspecified C -naph thalene, respectively. The enrichment of the semi-volatile fraction of the exhaust sample with aromatic hydrocarbons could be due to a process which selectively removes saturated hydrocarbons or to a process which converts larger aromatics to smaller aromatics. Conversion of alkylbenzenes to benzene and toluene in the spark-ignition engine has been observed (Kaiser et al., 1992). Together, the emission rate data presented in Tables 2 and 3 provide a profile of the gas-phase hydrocarbon emissions from a representative light duty diesel vehicle.
Total NMHC emissions were 32.9 mg km\, 80% in the light HC fraction and 20% in the semi-volatile HC fraction. Approximately 85% of the NMHC mass can be identified and additional mass can be characterized as either aromatic or paraffinic. Ethene, the most abundant NMHC species, accounts for 30% of the total mass. If we assume that the species ethene through m-/p-xylene, as shown in Table 2, all represent partial combustion products, then partial combustion products represent 53% and unburned fuel 47% of the NMHC in the diesel exhaust. Absent from the exhaust are the small alkanes (e.g., butane, 2-methylbutane, pentane, 2-methylpentane) and small branched olefins (e.g., 3-methylpropene) which are prevalent in gasoline vehicle exhaust. Toluene, frequently one of the most abundant hydrocarbons in gaso-
W.O. Siegl et al. / Atmospheric Environment 33 (1999) 797—805
line vehicle exhaust, was only observed at the 2% level in the NMHC exhaust from this diesel vehicle. The aldehydes and ketones were also collected and speciated using well-established methods (Swarin et al., 1992; Siegl et al., 1993). The speciated aldehyde and ketone emission rates are presented in Table 4 (the mass of oxygen is included in the emission rates). The two carbonyl species which are classified as air toxics, formaldehyde and acetaldehyde, account for 74% of the total.
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Formaldehyde alone accounts for 54% of the total. The formaldehyde and acetaldehyde emissions from this diesel vehicle are higher than the emission rates recently reported for a gasoline-powered vehicle fleet (FTP test): formaldehyde (6.6 mg km\ vs. 1.0 mg km\) and acetaldehyde (2.4 mg km\ vs. 0.8 mg km\) (Auto/Oil, 1992). If NMOG (non-methane organic gases) emissions are calculated (NMOG"NMHC#carbonyl species), the average NMOG emissions are 50.2 mg km\, and the carbonyl species (aldehydes#ketones) represent 34% of the total NMOG emissions. For gasoline vehicles, the carbonyl species typically represent about 5% of the total NMOG emissions.
4. Conclusions
Fig. 1. Distribution of n-alkanes in the fuel and exhaust samples.
The gas-phase organic emissions (light HC, semi-volatile HC, and carbonyl) from a recent model, dieselfueled, light-duty vehicle have been speciated and quantified. Combining the two HC speciation data sets provides a profile of the gas-phase HC emissions from a light duty diesel vehicle. Methane accounted for 33% of total hydrocarbon emissions. Of the non-methane hydrocarbons, 80% were present in the light hydrocarbon fraction (C —C ), and 20% in the semi-volatile fraction ('C ). Approximately, 85% of the NMHC mass can be identified. Ethene accounts for 30% of the total NMHC.
Fig. 2. Composition of the exhaust and fuel profiles of the n-C to n-C region (as recovered from Tenax traps).
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Table 4 Aldehyde and ketone emissions profile (mg km\) (MVEG cycle, 3-test average) Species
Formaldehyde Acetaldehyde Acetone Propionaldehyde Crotonaldehyde Methylethyl ketone Butyraldehyde Benzaldehyde Total Total NMOG
Average
% S.D.
9.23 3.51 1.80 0.86 0.65 0.00 0.49 0.65
5.5% 0.9% 8.6% 6.1% 6.7% 0.0% 17.6% 5.3%
17.19 50.20
6.3%
Partial combustion products represent 53%, and unburned fuel 47% of the NMHC in the diesel exhaust. The semi-volatile fraction extended only to about C . n Alkanes and methylnaphthalenes were the major components, accounting for approximately 37% of the semivolatile fraction. Although the semi-volatile HC fraction contained almost entirely fuel components, the ratio of species was changed with an enrichment of aromatic species observed in the exhaust. Formaldehyde and acetaldehyde, account for 74% of the total carbonyl emissions. The NMOG emission rate was 50.2 mg km\ with aldehydes and ketones accounting for 34% of total NMOG.
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