Characterization of Particulate Lead in Vehicle Exhaust-Experimental Techniques Discussions Follow Kamran Habibi Petroleum Laboratory, Organic Chemicals Department, E. I. du Pont de Nemours & Co., Wilmington, Del. 19898
rn Experimental techniques and equipment required to
provide representative samples of particulate matter from the exhaust of vehicles are considered. A system capable of sampling the exhaust from a vehicle under realistic operating conditions has been constructed. The vehicle is operated on a chassis dynamometer and controlled by magnetic tapes recorded on the road. The exhaust is diluted with air in a mixing tunnel and a near-proportional sample obtained using the variable dilution principle. During each run a number of samples are collected simultaneously to confirm the measurements made with different instruments, as well as to provide a mass balance. The sampling system can be used with a variety of instruments to characterize particulate matter emitted in the exhaust in terms of particle size, particle size distribution, physical structure, and composition. An exhaust filter has been developed to withstand exhaust gas temperatures. and is connected directly to the vehicle tail pipe. This filter has been shown to quantitatively collect exhaust lead compounds and thus provide the means to investigate the effect of vehicle characteristics and driving conditions on lead emissions.
0
ver the past 15 years, a large amount of effort has been devoted to the development of sampling and analytical techniques to determine the gaseous components present in automotive exhaust. To this end, methods have been devised to measure the hydrocarbons, carbon monoxide. carbon dioxide, nitrogen oxides, and various oxygenated compounds under a variety of vehicle test conditions. Development of these test methods has been a long, timeconsuming process requiring the definition of the factors affecting the accuracy, reproducibility, and meaningfulness of the test results. Vehicle operating conditions, exhaust sampling techniques, and analytical methods for specific exhaust components were all important considerations. In spite of the activity devoted to this work, further improvements in test procedures are still being contemplated. I n comparison, the amount of activity spent to develop methods to determine the quantity and composition of the particulate matter present in vehicle exhaust has been quite small. Hirschler, et al. (1957 and 1964) developed test methods involving electrostatic precipitation to isolate exhaust particulate matter for subsequent characterization. These methods were suitable for use with vehicles operated under both steady speed and cyclic conditions. Mueller. et 01. (1964) developed methods to sample
vehicle exhaust under constant speed conditions and effected particle size separation in impactors and the Goetz (1960) aerosol spectrometer. The purpose of this paper is to describe sampling and analytical procedures which are being developed at the Du Pont Petroleum Laboratory to determine the amount and characteristics of the particulate matter present in the exhaust of vehicles over a wide range of realistic operating conditions. The development activities described have been primarily directed towards the characterization of lead compounds present in the exhaust. E X P E R I M E N T A L TECHNIQUES Adequate characterization of the particulate lead emitted in vehicular exhaust requires two different types of sampling systems: A system capable of obtaining a representative proportional sample of the particulate matter under realistic operating conditions; and a second system capable of collecting all of the particulate matter under all operating conditions. The proportional sampling system permits measurements of size, physical characteristics, and chemical composition of the particulate matter. The total sampling system makes it possible to check that the amount sampled is representative of the actual amount emitted from the vehicle. Further, the total sampling system provides a ready means of determining the influence of factors such as mode of operation and type and extent of mileage accumulation on the amount of lead emitted from the tail pipe. The development of these two systems and their performance are described in the following sections.
PROPORTIONAL S 4 M P L I N G SYSTEM FOR EXHAUST PARTICULATE MATTER The basic requirements for obtaining representative samples of particulate matter in vehicle exhaust are described briefly under the following headings: Suitable Driving Cycle. Cyclic operation representative of actual motorist driving must be used. Although steady state operation would greatly simplify the design of a sampling system, it is not a representative mode of driving. Condensation of Water Vapor. The exhaust gas contains approximately 10% by weight of water vapor, and if this vapor condenses, the condensation would probably initiate on the particulate matter and affect its size and characteristics. Condensation can be prevented by maintaining the exhaust gas at elevated temperatures or by dilution of the exhaust with a relatively dry gas. Proportional Sampling. With cyclic vehicle operation, a proportional sample of the particulate matter in the fluctuating exhaust stream is required. Thus, the amount of Volume 4, Number 3, March 1970 239
particles of any specific size sampled must be proportional to the total amount of such particles emitted from the tail pipe, and the same ratio should hold for all particle sizes emitted from the vehicle. Isokinetic Sampling. Sampling particulate matter from a flowing gas stream requires the gas velocity inside a sampling probe to be equal to that of the main gas stream at the inlet to the probe. Nonisokinetic sampling can lead to errors, particularly with aerosols containing large particles, i.e., greater than 2 to 5 in size. Changes in Size or Loss of Particulate Matter. The sampling system should not alter the size or size distribution of the aerosol in any way. Further, it must not allow an appreciable loss of material which would not be accounted for in subsequent measurements. Sampling Duration. The amount of lead emitted from vehicles is not constant but fluctuates greatly, particularly on a short term basis. Thus, the sampling unit must be capable of sampling as long as necessary to obtain reproducible results. The capability of prolonged sampling duration is also important to obtain sufficient material for accurate analysis. Mass Balance. Although not directly related to problems of sampling, the overall system of the vehicle and sampling unit must produce an acceptable mass balance. This is the only real confirmation that the lead being measured represents an acceptable portion of the emitted lead. With the above requirements in mind, the sampling system shown in Figure 1 has been developed. The test vehicle is operated on a programmed chassis dynamometer and the operation of the vehicle and dynamometer is controlled from information stored on magnetic tapes. The tapes are made by recording the intake manifold vacuum and vehicle speed under actual driving conditions on the road. Thus city, suburban, expressway, or any suitable type of driving can be directly and accurately simulated on the dynamometer. The total exhaust stream from the vehicle is led into a large tunnel and mixed with a stream of filtered ambient air. Although this approach does not exactly simulate actual road driving. the situation is very similar in that the exhaust is immediately diluted with a fairly large proportion of turbulent air. Dew point calculations show that a dilution ratio of 4 to 1 is sufficient to prevent the condensation of exhaust water vapor for most road operating conditions. The sampling system operates at a much higher dilution (23 to 1 at 45 m.p.h. road load) in order to represent more closely the immediate dilution occurring on the road, reduce effectively the chances of further agglomeration of the lead particles, and provide a uniform gas temperature at the sampling section of the tunnel. The latter is a requirement for proportional sampling and will be discussed later. The tunnel volume flow based on the desired dilution was set at 1150 d.t.m. The tunnel diameter is 22 inches, giving a mean tunnel gas velocity of 435 feet per minute with maximum velocity of 490 feet per minute at the flat section of the velocity profile, This velocity was established so that sampling could be carried out isokinetically at the sample flows required for the size measuring instruments with probes ranging from 3/8- to 3/4-inch in diameter. The overall mixing length is 40 feet and was chosen after experiments indicated that thorough mixing of the exhaust and ambient air streams is achieved in this length. 240 Environmental Science & Technology
ORIFICE
FIBERGLASS' FILTER
SAMPLING PROBES
Figure 1. Details of the sampling system Proportional Sampling To obtain a proportional sample of the exhaust stream under cyclic operation, the variable dilution principle is used. The mixture, consisting of the total exhaust and the ambient air, is drawn through the tunnel and past the sampling point at a constant volume flow by the blower located at the downstream end of the tunnel. The system is quite similar in principle to that developed for mass emission analysis of gaseous exhaust components by Broering, et al. (1967). Thus, when the exhaust flow is high, a reduced portion of ambient air is drawn in and, at low exhaust flows, a greater portion of ambient air is utilized. By taking the gas samples at a constant volume flow rate, a true proportional sample of the exhaust particulate matter is obtained if the following conditions are met: a. There must be no change of the particulate aerosol (i.e., agglomeration or particle breakup), or loss of material in the sampling tunnel. b. There must be thorough mixing of the ambient air and the exhaust resulting in an "isotropic" distribution of the particulate matter in the gas stream at the sampling station. c. There must be isokinetic sampling of particulate matter from the gas stream. Considering first the possible changes of the aerosol by the sampling system, the high dilution of the exhaust effectively reduces further agglomeration of the particulate matter. Since turbulence also affects agglomeration, the mixing tunnel is operated at less than one-half the maximum Reynolds number of the exhaust in the tail pipe. The low tunnel gas velocity and controlled turbulence also reduce the chance of breakup of the very large agglomerates emitted from the car, although particle breakup is not considered a problem in this system. Some loss of particulate matter due to surface deposition is impossible to eliminate. It occurs primarily due to gravitational settling of some very large particles present in the exhaust stream (0.3 to 3 mm. in diameter) as well as turbulent deposition of smaller particles. However, the deposited material is not lost in the system. By suitably positioned microscope slides, it is possible to obtain a reasonable size count. Further, the amount deposited can be determined by extraction and analyses after a series of identical runs, or even after a single run if desirable. For the sampling system to operate satisfactorily, thorough mixing of the exhaust gas and diluting air stream is necessary. Further, a flat velocity profile is needed to enable simultaneous isokinetic sampling of the particulate matter at several locations under identical conditions. However, to achieve these conditions there are some conflicting requirements. For effective mixing of two gas streams in a minimum distance, mixing aids such as concentric tubes of reducing diameter or mesh grids are most efficient.
400 w RANGE OF VALUES
!i 5 350
5, IW
,.
k? 300 c0
CROSS-SECTIONAL VIEW OF TUNNEL
0
$ 250
**"*-*--*
200
2
4
\
AREA HEATED BY /THE SUN
J
6 8 IO 12 14 16 18 2 0 SAMPLING POINT- HORIZONTAL
22
Figure 2. Distortion of velocity profile
However, introduction of such devices into the gas stream results in surface deposition and eliminates their use in this application. Increase of turbulence is also prohibitive, as it leads to increased surface deposition and the possibility of a change in the aerosol size. In the absence of mixing aids or very high turbulence, an extended length of the tunnel can provide the required mixing. However, this results in increased surface deposition and reduces the flatness of the velocity profiles by surface friction. To maintain the low tunnel gas velocity and yet promote mixing in a reasonable length, a large-hole orifice plate was used at the point of introduction of the exhaust stream into the tunnel (see Figure 1 ) . As will be discussed later, the orifice plate proved to be very helpful in achieving the required mixing and flat velocity profile and allowing reduced tunnel length and deposition. Despite this orifice plate, appreciable tunnel length is required to dampen the eddies set up by the fluctuating exhaust flow. Samples withdrawn from the tunnel at constant flow rate are metered by rotameters located well downstream in the sample line. Thus, the sample flow measurement is made at a relatively constant temperature. However, the temperature of the gas stream in the tunnel varies during cyclic operation despite the high dilution ratio used. This variation is normally very small, but when the vehicle is operated at high speeds and loads, the variation can range from 30' F. to 45" F. above ambient temperature. This affects the proportionality of the samples in two ways. One, it results in a fluctuation of the velocity in the inlet section of the sampling probe and thus causes small deviations from the isokinetic sampling rate. Two, it changes the ratio of the sample flow to tunnel flow, and hence the proportion of total aerosol sampled. In view of the small overall temperature fluctuations, the error due to the nonisokinetic sampling is considered negligible. The error in terms of the variation of the portion of the tunnel's flow sampled is also small, but can be determined by monitoring the tunnel gas temperature at the sampling point. The use of a heat exchanger in the exhaust gas stream to reduce the final mixture temperature fluctuation is not considered applicable to this system due to the surface deposition and material loss caused by a heat exchanger. However, a heat exchanger downstream of the sampling section but before the blower could be used to feed virtually constant temperature gas to the tunnel blower.
Performance o f Sampling System Tests on the tunnel sampling system were made to determine the velocity profile and investigate the mixing characteristics of the two inlet streams. The velocity was measured using a thermal anemometer with a sensitive probe covering a length of one inch. Initial velocity profiles indicated that the surface temperature of the tunnel wall had a marked influence on the shape of the profile. The action of sunlight on one side of the stainless steel tunnel and the resulting wall temperature gradient was sufficient to distort the velocity profile completely as shown in Figure 2 . Insulation of the tunnel eliminated this problem. To investigate the mixing characteristics, a tracer gas (propane) was injected at the point where the exhaust normally enters the tunnel, Samples of the tunnel gas stream were taken at the sampling section along the diameter and the concentration of the tracer (and hence an indication of the mixing in the tunnel) was determined. In this way, the minimum length required for satisfactory mixing of the two gas streams was established. However, the length of the tunnel required produced a marked wall effect and destroyed the flatness of the turbulent profile. At this stage, the investigation of orifice plates at the tunnel inlet (Figure 1 ) was started. This configuration resulted in a region of good mixing at the vena contractu and then an expansion of the gas stream to the wall of the tunnel, resulting in the required tunnel gas velocity. Figures 3 and 4 compare the velocity profiles with and with-
350--
300 -
250 -
200
PROPANE CONCENTRATION
-
Ba SAMPLING POINT
- HORIZONTAL
Figure 3. Velocity profile and mixing after tunnel insulation I
500. 400 n
-
300 L
PROPANE CONCENTRATION
200 -
-
500
J!0 5
5Y 400 8 f 8 W
w
> 100 -
a
2
4
6
8 IO 12 14 16 SAMPLING POINT
18
20
22
Figure 4. Velocity profile and mixing with the orifice plate Volume 4, Number 3, March 1970 241
Figure 5. Scanning electron micrographs of tunnel floor deposits. (Arrow in top left micrograph points to an area which is magnified ten times as shown in the top right micrograph)
out an orifice plate. The installation of the orifice plates was accompanied by an increase in the tunnel blower capacity to produce the required exhaust dilution; hence the increase of the tunnel gas velocity in Figure 4. Also shown in Figure 4 is the concentration of the propane (tracer) gas across the diameter of the tunnel under normal operating conditions. The size of the orifice plate was chosen so that the gas velocity in the region of the vena contractu was approximately equal to the velocity of the exhaust gas issuing from the tail pipe for average driving conditions. Orifice diameters of six, seven, and eight inches have been investigated and the eight-inch orifice is currently used. To determine how well the system functioned with particulate matter, a test vehicle connected to the tunnel was operated under cyclic conditions and the concentration of lead determined at five to eight locations across the sampling cross-section of the tunnel. Samples were collected simultaneously along the horizontal and vertical diameter and on the 45O axes. The results of two such tests are presented in Table I. The variation in the lead concentration at the various locations was of the order of *7% of the mean value. The uniformity of lead concentration would indicate that the mixing of the exhaust particulate matter with the ambient air stream is satisfactory. Final proof will require the determination of the size and size distribution of samples taken at various locations across the diameter of the tunnel. 242 Environmental Science & Technology
Surface Deposition After several particulate mixing and size measurement runs, the tunnel sampling system was dismantled for inspection. The nature and location of deposits in the tunnel indicated that both turbulent and gravitational deposition had taken place. Heavy deposits of large “gritty” particles were observed along a 16- to 20-foot length of the tunnel, initiating at a location where the exhaust gas expanded out to the wall. These deposits were concentrated at the bottom of the tunnel only and appeared to be due to gravitational settling, owing to their size ~
~~
~~
Table I. Particulate Lead Concentration at Various Sampling Locations Across the Tunnel
Location number 1 2 3 4 5 6 7 8
Average lead concentration fig.;M$
Position Run 1 Horizontal 1540 Horizontal 1580 Vertical 1430 Vertical 1520 45 1400 45” 1590 45 a 1380 45 O ... Average 1490 O
Run 2
1590 1480 1410 ...
1580 ... ... 1550
1520
Figure 6 . Photomicrographs of tunnel wall deposits which ranged from 0.3 to 3 mm. (or 300 to 3000 p ) , in diameter. A number of these large particles were photographed using a scanning electron microscope and are shown in Figure 5. To obtain a complete view, only the smaller particles are shown. However, the surface of the larger particles was very similar to those shown on Figure 5 . As expected, the amount of these coarse deposits diminished along the length of the tunnel and terminated approximately 10 to 14 feet upstream of the sampling section. Over the remaining surface of the tunnel a light dusting of fine "powdery" material was deposited uniformly, indicating turbulent deposition. These particles were large compared to the average size of lead particles reported to be in the exhaust of vehicles, and ranged from 5 to 50 in diameter. This observation is consistent with published data indicating that turbulent deposition is most likely to occur with the larger particles of an aerosol (Postma, Schwendiman, 1960; Davies, 1965). Optical microscope photographs of the particles collected on coated slides placed along the sides and the top wall of the tunnel are shown on Figure 6. As in the case of gravitational settling, the amount of turbulent deposition decreased along the
length of the tunnel, and only very light deposits were evident at the sampling section. The total amount of lead deposited in the tunnel was equivalent to 10% of the lead consumed by the vehicle when the vehicle was operated under medium duty cyclic test conditions. Over 95% by weight of the deposited lead was as coarse particles in the 16- to 20-foot section, corresponding to the location of gravitational settling,
TOTAL E X H 4 U S T COLLECTION SYSTEM To obtain data on vehicle lead emission characteristics, and to obtain a mass balance over the complete vehiclesampling system, methods to collect all of the lead particulate matter from the vehicle exhaust were considered. Filtration seemed to be the most desirable method in view of its simplicity of operation and subsequent treatment to determine amount of material collected. A total exhaust filter was developed which can withstand the temperatures existing at the vehicle tail pipe and can be mounted directly on the tail pipe. The filter unit is a cylindrical drum, 18 inches in diameter, 24 inches long, and packed with a high efficiency fiber glass medium, as shown in Figure 7. The exhaust gas flows directly into the HOLDER TOP PAN
,FILTER
SUPPORT RODS HOLDING TOP AND BOTTOM PANS
2 FT.
/
u
VEHICLE EXHAUST
Figure 7. Total exhaust filter Volume 4, Number 3, March 1970
243
Table 11. Total Exhaust Filter Efficiency Tests Average Lead Concentration, pg./M3 Filtration Without With efficiency, Sample location filter filter Car A : Test duration 280 miles 1 3080 7.8 2 3175 10.1 3 2900 12.9 __ Average 3052 10.3 99.7
(A)
z
EXHAUST
__.
TUNNEL SAMPLING SYSTEM
PROBE
FILTER
Figure 8. Total exhaust filter efficiency tests
Car B: Test duration 280 miles 1
2 3 Average
cylinder, then passes outwards through the filter media, which is supported externally by a stainless steel grid. The unit is sealed by internal springs located at the top and bottom pans and also by a stainless steel strip over the seam, which is held in position by the internal springs. The pressure drop across the filter is quite low-less than 2 inches of water at 70 m.p.h. cruise. It increases with the accumulation of material on the filter, but is less than 6 inches of water after 500 miles of normal driving. Thus, the use of the filter does not affect the vehicle operation. After each test the unit is disassembled and the lead on the filter media is extracted in boiling hydrochloric acid. The small amount of lead deposited on the inlet pipe and the internal parts of the holder is extracted with Versene. The unit is easy to assemble. easy to use, and requires a minimum of processing at the end of each test. An important feature of this development is that the unit is readily adaptable to actual road operation. Hence, it can be used to determine how vehicle lead emission rates are affected by road vibration and the thermal fluctuations resulting from “start-stop” driving associated with normal motorist operation. Performance of Total Exhaust Filter A major consideration in the application of the total filter was assurance that it was indeed a total filter. Initial tests were carried out by encasing the filter in a drum and sampling the filtered exhaust gas to determine its lead content (see Figure 8a). The vehicle was operated under steady state and cyclic test conditions and sampling was accomplished by inserting a probe into the drum and drawing the exhaust through “absolute” Millipore filters to remove particulate lead material. In some runs the Millipore filter was maintained at 903 C. to prevent water vapor condensation, and in others the sampled exhaust was first passed through ice traps for water removal and subsequently filtered at ambient temperatures. The interior of the probe and the condensate traps were extracted and analyzed for lead. Other experiments were carried out using bubblers of nitric acid or Versene for lead extraction. In these runs the sampled exhaust was first passed through ice traps for water removal and subsequently passed through the reagents contained in bubblers filled with glass beads. As before, the inside of the probe, the condensate traps, 244 Environmental Science & Technology
710 915 620 -
4.5 1 .o 0.6
748
2.0
99.7
and the bubbler reagents were analyzed for lead. Finally, in case lead was escaping in a vapor form, iodine crystal traps and potassium iodide bubblers were used. In all these experiments, the quantity of lead found in the filtered exhaust sample was less than 1% of the amount of lead consumed by the vehicle. Final efficiency runs were carried out using the particulate tunnel sampling system as shown in Figure 8b. The filter holder was mounted on the end of the exhaust tail pipe in the tunnel. With no filter media in place, the concentration of lead at the sampling section of the tunnel was determined by sampling at three locations. This test was followed by a second run in which the filter holder was packed in the normal manner and the concentration of lead at the sampling section was again measured under identical conditions, Cyclic vehicle operation was used in these tests and the results for two different vehicles are shown in Table 11. In both cases, the filter efficiency was found to be greater than 99% in terms of lead removal. To test the total exhaust filter in actual run application, Car A, a 327 C.I.D. standard vehicle with automatic transmission, was used, Steady state operation at 20. 45, and 70 m.p.h. under road load was investigated. The tests ranged from 200 to 400 miles in duration and were run on a fuel containing 3 grams of lead per gallon as Motor Mix. N o problems were encountered in vehicle operation nor in the use of the filter. The results in terms of per cent of the
RANGE OF VALUES
1 1
Figure 9. Steady state operation at road load
--.
ANDERSEN IMPACTOR SYSTEM -__
MONSANTO IMPACTOR SYSTEM
SAMPLING TUNNEL
n btoU MANOMETER
FLOW CONTROL ROTAM ANDERSEN
F T Z $ Y O L
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ROTA-&.VACUUM PUMP METER
3/8 IN.
MONSANTO FILTER
(8)
FILTER
MANOMETER
“ (F)
’
VACUUM SAGE
Figure 10. Schematic diagram of impactor-type size measuring equipment
lead consumed emitted from the vehicle are shown on Figure 9. The values are in good agreement with data by Hirschler, et 01. (1957 and 1964) and indicate that an increase in the road speed was associated with an increase in the fraction of the burned lead emitted with the vehicle exhaust. C H A R A C T E R I Z A T I O N OF E X H A U S T LEADPARTICLE SIZE In characterizing the particulate emissions from vehicles, it is important to define both the physical properties and the chemical composition of the emitted material. Such physical characteristic3 as particle size and particle shape are of obvious interest. The proportional sampling system described in previous sections provides the basic means for obtaining suitable samples for physical and chemical analyses. However, additional systems must be used in conjunction with the proportional sampling system to obtain the specific information desired. One area of interest which has received considerable attention in the current study is that directed towards determining the particle size of the lead-containing particulate matter present in automotive exhaust. The choice of suitable size measuring instruments is a difficult one, as most commercially available units have some reported limitations. The final choice therefore depends very much on the application and the nature and concentration of the aerosol being sampled. The initial instruments selected for this work were impactor-type units, These instruments are simple to operate, widely used, and have been calibrated with considerable success. The main problem reported in their application appears to be re-entrainment of particles impacted on the various stages. This does not appear to be a serious problem with exhaust lead aerosols for two reasons. First, owing to the very sensitive analytical procedures available for lead, accurate measurement of the quantity of lead per stage can be made at stage loadings well below that at which re-entrainment begins to be a problem. Second. the particles appear to adhere firmly to the impactor plates, and tests with and without an “adhesive” coating of the impactor plates show no significant difference in their results. Two impactor units have been used simultaneously for size measurements. The Andersen Sampler (Andersen, equivalent 1966) covers the size range of 1 / 2 to 9 diameter and operates at a sample flow of 1 c.f.m. This instrument appears to be reasonably well suited for this application, although the wide size distribution of the aerosol limits the amount of lead sample retained on the various stages of the unit. The second instrument selected was the Monsanto Impactor (Brink, 1958). This unit can
size particles as small as 0.2 equivalent diameter at a flow rate of only 0.14 c.f.m. Using the two units simultaneously, an acceptable portion of the sampled lead is retained in the size measuring instruments. Particle size calibration data for the Andersen Sampler were provided by the manufacturer. The instrument was also calibrated by Flesch, et al. (1956) using monodispersed aerosols and by May (1964), who made microscopic size counts of material collected at each stage using polydisperse aerosols. The above calibration data are in good agreement particularly when applied to size measurements of aerosols with a wide size distribution. ‘The cut sizes used for the Andersen Sampler were based on the above data. Calibration of the Monsanto Impactor was based on the procedure developed by Ranz and Wong (1952) and described by Brink (1958) in relation to this unit. With this procedure, the cut size for each stage is calculated using measured jet diameters. the gas velocity through the jet, and physical properties of the particles and the gas stream. Diagrams of the flow systems used in conjunction with the two impactor units are presented in Figure 10. In the case of the Andersen unit, a constant flow stream of diluted exhaust from the “proportional sampling tunnel” is drawn through a 3/4-inch diameter probe into the Andersen unit ( A ) . The outlet stream from the unit flows through parallel absolute filter units (B) and then through the rotameter (C) and flow control valve ( D ) . The manometer ( E ) measures the pressure drop across the Andersen and provides a means of checking its operation while a calibrated gauge ( F ) measures the vacuum for rotameter flow correction. The flow system for the Monsanto Impactor is essentially the same except that the manometer is used to measure a specific pressure drop across the impactor for flow and stage cut-size calculations, In all size runs at least two additional samples are withdrawn from the tunnel simultaneously and simply filtered, using an “absolute” Millipore filter. The first is a “comparison” stream for the Andersen; thus a 3/4-inch probe is used and samples are withdrawn at a flow rate of 1 c.f.m. The second is a comparison stream for the Monsanto Impactor, using a 3/ 8-inch probe and sampling at approximately 0.14 c.f.m. The total quantity of lead collected in the comparison streams is expected to be in close agreement with the total amount collected in the various stages of the size nieasurement streams. Further, average concentration of lead in the various sample streams must be in good agreement, indicating satisfactory mixing in the sampling tunnel. Figure 11 shows a photograph of the sampling station set up for a size measurement run. Also shown is a photograph of the tunnel sampling system. Volume 4, Number 3, March 1970 245
Figure 11. The sampling station set up for size measurement runs (left) and the sampling tunnel (right)
To investigate our size measurement techniques, Car A was used with the tunnel sampling system and operated on a fuel containing 3 grams of lead per gallon as Motor Mix. As a suitable mode of driving the Modified AMA City Driving Schedule was selected. This driving cycle has been widely used in development work associated with control of vehicle hydrocarbon emissions. It is specified in the Federal Register (1968) describing Test Procedure for Certification of Vehicle Emission Control Devices and is considered representative of city-suburban type driving. Each AMA cycle consists of 40 miles of operation divided into 11 laps. The first nine laps represent driving at speeds ranging from 30 to 45 m.p.h. with appropriate starts, stops, accelerations, and decelerations. Laps 11 and 12 represent highway driving at 55 and 70 m.p.h., with two wide open throttle accelerations to 70 m.p.h. in the final lap. Using the above cycle, particle size of the lead emitted from the test car was measured during three separate series of runs. Initial r m s were carried out at a relatively low mileage on the vehicle exhaust system-3000 to 7500 miles. Later, size measurement runs were carried out at 16,000 and 21,000 miles. Between these tests periods, mileage accumulation was obtained on a dynamometer under AMA and constant speed driving conditions. The results of the size measurements are shown in Table 111. Each run consisted of 5 AMA cycles, i.e., 200 miles of operation. The results indicate that at any given mileage the repeatability of the size measuring system is quite satisfactory. There is also close agreement between the results of the Andersen and the Monsanto Impactor. The average size of the emitted lead particles, however, appears to increase significantly with mileage accumulation. The values for the Mass Median Equivalent diameter (diameter of an aerodynamically equivalent sphere of unit density) were obtained from log-probability plots of impactor-stage cut-size diameter against the cumulative weight per cent of the lead sampled. A selection of these size distribution plots is shown on Figures 12 through 15. The plots indicate a very wide particle size distribution. 246
Environmental Science & Technology
The steepness of the lines also make the values for the MMED somewhat sensitive to the total amount of lead deposited in the tunnel. Tunnel deposition values based on the lead accumulated during a series of identical runs were used in the calculations. Comparing the data in Table 111 with the values reported by Hirschler, et al. (1964) indicates a finer lead aerosol was measured in this work. Hirschler used three vehicles operating under a city-type driving cycle and at mileages ranging from 0 to 27,000 miles. His results indicate an average equivalent diameter of 10 to 12 p. Additional size measurement runs were also made under constant speed conditions. Operation at 45 m.p.h. was selected and the vehicle was “conditioned” for approximately 1000 miles at this speed. Three exhaust lead size measurement runs were then carried out and the results are shown in Table IV.
Table 111. Lead Particle Size in the Exhaust of Test Cat A Modified AMA Schedule on the Programmed Chassis Dynamometer Vehicle MMED”, mileage Andersen Monsanto 3,250 1.2 , . . 6,450 1.2 ... 6,650 I .1 , . . 7,050 1.2 ... 7,250 1 .o 1.7 7,450 1.1 1.3 Average 1 . 1 1.5 16,250 4.1 4.5 16,450 3.1 3.1 ~
Average 21,150 21,350
21,550
3.6 4.6 I .5 8.0 -
Average 4 . 7 5
Mass median equivalent diameter.
3.8
4.6 5.4 7.0 5.7
10.0. I- I
ANDERSEN
50
ANDERSEN
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a-a MONSANTO (AMA CYCLE)
50
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80 90 95 9899
60
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LEAD EMITTED-
miles)
Figure
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WEIGHT OVERSIZE
Figure 15. Andersen and Monsanto impactor results (21,150 miles)
a- a ANDERSEN o--0 MONSANTO
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(AMA CYCLE)
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Figure 13. Andersen and Monsanto impactor results (7450 miles)
'
100
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i
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Table IV. Lead Particle Size in the Exhaust of Test Car A Steady State Operation at 45 m.p.h., 19 Inch Vacuum MMEDI'. u
i
0.3
Vehicle mileage
Test duration,
miles
Andersen and Monsanto combined data
10,150 10,630 11,050
150 330 360
0.4 0.6 0 7
Average LEAD EMITTED-%
WEIGHT OVERSIZE (1
0.6
Mass median equivalent diameter.
Figure 14. Andersen and Monsanto impactor results (16,450 miles)
Tests at 45 m.p.h. resulted in a finer lead aerosol than that observed under AMA conditions although, as before, a wide particle size distribution is indicated-see Figure 16. These results show a MMED somewhat larger than values reported by Mueller (1964) for the same operating condition (0.3-p equivalent diameter). The difference is
small considering the possible effect of vehicles, mileage accumulation, and size measurement techniques. Included in the above series of size measurement runs were total exhaust filtration runs carried out to determine the overall lead balance. After accounting for the lead deposited in the tunnel sampling system, a good Volume 4, Number 3, March 1970 247
mass balance was obtained in terms of lead emitted from the vehicle and lead sized (or sampled) with the tunnel system.
SUMMARY Sampling techniques have been developed which enable the characterization of the particulate lead contained in vehicle exhaust. The tunnel sampling system used in conjunction with a car operated on a programmed chassis dynamometer offers a unique opportunity to examine the characteristics of lead particles under realistic operating conditions. Proportional samples of the exhaust particulate matter can be obtained and satisfactory operation of the system can be confirmed by a direct mass balance. Although only impactor sizing units have been used, the system is compatible with other sizing instruments. Physical properties and chemical composition of representative samples can be determined using the electron microscope, electron microprobe, X-ray diffraction, X-ray fluorescence, and other chemical analytical techniques. A total particulate lead filter has been developed which can be attached directly to the exhaust tail pipe of a vehicle. This filter provides a convenient means to investigate vehicle lead emissions under various types of vehicle operation. The filter is also a tool to check the adequacy of the mass balance obtained with the tunnel sampling system. Both the tunnel sampling system and the total filter have been shown to give reproducible measurements of the emitted lead. However, the particle size and amount of lead emitted vary widely with mode of vehicle operation, vehicle driving history, and probably other as yet undefined parameters, Extensive additional investigations will be required before the influence of all the important factors controlling lead emissions in the vehicle population can be defined. Such investigations are under way. Acknowledgment The author is indebted to J. B. Dunson, of Du Pont Engineering Department, for his advice and assistance in the general field of particulates.
Literature Cited Andersen, A. A., “A Sampler for Respiratory Health Hazard Assessment,” A m . Ind. Hygiene Assoc. J . 27, March, 1966. Brink, J. A., Jr., “Cascade Impactor for Adiabatic Measurements,” Znd. Eng. Chem. 50, 645 (1958). Broering, L. C., Jr., Werner, W. J., Rose, A. H., Jr., “Automotive Mass Emission Analysis by a Variable Dilution Technique,” presented at the Air Pollution Control Association Annual Meeting, Cleveland, Ohio, June, 1967. Davies, C. N., “The Rate of Deposition of Aerosol Particles from Turbulent Flow Through Ducts,” Ann. OCCUP. Hyg. 8, 239-45 (1965). Federal Register, “Control of Air Pollution from New Motor Vehicles and New Motor Vehicle Engines,” Vol. 33, No. 2, Part 11, Department of Health, Education, and Welfare, January, 1968. Flesch, J. P., Norris, C . H., and Nugent, A. E., Jr., “A Method for Calibrating Particulate Air Samplers with Monodisperse Aerosols: Application to the Andersen Cascade Impactor,” American Industrial Hygiene Conference, Pittsburgh, Pa., May, 1966. Goetz, A., Stevenson, H. J. R., Preiming, O., “The Design and Performance of the Aerosol Spectrometer,” Air Pollution Control Assoc. 10, 378-83 (1960). Hirschler, D. A., Gilbert, L. F., “Nature of Lead in Automobile Exhaust Gas,” Archives of Environmental Health, Symposium on Lead, February, 1964. Hirschler, D. A., Gilbert, L. F., Lamb, F. W., Niebylski, L. M., “Particulate Lead Compounds in Automobile Exhaust Gas,” Ind. Eng. Chem. 49, 1131-42 (1957). May, K. R., “Calibration of a Modified Andersen Bacterial Aerosol Sampler,” Appl. Microbiol. 12, 37 ( 1964). Mueller, P. K., Helwig, H. L., Alcocer, A. E., Gong, W. K., Jones, E. E., “Concentration of Fine Particles and Lead in Car Exhaust,” Symposium on Air Pollution Measurement Methods, Special Technical Publication No. 352, American Society for Testing and Materials, 1964. Postma, A. K., Schwendiman, L. C., “Studies in Micrometrics-Particle Deposition in Conduits as a Source of Error in Aerosol Sampling,” Report No. HW-65308, Hanford Atomic Products Operation, Hanford Atomic Products Operation, Richmond, Wash., May 12, 1960. Ranz, W. E., Wong, J. B., A M A Arch. Ind. Hyg. and Occup. Med. 5 , 464 (1952). Received f o r review June 20, 1969. Accepted October 7, 1969. Symposium on Air Conservation and Lead, Division of Water, Air, and Waste Chemistry, 157th National Meeting, ACS, Minneapolis, Minn., April 1963,
Discussion Characterization of Particulate Lead in Vehicle ExhaustExperimental Techniques P. K. Mueller Ph.D., Chief Air and Industrial Hygiene Laboratory, California State Department of Public Health, 2151 Berkeley Way, Berkeley, Calif. 94704
D
r. Habibi’s work stands as the first substantial advance in the technology of sampling particles emitted by the exhaust duct of cars in seven years. In view of the increasing urgency to set limits on lead emissions from cars ( I , 2 ) it is regrettable that past efforts on particulate matter emitted by vehicular exhausts have been so small. Seven years ago, in 1962, my group, at the California State Department of Public Health, set about to apply size-segregated collection methods to the sampling of exhaust aerosol 248 Environmental Science & Technolog?
from cars operated at cruise conditions under road load on an all-weather chassis dynamometer. The results of our work were presented to the scientific community ( I ) and published in great detail (2). In general terms, the results of our work then and subsequent unpublished work since demonstrated that cars burning 3 ml. of TEL or TML motor mix per gallon of fuel were emitting under cruise conditions about 5 to 60 pg. Pb per liter of exhaust depending on speed in the range of 25 to
