particulate matter in new technology diesel exhaust … · particulate matter in new technology...

ISSN: 1047-3289 J. Air & Waste Manage. Assoc. 61:894–913DOI: 10.1080/10473289.2011.599277Copyright 2011 Air & Waste Management Association

REVIEW PAPER

Particulate Matter in New Technology Diesel Exhaust (NTDE) isQuantitatively and Qualitatively Very Different from that Foundin Traditional Diesel Exhaust (TDE)

Thomas W. HesterbergNavistar, Inc., Chicago, IL

Christopher M. Long and Sonja N. SaxGradient, Cambridge, MA

Charles A. LapinLapin & Associates, Glendale, CA

Roger O. McClellanToxicology and Human Health Risk Analysis, Albuquerque, NM

William B. BunnNavistar, Inc., Chicago, IL

Peter A. ValbergGradient, Cambridge, MA

ABSTRACTDiesel exhaust (DE) characteristic of pre-1988 engines isclassified as a “probable” human carcinogen (Group 2A)by the International Agency for Research on Cancer(IARC), and the U.S. Environmental Protection Agencyhas classified DE as “likely to be carcinogenic to humans.”These classifications were based on the large body of healtheffect studies conducted on DE over the past 30 or so years.However, increasingly stringent U.S. emissions standards(1988–2010) for particulate matter (PM) and nitrogen oxi-des (NOx) in diesel exhaust have helped stimulate majortechnological advances in diesel engine technology anddiesel fuel/lubricant composition, resulting in the emer-gence of what has been termed New Technology DieselExhaust, or NTDE. NTDE is defined as DE from post-2006

and older retrofit diesel engines that incorporate a varietyof technological advancements, including electronic con-trols, ultra-low-sulfur diesel fuel, oxidation catalysts, andwall-flow diesel particulate filters (DPFs). As discussed in aprior review (T. W. Hesterberg et al.; Environ. Sci. Technol.2008, 42, 6437-6445), numerous emissions characteriza-tion studies have demonstratedmarked differences in regu-lated and unregulated emissions between NTDE and“traditional diesel exhaust” (TDE) from pre-1988 dieselengines. Now there exist even more data demonstratingsignificant chemical and physical distinctions betweenthe diesel exhaust particulate (DEP) in NTDE versus DEPfrom pre-2007 diesel technology, and its greater resem-blance to particulate emissions from compressed naturalgas (CNG) or gasoline engines. Furthermore, preliminarytoxicological data suggest that the changes to the physicaland chemical composition of NTDE lead to differences inbiological responses between NTDE versus TDE exposure.Ongoing studies are expected to address some of theremaining data gaps in the understanding of possibleNTDE health effects, but there is now sufficient evidenceto conclude that health effects studies of pre-2007 DE likelyhave little relevance in assessing the potential health risksof NTDE exposures.

INTRODUCTIONDiesel-engine exhaust (DE) exposures and health effectshave been studied extensively for decades. DE is a sourceof particulate matter (PM), nitrogen oxides (NOx), carbonmonoxide (CO), and a number of air toxics (e.g., aldehydes,

894 Journal of the Air & Waste Management Association Volume 61 September 2011

IMPLICATIONSBased on the distinct physical and chemical properties ofNew Technology Diesel Exhaust (NTDE), it has become clearthat findings from the health effects studies conducted ontraditional DE (TDE) over the last 30 years have little rele-vance to NTDE, which is more similar to the exhaust fromcompressed natural gas (CNG) or gasoline engine emissionsthan to traditional TDE. Once sufficient health effects dataare available for NTDE, it will thus be necessary to conductnew hazard and risk assessments for NTDE that are inde-pendent of the DE toxicological database acquired on emis-sions from pre–2007 diesel technology.

volatile organic compounds, polycyclic aromatic hydrocar-bons [PAHs]). Based primarily on the sizable toxicologicaldatabase characterizing DE from pre-1988 engines, severalregulatory agencies and scientific consensus groups haveconducted hazard assessments for DE (e.g., theInternational Agency for Research on Cancer [IARC],2 theU.S. Environmental Protection Agency [U.S. EPA],3 theU.S. National Toxicology Program,4 the U.S. NationalInstitute for Occupational Safety and Health,5 and theCalifornia Environmental Protection Agency6). Despitesome major uncertainties and limitations in both thehuman epidemiologic and laboratory animal evidencefrom the historical DE studies,7–10 these agencies have gen-erally concluded that sufficiently high DE exposures canincrease the risk of cancer (e.g., lung cancer) and noncancerhealth effects. Specifically, in 1989, IARC classified DE as a“probable” human carcinogen (Group 2A) based on “lim-ited” evidence in humans but “sufficient” evidence in rats.U.S. EPA in 2002 classified diesel exhaust as “likely to becarcinogenic to humans,” and in 2000 as a “mobile sourceair toxic.” Diesel exhaust particulate (DEP) was listed as a“toxic air contaminant” (TAC) by California EPA in 1998.

Given the regulatory concerns regarding DE healthrisks, U.S. EPA has implemented progressively more strin-gent DE standards for on-road heavy-duty diesel engines(HDDEs) since the promulgation of a smoke standard forthe 1970 model year.3 Limits on NOx, CO, and hydrocar-bons soon followed, and for the 1988 model year, U.S. EPAimplemented the first PMstandard forHDDEs (0.6 g/bhp�hr,using a transient test) that reduced overall diesel-fleet PMemissions by about 40%. As shown in Figure 1, U.S. EPA’sefforts culminated in a PM standard of 0.01 g/bph�hr for the2007 model year. NOx emissions standards have also beenreduced by approximately 99% compared to pre-1988engines, with the phase-in of aNOx standard of 0.2 g/bph�hrfor the 2010 model year. U.S. EPA PM emissions standardsfor off-road diesel engines have followed a different

schedule, taking effect in 1996 and 2000 for off-road equip-ment and locomotive engines, respectively.

The stringent emission limits prompted the develop-ment of significant technological advances in dieselengine technology, as well as major changes in the com-position of diesel fuel and engine lubricants. Theseadvances resulted in the emergence of what has beentermed “New Technology Diesel Exhaust”, or NTDE.1,7,8

As discussed further in the next section, NTDE has beendefined as emissions from post-2006 diesel engines andearlier-model diesel vehicles retrofitted with aftertreat-ment devices. It is a product of the innovative develop-ment of integrated, multicomponent emissions reductionsystems (engines, fuel injection systems, ultra-low-sulfurfuels, lubricants, and exhaust aftertreatment devices) tomeet the tightened U.S. EPA emissions standards.8 NTDEis distinct from, and contrasts in many ways from, “tradi-tional” DE (TDE), which refers to emissions from pre-1988diesel engines sold and in use prior to the U.S. EPA HDDEparticulate standards, and “transitional” DE, which con-sists of DE from 1988 to 2006 diesel engines manufacturedduring a time period of continuous improvements to theinternal design of the diesel engine, but prior to the full-scale implementation of multicomponent aftertreatmentsystems.

The widespread adoption of NTDE-compliant dieselengines in both the United States and Europe has stimu-lated a flurry of research characterizing pollutant emissionsin NTDE.1,11,12 We documented substantial reductions inemissions of a variety of regulated and unregulated species(e.g., PM, CO, nonmethane hydrocarbons, formaldehyde,benzene, acetaldehyde, and PAHs) in NTDE versus eitherTDE or “transitional”DE in a prior review of emissions datafrom transit buses, school buses, refuse trucks, and passen-ger cars.1 Since the 2008 publication of this review, resultsfrom numerous characterizations of NTDE emissions havebeen published, including Phase 1 findings from theAdvanced Collaborative Emissions Study (ACES),13,14 a ser-ies of findings from a collaborative study of the CaliforniaAir Resources Board (CARB) and the University of SouthernCalifornia (USC),15–21 and other detailed analyses.22–29

Importantly, these new studies have substantiallyimproved our understanding of how the DEP in NTDEdiffers from the DEP in TDE in terms of chemical andphysical properties that can affect toxicity. In particular,they address whether secondary emissions, such as nitro-PAHs and dioxins/furans, may be associated with exhaustaftertreatment systems. Other recent studies18,30–34 providepreliminary toxicological data for NTDE.

In our review, we thus assemble the evidence relevantto potential differences in the health risks of NTDE versusTDE. We focus on DEP emissions, and not diesel exhaustgases, given that DEP is considered to be a risk driver for DEhealth effects and has been a principal target of recentdiesel engine modifications and aftertreatments. As partof our analysis, we evaluate recent findings addressing thechemical composition and particle size distribution ofNTDE particulates, given that both chemical compositionand particle size distribution are regarded as key determi-nants of PM toxicity. We address two questions of specificinterest, namely whether there is evidence for the forma-tion of any new species of toxicological concern in NTDE,

Figure 1. U.S. EPA standards for particulate emissions from heavy-duty diesel trucks (t) or urban buses (ub), calculated as gramsparticulate matter emitted per brake-horsepower-hour (g/bhp�hr) andadjusted relative to pre-1988 unregulated engine emissions. FromHesterberg et al.8 and U.S. EPA Health Assessment Document forDiesel Engine Exhaust3 (Table 2-4, p. 2–16).

Hesterberg et al.

Volume 61 September 2011 Journal of the Air & Waste Management Association 895

and the effects of aftertreatment technologies on DEPnanoparticle emissions. We review the preliminary toxico-logical data for NTDE. An overarching purpose to thisreview is to address the question of the degree to whichthe prior DE hazard assessments do or do not apply toNTDE.

DEFINING NEW TECHNOLOGY DIESEL EXHAUST(NTDE)NTDE is the exhaust from diesel engines that incorporate avariety of technological advancements, including electro-nic controls, ultra-low-sulfur diesel (ULSD) fuel, oxidationcatalysts, and wall-flow diesel particulate filters (DPFs) cap-able of achieving U.S. EPA’s 2007 PM emissions standard of0.01 g/bhp�hr. Both DE from post-2006 diesel engines, aswell as DE from earlier engines retrofitted with a DPF andoperated with ULSD fuel (<15 ppm sulfur), are consideredto be representative of NTDE, providing that they canachieve the stringent 2007 PM standard. Although therecan be variation in their makeup, currently available multi-component aftertreatment systems have gained usage forcontrol of both DEP and gaseous DE emissions, consistingof a DPF for DEP control, a diesel oxidation catalyst (DOC)for hydrocarbon control, and advanced exhaust gas recir-culation (EGR) technology and/or absorbers or selectivecatalytic reduction technology (SCRT) for NOx control.Therefore, DE from a late-model diesel engine equippedwith only a DOC is not considered to be representative ofNTDE. ULSD fuel, which is now required in the UnitedStates and is essential for the proper functioning of DPFs,is also a critical component for an overall aftertreatmentsystem.

The DPF is now accepted as a centerpiece of NTDEaftertreatment systems needed to meet the stringent PMemission limits.11 As discussed by Maricq11 and Heebet al.,23 there are a variety of different DPF variationsdepending on the substrate material (e.g., cordierite or sili-con carbide [SiC] ceramic materials), regeneration strategy(e.g., continuous versus forced), and the use of catalysts(none, fuel-borne, coated or incorporated onto substrate).Heeb et al.23 discussed three major classes of DPFs, namely(i) porous or fibrous substrates coated with catalysts(e.g., catalyzed diesel particle filters, or C-DPFs), (ii)uncoated substrates that collect fuel-borne catalysts(e.g., transition metals such as platinum, iron, cerium),and (iii) uncoated filters that rely on active regeneration(e.g., fuel burners or electrical heaters). In particular, con-tinuously regenerating diesel particulate filters (CRDPFs),including Continuously Regenerating Trap (CRT; trade-mark of Johnson Matthey, Malvern, PA) and CatalyzedContinuously Regenerating Trap (CCRT; trademark ofJohnson Matthey) systems, which consist of an oxidationcatalyst followed by either an uncatalyzed or catalyzedDPF,have gained usage. It should thus be evident that there canbe numerous different aftertreatment configurations basedon DPF specifications alone, meaning that NTDE encom-passes the exhaust from a variety of different diesel enginetypes and aftertreatment configurations. All NTDE (post-2006 DE), however, is distinct from “traditional” (pre-1988) DE (TDE) and “transitional” (1988–2006) DE emittedfrom diesel engine technologies not compliant with the2007 HDDE emissions standards.

CHANGED CHEMICAL COMPOSITION OF DEPIN NTDEIt is now well-established that “order of magnitude” reduc-tions in PM mass emissions are typical ofNTDE.1,14,17,19,25,26 For example, for the recent CARB–USC testing of four heavy-duty and medium-duty dieselvehicles operated using six different aftertreatment config-urations (Table 1) and multiple test cycles, Biswas et al.17

reported consistent PM mass reductions of >90%, obser-ving in all cases PM emission rates of less than 0.01 g/mile(Figure 2). Given the role of chemical composition as a keyfactor affecting DEP toxicity, recent studies have furtherinvestigated how emissions of specific DEP constituentsare impacted by the use of different aftertreatmenttechnologies.

As discussed by Herner et al.,19 aftertreatment devicessuch as DPFs and SCRs can be viewed as “chemical reac-tors,” because they generate varying degrees of oxidizingand reducing conditions to promote PM filtration and com-bustion, and NOx conversion. Given that these conditionscan in turn have unintended effects on other DE species,there has been extensive speculation on whether new spe-cies of toxicological concern may be formed in NTDE andwhether any alteration or new formation of toxic speciesmay offset the public health benefits associated with thereported reductions in regulated and unregulated spe-cies.25,35 With the recent publication of a number of com-prehensive emissions studies, we are now in a betterposition to address this question and to characterize thekey distinctions in DEP chemical composition betweenNTDE versus pre-2007 DE.

Major Chemical SpeciesPhase I ACES findings13,14 provide some of the more com-prehensive data for demonstrating the major differences inDEP composition for NTDE versus pre-2007 DE. ACES isconsidered to provide a highly robust data set based onboth its measurement of a comprehensive set of regulatedand unregulated species across multiple engines/aftertreat-ment configurations, and its strong study design thataimed to minimize potential artifacts. The ACES studydesign was a product of highly detailed project planningthat benefited from independent oversight by both theHealth Effects Institute (HEI) and the CoordinatingResearch Council (CRC) and input from a wide range ofexperts and stakeholders on various advisory and steeringcommittees.13 Four new 2007-model-year diesel engineswere tested, including three engines equipped with aDOC and a C-DPF, and one engine equipped with anexhaust diesel fuel burner and C-DPF (see Table 1). UsingULSD fuel (sulfur content of 4.5 ppm; analyzed to assure nointerfering factors), each ACES engine was tested multipletimes on an engine dynamometer using the 20-min FederalTest Procedure (FTP) transient cycle, as well as a new 16-hrtransient cycle developed at West Virginia University thatcovers a complete engine operation with active regenera-tion events. Khalek et al.13 details the rigorous engine test-ing procedures and state-of-the-art analytical methodsemployed during the ACES emissions testing, whichincluded extensive conditioning of engines and samplingsystems and collection of blank samples in the constantvolume sampler (CVS) dilution tunnel.

Hesterberg et al.


Figure 3 contrasts the average DEP compositionreported by Khalek et al.13,14 for the four 2007-model-yearACES engines with a particle composition representative ofTDE.36 As illustrated by Figure 3, Khalek et al.14 observedPM from the 2007 ACES engines to be dominated by sul-fates (53%) and organic carbon (30%), with a substantiallyreduced elemental carbon content (13%) compared to thattypical of TDE (approximately 40% in Figure 3, but whichcan range as high as 90% depending on engine load andspeed). Table 2 summarizes the substantial reductions inDEP (and DE) constituents reported by Khalek et al.14 forthe ACES engines as compared to 2004 engines (1998engines for dioxins/furans), including 99%, 96%, and71% reductions for elemental carbon (EC), organic carbon(OC), and inorganic ions, respectively.

Findings from the CARB–USC diesel vehicle testingprogram provide further evidence of significant reductionsin both elemental and organic carbon emissions inNTDE.17,22 As part of this collaborative study, emissionsof size-resolved PM components from multiple HDDEswith and without various aftertreatment technologieswere characterized for different driving cycles (e.g., cruiseand the transient U.S. EPA Urban Dynamometer DrivingSchedule, or UDDS).17,22 Table 1 documents the fourheavy-duty diesel vehicles and variety of aftertreatmentconfigurations tested in the CARB–USC program, which

Table 1. Summary of diesel test vehicles in selected NTDE emissions testing studies.

Diesel Engine Aftertreatment

Vehicle Number/ID Vehicle Make Model YearSize (L) orPower (hp) Make and Type Description

CARB–USC diesel vehicle testing program (adapted from Herner et al.19)Veh1: Baseline Kenworth Cummins M11 1998 11 L None NoneVeh1: DPF1 Kenworth Cummins M11 1998 11 L JM CRT DOC þ uncatalyzed DPFVeh1: DPF1þV-SCR Kenworth Cummins M11 1998 11 L JM SCRT CRT þ vanadium-based SCR

þ small catalyst forammonia slip

Veh1: DPF1þZ-SCR Kenworth Cummins M11 1998 11 L JM SCRT CRT þ Fe-Z SCR þ smallcatalyst for ammonia slip

Veh2: DPF2 International International 1999 7.6 L Engelhard DPX Catalyzed DPFVeh3: DPF3 Thompson Cummins 2003 5.9 L Cleaire Horizon þ EGR Uncatalyzed DPF with electric

regeneration systemVeh4: DPF4 Gilig (35 ft

Hybrid)Cummins 2006 5.9 L JM CCRT DOC þ catalyzed DPF

ACES testing (adapted from Khalek et al.13,14)Engines treated as anonymous and data

generally reported on average basis acrossthe four engines

CaterpillarCumminsDetroit DieselVolvo Powertrain

CAT C13Cummins ISXDDC Series 60Mack MP7

2007200720072007

430 hp455 hp455 hp395 hp

Three of four engines equipped with DOC followed bywall-flow C-DPF; fourth engine equipped with anexhaust diesel fuel burner followed by a C-DPF; allengines equipped with EGR systems

Liu et al.28 organic species emissions testing2004 Engine Unspecified Unspecified 2004 15 L EGR system2007 Engine Unspecified Unspecified 2007 15 L EGR system, DPF (DOC and CSF),

crankcase emissions coalescerLiu et al.29 PCDD/F emissions testingEngine A Unspecified Unspecified 2010 8.9 L Either DOC-DPF or DOC-DPF-SCR (either Cu-Z or Fe-Z

SCR)Engine B Unspecified Unspecified 2010 12.9 L DOC-DPF-SCR (Cu-Z SCR)

Notes: JM¼ Johnson Matthey; CRT¼ continuous regeneration trap; DOC¼ diesel oxidation catalyst; DPF¼ diesel particulate filter; CSF¼ catalyzed soot filter; EGR¼ exhaustgas recirculation; SCR ¼ selective catalytic reduction; Cu-Z ¼ copper-zeolite; Fe-Z ¼ iron-zeolite; CCRT ¼ catalyzed continuous regeneration system.

Figure2. PMemissions fromCARB–USC test vehicle fleet equippedwith different aftertreatment technologies.17,18 Numbers indicate theremoval efficiencies (%) for the corresponding aftertreatment.Asterisks indicate authors’ note that these reduction efficienciesshould be given greater weight because these retrofits are directlycomparable to baseline (with no aftertreatment) due to the use of thesame vehicle. See Table 1 for vehicle and aftertreatment specifics.

Hesterberg et al.


included either no aftertreatment (baseline case, represen-tative of transitional DE) or one of several different cata-lyzed or noncatalyzed DPF systems.

As shown in Table 3, Biswas et al.17 reported largereductions in total carbon (TC) emissions, including both

the elemental carbon and organic carbon fractions, acrossthe retrofitted vehicles compared with the baseline, non-retrofit vehicle. For example, consistent reductions in EC of>99% were observed across the aftertreatment configura-tions for the UDDS cycle testing, with OC reductions of95% and higher.17 Other studies have also reported simi-larly high EC and OC reductions across a range of engines,aftertreatment configurations, test cycles, and fuel sulfurcontent.25,26,28,37,38 For example, Liu et al.28 observed>99% reductions for both EC and OC in their comprehen-sive study of emissions of more than 150 organic speciesfrom a 2007 engine representative of NTDE versus a 2004non-retrofit engine (see Table 1 for engine specifications).

Biswas et al.17 also reported consistently high reduc-tions (97% and higher) for the water-soluble organic carbon(WSOC) fraction across the various aftertreatment config-urations (Table 3). As discussed by Biswas et al.,17,18 WSOCcontent is thought to be relevant to DEP toxic potential,given research findings indicating that constituents of thisOC fraction, such as quinones, oxygenated PAHs, and alde-hydes, may contribute to the oxidative stress response asso-ciated with ambient PM. Although substantial reductions inWSOC were observed across the different aftertreatmentconfigurations, the ratio of WSOC to total OC was observedto depend on the aftertreatment configuration.17 In particu-lar, evidence of reduced relative OC solubility (WSOC/OC:8–25%) was observed for the retrofitted vehicles with cata-lyzed filters compared to the retrofitted vehicles with unca-talyzed filters (WSOC/OC: 60–100%).

As shown in Table 3, findings from the CARB–USCstudy indicate that there can be higher fractions of parti-culate sulfate and ammonium in NTDE as compared toTDE.17,18,22 For example, Biswas et al.18 reported baselinevehicle sulfate emissions of 0.09 and 0.36 mg/km for thecruise and UDDS cycles, respectively. For the retrofit

Figure 3. Representative compositions of particulate matter in NTDE (based on data from Khalek et al.14) and TDE (based on data fromKittelson36) from heavy-duty diesel engines tested in heavy-duty transient cycles. Note that there may be deviations from these compositions forparticular engines due to the variability in DEP composition based on such factors as engine model, operating conditions, and fuel and lube oilcompositions. Constituent labels are used as given in the actual study publications, recognizing that there are some differences in constituentcategories between the two studies. For example, “Unburnt Oil” differs from “Organic Carbon”, given there are sources of organic carbon in DE otherthan just unburnt oil. In addition, “Ash andOther” includes both elements and elemental oxides, whereas “Elements w/o sulfur” is more restrictive andrepresents just elemental contributions. Notwithstanding some differences in the data shown in the plots, they demonstrate that not only is less PMemitted in NTDE on a per-mile basis, but the emitted PM differs in composition from the PM emitted in TDE.

Table 2. Summary of average unregulated emissions for 12 repeats of the 16-hourcycles for all four ACES engines compared to CRC E55/E59 data for 2004 technologyengines (data from Khalek et al.14).

2004 Engines 2007 Engines

ChemicalSpecies

Average(mg/hr)

Average(mg/hr)

Average(mg/hr)

SD(mg/hr)

Average %Reduction

Relative to 2004Technology

Single ringaromatics

405 405 71.6 32.97 82%

PAHs 325 325 69.7 23.55 79%Alkanes 1030 1030 154.5 78.19 85%Hopanes/

steranes8.2 8.2 0.1 0.12 99%

Alcohols andorganicacids

555 555 107.4 25.43 81%

Nitro-PAH 0.3 0.3 0.1 0.03 81%Carbonyls 12500 12500 255.3 95.25 98%Inorganic ions 320 320 92.3 37.68 71%Metals and

elements400 400 6.7 3.01 98%

OC 1180 1180 52.8 47.1 96%EC 3445 3445 22.6 4.71 99%Dioxins/furans N/A N/A 6.20 �

10�55.20 �10�5

99%a

Notes: N/A ¼ not available. aRelative to 1998 technology engines.

Hesterberg et al.


vehicles, sulfate emissions ranged from 0.005 (Veh3:DPF3) to 5.6 (Veh1: DPF1þV-SCR) mg/km for theUDDS cycle, and from 0.02 (Veh3: DPF3) to 1.5 mg/km(Veh1: DPF1) mg/km for the cruise cycle (see Table 1 forretrofit vehicle IDs and specifications). Particulate ammo-nium emissions were shown to range as high as 30 timesabove the baseline case. Herner et al.22 provides furtherinsights on sulfate emissions from the CARB–USC retrofitvehicles, reporting correlations between the particle massand sulfate fractions and the formation of nucleation-mode particles. As discussed more later, Herner et al.22

demonstrated the formation of volatile sulfur-basednucleation-mode particles after the exhaust aftertreat-ment systems via oxidation of SO2 (formed from sulfurin fuel and engine oil) to SO3 for four of the retrofitvehicles meeting certain conditions in the aftertreatment(level of catalyst, exhaust temperature, and sulfur sto-rage). They demonstrated that sufficient numbers ofthese particles, consisting primarily of sulfuric acid andammonium sulfate with some organic coatings, could beformed such that they could dominate DEP mass frac-tions (up to 62% of reconstructed mass).

Other studies have also reported evidence of sulfateformation inNTDE, althoughwith some variability in find-ings.25,38–40 This may be due to some aftertreatment con-figurations not meeting the conditions needed for sulfatenucleation, or it may also be due to differences in experi-mental factors (e.g., test cycles, temperatures, dilutionratios, residence times, and sampling techniques) betweenstudies. Grose et al.39 and Kittelson et al.40 documented thegeneration of nucleation-mode sulfate particles during on-road testing of heavy-duty diesel engines equipped with aCRT and operated with fuels containing either 15 or 49ppm sulfur. They observed greater particle number emis-sions for engine loads sufficiently high to raise the exhausttemperature above 300 �C and for the higher sulfur fuel,with chemical analysis confirming that these particles con-sisted primarily of sulfates. Based on analyses of the volati-lity profile of the DEP emissions, Grose et al.39 furtherconcluded that these nanoparticles may exist as ammo-nium sulfates, which would be fully water-soluble. For

testing of a 2001-model-year HDDE (10.8 L), Liu et al.25

reported no detectable sulfate emissions for use of a C-DPF system, but increased sulfate emissions with the useof a SCR aftertreatment system.

PAHs and Other Organic SpeciesBecause PAHs are recognized as some of the more toxic DEconstituents, a number of studies have characterized PAHemissions inNTDE relative to pre-2007DE. Hesterberg et al.1

previously reported the highest emissions of total 2-, 3-, and�4-ring PAH compounds in diesel transit and school buseslacking aftertreatment. With use of aftertreatment,Hesterberg et al.1 reported that PAH emissions for transitbuses were significantly reduced (on average 60–97%reductions, depending on the aftertreatment type and thenumber of PAH rings) and of a similar magnitude, andsometimes lower, than buses operating with compressednatural gas (CNG) fuel with and without aftertreatment.Several recent studies have characterized emissions of asuite of individual PAH species in NTDE, including thosesuspected to be carcinogenic. Given that nitro-PAHs haveelevated genotoxic potential, recent studies have alsoaddressed concerns that the elevated temperatures andreactive compounds or catalysts used in DPFsmay promotenitro-PAH formation.11

As part of ACES, Khalek et al.13,14 measured emissions(gaseous and particulate) of 18 PAHs and 9 nitro-PAHs forthe four 2007-model-year ACES engines. Based on averagedemissions data representing 12 repeats of the 16-hr cyclesrun for each engine, Khalek et al.14 reported an overallaverage PAH reduction of 79% and an overall nitro-PAHreduction of 81% for the ACES engines compared to 2004-model-year engine technology (Table 2). Compared to datafor a 2000-model-year technology engine, reductions forindividual PAHs ranged from 80% (for naphthalene) to>99% (Table 4). Khalek et al.14 also reported consistentreductions of 92% and higher (compared to year 2000technology engine data) for all nine of the nitro-PAHsincluded as test analytes (Table 4).

As part of the CARB–USC testing program, Pakbinet al.20 reported emissions of particle-bound organic species

Table 3. Percent removal efficiency for DEP chemical species compared to baseline vehicle with no aftertreatment (UDDS cycle, mg/km basis).

Chemical Species Veh1: DPF1 Veh1: DPF1þV-SCR Veh1: DPF1þZ-SCR Veh2: DPF2 Veh3: DPF3 Veh4: DPF4

EC >99% >99% >99% >99% >99% >99%OC 95% 99% 99% >99% >99% >99%WSOC >99% 97% 98% >99% 98% >99%Nitrate >99% 23% 84% 15% 98% 41%Sulfate (596%) (1445%) (50%) (440%) 99% 92%Ammonium (2884%) (2362%) 50% (1216%) >99% >99%Potassium 47% 82% 87% 80% 97% 93%Chloride (2644%) >99% (206%) (167%) >99% 17%Alkanes >99% >99% >99% >99% >99% >99%PAHs >99% >99% >99% >99% >99% >99%

Notes: Results from the CARB–USC test vehicle fleet equipped with different aftertreatment technologies (see Table 1). Data from Biswas et al.18 EC¼ elemental carbon; OC¼organic carbon; WSOC ¼ water soluble organic carbon; PAHs ¼ polycyclic aromatic hydrocarbons; DPF ¼ diesel particulate matter filter; DOC ¼ diesel oxidation catalyst; V-SCRT¼ vanadium-based selective catalytic reduction technology; Z-SCRT¼ iron-based zeolite selective catalytic reduction technology; numbers in parentheses¼ increase inemissions with aftertreatment. Vehicle details in Table 1.

Hesterberg et al.


for three of the retrofit configurations (either a CRT system,a vanadium-based SCRT, or a zirconium-based SCRT, corre-sponding to the Veh1: DPF1, Veh1: DPF1þV-SCR, andVeh1: DPF1þZ-SCR IDs, respectively, in Table 1). For thebaseline case that is representative of transitional DE, emis-sion factors for PAH compounds ranged from 2.4 to 49 mg/km for the UDDS cycle, and 0.4 to 9 mg/km for the cruisecycle. In contrast, emission factors for NTDEwere below 0.1mg/km for the CRT system and below 0.02 mg/km with theSCRT systems, totaling reductions of >99% regardless of thecycle tested and the PAH species. The high PAH removalefficiencies of the CRT system have been attributed to itsability to both oxidize (DOC component) and filter (DPFcomponent), with the enhanced performance of the SCRTattributed to its ability to break apart heavy hydrocarbonsinto smaller molecules.20,25

Other studies have reported similarly high and consis-tent PAH removal efficiencies for C-DPF-retrofitted

engines,25,37,41,42 whereas other studies have observedmore variable removal efficiencies, in particular for themore volatile PAHs.23,24,28,43 For example, Heeb et al.23

characterized emissions of eight 4- to 6-ring PAHs from aHDDE (6.11 L) operated with two different cordierite-basedDPF systems and fuel-borne catalysts (iron- or copper/iron-based). For testing using the eight-stage InternationalOrganization for Standardization (ISO) 8178/4 C1 testcycle (representing a mix of full and partial engine loadoperation typical of construction machinery), theyobserved 40–90% reductions for the PAH test analytes,including 60–90% reductions for the six carcinogenicPAHs included in their testing. They found that the morereactive and less volatile PAHs were retained more effec-tively by the DPF, with greater reductions for pyrene (80%)as compared to the more volatile fluoranthene (40%). Infollow-up study of the performance of 14 different DPFs forthe removal of the same eight 4- to 6-ring PAHs using the

Table 4. Summary of PAH and nitro-PAH average emissions for the ACES testing (data from Khalek et al.14) and for the Liu et al.28 testing.

Khalek et al.14 Liu et al.28

PAH and nitro-PAHCompounds

2000 Technology Enginea

(mg/bhp�hr)2007 ACES Enginesa

(mg/bhp�hr)%

Reduction2004 Technology Engineb

(mg/bhp�hr)2007

Engineb

(mg/bhp�hr)%

Reduction

PAHs

Naphthalene 0.4829 0.0982 80 0.719 0.122 83Acenaphthylene 0.0524 0.0003 >99 0.0305 0.00218 93Acenaphthene 0.0215 0.0004 98 0.0455 0.0220 52Fluorene 0.0425 0.0013 97 0.131 0.0129 90Phenanthrene 0.05 0.0055 89 0.0786 0.0123 84Anthracene 0.0121 0.0004 97 0.00738 0.000862 88Fluoranthene 0.0041 0.0003 93 0.00431 0.00113 74Pyrene 0.0101 0.0004 96 0.0117 0.000979 92Benzo(a)anthracene 0.0004 <0.0000001 >99 0.000586 0.0000632 89Chrysene 0.0004 <0.0000001 >99 0.00105 0.000123 88Benzo(b)fluoranthene <0.0003 <0.0000001 >99 — — —

Benzo(k)fluoranthene <0.0003 <0.0000001 >99 — — —

Benzo(b,k,j)fluoranthene — — — 0.00024 0.00000776 97Benzo(g,h,i)fluoranthene — — — 0.000607 0.000258 58Benzo(e)pyrene <0.0003 <0.0000001 >99 0.000232 0.00000374 98Benzo(a)pyrene <0.0003 <0.0000001 >99 0.0000797 0.00000613 92Perylene <0.0003 <0.0000001 >99 — — —

Indeno(1,2,3-c,d)pyrene <0.0003 <0.0000001 >99 — — —

Dibenz(a,h)anthracene <0.0003 <0.0000001 >99 — — —

Benzo(g,h,i)perylene <0.0003 <0.0000001 >99 0.0000724 0.0000168 77

Nitro-PAHs

1-Nitronaphthalene — — — 0.000361 0.0000858 762-Nitronaphthalene — — — 0.000531 0.0000478 912- Nitrofluorene 0.000065 0.0000009 99 — — —

9-Nitroanthracene 0.0007817 0.0000031 >99 0.000192 0.00004032-Nitroanthracene 0.0000067 <0.00000001 >99 — — —

9-Nitrophenanthrene 0.0001945 0.0000153 92 — — —

4-Nitropyrene 0.0000216 <0.00000001 >99 — — —

1-Nitropyrene 0.0006318 0.00002 97 0.000055 <0.00000025 997-Nitrobenz(a)anthracene 0.0000152 0.0000002 99 — — —

6-Nitrochrysene 0.0000023 <0.00000001 >99 — — —

6-Nitrobenzo(a)pyrene 0.0000038 <0.00000001 >99 — — —

Notes: aAs discussed in Khalek et al.,14 averages are for 12 repeats of the 16-hr cycles for all four 2007 ACES engines, and for the 2000 technology engine operated over the FTPtransient cycle. The significant figures signify the detection limits in mg/bhp�hr. bAs discussed in Liu et al.,28 averages are for at least three tests where engines were operatedover the FTP transient cycle. Chrysene emissions are the sum of chrysene and triphenylene emissions.

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same HDDE and test cycle, Heeb et al.24 reported removalefficiencies of 31–87% for the DPFs they characterized asbeing of “low oxidation potential”, and 75–98% for theDPFs of “high oxidation potential”. For their study ofparticle-phase and semivolatile organic compound emis-sions from a model-year 2004 baseline HDDE and amodel-year 2007 engine with aftertreatment components(Table 1), Liu et al.28 reported PAH removal efficiencies inNTDE that varied between 52% and 98% for more than 20PAH compounds (Table 4).

Most studies have also generally reported high removalefficiencies for nitro-PAHs in NTDE, althoughwith variableresults for some specific nitro-PAHs, and evidence of possi-ble nitro-PAH formation.12,14,23,24,28,37,42,44,45 In interpret-ing these results, findings suggesting formation of nitro-PAH artifacts during sampling and PM collection cannot beoverlooked.46 Similar to the high nitro-PAH removal effi-ciencies reported for the ACES testing, Liu et al.28 reportedhigh removal efficiencies (76–99.5%) for the majority ofnitro-PAHs in NTDE from their 2007 engine compared toa 2004 technology (non-retrofit) engine (Table 3).However, Liu et al.28 also reported a “small increase” for 9-nitrophenanthrene emissions (data not provided). Heebet al.23 also reported increased emissions of several smaller,more volatile nitro-PAHs, including 9-nitrophenanthrene(and 1- and 2-nitronaphthalene, 9-nitroanthracene), for aDPF-equipped engine compared to a baseline engine. Heebet al.24 observed evidence for formation of 1-nitropyrene in“low oxidation potential”DPFs (63% increase in emissionscompared to the baseline case with no aftertreatment), butsubstantial reductions of this samemutagenic nitro-PAHby“high oxidation potential” DPFs. Ratcliff et al.42 did notobserve formation of 1-nitropyrene, although theyobserved it to be the most abundant nitro-PAH and tohave a reduced removal efficiency compared to other mea-sured nitro-PAHs (35% compared to >90%). These findingsmerit scrutiny due to the mutagenic properties of nitro-PAHs, but it is important to note that study findings gen-erally demonstrate significant reductions of nitro-PAHs inNTDE, and even for those specific nitro-PAHs that may notbe removed effectively, emission levels are very low (spe-ciated nitro-PAH emissions are generally in the ng/bhp�hrrange37).

Several recent studies support efficient removal ofother particle-phase and semivolatile organic species byDPFs and other aftertreatment components.14,20,28 In par-ticular, Liu et al.28 observed reductions of >90% for mostC1, C2, andC10 throughC33 particle-phase and semivolatileorganic compounds in NTDE, including >99% for all C11–

C24 n-alkanes and 97% and higher for hopanes andsteranes.

Given findings from Swiss researchers suggestingincreased dioxin/furan (PCDD/F) emissions with the useof copper catalyst materials, in particular in the presenceof elevated chlorine levels in fuel,35,47 a number of recentstudies have investigated PCDD/F emissions in NTDE. Ingeneral, studies have not replicated the Swiss findings,instead reporting consistent reductions in PCDD/F emis-sions for various aftertreatment configurations, includingfor engines operated with copper-zeolite (Cu-zeolite) andiron-zeolite (Fe-zeolite) SCR systems, copper/iron-based

fuelborne catalysts, and diesel fuels doped with elevatedchlorine levels.14,29,48–51 In particular, for a comprehensivetesting strategy that employed two different 2010 enginesoperated over a range of different exhaust aftertreatmentconfigurations (including Cu-zeolite SCRs operated withand without urea; see Table 1) using chlorine-doped dieselfuels (0.6 and 8.4 ppm), Liu et al.29 demonstrated signifi-cant reductions (60–80%) in PCDD/F emissions for all after-treatment configurations, with no impact of elevatedchlorine fuel levels (Figure 4). In addition, Khalek et al.14

reported 99% reductions in dioxin/furan emissions fromthe four 2007 ACES engines compared to 1998 technologyengines (Table 2).

Trace MetalsTrace metals make up a small fraction (e.g., <1%) of thetotal PMmass emissions from diesel exhaust, but are still ofparticular interest given the toxic potential of such speciesas lead (Pb), manganese (Mn), arsenic (As), and chromium(Cr).52 In addition, concerns have been raised thatincreased emissions of several metal species, such as vana-dium, copper, and iron, may be associated with the cata-lysts in some diesel exhaust aftertreatment systems.16,21

Although data from the CARB–USC testing program pro-vide some evidence of possible releases of catalystmetals,16,21 recent studies generally show that particulate-bound metals are reduced significantly with the use ofDPFs, and that further reductions may be achieved withSCR systems. Table 5 summarizes the significant reductionsreported by Hu et al.16 for most metal species across the

Figure 4. Comparison of PCDD/F emissions from multiple engines(A and B) and aftertreatment configurations expressed using the 1998World Health Organization (WHO) toxic equivalency quotients (TEQs)(adapted from Liu et al.29). The comparison assumes that nondetectsare present at the detection limit. Unless noted otherwise, all SCRsystems are Cu-zeolite SCRs and diesel fuel is doped to 0.6 ppmchlorine.

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CARB–USC aftertreatment configurations. Hu et al.16

observed some influence of driving cycle onmetal removalefficiencies, although high overall total trace metals reduc-tions were achieved for all DPFs for both the cruise andUDDS cycles (>85 and >95%, respectively). In addition,Khalek et al.14 reported an average reduction of 98% formetals and elements for the ACES 2007-compliant enginesas compared to 2004 technology engines (Table 2). Liuet al.25 reported a 93% reduction in total metals emissionswith a DPF, and an additional 25% reduction with a SCRsystem.

As shown in Table 5, Hu et al.16 reported increasedemissions of some metals (e.g., vanadium, platinum) forthe CARB–USC vehicles equipped with vanadium- andzeolite-based SCRT systems, thus providing some evidenceof possible releases of catalyst metals, particularly at thehigh temperature conditions of the cruise cycle. Theauthors emphasized, however, that these findings are likelyof little significance to real-world exposures, given thatconcentrations of these metals in the NTDE were still 2–3times lower than the levels of these metals measured inroadway tunnel studies.16 In contrast to the CARB–USCfindings, Chapman et al.53 reported no evidence of vana-dium losses in a laboratory experiment of a vanadium-based SCR intended to simulate lifetime catalyst exposure.For analyses of the water-soluble fraction of elements inDEP samples collected from many of the same enginesand aftertreatment configurations as Hu et al.,16 Vermaet al.21 reported consistent increases in the relative abun-dance of a number of redox-active transition metals (V, Fe,Mn, Ni, Cu, andCr) compared to data for the baseline (non-retrofit) truck. These increases were observed for retrofitswith and without SCRT systems, suggesting that theenrichment of these trace metals is associated with boththe oxidation catalysts and/or the embedded catalysts onthe DPFs, as well as the SCRT systems.

Summary on Changes to DEP ChemicalComposition

In summary, there is now an abundance of data demon-strating the significant reductions in specific DEP compo-nents of toxicological concern (e.g., elemental carbon,organic carbon, PAHs, nitro-PAHs, dioxins/furans, metals)that correspond to the order-of-magnitude reductions intotal PM mass typical of NTDE. Sulfate is one of the fewDEP species with some consistent findings of increasedemissions in NTDE as compared to TDE, but this species isgenerally regarded to be of low toxicity.54–56 In fact, ashypothesized by Grose et al.,39 the enrichment of ammo-nium sulfate particles in NTDE, rather than organic com-pounds, is more likely to result in decreased toxicity forNTDE compared to TDE. Although there is a need formore toxicological study to confirm this idea, Herneret al.22 reported an inverse correlation between numbersof volatile sulfur-based nucleation-mode particles andmea-sures of oxidation potential from chemical and cellularassays (from the Biswas et al.18 and Verma et al.21 studiesthat are discussed later), concluding that this increasedsulfate fraction was associated with reduced toxicity.Moreover, there remains no reliable evidence that speciesof toxicological concern are formed as a result of aftertreat-ment technologies. The preliminary toxicological data arereviewed later, but there is sufficient evidence from theDEPchemical characterization data alone to support the con-clusion that NTDE is toxicologically distinct from TDE andtransitional DE.

IMPACTS OF AFTERTREATMENT ON ULTRAFINEPARTICLE (UFP) EMISSIONSGiven that the major reductions in DEP mass emissionscharacteristic of NTDE are associated with correspondingreductions in “condensation surfaces”, it has beenhypothesized that advanced emission control

Table 5. Percent removal efficiencies for metal species, compared to a baseline vehicle lacking aftertreatment (CARB–USC diesel vehicle testing program data from Hu et al.16;on a ng/km basis).

Veh1: DPF 1 Veh1: DPF1þV-SCR Veh1: DPF1þZ-SCR Veh2: DPF2 Veh3: DPF3 Veh4: DPF4

Trace Metal Cruise UDDS Cruise UDDS Cruise UDDS Cruise UDDS Cruise UDDS Cruise UDDS

Ca >99% 95% 78% >99% 95% >99% 94% >99% >99% >99% NM >99%Zn >99% 99% 98% >99% 99% >99% 99% >99% >99% >99% NM >99%Cr 40% 92% 16% 89% (112%) 94% 89% 99% 93% >99% NM >99%Mn 93% 93% 72% 80% 81% 93% 98% >99% >99% >99% NM >99%Ni 50% 73% (464%) (35%) (307%) 78% 96% 90% >99% 95% NM >99%Pb 84% 92% 74% 80% 65% 91% 98% >99% >99% 84% NM 97%Ti 92% 92% (283%) 70% 71% 97% 94% 97% >99% >99% NM 98%V (57%) 50% (7929%) (1061%) (429%) 11% >99% (28%) >99% 94% NM 83%Cu 99% 99% 44% 45% (997%) 23% 88% 66% 10% >99% NM >99%Fe 69% 84% (30%) 5% 48% 92% 96% 99% 99% 98% NM >99%Pt (>100%*) 10% (>100%*) 90% (>100%*) 80% (>100%*) 90% ** >99% NM >99%S (306%) (143%) (868%) (51%) (472%) 85% INC 41% >99% >99% NM >99%

Notes: Percentages are relative to “0%”, which would signify an aftertreatment technology that yielded no change in emissions. Percentages in parentheses¼ percent increasein emissions with exhaust aftertreatment. *Pt was not detected in baseline vehicle emissions. **Pt was not detected with aftertreatment. NM¼ not measured. Vehicle details inTable 1.

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technologies, and DPFs in particular, may promote nuclea-tion and contribute to increased nanoparticle number con-centrations.36 These hypotheses were stimulated by a 1996Health Effects Institute (HEI) report57 that observed order-of-magnitude increases in nanoparticle emissions for a1991-model-year high-pressure direct-injection dieselengine equipped with an oxidation catalytic converter(OCC), compared to emissions from a 1988-model-yearbaseline diesel engine. Since the 1996 HEI report, a largenumber of diesel engine emissions studies have focused onnanoparticle emissions, in part due to the heightened inter-est in the potential health risks posed by nano-sizedparticles.9,58

There is now a sizeable emissions database for dieselnanoparticles in NTDE, although the lack of consistent pro-tocols for measurement of nanoparticle emissions fromdiesel engines is amajor limitation affecting their interpreta-tion, and in particular the comparisons of findings amongstudies. Evenmore basic are the differences among studies inhow diesel nanoparticles are defined, for example, DEP withdiameters <180 nm,15 DEP with diameters in the range of3–30nm,40DEPwithdiameters <50nm,57 etc. In addition tothe typical parameters of interest (e.g., engine specifications,aftertreatment configurations, fuel type, operating condi-tions), it is now well known that many experimental factorscan influence nanoparticle concentrations (both formationand decay), including relative humidity, temperature, dilu-tion ratio, dilution rate, and residence times, and may con-tribute to possible nanoparticle artifacts.59–61 Recognizingthe remaining uncertainties in the available studies and theneed for additional measurements using standard protocols,we highlight below some key findings related to diesel nano-particles in NTDE versus baseline diesel exhaust (i.e., TDE).As discussed in this section, there is evidence demonstratingthat DPFs can effectively remove diesel nanoparticles, butthat some aftertreatment configurations and operating con-ditions may contribute to formation of nucleation-modesulfate particles after the control devices.11,15,22,39,40,62

DPFs Can Effectively Remove Diesel NanoparticlesSome of the earlier evidence demonstrating the effectiveremoval of diesel nanoparticles by DPFs was provided bystudies investigating particle number emissions from dieseland CNG buses.63–65 In particular, for steady-state testing(both at idle and cruise) conducted using two dilution con-ditions (a minidiluter and a CVS), Holmén and Ayala63

reported that the use of a CRT typically reduced numberconcentrations of bothnucleation-mode and accumulation-mode particles by factors of 10–100. In a follow-up publica-tion that reported results for transient cycle testing (usingthree test cycles, including the New York Bus [NYB], UrbanDynamometer Driving Schedule [UDDS], and CentralBusiness District [CBD] cycles), Holmén and Qu64 againreported results indicating the strong performance of theCRT in reducing particle number concentrations.Compared to the baseline diesel engine, they observed con-sistent reductions in particle number concentrations ofmore than 2 orders of magnitude for the CRT-equippedbus. Similar to these findings, Nylund et al.65 reported thatparticle number concentrations were 2 orders of magnitudelower for a CRT-equipped bus than a baseline (non-retrofit)

bus in a study conducted at the VTT Technical ResearchCentre of Finland using three 2003-model-year Euro 3 dieselbuses (all the same model, but either having no aftertreat-ment, aDOC, or aCRT). Nylund et al.65 further reported thatthe CRT was highly effective at removing particles of all sizeclasses. Both studies generally reported similar levels of par-ticle number emissions among the retrofitted diesel busesand CNG buses.63–65

Other recent studies14,15,22,40,66–69 have also reportedsignificant reductions in particle number emissions inNTDE from DPF-equipped vehicles compared to vehicleswithout aftertreatment, thus demonstrating high removalefficiencies for DEP nanoparticles given that they typicallydominate DEP emissions on a particle number basis.9 Forexample, as shown in Figure 5, Khalek et al.14 reportedsignificant reductions in average total particle number con-centrations for the 2007 ACES engines, both with (89% forthe West Virginia University (WVU) 16-hr transient cyclethat included C-DPF-active regeneration events) and with-out regeneration (99% for the FTP cycle), compared toemissions from 2004 technology engines. For steady-state(50 mph cruise and idle) and transient (CBD and NYBcycles) testing of a 2000-model-year Isuzu medium–

heavy-duty delivery truck equipped with a CRT and fueledwithULSD, Ayala andHerner67 reported 98–99.9% removalof particles with diameters of less than 100 nm, and overallreductions in total number concentrations of 2–3 orders ofmagnitude. Similarly, based on testing of four current pro-duction European vehicles using two transient drivingcycles (the Common Artemis Driving Cycles and the NewEuropean Driving Cycle), Bosteels et al.68 consistentlyreported particle number emissions for a DPF-equippedvehicle that were more than 3 orders of magnitude smallerthan those for two diesel vehicles without aftertreatment,and at least 1 order of magnitude smaller than thoseobserved for a gasoline vehicle.

With the exception of several catalyzed aftertreatmentconfigurations that were associated with formation ofnucleation-mode sulfate particles (discussed in next sec-tion), findings from the recent CARB–USC testing programalso provide evidence of reduced nanoparticle emissions inNTDE from DPF-equipped vehicles.15,22 For the two non-nucleation configurations (Veh3: DPF3, which is the dieselbus equipped with a Cleaire Horizon electric particle filter,and Veh4: DPF4, which is the diesel hybrid electric busequipped with a CCRT), particle number emissions wereapproximately 3 orders of magnitude lower than for thebaseline (non-retrofit) vehicle for both cruise and UDDScycle testing (�1011 particles/mile vs. �1015 particles/mile).22 As discussed more in the next section, increasedparticle number emissions, and specifically formation ofnucleation-mode sulfate particles, was observed for mosttest aftertreatment configurations (Veh1: DPF1, Veh1:DPF1þV-SCR, Veh1: DPF1þZ-SCR, Veh2: DPF2) duringboth cruise (50 mph) and UDDS cycle testing.15,22 Thisformation of nucleation-mode sulfate particles, which con-tributed to increases in particle number concentrations aslarge as 20 times higher than the baseline case,22 maskedthe removal of DEP particles by DPFs. Importantly, emis-sions data for testing during idle cycles provide strong evi-dence for the high removal efficiencies of DPFs, withunmeasurable (less than background) particle number

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concentrations for all DPF-equipped configurations com-pared to particle number emissions of 1.60 � 0.05 � 1016

particles/hr for the baseline vehicle case.22

Evidence of Nucleation-Mode Particle Formationin NTDE

Although there is thus strong evidence demonstrating theeffectiveness of DPFs for removal of DEP nanoparticle emis-sions, there are now findings from several studies indicat-ing that some aftertreatment configurations may promoteformation of nucleation-mode particles in NTDE, such thatthere can be large overall increases in total particle numberconcentrations. In particular, several experimental studieshave reported that the formation of sulfate nucleation-mode particles is enhanced by the catalytic oxidation ofSO2 to SO3 that can occur in C-DPFs, as well as uncatalyzedDPFs with DOCs or SCRT systems.15,17,22,40,70 Enhancednucleation has now been demonstrated for a variety ofaftertreatment configurations, with study findings suggest-ing that its importance depends on both the aftertreatmentspecifications (e.g., catalytic loading, sulfur exposure his-tory), operating conditions (driving cycle, and more speci-fically, exhaust temperature and load), and fuel and engineoil sulfur content.22 Evidence of enhanced nucleation isreviewed below, with the caveat that experimental testconditions (e.g., dilution ratios, dilution rates, temperature,relative humidity, time lapse) may also be key factors driv-ing the observed nucleation that has been observed tooccur following exhaust aftertreatment devices.

Given its testing of a variety of aftertreatment config-urations over multiple driving cycles, the CARB–USC test-ing program offers some of the most useful data forcharacterizing the range of conditions under which

nucleation may be enhanced in NTDE. As mentionedabove, Biswas et al.15 and Herner et al.22 reported evidenceof dominant nucleation modes for several catalyzed after-treatment configurations, includingCRT and SCRT systemsand a C-DPF, generally for both the cruise and UDDS testcycles. Given that the formation of a dominant nucleationmode was only observed for configurations containing cat-alyzed aftertreatment, Herner et al.22 concluded that thepresence of catalytic surface was a necessary condition foroccurrence of a nucleation mode. Importantly, given thelack of a nucleationmode for the Veh4: DPF4 configurationthat contained themost heavily catalyzed aftertreatment inthe study (a CCRT system; see Table 1), Herner et al.22

concluded that the presence of catalytic surface was not asufficient condition on its own for formation of nucleation-mode particles. In addition, they did not always observeincreased particle number emissions for nucleating after-treatment configurations.

Based on their test results, Herner et al.22 identifiedsulfur storage and exhaust temperature to be other keydetermining factors of the magnitude of nucleation. Forexample, they attributed the low particle number emis-sions of the CCRT-equipped bus (Veh4: DPF4) to the initialcapacity of the new trap to store sulfates, hypothesizingthat once the trap had aged sufficiently and its storagesites had become saturated, formation of nuclei-mode sul-fate particles would occur.15,22 In support of the role ofexhaust temperature, Herner et al.22 pointed to evidenceof increased nucleation (as much as 20 times greater parti-cle number emissions than the uncontrolled baseline vehi-cle) during the continuous high temperatures achieved forthe constant, high-speed cruise cycle testing, contrastingthis with the more variable evidence of nucleation (which

Figure 5. Average particle number emissions (note the logarithmic scale) for 2007 ACES engines (with and without C-DPF regeneration) versus a2004 technology engine.14 As discussed in Khalek et al.,14 data for the 2007 ACES engines were based on 12 repeats of the 20-min Federal TestProcedure transient cycle (FTP-w) or 12 repeats of the 16-hr cycle, each for all four ACES engines and for sampling from an unoccupied animalexposure chamber set up on a constant volume sampler (CVS). Data for the 2004 technology engine were based on six repeats of the FTP transientcycle from a full flow CVS.

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ranged between 75% fewer to 60% greater emissions thanthe uncontrolled baseline) during the UDDS cycle testingwhen critical temperatures needed to oxidize SO2 to SO3

would only be achieved for short time periods. AlthoughHerner et al.22 concluded that nucleation was moretemperature-dependent than load-dependent, Vaaraslahtiet al.70 previously reported the presence of an enhancednucleation mode at high engine loads for a 1996-model-year HDDE operated using a continuously regeneratingDPF (CRDPF, with an upstream DOC).

Findings from the CARB–USC testing15,22 confirmedhypotheses that SCRT systems, designed to reduce NOx byreaction with ammonia, can have a secondary effect onsulfate nucleation.25,27 By conducting experiments wherethey bypassed the SCR portion of the aftertreatment sys-tem, Biswas et al.15 directly observed the impact of the SCRcatalyst on nucleation, with factor of 2–3 reductions inparticle number concentrations. In contrast to the CARB–USC findings for the SCRT, Guo et al.71 observed no evi-dence of nucleation for an exhaust system equipped withtwo active lean NOx (ALN) catalysts in series followed by aC-DPF. For steady-state testing and normal operating con-ditions, Guo et al.71 reported >99% trapping efficiency forthe C-DPF for two diesel fuels with differing sulfur content(340 ppm and 4 ppm) based on particle number concentra-tion measurements.

Investigators at the University of Minnesota have alsoreported findings that support a key role of sulfur storage onnucleation potential.40,72 In particular, for on-road testing ofa CCRT using the University of Minnesota Mobile EmissionLaboratory (which consisted of a Volvo diesel engine fueledwith low-sulfur [15 ppm] diesel fuel, and lubricated withlow-sulfur [1300 ppm] oil), Kittelson et al.40 reported reduc-tions in particle number concentrations to levels not detect-able above background. They attributed these reductions tosulfate storage on the washcoat used on the catalyst filter.They further hypothesized that as the CCRT aged and thesesulfate storage sites become filled over time, nucleation-mode nanoparticle generation would be observed.40

Building upon these findings, Swanson et al.72 demon-strated variable CRT performance depending on DOC age,with no nucleation modes observed for a new CRT systemtested at exhaust temperatures of almost 400 �C, but ele-vated number concentrations of nucleation-mode particlesfor both a usedCRT system and a usedDOC/newDPF. Basedon their findings, Swanson et al.72 hypothesized that nano-particle emissions associated with use of a CRT are due torelease of stored sulfates from the DOC rather than from theuncatalyzed DPF.

Although the CARB–USC testing15,22 did not addressthe potential role of active regeneration in DEP nanoparti-cle formation, several other studies have investigated thisquestion. In particular, Khalek et al.14 reported an increasein total particle number concentrations, and specifically inthe concentrations of sub-30-nm volatile nanoparticles,during C-DPF active regeneration events for the ACES2007-model-year engines. Due in part to the apparentincrease in nanoparticle concentrations during C-DPFactive regeneration, they observed an 88% increase in aver-age total particle number concentrations for their 16-hrtests, which included C-DPF active regeneration, versusthe 20-min FTP tests that did not include active

regeneration (Figure 5). They hypothesized that the releaseand subsequent nucleation of sulfates stored on the DOCand C-DPF was the source of the volatile nanoparticlesduring active regeneration. For their testing of severalDPF-equipped diesel passenger cars operated with ULSDfuel (<10 ppm sulfur), Mohr et al.73 also reported the gen-eration of a nucleation mode during regeneration, withorder-of-magnitude increases in particle number concen-trations. In contrast to the Khalek et al.14 and Mohr et al.73

findings, Guo et al.71 did not see any increase in particlenumber emissions above engine-out levels during regenera-tion for their testing of a 2.5-L York diesel engine operatedwith a C-DPF (and two ALN catalysts) and low-sulfur dieselfuel (4 ppm). Guo et al.71 did report a temporary nucleationmode during regeneration for tests where diesel fuel with asulfur content of 340 ppm was used.

Lastly, several studies have reported findings support-ing the role of fuel sulfur content on nucleation potential,observing reduced nucleation for lower fuel sulfur con-tents.61,66,70,71 In particular, Vaaraslahti et al.70 observed asmaller nucleationmode with low-sulfur fuel (2 ppm) com-pared to a higher fuel sulfur content (40 ppm).

Nanoparticles in NTDE Differ from Nanoparticlesin TDE

As discussed earlier, there is a growing body of evidencedemonstrating that nanoparticle emissions in NTDE have asulfate-rich composition because they are primarily asso-ciated with nucleation of sulfates after the controldevices.11,17,22,39,40,62 Importantly, this sulfate-rich com-position differs from the hydrocarbon-rich compositionthat studies have reported for the nanoparticles emittedfrom conventional diesel engines when a DPF is not pre-sent.22,74 In addition, Herner et al.22 reported evidence of ashift in the size distribution of nanoparticle emissions fornucleating aftertreatment configurations, with dominantemissions of particles smaller than 20 nm rather than the40–70-nm particles typical of the uncontrolled baselinecase. As briefly discussed below, these shifts in the compo-sition and size of nanoparticles in NTDE likely result in ashift in their toxicological potential.

An abundance of toxicological data supports the lowtoxicity of sulfate particles, which will tend to undergo dis-solution in the lungs regardless of their size.39,41–43,62 Incontrast, Tobias et al.74 concluded that diesel nanoparticlemass in engines lacking aftertreatment also contains somesulfate, but is dominated by insoluble elemental carbon andbranched alkanes and alkyl-substituted cycloalkanes fromunburned fuel and/or lubricating oil, some of which is likelyto remain in particulate form. Because the sulfate PM foundinNTDE nanoparticles is likely to be relatively soluble in thelung, it is not likely to persist there like the insoluble ECmaking up a large portion of the PM of TDE nanoparticles(see Figure 3). Although toxicological studies are needed, thesulfate-rich composition of NTDE nanoparticles thusmay contribute to their reduced toxicity compared tohydrocarbon-rich TDE nanoparticles, possibly mitigatingany potential health risks associated with their greater num-bers. As discussed earlier, the Herner et al.22 finding of aninverse correlation between numbers of volatile sulfur-basednucleation-mode particles andmeasures of oxidation poten-tial from chemical and cellular assays supports this idea.

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Finally, citing the increased volatility of the nucleation-mode sulfate particles compared to the solid soot particlescharacteristic of non-retrofit pre-2007 diesel engines, Herneret al.22 highlight factors that will likely contribute to theirreduced exposure potential, including shorter atmosphericlifetimes and rapid drop-offs in concentrationsmoving awayfrom sources (e.g., roadways).

Given the large uncertainties regarding the role of par-ticle size relative to other physical-chemical properties(e.g., surface area, surface chemistry, surface charge,shape, agglomeration state, chemical composition, crystalstructure, and solubility) in determining nanoparticle toxi-city,75 it is uncertain how the shift to smaller particle sizeswill affect the toxicity of NTDE relative toDE frompre-2007engines.

Summary on NTDE Nanoparticle EmissionsAs discussed above, there is now a bodyof study findings notonly demonstrating the effective filtration of nano-sizedparticles by a variety of different DPFs, but also indicatingthe potential for formation of a nucleation mode after thetailpipe for certain aftertreatment configurations and oper-ating conditions. It remains difficult, however, to interpretthis body of findings given the lack of standard protocols formeasuring DEP nanoparticle emissions and the potential fornanoparticle artifacts due to variable and sometimes unrea-listic experimental conditions, including dilution rates,dilution ratios, temperatures, measurement time since for-mation, and relative humidities. Notwithstanding theremaining uncertainties in the available data and theneed for toxicological confirmation, the shift from ahydrocarbon-rich composition to a sulfate-rich compositioncan be expected to contribute to an overall reduction in thehealth risks posed by DEP nanoparticles in NTDE, despitethe potential for increased emissions under some condi-tions. For additional perspective on the health significanceof DEP nanoparticles in NTDE, it is also important to notethat the presence of a nucleation mode is not unique toNTDE, as studies have reported similar and sometimesgreater particle number emissions in gasoline engineexhaust (GEE) and the exhaust of CNG engines than inNTDE.63–65,68,76–78

PRELIMINARY TOXICOLOGICAL DATA FOR NTDEAlthough there has been a dramatic increase in the numberof emissions characterization studies for NTDE, there stillremain relatively few toxicological data for NTDE. Theneed for a large-scale toxicological evaluation of NTDE hasbeen well recognized for a number of years, serving as amotivating factor for the design of the comprehensivehealth effects components of the ongoing AdvancedCollaborative Emissions Study (ACES). As described in theproject plan for ACES79 and a recent presentation,80 theACES research plan includes biological screening assays forboth mice and rats, both cancer and noncancer healtheffects, and both short-term and long-term exposures. Acore component of ACES is a chronic rat bioassay whererats are exposed via inhalation for 24 or 30 months, withinterim sacrifices at 1, 3, 12, and 24 months, to three dilu-tions of whole emissions from a 2007-compliant dieselengine with advanced emission control technologies (andclean air controls). The specific engine being used is similar

to one of the four engines evaluated by Khalek et al.,13,14

with exposures taking place in an animal exposure facilityspecifically developed for ACES by the Lovelace RespiratoryResearch Institute (LRRI). Health endpoints of interestinclude not only carcinogenicity but also pulmonary func-tion, pulmonary inflammation, oxidative damage, lung cellproliferation, histopathological changes, and hematologicaleffects. In addition to the chronic rat bioassay, LRRI is alsoconducting a 13-week subchronic mouse bioassay, againwith three dilutions of whole emissions and a similar set ofhealth endpoints (excluding carcinogenicity and pulmon-ary function).

Until health-effects findings from ACES are available(2011–2014 time frame), we can rely on a preliminary set oftoxicological findings for NTDE available from a limitednumber of human controlled-exposure studies, animal stu-dies, and in vitro bioassays. These are discussed below inorder of their relevance for assessing potential humanhealth risk to NTDE. Given that most have involved expo-sures to whole emissions rather than just the DEP fraction,they provide insights on the biological activity of both theparticulate and gaseous species in NTDE. As discussedbelow, although limited in number, these studies are con-sistent in supporting toxicological distinctions betweenNTDE, including its DEP fraction, and pre-2007 DE.

Preliminary Findings from HumanControlled-Exposure Studies

As discussed in a previous review,81 human controlled-exposure studies are considered to provide some of themost relevant data for assessing the potential health risksof DE exposure given that they directly study human sub-jects, use well-defined exposure concentrations and dura-tions, and precisely measure health outcomes includingsubtle biological responses. We identified only a singlepeer-reviewed journal publication of human clinical dataof relevance toNTDE, as well as two abstracts from the sameresearch team.30–32 As described in these publications, aresearch team led by researchers at Umeå University andthe University of Edinburgh has conducted human clinicalexperiments of adverse vascular and prothrombotic effectsusing DE from a Volvo diesel engine (Volvo TD40 GJE, 4.0L, 4 cylinders) operated under transient conditions withand without a CRT particle trap. Using a randomized,double-blind, three-way crossover design, 19 healthy malevolunteers (mean age, 25 � 3 yrs) were exposed to filteredair, unfiltered dilute diesel engine exhaust, and dilute dieselengine exhaust during 1-hr periods of alternatingmoderateexercise and rest. These investigators assessed responses to anumber of surrogate measures of adverse cardiovasculareffects that they had previously shown to be affected byelevated whole-DE exposures without a particle trap inplace,82–85 including endothelial vasomotor and fibrinoly-tic function and ex vivo thrombus formation.

As discussed in Lucking et al.,32 DEP mass concentra-tions in the exposure chamber were reduced 98% with useof the particle trap (from 320 � 10 to 7.2 � 2.0 mg/m3; P <0.0001), and fine (<1 mm) particle number concentrationswere reduced by >99.8% (from 150,000–300,000/cm3 to30–300/cm3; P < 0.001). Use of the particle trap was foundto reduce the elevations in thrombus formation and toeliminate the impaired vasodilation and fibrinolytic

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function observed for unfiltered DE exposures.30–32 Basedon these consistent findings showing not only improvedresponses but also normalization, the investigators con-cluded that retrofit particle traps appear to have “beneficialeffects” on surrogate biomarkers of cardiovascular health.32

Limited Laboratory Animal EvidenceWe identified just a single laboratory animal study that hasinvestigated the health effects of an inhaled diesel exhaustmixture representative of NTDE, namely the McDonaldet al.33 study. McDonald et al.33 investigated the relativetoxicity of acute inhalation exposures (6 hrs per day over 7days) for a baseline uncontrolled, TDE emissions case(approximately 200 mg/m3 DEP) versus an emissions reduc-tion case (low-sulfur fuel, catalyzed ceramic trap) on a suiteof sensitive measures of acute lung toxicity in mice, includ-ing lung inflammation, respiratory syncytial virus (RSV)resistance, and oxidative stress. McDonald et al.33 reportedthat the use of the in-line catalyst trap on the test engine (aYanmar single-cylinder diesel engine generator), as well aslow-sulfur fuel, reducedmost DE exposuremeasures, includ-ing particle mass and number concentrations, elementalcarbon, and particle-bound PAHs, to near background levelsin their exposure chamber (with NOx being a notable excep-tion, where only a 10% reduction was observed—from 2.1ppm down to 1.9 ppm). For the baseline TDE case,McDonald et al.33 observed statistically significant DE-induced effects for each class of responses, whereas theseeffects were either nearly or completely eliminated for theemissions reduction case. Despite the need to confirm thesefindings for a broader range of engines, aftertreatment con-figurations, operating conditions, and classes of health end-points (e.g., cardiovascular effects, allergenic effects),McDonald et al.33 concluded that their findings suggestthat aftertreatment technologies can mitigate potentialhealth hazards of the associated DE exposures.

Recently, Tzamkiozis et al.34 reported the results of astudy where mice were exposed via intratracheal instilla-tion to waterborne suspensions of exhaust PM collectedfrom several different diesel and gasoline vehicles, includ-ing a Euro 4 diesel car (TDE-like) and the same car retro-fitted with a DPF and representative of a Euro 4þ vehicle(NTDE-like). No evidence of a local inflammatory cellularresponse for the Euro 4þ samples was observed comparedto sham controls (as assessed by influx of polymorphonuc-lear neutrophils [PMNs] in bronchoalveolar lavage [BAL]),but the investigators reported a statistically significantincrease in BALprotein levels, an indicator of alveolar tissueinjury, for the highest of the two test doses. A similarresponsewas observed for PM samples from all test engines,with the greatest response for the Euro 4 diesel car.Importantly, the exhaust stream for the Euro 4þ diesel carwas diluted 100-fold less than that of the Euro 4þ diesel carand the other diesel test vehicles (12,000:1 vs. 120:1), indi-cating that its response on a per-mile-traveled basis wouldbe significantly less that of the other diesel test vehicles.Overall, these results are of indeterminate human healthrelevance due to the unrealistic exposure scenario (water-borne PM suspensions, intratracheal instillation) and theuncertain clinical significance of the biological responses.

Results from In Vitro Studies of NTDEAlthough of limited relevance to human health risk andthemselves few in number, in vitro studies currently offerthe largest amount of toxicological data for NTDE. In vitrostudies can offer some insight on the relative toxicity ofNTDE versus TDE, although they are also limited in thisregard due to the numerous differences from the in vivosituation that affect the interpretation of their findings forDEP, including (1) absence of the normal lung-defensemechanisms (e.g., macrophage mediated and mucocilliaryclearance); (2) absence of cellular protective mechanismssuch as antioxidants and DNA repair that act to preventthe expression of intracellular damage or DNA mutations;(3) extremely high doses compared to what is deposited inthe alveolar regions of the lung after inhalation, thus elicit-ing high-dose responses not mechanistically relevant tolower doses; (4) dosing with compounds concentratedfrom DEP by high-temperature, organic solvent extraction,that is, compounds that appear to be much less bioavailablefrom inhaled and lung-retainedDEP; and (5) possible dosingwith reactive artifacts—for example, nitrated organic com-pounds—formed on filters due to the extended collectiontimes needed to obtain enough DEP mass.7,8 Recognizingtheir inherent limitations, we briefly review findings fromin vitro studies of NTDE below.

Several of the recent in vitro studies of NTDE haveassessed the oxidative potential of the DEP fraction ofNTDE versus that of other engine exhausts, including onestudy that used a sensitive macrophage-based in vitroassay21 and two that used the molecular (cell-free) dithio-threitol (DTT) assay18,86 to assess reactive oxygen species(ROS) activity. For a range of different engines and after-treatment configurations, these studies reported consis-tent, large reductions in overall ROS activity (expressed ona per-distance-traveled basis) for diesel retrofits comparedto baseline (non-retrofit) engines. Using a rat macrophage-based assay, Verma et al.21 observed significant reductionsin overall ROS activity (per distance traveled for cruise andUDDS test cycles and per hour for idle) for the variousaftertreatment configurations included in the CARB–USCtesting program, compared to the non-retrofit baselinevehicle. Also for the CARB–USC vehicles but for a DTTacellular assay, Biswas et al.18 reported uniformly highreductions (60–98%) in oxidative potential expressed perunit vehicle distance traveled for the retrofitted vehicles,including some of the highest reductions for the two SCRTsystems. As discussed by the study authors, these resultssuggest that the SCRT systemsmay contribute to the reduc-tion and removal of toxicologically important organic com-pounds. Furthermore, as discussed previously, Herneret al.22 examined the relationship between the two mea-sures of ROS activity and particle number emissions ofvolatile sulfur-based nucleation-mode particles, observinga negative correlation and concluding that nucleationevents in catalyzed aftertreatment systems may contributeto reduced NTDE toxicity. In a study of three light-dutyvehicles in five different configurations, Cheung et al.86

reported that a DPF-equipped Euro 4þ Honda Accord (2.2L, i-CDTi) diesel car had the lowest per km oxidative poten-tial (as assessed using the acellular DTT assay) among theirtest vehicles, which included a Euro 3 gasoline vehicle.

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Both Biswas et al.18 and Verma et al.22 also reportedfindings indicating an increase in per-PM-mass oxidativeactivity for most aftertreatment configurations comparedto the CARB–USC baseline vehicle. However, they bothemphasize the reductions in particulate mass emissionsfor all aftertreatment configurations that contribute to theoverall reduced oxidative activity expressed on a per-distance-traveled basis for the DEP in NTDE versus baselineengines. Interestingly, Verma et al.22 reported findings sug-gesting that the increases in the per-PM-mass oxidativeactivity for the retrofitted configurations may be tied toincreases in their fractions of redox-active transitionmetals(e.g., Mn, V, Ni, Cu, Fe, and Cr).

Other studies43,65,87–91 have investigated the effect ofaftertreatment on DEP mutagenic activity, despite the evi-dence indicating that whole DEP (as opposed to solventextracts of DEP) in TDE is not genotoxic to cells in culturedue to the minimal bioavailability of the mutagenic com-pounds (e.g., PAHs, nitro-PAHs) in DEP in lung fluids.7,8 Asdiscussed in two prior reviews7,8, in vitro evidence for themutagenic activity of DEP primarily comes from studiesthat used hot organic solvents to extract the organicsfrom DEP, and that also may have been affected by artifac-tual formation of nitro-PAH on DEP filter samples. Theselimitations apply to the two studies that providemutageni-city test results for combinations of engine systems, after-treatment devices, and fuel types representative of NTDE,namely the CARB43,87 and VTT Technical Research Centreof Finland65 studies where DEP emissions from moderndiesel buses operated with either no aftertreatment, an

oxidation catalyst, or a CRT filter were assessed using amodified Ames test (Salmonella/microsome test). Figure 6shows that the lowest mutagen emissions in these studieswere generally observed for the CRT-equipped buses. Bothstudies, however, reported the highest findings for specificmutagenic activity (SMA), which is defined as the numberof revertant bacteria (rev) per mass of PM collected(e.g., rev/mg PM), for the CRT-equipped buses. For perspec-tive, Figure 6 shows that CARB researchers have observedapproximately 2–3-fold higher SMAs for particulate emis-sions from a CNG bus with aftertreatment (catalyzed muf-fler) than for the CRT-equipped buses.91

Several recent studies have used reporter gene bioas-says as screening tools for assessing the biological activity offiltered and unfiltered DE.23,24,48 Based on findings thatinclude substantially reduced responses in multiple bioas-says for DEP fromDPF-equipped engines compared to base-line diesel engines lacking aftertreatment (e.g., 80–90% and55–66% reductions in responses for the aryl hydrocarbonreceptor [AHR] and estradiol receptor [ER], respectively24),they provide support for the reduced biological activity ofDEP in NTDE.

Although not without its own limitations, the recentHasson et al.92 study merits some comment given that itprovides findings relevant to the potential role of regenera-tion events in affecting NTDE toxicity. More specifically,Hassan et al.92 used a rat lung slice organotypic in vitromodel to assess the acute toxicity of NTDE emitted froman advanced emission control technology diesel engine(2.8 L) equipped with a C-DPF and operated with and

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Figure 6. Results of Ames bacterial mutagenicity test results from the Finnish VTT study65 of diesel buses with and without aftertreatment (Euro 3buses) and from the CARB study43,87 of diesel buses with and without aftertreatment operated using three different diesel fuels (ECD, ECD1, andCARB fuels). Data shown are for the Salmonella strain TA98 with metabolic activation (þS9) and the particulate fraction only. The VTT study buseswere operated using the European Braunschweig bus cycle, whereas the CARB study buses were operated using the Central Business District(CBD) cycle for transit buses. Average CNG particle-associated mutagenic activity and emissions are from CARB testing91 of a CNG transit buseswith aftertreatment (a catalyzed muffler).

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without regeneration events. Despite high acute exposuresof lung slices to 1%, 5%, or 10% exhaust mass fractions and3–4-fold increases in emissions of 30-nm nanoparticles and100–200-nm fine particles during regeneration, Hassanet al.92 reported the absence of a significant acute biologicalresponse (based on markers of tissue viability, oxidativestress, proinflammatory cytokine release, and exhaust oxi-dant potential) for NTDE emitted during the transient NewEuropean Driving Cycle (NEDC) with and without regen-eration events.

Conclusions Regarding Weight of CurrentToxicological Evidence

Although there remain far fewer toxicological data forNTDE than for TDE and transitional DE, limited data arenow available for multiple lines of toxicological investiga-tion, including human clinical studies, laboratory animalstudies, and in vitro bioassays. These toxicological data arecurrently limited and insufficient on their own to supportreliable conclusions regarding the toxicological potency ofNTDE compared to that of TDE or transitionalDE. Nevertheless, they are consistent in supporting theconclusion that there are toxicological differences betweenNTDE and both TDE and transitional DE. In particular,available human clinical data for NTDE exposures showan absence of some biological responses previouslyobserved in studies of older diesel engine technologies.30–32

Although additional human clinical studies are clearlyneeded to address a larger suite of health endpoints, theavailable study findings are more relevant to the risk assess-ment ofNTDE thanexperimental findings, in particular thosefrom in vitro studies. In addition, with its well-characterizedexposure atmosphere and its assessment of a suite of sensitivemeasures of acute lung toxicity, the McDonald et al. animalstudy33 provides some of the strongest evidence of thereduced health risks posed by NTDE. However, as a singlestudy that addressed only acute lung toxicity for a singleengine, aftertreatment configuration, and animal species,there is clearly a need for more comprehensive toxicologicalinvestigations of NTDE to confirm the toxicological differ-ences between NTDE and TDE indicated by the chemicalandphysical characterizationdata.On this note, it is expectedthat ACES will soon provide a wealth of information forassessing the potential carcinogenicity, and the subchronicand chronic noncancer toxicity of NTDE.

Given that studies such as McDonald et al.33 have gen-erally employed the same dilution rates for NTDE as for base-line diesel exposures, it is unclear from the currenttoxicological data the extent to which the observations ofreduced NTDE potency are simply due to the reduced expo-sures to the various DE constituents versus changes to thechemical and physical properties of DEP in NTDE. There issome evidence from the in vitro tests of mutagenicity andoxidation potential that the biological activity (on a per-massbasis—i.e., permg of DEP) of DEP inNTDE is not significantlyreduced, and may even be higher, than that of DEP emittedfrom baseline diesel engines lacking aftertreatment.However, findings demonstrating reductions in oxidationpotential and mutagen emissions for NTDE expressed on aper-distance-traveled basis have greater relevance for asses-sing potential health risk implications.18,22 In addition, it isimportant to note that these in vitro screening assays have

some utility as indicators of potentially toxic compounds,but given their well-known limitations, it is not possible toextrapolate human health risks from their findings.

CONCLUSIONS AND IMPLICATIONSAs discussed in detail in this review, there now exists a largebody of evidence showing that the DEP in NTDE is chemi-cally and physically distinct from theDEP in TDE and transi-tional DE. There are also preliminary toxicological datasuggesting that these differences in DEP emissions, both interms of emissions levels (on a mass and number basis) andin terms of chemical and physical properties, contribute todifferences in the risk profile of NTDE versus TDE exposures.There remain some large data gaps that limit the currentunderstanding of the toxicological potency of NTDE, butboth the emissions differences and preliminary toxicologicaldata support reductions in the potential health risks posedby NTDE in real-world exposure scenarios. Moreover, thereis clearly now a greater level of support for the idea that thehistorical data from animal laboratory and human epide-miological studies of TDE have only limited relevance inassessing the potential health risks of NTDE exposures. Infact, U.S. EPA emphasized in the 2002 Health AssessmentDocument for Diesel Engine Exhaust that its findingsapplied only to engines that were manufactured prior to1995 and that newer technology engines would require areevaluation with regard to potential health impacts.3

Furthermore, there is a growing body of data indicatingthat PM emissions in NTDE have a greater resemblance toPM emissions in CNG exhaust and GEE than to TDE, bothin terms of mass emissions and particle composition. In aprior review,1 we demonstrated that exhaust-aftertreatment technologies reduce emissions for numer-ous regulated and unregulated species in diesel buses tosimilar levels for CNG-fueled buses. Figure 7A illustratesthis, showing that particulate mass emissions (g/mile) inNTDE from transit buses tested with the Central BusinessDistrict cycle are more comparable to the emission levelsfrom CNG-fueled transit buses than from TDEbuses.47,93–100 Figure 7B shows that for particulate massemissions (g/mile) in passenger cars, NTDE emission levelsare more comparable to gasoline- and CNG-fueled vehi-cles.76,101,102 In summary, the transit bus and passengercar data show that NTDE mass particulate emissions are20–70 times lower than those for TDE, and are in therange, if not lower than, particulate mass emission levelsreported for CNG- and gasoline-fueled vehicles.

In addition, Figure 8 shows data from the Cheung et al.study86 that included emissions testing of a Euro 3 gasolinevehicle in addition to two diesel vehicles with varyinglevels of aftertreatment (Euro 4þ with a DPF, DOC, andEGR, which is representative of NTDE; and Euro 1 with noaftertreatment, which is representative of TDE). This figureillustrates that the emissions permile were lowest for NTDEfrom the Euro 4þ diesel vehicle for all compounds tested(total PM mass, organic carbon, elemental carbon, water-soluble carbon, sulfate, ammonium, and sum of all inor-ganic species), except one (nitrate). In their emissions char-acterization study of four current production Europeanvehicles, Bosteels et al.68 reported comparable, and moreoften lower, emissions of both particulate- and vapor-phasePAHs for a DPF-equipped diesel vehicle versus a gasoline

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vehicle. As discussed earlier, findings from a number ofstudies indicate that nanoparticle emissions (and total par-ticle number emissions) are now comparable, if not lower,for DPF-equipped diesels versus CNG and GEE vehi-cles.63–65,68,76–78 Finally, Figure 9 compares the ratio of ele-mental carbon (EC) to total carbon (TC; where TC ¼ EC þorganic carbon [OC]) for different engine exhaust types,providing further evidence that NTDE particulate is morecomparable to engine exhaust particulate from CNG- andgasoline-fueled engines than to TDE particulate.63,103,104 Asshown in Figure 9, EC/TC ratios for NTDE are substantiallylower than those for both TDE and gasoline-engine parti-culate for both transient and steady-state test cycles, bear-ing the greatest resemblance to those for CNG particulate.

Just as one would conclude that the health studies con-ducted on TDE over the last 30 years have little relevance toCNG engine exhaust andGEE, there is now evidence indicat-ing that they have little relevance to NTDE. Additional tox-icological data are clearly needed to conduct a reliable hazardassessment for NTDE given the uncertainties in the limiteddata that are currently available, but the available emissionsand toxicology data indicate that the historical DE hazard

Figure 7. Particulate emissions (PM; g/mile) for transit buses (A) andpassenger cars (B) of different engine technologies. Data for transitbuses include TDE, NTDE, and CNG exhaust (both withoutaftertreatment and with oxidation catalyst; CNGþOC), with all testingfor the Central Business District test cycle (means, standard errorsplotted).44,93–100 Data for passenger cars include TDE, NTDE, andCNG and gasoline exhaust, with pooling of data from a variety oftransient test cycles (means, standard errors plotted).76,102

Figure 8. Comparison of mass and chemical species emissions(mg/mile, note the logarithmic scale) for light-duty vehiclesrepresentative of NTDE, GEE, and TDE tested on a chassisdynamometer for a cold-start New European Driving Cycle (NEDC)and a series of Artemis cycles.86 Specific vehicle configurationsinclude a Euro 4þ Honda Accord (2.2 L, i-CDTi) equipped with aceramic-catalyzed diesel particulate filter (DPF), a closed-coupledoxidation catalyst (pre-cat), and exhaust gas recirculation (EGR),operated using low-sulfur (<10 ppm) diesel fuel and lube oil with asulfur content of 8900 ppm wt (considered to be NTDE); a Euro 3Toyota Corolla (1.8 L) equipped with a three-way catalytic converterand operated using unleaded gasoline with a research octane number(RON) of 95 and fully synthetic lube oil (considered to be GEE); and aEuro 1 compliant Volkswagen Golf (TDI; 1.9 L) operated using dieselfuel with a nominal sulfur content of 50 ppmwt (considered to be TDE).

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assessments relied upon by various regulatory agencies andscientific panels are of limited, if any, relevance forNTDE. New hazard and risk assessments for NTDE need tobe conducted that are independent from the toxicologicaldatabase regarding pre-2007 diesel engines. ACES promisesto provide some of the data needed to better assess thepotential carcinogenicity and noncancer toxicity of NTDE,and the toxicological database could be further bolstered ifmore DE health effects studies focus on NTDE than onTDE. As the diesel fleet continues to turn over and NTDE-emitting vehicles with modern aftertreatment systemsreplace older TDE-emitting vehicles, the utility of the currentDE health effects database for assessing potential health risksto DE constituents in the ambient environment will becomeless and less relevant. Furthermore, based on epidemiologicalstudies of near-roadwaypopulations that provide evidence oflinks between traffic-related air pollution and adverse cardi-opulmonary health effects,105 the turnover of the diesel fleetwith NTDE-emitting vehicles is likely to have significantpublic health benefits. Although both are emitted fromdiesel-powered engines, NTDE is a different substance thanTDE, requiring its own toxicological investigation and denovo hazard and risk assessments.

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About the AuthorsThomas W. Hesterberg, Ph.D., is Director of ProductStewardship and Environmental Health and WilliamB. Bunn, III, MD, JD, MPH, is Vice President of Health,Safety, Security, and Productivity, both with Navistar Inc.,Chicago, IL. Christopher M. Long, Sc.D., Sonja N. Sax,Sc.D., and Peter A. Valberg, Ph.D, are consultants in theCambridge, MA, office of Gradient. Charles A. Lapin, Ph.D.,is a toxicology consultant with Lapin & Associates, Glendale,CA. Roger O. McClellan, DVM, is a toxicology consultant inAlbuquerque, NM. Please address correspondence toChristopher M. Long, Gradient, 20 University Road,Cambridge, MA 02138; phone: (617) 395-5000; fax: (617)395-5001; [email protected].

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