Particulate Matter in New Technology Diesel Exhaust (NTDE) is Quantitatively and Qualitatively Very Different from that Found in Traditional Diesel Exhaust (TDE)

ABSTRACT

Diesel exhaust (DE) characteristic of pre-1988 engines is classified as a “probable” human carcinogen (Group 2A) by the International Agency for Research on Cancer (IARC), and the U.S. Environmental Protection Agency has classified DE as “likely to be carcinogenic to humans.” These classifications were based on the large body of health effect 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 oxides (NO_x) in diesel exhaust have helped stimulate major technological advances in diesel engine technology and diesel fuel/lubricant composition, resulting in the emergence of what has been termed New Technology Diesel Exhaust, or NTDE. NTDE is defined as DE from post-2006 and older retrofit diesel engines that incorporate a variety of technological advancements, including electronic controls, ultra-low-sulfur diesel fuel, oxidation catalysts, and wall-flow diesel particulate filters (DPFs). As discussed in a prior review (T. W. Hesterberg et al.; Environ. Sci. Technol. 2008, 42, 6437-6445), numerous emissions characterization studies have demonstrated marked differences in regulated and unregulated emissions between NTDE and “traditional diesel exhaust” (TDE) from pre-1988 diesel engines. Now there exist even more data demonstrating significant chemical and physical distinctions between the diesel exhaust particulate (DEP) in NTDE versus DEP from pre-2007 diesel technology, and its greater resemblance to particulate emissions from compressed natural gas (CNG) or gasoline engines. Furthermore, preliminary toxicological data suggest that the changes to the physical and chemical composition of NTDE lead to differences in biological responses between NTDE versus TDE exposure. Ongoing studies are expected to address some of the remaining data gaps in the understanding of possible NTDE health effects, but there is now sufficient evidence to conclude that health effects studies of pre-2007 DE likely have little relevance in assessing the potential health risks of NTDE exposures.

IMPLICATIONS

Based on the distinct physical and chemical properties of New Technology Diesel Exhaust (NTDE), it has become clear that findings from the health effects studies conducted on traditional DE (TDE) over the last 30 years have little relevance to NTDE, which is more similar to the exhaust from compressed natural gas (CNG) or gasoline engine emissions than to traditional TDE. Once sufficient health effects data are available for NTDE, it will thus be necessary to conduct new hazard and risk assessments for NTDE that are independent of the DE toxicological database acquired on emissions from pre–2007 diesel technology.

INTRODUCTION

Diesel-engine exhaust (DE) exposures and health effects have been studied extensively for decades. DE is a source of particulate matter (PM), nitrogen oxides (NO_x), carbon monoxide (CO), and a number of air toxics (e.g., aldehydes, volatile organic compounds, polycyclic aromatic hydrocarbons [PAHs]). Based primarily on the sizable toxicological database characterizing DE from pre-1988 engines, several regulatory agencies and scientific consensus groups have conducted hazard assessments for DE (e.g., the International Agency for Research on Cancer [IARC],2 the U.S. Environmental Protection Agency [U.S. EPA],3 the U.S. National Toxicology Program,4 the U.S. National Institute for Occupational Safety and Health,5 and the California Environmental Protection Agency6). Despite some major uncertainties and limitations in both the human epidemiologic and laboratory animal evidence from the historical DE studies,7 8 9–10 these agencies have generally concluded that sufficiently high DE exposures can increase the risk of cancer (e.g., lung cancer) and noncancer health effects. Specifically, in 1989, IARC classified DE as a “probable” human carcinogen (Group 2A) based on “limited” evidence in humans but “sufficient” evidence in rats. U.S. EPA in 2002 classified diesel exhaust as “likely to be carcinogenic to humans,” and in 2000 as a “mobile source air toxic.” Diesel exhaust particulate (DEP) was listed as a “toxic air contaminant” (TAC) by California EPA in 1998.

Given the regulatory concerns regarding DE health risks, U.S. EPA has implemented progressively more stringent DE standards for on-road heavy-duty diesel engines (HDDEs) since the promulgation of a smoke standard for the 1970 model year.3 Limits on NO_x, CO, and hydrocarbons soon followed, and for the 1988 model year, U.S. EPA implemented the first PM standard for HDDEs (0.6 g/bhp·hr, using a transient test) that reduced overall diesel-fleet PM emissions by about 40%. As shown in Figure 1, U.S. EPA's efforts culminated in a PM standard of 0.01 g/bph·hr for the 2007 model year. NO_x emissions standards have also been reduced by approximately 99% compared to pre-1988 engines, with the phase-in of a NO_x standard of 0.2 g/bph·hr for the 2010 model year. U.S. EPA PM emissions standards for off-road diesel engines have followed a different schedule, taking effect in 1996 and 2000 for off-road equipment and locomotive engines, respectively.

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

The stringent emission limits prompted the development of significant technological advances in diesel engine technology, as well as major changes in the composition of diesel fuel and engine lubricants. These advances resulted in the emergence of what has been termed “New Technology Diesel Exhaust”, or NTDE.1,7,8 As discussed further in the next section, NTDE has been defined as emissions from post-2006 diesel engines and earlier-model diesel vehicles retrofitted with aftertreatment devices. It is a product of the innovative development of integrated, multicomponent emissions reduction systems (engines, fuel injection systems, ultra-low-sulfur fuels, lubricants, and exhaust aftertreatment devices) to meet the tightened U.S. EPA emissions standards.8 NTDE is distinct from, and contrasts in many ways from, “traditional” DE (TDE), which refers to emissions from pre-1988 diesel engines sold and in use prior to the U.S. EPA HDDE particulate standards, and “transitional” DE, which consists of DE from 1988 to 2006 diesel engines manufactured during a time period of continuous improvements to the internal design of the diesel engine, but prior to the full-scale implementation of multicomponent aftertreatment systems.

The widespread adoption of NTDE-compliant diesel engines in both the United States and Europe has stimulated a flurry of research characterizing pollutant emissions in NTDE.1,11,12 We documented substantial reductions in emissions of a variety of regulated and unregulated species (e.g., PM, CO, nonmethane hydrocarbons, formaldehyde, benzene, acetaldehyde, and PAHs) in NTDE versus either TDE or “transitional” DE in a prior review of emissions data from transit buses, school buses, refuse trucks, and passenger cars.1 Since the 2008 publication of this review, results from numerous characterizations of NTDE emissions have been published, including Phase 1 findings from the Advanced Collaborative Emissions Study (ACES),13,14 a series of findings from a collaborative study of the California Air Resources Board (CARB) and the University of Southern California (USC),15–21 and other detailed analyses.22–29 Importantly, these new studies have substantially improved our understanding of how the DEP in NTDE differs from the DEP in TDE in terms of chemical and physical properties that can affect toxicity. In particular, they address whether secondary emissions, such as nitro-PAHs and dioxins/furans, may be associated with exhaust aftertreatment systems. Other recent studies18,30–34 provide preliminary toxicological data for NTDE.

In our review, we thus assemble the evidence relevant to potential differences in the health risks of NTDE versus TDE. We focus on DEP emissions, and not diesel exhaust gases, given that DEP is considered to be a risk driver for DE health effects and has been a principal target of recent diesel engine modifications and aftertreatments. As part of our analysis, we evaluate recent findings addressing the chemical composition and particle size distribution of NTDE particulates, given that both chemical composition and particle size distribution are regarded as key determinants of PM toxicity. We address two questions of specific interest, namely whether there is evidence for the formation of any new species of toxicological concern in NTDE, and the effects of aftertreatment technologies on DEP nanoparticle emissions. We review the preliminary toxicological data for NTDE. An overarching purpose to this review is to address the question of the degree to which the prior DE hazard assessments do or do not apply to NTDE.

DEFINING NEW TECHNOLOGY DIESEL EXHAUST (NTDE)

NTDE is the exhaust from diesel engines that incorporate a variety of technological advancements, including electronic controls, ultra-low-sulfur diesel (ULSD) fuel, oxidation catalysts, and wall-flow diesel particulate filters (DPFs) capable of achieving U.S. EPA's 2007 PM emissions standard of 0.01 g/bhp·hr. Both DE from post-2006 diesel engines, as well as DE from earlier engines retrofitted with a DPF and operated with ULSD fuel (<15 ppm sulfur), are considered to be representative of NTDE, providing that they can achieve the stringent 2007 PM standard. Although there can be variation in their makeup, currently available multicomponent aftertreatment systems have gained usage for control of both DEP and gaseous DE emissions, consisting of a DPF for DEP control, a diesel oxidation catalyst (DOC) for hydrocarbon control, and advanced exhaust gas recirculation (EGR) technology and/or absorbers or selective catalytic reduction technology (SCRT) for NO_x control. Therefore, DE from a late-model diesel engine equipped with only a DOC is not considered to be representative of NTDE. ULSD fuel, which is now required in the United States and is essential for the proper functioning of DPFs, is also a critical component for an overall aftertreatment system.

The DPF is now accepted as a centerpiece of NTDE aftertreatment systems needed to meet the stringent PM emission limits.11 As discussed by Maricq11 and Heeb et al.,23 there are a variety of different DPF variations depending on the substrate material (e.g., cordierite or silicon 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, continuously regenerating diesel particulate filters (CRDPFs), including Continuously Regenerating Trap (CRT; trademark of Johnson Matthey, Malvern, PA) and Catalyzed Continuously Regenerating Trap (CCRT; trademark of Johnson Matthey) systems, which consist of an oxidation catalyst followed by either an uncatalyzed or catalyzed DPF, have gained usage. It should thus be evident that there can be numerous different aftertreatment configurations based on DPF specifications alone, meaning that NTDE encompasses the exhaust from a variety of different diesel engine types and aftertreatment configurations. All NTDE (post-2006 DE), however, is distinct from “traditional” (pre-1988) DE (TDE) and “transitional” (1988–2006) DE emitted from diesel engine technologies not compliant with the 2007 HDDE emissions standards.

CHANGED CHEMICAL COMPOSITION OF DEP IN NTDE

It is now well-established that “order of magnitude” reductions in PM mass emissions are typical of NTDE.1,14,17,19,25,26 For example, for the recent CARB–USC testing of four heavy-duty and medium-duty diesel vehicles operated using six different aftertreatment configurations (Table 1) and multiple test cycles, Biswas et al.17 reported consistent PM mass reductions of >90%, observing in all cases PM emission rates of less than 0.01 g/mile (Figure 2). Given the role of chemical composition as a key factor affecting DEP toxicity, recent studies have further investigated how emissions of specific DEP constituents are impacted by the use of different aftertreatment technologies.

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

		Diesel Engine			Aftertreatment
Vehicle Number/ID	Vehicle Make	Model	Year	Size (L) or Power (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	None
Veh1: DPF1	Kenworth	Cummins M11	1998	11 L	JM CRT	DOC + uncatalyzed DPF
Veh1: DPF1+V-SCR	Kenworth	Cummins M11	1998	11 L	JM SCRT	CRT + vanadium-based SCR + small catalyst for ammonia slip
Veh1: DPF1+Z-SCR	Kenworth	Cummins M11	1998	11 L	JM SCRT	CRT + Fe-Z SCR + small catalyst for ammonia slip
Veh2: DPF2	International	International	1999	7.6 L	Engelhard DPX	Catalyzed DPF
Veh3: DPF3	Thompson	Cummins	2003	5.9 L	Cleaire Horizon + EGR	Uncatalyzed DPF with electric regeneration system
Veh4: DPF4	Gilig (35 ft Hybrid)	Cummins	2006	5.9 L	JM CCRT	DOC + catalyzed DPF
ACES testing (adapted from Khalek et al. 13,14)
Engines treated as anonymous and data generally reported on average basis across the four engines	Caterpillar CumminsDetroit DieselVolvo Powertrain	CAT C13 Cummins ISXDDC Series 60 Mack MP7	2007 2007 2007 2007	430 hp455 hp455 hp395 hp	Three of four engines equipped with DOC followed by wall-flow C-DPF; fourth engine equipped with an exhaust diesel fuel burner followed by a C-DPF; all engines equipped with EGR systems
Liu et al. 28 organic species emissions testing
2004 Engine	Unspecified	Unspecified	2004	15 L	EGR system
2007 Engine	Unspecified	Unspecified	2007	15 L	EGR system, DPF (DOC and CSF), crankcase emissions coalescer
Liu et al. 29 PCDD/F emissions testing
Engine 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: JM = Johnson Matthey; CRT = continuous regeneration trap; DOC = diesel oxidation catalyst; DPF = diesel particulate filter; CSF = catalyzed soot filter; EGR = exhaust gas recirculation; SCR = selective catalytic reduction; Cu-Z = copper-zeolite; Fe-Z = iron-zeolite; CCRT = catalyzed continuous regeneration system.

Figure 2. PM emissions from CARB–USC test vehicle fleet equipped with different aftertreatment technologies.17,18 Numbers indicate the removal efficiencies (%) for the corresponding aftertreatment. Asterisks indicate authors’ note that these reduction efficiencies should be given greater weight because these retrofits are directly comparable to baseline (with no aftertreatment) due to the use of the same vehicle. See Table 1 for vehicle and aftertreatment specifics.

As discussed by Herner et al.,19 aftertreatment devices such as DPFs and SCRs can be viewed as “chemical reactors,” because they generate varying degrees of oxidizing and reducing conditions to promote PM filtration and combustion, and NO_x conversion. Given that these conditions can in turn have unintended effects on other DE species, there has been extensive speculation on whether new species of toxicological concern may be formed in NTDE and whether any alteration or new formation of toxic species may offset the public health benefits associated with the reported reductions in regulated and unregulated species.25,35 With the recent publication of a number of comprehensive emissions studies, we are now in a better position to address this question and to characterize the key distinctions in DEP chemical composition between NTDE versus pre-2007 DE.

Major Chemical Species

Phase I ACES findings13,14 provide some of the more comprehensive data for demonstrating the major differences in DEP composition for NTDE versus pre-2007 DE. ACES is considered to provide a highly robust data set based on both its measurement of a comprehensive set of regulated and unregulated species across multiple engines/aftertreatment configurations, and its strong study design that aimed to minimize potential artifacts. The ACES study design was a product of highly detailed project planning that benefited from independent oversight by both the Health Effects Institute (HEI) and the Coordinating Research Council (CRC) and input from a wide range of experts and stakeholders on various advisory and steering committees.13 Four new 2007-model-year diesel engines were tested, including three engines equipped with a DOC and a C-DPF, and one engine equipped with an exhaust diesel fuel burner and C-DPF (see Table 1). Using ULSD fuel (sulfur content of 4.5 ppm; analyzed to assure no interfering factors), each ACES engine was tested multiple times on an engine dynamometer using the 20-min Federal Test Procedure (FTP) transient cycle, as well as a new 16-hr transient cycle developed at West Virginia University that covers a complete engine operation with active regeneration events. Khalek et al.13 details the rigorous engine testing procedures and state-of-the-art analytical methods employed during the ACES emissions testing, which included extensive conditioning of engines and sampling systems and collection of blank samples in the constant volume sampler (CVS) dilution tunnel.

Figure 3 contrasts the average DEP composition reported by Khalek et al.13,14 for the four 2007-model-year ACES engines with a particle composition representative of TDE.36 As illustrated by Figure 3, Khalek et al.14 observed PM from the 2007 ACES engines to be dominated by sulfates (53%) and organic carbon (30%), with a substantially reduced elemental carbon content (13%) compared to that typical of TDE (approximately 40% in Figure 3, but which can range as high as 90% depending on engine load and speed). Table 2 summarizes the substantial reductions in DEP (and DE) constituents reported by Khalek et al.14 for the ACES engines as compared to 2004 engines (1998 engines for dioxins/furans), including 99%, 96%, and 71% reductions for elemental carbon (EC), organic carbon (OC), and inorganic ions, respectively.

Figure 3. Representative compositions of particulate matter in NTDE (based on data from Khalek et al.14) and TDE (based on data from Kittelson36) from heavy-duty diesel engines tested in heavy-duty transient cycles. Note that there may be deviations from these compositions for particular engines due to the variability in DEP composition based on such factors as engine model, operating conditions, and fuel and lube oil compositions. Constituent labels are used as given in the actual study publications, recognizing that there are some differences in constituent categories between the two studies. For example, “Unburnt Oil” differs from “Organic Carbon”, given there are sources of organic carbon in DE other than just unburnt oil. In addition, “Ash and Other” includes both elements and elemental oxides, whereas “Elements w/o sulfur” is more restrictive and represents just elemental contributions. Notwithstanding some differences in the data shown in the plots, they demonstrate that not only is less PM emitted 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-hour cycles for all four ACES engines compared to CRC E55/E59 data for 2004 technology engines (data from Khalek et al.14)

	2004 Engines		2007 Engines
Chemical Species	Average(mg/hr)	Average(mg/hr)	Average(mg/hr)	SD(mg/hr)	Average % Reduction Relative to 2004 Technology
Single ring aromatics	405	405	71.6	32.97	82%
PAHs	325	325	69.7	23.55	79%
Alkanes	1030	1030	154.5	78.19	85%
Hopanes/steranes	8.2	8.2	0.1	0.12	99%
Alcohols and organic acids	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 elements	400	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^−5	5.20 × 10^−5	99% ^a
Notes: N/A = not available. ^aRelative to 1998 technology engines.

Findings from the CARB–USC diesel vehicle testing program provide further evidence of significant reductions in both elemental and organic carbon emissions in NTDE.17,22 As part of this collaborative study, emissions of size-resolved PM components from multiple HDDEs with and without various aftertreatment technologies were characterized for different driving cycles (e.g., cruise and the transient U.S. EPA Urban Dynamometer Driving Schedule, or UDDS).17,22 Table 1 documents the four heavy-duty diesel vehicles and variety of aftertreatment configurations tested in the CARB–USC program, which included either no aftertreatment (baseline case, representative of transitional DE) or one of several different catalyzed or noncatalyzed DPF systems.

As shown in Table 3, Biswas et al.17 reported large reductions in total carbon (TC) emissions, including both the elemental carbon and organic carbon fractions, across the retrofitted vehicles compared with the baseline, non-retrofit vehicle. For example, consistent reductions in EC of >99% were observed across the aftertreatment configurations for the UDDS cycle testing, with OC reductions of 95% and higher.17 Other studies have also reported similarly high EC and OC reductions across a range of engines, aftertreatment configurations, test cycles, and fuel sulfur content.25,26,28,37,38 For example, Liu et al.28 observed >99% reductions for both EC and OC in their comprehensive study of emissions of more than 150 organic species from a 2007 engine representative of NTDE versus a 2004 non-retrofit engine (see Table 1 for engine specifications).

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

Chemical Species	Veh1: DPF1	Veh1: DPF1+V-SCR	Veh1: DPF1+Z-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 EC = elemental carbon; OC = organic carbon; WSOC = water soluble organic carbon; PAHs = polycyclic aromatic hydrocarbons; DPF = diesel particulate matter filter; DOC = diesel oxidation catalyst; V-SCRT = vanadium-based selective catalytic reduction technology; Z-SCRT = iron-based zeolite selective catalytic reduction technology; numbers in parentheses = increase in emissions with aftertreatment. Vehicle details in Table 1.

Biswas et al.17 also reported consistently high reductions (97% and higher) for the water-soluble organic carbon (WSOC) fraction across the various aftertreatment configurations (Table 3). As discussed by Biswas et al.,17,18 WSOC content is thought to be relevant to DEP toxic potential, given research findings indicating that constituents of this OC fraction, such as quinones, oxygenated PAHs, and aldehydes, may contribute to the oxidative stress response associated with ambient PM. Although substantial reductions in WSOC were observed across the different aftertreatment configurations, the ratio of WSOC to total OC was observed to depend on the aftertreatment configuration.17 In particular, evidence of reduced relative OC solubility (WSOC/OC: 8–25%) was observed for the retrofitted vehicles with catalyzed filters compared to the retrofitted vehicles with uncatalyzed filters (WSOC/OC: 60–100%).

As shown in Table 3, findings from the CARB–USC study indicate that there can be higher fractions of particulate sulfate and ammonium in NTDE as compared to TDE.17,18,22 For example, Biswas et al.18 reported baseline vehicle sulfate emissions of 0.09 and 0.36 mg/km for the cruise and UDDS cycles, respectively. For the retrofit vehicles, sulfate emissions ranged from 0.005 (Veh3: DPF3) to 5.6 (Veh1: DPF1+V-SCR) mg/km for the UDDS cycle, and from 0.02 (Veh3: DPF3) to 1.5 mg/km (Veh1: DPF1) mg/km for the cruise cycle (see Table 1 for retrofit vehicle IDs and specifications). Particulate ammonium emissions were shown to range as high as 30 times above the baseline case. Herner et al.22 provides further insights on sulfate emissions from the CARB–USC retrofit vehicles, reporting correlations between the particle mass and sulfate fractions and the formation of nucleation-mode particles. As discussed more later, Herner et al.22 demonstrated the formation of volatile sulfur-based nucleation-mode particles after the exhaust aftertreatment systems via oxidation of SO₂ (formed from sulfur in fuel and engine oil) to SO₃ for four of the retrofit vehicles meeting certain conditions in the aftertreatment (level of catalyst, exhaust temperature, and sulfur storage). They demonstrated that sufficient numbers of these particles, consisting primarily of sulfuric acid and ammonium sulfate with some organic coatings, could be formed such that they could dominate DEP mass fractions (up to 62% of reconstructed mass).

Other studies have also reported evidence of sulfate formation in NTDE, although with some variability in findings.25,38 39–40 This may be due to some aftertreatment configurations not meeting the conditions needed for sulfate nucleation, or it may also be due to differences in experimental factors (e.g., test cycles, temperatures, dilution ratios, residence times, and sampling techniques) between studies. Grose et al.39 and Kittelson et al.40 documented the generation of nucleation-mode sulfate particles during on-road testing of heavy-duty diesel engines equipped with a CRT and operated with fuels containing either 15 or 49 ppm sulfur. They observed greater particle number emissions for engine loads sufficiently high to raise the exhaust temperature above 300 °C and for the higher sulfur fuel, with chemical analysis confirming that these particles consisted primarily of sulfates. Based on analyses of the volatility profile of the DEP emissions, Grose et al.39 further concluded that these nanoparticles may exist as ammonium 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 use of a SCR aftertreatment system.

PAHs and Other Organic Species

Because PAHs are recognized as some of the more toxic DE constituents, a number of studies have characterized PAH emissions in NTDE relative to pre-2007 DE. Hesterberg et al.1 previously reported the highest emissions of total 2-, 3-, and ≥4-ring PAH compounds in diesel transit and school buses lacking aftertreatment. With use of aftertreatment, Hesterberg et al.1 reported that PAH emissions for transit buses were significantly reduced (on average 60–97% reductions, depending on the aftertreatment type and the number of PAH rings) and of a similar magnitude, and sometimes lower, than buses operating with compressed natural gas (CNG) fuel with and without aftertreatment. Several recent studies have characterized emissions of a suite of individual PAH species in NTDE, including those suspected to be carcinogenic. Given that nitro-PAHs have elevated genotoxic potential, recent studies have also addressed concerns that the elevated temperatures and reactive compounds or catalysts used in DPFs may promote nitro-PAH formation.11

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

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-PAH Compounds	2000 Technology Engine ^a (mg/bhp·hr)	2007 ACES Engines ^a (mg/bhp·hr)	% Reduction	2004 Technology Engine ^b (mg/bhp·hr)	2007 Engine ^b (mg/bhp·hr)	% Reduction
PAHs
Naphthalene	0.4829	0.0982	80	0.719	0.122	83
Acenaphthylene	0.0524	0.0003	>99	0.0305	0.00218	93
Acenaphthene	0.0215	0.0004	98	0.0455	0.0220	52
Fluorene	0.0425	0.0013	97	0.131	0.0129	90
Phenanthrene	0.05	0.0055	89	0.0786	0.0123	84
Anthracene	0.0121	0.0004	97	0.00738	0.000862	88
Fluoranthene	0.0041	0.0003	93	0.00431	0.00113	74
Pyrene	0.0101	0.0004	96	0.0117	0.000979	92
Benzo(a)anthracene	0.0004	<0.0000001	>99	0.000586	0.0000632	89
Chrysene	0.0004	<0.0000001	>99	0.00105	0.000123	88
Benzo(b)fluoranthene	<0.0003	<0.0000001	>99	—	—	—
Benzo(k)fluoranthene	<0.0003	<0.0000001	>99	—	—	—
Benzo(b,k,j)fluoranthene	—	—	—	0.00024	0.00000776	97
Benzo(g,h,i)fluoranthene	—	—	—	0.000607	0.000258	58
Benzo(e)pyrene	<0.0003	<0.0000001	>99	0.000232	0.00000374	98
Benzo(a)pyrene	<0.0003	<0.0000001	>99	0.0000797	0.00000613	92
Perylene	<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	76
2-Nitronaphthalene	—	—	—	0.000531	0.0000478	91
2- Nitrofluorene	0.000065	0.0000009	99	—	—	—
9-Nitroanthracene	0.0007817	0.0000031	>99	0.000192	0.0000403
2-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	99
7-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 FTP transient 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 operated over the FTP transient cycle. Chrysene emissions are the sum of chrysene and triphenylene emissions.

As part of the CARB–USC testing program, Pakbin et al.20 reported emissions of particle-bound organic species for three of the retrofit configurations (either a CRT system, a vanadium-based SCRT, or a zirconium-based SCRT, corresponding to the Veh1: DPF1, Veh1: DPF1+V-SCR, and Veh1: DPF1+Z-SCR IDs, respectively, in Table 1). For the baseline case that is representative of transitional DE, emission factors for PAH compounds ranged from 2.4 to 49 μg/km for the UDDS cycle, and 0.4 to 9 μg/km for the cruise cycle. In contrast, emission factors for NTDE were below 0.1 μg/km for the CRT system and below 0.02 μg/km with the SCRT systems, totaling reductions of >99% regardless of the cycle tested and the PAH species. The high PAH removal efficiencies of the CRT system have been attributed to its ability to both oxidize (DOC component) and filter (DPF component), with the enhanced performance of the SCRT attributed to its ability to break apart heavy hydrocarbons into smaller molecules.20,25

Other studies have reported similarly high and consistent PAH removal efficiencies for C-DPF-retrofitted engines,25,37,41,42 whereas other studies have observed more variable removal efficiencies, in particular for the more volatile PAHs.23,24,28,43 For example, Heeb et al.23 characterized emissions of eight 4- to 6-ring PAHs from a HDDE (6.11 L) operated with two different cordierite-based DPF systems and fuel-borne catalysts (iron- or copper/iron-based). For testing using the eight-stage International Organization for Standardization (ISO) 8178/4 C1 test cycle (representing a mix of full and partial engine load operation typical of construction machinery), they observed 40–90% reductions for the PAH test analytes, including 60–90% reductions for the six carcinogenic PAHs included in their testing. They found that the more reactive and less volatile PAHs were retained more effectively by the DPF, with greater reductions for pyrene (80%) as compared to the more volatile fluoranthene (40%). In follow-up study of the performance of 14 different DPFs for the removal of the same eight 4- to 6-ring PAHs using the same HDDE and test cycle, Heeb et al.24 reported removal efficiencies of 31–87% for the DPFs they characterized as being of “low oxidation potential”, and 75–98% for the DPFs of “high oxidation potential”. For their study of particle-phase and semivolatile organic compound emissions from a model-year 2004 baseline HDDE and a model-year 2007 engine with aftertreatment components (Table 1), Liu et al.28 reported PAH removal efficiencies in NTDE that varied between 52% and 98% for more than 20 PAH compounds (Table 4).

Most studies have also generally reported high removal efficiencies for nitro-PAHs in NTDE, although with variable results for some specific nitro-PAHs, and evidence of possible nitro-PAH formation.12,14,23,24,28,37,42,44,45 In interpreting these results, findings suggesting formation of nitro-PAH artifacts during sampling and PM collection cannot be overlooked.46 Similar to the high nitro-PAH removal efficiencies reported for the ACES testing, Liu et al.28 reported high removal efficiencies (76–99.5%) for the majority of nitro-PAHs in NTDE from their 2007 engine compared to a 2004 technology (non-retrofit) engine (Table 3). However, Liu et al.28 also reported a “small increase” for 9-nitrophenanthrene emissions (data not provided). Heeb et al.23 also reported increased emissions of several smaller, more volatile nitro-PAHs, including 9-nitrophenanthrene (and 1- and 2-nitronaphthalene, 9-nitroanthracene), for a DPF-equipped engine compared to a baseline engine. Heeb et al.24 observed evidence for formation of 1-nitropyrene in “low oxidation potential” DPFs (63% increase in emissions compared to the baseline case with no aftertreatment), but substantial reductions of this same mutagenic nitro-PAH by “high oxidation potential” DPFs. Ratcliff et al.42 did not observe formation of 1-nitropyrene, although they observed it to be the most abundant nitro-PAH and to have a reduced removal efficiency compared to other measured nitro-PAHs (35% compared to >90%). These findings merit scrutiny due to the mutagenic properties of nitro-PAHs, but it is important to note that study findings generally demonstrate significant reductions of nitro-PAHs in NTDE, and even for those specific nitro-PAHs that may not be removed effectively, emission levels are very low (speciated nitro-PAH emissions are generally in the ng/bhp·hr range37).

Several recent studies support efficient removal of other particle-phase and semivolatile organic species by DPFs and other aftertreatment components.14,20,28 In particular, Liu et al.28 observed reductions of >90% for most C₁, C₂, and C₁₀ through C₃₃ particle-phase and semivolatile organic compounds in NTDE, including >99% for all C₁₁–C₂₄ n-alkanes and 97% and higher for hopanes and steranes.

Given findings from Swiss researchers suggesting increased dioxin/furan (PCDD/F) emissions with the use of copper catalyst materials, in particular in the presence of elevated chlorine levels in fuel,35,47 a number of recent studies have investigated PCDD/F emissions in NTDE. In general, studies have not replicated the Swiss findings, instead reporting consistent reductions in PCDD/F emissions for various aftertreatment configurations, including for engines operated with copper-zeolite (Cu-zeolite) and iron-zeolite (Fe-zeolite) SCR systems, copper/iron-based fuelborne catalysts, and diesel fuels doped with elevated chlorine levels.14,29,48 49 50–51 In particular, for a comprehensive testing strategy that employed two different 2010 engines operated over a range of different exhaust aftertreatment configurations (including Cu-zeolite SCRs operated with and without urea; see Table 1) using chlorine-doped diesel fuels (0.6 and 8.4 ppm), Liu et al.29 demonstrated significant reductions (60–80%) in PCDD/F emissions for all aftertreatment configurations, with no impact of elevated chlorine fuel levels (Figure 4). In addition, Khalek et al.14 reported 99% reductions in dioxin/furan emissions from the four 2007 ACES engines compared to 1998 technology engines (Table 2).

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

Trace Metals

Trace metals make up a small fraction (e.g., <1%) of the total PM mass emissions from diesel exhaust, but are still of particular interest given the toxic potential of such species as lead (Pb), manganese (Mn), arsenic (As), and chromium (Cr).52 In addition, concerns have been raised that increased emissions of several metal species, such as vanadium, copper, and iron, may be associated with the catalysts in some diesel exhaust aftertreatment systems.16,21 Although data from the CARB–USC testing program provide some evidence of possible releases of catalyst metals,16,21 recent studies generally show that particulate-bound metals are reduced significantly with the use of DPFs, and that further reductions may be achieved with SCR systems. Table 5 summarizes the significant reductions reported by Hu et al.16 for most metal species across the CARB–USC aftertreatment configurations. Hu et al.16 observed some influence of driving cycle on metal removal efficiencies, although high overall total trace metals reductions were achieved for all DPFs for both the cruise and UDDS cycles (>85 and >95%, respectively). In addition, Khalek et al.14 reported an average reduction of 98% for metals and elements for the ACES 2007-compliant engines as compared to 2004 technology engines (Table 2). Liu et al.25 reported a 93% reduction in total metals emissions with a DPF, and an additional 25% reduction with a SCR system.

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: DPF1+V-SCR		Veh1: DPF1+Z-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 parentheses = percent increase in emissions with exhaust aftertreatment. *Pt was not detected in baseline vehicle emissions. **Pt was not detected with aftertreatment. NM = not measured. Vehicle details in Table 1.

As shown in Table 5, Hu et al.16 reported increased emissions of some metals (e.g., vanadium, platinum) for the CARB–USC vehicles equipped with vanadium- and zeolite-based SCRT systems, thus providing some evidence of possible releases of catalyst metals, particularly at the high temperature conditions of the cruise cycle. The authors emphasized, however, that these findings are likely of little significance to real-world exposures, given that concentrations of these metals in the NTDE were still 2–3 times lower than the levels of these metals measured in roadway tunnel studies.16 In contrast to the CARB–USC findings, Chapman et al.53 reported no evidence of vanadium 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 in DEP samples collected from many of the same engines and aftertreatment configurations as Hu et al.,16 Verma et al.21 reported consistent increases in the relative abundance of a number of redox-active transition metals (V, Fe, Mn, Ni, Cu, and Cr) compared to data for the baseline (non-retrofit) truck. These increases were observed for retrofits with and without SCRT systems, suggesting that the enrichment of these trace metals is associated with both the oxidation catalysts and/or the embedded catalysts on the DPFs, as well as the SCRT systems.

Summary on Changes to DEP Chemical Composition

In summary, there is now an abundance of data demonstrating the significant reductions in specific DEP components of toxicological concern (e.g., elemental carbon, organic carbon, PAHs, nitro-PAHs, dioxins/furans, metals) that correspond to the order-of-magnitude reductions in total PM mass typical of NTDE. Sulfate is one of the few DEP species with some consistent findings of increased emissions in NTDE as compared to TDE, but this species is generally regarded to be of low toxicity.54 55–56 In fact, as hypothesized by Grose et al.,39 the enrichment of ammonium sulfate particles in NTDE, rather than organic compounds, is more likely to result in decreased toxicity for NTDE compared to TDE. Although there is a need for more toxicological study to confirm this idea, Herner et al.22 reported an inverse correlation between numbers of volatile sulfur-based nucleation-mode particles and measures of oxidation potential from chemical and cellular assays (from the Biswas et al.18 and Verma et al.21 studies that are discussed later), concluding that this increased sulfate fraction was associated with reduced toxicity. Moreover, there remains no reliable evidence that species of toxicological concern are formed as a result of aftertreatment technologies. The preliminary toxicological data are reviewed later, but there is sufficient evidence from the DEP chemical characterization data alone to support the conclusion that NTDE is toxicologically distinct from TDE and transitional DE.

IMPACTS OF AFTERTREATMENT ON ULTRAFINE PARTICLE (UFP) EMISSIONS

Given that the major reductions in DEP mass emissions characteristic of NTDE are associated with corresponding reductions in “condensation surfaces”, it has been hypothesized that advanced emission control technologies, and DPFs in particular, may promote nucleation and contribute to increased nanoparticle number concentrations.36 These hypotheses were stimulated by a 1996 Health Effects Institute (HEI) report57 that observed order-of-magnitude increases in nanoparticle emissions for a 1991-model-year high-pressure direct-injection diesel engine equipped with an oxidation catalytic converter (OCC), compared to emissions from a 1988-model-year baseline diesel engine. Since the 1996 HEI report, a large number of diesel engine emissions studies have focused on nanoparticle emissions, in part due to the heightened interest in the potential health risks posed by nano-sized particles.9,58

There is now a sizeable emissions database for diesel nanoparticles in NTDE, although the lack of consistent protocols for measurement of nanoparticle emissions from diesel engines is a major limitation affecting their interpretation, and in particular the comparisons of findings among studies. Even more basic are the differences among studies in how diesel nanoparticles are defined, for example, DEP with diameters <180 nm,15 DEP with diameters in the range of 3–30 nm,40 DEP with diameters <50 nm,57 etc. In addition to the typical parameters of interest (e.g., engine specifications, aftertreatment configurations, fuel type, operating conditions), it is now well known that many experimental factors can influence nanoparticle concentrations (both formation and decay), including relative humidity, temperature, dilution ratio, dilution rate, and residence times, and may contribute to possible nanoparticle artifacts.59 60–61 Recognizing the remaining uncertainties in the available studies and the need for additional measurements using standard protocols, we highlight below some key findings related to diesel nanoparticles in NTDE versus baseline diesel exhaust (i.e., TDE). As discussed in this section, there is evidence demonstrating that DPFs can effectively remove diesel nanoparticles, but that some aftertreatment configurations and operating conditions may contribute to formation of nucleation-mode sulfate particles after the control devices.11,15,22,39,40,62

DPFs Can Effectively Remove Diesel Nanoparticles

Some of the earlier evidence demonstrating the effective removal of diesel nanoparticles by DPFs was provided by studies investigating particle number emissions from diesel and CNG buses.63 64–65 In particular, for steady-state testing (both at idle and cruise) conducted using two dilution conditions (a minidiluter and a CVS), Holmén and Ayala63 reported that the use of a CRT typically reduced number concentrations of both nucleation-mode and accumulation-mode particles by factors of 10–100. In a follow-up publication that reported results for transient cycle testing (using three test cycles, including the New York Bus [NYB], Urban Dynamometer Driving Schedule [UDDS], and Central Business District [CBD] cycles), Holmén and Qu64 again reported results indicating the strong performance of the CRT in reducing particle number concentrations. Compared to the baseline diesel engine, they observed consistent reductions in particle number concentrations of more than 2 orders of magnitude for the CRT-equipped bus. Similar to these findings, Nylund et al.65 reported that particle number concentrations were 2 orders of magnitude lower for a CRT-equipped bus than a baseline (non-retrofit) bus in a study conducted at the VTT Technical Research Centre of Finland using three 2003-model-year Euro 3 diesel buses (all the same model, but either having no aftertreatment, a DOC, or a CRT). Nylund et al.65 further reported that the CRT was highly effective at removing particles of all size classes. Both studies generally reported similar levels of particle number emissions among the retrofitted diesel buses and CNG buses.63 64–65

Other recent studies14,15,22,40,66 67 68–69 have also reported significant reductions in particle number emissions in NTDE from DPF-equipped vehicles compared to vehicles without aftertreatment, thus demonstrating high removal efficiencies for DEP nanoparticles given that they typically dominate DEP emissions on a particle number basis.9 For example, as shown in Figure 5, Khalek et al.14 reported significant reductions in average total particle number concentrations for the 2007 ACES engines, both with (89% for the West Virginia University (WVU) 16-hr transient cycle that included C-DPF-active regeneration events) and without regeneration (99% for the FTP cycle), compared to emissions from 2004 technology engines. For steady-state (50 mph cruise and idle) and transient (CBD and NYB cycles) testing of a 2000-model-year Isuzu medium–heavy-duty delivery truck equipped with a CRT and fueled with ULSD, Ayala and Herner67 reported 98–99.9% removal of particles with diameters of less than 100 nm, and overall reductions in total number concentrations of 2–3 orders of magnitude. Similarly, based on testing of four current production European vehicles using two transient driving cycles (the Common Artemis Driving Cycles and the New European Driving Cycle), Bosteels et al.68 consistently reported particle number emissions for a DPF-equipped vehicle that were more than 3 orders of magnitude smaller than those for two diesel vehicles without aftertreatment, and at least 1 order of magnitude smaller than those observed for a gasoline vehicle.

Figure 5. Average particle number emissions (note the logarithmic scale) for 2007 ACES engines (with and without C-DPF regeneration) versus a 2004 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 Test Procedure transient cycle (FTP-w) or 12 repeats of the 16-hr cycle, each for all four ACES engines and for sampling from an unoccupied animal exposure chamber set up on a constant volume sampler (CVS). Data for the 2004 technology engine were based on six repeats of the FTP transient cycle from a full flow CVS.

With the exception of several catalyzed aftertreatment configurations that were associated with formation of nucleation-mode sulfate particles (discussed in next section), findings from the recent CARB–USC testing program also provide evidence of reduced nanoparticle emissions in NTDE from DPF-equipped vehicles.15,22 For the two non-nucleation configurations (Veh3: DPF3, which is the diesel bus equipped with a Cleaire Horizon electric particle filter, and Veh4: DPF4, which is the diesel hybrid electric bus equipped with a CCRT), particle number emissions were approximately 3 orders of magnitude lower than for the baseline (non-retrofit) vehicle for both cruise and UDDS cycle testing (∼1011 particles/mile vs. ∼1015 particles/mile).22 As discussed more in the next section, increased particle number emissions, and specifically formation of nucleation-mode sulfate particles, was observed for most test aftertreatment configurations (Veh1: DPF1, Veh1: DPF1+V-SCR, Veh1: DPF1+Z-SCR, Veh2: DPF2) during both cruise (50 mph) and UDDS cycle testing.15,22 This formation of nucleation-mode sulfate particles, which contributed to increases in particle number concentrations as large as 20 times higher than the baseline case,22 masked the removal of DEP particles by DPFs. Importantly, emissions data for testing during idle cycles provide strong evidence for the high removal efficiencies of DPFs, with unmeasurable (less than background) particle number concentrations for all DPF-equipped configurations compared to particle number emissions of 1.60 ± 0.05 × 1016 particles/hr for the baseline vehicle case.22

Evidence of Nucleation-Mode Particle Formation in NTDE

Although there is thus strong evidence demonstrating the effectiveness of DPFs for removal of DEP nanoparticle emissions, there are now findings from several studies indicating that some aftertreatment configurations may promote formation of nucleation-mode particles in NTDE, such that there can be large overall increases in total particle number concentrations. In particular, several experimental studies have reported that the formation of sulfate nucleation-mode particles is enhanced by the catalytic oxidation of SO₂ to SO₃ that can occur in C-DPFs, as well as uncatalyzed DPFs with DOCs or SCRT systems.15,17,22,40,70 Enhanced nucleation has now been demonstrated for a variety of aftertreatment configurations, with study findings suggesting that its importance depends on both the aftertreatment specifications (e.g., catalytic loading, sulfur exposure history), operating conditions (driving cycle, and more specifically, exhaust temperature and load), and fuel and engine oil sulfur content.22 Evidence of enhanced nucleation is reviewed below, with the caveat that experimental test conditions (e.g., dilution ratios, dilution rates, temperature, relative humidity, time lapse) may also be key factors driving the observed nucleation that has been observed to occur following exhaust aftertreatment devices.

Given its testing of a variety of aftertreatment configurations over multiple driving cycles, the CARB–USC testing program offers some of the most useful data for characterizing the range of conditions under which nucleation may be enhanced in NTDE. As mentioned above, Biswas et al.15 and Herner et al.22 reported evidence of dominant nucleation modes for several catalyzed aftertreatment configurations, including CRT and SCRT systems and a C-DPF, generally for both the cruise and UDDS test cycles. Given that the formation of a dominant nucleation mode was only observed for configurations containing catalyzed aftertreatment, Herner et al.22 concluded that the presence of catalytic surface was a necessary condition for occurrence of a nucleation mode. Importantly, given the lack of a nucleation mode for the Veh4: DPF4 configuration that contained the most heavily catalyzed aftertreatment in the study (a CCRT system; see Table 1), Herner et al.22 concluded that the presence of catalytic surface was not a sufficient condition on its own for formation of nucleation-mode particles. In addition, they did not always observe increased particle number emissions for nucleating aftertreatment configurations.

Based on their test results, Herner et al.22 identified sulfur storage and exhaust temperature to be other key determining factors of the magnitude of nucleation. For example, they attributed the low particle number emissions of the CCRT-equipped bus (Veh4: DPF4) to the initial capacity of the new trap to store sulfates, hypothesizing that once the trap had aged sufficiently and its storage sites had become saturated, formation of nuclei-mode sulfate particles would occur.15,22 In support of the role of exhaust temperature, Herner et al.22 pointed to evidence of increased nucleation (as much as 20 times greater particle number emissions than the uncontrolled baseline vehicle) during the continuous high temperatures achieved for the constant, high-speed cruise cycle testing, contrasting this with the more variable evidence of nucleation (which ranged between 75% fewer to 60% greater emissions than the uncontrolled baseline) during the UDDS cycle testing when critical temperatures needed to oxidize SO₂ to SO₃ would only be achieved for short time periods. Although Herner et al.22 concluded that nucleation was more temperature-dependent than load-dependent, Vaaraslahti et al.70 previously reported the presence of an enhanced nucleation mode at high engine loads for a 1996-model-year HDDE operated using a continuously regenerating DPF (CRDPF, with an upstream DOC).

Findings from the CARB–USC testing15,22 confirmed hypotheses that SCRT systems, designed to reduce NO_x by reaction with ammonia, can have a secondary effect on sulfate nucleation.25,27 By conducting experiments where they bypassed the SCR portion of the aftertreatment system, Biswas et al.15 directly observed the impact of the SCR catalyst on nucleation, with factor of 2–3 reductions in particle number concentrations. In contrast to the CARB–USC findings for the SCRT, Guo et al.71 observed no evidence of nucleation for an exhaust system equipped with two active lean NO_x (ALN) catalysts in series followed by a C-DPF. For steady-state testing and normal operating conditions, Guo et al.71 reported >99% trapping efficiency for the C-DPF for two diesel fuels with differing sulfur content (340 ppm and 4 ppm) based on particle number concentration measurements.

Investigators at the University of Minnesota have also reported findings that support a key role of sulfur storage on nucleation potential.40,72 In particular, for on-road testing of a CCRT using the University of Minnesota Mobile Emission Laboratory (which consisted of a Volvo diesel engine fueled with low-sulfur [15 ppm] diesel fuel, and lubricated with low-sulfur [1300 ppm] oil), Kittelson et al.40 reported reductions in particle number concentrations to levels not detectable above background. They attributed these reductions to sulfate storage on the washcoat used on the catalyst filter. They further hypothesized that as the CCRT aged and these sulfate storage sites become filled over time, nucleation-mode nanoparticle generation would be observed.40 Building upon these findings, Swanson et al.72 demonstrated variable CRT performance depending on DOC age, with no nucleation modes observed for a new CRT system tested at exhaust temperatures of almost 400 °C, but elevated number concentrations of nucleation-mode particles for both a used CRT system and a used DOC/new DPF. Based on their findings, Swanson et al.72 hypothesized that nanoparticle emissions associated with use of a CRT are due to release of stored sulfates from the DOC rather than from the uncatalyzed DPF.

Although the CARB–USC testing15,22 did not address the potential role of active regeneration in DEP nanoparticle formation, several other studies have investigated this question. In particular, Khalek et al.14 reported an increase in total particle number concentrations, and specifically in the concentrations of sub-30-nm volatile nanoparticles, during C-DPF active regeneration events for the ACES 2007-model-year engines. Due in part to the apparent increase in nanoparticle concentrations during C-DPF active regeneration, they observed an 88% increase in average total particle number concentrations for their 16-hr tests, which included C-DPF active regeneration, versus the 20-min FTP tests that did not include active regeneration (Figure 5). They hypothesized that the release and subsequent nucleation of sulfates stored on the DOC and C-DPF was the source of the volatile nanoparticles during active regeneration. For their testing of several DPF-equipped diesel passenger cars operated with ULSD fuel (<10 ppm sulfur), Mohr et al.73 also reported the generation of a nucleation mode during regeneration, with order-of-magnitude increases in particle number concentrations. In contrast to the Khalek et al.14 and Mohr et al.73 findings, Guo et al.71 did not see any increase in particle number emissions above engine-out levels during regeneration for their testing of a 2.5-L York diesel engine operated with a C-DPF (and two ALN catalysts) and low-sulfur diesel fuel (4 ppm). Guo et al.71 did report a temporary nucleation mode during regeneration for tests where diesel fuel with a sulfur content of 340 ppm was used.

Lastly, several studies have reported findings supporting the role of fuel sulfur content on nucleation potential, observing reduced nucleation for lower fuel sulfur contents.61,66,70,71 In particular, Vaaraslahti et al.70 observed a smaller nucleation mode with low-sulfur fuel (2 ppm) compared to a higher fuel sulfur content (40 ppm).

Nanoparticles in NTDE Differ from Nanoparticles in TDE

As discussed earlier, there is a growing body of evidence demonstrating that nanoparticle emissions in NTDE have a sulfate-rich composition because they are primarily associated with nucleation of sulfates after the control devices.11,17,22,39,40,62 Importantly, this sulfate-rich composition differs from the hydrocarbon-rich composition that studies have reported for the nanoparticles emitted from conventional diesel engines when a DPF is not present.22,74 In addition, Herner et al.22 reported evidence of a shift in the size distribution of nanoparticle emissions for nucleating aftertreatment configurations, with dominant emissions of particles smaller than 20 nm rather than the 40–70-nm particles typical of the uncontrolled baseline case. As briefly discussed below, these shifts in the composition and size of nanoparticles in NTDE likely result in a shift in their toxicological potential.

An abundance of toxicological data supports the low toxicity of sulfate particles, which will tend to undergo dissolution in the lungs regardless of their size.39,41 42 –43,62 In contrast, Tobias et al.74 concluded that diesel nanoparticle mass in engines lacking aftertreatment also contains some sulfate, but is dominated by insoluble elemental carbon and branched alkanes and alkyl-substituted cycloalkanes from unburned fuel and/or lubricating oil, some of which is likely to remain in particulate form. Because the sulfate PM found in NTDE nanoparticles is likely to be relatively soluble in the lung, it is not likely to persist there like the insoluble EC making up a large portion of the PM of TDE nanoparticles (see Figure 3). Although toxicological studies are needed, the sulfate-rich composition of NTDE nanoparticles thus may contribute to their reduced toxicity compared to hydrocarbon-rich TDE nanoparticles, possibly mitigating any potential health risks associated with their greater numbers. As discussed earlier, the Herner et al.22 finding of an inverse correlation between numbers of volatile sulfur-based nucleation-mode particles and measures of oxidation potential from chemical and cellular assays supports this idea. Finally, citing the increased volatility of the nucleation-mode sulfate particles compared to the solid soot particles characteristic of non-retrofit pre-2007 diesel engines, Herner et al.22 highlight factors that will likely contribute to their reduced exposure potential, including shorter atmospheric lifetimes and rapid drop-offs in concentrations moving away from sources (e.g., roadways).

Given the large uncertainties regarding the role of particle size relative to other physical-chemical properties (e.g., surface area, surface chemistry, surface charge, shape, agglomeration state, chemical composition, crystal structure, and solubility) in determining nanoparticle toxicity,75 it is uncertain how the shift to smaller particle sizes will affect the toxicity of NTDE relative to DE from pre-2007 engines.

Summary on NTDE Nanoparticle Emissions

As discussed above, there is now a body of study findings not only demonstrating the effective filtration of nano-sized particles by a variety of different DPFs, but also indicating the potential for formation of a nucleation mode after the tailpipe for certain aftertreatment configurations and operating conditions. It remains difficult, however, to interpret this body of findings given the lack of standard protocols for measuring DEP nanoparticle emissions and the potential for nanoparticle artifacts due to variable and sometimes unrealistic experimental conditions, including dilution rates, dilution ratios, temperatures, measurement time since formation, and relative humidities. Notwithstanding the remaining uncertainties in the available data and the need for toxicological confirmation, the shift from a hydrocarbon-rich composition to a sulfate-rich composition can be expected to contribute to an overall reduction in the health risks posed by DEP nanoparticles in NTDE, despite the potential for increased emissions under some conditions. For additional perspective on the health significance of DEP nanoparticles in NTDE, it is also important to note that the presence of a nucleation mode is not unique to NTDE, as studies have reported similar and sometimes greater particle number emissions in gasoline engine exhaust (GEE) and the exhaust of CNG engines than in NTDE.63 64 –65,68,76 77–78

PRELIMINARY TOXICOLOGICAL DATA FOR NTDE

Although there has been a dramatic increase in the number of emissions characterization studies for NTDE, there still remain relatively few toxicological data for NTDE. The need for a large-scale toxicological evaluation of NTDE has been well recognized for a number of years, serving as a motivating factor for the design of the comprehensive health effects components of the ongoing Advanced Collaborative Emissions Study (ACES). As described in the project plan for ACES79 and a recent presentation,80 the ACES research plan includes biological screening assays for both mice and rats, both cancer and noncancer health effects, and both short-term and long-term exposures. A core component of ACES is a chronic rat bioassay where rats are exposed via inhalation for 24 or 30 months, with interim sacrifices at 1, 3, 12, and 24 months, to three dilutions of whole emissions from a 2007-compliant diesel engine with advanced emission control technologies (and clean 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 facility specifically developed for ACES by the Lovelace Respiratory Research Institute (LRRI). Health endpoints of interest include not only carcinogenicity but also pulmonary function, pulmonary inflammation, oxidative damage, lung cell proliferation, histopathological changes, and hematological effects. In addition to the chronic rat bioassay, LRRI is also conducting a 13-week subchronic mouse bioassay, again with three dilutions of whole emissions and a similar set of health endpoints (excluding carcinogenicity and pulmonary function).

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

Preliminary Findings from Human Controlled-Exposure Studies

As discussed in a previous review,81 human controlled-exposure studies are considered to provide some of the most relevant data for assessing the potential health risks of DE exposure given that they directly study human subjects, use well-defined exposure concentrations and durations, and precisely measure health outcomes including subtle biological responses. We identified only a single peer-reviewed journal publication of human clinical data of relevance to NTDE, as well as two abstracts from the same research team.30 31–32 As described in these publications, a research team led by researchers at Umeå University and the University of Edinburgh has conducted human clinical experiments of adverse vascular and prothrombotic effects using DE from a Volvo diesel engine (Volvo TD40 GJE, 4.0 L, 4 cylinders) operated under transient conditions with and without a CRT particle trap. Using a randomized, double-blind, three-way crossover design, 19 healthy male volunteers (mean age, 25 ± 3 yrs) were exposed to filtered air, unfiltered dilute diesel engine exhaust, and dilute diesel engine exhaust during 1-hr periods of alternating moderate exercise and rest. These investigators assessed responses to a number of surrogate measures of adverse cardiovascular effects that they had previously shown to be affected by elevated whole-DE exposures without a particle trap in place,82 83 84–85 including endothelial vasomotor and fibrinolytic function and ex vivo thrombus formation.

As discussed in Lucking et al.,32 DEP mass concentrations in the exposure chamber were reduced 98% with use of the particle trap (from 320 ± 10 to 7.2 ± 2.0 μg/m3; P < 0.0001), and fine (<1 mm) particle number concentrations were reduced by >99.8% (from 150,000–300,000/cm3 to 30–300/cm3; P < 0.001). Use of the particle trap was found to reduce the elevations in thrombus formation and to eliminate the impaired vasodilation and fibrinolytic function observed for unfiltered DE exposures.30 31 –32 Based on these consistent findings showing not only improved responses but also normalization, the investigators concluded that retrofit particle traps appear to have “beneficial effects” on surrogate biomarkers of cardiovascular health.32

Limited Laboratory Animal Evidence

We identified just a single laboratory animal study that has investigated the health effects of an inhaled diesel exhaust mixture representative of NTDE, namely the McDonald et al.33 study. McDonald et al.33 investigated the relative toxicity of acute inhalation exposures (6 hrs per day over 7 days) for a baseline uncontrolled, TDE emissions case (approximately 200 μg/m3 DEP) versus an emissions reduction case (low-sulfur fuel, catalyzed ceramic trap) on a suite of sensitive measures of acute lung toxicity in mice, including lung inflammation, respiratory syncytial virus (RSV) resistance, and oxidative stress. McDonald et al.33 reported that the use of the in-line catalyst trap on the test engine (a Yanmar single-cylinder diesel engine generator), as well as low-sulfur fuel, reduced most DE exposure measures, including particle mass and number concentrations, elemental carbon, and particle-bound PAHs, to near background levels in their exposure chamber (with NO_x being a notable exception, where only a 10% reduction was observed—from 2.1 ppm 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 these effects were either nearly or completely eliminated for the emissions reduction case. Despite the need to confirm these findings for a broader range of engines, aftertreatment configurations, operating conditions, and classes of health endpoints (e.g., cardiovascular effects, allergenic effects), McDonald et al.33 concluded that their findings suggest that aftertreatment technologies can mitigate potential health hazards of the associated DE exposures.

Recently, Tzamkiozis et al.34 reported the results of a study where mice were exposed via intratracheal instillation to waterborne suspensions of exhaust PM collected from several different diesel and gasoline vehicles, including a Euro 4 diesel car (TDE-like) and the same car retrofitted with a DPF and representative of a Euro 4+ vehicle (NTDE-like). No evidence of a local inflammatory cellular response for the Euro 4+ samples was observed compared to sham controls (as assessed by influx of polymorphonuclear neutrophils [PMNs] in bronchoalveolar lavage [BAL]), but the investigators reported a statistically significant increase in BAL protein levels, an indicator of alveolar tissue injury, for the highest of the two test doses. A similar response was 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 4+ diesel car was diluted 100-fold less than that of the Euro 4+ diesel car and the other diesel test vehicles (12,000:1 vs. 120:1), indicating that its response on a per-mile-traveled basis would be significantly less that of the other diesel test vehicles. Overall, these results are of indeterminate human health relevance due to the unrealistic exposure scenario (waterborne PM suspensions, intratracheal instillation) and the uncertain clinical significance of the biological responses.

Results from In Vitro Studies of NTDE

Although of limited relevance to human health risk and themselves few in number, in vitro studies currently offer the largest amount of toxicological data for NTDE. In vitro studies can offer some insight on the relative toxicity of NTDE versus TDE, although they are also limited in this regard due to the numerous differences from the in vivo situation that affect the interpretation of their findings for DEP, including (1) absence of the normal lung-defense mechanisms (e.g., macrophage mediated and mucocilliary clearance); (2) absence of cellular protective mechanisms such as antioxidants and DNA repair that act to prevent the expression of intracellular damage or DNA mutations; (3) extremely high doses compared to what is deposited in the alveolar regions of the lung after inhalation, thus eliciting high-dose responses not mechanistically relevant to lower doses; (4) dosing with compounds concentrated from DEP by high-temperature, organic solvent extraction, that is, compounds that appear to be much less bioavailable from inhaled and lung-retained DEP; and (5) possible dosing with reactive artifacts—for example, nitrated organic compounds—formed on filters due to the extended collection times needed to obtain enough DEP mass.7,8 Recognizing their inherent limitations, we briefly review findings from in vitro studies of NTDE below.

Several of the recent in vitro studies of NTDE have assessed the oxidative potential of the DEP fraction of NTDE versus that of other engine exhausts, including one study that used a sensitive macrophage-based in vitro assay21 and two that used the molecular (cell-free) dithiothreitol (DTT) assay18,86 to assess reactive oxygen species (ROS) activity. For a range of different engines and aftertreatment configurations, these studies reported consistent, large reductions in overall ROS activity (expressed on a per-distance-traveled basis) for diesel retrofits compared to baseline (non-retrofit) engines. Using a rat macrophage-based assay, Verma et al.21 observed significant reductions in overall ROS activity (per distance traveled for cruise and UDDS test cycles and per hour for idle) for the various aftertreatment configurations included in the CARB–USC testing program, compared to the non-retrofit baseline vehicle. Also for the CARB–USC vehicles but for a DTT acellular assay, Biswas et al.18 reported uniformly high reductions (60–98%) in oxidative potential expressed per unit vehicle distance traveled for the retrofitted vehicles, including some of the highest reductions for the two SCRT systems. As discussed by the study authors, these results suggest that the SCRT systems may contribute to the reduction and removal of toxicologically important organic compounds. Furthermore, as discussed previously, Herner et al.22 examined the relationship between the two measures of ROS activity and particle number emissions of volatile sulfur-based nucleation-mode particles, observing a negative correlation and concluding that nucleation events in catalyzed aftertreatment systems may contribute to reduced NTDE toxicity. In a study of three light-duty vehicles in five different configurations, Cheung et al.86 reported that a DPF-equipped Euro 4+ Honda Accord (2.2 L, i-CDTi) diesel car had the lowest per km oxidative potential (as assessed using the acellular DTT assay) among their test vehicles, which included a Euro 3 gasoline vehicle.

Both Biswas et al.18 and Verma et al.22 also reported findings indicating an increase in per-PM-mass oxidative activity for most aftertreatment configurations compared to the CARB–USC baseline vehicle. However, they both emphasize the reductions in particulate mass emissions for all aftertreatment configurations that contribute to the overall reduced oxidative activity expressed on a per-distance-traveled basis for the DEP in NTDE versus baseline engines. Interestingly, Verma et al.22 reported findings suggesting that the increases in the per-PM-mass oxidative activity for the retrofitted configurations may be tied to increases in their fractions of redox-active transition metals (e.g., Mn, V, Ni, Cu, Fe, and Cr).

Other studies43,65,87 88 89 90 –91 have investigated the effect of aftertreatment on DEP mutagenic activity, despite the evidence indicating that whole DEP (as opposed to solvent extracts of DEP) in TDE is not genotoxic to cells in culture due to the minimal bioavailability of the mutagenic compounds (e.g., PAHs, nitro-PAHs) in DEP in lung fluids.7,8 As discussed in two prior reviews7,8, in vitro evidence for the mutagenic activity of DEP primarily comes from studies that used hot organic solvents to extract the organics from DEP, and that also may have been affected by artifactual formation of nitro-PAH on DEP filter samples. These limitations apply to the two studies that provide mutagenicity test results for combinations of engine systems, aftertreatment devices, and fuel types representative of NTDE, namely the CARB43,87 and VTT Technical Research Centre of Finland65 studies where DEP emissions from modern diesel buses operated with either no aftertreatment, an oxidation catalyst, or a CRT filter were assessed using a modified Ames test (Salmonella/microsome test). Figure 6 shows that the lowest mutagen emissions in these studies were generally observed for the CRT-equipped buses. Both studies, however, reported the highest findings for specific mutagenic activity (SMA), which is defined as the number of revertant bacteria (rev) per mass of PM collected (e.g., rev/μg PM), for the CRT-equipped buses. For perspective, Figure 6 shows that CARB researchers have observed approximately 2–3-fold higher SMAs for particulate emissions from a CNG bus with aftertreatment (catalyzed muffler) than for the CRT-equipped buses.91

Figure 6. Results of Ames bacterial mutagenicity test results from the Finnish VTT study65 of diesel buses with and without aftertreatment (Euro 3 buses) and from the CARB study43,87 of diesel buses with and without aftertreatment operated using three different diesel fuels (ECD, ECD1, and CARB fuels). Data shown are for the Salmonella strain TA98 with metabolic activation (+S9) and the particulate fraction only. The VTT study buses were 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 buses with aftertreatment (a catalyzed muffler).

Several recent studies have used reporter gene bioassays as screening tools for assessing the biological activity of filtered and unfiltered DE.23,24,48 Based on findings that include substantially reduced responses in multiple bioassays for DEP from DPF-equipped engines compared to baseline diesel engines lacking aftertreatment (e.g., 80–90% and 55–66% reductions in responses for the aryl hydrocarbon receptor [AHR] and estradiol receptor [ER], respectively24), they provide support for the reduced biological activity of DEP in NTDE.

Although not without its own limitations, the recent Hasson et al.92 study merits some comment given that it provides findings relevant to the potential role of regeneration events in affecting NTDE toxicity. More specifically, Hassan et al.92 used a rat lung slice organotypic in vitro model to assess the acute toxicity of NTDE emitted from an advanced emission control technology diesel engine (2.8 L) equipped with a C-DPF and operated with and without regeneration events. Despite high acute exposures of lung slices to 1%, 5%, or 10% exhaust mass fractions and 3–4-fold increases in emissions of 30-nm nanoparticles and 100–200-nm fine particles during regeneration, Hassan et al.92 reported the absence of a significant acute biological response (based on markers of tissue viability, oxidative stress, proinflammatory cytokine release, and exhaust oxidant potential) for NTDE emitted during the transient New European Driving Cycle (NEDC) with and without regeneration events.

Conclusions Regarding Weight of Current Toxicological Evidence

Although there remain far fewer toxicological data for NTDE than for TDE and transitional DE, limited data are now available for multiple lines of toxicological investigation, including human clinical studies, laboratory animal studies, and in vitro bioassays. These toxicological data are currently limited and insufficient on their own to support reliable conclusions regarding the toxicological potency of NTDE compared to that of TDE or transitional DE. Nevertheless, they are consistent in supporting the conclusion that there are toxicological differences between NTDE and both TDE and transitional DE. In particular, available human clinical data for NTDE exposures show an absence of some biological responses previously observed in studies of older diesel engine technologies.30 31–32

Although additional human clinical studies are clearly needed to address a larger suite of health endpoints, the available study findings are more relevant to the risk assessment of NTDE than experimental findings, in particular those from in vitro studies. In addition, with its well-characterized exposure atmosphere and its assessment of a suite of sensitive measures of acute lung toxicity, the McDonald et al. animal study33 provides some of the strongest evidence of the reduced health risks posed by NTDE. However, as a single study that addressed only acute lung toxicity for a single engine, aftertreatment configuration, and animal species, there is clearly a need for more comprehensive toxicological investigations of NTDE to confirm the toxicological differences between NTDE and TDE indicated by the chemical and physical characterization data. On this note, it is expected that ACES will soon provide a wealth of information for assessing the potential carcinogenicity, and the subchronic and chronic noncancer toxicity of NTDE.

Given that studies such as McDonald et al.33 have generally employed the same dilution rates for NTDE as for baseline diesel exposures, it is unclear from the current toxicological data the extent to which the observations of reduced NTDE potency are simply due to the reduced exposures to the various DE constituents versus changes to the chemical and physical properties of DEP in NTDE. There is some evidence from the in vitro tests of mutagenicity and oxidation potential that the biological activity (on a per-mass basis—i.e., per μg of DEP) of DEP in NTDE is not significantly reduced, and may even be higher, than that of DEP emitted from baseline diesel engines lacking aftertreatment. However, findings demonstrating reductions in oxidation potential and mutagen emissions for NTDE expressed on a per-distance-traveled basis have greater relevance for assessing potential health risk implications.18,22 In addition, it is important 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 to extrapolate human health risks from their findings.

CONCLUSIONS AND IMPLICATIONS

As discussed in detail in this review, there now exists a large body of evidence showing that the DEP in NTDE is chemically and physically distinct from the DEP in TDE and transitional DE. There are also preliminary toxicological data suggesting that these differences in DEP emissions, both in terms of emissions levels (on a mass and number basis) and in terms of chemical and physical properties, contribute to differences in the risk profile of NTDE versus TDE exposures. There remain some large data gaps that limit the current understanding of the toxicological potency of NTDE, but both the emissions differences and preliminary toxicological data support reductions in the potential health risks posed by NTDE in real-world exposure scenarios. Moreover, there is clearly now a greater level of support for the idea that the historical data from animal laboratory and human epidemiological studies of TDE have only limited relevance in assessing the potential health risks of NTDE exposures. In fact, U.S. EPA emphasized in the 2002 Health Assessment Document for Diesel Engine Exhaust that its findings applied only to engines that were manufactured prior to 1995 and that newer technology engines would require a reevaluation with regard to potential health impacts.3

Furthermore, there is a growing body of data indicating that PM emissions in NTDE have a greater resemblance to PM emissions in CNG exhaust and GEE than to TDE, both in terms of mass emissions and particle composition. In a prior review,1 we demonstrated that exhaust-aftertreatment technologies reduce emissions for numerous regulated and unregulated species in diesel buses to similar levels for CNG-fueled buses. Figure 7A illustrates this, showing that particulate mass emissions (g/mile) in NTDE from transit buses tested with the Central Business District cycle are more comparable to the emission levels from CNG-fueled transit buses than from TDE buses.47,93 94 95 96 97 98 99–100 Figure 7B shows that for particulate mass emissions (g/mile) in passenger cars, NTDE emission levels are more comparable to gasoline- and CNG-fueled vehicles.76,101,102 In summary, the transit bus and passenger car data show that NTDE mass particulate emissions are 20–70 times lower than those for TDE, and are in the range, if not lower than, particulate mass emission levels reported for CNG- and gasoline-fueled vehicles.

Figure 7. Particulate emissions (PM; g/mile) for transit buses (A) and passenger cars (B) of different engine technologies. Data for transit buses include TDE, NTDE, and CNG exhaust (both without aftertreatment and with oxidation catalyst; CNG+OC), with all testing for the Central Business District test cycle (means, standard errors plotted).44,93 94 95 96 97 98 99–100 Data for passenger cars include TDE, NTDE, and CNG and gasoline exhaust, with pooling of data from a variety of transient test cycles (means, standard errors plotted).76,102

In addition, Figure 8 shows data from the Cheung et al. study86 that included emissions testing of a Euro 3 gasoline vehicle in addition to two diesel vehicles with varying levels of aftertreatment (Euro 4+ with a DPF, DOC, and EGR, which is representative of NTDE; and Euro 1 with no aftertreatment, which is representative of TDE). This figure illustrates that the emissions per mile were lowest for NTDE from the Euro 4+ diesel vehicle for all compounds tested (total PM mass, organic carbon, elemental carbon, water-soluble carbon, sulfate, ammonium, and sum of all inorganic species), except one (nitrate). In their emissions characterization study of four current production European vehicles, Bosteels et al.68 reported comparable, and more often lower, emissions of both particulate- and vapor-phase PAHs for a DPF-equipped diesel vehicle versus a gasoline vehicle. As discussed earlier, findings from a number of studies indicate that nanoparticle emissions (and total particle number emissions) are now comparable, if not lower, for DPF-equipped diesels versus CNG and GEE vehicles.63 64–65,68,76 77–78 Finally, Figure 9 compares the ratio of elemental carbon (EC) to total carbon (TC; where TC = EC + organic carbon [OC]) for different engine exhaust types, providing further evidence that NTDE particulate is more comparable to engine exhaust particulate from CNG- and gasoline-fueled engines than to TDE particulate.63,103,104 As shown in Figure 9, EC/TC ratios for NTDE are substantially lower than those for both TDE and gasoline-engine particulate for both transient and steady-state test cycles, bearing the greatest resemblance to those for CNG particulate.

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

Figure 9. The elemental carbon proportions of total carbon from particulate emissions for transit buses (TDE, NTDE, and CNG) and passenger cars (gasoline) tested under transient test cycles and steady-state conditions. The transient test cycle for the transit buses was the Central Business District test cycle,103 whereas the Unified Driving Cycle was used for the cars.104 The steady-state data for transit buses and cars are from Holmén and Ayala63 and Schauer et al.,104 respectively.

Just as one would conclude that the health studies conducted on TDE over the last 30 years have little relevance to CNG engine exhaust and GEE, there is now evidence indicating that they have little relevance to NTDE. Additional toxicological data are clearly needed to conduct a reliable hazard assessment for NTDE given the uncertainties in the limited data that are currently available, but the available emissions and toxicology data indicate that the historical DE hazard assessments relied upon by various regulatory agencies and scientific panels are of limited, if any, relevance for NTDE. New hazard and risk assessments for NTDE need to be conducted that are independent from the toxicological database regarding pre-2007 diesel engines. ACES promises to provide some of the data needed to better assess the potential carcinogenicity and noncancer toxicity of NTDE, and the toxicological database could be further bolstered if more DE health effects studies focus on NTDE than on TDE. As the diesel fleet continues to turn over and NTDE-emitting vehicles with modern aftertreatment systems replace older TDE-emitting vehicles, the utility of the current DE health effects database for assessing potential health risks to DE constituents in the ambient environment will become less and less relevant. Furthermore, based on epidemiological studies of near-roadway populations that provide evidence of links between traffic-related air pollution and adverse cardiopulmonary health effects,105 the turnover of the diesel fleet with NTDE-emitting vehicles is likely to have significant public health benefits. Although both are emitted from diesel-powered engines, NTDE is a different substance than TDE, requiring its own toxicological investigation and de novo hazard and risk assessments.