Environ. Sci. Technol. 2007, 41, 4593-4599
Influence of Engine Operating Conditions on Diesel Particulate Matter Emissions in Relation to Transient and Steady-State Conditions Z . G E R A L D L I U , * ,† D E N I S E C . F O R D , † VICTORIA N. VASYS,† DA-REN CHEN,‡ AND TIMOTHY R. JOHNSON§ Cummins Filtration and Emission Solutions, Stoughton, Wisconsin 53589, Washington University, St. Louis, Missouri 63130, and TSI Incorporated, Shoreview, Minnesota 55126
Airborne particulate matter is an important pollutant affecting air quality. Currently, diesel PM regulations are based on emitted particle mass; however, the particle size distributions are also important factors in air quality. While the distributions of particulate emissions under steadystate conditions are well-known and have been generalized, varying distributions under transient conditions are not wellunderstood. This study investigates the size distributions of PM, focusing on the nuclei- and accumulation-modes, emitted from diesel engines under transient operations. Some engine conditions during transient testing produced particle size distributions that were notably different from those produced under steady-state conditions. During transient operation, the size distributions were either mono- or bimodal with peaks that were able to switch quickly between the nuclei- and accumulation-modes. These distributions have not been observed during steady-state testing but are significant because environmental and health effects and emission control solutions are highly dependent on particle size.
Introduction The concentration of airborne particulate matter (PM) in the atmosphere is a major aspect of air quality issues. Many negative effects of PM on both environment and human health are size dependent. For example, the small size institutes global effects, as long residence times permit the airborne travel of particles. As PM becomes smaller, especially in the submicrometer range, the effects on human health grow larger (1-4). The internal combustion engine, particularly the diesel engine, emits large amounts of submicrometer PM into the atmosphere. Diesel particles can be categorized into three general size ranges, as explained by Kittelson (5) for steadystate operation. In Kittelson’s generalized distribution, the nuclei-mode (Dp 500 nm) contributes the least number and mass to diesel particulate emissions and is comprised of accumulationmode particles that have agglomerated while deposited on engine and exhaust surfaces. Engine pulse, vibration, and exhaust flow re-entrain these particles, resulting in coarsemode emissions. These three groups of particles form trimodal and log normal distributions (5). Knowledge of the detailed PM size distribution of an engine’s emissions is crucial for the development of emission control devices. Nuclei-mode particles are most responsive to diffusion forces, and coarse-mode particles are most responsive to interception and inertial impaction. However, the majority of accumulation-mode particles is only marginally responsive to any of the aforementioned forces and is often responsible for the minimum efficiency of each of the currently existing filter types (6). Although PM size distributions have been generalized for steady-state conditions, subsequent studies have shown that during transient operation, changing engine parameters strongly affect both the size distribution and the concentration of emitted particles (7, 8). As after-treatment devices are integrated into diesel exhaust systems, the transient emission response could alter their effectiveness. Since engines are frequently operated under transient conditions, the understanding of particle distributions under transient conditions is essential. Nations around the world are regulating diesel engine emissions, and transient testing cycles are often used for this purpose. For example, the U.S. Environmental Protection Agency (EPA) uses a 1200 s standard transient pattern, known as the Federal Transient Testing Procedure Heavy-Duty (FTP HD) to certify heavyduty engines. Measurement of an engine operating transiently is more difficult, however, because both the speed and the load of the engine change as a function of time. These factors result in changing the air/fuel ratio, mixing pattern, and in-cylinder temperature, which in turn influence the formation and composition of PM inside of the engine (10). Additionally, the dilution and cooling of the exhaust are effected by the changing engine conditions, thus altering the particles in the exhaust stream (4, 11). Because these transient conditions change continuously, the measurement of PM size distributions requires the use of real-time instrumentation. This eliminates several instruments used for steady-state testing and makes others marginally useful. In this paper, we investigate transient engine out PM size distributions and compare them with those of steady-state operation. Two studies were conducted to investigate the emission response to transient conditions, each using a different primary particle sizing instrument. A third, more established, particle sizing instrument was also used for steady-state testing to validate the other sizers. From these data, particle size distributions of transient operation are described, and the influence of the engine operating conditions is discussed. VOL. 41, NO. 13, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4593
TABLE 1. Engine Parameters parameters
engine 1
engine 2
Model model year Cylinders Configuration Aspiration
Cummins ISM 435V 2001 6 inline-6 turbocharger/air to air cooler combustion type direct injection (DI) Displacement 10.8 L max torque 1967 N m (1450 ft lbs) at 1200 rpm max power 325 kW (435 bhp) at 1600 rpm
International 7.3 L IDI 1992 8 V8 naturally aspirated indirect injection (IDI) 7.3 L 412 N m (304 ft lbs) at 1600 rpm 115 kW (155 bhp) at 2700 rpm
TABLE 2. Parameters of Particle Sizing Instrumentation SMPS resolution (channels per decade) particle size range (nm) up-scan time (s) particle classification particle counting
N-ASA
EEPS
32
32
16
7.0-294 120 long DMA UCPC
6.0-90 5 nano-DMA electrometer
5.6-560 0.1 nano-DMA electrometer
Experimental Procedures Engines, Dynamometers, and Fuel. To gather data for this study, two engines and dynamometers were used in two setups. Engine 1 (described in Table 1) was used for the first transient test and was controlled by a General Electric direct current dynamometer rated at 525 kW. This engine was fueled with CENEX brand conventional No. 2 diesel fuel composed of greater than 300 ppm sulfur, which was similar to fuels used in nearly all on-road diesel vehicles in the U.S. at the time of the tests. Engine 2, also described in Table 1, was used for the second transient test. This engine previously provided power to a school bus and was designed with early 1990s emissions technology, producing more particles than engine 1. Engine 2 was controlled by a General Electric direct current dynamometer rated at 155 kW. This engine was fueled by Chevron Phillips ultra-low sulfur diesel fuel, composed of less than 10 ppm sulfur, and was used in the transient 2 test and a repeat of the steady-state test. Experimental Setup. A TSI model 3936 scanning mobility particle sizer (SMPS) was used as the reference particle sizing instrument in both tests. It consists of a long differential mobility analyzer (DMA) and an ultra-fine condensation particle counter (UCPC). Its up-scan time is 120 s; thus, it cannot be used in transient testing. However, the other instruments’ data were validated by comparison to the SMPS data during steady-state-modes. Each instruments’ parameters are listed in Table 2. The first transient test was conducted to determine whether transient conditions had any effect not seen during steady-state testing. The experimental apparatus used in this test is shown in Figure 1a. The particle sizer used was a nanoaerosol size analyzer (n-ASA) with a faster, although not real-time, up-scan time than the SMPS. This instrument had previously been employed to characterize jet engine emissions (13). It consists of a bipolar charger, a nanoDMA column, and an aerosol electrometer. The nanoDMA column can sample particles in the size range of 3-300 nm, and a range of 6-90 nm was used to further reduce the up-scan time to approximately 5 s. As this study was the first application of n-ASA to the characterization of diesel engine emissions, a TSI 3022A CPC and a Dekati electrical lowpressure impactor (ELPI) were used for reference in addition to the SMPS. The n-ASA relies on high concentrations of particles for measurement; thus, the sampled exhaust stream 4594
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 13, 2007
was only diluted by a factor of 2. This high concentration was not tolerable in the CPC; thus, the exhaust sample entered a closed loop dilution system before proceeding to it to avoid overloading the instrument. The results from transient 1 proved to differ from steadystate results; thus, a second more precise transient test was conducted with a real-time particle sizing instrument, a TSI Model 3090 engine exhaust particle spectrometer (EEPS). The EEPS can sample particles with diameters between 5.6 and 560 nm at 10 samples per second. Although the scan time is extremely quick for EEPS, it does, however, suffer from reduced resolution as compared to SMPS (14). Therefore, SMPS and 3022A CPC were used for reference during testing. The engine available for use in this test, engine 2, produced a large amount of particles, and EEPS requires low particle concentrations; thus, a three stage dilution system was utilized. The transient 2 testing setup is displayed in Figure 1b. The exhaust stream-flowed first through an 8:1 first-stage diluter and was heated with dry dilution air (300 °C) and a heat-blanket to minimize nucleation of volatile species. The diluted stream subsequently traveled through second- and third-stage diluters with 8:1 and 15:1 dilution ratios, respectively, and finally entered the particle sizing instruments for data collection. Test Cycles. For the transient testing situations, the appropriate engine was started and warmed up to operating temperatures for 30 min at 50% of its maximum load and an intermediate speed. Next, FTP HD cycles were run on the engine to simulate transient operation. The typical time sequence of engine speed and torque during the FTP HD cycle was defined in the 1998 Code of Federal Regulations (CFR) (12). The cycle consists of four phases, simulating a series of freeway and non-freeway operations. Phase 1, the New York Non-Freeway (NYNF) phase, simulates the stopping, starting, and idling of city traffic. Phase II, the Los Angeles Non-Freeway (LANF) section, is based on crowded urban traffic with less idling time. Phase III, the prolonged higher speed section of the cycle known as the Los Angles Freeway (LAFY) section, simulates nearly constantly changing conditions during crowded freeway driving. Last, Phase IV is the NYNF section repeated. For the steady-state test, the appropriate engine was started and allowed to stabilize at an intermediate speed and 50% of its maximum load for 60 min. The 11 steadystate-modes of ISO-8178 (15) were then begun with measurements starting after the engine was stabilized.
Results and Discussion Instrumentation Validation. Prior to testing, SMPS, n-ASA, and EEPS were individually calibrated by the manufacturers. The SMPS calibration was also validated at the testing facility by measuring the flow rates, checking the electrostatic charges, and monitoring the temperatures. The n-ASA and EEPS were then validated by critiquing them against the TSI standard SMPS. In general, the data from both instruments agreed well with SMPS, although the size distributions measured by EEPS shifted slightly toward a smaller size range. Transient Analysis. Transients 1 and 2 tests were not predicted to be comparable due to the use of different engines, fuels, and dilution systems. However, the results show similar emission patterns, many of which are not seen in steady-state results. The four figures used to display particle size distributions for the tests were chosen from four representative points in the FTP HD cycle. For ease of comparison between the two tests, each figure contains one n-ASA plot and three EEPS plots, taken over a 5 s time span. Because different engines were used for the transients 1 and 2 tests, as described in the Experimental Procedures, the engine conditions during each point in the FTP cycle may have varied between the two transient tests; therefore, a
FIGURE 1. Transient and steady-state sampling systems (a and b). snapshot of the engine conditions for each plot is also provided. Although there are several differences in the experimental conditions for the two tests, many of the PM distribution trends remain the same and are explained next. The end of a deceleration occurred at 42 s into the FTP HD cycle, causing engine loads to be negative, speeds around 40% of maximum, and EGTs relatively low. The PM size distributions resulting from these engine conditions, displayed in Figure 2, begin with a large peak in the nucleimode. Several seconds further into the FTP cycle, a large peak of accumulation-mode particles emerged as the engines began to accelerate, evidenced by the growth of the accumulation-mode in the 44.5 and 47 s transient 2 size distributions. As the loads on the engines increased, more fuel was injected into the engines, thus decreasing the air/ fuel ratios, increasing the soot emissions, and accelerating the rate of particulate agglomeration (10). Because the transient 1 PM size distribution curve was scanned over a 5 s time span, a single bimodal distribution was the result of these rapidly changing engine conditions, with the beginning capturing the nuclei-mode particles created by the deceleration and the end capturing the accumulation-mode particles created by the acceleration. However, the nearly real-time EEPS measurements show the progression of the PM size distribution from monomodal nuclei to monomodal accumulation as the engine conditions changed. At about 226 s into the FTP cycle, engine 1 accelerated to 2050 rpm (nearly rated speed), and the torque dropped from 1725 to 750 N m. Data were taken at 226, 228.5, and 231 s for the transient 2 test, in which the engine speed increased from about 2850 to 3000 rpm and the torque changed from being periodic around 300 N m to approximately steady at 150 N m. Exhaust gas temperatures (EGT) quickly increased, then slowly decreased for both tests, ranging from about 265-311 and 190-267 °C for engines 1 and 2, respectively.
The PM size distributions for these engine conditions are show in Figure 3. A tiny bump in the nuclei region and a large peak in the accumulation region appear in each distribution at 226 s, which can be explained by the mediumhigh torque being applied to both engines at this time. As time progressed and the torque and EGT decreased, however, a nuclei peak grew in the transient 2 distributions. This was not witnessed in the transient 1 distribution because n-ASA had already finished scanning for nuclei particles by the time the torque and EGT began to drop. Throughout this time interval, the accumulation-mode peak remained large for both tests because although the engine torques and EGTs dropped, they remained positive and midrange. In the beginning of the 240 s measurement period, both engines experienced nearly steady-state conditions: engine 1 was operating at a relatively constant 2100 rpm, 750 N m, and 250 °C, and engine 2 was operating at 3000 rpm, 125 N m, and 235 °C. The transient 1 particle size distribution, displayed in Figure 4, now exhibits the nuclei-mode that could not be captured during the 226 s measurement. The transient 2 particle distribution was bimodal, similar to the 231 s measurement, only more balanced now because more particles nucleated due to the lower, stabilized EGT. Shortly after 240 s into the FTP cycle, a large drop in load occurred in both tests, which left the torque near zero for engine 1 and largely negative for engine 2. This change is evidenced by the near disappearance of the accumulation-mode and the left shift in the nuclei-mode in the transient 2 PM size distribution at second 242.5. Approaching 245 s, a sharp increase in torque occurred in both tests, which increased the total amount of particles produced inside of the cylinder, in turn increasing the amount of accumulation-mode particles emitted. Notice, however, that the magnitude of the nuclei-mode also increased during this acceleration. Overfueling may have occurred, which results in inefficient mixing VOL. 41, NO. 13, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4595
FIGURE 2. Particle size distribution for transient 1 at 42 s and for transient 2 at 42, 44.5, and 47 s. Note the difference in the y-axis scale between transient 1 and transient 2 distributions.
FIGURE 3. Particle size distribution for transient 1 at 226 s and for transient 2 at 226, 228.5, and 231 s. Note the difference in the y-axis scale between transient 1 and transient 2 distributions. of air and fuel inside of the engine and increased hydrocarbon emissions (10). Since the EGT remained low throughout this measurement period, increased nucleation and condensation were likely. Finally, at 892 s into the FTP HD cycle, the engine speeds were slowly declining to 1680 and 2400 rpm for engines 1 and 2, respectively. The loads on the engines fluctuated rapidly, and the EGTs were relatively high. The transient 2 4596
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 13, 2007
PM size distributions shown in Figure 5 for 892, 894.5, and 897 s have a large lower accumulation-mode peak that shifted in location as the load on the engine was adjusted. At 894.5 s, the speed and load were both decreasing, but the EGT remained high, thus creating a low hump of particles, peaking in the lower accumulation-mode. A distinct peak was not seen in the nuclei-mode because the high EGT hindered the nucleation of gases and their conversion to particles. A plot
FIGURE 4. Particle size distribution for transient 1 at 240 s and for transient 2 at 240, 242.5, and 245 s. Note the difference in the y-axis scale between transient 1 and transient 2 distributions.
FIGURE 5. Particle size distribution for transient 1 at 892 s and for transient 2 at 892, 894.5, and 897 s. Note the difference in the y-axis scale between transient 1 and transient 2 distributions. of the transient 2 distribution at 904 s is included because this is where a significant nuclei-mode peak began to form. At this point, the load was still roughly the same as it was at 897 s, but the EGT decreased from 252 to 219 °C, and the speed increased from 1550 to 1762 rpm. Although similar emission patterns are seen in the transients 1 and 2 tests, several differences due to the use of different engines, fuels, and dilution systems should be noted. In all cases, the particle distributions’ peaks were shifted to smaller diameters for the transient 2 test. This is in part a result of the instrumentation differences but also because
the temperatures occurring in the transient 2 engine were lower than those in the transient 1 engine. Fewer nucleimode particles were generally witnessed in the transient 2 test because a lower sulfur content fuel was used in the transient 2 test, reducing the amount of sulfur available for nucleation. Also, a heated first-stage diluter was used in the transient 2 test. Wong et al. explained that the use of a heated first-stage dilutor prevented the partial vapor pressure of volatile components from increasing during dilution, which suppressed additional nuclei-mode particle formation (19). Furthermore, by preventing the formation of a saturated VOL. 41, NO. 13, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4597
mixture in the first dilutor, condensation of volatiles and additional growth of particles in the subsequent dilutors were also reduced. Transient versus Steady-State Emission Patterns. The amount of information derived from the transient tests was significantly greater than the amount derived from the steadystate test. The transient 1 test allowed us to see how changing engine conditions affected particle size distributions, and the transient 2 test demonstrated how specific, nearly static engine conditions actually occurring during the FTP HD cycle affected particle size distributions. Overall, similar results were produced for each transient test, so only one comprehensive distribution will be displayed and discussed in this section. Since the transient 2 test used the same instrument, dilution system, engine, and fuel as the steadystate test, its distribution was chosen for this purpose and is displayed in Figure 6. To avoid congesting the figure, only 60 frames of the entire FTP data are shown in Figure 6a. The EEPS’ ability to output 12 000 particle size distributions during one cycle allows for a detailed view of emission patterns during short time segments. Focused displays of a period of acceleration and a period of deceleration are shown in Figure 6b,c, respectively. During these strongly transient conditions, the engine encountered combinations of loads and speeds similar to those of ISO-modes 8, 10, and 11 and 7, 8, and 9 for the acceleration and deceleration, respectively. These steady-state distributions are also displayed in Figure 6b,c, embedded at the moment of the transient cycle when the engine conditions correspond, as indicated by arrows. A variety of particle distributions is seen in the transient test results that are not seen in the results from any of the 11 ISO-modes. In the steady-state study, mostly accumulation-mode particles were formed when ultra-low sulfur fuel was used. In the transient tests, however, a large amount of nuclei-mode particles was still formed. As we have detailed in Transient Analysis, these large nuclei-mode distributions occurred predominately when the engine loads and EGTs were low or declining. The low load results for ISO-modes 10 and 11 have been seen in steady-state analysis (5, 7, 17, 20, 21) before. Kittelson (5) previously explained that most nuclei-mode particles form during the dilution process or during expansion and cooling in the engine after combustion is complete; this also explains why a large amount of nuclei particles is formed when the load is being reduced and not just when the engine is idling. Liu et al. (20) also explained that low exhaust gas temperatures, occurring during idle and declining loads, create favorable conditions for the formation of nuclei-mode particles. In addition to exhaust conditions, previous studies (18, 22, 23) have positively correlated the formation of nuclei-mode particles to unburned hydrocarbon emissions during low and declining load conditions. Finally, we have also seen nuclei-mode particles occasionally forming during accelerations. Heywood (10) explained that if overfueling occurs during acceleration, hydrocarbon emissions will be increased. In many sections of the FTP HD cycle, a significant accumulation-mode is seen. This happens at high, medium, and increasing loads and EGTs. As the engine load and EGT increase, volatile compounds have less opportunity to nucleate, and more accumulation-mode particles are emitted due to higher fuel consumption inside of the cylinder (5, 10). This is especially true for increasing load conditions such as accelerations, which do not exist in steady-state analyses. Finally, several bimodal distributions have appeared throughout the FTP cycle and are usually seen during quickly changing engine conditions. When the engine is accelerating, decelerating, or the load and EGT are changing from increasing to decreasing, or vise versa, conditions are often favorable for both types of particles to form. The relative magnitudes of these modes then depend on the precise load, 4598
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 13, 2007
FIGURE 6. Particle size distribution throughout the FTP HD cycle for transient 2 test as compared to the particle size distributions produced from steady-state testing (a-c). speed, fuel consumption, and EGT at the time of particle formation. The results of these quickly changing conditions rarely can be seen during steady-state testing.
Acknowledgments The authors thank Eddie Thurow, Tom Wosikowski, and Thaddeus Swor of Cummins Filtration and Emission Solutions for their assistance in test cell preparations and dynamometer control. The authors also express gratitude to Nalin Perera and Georg Pingen of Washington University and Robert Caldow, Jeremy Kolb, and Nick Velander of TSI for their help with instrumentation.
Literature Cited (1) Harrison, R. M. Airborne Particulate Matter in the United Kingdom, Third Report of the Quality of Urban Air Review Group; The University of Birmingham: Edgbaston, England, 1996. (2) Scherrer, H. C.; Kittelson, D. B. Light Absorption Cross-Sections of Diesel Particles; SAE Paper No. 810181; 1981. (3) Kittelson, D. B.; Kadue, P. A.; Scherrer, H. C.; Lovrien, R. Characterization of Diesel Particles in the Atmosphere; Final Report, Coordinating Research Council: 1988. (4) Finlayson-Pitts, B. J.; Pitts, J. N. J. Chemistry of the Upper and Lower Atmosphere; Academic Press: San Diego, 2000. (5) Kittelson, D. B. Engines and nanoparticles: A review. J. Aerosol Sci. 1998, 29, 575-588. (6) Hinds, W. C. Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles; John Wiley and Sons: New York, 1982. (7) Liu, Z. G.; Thurow, E. M.; Lincoln, J. C.; Chen, D.-R.; Perera, N.; Pingen, G. Transient Analysis of Engine Nanoparticles using a Fast-Scanning Differential Mobility Particle Analyzer; SAE Paper No. 2004-01-0971: 2004. (8) Liu, Z. G.; Thurow, E. M.; Caldow, R.; Johnson, T. R. Transient Performance of Diesel Particulate Filters as Measured by an Engine Exhaust Particle Size Spectrometer; SAE Paper No. 200501-0185: 2005. (9) Merrion, D. F. Heavy-Duty Diesel Emission RegulationssPast, Present, and Future; SAE Paper No. 2003-01-0040: 2003. (10) Heywood, J. B. Internal Combustion Engine Fundamentals; McGraw-Hill, Inc.: New York, 1988. (11) Eastwood, P. Critical Topics in Exhaust Gas After Treatment; Research Studies Press Ltd.: Baldock, 2000. (12) Code of Federal Regulations. Protection of the Environment, Title 40, Part 86; 1998.
(13) Han, H.-S.; Chen, D.-R.; Pui, D. Y. H.; Anderson, B. E. A nanometer aerosol size analyzer for rapid size distribution measurements. J. Nanoparticle Res. 2000, 2, 43-52. (14) Johnson, T.; Caldow, R.; Po¨cher, A.; Mirme, A.; Kittelson, D. B. A New Electrical Mobility Particle Sizer Spectrometer for Engine Exhaust Particle Measurements; SAE Paper No. 2004-01-1341: 2004. (15) ISO 8178-4. Reciprocating Internal Combustion EnginessExhaust Emission Measurement Cycles for Different Engine Applications; 1996. (16) Abdul-Khalek, I. S.; Kittelson, D. B. Real-Time Measurement of Volatile and Solid Exhaust Particles using a Catalytic Stripper; SAE Paper No. 950236: 1995. (17) Abdul-Khalek, I. S.; Kittelson, D. B.; Brear, F. Diesel Trap Performance: Particle Size Measurements and Trends; SAE Paper No. 982599: 1998. (18) Vaaraslahti, K.; Virtanen, A.; Ristimaki, J.; Keskinen, J. Nucleation mode formation in heavy-duty exhaust with and without a particulate filter. Environ. Sci. Technol. 2004, 38, 4884-4890. (19) Wong, C. P.; Chan, T. L.; Leung, C. W. Characterization of diesel exhaust particle number and size distributions using minidilution tunnel and ejector-dilutor measurement techniques. Atmos. Environ. 2003, 37, 4435-4446. (20) Liu, Z. G.; Skemp, M. D.; Lincoln, J. C. Diesel Particulate Filters: Trends and Implications of Particle Size Distribution Measurement; SAE Paper No. 2003-01-0046: 2003. (21) Saito, K.; Shinozaki, O.; Seto, T.; Kim, C.-S.; Okuyama, K.; Kwon, S.-B.; Lee, K. W. The Origins of Nanoparticle Modes in the Number Distribution of Diesel Particulate Matter; SAE Paper No. 200201-1008: 2002. (22) Shi, J. P.; Harrison, R. M.; Brear, F. Particle size distribution from a modern heavy-duty diesel engine. Sci. Total Environ. 1999, 235, 305-317. (23) Ronkko, T.; Virtanen, A.; Vaaraslahti, K.; Keskinen, J.; Pirjola, L.; Lappi, M. Effect of dilution conditions and driving parameters on nucleation mode particles in diesel exhaust: Laboratory and on-road study. Atmos. Environ. 2006, 40, 2893-2901.
Received for review July 7, 2006. Revised manuscript received December 22, 2006. Accepted April 30, 2007. ES0616229
VOL. 41, NO. 13, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4599
