Hydrocarbon gases emitted from vehicles on the road. 2

Dec 1, 1983 - Hydrocarbon gases emitted from vehicles on the road. 2. Determination of emission rates from diesel and spark-ignition vehicles. Christi...
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Hydrocarbon Gases Emitted from Vehicles on the Road. 2. Determination of Emission Rates from Diesel and Spark-Ignition Vehicles Christine V. Hampton,* William R. Plerson, Dennis Schuetzle, and T. Michael Harveyt

Research Staff, Ford Motor Company, Dearborn, Michigan 48121 Emission rates of 22 gas-phase hydrocarbon compounds from motor vehicles in highway operation were determined in the Allegheny Mountain Tunnel of the Pennsylvania Turnpike with Tenax adsorbent. Concurrently, particulate-phase components were collected on quartz-fiber filters. The emission rates of n-alkanes C8H18through Cl7H% and of phenol, toluene, ethylbenzene, propylbenzene, styrene, methylstyrene, ethynylbenzene, indan, indene, naphthalene, 1-methylnaphthalene, and 2-methylnaphthalene are given for heavy-duty diesel and light-duty gasoline-powered vehicles. Emission rates determined from this study are generally consistent with tailpipe emission rates determined from published dynamometer studies. The distribution of the n-alkanes between the gas and particulate phases appears to be related to vapor pressure; i.e., the gas/particulate mass ratio is approximately proportional to the vapor pressure in the limited range that could be tested (C1+through C17). Qualitative results from the same study, identifying (completely or partially) more than 300 gas-phase hydrocarbon species, were reported in a previous article. Introduction Motor vehicles are said (1) to be a leading source of man-made emissions of hydrocarbon gases in the United States, contributing an estimated (2) 35% of the 1977 national total. Yet, few studies have been conducted in an on-road setting to provide estimates of the automotive contribution to concentrations of individual hydrocarbons in the atmosphere. Typically, (3),ambient levels of certain species near a freeway are determined but without any means either to (a) resolve the automotive contribution from the contributions of other sources or (b) determine emission rates for application in any other situation. For the past decade, investigators have followed the opposite approach of simulating atmospheric conditions by measuring the diluted exhaust of vehicles or engines running on dynamometers (4-10). These measurements yield emission-rate estimates (say, milligrams per vehicle kilometer driven) under specified driving conditions such as the urban driving schedule or FTP ( l l ) , the Highway Fuel Economy Test (12), and so forth. Atmospheric models are used to construct the link between emission rates and atmospheric concentrations. Both approaches have in the past suffered from the lack of suitable chemical analytical methods. Complex gas Present address: Harvey Laboratories, Inc., Charlottesville,VA 22905.

0013-936X/83/0917-0699$01.50/0

chromatograms have compelled investigators to resort to a molecular weight “cut” technique to estimate emission rates of species found in the particulate phase (6). Although useful in determining emission rates of molecular weight groups, this technique gives no quantitation of individual compounds. The vapor-phase organic species also are not fully separated according to gas chromatographic retention time. Though definitive information regarding partitioning of a given component between the vapor and particulate phases in urban air is available (13, 14), only a few partitioning measurements on automotive exhaust have been reported (15-1 7). The main object of the present study was to obtain atmospheric gas hydrocarbon concentrations in an environment where traffic composition, traffic volume, and vehicle operating modes were known and atmospheric dilution was well defined, and where vehicle emissions could be differentiated from emissions of other sources, so that emission rates from vehicles could be calculated. The second objective was to determine gas/particulate ratios for some of the series of n-alkanes. The Allegheny Mountain Tunnel of the Pennsylvania Turnpike is suited to these objectives and has been utilized for a number of on-road emissions studies since 1970 (18-36). The present paper is based on the second of two 1979 Allegheny field experiments in which a number of emissions questions were investigated (18-32), including hydrocarbon gases (30). Over 400 gas-phase organic compounds were detected by gas chromatography/mass spectrometry (GC/MS), and complete or partial identifications were reported on more than 300 of these (30). As a sequel to ref 30, the present paper extends the GC/MS work to emission-rate quantitation in the case of 22 selected gas-phase n-alkanes and aromatic hydrocarbons and evaluates gas/particulate partitioning for some of the n-alkanes. The results would be appropriate for a highway driving condition at -80 km/h on straight and nearly level highway. Experimental Section Some of the experimental details are listed in Table I. The full field experiment ran from Aug 25-Sept 7,1979. Sampling Site and General Approach. The Allegheny Mountain Tunnel (Figure 1) has two two-lane tubes through Allegheny Mountain. All emission measurements were made in the south tube which carried the eastbound traffic. The tunnel is 1.85 km long with a cross section of 48.0 m2, at an elevation 707 above sea level. The roadbed is essentially straight, but there is an average downgrade

0 1983 American Chemical Society

Environ. Sci. Technol., Vol. 17, No. 12, 1983 690

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Flgure 1. Allegheny Mountain Tunnel. Crosses indlcate East Portal sampllng locatlons in eastbound tube (no. 1 = right slde; no. 2 = left slde). Tenax cartrldge samplers were on the left sMe between no. 2 and the portal. Overhead air ducts Indicated in the end view are for intake ventilation air.

of 0.5% and a tailwind increasing through the tunnel to an average 26 km/h at the exit. Vehicle speeds are fairly constant at close to the posted 55 mile/h (89 km/h) limit, but the grade and tailwind enhance the fuel economy by a calculated (35)factor of 1.7. Experimentally the factor was found to be 1.3 f 0.1 for a gasoline-powered automobile with electronic fuel injection and fuel-economy readout. Heavy-duty diesel trucks and light-duty spark-ignition vehicles were effectively the only two types of vehicles using the tunnel in 1979, together comprising 96.8% of all vehicles. Turnpike fuel sales for Sept 1979 show that 53.5% of the gasoline was unleaded and presumably therefore consumed in catalyst-equipped vehicles. Turnpike toll plaza scale records were used to determine that the average heavy-duty diesel at Allegheny had a gross weight of 26 metric tons. The traffic densities of the two vehicle types follow quite dissimilar diurnal and weekly patterns (Figure 2), rendering it possible to distinguish their contributions by the algebraic procedures described below. About 40% of the air entering the eastbound tunnel is drawn in through the west or entrance portal by the r a m ming effect of the traffic, in concert with the prevailing westerly wind; the remainder of the inflow is equally divided between the east and west intake fan rooms (Table I). There is no exhaust system as such; all of the air in the eastbound tunnel goes out through the east or exit portal, at flow rates -20000 m3/min. Airborne concentrations measured at stations just inside the exit (Figure 1)therefore represent the cumulative emissions throughout the length of the tunnel, together with whatever was in the Environ. Scl. Technol., Vol. 17, No. 12, 1983

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WED THUR FRI 8/31 9/1 9/2 DAY 9/4 915 916 917 Flgure 2. Visual counts of eastbound traffic through the Allegheny Mountain Tunnel during part of the AugISept 1979 experlment. Upper = spark-ignition vehicles; lower = dlesel trucks.

intake air. The concentration in the intake air is subtracted from that in the tunnel to give the concentration just attributable to vehicles. In making the subtraction the intake average .concentration between the east and west intake fan rooms is weighted in accord with the fact that most of the tunnel air comes from the west side of the mountain (intake fans plus entrance portal, Table I). The intake air is ambient air slightly contaminated by local automotive emissions; P b and CO concentrations in the fan rooms are usually -6% of those in the tunnel, and air entering the west portal is somewhat more contaminated, compared to sites farther away up the mountain. For emission-rate calculations one must subtract the actual intake-air concentrations and not the uncontaminated ambient-air concentrations. Concentrations of particulate and gas-phase emissions measured at the right-side station in the tunnel (no. 1, Figure 1) generally exceed those at the left-side station (20, 29,31, 35), by an amount depending on the traffic conditions and the species under consideration. For measurements on the left side only, as was the case with the gas-phase hydrocarbons, this effect is taken into account by means of an asymmetry correction factor (Table I). This factor is calculated from the intake-corrected left-side and right-side concentrations of particulate mass and, for a second estimate, the (intake-corrected) dichloromethane-extractable particulate mass. The factor is the left-right'average divided by the left-side value. The asymmetry correction factor for the (intake-corrected) hydrocarbon gases is presumed to be the same as that for the particulate mass or the extractable particulate mass, or rather their arithmetic average. The equations describing the air quality in the Allegheny Tunnel were derived in an earlier report (27)and elsewhere (22,32). For each sampling period or "run" the emission rate E (mg/km) averaged over all vehicles without regard to type is E A*V/(@) (1) and (if only the two principal vehicle types are significant) E = X G + (1 - x)D (2) where A = intake-corrected and asymmetry-corrected concentration (mg/m3) average in the tunnel during the run, for the species under consideration, V = total air flow

Table I. Experimental Conditions, Allegheny Tunnel Eastbound, AugISept 1979 run 10 run 14

run 17

Tenax sampling time 1125, Tues, 914 2140, Wed, 915 1310, Sun, 912 start, EDT 240 220 220 duration, min 72 66 66 sample volume, L tunnel air throughput 426 x 104 486 x 104 524 x 104 total . . m3 . . % in via intake fans 33 29 29 east 33 29 29 west % in via west portal 34 42 42 asymmetry correction factora 1.12 1.34 1.05 vehicle count total no.6 2721 3057 1065 gasoline powered, % 93.6 81.9 31.5 HiVol filterC sampling time start, EDT 1945, Wed, 915 duration, min 692 sample volume, m3 70 2 tunnel air throughput total m3 150 x 105 % in via intake fans: east 29.5 west 29.5 41 % in via west portal asymmetry correction factora 1.05 vehicle count total no.6 2829 gasoline powered, % 34.7 a ( R + L)/2L, where R and L are right- and left-side kg/m3concentrations of vehicle emissions at the tunnel sampling stations (intake corrected). Left-side concentrations are multiplied by these factors to get best estimates for the east portal (see text). Multiply by the tunnel length (1.85 km) to obtain vehicle kilometers driven in the tunnel during the sampling period. N o GC/MS analysis on run 10 or run 14 filter extracts.

(m3) through the tunnel during the run, ut = number of vehicles that went through the tunnel during the run, L = tunnel length (km), x = spark-ignition vehicle fraction of the total vehicle count for one run,G = average emission rate (mg/km) for spark-ignition vehicles, and D = average emission rate (mg/km) for the heavy-duty diesels. From (2) it is obvious that E is a linear function of x, with intercepts E = Gatx = 1 E =Datx =0 (3)

so that one can determine G and D from two sampling runs with different x values. If there are more than two sampling runs, simple linear least-squares regression of E vs. x gives G and D. The process is illustrated graphically in Figure 3, showing the determination of Ba emission rates from spark-ignition and heavy-duty diesel vehicles in an earlier experiment. In practice one should apply a correction for diesel passenger cars, which during the 1979 experiment comprised (by count) 2.36% of the automobile traffic. The corrected value G' is

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Derivation of (4)requires the assumption that the emission rate of the species under consideration is - 1 / 3 as much from light-duty diesels as from heavy-duty diesels, 1 / 3 being the approximate ratio of their fuel-consumption rates. For species mostly associated with spark-ignition vehicles, this refinement is inconsequential. For calculating the gas/particulate partitioning, it is not necessary to apply asymmetry factors or subtract intake concentrations. The gas-phase concentration, in the tunnel, of the species under consideration is compared directly with ita concurrently measured particulate-phase concen-

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Flgure 3. Plot of milligramsof Ba per kllometer vs. traffic composition, 11 consecutlve halfday filters, Tuscarora Mountain Tunnel, Aug 1976. Intercepts at 0% and 100% spark ignition (gasoline powered) correspond to the Ba emlssion rates from dlesel and spark-ignition vehicles, respectlvely.

tration. This comparison was actually carried out in only one case, namely, tunnel left-side run 17 (Table I). Since the gas sampling period was actually not identical with the particulate sampling period (Table I), the average traffic density is not identical during the two measurements, nor is the tunnel wind speed, so a small (7%)correction to the concentration ratio is required. Average temperature was -20 OC during the period of sampling for the evaluation of partitioning. Sampling Procedures. Vapor sampling stations were set up several meters inside the east portal of the eastbound tube on the left side (no. 2 in Figure 1) and in the east and west intake fan rooms. Particulate sampling Envlron. Sci. Technol., Vol. 17, No. 12, 1983 701

stations were set up at these locations and also on the right side in the tunnel (no. 1,Figure l), though only one sample was to be analyzed by GC/MS for the present study (Table I). The sampling inlets were -2 m above the roadway. The gases were collected in tandem cartridges packed with Tenax adsorbent as described in the earlier paper (30). Tunnel samples were obtained in triplicate with upstream filtering and in triplicate without filtering as well in order to evaluate the contribution of condensed-phase material captured in the sampling process. Each sample stream contained two Tenax cartridges in series to evaluate the completeness of adsorption on the front cartridge. The sampling arrangement in the intake fan rooms was the same except that the streams were not filtered. Particulate samples were collected on Teflon-impregnated glass-fiber (Pallflex T60A20) HiVol filters as described elsewhere (32). The HiVol samplers were equipped with cyclones which excluded large (nominally > 5.5 pm) particles. Traffic composition was determined by 10-min visual count every hour. The vehicles were classified into 18 categories according to vehicle type (motorcycles,passenger cars, buses, and trucks), engine (gasoline or diesel), and number of axles (automobile pulling two-axle trailer, three-axle diesel bus, five-axle diesel truck, etc.). Axles we)e counted with a Leupold/Stevens road-tube print/punch traffic counter with totals read out onto punch tape every 15 min. The axle count was combined with the visual-count axles per vehicle to calculate the vehicle total for the sampling run. Multiplication by 1.85 km, the length of the tunnel, gave the number of vehicle kilometers driven in the tunnel during the sampling run. Tunnel air flow was measured with a Gill anemometer stationed 47 m inside the east portal. The output was integrated over the sampling run and multiplied by the 48.0-m2 tunnel cross section to give the total air flow volume (m3). Traffic volume and composition are correct to within 1% to a few percent depending on volume, as shown by the close agreement between the axle count recorded and that inferred from the visual counts. The uncertainty associated with spatial gradients in concentration, or spatial gradients and measurement errors in air flux, is probably 115%. Traverses with a hand-held wind run meter established that the measurement by the Gill anemometer was representative of the tunnel cross section. An error estimated to vary between 0 and 15% is caused by the fact that the fraction of the tunnel flow being sampled is not quite constant since the sample flow rates are constant and the tunnel air flow rate is not. The overall emission-rate uncertainty would be a combination of the sampling errors and analytical errors. The latter depend on the species and the analytical method. Generally the random errors of sampling and analysis (though not the systematic errors) should be reflected in the calculated errors in the regression analyses. Analytical Procedures. The GC/MS analysis of the Tenax samples has been discussed in the earlier paper (30). The Tenax samples were analyzed by thermal-desorption gas chromatography/mass spectrometry. The GC column was a 50-m SF96 glass capillary, temperature-programmed from -20 (1-min hold) to 230 OC at 3 OC/min. Hexafluorobenzene (HFB) was added as an internal standard to the cartridges before analysis. Efficient desorption was verified by repeating the desorption on a few of the cartridges. The filter samples were extracted in a Soxhlet apparatus for 12-18 h with dichloromethane as previously described 702

Environ. Sci. Technol., Vol. 17, No. 12, 1983

(32). The extract of the one filter (run 17, tunnel left side) was analyzed by direct liquid injection GC/MS on a VG MM16 mass spectrometer. A 6-ft glass column packed with Dexsil300 was temperature programmed from 90 to 300 OC at 4 OC/min. Before analysis, octafluoronaphthalene (OFN) was added to the extract as an internal standard. Compound concentrations in either the vapor phase or the particulate phase were calculated as follows. For any compound, the concentration C (pg/m3) is

where A- = peak area calculated for the compound under consideration, A s =~ mass chromatographic peak area for the internal standard (HFB for the Tenax samples and OFN for the filter extracts), Wunk= molecular weight of the compound under consideration, W,, = molecular weight of the internal standard, QsM = mass of internal standard added (pg), R = molar response factor of the compound under consideration, relative to the molar response of the internal standard, and u = volume of air sampled (m3). Response factors were determined experimentally for the n-alkanes relative to OFN and for the n-alkanes Cl0, C12,C13, CI4,Cls, and C17 relative to HFB. The other response factors relative to HFB are from the literature (37). The OFN response factor relative to HFB was also determined and found to be 1.6. The experimentally determined response factors agree with published values (37) within =t35%.

Results Gas-Phase Concentrations. Vapor concentrations are listed in Table 11. Compounds were chosen for inclusion in Table I1 on the basis of adequate retention on the Tenax cartridges (15% breakthrough), reproducibility among replicates, ample ion abundance, and a mass spectrum free of interference from coeluting compounds. The compounds chosen all had chromatographic peaks lying within region B of Figure 4. As is apparent in the backup-cartridge chromatogram, compounds with peaks in region A-essentially everything more volatile than n-octanewere incompletely retained on the Tenax. The only compound in Table I1 with >5% breakthrough is toluene (15% breakthrough). Values in Table I1 include the backup cartridges. Region C of Figure 4, extending from octadecane through octacosane, is characterized by irreproducibility problems. The inserted chromatogram portions from triplicate runs illustrate the large ion-current variations typical of this region. Problems in addition to irreproducibility (>30%) among results from triplicate cartridges include sporadic unexpected trends of concentration vs. carbon number and values from filtered units exceeding those from unfiltered ones. Other authors have encountered problems of a similar nature (6). Because of this situation we did not attempt to quantitate the vapor-phase alkanes beyond C17. In run 17, vapor-phase concentrations of the n-alkanes C15, C16, and C1, when sampled with prefilters are substantially lower than when sampled without prefilters. Such behavior is less evident in run 10 and not at all evident in run 14, and thus, the case for a filtration effect in this molecular weight range is equivocal. AB will be seen below, partitioning to the particulate phase is only slight in the C15 to C17 range; thus, the filter effect in that range should be small. Benzaldehyde was observed in the tunnel, at concentrations exceeding those in the intake ran rooms, but the

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Figure 5. Vapor-phase mass emissions in milligrams per kilometer vs. percent spark-ignition (gasoilnapowered) vehicles for ndodecane and ethylbenzene. Tenax units filtered. I t can be seen that ndodecane emission rates are higher from diesel trucks, and ethylbenzene emission rates are higher from spark-ignition vehicles. 1000

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Flgure 4. GC/MS total-ion-current (RIC) chromatograms of gas samples collected on Tenax In the eastbound tube, Allegheny Mountain Tunnel, 1310 to 1650 EDT, Sunday, 9/2/79(run lo),without upstream prefilter. Sampling rate 300 mL/min. Top = main cartridge: bottom = backup cartridge. Insets and areas A, B, and C are explained In the text. Peak marked (X) Is a contaminant.

lack of consistency in concentration on backup cartridges suggests the possibility of reaction(s) between the Tenax and some gaseous component(s) in the tunnel air. Other compounds of environmental interest-dimethylnitrosamine, trichloroethylene, and fluorene-were not found, though we lack calibrations at low enough levels to establish detection limits. For the compounds in Table 11,agreement was generally in the neighborhood of f10% (one standard deviation) among replicate tunnel samples. The intake concentrations were so low by comparison that their errors were inconsequential in the subsequent emission-rate calculations (Table 111). Peak-area reproducibility of the HFB standard was -2%. Errors in Rf would be systematic errors which would affect all numbers for a given compound in Table I1 by a constant factor; as already mentioned, agreement with literature Rf values is within f35%. Certain species in Table I1 seem to depart from the rule that local traffic should result in fan-room concentrations -6% as high as concentrations in the tunnel. Intake concentrations on the low side possibly reflect losses, as the amounts in question are all very low. The n-alkanes from c8 and CI2show a fan-room excess, suggesting in each case an ambient contribution in addition to the contamination, but we have no remote-site data to verify this. Gas-Phase Emission Rates. Emission rates, calculated from the observed concentrations (eq 5) by regression analysis according to the procedure of eq 1-4, are listed in Table 111. Figure 5 illustrates the process for two of the compounds, n-dodecane and ethylbenzene. From the slopes or intercepts it is apparent that dodecane is a species more characteristic of diesel emissions while ethylbenzene is associated chiefly with spark-ignition vehicles. Several trends are evident in Table 111. One is the association of compound type with vehicle type-alkanes with diesels and aromatic compounds with spark-ignition vehicles. Second, the n-alkanes from spark-ignition ve-

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Figure 8. Mass emisslon rates of vapor-phase nalkanes from dlesel and spark-Ignition (gasoline-powered) vehicles. Left: samples collected on Tenax cartridges with upstream filtering. Right: samples collected without upstream filtering.

hicles tend to be of lower molecular weight than those from diesels; the peak of the molecular weight distribution lies probably below c8 for spark-ignition vehicles and approximately CI2 to C14 for diesels. Maximum diesel/ spark-ignition emission-rate ratio occurs at approximately C15. This trend is illustrated in Figure 6, where also the diesel patterns suggest a filtration effect beginning at CIS or C14. Consonant with this trend is the homologous series toluenefethylbenzenefn-propylbenzene; although these compounds are, like aromatic compounds in general, associated mostly with spark-ignition vehicles, the strength of the association declines with increasing length of the aliphatic side chain (respective spark-ignition/ diesel ratios -5, 3.7, and 1.4). The emission rates in Table I11 pertain to a situation in which the vehicles, because of the tailwind and roadway grade in the tunnel, are operating at enhanced fuel economy as discussed earlier. On the assumption that the fuel-specific emission rate (mg emitted/g of fuel consumed) is independent of engine load in the intermediate-load region, the emission rates at the Allegheny speed but on Environ. Sci. Technoi., Vol. 17,No. 12, 1983 703

Table 11. Tunnel and Intake-Air Gas-Phase Concentrations, pg/m3,at Alleghenya‘ tunnel (east portal left side) intake fan roomsd prefilterb conditions run 1 0 run 1 4 run 17 run 10 run 14 run 17 RfC n-octane F 1.2ge 6.8 6.6 1.9 NF 7.0 2.1 7.1 0.8 0.4 0.9 n-nonane F 1.41e 4.5 4.3 1.9 NF 5.2 4.0 2.0 0.9 1.1 0.3 n-decane F 1.39 3.3 3.4 2.2 NF 3.9 3.0 2.3 1.5 0.4 1.1 n-undecane F 1.35e 4.6 5.2 4.0 NF 4.7 4.6 4.6 1.4 1.3 0.5 n-dodecane F 1.41 4.1 4.8 4.6 NF 3.6 4.8 0.3 3.9 0.7 0.4 n-tridecane F 1.44 2.9 3.5 4.1 NF 2.9 3.3 5.0 0.1 0.2 0.4 F n-tetradecane 1.65 2.5 3.0 3.4 NF 2.1 0.1 0.1 2.6 0.25 4.3 n-pentadecane F 1.41e 2.3 2.7 3.0 NF 2.55 0.2 5.1 2.5 0.2 0.3 n-hexadecane F 1.80 1.8 1.6 1.6 NF