Effect of Advanced Aftertreatment for PM and NOx Reduction on

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Effect of Advanced Aftertreatment for PM and NOx Reduction on Heavy-Duty Diesel Engine Ultrafine Particle Emissions Jorn Dinh Herner,* Shaohua Hu, William H. Robertson, Tao Huai, M.-C. Oliver Chang, Paul Rieger, and Alberto Ayala California Air Resources Board, 1001 “I” Street, P.O. Box 2815, Sacramento, California 95812, United States

bS Supporting Information ABSTRACT: Four heavy-duty and medium-duty diesel vehicles were tested in six different aftertreament configurations using a chassis dynamometer to characterize the occurrence of nucleation (the conversion of exhaust gases to particles upon dilution). The aftertreatment included four different diesel particulate filters and two selective catalytic reduction (SCR) devices. All DPFs reduced the emissions of solid particles by several orders of magnitude, but in certain cases the occurrence of a volatile nucleation mode could increase total particle number emissions. The occurrence of a nucleation mode could be predicted based on the level of catalyst in the aftertreatment, the prevailing temperature in the aftertreatment, and the age of the aftertreatment. The particles measured during nucleation had a high fraction of sulfate, up to 62% of reconstructed mass. Additionally the catalyst reduced the toxicity measured in chemical and cellular assays suggesting a pathway for an inverse correlation between particle number and toxicity. The results have implications for exposure to and toxicity of diesel PM.

’ INTRODUCTION In response to health concerns, regulators have promulgated ever stricter emissions standards for particulate matter (PM) from diesel engines. In the United States, the standard for PM mass from heavy duty diesel engines (HDDE) has been lowered from 1.0 g per brake horsepower-hour (g/bhp-hr) prior to 1988, to the current 0.01 g/bhp-hr for 2007 and later model year engines. These standards have been effective in lowering diesel PM mass emissions over the preceding decades. The effect on particle number emissions, chiefly ultrafine particles (UFP, defined as particles with aerodynamic diameter 30,000 miles and have therefore saturated their initial sulfur storage capacity. They produce a nucleation mode when the temperature in the aftertreatment reaches the critical temperature threshold discussed

previously. Veh3-DPF3 did not produce nucleation mode particles because it uses an uncatalyzed DPF, which does not oxidize SO2 to SO3. Veh4-DPF4 is equipped with a heavily catalyzed DPF, but the vehicle had only been in service for 1000 miles when tested. Therefore the DPF has plenty of capacity for storing sulfur and as a result did not produce a nucleation mode. It is expected that Veh4-DPF4 will cause nucleation once the initial capacity for storing sulfur on the catalytic surface has been reached.19 Figure 4 shows the mass fraction of each chemical constituent measured in the emitted PM for the cruise, UDDS, and idle cycles. Table S2 shows the emission factors in mg/mi and mg/h. PM emitted from the uncontrolled baseline, Veh1-baseline, consists almost exclusively of OC and EC irrespective of cycle, though as expected the relative proportion of each change with the load of the cycle. The PM from the nucleating configurations, Veh1-DPF1, Veh1-DPF1þSCR1, Veh1-DPF1þSCR2, and Veh2þDPF2, contained significant fractions of sulfate, ammonium, and some nitrate. Both the mass and total fraction of sulfate measured correlated well with the magnitude of the nucleation mode seen in Figure 1 and further corroborates the suggested pathway for nucleation. During cruise the fraction of sulfate reached as much as 62% of total reconstructed PM mass in Veh1-DPF1þSCR1, the most heavily catalyzed nucleating system tested. The fuel economy of Vehicle1 has been reported previously.3 Average engine oil use can be calculated from the vehicle maintenance records. Combining the fuel and engine oil use with the measured sulfur content of each, sulfur emissions from Veh1 can be calculated to be 3.87 mgS/mi and 3.07 mgS/mi during the cruise and UDDS, respectively. This sulfur will be emitted either in the gas phase as SO2 or converted to SO3 in the aftertreatment and measured as SO4 in the particle phase or in some combination of gas and particle phase. Should all the sulfur be emitted in the particle phase, Veh1 has the potential to emit 2416

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Figure 4. Mass fraction of each chemical measured in emitted PM. * Denotes a nucleating configuration/cycle combination.

Figure 5. Total number emissions for particles with a diameter up to 20 nm (nuclei mode particle, gray bar) and greater than 20 nm (soot particle, black bar). * Denotes a nucleating configuration/cycle combination.

11.6 mgSO4/mi and 9.2 mgSO4/mi from the cruise and UDDS cycles, respectively. As expected, very little SO4 was measured in the PM of the baseline configuration, less than 1% of the calculated potential during cruise (Table S2). The amount of sulfur in the PM increases to 76% of this potential (8.82 mgSO4/mi) for Veh1-DPF1þSCR1, the greatest emitter of nucleation mode particles, during the cruise. The other configurations and cycles fall in between these two extremes, but their relative level can be predicted by how heavily they are catalyzed and the level of nucleation seen in Figure 1. It is expected that an even greater conversion of SO2 to SO3 could occur in configuration Veh1-DPF1þSCR1 under heavier loads that would lead to higher temperatures in the aftertreatment. Hence, the number emissions from these vehicles is determined in the first instance by whether or not conditions exist for nucleation, and if they do, on the amount of sulfur in the fuel and oil and on the amount of catalyst available to convert SO2 to SO3. Figure 5 shows the total particle (solid þ volatile) number emissions for the cruise and UDDS cycles and provides a better understanding of the total particle number emissions from DPF equipped vs uncontrolled HDDEs. In general, with the amount of sulfur present in the engine oil and fuel tested, the configurations with catalyzed aftertreatment had the potential to emit more particles than the baseline but did not always do so. The

nucleating configurations all emitted a greater number of particles during the cruise mode when high temperatures promoted continuous particle formation. The highest particle number emitter, Veh1-DPF1-SCR1, emitted 20 times as many particles as the uncontrolled baseline during this cycle. During the UDDS cycle, when temperatures were only high enough to oxidize SO2 to SO3 and form a nucleation mode for short periods, the nucleating configurations emitted on average an equal amount of particles as the uncontrolled baseline, ranging between emitting 75% less to 60% more, dependent on the configuration. It is important to note that the non-nucleating configurations emitted three to 4 orders of magnitude less particles than the baseline during these two cycles and that all DPF equipped configurations emitted an unmeasurable (less than background) number of particles during the idle cycle, while the baseline emitted 1.60 ( 0.05  1016 particles/h. In addition to comparing the number of particles emitted from configurations with aftertreatment vs the uncontrolled baseline, Figure 5 also differentiates between emissions of particles smaller than 20 nm and larger than 20 nm, underscoring a fundamental difference in the types of particles emitted from DPF equipped vs uncontrolled HDDEs. Emissions from Veh1-Baseline are dominated by particles larger than 20 nm, while those of the DPF equipped configurations are dominated by particles smaller than 20 nm. These particles are all considered UFP and could thus be lumped together as such. However, UFP particles from pre-2007 HDDEs consist of a solid soot core, coated with organics and sulfuric acid, while nucleation mode UFP from post-2007 HDDEs are smaller, consisting mainly of volatile material, such as sulfuric acid and ammonium sulfate coated with some organics. Since emission standards were inititally lowered individual particles have become smaller but remained morphologically the same, consisting of a solid soot core with adsorbed organics and ions. Once DPFs were introduced in 2007, particles are either largely absent from diesel emissions, or if the conditions exist for nucleation, the emitted particles are smaller and different from traditional diesel particles, being volatile, sulfurbased, and lacking a solid soot core. This research has important implications. First, all DPFs reduce the emissions of solid particles by several orders of magnitude. The nucleation mode particles that can be emitted under certain conditions are small, less than 20 nm, and different in composition from traditional diesel UFP, as they lack a solid soot core, are 2417

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Figure 6. Total particle number and expression in the DTT acellular and Macrophage ROS in vitro assays during cruise at 50 mph and UDDS. For the nucleating configurations higher levels of catalyst in the aftertreatment leads to both higher particle number and lower expression in these two assays.

volatile, and consist of ions and OC rather than EC. They are also different from the organic based nucleation mode particles that can be emitted from uncontrolled diesel engines and that are currently observed on-road. Second, exposure to high particle number concentrations derived from diesel engines with retrofit aftertreatment for PM and NOx control will occur mainly on or near those roads where temperatures in the aftertreatment reach the critical levels needed for nucleation, such as freeways and perhaps some major arterials or steep upgrades. These very small volatile particles have relatively short atmospheric lifetimes and are quickly removed as one moves away from the roadways.22-24 The smaller size and chemical composition will most likely affect the toxicity of post-2007 HDDE UFP. Figure 6 shows two measures of toxicity vs overall particle number emissions. In the current study the toxicity was determined by testing for reactive oxygen species (ROS) activity, by measuring the dithiothreitol (DTT) consumption rate12 and by in vitro exposure to rat alveolar macrophages.25 It has been suggested that particle number might be an indicator of toxicity. However, the particle number emissions and measures of toxicity measured in this study and shown in Figure 6 do not suggest such a relationship for either DTT or Macrophage ROS. The presence of catalytic aftertreatment, which encourages nucleation and therefore high particle number emissions, also appears to reduce the toxicity of emissions. For example the presence of catalyst effectively removes the water-soluble organics that have been shown to correlate well with DTT expression. This would explain the apparent inverse relationship between particle number emissions and toxicity seen in nucleating configurations in Figure 6. No sweeping conclusions can be reached from this result, and more measures of toxicity and health effects of diesel PM need to be made for a complete analysis. It does however suggest a rethinking of the health effects of particle number emissions (solid and volatile) from diesel engines. The aftertreatment tested in the current study are mainly retrofit devices and aside from DPF3 all rely on passive regeneration, meaning that the collected soot is removed slowly and continually without introducing additional energy to the system. OEM installed DPFs are widely expected to employ both passive and active regeneration, the latter in the form of either a diesel fuel burner or diesel fuel injector installed upstream of the DPF, used to temporarily increase the temperature in short discrete

events, as needed, to burn of the collected soot. During the discrete active regeneration events ATout temperatures can reach >500 °C which will most likely also release sulfur stored in the DPF and lead to nucleation. These devices will likely subsequently have the capacity store sulfur and thus repress nucleation for a considerable amount of time after each active regeneration.

’ ASSOCIATED CONTENT

bS

Supporting Information. Figure S1 shows the complete laboratory sampling setup, while Figure S2 shows the speed vs time trace of the UDDS cycle, and Figure S3 shows the particle size distribution measured in the CSV tunnel during idle. Table S1 shows the complete details of the tested vehicles, aftertreatment, and test configurations. Table S2 shows the emissions factors in mg/mi or mg/h. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: (916)324-9299. Fax: (916)322-4357/(916)323-1045. E-mail: [email protected]. Corresponding author address: Research Division, California Air Resources Board P.O. Box 2815, Sacramento, CA 95812.

’ ACKNOWLEDGMENT This project was funded by the California Air Resources Board, the South Coast Air Quality Management District, and the California Energy Commission. BP provided the diesel fuel for the study, and the California Department of Transportation, the San Joaquin Regional Transit District, the Elk Grove Unified School District, and the Sanitation Districts of Los Angeles County provided test vehicles. The authors would like to thank Dr. Subhasis Biswas, Dr. Harish Phuleria, Dr. Michael Geller, Dr. Constantinos Sioutas, Ning Zhi, Payam Pakbin, Mohammad Arhami, Julia Sandoval, Luzviminda Salazar, Ralph Rodas, George Gatt, and Keshav Sahay for their valuable support during experimental phase. We also acknowledge the retrofit device makers, whose technologies made this investigation possible. The statements and opinions expressed in this paper are solely the authors’ and do not represent the official position of the California 2418

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Environmental Science & Technology Air Resources Board. The mention of trade names, products, and organizations does not constitute endorsement or recommendation for use. The Air Resources Board is a department of the California Environmental Protection Agency. CARB’s mission is to promote and protect public health, welfare, and ecological resources through effective reduction of air pollutants while recognizing and considering effects on the economy. CARB oversees all air pollution control efforts in California to attain and maintain health-based air quality standards.

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