Environ. Sci. Technol. 2008, 42, 7957–7962
Diesel Nucleation Mode Particles: Semivolatile or Solid? ANDREA DE FILIPPO AND M. MATTI MARICQ* Dipartimento di Ingegneria Chimica, Universita` di Napoli “Federico II”, Naples, Italy, and Research & Advanced Engineering, Ford Motor Company P.O. Box 2053, MD 3179, Dearborn, Michigan 48121
Received April 15, 2008. Revised manuscript received July 7, 2008. Accepted August 13, 2008.
Although the preponderance of current data points to semivolatile diesel nuclei particles composed of sulfuric acid and heavy hydrocarbons, the question remains as to what extent, if any, they contain solid cores. We present evidence here of a “solid” particle nucleation mode that accompanies normal soot emissions in the case of two modern light-duty diesel vehicles run with ultralow sulfur fuel. This mode is most prominent at idle, but also appears at speeds below ∼30 mph, and is highly sensitive to the level of exhaust gas recirculation (EGR). The nuclei particles are examined for their volatility and electrical charge. In stark contrast to “conventional” nuclei particles, they remain nonvolatile to >400 °C and exhibit a bipolar charge with a Boltzmann temperature of 580 °C. Their nonvolatile nature rules out sulfate and heavy hydrocarbons as primary constituents, and their electrical charge requires formationinahigh-temperatureenvironmentcapableofgenerating bipolar ions. This suggests that “solid” nuclei particles form during combustion but remain distinct from soot particles, analogous to what has been found recently in flames. As concerns about potential emissions of nonvolatile nanoparticles have already surfaced, an important conclusion of this study is that diesel particulate filters remove the “solid” nucleation mode with an efficiency comparable to soot.
Introduction Diesel engine technology lies at the crossroads of two current environmental issues: air quality and climate change. Technology improvements, including direct injection, common rail, turbocharging and multipulse fuel injections, have helped overcome the low power density and high noise disadvantages of traditional diesel engines (1, 2). Diesel technology’s fuel efficiency benefit makes it a viable near-term alternative for greenhouse gas abatement from the mobile source sector. A major hurdle to its widespread adaptation has been the stringent tailpipe emissions standards being implemented as a result of potential fine particle related health effects, for example, the present U.S. EPA 2007 and Euro 5 limits. Efforts to overcome these hurdles have stimulated considerable interest not only in diesel aftertreatment systems but also in the study of the PM emissions themselves (3, 4). Diesel exhaust PM is a complex mixture that depends on engine operation, fuel composition, lube oil, aftertreatment technology, and exhaust sampling procedure. Engines combusting inhomogeneous air/fuel mixtures produce soot with * Corresponding author e-mail:
[email protected]. 10.1021/es8010332 CCC: $40.75
Published on Web 10/04/2008
2008 American Chemical Society
a characteristic log-normal distribution of 20-300 nm particles (5). Ash from trace metals, or cerium and iron fuel additives sometimes used to catalyze diesel particulate filter (DPF) regeneration (6), usually becomes incorporated into the soot. Together these constitute the solid soot component of diesel PM. As exhaust exits the engine and cools organic material and sulfate condense onto the soot, yielding internally mixed particles containing solid and semivolatile fractions (7-9). Alternatively, as exhaust cools. semivolatile materials can nucleate to produce a second smaller diameter PM mode. This mode, perhaps prompted by its fickle nature, has received extensive study. As shown by Shi and Harrision (10), sulfate plays an important role, but one very sensitive to dilution ratio and humidity. In light-duty diesel exhaust, nucleation depends synergistically on the presence of high sulfur fuel (350 ppm) and a diesel oxidation catalyst (DOC) (11, 12). But for heavy-duty diesel engines, particle mass spectrometry by Tobias et al. suggests that nuclei particles primarily originate from lube oil (∼95%) (13). Exhaust aftertreatment complicates the picture. A DOC can reduce PM mass by removing condensable organic material (14). But, by oxidizing SO2 to SO3 it can also increase the propensity for nucleation by sulfate (15). Implicit in this discussion is the notion that nucleation mode particles are liquid, one supported by volatility and chemical analysis (16-18). However, the question has remained as to whether nuclei particles contain solid cores. A few recent observations suggest that they can. Kittelson et al. (16) found residual nonvolatile material in nuclei mode PM emissions from a heavy-duty diesel engine. And Ro¨nkko¨ et al. observed nonvolatile cores over a range of driving conditions in the exhaust of a 2005 model heavy-duty diesel truck, one that achieves Euro IV emission standards by advanced fuel injection techniques and cooled exhaust gas recirculation (EGR) and thus produces PM with an unusually high soluble organic fraction (SOF) (19). The source of nonvolatile cores remains unknown. Possibly they originate in the engine from pyrolyzed hydrocarbons or lube oil derived ash. But they are not limited to engines. Recent work shows that a separate nucleation mode of nonvolatile particles is also found in sooting premixed flames, with a propensity for lower temperature flames (20-23). These are characterized as 2-5 nm spherical particles exhibiting translucent transmission electron microscope (TEM) images, immature particles with relatively low C/H ratio that have not yet completed the carbonization process to soot (24). Engine exhaust and flame nanoparticles have recently been compared by Sgro et al. (25), but primarily via UV extinction and size measurement of water collected exhaust particles. Our objective here is to present data from two modern light-duty diesel vehicles that exhibit nucleation mode emissions distinctly different from conventional. We examine the volatility and electrical properties of these disparate nucleation modes to learn something about their nature and origin. While nonvolatile nuclei particle emissions are most prominent at idle, we explore the impacts of EGR and vehicle speed. Finally, to address potential environmental impact, we investigate the effectiveness of diesel exhaust aftertreatment to reduce “solid” nuclei particle emissions.
Materials and Methods Test Vehicles and PM Sampling. PM emissions are examined from three light-duty diesel test vehicles run on ultralow sulfur fuel. All are of modern design, including common rail, VOL. 42, NO. 21, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1 LD diesel 1 year engine (L) common rail turbocharged no. of injections fuel sulfur (ppm) aftertreatmenta
2003 1.8 I4 yes yes 1 90% above 65 nm. After correction for penetration efficiency and dilution, the results are reported as particle concentration in engine exhaust. Vehicle 1 data were recorded in an earlier campaign using the TSI 3081 “long” DMA and TSI 3010 and 3025 CPCs. Although more limited in the sub 10 nm range (see the Supporting Information), this instrumentation more than adequately demonstrates the distinction between the nucleation modes of LD diesel 1 versus 2 & 3. Particle volatility is measured by comparing particle size before and after passing the diluted aerosol through a heater to evaporate semivolatile material. LD diesel 1 measurements 7958
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used a thermodenuder (Dekati), in which a heating tube is followed by an annular tube lined with active charcoal to adsorb the volatized gases so that they do not renucleate or condense. Diffusion and thermophoretic losses are reported to be ∼30% over the 20-250 nm range, but are not well characterized below 20 nm (27). Because of concern for increased particle losses below 10 nm, a simpler variant is applied for volatility measurements with the nanoDMA. A heat pipe (20 cm long, 0.9 cm diameter) evaporates semivolatile material, and the denuder section is omitted. Instead, the dilution ratio is kept sufficiently high to prevent renucleation of evaporated material as the aerosol cools downstream of the heat pipe. The disappearance of poly R-olefin (PAO) oil droplets as the thermodesorber temperature is raised to 260 °C (see Figure 3) demonstrates the method’s efficacy and verifies the lack of renucleation. Losses in this approach are not separately measured, but can be ascertained to be 400 °C, draws a further analogy to the “transparent” nanoparticles observed from flames (24). How two modes of nonvolatile particles can originate during diesel combustion is currently not known. Possibly the multiple fuel injection pulses per engine cycle used in new technology diesel engines to control soot formation and noise can produce separate particle populations. The dependence of nonvolatile nuclei particles on EGR level is perhaps easier to explain. EGR is used to reduce NOx formation by displacing air in the cylinder charge, whereby the reduced oxygen content lowers NOx formation (31, 32). However, this also reduces soot burn-out during the exhaust stroke leading to the well-known NOx-PM tradeoff. This is evident in Figure 2 (lower panel; also see the Supporting Information, Figure S3) by the substantial increase of soot mode particles when EGR is on. In this case, even if nuclei particles are formed, their removal by coagulation with the soot mode would be greatly enhanced. Similarly, increased soot formation at higher vehicle speed and load may help mask a nonvolatile nucleation mode if present. Although debate about their exact origins and characteristics may continue, it is possible to address the fate of nonvolatile nuclei particles prior to their exiting the tailpipe. Unlike the more common semivolatile nucleation mode, these nonvolatile particles are present engine-out; thus, they must pass through the entire aftertreatment system. They are largely unaffected by the DOC but DPFs can trap them with essentially the same >99% efficiency as the soot (accumulation) mode and significantly limit their release to the atmosphere.
Acknowledgments The authors thank Sandip Shah, Mike Loos, Adolfo Mauti, and Joe Szente for running the chassis dynamometer and helping with the experimental setup. We thank Paul Tennison and Gang Guo for acquiring EGR data from the engine control module. And we thank Yi Liu (Wayne State University) for help producing the TEM images. A. De Filippo thanks Prof. Antonio D’Alessio for his priceless advice and the “Programma di scambi internazionali per VOL. 42, NO. 21, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Mobilita` di breve durata” by Universita` di Napoli “Federico II” for financial support.
Supporting Information Available Experimental section and additional figures (PDF). This material is available free of charge via the Internet at http:// pubs.acs.org.
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