Heavy Duty Diesel Engine Exhaust Aerosol Particle and Ion

Dec 9, 2008 - Technology, Helsinki Polytechnic, P.O. Box 4020, FI-00099. Helsinki, Finland, Department of Physical Sciences, University of. Helsinki, ...
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Environ. Sci. Technol. 2009, 43, 163–168

Heavy Duty Diesel Engine Exhaust Aerosol Particle and Ion Measurements ¨ H D E , †,⊥ T O P I R O ¨ NKKO ¨ ,† TERO LA ANNELE VIRTANEN,† T A N J A J . S C H U C K , ‡,∇ L I I S A P I R J O L A , §,| ¨ M E R I , |,⊥ M A R K K U K U L M A L A , | KAARLE HA FRANK ARNOLD,‡ DIETER ROTHE,# AND J O R M A K E S K I N E N * ,† Aerosol Physics Laboratory, Department of Physics, Tampere University of Technology, P. O. Box 692, FIN -33101, Tampere, Finland, Atmospheric Physics Division, Max Planck Institute for Nuclear Physis (MPIK), P.O. Box103980, D-69029, Heidelberg, Germany, Department of Technology, Helsinki Polytechnic, P.O. Box 4020, FI-00099 Helsinki, Finland, Department of Physical Sciences, University of Helsinki, P.O. Box 64, FI-00014 Helsinki, Finland, Finnish Institute of Occupational Health, FI-00250 Helsinki, Finland, and MAN Nutzfahrzeuge AG, Abt. MTVN, Abgasnachbehandlung, Vogelweiherstr. 33, D-90441 Nu ¨rnberg, Germany

Received June 19, 2008. Revised manuscript received September 25, 2008. Accepted October 21, 2008.

Heavy duty EURO 4 diesel engine exhaust particle and ion sizedistributionsweremeasuredatthetailpipeusingdynamometer testing. Measurements of particle volatility and electrical charge were undertaken to clarify diesel exhaust nucleation mode characteristics with different exhaust after-treatment systems. Nucleation mode particle volatility and charging probability were dependent on exhaust after-treatment: particles were volatile and uncharged when the engine was equipped with diesel particulate filter and partly volatile and partly charged in exhaust without any after-treatment or with an oxidation catalyst only. The absence of charged particles in the nucleation mode of diesel particulate filtered exhaust excludes the ion mediated process as a nucleation particle formation mechanism.

Introduction Diesel exhaust particle size distribution is often measured to be bimodal, with distinctive nucleation and soot modes. The nonvolatile soot mode particles, with a modal diameter of approximately 50 nm, are formed already in the combustion chamber. The nucleation mode particles, usually with diameter below 30 nm, are generally proposed to be formed during the dilution and cooling process in ambient air or in the sampling setup. The mechanism for diesel exhaust nucleation is traditionally considered to be a binary homogeneous sulfuric acid-water nucleation (BHN), followed by growth of particles by hydrocarbon condensation (1-3). * Corresponding author phone: +358 3 3115 2676; fax: +358 3 3115 2600; e-mail: [email protected]. † Tampere University of Technology. ⊥ Finnish Institute of Occupational Health. ‡ Max Planck Institute for Nuclear Physis (MPIK). § Helsinki Polytechnic. | University of Helsinki. # MAN Nutzfahrzeuge AG. ∇ Now at Max-Planck-Institute for Chemistry, Mainz. 10.1021/es801690h CCC: $40.75

Published on Web 12/09/2008

 2009 American Chemical Society

For low fuel sulfur content (FSC), the nucleation rate and the growth rate of nucleation mode particles are correctly predicted by BHN models when the engine load is high and there is an after-treatment system with oxidation catalyst component (3, 4). However, BHN fails to explain some of the features of experimental results on nucleation mode particles. Nucleation mode particles are also found in diesel engine exhaust without after-treatment with low sulfur content fuel, i.e., in systems where sulfuric acid concentrations are too low for BHN (5, 6). At low loads volatile nucleation mode is rather linked with the concentration of hydrocarbon species in the exhaust, not with the sulfur content of the fuel (5). Ion-mediated nucleation (IMN) process has been proposed as an alternative formation mechanism of the volatile diesel nucleation mode particles (7, 8). The ions are believed to be formed in the diesel combustion flame as molecular ions through a chemi-ionization route. Consequently, they are often called chemiions. For IMN to take place, the chemiions should survive through the exhaust line and the possible after-treatment systems and exit the tailpipe. At the exit, either to ambient air or into a dilution system, the exhaust cools down. At ambient temperatures, the electrostatic interaction causes attachment of neutral molecules onto the ions. The resulting charged clusters are usually called cluster ions. In favorable conditions, the growth may lead to nucleation as the electrical charge lowers the barrier for nucleation (9). In this case, the nucleated particles will be charged, and they are sometimes called aerosol ions. For measurement purposes, the molecular chemiions have the highest electrical mobility, the cluster ions somewhat lower, and the aerosol ions generally much lower. Small nucleation rate changes observed between fuels with different sulfur content may be reasoned with IMN. As the chemiions in IMN are grown with sulfuric acid, the sulfuric acid concentrations control chemiion sizes and thereby also chemiion deposition rates. Hence, lower sulfuric acid concentrations cause slower growth of chemiions and higher chemiion deposition rates and, therefore, affect nucleation rate by limiting the number of clusters reaching critical size. In fact, IMN-model results are shown to fit to some diesel particulate total concentration measurements, both with filter after-treatment and without exhaust after-treatment (7, 8). An obvious test for the IMN process is the charge of the nucleation mode particles: if the process is chemiion mediated, a high portion of the volatile particles should be charged. The IMN-mechanism has been tested with mediumduty diesel engine, with doped diesel fuel without aftertreatment (10). Additives, in the form of cerium or lubricating oil, elevated the charge state of soot mode particles and produced a charged nucleation mode. However, nucleation mode was not measured when undoped fuel was used. Measured, charged nucleation mode was, in fact, nonvolatile and it was concluded in the study that the source of charged nucleation particles was combustion ion attachment to postcombustion nucleated metal particles and not the IMN process in the dilution system. Maricq (11) studied diesel exhaust particulate charge characteristics for a diesel particulate trap equipped light duty diesel truck and a light duty diesel car without a particle trap, and without fuel additives. The basic finding was that the nucleation mode particles were typically neutral in both cases. An exception was shown for the car without a particle trap. Normally, an uncharged nucleation mode was observed, with a geometric mean diameter below 5 nm. However, VOL. 43, NO. 1, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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another data set is also shown where the charged particle distribution is also bimodal. The reason for this was left unclear. Although a charged nucleation mode has not been measured in diesel exhaust studies, high concentrations of chemiions found from turbine combustion exhaust, for example (12, 13), suggest that chemiions may effect also diesel exhaust volatile compound dynamics in ambient air or in dilution systems. High concentrations of chemiions found from hydrocarbon flames as well as current measurement of diesel engine combustion chamber ionization promote the assumption (14, 15). If the volatile nucleation mode particles in diesel exhaust form through IMN, their concentration should strongly depend on the fraction of the chemiions surviving through the exhaust tube. This work extends diesel exhaust particle charge measurements down to few nanometers. The experimental part of the study was made with a Euro 4 heavy duty diesel engine in dynamometer facilities. Effect of after-treatment systems were studied with steady state engine loads without fuel additives. The aim of this work is to simultaneously study diesel exhaust aerosol ion characteristics and nucleation mode particle characteristics. The ion size distribution measurements have been previously used to study new particle formation in the atmosphere, see, e.g., Kulmala and Tammet (16). However, this is the first time when a ion spectrometer is utilized in diesel exhaust investigations.

Materials and Methods Engine, After-Treatment Systems and Fuels. Measurements were performed in the MAN motor company engine laboratory. The engine was 440 PS/323 kW 6 cylinder turbo charged common rail engine with EGR and it meets Euro 4 emission standards. Engine displacement was 10.6 L and peak torque 2220 Nm. Our test cycle consisted of four steady-state driving conditions with 25, 50, 100, and 75% engine loads. The loads points correspond to European steady cycle (ESC) modes 11, 3, 10, and 12 (see the Supporting Information). Before each cycle, 75% load was used to warm the engine. The order and durations of each mode periods were kept similar in all tests, to ensure the repeatability of test set and the comparability of runs with different after-treatment systems (6). Emissions of the engine were measured without aftertreatment systems, with an oxidation (DOC) catalyst and with a catalyzed diesel particle filter (DPF). The catalyzing material in both the DOC and the DPF was platinum, with loadings of 40 g/foot3 (1.4 g/dm3) and 20 g/foot3 (0.7 g/dm3), respectively. The sulfur content of the fuel (FSC) was 36 ppm. Measurement Equipment. The ion distributions were measured with an air ion spectrometer (AIS, AIREL Ltd.) (17). The AIS measures the aerosol ion concentration distributions with two parallel aspiration-type differential mobility analyzers (DMAs), one for each polarity. Each of the DMAs works in principle like those of a scanning mobility particle sizer (SMPS, see below). However, there are some differences: (1) The sample flow is not passed through a charger or neutralizer. (2) Each of the DMAs operates at a rather high flow rate of 60 LPM. Of this, the fraction of sample flow is high, 30 LPM (making up a total sample flow rate of 60 LPM). (3) The inner electrode acts as the voltage electrode, and the outer electrode is divided into 21 sections. Each section is connected to a sensitive current amplifier (electrometer). Instead of passing a known mobility fraction out of the DMA to be detected, the detection is based on measuring the electric currents carried to these sections by the precipitated ions. As each mobility size is measured in parallel, there is no voltage scanning. However, measurement times of several minutes are needed at low concentrations to increase the signal-to-noise ratio. The spectrometer mobility range was from 3.16 to 0.00134 cm2V-1s-1 corresponding to a diameter 164

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FIGURE 1. Experimental setup. range of 0.34-40 nm (17). The diameter range may be divided in two sections: cluster ion range from 0.34 to 1.6 nm and aerosol ion range from 1.6 to 40 nm. Both negative and positive polarity was measured, and the electrical signals were inverted to charge concentration data (AIS manual, AIREL LTD). A measurement time of 5 min was used in all the measurements. Two scanning mobility particle sizer (SMPS) were used in particle size distribution measurements (18). Nano-SMPS equipped with DMA 3085 and UCPC 3025 (TSI Inc., ref 19) was used in a size range from 3 to 60 nm and Long-SMPS equipped with DMA 3071 and UCPC 3025 (TSI Inc.) was used in a size range from 10 to 430 nm. The Nano-SMPS particle size distribution data were also corrected for the DMA diffusion losses and for the CPC efficiency. Following the example of ref 20, the difference between Nano-SMPS and Long-SMPS soot mode concentration was minimized with a numerical algorithm so that the soot mode concentrations were equal in the equipment. Also electrical low pressure impactor (ELPI, Dekati, Inc.) with filter stage was used in the study to follow the stability of the particle emission during the SMPS scans (21, 22). Particle volatility was studied by using thermodenuder (TD) at 75% engine load. In the TD, the diluted sample is led through the heated metal tube. After the heater, evaporated compounds are absorbed into the active charcoal. In our studies the total dilution ratio of the sample flow entering the thermodenuder was approximately 800. This prevents the renucleation of vaporized compounds after the TD even if the absorption to active charcoal is incomplete. During these measurements TD temperature was 270 °C. Additionally, size distribution measurements were done with different thermodenuder temperatures with 75% load and DOC aftertreatment. The heat-treated sample was measured with Nano-SMPS and ELPI. The losses in the thermodenuder were quite high. Loss measurements made with silver particles show that loss percentage was 94, 74, and 28-40% for 5, 10, and 30 nm particle diameter, respectively. The losses were corrected for particle size distributions when the sample was treated with thermodenuder. Sampling System. The sampling system is shown in Figure 1. The sample was taken from the exhaust tube, 40 cm behind the after-treatment system, directly to a primary diluter with a dilution ratio of 12. After passing through a residence time chamber, the sample was diluted further with ejectors (symbol ]), resulting in an overall dilution ratio close to 800. Supplement dilution air, 25 LPM, was used in the AIS sampling line ejector to ensure sufficient sample flow (60 LPM) to the AIS. The sampling system diffusion losses were corrected by assuming laminar flow. The primary diluter is of the porous tube type (23). The whole sampling system is a modified version of partial flow sampling system (PFSS) (24) and similar to that used, e.g., in 5, 6. The dilution air temperature was kept at 30 °C. The primary DR was adjusted

FIGURE 2. Measured particle size distributions without exhaust after-treatment (w/o), with DOC, and with DPF, 75% load. Both without after-treatment (none) and with DOC also dry particle size distributions are bimodal. For he DPF case, nucleation mode is completely volatilized in the thermodenuder. by changing the flow rate of the sample gas while keeping the dilution air flow constant. Primary dilution ratio values were calculated using measured CO2 concentrations.

Results Nucleation Mode Particles. Nucleation mode was observed with most of the load points, regardless of the after-treatment scheme. We will have a closer look at the particle and ion characteristics at the 75% load point. The characteristics of the nucleation mode for all the load points are summarized in the Supporting Information. A significant nucleation mode was observed at 75% load regardless of the exhaust aftertreatment system (Figure 2). Without after-treatment, the sulfuric acid concentration in the exhaust can be assumed to be low. Thus the existence of the nucleation mode indicates that there has to be some other nucleation process than homogeneous H2SO4-H2O nucleation (3). The particle volatility measurements reveal that the nucleation mode persists through the thermodenuder in this case. This indicates that the nucleation mode particles have a nonvolatile core. Similar results were reported earlier by Ro¨nkko¨ et al. (5). Also Kittelson et al. (25) reported nucleation mode particles with nonvolatile core, but only for idle conditions. The right panel of Figure 2 shows that the nucleation mode particles were partially nonvolatile, also, in the case of DOC. In contrast, when diesel particle filter (DPF) was used, practically all particles were removed by the thermodenuder. This indicates that the DPF removes the nonvolatile cores, and the nucleation mode particles post-DPF consist of only volatile compounds. The thermodenuder temperature was set at 270 °C during these tests. The results shown in Figure 2 are corrected for the TD losses. Measurements with DOC and 75% engine load were also performed by varying the TD temperature. The nucleation particle concentration reduced by 90% and the mode diameter decreased from 8 to 5 nm when the temperature was increased to 150 °C. However, no changes were observed for further temperature increase to 250 °C. Similar behavior has been observed when no after-treatment is used (5, 26). However, nonvolatile nucleation mode has not been earlier reported for DOC after-treatment. Characteristics of Diesel Exhaust Ions. Aerosol ions (i.e., charged aerosol particles) were measured by using the AIS. Figure 3 represents an example of the measured particle and ion size distributions. The measured distributions were bimodal for engine exhaust without after-treatment (NONE)

FIGURE 3. Example of measured aerosol particle (nSMPS) and negative C(-) and positive C(+) ion distributions with different after-treatment systems with 75% engine load. Ion concentration standard deviations (STD) are also shown with vertical black lines. The light gray line divides the x-axis to cluster ions (1.6 nm). and with oxidation catalyst (DOC) (Figure 3). In the case of DPF, the particle charge level was significantly lower than in two other cases. As a matter of fact, the measured ion distribution was unstable and similar to the measured background concentration with all loads when after-treatment was DPF, although high concentration nucleation mode was measured repeatedly. Nevertheless, bimodal aerosol ion distributions were stable in both DOC treated exhaust and without aftertreatment. The measured aerosol ion size range was limited to approximately 40 nm, and thereby ion distribution in the size range of soot mode was measured only partially. However, soot mode has been shown to be charged. Charge fraction of soot resembles Boltzmann charge probabilities around temperatures from 1000 to 1500 K (11, 27). On the other hand, nucleation mode is previously considered to be neutral. Here, a clear aerosol ion mode is measured in the particle nucleation mode size range always when the nucleation mode exists for the cases without after-treatment and with oxidation catalyst. To consider the nucleation mode charge, it should be noted that the multiple charged soot mode particles may affect the charge measurement of nucleation mode size range. The effect of multiple charged soot mode particles on ion distribution measurement in nucleation mode size range is considered here by a simplified method. First a bimodal lognormal number size distribution was fitted to the measured SMPS size distribution to separate the nucleation and soot modes. The soot mode particle charge distribution was assumed to obey Boltzmann distribution at 1100 K (11). The fitted soot mode number size distribution was then converted to a charge based mobility distribution, taking into account the multiple charges up to four elementary charges. This charge distribution was then divided into mobility bins that correspond to the AIS measurement channels. Finally, the obtained charge values were subtracted from the measured values for each AIS channel. The remainder on each channel was then interpreted to represent an estimate for the charge distribution of the nucleation mode (Supporting Information). The extraction of the charged soot mode from AIS concentration spectrum is an idealization. Taking into account differences in used equipment as well as in sampling system, there is uncertainty in how the reduced concentration spectrums represent nucleation mode charge characteristics quantitatively. However, when log-normal size distributions were fitted to the nucleation mode particle distribution and to reduced ion spectrum in the same size range, nucleation VOL. 43, NO. 1, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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mode geometric mean diameter (nGMD) and ion distribution GMD (iGMD) behave similarly. The dependence of iGMD to nGMD was linear with correlation coefficients r2 ) 0.976 and r2 ) 0.978 for negative and positive polarity, respectively. The nGMD was 1 nm smaller than iGMD when nGMD < 8 and 1 nm larger when nGMD > 8nm. The difference is believed to be caused by the different measurement equipment and the used log-normal fit, not by physical processes. As AIS detection efficiency continues well below the detection limit of the SMPS, the ion mode is weighted by smaller size classes than particle mode. On the other hand, the nucleation mode above 8 nm overlap with the soot mode and the log-normal fit loses its accuracy. Taking all this into account, the nucleation mode and ion mode were essentially of the same size, and the measured ion mode is interpreted as the charged fraction of the nucleation mode. The AIS instruments allow measuring ions down to cluster ion size range (0.34-1.6 nm mass diameter with Tammet correction); however, the losses in the sampling systems are estimated to be too high for reliable measurement. The measured concentrations in this size range were mainly unstable and close to background concentrations. Below, we present a simplified calculation of combustionoriginated chemiion loss rates in the exhaust tube to estimate whether the ions could survive through the exhaust tube with short residence times (5 × 107 cm-3 with all loads without after-treatment), it is clear that the ion concentrations surviving through the studied exhaust system are too low to support the IMN. According to calculations, ion number concentration is too low, even if the nucleation through ion process in the sampling system would be ideal. This, however, does not rule out the possibility that ions may, through the attachment processes, have an effect on measured nonvolatile nucleation mode.

Discussion Both the volatility of the nucleation mode and the ion distribution characteristics depend on the used aftertreatment system. When the catalyzed DPF was used, the nucleation mode was highly volatile and aerosol ion distribution was close to the measured background ion distribution. The nucleation mode was uncharged, indicating no role of ion mediated nucleation process (IMN). The present findings do not challenge the role of binary homogeneous sulfuric acid-water nucleation (BHN) when there is a catalyzed diesel particle filter. Particle nucleation mode with nonvolatile core and bimodal ion distribution were measured without aftertreatment and with oxidation catalyst after-treatment. The ion distribution characteristics followed the particle distribution characteristics. In fact, clear ion mode was found in particle nucleation mode size range in all cases when nonvolatile nucleation mode core was measured. The connection between the nonvolatile core and the ion nucleation mode imply that the nucleation mode is not formed through nucleation of volatile compounds in the sampling system. Instead, the results imply that the volatile compounds condense on nonvolatile nuclei, several nanometers in size formed in a strongly ionizing, high temperature environment. The finding of partially charged, nonvolatile core particles helps to explain the nucleation mode when no after-treatment is used and the sulfuric acid concentration is estimated to

be too low for BHN. The case of oxidation catalyst is interesting: the SO2 f SO3 conversion by the catalyst is potentially enough for BHN, but at the same time there are nonvolatile particles. In the present case it seems that the nucleation mode is formed by condensation of vapors onto the nonvolatile core and not via BHN. The ion mediated process (IMN) clearly fails to interpret the nucleation process for the present case. Even if the nonvolatile, charged nucleation mode would be interpreted to be caused by IMN, the measured charge levels were lower than would be expected for the IMN. Further, the theoretical considerations also suggest that the combustion originated chemiions could not survive to the dilution system to explain the measured number concentrations of nucleation mode particles. Unfortunately, measurement setup details complicate direct conclusions of nucleation mode particle charge state. The differences in measurement equipment efficiencies and measurement ranges (see refs 19 and 17) complicate the charge fraction calculations. Moreover, the sampling setup did not allow the simultaneous AIS and Nano-SMPS measurement of thermodenuder treated sample. This hinders the determination of the nonvolatile nucleation mode charge state as well as size of the particles. Future work should include, at least, nonvolatile nucleation core charge state and particulate number measurements with similar equipment. Also measurement of cluster size ions with a different type of sampling setup is strongly encouraged.

Acknowledgments We thank Mr. Pasi Perhoniemi for taking care of practical issues of the measurement trip. This work was supported by Tekes, The Finnish Funding Agency for Technology and Innovation, and the Ministry of Transport and Communications, Finland.

Supporting Information Available Additional details are shown in three tables and a figure. This material is available free of charge via the Internet at http://pubs.acs.org.

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