Exhaust Gas Particle Mass Estimation Using an Electrical Low

Effect of the valve timing and the coolant temperature on particulate emissions from a gasoline direct-injection engine fuelled with gasoline and with...
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Energy & Fuels 2006, 20, 498-503

Exhaust Gas Particle Mass Estimation Using an Electrical Low Pressure Impactor E. Zervas,*,† P. Dorlhe`ne,† L. Forti,‡ C. Perrin,‡ J. C. Momique,§ R. Monier,§ H. Ing,| and B. Lopez| Renault, 1 Alle´ e Cornuel, 91510 Lardy, France, Institut Franc¸ ais du Pe´ trole (IFP), 1 et 4 AVenue du Bois Pre´ au, 92500 Rueil-Malmaison, France, PSA Peugeot Citro¨en, 18 rue des FauVelles, 92250 La Garenne-Colombes, France, and UTAC, Autodrome de Linas-Montlhe´ ry, 91310 Montlhe´ ry, France ReceiVed October 6, 2005. ReVised Manuscript ReceiVed December 12, 2005

The electrical low-pressure impactor (ELPI) is employed to measure the particle number and size distribution of the exhaust gas of internal combustion engines. If appropriate values of particle density are available, the emitted particle mass may be estimated with this method. For this work, three Euro3 passenger cars (one gasoline car operating under stoichiometric conditions, one diesel car, and one diesel car equipped with a diesel particulate filter, DPF) were tested on the New European Driving Cycle. The exhaust particles were measured using the current European regulations (gravimetric method) and estimated from the ELPI particle number and size distribution. Different particle density values were used to estimate the particle mass using all ELPI stages or only some of them. The results show that the particle mass estimated with ELPI is quite well correlated with the mass collected on filters in the case of a Euro3 diesel passenger car without a diesel particulate filter; however, this method fails to estimate the particle mass emitted from gasoline or DPFequipped diesel passenger cars.

Introduction Current European regulations are based on a gravimetric measurement of the particles emitted from diesel passenger cars.1 However, as the exhaust emission levels decrease, this method reaches its limits and other methods, based on particle number measurements are proposed to replace it. Currently, there are many methods for particle number and size determination. The most common methods used for vehicle exhaust gas are the electrical low-pressure impactor2 (ELPI), the scanning mobility particle sizer3 (SMPS), and particle counters,4 but many others are also presented. Burtscher5 and Mohr6 give a detailed description of several analytical methods. It must be noted that SMPS cannot be used for the analysis of particle distribution on the New European Driving Cycle (NEDC) because it has an insufficient resolution time (several minutes). Particle counters can determine total particle number but not the size distribution. ELPI has a sufficient time resolution to follow the * To whom correspondence should be addressed. E-mail: [email protected]. Phone: +331-76 87 84 77. Fax: +331-76 87 82 92. † Renault. ‡ Institut Franc ¸ ais du Pe´trole (IFP). § PSA Peugeot Citro ¨ en. | UTAC. (1) Directive 70/220, www.europa.eu.int. (2) Keskinen, J.; Pietarinen, K.; Lehtimaki, M. J. Aerosol Sci. 1992, 23, 353-360. (3) Wang, S. C.; Flagan, R. C. Aerosol Sci. Technol. 1990, 13, 230240. (4) Hinds, W. C. Aerosol Technology; J. Wiley and Sons: New York, 1999. (5) Burtscher, H. J. Aerosol Sci. 2005, 36, 896-932. (6) Mohr, M.; Lehmann, U. Comparison Study of Particle Measurement Systems for Future Type ApproVal Application; Research Report No. 202779; EMPA: Du¨bendorf, Switzerland, 2003; http://www.empa.ch/plugin/template/empa/23/ 20988?tfid)23&detail)1&det_tfid)23&newsearch)0.

NEDC; however, its size resolution is lower than the resolution of SMPS. The principle of the ELPI is described in the literature.2 This device is used to determine particle number and distribution in vehicles’ exhaust gas.7,8,9,10,11 A detailed review of previous works using ELPI to measure exhaust particle number and distribution is presented in a previous work.11 Particle mass can be estimated using the particle size distribution of ELPI, if the particle density is known.7,9 A comparison with the mass collected on filters and the mass estimated by the ELPI was sometimes performed; however, these studies used quite old vehicles with relatively high particulate mass emissions.8,12 These studies showed that the mass estimated by ELPI is higher than the mass collected on filters. The estimated particle mass depends on the values of particle density used. The values reported in the literature are very scattered: an average density of 1.0 g/cm,3,8 1.7 g/cm,3,13 or 0.5 g/cm3,14 has been proposed. But, in the case of diesel exhaust gas, particle density is a function of size.7,12,14 It is generally assumed that nucleation particles are spherical and that their diameter determines the particle density; however, the bigger particles contain several nucleation particles and thus (7) Ahlvik, P.; Ntziachristos, L.; Keskinen, J.; Virtanen, A. SAE Tech. Pap. Ser. 1998, 980410. (8) Shi, J. P.; Harrison, R. M.; Brear, F. Sci. Total EnViron. 1999, 235, 305-307. (9) Maricq, M. M.; Podsiadlik, D. H.; Chase, R. E. Aerosol Sci. Technol. 2000, 33, 239-260. (10) Zervas, E.; Dorlhe`ne, P.; Daviau, R.; Dionnet, B. SAE Tech. Pap. Ser. 2004, 2004-01-1983. (11) Zervas, E.; Dorlhe`ne, P.; Forti, L.; Perrin, C.; Momique, J. C.; Monier, R.; Ing, H.; Lopez, B. Aerosol Sci. Technol. 2005, 39, 333-346. (12) Andrews, G. E.; Clarke, A. G.; Rojas, N. Y.; Sale, T.; Gregory, D. SAE Tech. Pap. Ser. 2001, 2001-01-1946. (13) Ulfvarson, U.; Figler, B.; Krantz, S. SAE Tech. Pap. Ser. 1997, 970759. (14) Witze, P. O.; Chase, R. E.; Maricq, M. M.; Podsiadlik, D. H.; Xu, N. SAE Tech. Pap. Ser. 2004, 2004-01-0964.

10.1021/ef050330a CCC: $33.50 © 2006 American Chemical Society Published on Web 01/13/2006

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Table 1. Main Characteristics of the Three Passenger Cars Useda

fuel type inertia class (kg) displacement (L) no. of cylinders valves/cylinder injection system emission limits after-treatment device

vehicle 1

vehicle 2

vehicle 3

diesel Peugeot 307 1360 2.0 4 2 Common Rail Euro3 DOC

diesel Peugeot 307 1360 2.0 4 4 Common Rail Euro3 DOC + DPF

gasoline Renault Me´gane 1130 1.6 4 4 MPI Euro3 TWC

a MPI: multipoint injection. DOC: diesel oxidation catalyst. TWC: three-way catalyst.

have much lower values of density because of empty space.7 Values of 1.0 g/cm3 at 50 nm and 0.3 g/cm3 at 300 nm,14 1.50.2 g/cm,3,12 1.2-0.3 g/cm,3,15 or 1.6-0.2 g/cm3 7have been suggested. A recent article16 presented a review of several aspects of particle density and morphology. Van Gulijk et al.17,18 presented a list of nonideal behavior of the ELPI, such as particle bounce, wall or inter-stage loss, overloading or surface built up, and losses caused by electrostatic effects and charger nonideal efficiency. It was suggested that the ELPI overestimates particle numbers, especially for particles bigger than 1 µm.12 Another critical point is the repeatability of ELPI,11 which is very poor at the upper stages because of very low numbers. For the above reasons, Witze et al.14 used only the 1-6 ELPI stages and an empirical method to adjust the particle density to obtain the same mass as on filters. In this last work, an average density of 0.5 g/cm3 was estimated. In this article, the ELPI is used to estimate the exhaust particle mass on the NEDC using the particle number and distribution of three Euro3 passenger cars (PC): a gasoline vehicle and two diesel vehicles, one with and one without a diesel particulate filter (DPF). The target is to find out if ELPI can be used to estimate the exhaust particulate mass, especially the particulate mass of the low-emitting vehicles (gasoline and diesel with DPF), using the particle density presented in the literature or empirically correlated values. Four laboratories participated: Institut Franc¸ ais du Pe´trole (IFP), PSA Peugeot-Citro¨en, Renault, and Union Technique de l′Automobile, du Motocycle et du Cycle (UTAC). The estimated mass is obtained using different assumptions and compared with the particle mass collected on filters using the current European regulatory gravimetric method. Experimental Section Vehicles and Test Conditions Used. Three Euro3 PCs were used in this study: a diesel PC (vehicle 1), a diesel PC equipped with a DPF (vehicle 2), and a gasoline PC operating under stoichiometric conditions (vehicle 3). Each vehicle is tested in every laboratory. Table 1 provides their main characteristics. These vehicles had a mileage of less than 10 000 km. Low sulfur content fuels (less than 10 ppm) were used in this study. The main characteristics of these fuels are presented elsewhere.11 The same lubricant was used for all three vehicles: it contains less than 0.4% sulfur. Twenty-five parts per million of a commercially used Cebased additive were added to the fuel in the case of the DPFequipped vehicle to decrease the necessary temperature for the DPF regeneration. (15) Maricq, M. M.; Xu, N. Aerosol Sci. Technol. 2004, 35, 1251-1274. (16) DeCarlo, P. F.; Slowik, J. G.; Worsnop, D. R.; Davidovits, P.; Jimenez, J. L. Aerosol Sci. Technol. 2004, 38, 1185-1205. (17) Van Gulijk, C.; Marijnissen, J. C. M.; Makkee, M.; Moulijn, J. A. J. Aerosol Sci. 2001, 32, 1117-1130. (18) Van Gulijk, C.; Marijnissen, J. C. M.; Makkee, M.; Moulijn, J. A.; Schmidt-Ott, A. J. Aerosol Sci. 2004, 35, 633-655.

Table 2. Lower, Intermediate, and Upper Diameters of Each ELPI Stage stage number

DL (nm)

DI (nm)

DU (nm)

1 2 3 4 5 6 7 8 9 10 11 12

7 28 53 91 155 266 382 614 949 1066 2390 4000

14 38 69 119 203 319 484 763 1006 1596 3092 6299

28 53 91 155 266 382 614 949 1066 2390 4000 9920

The tests were performed on the NEDC and regulated pollutants, as well as CO2, were measured according to current European regulations.1 The experimental procedure used was a cold NEDC followed by an extra urban driving cycle (EUDC, which was not taken into account in these calculations), 24 h of conditioning at 20 °C, and three cold NEDCs with a conditioning of 24 h between each cycle. In the case of the DPF-equipped Diesel PC, a DPF regeneration was performed before these cycles, to have the same particulate mass (PM) load on the DPF and consequently the same conditions. The constant flow sampler (CVS) flow was 9 m3/min for the gasoline vehicle and 12 m3/min for the two diesel ones. The flow through the filters was 30 L/min. Temperature in the weighing chambers was 21 ( 2, 21 ( 1, 25 ( 5, and 22 ( 2 °C, respectively, for Lab1, 2, 3, and 4, while the humidity was 55 ( 5, 50 ( 5, 45 ( 10, and 50 ( 10, respectively. ELPI Configuration. Number and size distribution measurements were performed using a DEKATI ELPI with greased aluminum foils and a 47 mm Pallflex TX40 filter stage, covering particle cut sizes from 7 nm to 10 µm (Table 2). The ELPI flow rate was 10 L/min in three labs and 20 L/min in the last lab. As the use of a thermodenuder can induce high particle losses,10 a DEKATI ejector-type dilutor heated at 130 °C with hot nitrogen was used to dilute the exhaust gas and minimize the nucleation particles. A dilution rate of 10 was used, while the flow of the exhaust gas before the diluter was 5 L/min. The total length of the antistatic tube, used to connect the sample probe installed on the dilution tunnel and the ELPI, was less than 3 m. An electric zero at every ELPI stage was performed automatically before each test, using purified air. It must be noted that in several tests, especially those of the gasoline and diesel with DPF vehicles, the number of large particles were very close to ELPI detection limits.11 Procedure Used to Correlate the Particle Mass Estimated by ELPI and the Mass Collected on Filters. To correlate the particle mass estimated by the ELPI and the mass collected on filters, the following formula is used k

me )

∑ NVF

i i i

i)j

where me is the total estimated particulate mass, Ni is the particle number of the i ELPI stage, Vi is the particle volume of the i ELPI stage, Fi is the particle density of the i ELPI stage, and j and k are the initial and final ELPI stages taken into consideration. All particles are considered to be spherical using the ELPI diameter as the physical diameter of the sphere. The term “particle density” used in this study is defined as the density of a spherical particle. This term is also defined as “effective density”.16 Different numbers of ELPI stages were taken into account: all ELPI stages (stages 1 to 12), only the first 7 stages (stages 1 to 7), and all ELPI stages except for 1st stage (stages 2-12). The second configuration was tested because the particle number of the stages 1-7 corresponds to 99.99-99.999% of the total particle number. The third configuration was tested because the 1st ELPI stage has an important particle number; however, the corresponding particle mass is very low, because of the very small diameter.

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Results and Discussion

Figure 1. Initial and simulated density distributions used in this study, when the same particle density is assumed for the three types of vehicles: Initial, initial values; ALL, simulated values using all ELPI stages; 1-7, simulated values using the first 7 ELPI stages; L, lower; I, intermediate; U, upper ELPI diameter.

To calculate the volume of each particle, three diameters were used: the lower diameter of each ELPI stage (DL), the intermediate diameter (DI), and the upper one (DU, Table 2). DI is defined as DI ) xDLDU On the basis of the literature data, four values of particle density distributions were tested. The first one uses a density of 1.0 g/cm3, which is constant with the particle size. The density of the other three distributions decreases with the particle size because the smaller particles are more spherical and consequently have a higher density, while the larger particles are aggregates of small particles and thus have more internal empty space. Three such types of density distribution were used: one depending linearly on size (1.2 g/cm3 at the first ELPI stage and 0.1 g/cm3 at the 12th) one depending linearly on the log of the particle diameter, and the values proposed by Ahlvik at al. (1998). Figure 1 shows the density distributions used. These values are named “initial density values”. However, two other density distributions were used. These distributions were based on the four previous density distributions, multiplied by a coefficient R (the same for each ELPI stage) to adjust the estimated ELPI mass values to the experimental ones. The R is estimated with a least-squares minimization method using all experimental points: ∑(mm - a∑i)jkNiViFi)2 ) min (with mm being the measured particulate mass). In the first case, as proposed in the literature, diesel and gasoline exhaust particles have the same density15 (however, a direct injection spark ignition engine was used in this study), this coefficient is the same for the three types of vehicles. In the second case, this coefficient is different for each type of vehicle and is named RD, RD1, and RG for the diesel, diesel with DPF, and gasoline vehicles, respectively. Thus, eight other density distributions were examined (Figures 1 and 2). These density values are named “simulated density values”. The estimated/ measured Values ratio is calculated and used to compare each case. A correlation between the particle number and particulate mass of each ELPI stage is searched, but unfortunately, the measurement of the particulate mass of each ELPI stage in the case of the NEDC experiments could not be performed with accuracy. The aluminum foils were often partially destroyed when they were extracted from the impactor, and their weighing was not repeatable because of the very low collected PM mass. For this reason, only the total PM mass over the entire NEDC was taken into consideration, and the correlation between the PM mass and number of each ELPI stage is not attempted.

Regulated Pollutants and Particle Number Emissions. The emission of regulated pollutants and particle number of these three vehicles and their reproducibility and repeatability values is presented elsewhere.11 The definitions of reproducibility and repeatability are given in this last work.11 The average PM emissions were 0.0010 g/km for the gasoline PC, 0.0284 g/km for the diesel PC, and 0.0008 g/km for the diesel PC equipped with DPF. For these three vehicles, the corresponding reproducibility values were 67, 29, and 164%, respectively, and the corresponding repeatability values were 60, 12.7, and 191%, respectively. The average particle numbers were 1.3 × 1012 km-1 for the gasoline passenger car, 1.3 × 1014 km-1 for the diesel PC, and 1.8 × 1011 km-1 for the diesel PC equipped with DPF. The reproducibility values were 59, 47, and 131%, respectively, and the corresponding repeatability values were 45, 14, and 96%. The average masses collected on the filters were 0.025, 0.7, and 0.021 mg, respectively, for the gasoline, diesel and diesel with DPF PCs. Estimation of PM Mass using ELPI. Number and Mass Distributions. Figure 3 shows, for the three vehicles used in this study, the number emitted and mass distributions estimated using different density values (“initial” and “simulated”). The PM mass of the four initial distributions follow the order AhlVik > linear log ≈ linear > constant. The log and linear particle density distributions give quite similar results. It must be noted that all simulated values give quite similar results, for this reason only one curve is shown in this figure. The simulated density distributions give lower particle masses than the initial ones. For the three vehicles used, the PM masses of the last ELPI stages are higher than the PM masses of the first ones, because of the bigger particles (even if the number of the last stage is much lower than the number of the first one). The mass distribution of the diesel and the gasoline vehicles presents one intermediate peak at the 5th ELPI stage (155-266 nm), while the mass distribution of the diesel with DPF vehicle is quite linear with PM size. It must be noted that 60-90% of the PM mass is found at the last three (10-12) ELPI stages. Initial Density Values. Figure 4 shows the estimated versus measured PM mass for the four types of initial density distributions when all and 1-7 ELPI stages are taken into consideration. A particular point must be noted. The results of all ELPI stages or of stages 2-12 are almost identical for all cases. This is the result of the very low contribution of the first stage to the total particle mass because of the very low particle diameter. For this reason, the results of the 2-12 ELPI stages are not presented in this figure and the rest of this study. This figure shows that the use of all ELPI stages overestimates the particle mass in all cases examined. These overestimations are, using average values, 4.8, 5.8, 5.8, and 7.6 times for the constant, linear, linear log, and Ahlvik diameter distributions, respectively, in the case of lower diameter, 14, 16.8, 16.8, and 22 times for the four types of density distribution in the case of intermediate diameter, and 45, 54, 54, and 70 times in the case of upper diameter. The results of the 1-7 ELPI stages are closer to the line y ) x, but more complex. The use of the lower diameter underestimates the emitted particle mass by 40, 28, 28, and 6% for the constant, linear, linear log, and Ahlvik density distributions, respectively. Even if the last value is quite low, Figure 4 shows an important data scattering at the low particle mass points corresponding to gasoline and diesel with DPF vehicles. The use of the intermediate diameter overestimates the particle mass by 20, 44, 44, and 90%, while the use of the upper diameter overestimates it by 2.5, 2.9, 2.9, and 3.8 times.

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Figure 2. Simulated density distributions used in this study, when different particle densities are assumed for each of the three types of vehicles. Because of large differences, the y axis of the left figure is presented in logarithmic form: D, diesel PC; D1, diesel with DPF PC; G, gasoline PC; L, lower; I, intermediate; U, upper ELPI diameter.

Figure 3. Number and mass distributions for the three vehicles used for the initial and simulated density values.

In all of these cases, the use of the linear or linear log density distribution gives quite similar results. These results suggest that a method based on those density distributions cannot be used to estimate the emitted particle mass for the three types of vehicles and other density values must be used. Simulated Density Values. Same Particle Density Values for the Three Types of Vehicles. Figure 1 shows the simulated density distributions used when it is assumed that the exhaust particle density of the three vehicles is the same. In all of these cases, the particle diameter used influences the obtained particle density following the order lower > intermediate > upper,

because a higher particle density is necessary to adjust the smaller particle size when lower diameters are taken into account. To give the same total mass, the use of the 1-7 ELPI stages requires higher density than the use of all stages, because the 1-7 stages contain only the smaller particles. The simulated particle density using all ELPI stages, assuming the same density for all three types of vehicles, is always lower than the initial values and is quite low compared to the values presented in the literature (see Introduction). The coefficient R is 0.1-0.17 for the lower diameter, 0.06-0.04 for the intermediate diameter, and only 0.013-0.020 for the upper one. At this point, it is very difficult to explain why, on NEDC, particles collected on filters are found to be much less dense than those presented in the literature. Some probable reasons are listed here: • it is suggested that the ELPI overestimates particle numbers.12,18 For this reason, lower particle density is necessary to estimate the total particle mass which corresponds to more particles than those really emitted, • Van Gulijk et al.18 suggested that ELPI underestimates the apparent size of particles. If the real particle size is bigger, lower density is necessary to estimate the particle mass collected on filters, • the ELPI charger efficiency is probably not the same for particles with different sizes18 leading to an erroneous particle distribution, • the ELPI diameter is quite different from the physical diameter of particles. The simulated particle density using the 1-7 ELPI stages is always lower than the initial values, but it is higher than those obtained using all ELPI stages. The density values of the 1-7 ELPI stages are now closer to the density values presented in the literature. For the four types of density distributions, the coefficient R is 0.4-0.62 for the lower diameter, 0.20-0.32 for the intermediate diameter, and 0.10-0.16 for the upper diameter. Witze et al.14 estimated the apparent particle density adjusting the estimated particle mass to the collected one, and he proposed an average density of 0.5 g/cm3. The same author used only the first 6 ELPI stages. In our study, using the first 7 ELPI stages, the lower ELPI diameter, and an initial particle density of 1.0 g/cm3 (constant with the particle size), we found a simulated particle density of 0.62 g/cm3, which is quite close to the value proposed by Witze et al.14

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Figure 4. Estimated versus measured values for the four initial density distributions used. Left: all ELPI stages. Right: only the first 7 ELPI stages. The points circled in red correspond to the diesel passenger car, while the points circled in blue correspond to the gasoline and diesel with DPF passenger cars.

Different Particle Density Values for Each Type of Vehicle. The second type of simulations used assumes different density values for each type of vehicle (Figure 2). Using all ELPI stages, the diesel vehicle particle density is almost identical to the density values when the density of the exhaust particles emitted from the three vehicles is assumed to be the same. This is the result of the high specific weight of the particle mass values of this vehicle in the simulations. The diesel with DPF vehicle particle density is always higher than that of the diesel one (1.7, 1.5, and 1.4 times higher for the lower, intermediate, and upper diameters for all four types of density distributions). The gasoline vehicle particle density is even higher (2.2, 1.9, and 1.7 times higher for the three diameters and for all four types of density distributions). Figure 3 shows the mean distribution of particle numbers for the three types of vehicles. It is shown that the two low particle-emitting vehicles (diesel with DPF and gasoline PCs) have a more important percentage of the number of particles at lower stages (which are more spherical and consequently have higher density than the bigger particles which are aggregates of smaller particles and have more internal void space) than the diesel vehicle. That is the reason higher particle density is necessary to estimate the collected particle mass. If only the the first 7 ELPI stages are taken into consideration, the tendencies are different. The diesel vehicle particle density is now only 60% of the density obtained when it is assumed that the particle density of all three vehicles is the same, while the particle density of the diesel with DPF and gasoline vehicles is respectively 19-20 and 9-10 times higher than that of the diesel vehicle. As the bigger particles are now eliminated from the particle distribution, the total number remains practically the same: a much higher particle density is necessary to estimate the collected particle mass in the case of diesel with DPF and gasoline vehicles. The gasoline density is now lower than that of the diesel with DPF one because the number of particles of the 1st ELPI stage, which has the higher number of particles, is lower (Figure 3). Those very high-density values are much higher than the values reported in the literature; this statement suggests that the method using the first 7 ELPI stages with different particle densities for each type of vehicle is not the best method to estimate the PM mass. Estimated Versus Measured Values. Figure 5 shows, for each of the above cases, the mean ratio between the estimated and

Figure 5. Lower bars: Mean ratio between the estimated and measured particle mass: I, initial values; SA, simulations assuming the same values for all three types of vehicles; SD, simulations assuming different density between the three types of vehicles; A, all ELPI stages; 7, the first 7 ELPI stages.

measured values of particle mass. This figure shows that these ratios follow the order upper > intermediate > lower diameters for all and 1-7 ELPI stages; the use of 1-7 ELPI stages instead of all ELPI stages always gives lower ratios, and the use of linear or linear log initial density distributions gives quite similar results. The use of simulated values when the same particle density is assumed for the three types of vehicles gives lower ratios than the initial ones. For all four density distributions, the simulated values are quite close, and the differences between the lower, intermediate, and upper diameters are not very significant. The use of the same particle density for all three types of vehicles and all ELPI stages underestimates the particle mass by 23-24%, while that of the first 7 stages underestimates it by 60%. The use of different particle density values for the three types of vehicles gives better results: these ratios are always very close to 1.0 (overestimation of 10-13% for all ELPI stages and 8-9% for the first 7 stages). According to the above results, the simulated values with different particle densities for each type of vehicle have a ratio

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Figure 6. Estimated versus measured values using lower ELPI diameter and Ahlvik density values. Left: all ELPI stages. Right: the first 7 ELPI stages.

very near 1.0. However, a closer analysis is presented in Figure 6. This figure shows the estimated versus measured values of PM mass in one of the above cases: using the lower ELPI diameter and the Ahlvik density distribution for all and the first 7 ELPI stages. This figure shows that in the case of the initial Ahlvik particle density, the estimated particle mass values are much higher than the measured mass values in the case of all ELPI stages. If only the first 7 ELPI stages are taken into account, the diesel particle mass is overestimated, and the mass of the two other vehicles is underestimated. The estimated mass values of the diesel vehicle generally fit the measured values quite well with simulated density values (the same or different for the three types of vehicles) and all or the first 7 ELPI stages. However, when the same density and all ELPI stages are used, quite an important dispersion is observed for the diesel with DPF vehicle, while the particle mass of the gasoline vehicle is overestimated. In the case of the first 7 ELPI stages, the mass of those last two vehicles is dispersed and underestimated. When different density values are used for each type of vehicle, the results between all and the first 7 ELPI stages are almost identical; in this case, the results of the two low-emitting vehicles (gasoline and diesel with DPF) are better but remain quite dispersed. Similar results are observed in the other “good” cases of Figure 5 (mean ratio near 1.0). Those results indicate that the

ELPI can be successfully used to estimate the particle mass of a Euro3 Diesel vehicle without DPF, but it generally fails to estimate the particle mass of vehicles emitting lower PM masses. Conclusions The ELPI is used to estimate the mass of the particles emitted on the NEDC from three Euro3 passenger cars. The four initial values of density distributions (constant, 1.0 g/cm3; linear and linear log, 1.2 g/cm3 at the first ELPI stage and 0.1 g/cm3 at the 12th one, and the values of Ahlvik at al.7) overestimate the particle mass if all ELPI stages are used. If only the first 7 ELPI stages are taken into consideration, these results are better, but the dispersion remains high. When all ELPI stages are used, the simulated density is lower than the values presented in the literature. In the case of the first 7 ELPI stages, these values are higher, but they still lower than the reported values. When a different particle density is simulated for each type of vehicle, the obtained results are better, indicating that each type of vehicle does not emit the same type of particles. The ELPI can be used to estimate the particle mass of a Euro3 Diesel passenger car without DPF, but it generally fails to estimate the particle mass of the lower particle-emitting passenger cars (gasoline and diesel with DPF). EF050330A