- Email: [email protected]
Oil-soaked sintered impactors for the ELPI in diesel particulate measurements
Aerosol Science 34 (2003) 635 – 640 www.elsevier.com/locate/jaerosci
Technical note
Oil-soaked sintered impactors for the ELPI in diesel particulate measurements C. van Gulijk∗ , J.C.M. Marijnissen, M. Makkee, J.A. Moulijn DelftChemTech, Delft University of Technology, Julianalaan 136, NL-2628BL Delft, The Netherlands Received 3 June 2002; received in revised form 2 December 2002; accepted 3 December 2002
Abstract Diesel soot overloads the ELPI-impactor rapidly if it is equipped with the standard 4at-surface impactors. This non-ideal behaviour was studied recently (J. Aerosol Sci. 32 (2001) 1117). It was found that rapid overloading, or surface build-up, is a result of a 4u8y bed of soot particles that covers the impactor surfaces and starts to 9lter airborne soot particles. This paper reports additional results with oil-soaked sintered impactors for the ELPI. It is demonstrated that (rapid) overloading is eliminated with oil-soaked sintered impactors. The maximum allowed mass load for the ELPI impactor is increased 50-fold. ? 2003 Elsevier Science Ltd. All rights reserved. Keywords: Diesel soot; ELPI; Instrument evaluation
1. Introduction The electrical low-pressure impactor (ELPI: Keskinen, Pietarinen, & Lehtmaki, 1992; Marjam?aki, Keskinen, Chen, & Pui, 2000a) gains popularity as an aerosol particle-sizer in automotive engineering. An increasing number of workers report their 9ndings on ELPI measurements: e.g. Ahlvik, Ntziachrostos, Keskinen, and Virtanen (1998); Pattas, Kyriakis, Samaras, Pistikopoulos, and Ntziachristos (1998); Maricq, Podsiadlik, and Chase (2000); Andrews, Clarke, Rojas, Sale, and Gregory (2001). Van Gulijk, Schouten, Marijnissen, Makkee, and Moulijn (2001) reports on impactor overloading, which poses a severe restriction for the measurement of size and numbers of a diluted diesel soot aerosol if the ELPI is used in standard con9guration (viz. with 4at impaction surfaces). Oil-soaked sintered impactor stages supplied by the ELPI-manufacturer (Dekati Ltd., Finland) may be a promising solution to circumvent impactor overloading of the ELPI ∗
Corresponding author. Tel.: +31-15-284-3605. E-mail address: [email protected] (C. van Gulijk).
0021-8502/03/$ - see front matter ? 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0021-8502(02)00212-4
636
C. van Gulijk et al. / Aerosol Science 34 (2003) 635 – 640
(Marjam?aki et al., 2000b). In this paper these oil-soaked sintered impactor stages are evaluated for the measurement of diesel soot aerosols. The results demonstrate that impactor overloading can be eliminated. 2. Theoretical background The problem with the measurement of a diluted diesel soot aerosol is that overloading or surface build-up occurs with the ELPI in the standard con9guration (Van Gulijk et al., 2001), resulting in dynamic signals which lead to incorrect size distribution and an incorrect number measurement. Overloading occurs at very low mass loads due to the fractal-like structure of dry diesel soot. A 4u8y bed of fractal-like soot particles covers the impactor surface in 60 s and it starts to 9lter soot particles. Thus, for a more or less constant aerosol 4ow, the measured particle size distribution changes over time. Because the signals from the ELPI changed very fast the maximum time of operation appeared to be 180 s (total mass load 15 g). After operation times of about 1800 s, conical moulds of particles form on the impactor surface. This marker for impactor overloading is generally accepted and comprehensively described by Hinds (1999). The problem of overloading has been acknowledged in the past by the development of special impactors that are less prone to overloading e.g. the MOUDI impactor (Marple, Rubow, & Behm, 1991) and the Lundgren impactor (Lundgren, 1967). Another relevant development was the introduction of oil-soaked sintered impactors (Reischl & John, 1978). These enable higher particle loads on the impactor stages by wetting multiple layers of particles. According to Sioutas, Chang, Kim, Koutrakis, and Ferguson (1999) particle bounce is also reduced by using oil-soaked sintered impactor stages. The manufacturer of the ELPI (Dekati Ltd., Tampere, Finland) has also acknowledged the problem of overloading, and therefore, they have produced special ELPI-impactor stages viz. sintered-stainless steel impactors that can be 9lled with oil to give oil-soaked sintered impactors. Sintered metal impactors having average pore sizes of 5, 20, and 40 m are available (Marjam?aki et al., 2000b). In this work the 40 m sintered-steel impactor stages were tested. The use of oil-soaked sintered impactor stages changes the cut-o8 sizes of the ELPI impactor. This is due to the fact that the surface characteristics of the sintered metal are di8erent from smooth aluminium foils. Table 1 shows the cut-o8 sizes for the ELPI system as supplied by the manufacturer (calibration by the manufacturer). Fig. 1 shows a standard impactor stage with aluminium foils (left) along with a sintered impactor stage (right). 3. Experimental The experimental set-up has been described extensively by Van Gulijk et al. (2001) so only a brief description is given here. The set-up consisted of the following parts: a small diesel engine that operated at a constant engine load of 50% which functioned adequately as a reproducible soot generator (LPW-2; 900 cm3 ; 5:7 kW by Lyster–Petter); a 9lter test bench in which the exhaust 4ow-rate was controlled with a membrane pump (PM 15368-0.15.0 by NKF-Verder); a two-step aerosol diluter system (Dekati diluters by Dekati Ltd.) that sampled the aerosol in the 9lter test bench; and the ELPI that measured the diluted diesel soot aerosol (ELPI-10 by Dekati Ltd.). Two types of ELPI-impactors were tested in measurements of 9000 s (2:5 h): aluminium foils coated with
C. van Gulijk et al. / Aerosol Science 34 (2003) 635 – 640
637
Table 1 Manufacturer calibration of cut-o8 sizes for the ELPI impactor in standard con9guration and with oil-soaked sintered impactors Stage number
Standard con9guration impactor cut-o8 size/(nm)
Sintered metal impactors impactor cut-o8 size/(nm)
Pre-impactor 1 2 3 4 5 6 7 8 9 10 11 12
10:3 × 103 6:79 × 103 4:03 × 103 2:49 × 103 1:63 × 103 1:01 × 103 647 399 259 169 105 60 30
9:75 × 103 6:43 × 103 3:82 × 103 2:36 × 103 1:54 × 103 956 612 333 210 106 56.0 32.9 25.3
Source: ELPI calibration sheet, Dekati Ltd., Finland.
Fig. 1. Impactor stage from ELPI standard con9guration (left), and oil-soaked sintered impactor stage (right).
Apiezon L vacuum grease (ELPI standard con9guration); and oil-soaked sintered impactors, these ? P3, by were the 40 m sintered-steel impactor stages impregnated with 3–5 drops of vacuum oil (Ol Pfei8er). Before starting the experiments, the set-up including the ELPI was allowed to heat up and
638
C. van Gulijk et al. / Aerosol Science 34 (2003) 635 – 640
stabilise for 1 h to avoid start-up e8ects. The ELPI was in 4ush mode during this time i.e. the ELPI reverses the air4ow in the inlet nozzle, thus ensuring that no particles enter the ELPI. By stopping the 4ush mode the experiments were started. Data from the ELPI were stored with a frequency of 1 Hz on a personal computer. The measured particle size distributions were averaged over 35 data points. The average variance in the measured aerosol concentration for each individual size-range of the ELPI was 2.4%; the largest variance was 5.9%. This indicates a good repeatability for the measurements.
4. Results Fig. 2 shows the measured particle size distribution for the ELPI in the standard con9guration as a log-probability plot. Fig. 3 shows the measured particle size distribution with oil-soaked sintered collectors. The count median diameter (CMD) is used as a tool for comparison of the results (Hinds, 1999). It is clear that the measured particle size distribution changes as a function of time with the ELPI in standard con9guration (Fig. 2). At the start of the measurement (60 s) the CMD was 66 nm, after 900 s it changed to 77 nm, after 3600 s it changed to 88 nm, and after 9000 s (2:5 h) it reached a value of 101 nm. Fig. 3 shows steady-state behaviour for the oil-soaked sintered impactors: the particle size distributions are also shown at 60, 900, 3600, and 9000 s but they overlap. The CMD was determined by averaging the CMDs at 60, 120, 180, 900, 1800, 3600, and 9000 s, which gives a value of 56:5 nm with a standard deviation of only 1:3 nm! The measured particle size distribution does not change during the entire measurement. 1000
60 s 900 s 3600 s 9000 s
aerodynamic diameter/[nm]
aerodynamic diameter/[nm]
1000
100
10
60 s 900 s 3600 s 9000 s
100
10 20
50
80 90 95
99
99.9
cumulative count fraction/[%] Fig. 2. Log-probability plot of the particle size distribution as a function of time with Apiezon L on aluminium foils (smooth surfaces). The measurement does not give stable results for a constant aerosol 4ow.
20
50
80 90 95
99
99.9
cumulative fraction/[%] Fig. 3. Log-probability plot of the particle size distribution as a function of time with oil-soaked sintered impactors. The results are stable.
C. van Gulijk et al. / Aerosol Science 34 (2003) 635 – 640
639
5. Discussion The dynamic behaviour that is found with the ELPI in standard con9guration is due to overloading. In this case, overloading falls apart in two phenomena. The 9rst phenomenon is rather well known: conical moulds form on the impactor surface. The conical moulds change the gas-4ow on the impactor and thereby change the cut-o8 size of the impactor. This phenomenon takes e8ect after a “long” operation time of the impactor. The second phenomenon is less well known: a very 9ne layer of 4u8y soot particles on the impactor surface 9lter the particle-laden gas stream that 4ows over the impactor. Thus, particles that are very small deposit on impactor stages that should only trap larger particles. This phenomenon takes e8ect almost immediately after starting an experiment (Van Gulijk et al., 2001). When the ELPI is equipped with oil-soaked sintered impactor stages no dynamic e8ects are observed, which indicates that overloading is eliminated. The following discussion gives insight. Capillary forces of the oil act on the particles: the oil wets the particles and draws them into the oily phase. The oil reservoir is large enough to wet many layers of particles so that the formation of conical moulds is postponed for at least 9000 s with the current set-up. The hypothesis was visually con9rmed: no conical moulds, which mark impactor overloading, were formed on the surface after 9000 s. The elimination of a 4u8y bed of particles on the impactor surface is more diPcult to prove: the samples could not be placed in the electron microscope. Instead, the CMD is used as an indicator. The value of the CMD that is found after 60 s is lower for oil-soaked sintered collectors (57 nm) than the value found for aluminium foils coated with Apiezon L vacuum grease (66 nm). Fig. 2 and our previous work show that the CMD increases by the 9ltering action of a 4u8y bed of soot particles. Therefore, the lower value of the CMD for oil-soaked impactors indicates that the e8ect of 9ltration is smaller than with standard impactors. Probably, the 4u8y bed of particles is eliminated completely because the particles are drawn into the oily phase. An argument in favour of this conclusion is that the conical moulds of particles are also drawn into the oily phase. The results show that the ELPI impactor can be used safely for 9000 s with the oil-soaked sintered impactors, which is equivalent to a total soot load in the ELPI of about 750 g, as opposed to 15 g with the original 4at impactors. This is a 50-fold increase, clearly a great improvement. A diPculty with using oil-soaked sintered impactors might be that the gas 4ow from the impactor nozzles could penetrate into the porous structure of the sintered metal. Recent work by Huang, Tsai, and Shih (2000) con9rmed this hypothesis. This phenomenon is thought to be incorporated in the calibration for the oil-soaked sintered impactor stages (lower cut-o8 sizes, see Table 1). Thus, it does not interfere with our results. Furthermore, the gas in not expected to penetrate deeply into the sintered metal because the pores are that small that the oil will 9ll the pores due to capillary forces. The experimental evidence in Fig. 3 shows that the measured signals do not change during the experiment. If 9ltration had been active a dynamic behaviour, such as encountered in Fig. 2, is expected. Rapid overloading is related to the 4u8y or fractal-like structure of the diesel soot particles (Van Gulijk et al., 2001). Thus, oil-soaked sintered impactors eliminate overloading of fractal-like agglomerate particles. This includes aerosols from diesel engines, aerosols from combustion processes, and other aerosols that are known to consist of agglomerates.
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C. van Gulijk et al. / Aerosol Science 34 (2003) 635 – 640
6. Conclusions The tests with oil-soaked impactors lead to a clear conclusion: oil-soaked sintered metal impactors must be used to measure particle size distributions of diesel soot with the ELPI. The CMD is stable during 9000 s with a 1:3 nm variance on a mean CMD of 57 nm. This indicates that the repeatability of the experiments is very good. Clearly, the concept of wetting multiple layers of particles by oil-soaked sintered impactors by Reischl and John (1978) is very useful for diesel soot measurements with the ELPI. Acknowledgements The authors gratefully acknowledge Dekati Ltd in Finland for the supply of sintered collectors. This work was funded by the Technology Foundation STW under grant number 349-3567. References Ahlvik, P., Ntziachrostos, L., Keskinen, J., & Virtanen, A. (1998). Real time measurements of diesel particle size distribution with an electrical low pressure impactor. SAE 980410. Andrews, G. E., Clarke, A. G., Rojas, N. J., Sale, T., & Gregory, D. (2001). Diesel particulate size distribution: The conversion of particle number size distribution to mass distribution. SAE 2001-01-1946. Hinds, W. C. (1999). Aerosol Technology. New York: Wiley. Huang, C. H., Tsai, C. J., & Shih, T. S. (2000). Excess particle collection ePciency by a porous-metal substrate during inertial impaction process. Journal of Aerosol Science, 31(Suppl. 1), S130–S131. Keskinen, J., Pietarinen, K., & Lehtm?aki, M. (1992). Electrical low pressure impactor. Journal of Aerosol Science, 23, 353–360. Lundgren, D. A. (1967). An aerosol sampler for determination of particle concentration as a function of size and time. Journal of the Air Pollution Control Association, 17, 225–229. Maricq, M. M., Podsiadlik, D. H., & Chase, R. E. (2000). Size distributions of motor vehicle exhaust pm: A comparison between ELPI and SMPS measurements. Aerosol Science and Technology, 33, 239–260. Marjam?aki, M., Keskinen, J., Chen, D., & Pui, D. Y. H. (2000a). Performance evaluation of the electrical low-pressure impactor (ELPI). Journal of Aerosol Science, 31, 249–261. Marjam?aki, M., Ristim?aki, J., Virtanen, A., Moisio, M., Luoma, R., & Keskinen, J. (2000b). Testing porous metal as a collection substrate in ELPI. Journal of Aerosol Science, 31(Suppl. 1), S76–S77. Marple, V. A., Rubow, K. L., & Behm, S. M. (1991). A mircoori9ce uniform deposit impactor (MOUDI): Description, calibration, and use. Aerosol Science and Technology, 14, 434–446. Pattas, K., Kyriakis, N., Samaras, Z., Pistikopoulos, P., & Ntziachristos, L. (1998). E9ect of DPF on particulate size distribution using an electrical low pressure impactor. SAE 980544. Reischl, G. P., & John, W. (1978). The collection ePciency of impaction surfaces: A new impaction surface. Staub-Reinhaltung Der Luft., 38, 55. Sioutas, C., Chang, M., Kim, S., Koutrakis, P., & Ferguson, S. T. (1999). Design and experimental characterization of a PM 1 and a PM 2.5 personal sampler. Journal of Aerosol Science, 30, 693–707. Van Gulijk, C., Schouten, H., Marijnissen, J., Makkee, M., & Moulijn, J. A. (2001). Restriction for the ELPI for diesel soot measurements. Journal of Aerosol Science, 32, 1117–1130.
Technical note
Oil-soaked sintered impactors for the ELPI in diesel particulate measurements C. van Gulijk∗ , J.C.M. Marijnissen, M. Makkee, J.A. Moulijn DelftChemTech, Delft University of Technology, Julianalaan 136, NL-2628BL Delft, The Netherlands Received 3 June 2002; received in revised form 2 December 2002; accepted 3 December 2002
Abstract Diesel soot overloads the ELPI-impactor rapidly if it is equipped with the standard 4at-surface impactors. This non-ideal behaviour was studied recently (J. Aerosol Sci. 32 (2001) 1117). It was found that rapid overloading, or surface build-up, is a result of a 4u8y bed of soot particles that covers the impactor surfaces and starts to 9lter airborne soot particles. This paper reports additional results with oil-soaked sintered impactors for the ELPI. It is demonstrated that (rapid) overloading is eliminated with oil-soaked sintered impactors. The maximum allowed mass load for the ELPI impactor is increased 50-fold. ? 2003 Elsevier Science Ltd. All rights reserved. Keywords: Diesel soot; ELPI; Instrument evaluation
1. Introduction The electrical low-pressure impactor (ELPI: Keskinen, Pietarinen, & Lehtmaki, 1992; Marjam?aki, Keskinen, Chen, & Pui, 2000a) gains popularity as an aerosol particle-sizer in automotive engineering. An increasing number of workers report their 9ndings on ELPI measurements: e.g. Ahlvik, Ntziachrostos, Keskinen, and Virtanen (1998); Pattas, Kyriakis, Samaras, Pistikopoulos, and Ntziachristos (1998); Maricq, Podsiadlik, and Chase (2000); Andrews, Clarke, Rojas, Sale, and Gregory (2001). Van Gulijk, Schouten, Marijnissen, Makkee, and Moulijn (2001) reports on impactor overloading, which poses a severe restriction for the measurement of size and numbers of a diluted diesel soot aerosol if the ELPI is used in standard con9guration (viz. with 4at impaction surfaces). Oil-soaked sintered impactor stages supplied by the ELPI-manufacturer (Dekati Ltd., Finland) may be a promising solution to circumvent impactor overloading of the ELPI ∗
Corresponding author. Tel.: +31-15-284-3605. E-mail address: [email protected] (C. van Gulijk).
0021-8502/03/$ - see front matter ? 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0021-8502(02)00212-4
636
C. van Gulijk et al. / Aerosol Science 34 (2003) 635 – 640
(Marjam?aki et al., 2000b). In this paper these oil-soaked sintered impactor stages are evaluated for the measurement of diesel soot aerosols. The results demonstrate that impactor overloading can be eliminated. 2. Theoretical background The problem with the measurement of a diluted diesel soot aerosol is that overloading or surface build-up occurs with the ELPI in the standard con9guration (Van Gulijk et al., 2001), resulting in dynamic signals which lead to incorrect size distribution and an incorrect number measurement. Overloading occurs at very low mass loads due to the fractal-like structure of dry diesel soot. A 4u8y bed of fractal-like soot particles covers the impactor surface in 60 s and it starts to 9lter soot particles. Thus, for a more or less constant aerosol 4ow, the measured particle size distribution changes over time. Because the signals from the ELPI changed very fast the maximum time of operation appeared to be 180 s (total mass load 15 g). After operation times of about 1800 s, conical moulds of particles form on the impactor surface. This marker for impactor overloading is generally accepted and comprehensively described by Hinds (1999). The problem of overloading has been acknowledged in the past by the development of special impactors that are less prone to overloading e.g. the MOUDI impactor (Marple, Rubow, & Behm, 1991) and the Lundgren impactor (Lundgren, 1967). Another relevant development was the introduction of oil-soaked sintered impactors (Reischl & John, 1978). These enable higher particle loads on the impactor stages by wetting multiple layers of particles. According to Sioutas, Chang, Kim, Koutrakis, and Ferguson (1999) particle bounce is also reduced by using oil-soaked sintered impactor stages. The manufacturer of the ELPI (Dekati Ltd., Tampere, Finland) has also acknowledged the problem of overloading, and therefore, they have produced special ELPI-impactor stages viz. sintered-stainless steel impactors that can be 9lled with oil to give oil-soaked sintered impactors. Sintered metal impactors having average pore sizes of 5, 20, and 40 m are available (Marjam?aki et al., 2000b). In this work the 40 m sintered-steel impactor stages were tested. The use of oil-soaked sintered impactor stages changes the cut-o8 sizes of the ELPI impactor. This is due to the fact that the surface characteristics of the sintered metal are di8erent from smooth aluminium foils. Table 1 shows the cut-o8 sizes for the ELPI system as supplied by the manufacturer (calibration by the manufacturer). Fig. 1 shows a standard impactor stage with aluminium foils (left) along with a sintered impactor stage (right). 3. Experimental The experimental set-up has been described extensively by Van Gulijk et al. (2001) so only a brief description is given here. The set-up consisted of the following parts: a small diesel engine that operated at a constant engine load of 50% which functioned adequately as a reproducible soot generator (LPW-2; 900 cm3 ; 5:7 kW by Lyster–Petter); a 9lter test bench in which the exhaust 4ow-rate was controlled with a membrane pump (PM 15368-0.15.0 by NKF-Verder); a two-step aerosol diluter system (Dekati diluters by Dekati Ltd.) that sampled the aerosol in the 9lter test bench; and the ELPI that measured the diluted diesel soot aerosol (ELPI-10 by Dekati Ltd.). Two types of ELPI-impactors were tested in measurements of 9000 s (2:5 h): aluminium foils coated with
C. van Gulijk et al. / Aerosol Science 34 (2003) 635 – 640
637
Table 1 Manufacturer calibration of cut-o8 sizes for the ELPI impactor in standard con9guration and with oil-soaked sintered impactors Stage number
Standard con9guration impactor cut-o8 size/(nm)
Sintered metal impactors impactor cut-o8 size/(nm)
Pre-impactor 1 2 3 4 5 6 7 8 9 10 11 12
10:3 × 103 6:79 × 103 4:03 × 103 2:49 × 103 1:63 × 103 1:01 × 103 647 399 259 169 105 60 30
9:75 × 103 6:43 × 103 3:82 × 103 2:36 × 103 1:54 × 103 956 612 333 210 106 56.0 32.9 25.3
Source: ELPI calibration sheet, Dekati Ltd., Finland.
Fig. 1. Impactor stage from ELPI standard con9guration (left), and oil-soaked sintered impactor stage (right).
Apiezon L vacuum grease (ELPI standard con9guration); and oil-soaked sintered impactors, these ? P3, by were the 40 m sintered-steel impactor stages impregnated with 3–5 drops of vacuum oil (Ol Pfei8er). Before starting the experiments, the set-up including the ELPI was allowed to heat up and
638
C. van Gulijk et al. / Aerosol Science 34 (2003) 635 – 640
stabilise for 1 h to avoid start-up e8ects. The ELPI was in 4ush mode during this time i.e. the ELPI reverses the air4ow in the inlet nozzle, thus ensuring that no particles enter the ELPI. By stopping the 4ush mode the experiments were started. Data from the ELPI were stored with a frequency of 1 Hz on a personal computer. The measured particle size distributions were averaged over 35 data points. The average variance in the measured aerosol concentration for each individual size-range of the ELPI was 2.4%; the largest variance was 5.9%. This indicates a good repeatability for the measurements.
4. Results Fig. 2 shows the measured particle size distribution for the ELPI in the standard con9guration as a log-probability plot. Fig. 3 shows the measured particle size distribution with oil-soaked sintered collectors. The count median diameter (CMD) is used as a tool for comparison of the results (Hinds, 1999). It is clear that the measured particle size distribution changes as a function of time with the ELPI in standard con9guration (Fig. 2). At the start of the measurement (60 s) the CMD was 66 nm, after 900 s it changed to 77 nm, after 3600 s it changed to 88 nm, and after 9000 s (2:5 h) it reached a value of 101 nm. Fig. 3 shows steady-state behaviour for the oil-soaked sintered impactors: the particle size distributions are also shown at 60, 900, 3600, and 9000 s but they overlap. The CMD was determined by averaging the CMDs at 60, 120, 180, 900, 1800, 3600, and 9000 s, which gives a value of 56:5 nm with a standard deviation of only 1:3 nm! The measured particle size distribution does not change during the entire measurement. 1000
60 s 900 s 3600 s 9000 s
aerodynamic diameter/[nm]
aerodynamic diameter/[nm]
1000
100
10
60 s 900 s 3600 s 9000 s
100
10 20
50
80 90 95
99
99.9
cumulative count fraction/[%] Fig. 2. Log-probability plot of the particle size distribution as a function of time with Apiezon L on aluminium foils (smooth surfaces). The measurement does not give stable results for a constant aerosol 4ow.
20
50
80 90 95
99
99.9
cumulative fraction/[%] Fig. 3. Log-probability plot of the particle size distribution as a function of time with oil-soaked sintered impactors. The results are stable.
C. van Gulijk et al. / Aerosol Science 34 (2003) 635 – 640
639
5. Discussion The dynamic behaviour that is found with the ELPI in standard con9guration is due to overloading. In this case, overloading falls apart in two phenomena. The 9rst phenomenon is rather well known: conical moulds form on the impactor surface. The conical moulds change the gas-4ow on the impactor and thereby change the cut-o8 size of the impactor. This phenomenon takes e8ect after a “long” operation time of the impactor. The second phenomenon is less well known: a very 9ne layer of 4u8y soot particles on the impactor surface 9lter the particle-laden gas stream that 4ows over the impactor. Thus, particles that are very small deposit on impactor stages that should only trap larger particles. This phenomenon takes e8ect almost immediately after starting an experiment (Van Gulijk et al., 2001). When the ELPI is equipped with oil-soaked sintered impactor stages no dynamic e8ects are observed, which indicates that overloading is eliminated. The following discussion gives insight. Capillary forces of the oil act on the particles: the oil wets the particles and draws them into the oily phase. The oil reservoir is large enough to wet many layers of particles so that the formation of conical moulds is postponed for at least 9000 s with the current set-up. The hypothesis was visually con9rmed: no conical moulds, which mark impactor overloading, were formed on the surface after 9000 s. The elimination of a 4u8y bed of particles on the impactor surface is more diPcult to prove: the samples could not be placed in the electron microscope. Instead, the CMD is used as an indicator. The value of the CMD that is found after 60 s is lower for oil-soaked sintered collectors (57 nm) than the value found for aluminium foils coated with Apiezon L vacuum grease (66 nm). Fig. 2 and our previous work show that the CMD increases by the 9ltering action of a 4u8y bed of soot particles. Therefore, the lower value of the CMD for oil-soaked impactors indicates that the e8ect of 9ltration is smaller than with standard impactors. Probably, the 4u8y bed of particles is eliminated completely because the particles are drawn into the oily phase. An argument in favour of this conclusion is that the conical moulds of particles are also drawn into the oily phase. The results show that the ELPI impactor can be used safely for 9000 s with the oil-soaked sintered impactors, which is equivalent to a total soot load in the ELPI of about 750 g, as opposed to 15 g with the original 4at impactors. This is a 50-fold increase, clearly a great improvement. A diPculty with using oil-soaked sintered impactors might be that the gas 4ow from the impactor nozzles could penetrate into the porous structure of the sintered metal. Recent work by Huang, Tsai, and Shih (2000) con9rmed this hypothesis. This phenomenon is thought to be incorporated in the calibration for the oil-soaked sintered impactor stages (lower cut-o8 sizes, see Table 1). Thus, it does not interfere with our results. Furthermore, the gas in not expected to penetrate deeply into the sintered metal because the pores are that small that the oil will 9ll the pores due to capillary forces. The experimental evidence in Fig. 3 shows that the measured signals do not change during the experiment. If 9ltration had been active a dynamic behaviour, such as encountered in Fig. 2, is expected. Rapid overloading is related to the 4u8y or fractal-like structure of the diesel soot particles (Van Gulijk et al., 2001). Thus, oil-soaked sintered impactors eliminate overloading of fractal-like agglomerate particles. This includes aerosols from diesel engines, aerosols from combustion processes, and other aerosols that are known to consist of agglomerates.
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6. Conclusions The tests with oil-soaked impactors lead to a clear conclusion: oil-soaked sintered metal impactors must be used to measure particle size distributions of diesel soot with the ELPI. The CMD is stable during 9000 s with a 1:3 nm variance on a mean CMD of 57 nm. This indicates that the repeatability of the experiments is very good. Clearly, the concept of wetting multiple layers of particles by oil-soaked sintered impactors by Reischl and John (1978) is very useful for diesel soot measurements with the ELPI. Acknowledgements The authors gratefully acknowledge Dekati Ltd in Finland for the supply of sintered collectors. This work was funded by the Technology Foundation STW under grant number 349-3567. References Ahlvik, P., Ntziachrostos, L., Keskinen, J., & Virtanen, A. (1998). Real time measurements of diesel particle size distribution with an electrical low pressure impactor. SAE 980410. Andrews, G. E., Clarke, A. G., Rojas, N. J., Sale, T., & Gregory, D. (2001). Diesel particulate size distribution: The conversion of particle number size distribution to mass distribution. SAE 2001-01-1946. Hinds, W. C. (1999). Aerosol Technology. New York: Wiley. Huang, C. H., Tsai, C. J., & Shih, T. S. (2000). Excess particle collection ePciency by a porous-metal substrate during inertial impaction process. Journal of Aerosol Science, 31(Suppl. 1), S130–S131. Keskinen, J., Pietarinen, K., & Lehtm?aki, M. (1992). Electrical low pressure impactor. Journal of Aerosol Science, 23, 353–360. Lundgren, D. A. (1967). An aerosol sampler for determination of particle concentration as a function of size and time. Journal of the Air Pollution Control Association, 17, 225–229. Maricq, M. M., Podsiadlik, D. H., & Chase, R. E. (2000). Size distributions of motor vehicle exhaust pm: A comparison between ELPI and SMPS measurements. Aerosol Science and Technology, 33, 239–260. Marjam?aki, M., Keskinen, J., Chen, D., & Pui, D. Y. H. (2000a). Performance evaluation of the electrical low-pressure impactor (ELPI). Journal of Aerosol Science, 31, 249–261. Marjam?aki, M., Ristim?aki, J., Virtanen, A., Moisio, M., Luoma, R., & Keskinen, J. (2000b). Testing porous metal as a collection substrate in ELPI. Journal of Aerosol Science, 31(Suppl. 1), S76–S77. Marple, V. A., Rubow, K. L., & Behm, S. M. (1991). A mircoori9ce uniform deposit impactor (MOUDI): Description, calibration, and use. Aerosol Science and Technology, 14, 434–446. Pattas, K., Kyriakis, N., Samaras, Z., Pistikopoulos, P., & Ntziachristos, L. (1998). E9ect of DPF on particulate size distribution using an electrical low pressure impactor. SAE 980544. Reischl, G. P., & John, W. (1978). The collection ePciency of impaction surfaces: A new impaction surface. Staub-Reinhaltung Der Luft., 38, 55. Sioutas, C., Chang, M., Kim, S., Koutrakis, P., & Ferguson, S. T. (1999). Design and experimental characterization of a PM 1 and a PM 2.5 personal sampler. Journal of Aerosol Science, 30, 693–707. Van Gulijk, C., Schouten, H., Marijnissen, J., Makkee, M., & Moulijn, J. A. (2001). Restriction for the ELPI for diesel soot measurements. Journal of Aerosol Science, 32, 1117–1130.