proximately 60-80% of the ultimate capacity was utilized within a few hours in the case of granular carbon. The remaining capacity, however, took several weeks to be exhausted. The powdered carbon was shown to reach equilibrium in 3 to 5 days of contact. The experimental isotherms were shown to be independent of the type of buffer used, the carbon particle size, and the initial adsorbate concentration. Both phenol and o-chlorophenol isotherms were well fitted by the Freundlich equation over the range studied. The reason for the apparent lack of consistency among isotherms previously reported in the literature is attributed to difficulty in defining the attainment of true equilibrium. The slow uptake can account for a significant proportion of the carbon capacity and can easily be overlooked when performing an isotherm study. Disregard of this capacity or equivalently nonallowance of sufficient time for equilibrium can be the cause of previously unexplained observations such as the variation of isotherm capacity with the initial concentration of adsorbate. A model describing adsorption in terms of rapid radial diffusion in series with slower diffusion into branch pores fits experimental data well and can be used to assess potential effects of varying particle size, initial concentration, and so forth. I t is suggested that granular carbon isotherms be evaluated over periods of a t least 30 days unless rigorous studies are carried out to ensure equilibrium is obtained. Where possible, powdered carbon obt.ained from the granular carbon of interest should be used in the isotherm evaluations and these
should be conducted for a t least 3 days.
Literature Cited (1) Snoeyink, V. L., Weber, W. J., Mark, H. B., Enuiron. Sci. Tech-
nol., 3, 918 (1969). (2) Zogorski, J. S., Faust, S. D., Haas, J. H., J . Colloid Interface Sci., 55,329 (1976). (3) Ying, W., Ph.D. Thesis, University of Michigan, Ann Arbor, Mich., 1978. (4) Myers, A. L., Zolandz, R. R., 171st National Meeting of the American Chemical Society, Miami, Fla., Sept 1978. (5) Rankin. P. R.. M.Enz. Thesis. McMaster Universitv. Hamilton. 'Ontario, 1975. ' (6) Vermeulen, T., Adu. Chem. Eng., 2, 147 (1958). (7) Fleck, R. D., Kirwan, D. J., Hall, K. R., Ind. Eng. C'hem.Fund., 12(1),95 (1973). (8) Crittenden, J. C., Weber, W. J., J . Enuiron. Eng. Diu., Am. Soc. Civ. Eng., 104, 185 (1978). (9) Wedin, J., M.Eng. Thesis, Royal Institute of Technology, Stockholm, Swedenr1976. (10) Peel, R. G., Ph.D. Thesis, McMaster University, Hamilton, Ontario, 1979. (11) Usinowicz, P., Ph.D. Thesis, University of Michigan, Ann Arbor, 1972. (12) Weber, W. J., Morris, J. C., J . San. Eng. Diu., Am. Soc. Ciu. Eng., 90 (SA3),79 (1964). (13) Dedrick, R. L., Beckmann, R. B., CEP Symp. Ser., 63(74), 68
-
(1967). \-ll.,.
(14) Martin, R. J., Al-Bahrani, K. S., Water Res., 12,879 (1978).
(15) Huang, J., Garrett, J. T., Water Sewage Works, 124, 64 (1977). (16) Mathews, A. P., Ph.D. Thesis, University of Michigan, Ann Arbor, 1974.
Received for review July 9, 1979. Accepted October 4 , 1979.
Effects of Fuel, Lubricant, and Engine Operating Parameters on the Emission of Polycyclic Aromatic Hydrocarbons Peter S. Pedersen* and Jan lngwersen Laboratory for Energetics, Technical University of Denmark, Building 403, DK-2800 Lyngby, Denmark
Torben Nielsen and Elfinn Larsen Chemistry Department, Ris0 National Laboratory, DK-4000 Roskilde. Denmark
The effects of fuel, lubricant, and engine operating parameters on the emission of polycyclic aromatic hydrocarbons (PAHs) associated with particles were studied by means of an automobile engine operated with a number of special fuels under steady-state conditions. More than 90% of the mass of the heavier PAHs (e.g., benzo[a]pyrene) was found on particles with a diameter below 1pm, while for the light PAHs, a substantial part was in the vapor phase. The aromatic content, the type of aromatic fraction, and the PAH content of the fuel and lubricant had a strong effect on particle-bonded PAH emission; in comparison to this, the lead content of the fuel was of minor importance. The lubricant itself had only slight influence on particle-bonded PAH emission. This varied with the air:fuel ratio in a manner similar to.particulate matter and to unburned hydrocarbons, showing high emissions a t very rich as well as very lean mixtures. On increasing the engine load, the particle-bonded PAH emission increased. H
Lung cancer has been one of the highest causes of death of humans in the industrialized countries (1, 2 ) . The existence of an urban factor in the risk of lung cancer seems a t least partly to be caused by atmospheric pollution (2-4). In general, 0013-936X/80/0914-71$01 .OO/O
@ 1980 American Chemical Society
the atmospheric levels of polycyclic aromatic hydrocarbons (PAHs) are orders of magnitude higher in urban areas than in rural areas ( 5 , 6 ) . Some of the PAHs are considered to be strong carcinogenics (5, 7). The main part of the carcinogenic PAHs in the air has been found to be associated with particles ( 4 9 ) .Since particles in the low micrometer range are deposited in the pulmonary region of the lungs while larger particles are impacted in the upper respiratory tract, removed by mucociliary action, and swallowed (IO),the health hazards associated with inhalation of the two size ranges are different. The emission of PAHs from automotive sources has been estimated to be 2% of the total amount emitted to the atmosphere in the U S . ( 5 ) .Changes in the traffic intensity and in the extent to which coal and fuel oil are used for domestic heating and power generation might influence this significantly; in areas close to congested roads and streets, automotive emissions can be the dominant source (5,11-13). The worldwide trend to reduce lead addition to gasoline in order to reduce lead emission to the atmosphere is likely to have the effect of increasing the content of aromatic hydrocarbons in order to maintain the knock resistance of the fuel (14);a number of investigations have shown that this leads Volume 14, Number 1, January 1980 71
to increased PAH emission (15-20). Other investigations (21, 22) indicate that fuel PAH content has a stronger influence on PAH emission than does fuel aromatic content. However, all these investigations h w e concentrated on the effects of fuel (and to a smaller extent lubricant) parameters on the total emission of PAH, while engine operating parameters and the particle size distribution of the emitted PAHs have been dealt with only to a very limited extent; Melton et al. (23) detected BaP associated with particles of less than 1pm in diameter, sampled from the exhaust of two engines running on a commercial gasoline, doped with BaP in extremely high concentrations (1000 ppm). In a similar experiment performed later (24), BaP was found to be associated with only the smallest particle size fraction (0.25 pm). Foster et al. (25) in a single experiment detected the presence of 21 different PAHs in the total particulate sample. In the investigations dealing with the total PAH emission (15-22) in general it has not been possible to vary only one important parameter a t a time. As an example of this, the investigation of the effects on PAH emission of fuel lead content has been performed with a simultaneous variation in octane number, which is a general measure of the reaction rate of the fuel, a t least with regard to the preflame reactions. The simultaneous variation in octane number might, therefore, have influenced the PAH emission, as the PAH is formed in the quench layer a t the combustion chamber walls (15,26). The present study therefore concentrated on the particle size distribution of the emitted PAHs as affected by fuel, lubricant, and engine operating parameters; a number of special fuels were used, which allowed single parameter variations to be performed without changing the octane number. Experimental Setup and Techniques
General Description. In order to control the engine operating conditions as effectively as possible, a single engine in an engine/dynamometer setup was chosen. The Ford Escort l100-cm3 4-cylinder gasoline engine was connected to an eddy-current brake, which absorbed the power of the engine. The exhaust gas was taken from the exhaust manifold to a three-way cock, which conducted the exhaust gas either directly to the laboratory exhaust plant (when measurements were not in progress) or through the cooled exhaust pipe to a dilution tube. Here the exhaust gas was diluted with filtered ambient air to simulate the natural cooling and dilution process. A substream of the diluted exhaust gas was led through a cyclone assembly and an absolute filter for the collection of particles. The general emission characteristics of the engine were determined by the use of NDIR analyzers for CO, CO2, and hydrocarbons (hexane equivalent), a FID analyzer for total hydrocarbons (C1 equivalent), a chemiluminescence NO/NO, analyzer, and a paramagnetic 0 2 analyzer. In the following, a brief description will be given of the most important parts of the equipment. A detailed description can be found elsewhere (27). The design of the particle collection equipment was intended to reproduce actual road driving conditions, as the formation of particles depends on the conditions in which the exhaust is cooled. T o avoid condensation of water vapor, the exhaust gas was cooled only down to about 160 “C in the exhaust pipe, which corresponds to the temperature of the exhaust gas a t the tail pipe during road driving. After this partial cooling, the exhaust gas was led into the dilution tube and mixed with filtered air (ratio 1:19) to simulate the dilution and cooling that take place when the exhaust gas leaves the exhaust pipe of a vehicle driving on the road. By this, condensation of water vapor was avoided, a realistic temperaturetime history for the exhaust gas was achieved, and the temperature of the diluted gas a t the sampling position of the 72
Environmental Science & Technology
dilution tube was kept a t 30-35 “C. Particles for the subsequent analysis of particle-bonded PAHs were collected using an isokinetic probe in the diluted exhaust gas stream. The sample was directed through a cyclone battery with three cyclones in parallel, collecting particles larger than 1l m in diameter, and subsequently through a large glass-fiber absolute filter (Whatman GF/A) for collecting the remaining particles. This equipment was used for all experiments. However, two experiments were performed in order to determine the amount of PAHs not collected by this equipment, Le., vapor phase PAHs. Two large Pyrex freeze traps (gas washing bottle type, 100-mm diameter, 750-mm length) were placed in series after the filter. They were cooled by dry ice/alcohol and were able to cool the sample down to -3 “C a t the exit from the second freeze trap, using the same flow rate as in all other experiments (45 m3/h). For all experiments, the contents of seven PAHs were determined on the filters and in the cyclones. These were anthracene (A), fluoranthene, 1-methylanthracene (1-meA), pyrene, benzo[a]anthracene (BaA), benzo[a]pyrene (BaP), and benzo[ghi]perylene (BghiP). Filters and cyclones were extracted ultrasonically in cyclohexane, and the extracts were concentrated by evaporation. The PAHs in the concentrated extracts were separated by thin-layer chromatography from up to six other fluorescent fractions. The PAH fraction was analyzed by high-performance liquid chromatography with on-line fluorescence detection. The column used was Zorbax ODs, and the eluant was methanol-water (8:l).The method is described in detail elsewhere (28). Research Fuels. In order to make an extended variation of fuel parameters possible without varying more than one important fuel parameter at a time, a series of special fuels (see Table I) was prepared by blending isooctane (2,2,4-trimethylpentane), n-heptane, benzene, toluene, o-xylene, and heavy aromatic discards (the latter more than 99% Cg and C ~ aroO matics). All fuels had nominal octane ratings of 97 RON; fuels having tetramethyllead (TML) and tetraethyllead (TEL) addition used a fixed TML: T E L ratio of 3:l. Except for test fuels A3-A6, the aromatic fraction was a fixed mixture of C6-C7-C8-C9 Cl0 = 8:30:40:22, which corresponds closely to the composition of the aromatic fraction of a 97 RON Catalytic Reformate. n-Heptane and isooctane were chosen because they have nearly the same boiling point and density, and are not too different in chemical structure; last, but not least, the changing of the isooctane:n-heptane ratio in the fuel is a powerful tool for maintaining a constant octane rating while making considerable variations of other fuel parameters. Owing to the chemical structure of the two compounds, changes in the isooctane:n-heptane ratio were not expected to influence significantly PAH formation in the engine and its emission. By appropriate blending within groups of the special fuels, the significance of aromatic content (Al-A2), lead content (Pbl-Pb2), and combined lead and aromatic content (Cl-C2) was investigated, while base fuel A was used for the investigation of engine and lubricant parameters and test fuels A3-A6 were used to investigate the significance of aromatic composition. The special fuels were designed by the authors and supplied by the B P Research Centre, Sunbury, U.K. Research Lubricants. I t has been found (18)that retention of PAHs in the lubricant plays a role for the subsequent emission. The retention capacity of a lubricant may depend on its chemical structure, which may also be important for PAH formation resulting from lubricant combustion. Therefore, the use of a synthetic lubricant was considered, but it was felt that the synthetic lubricant in question (BP Enerjet 523, based on “hindered” esters (29)),if used for the entire
+
Table 1. Limited Inspection Data of Research Fuels isooctane, vol %
blend composition n-heptane, vol % aromatics, vol %
lead cont., g of Pb/L
type of aromatics
base fuel A test fuel A1 test fuel A2 test fuel A3 test fuel A4 test fuel A5
44 20 87 45 40 50
16 20 13 15 20 10
40 60 0 40 40 40
0.4 0.4 0.4 0.4 0.4 0.4
mixture mixture
test test test test test
38 51 38 25 60
22 9 22 15 20
40 40 40 60 20
0.4 0.0 0.8 0.0 0.8
c9
fuel A6 fuel P b l fuel Pb2 fuel C1 fuel C2
investigation, might cause piston-ring difficulties. T o avoid this, a commercial SAE 15W/50 multigrade engine oil was used, and sufficient amounts were acquired in order to ensure that the lubricant was identical in all measurements. The synthetic lubricant was used only for the last two measurements in order to investigate the effects of the chemical structure of the lubricant. For lubricant specifications, see ref 27. Experimental Conditions. The investigation of PAHs associated with small and large particles requires exhaust gas dilution and isokinetic sampling from the diluted exhaust gas. In order to separate particles as planned, the cyclones must be operated a t a constant flow rate, and since isokinetic sampling requires that a fixed proportion of the diluted exhaust gas be sampled, the total flow rate in the dilution tube must be maintained constant. I t is also desirable to maintain the dilution ratio and sample temperature constant in order to not influence the PAH vapor phase:particle-bonded ratio. Consequently, a steady-state engine condition must be chosen, although an engine load cycle might represent actual driving conditions better. However, the effects of engine operating parameters cannot be studied if an engine load cycle was used, and the reproducibility of such experiments (and experiments with cars on a chassis dynamometer using driving schedules like the ECE R.15 or the FTP) is inferior to steady-state experiments. Thus, an engine condition corresponding to 60 km/h constant speed on level road was chosen for the experiments. This engine load is within the normal range for driving in urban areas. Experimental Procedure. Earlier investigations (26) showed that soot deposits in the combustion chamber could accumulate PAHs that could be reliberated and emitted. As discussed also in ref 30, it is therefore important to stabilize deposits before a measurement. This was done using a six-step conditioning cycle designed to simulate the average use of a typical car in Europe, where city driving accounts for about 40-50% of the annual mileage. Four of the six steps were used to simulate city driving conditions, one for open road driving (90 km/h) and the last for motorway driving (120 km/h). The engine was operated automatically for 4 h in this conditioning cycle prior to making the measurements in order to create and stabilize soot deposits corresponding to the actual fuel composition and to the average use of a car. Following that, it was switched to the constant speed condition, the exhaust gas was directed into the dilution tube (provided this was not already the case during conditioning, e.g., at the experiments with doped fuel and other fuel parameter variations), and the setup allowed to stabilize. Following that, particles were collected for the chemical analysis of PAHs by isokinetic sampling of 90 m3 (10%) of the diluted exhaust gas. The duration of the collection was 2 h in each experiment. Before conditioning was
benzene toluene o-xylene
+
ClO mixture mixture mixture mixture
BaP content, pg/mL
octane number, RON
0.12 0.16 0.003 0.004 0.002 0.001
97.8 97.0 97.2 98.2 97.5 98.2
0.48 0.1 1 0.1 1 0.16 0.046
97.9 97.3 97.2 97.0 97.1
started the engine was supplied with a new charge of lubricant, and after the measurement this charge was again drained off the engine while hot.
Experimental Results General Remarks. The following section deals with the experimental results and compares them with those of previous work reported in the literature. It must be borne in mind, however, that in most other studies the total PAH emission was investigated, whereas in the present study PAH in the form of or associated with particles was investigated. Moreover, a number of test conditions differed, especially the engine load condition (steady state, warm engine a t constant load), where a number of other investigations employed a city driving schedule, in some cases with cold start. This latter case, in particular, is known to be accompanied by a considerable increase in PAH emission (30). Another important point is that PAHs, e.g., BaP, captured on filter media may decompose during further collection by reaction with oxidants, e.g., ozone and NO2 present in the air passing through the filter (31,32).As the cNO2/cNO ratio was low, it was, however, considered that the levels of oxidants in the unirradiated diluted exhaust were low. On the basis of present knowledge about reactivities of the different PAHs toward oxidants and nitrating species (33-36), it was not possible to find any significant trend in the results to indicate that chemical decomposition during sampling was important in the present study. Possible decomposition of the captured PAHs in the investigation of the effects of fuel and lubricant parameters on PAH emission would affect neither the relative variations in the results nor the conclusions. The results are presented in a general form expressed as a percentage of a reference emission of the seven PAHs. The reference values represent mean values of the results from 30 experiments. Furthermore, the results are presented as micrograms/kilogram of fuel burned in order to show the absolute levels. General Results. Vapor Phase PAHs. The two experiments with collection in freeze traps showed that the percentage of particle-bonded PAHs increased with the molecular weight of the compound. Only for the three least volatile PAHs (BaA, BaP, and BghiP) was the amount collected in the freeze traps so small that it could be assumed a total collection was obtained. For these three compounds, 92-95,97-98, and 99%, respectively, were particle bonded, while for the lower PAHs the dominant amount was in the vapor phase. However, since the same experimental procedure was used for each experiment in the main part of the investigation, this does not influence the conclusions to be drawn from the measurements of particle-bonded PAH emission. Except for a few alkylsubstituted PAHs, as, e.g., g,lO-dimethylanthracene, all the Volume 14, Number 1, January 1980
73
PAHs, which are considered to be carcinogenic, belong to the higher PAHs, such as BaA, BaP, and BghiP (5, 7). The main part of the carcinogenic PAHs should therefore be expected to be found adsorbed on particles. Recently, it was found (8)that the dominant amount of the lower PAHs in the urban air is in the vapor phase. This might suggest that a prolonged residence time of the diluted exhaust in the dilution tube would not have given significantly different results. Particle Size Distribution. The dominant mass fractions of the particle-bonded emission of all 7 PAHs analyzed were found to be associated with particles less than 1 pm in equivalent aerodynamic diameter, which is also the dominant size fraction of particles emitted from gasoline engines (2325). Melton et al. (23) and Foster et al. (24)detected BaP only on small particles (
