To meet increasingly stringent regulations for diesel engines, technologies such as combustion strategies, aftertreatment components, and fuel composition have continually evolved. The emissions reductionachieved by individual aftertreatment components using the same engine and fuel has been assessed and published previously (Liu et al., 2008a,b,c ). The present study instead adopted a systems approach to evaluate the net effect of the corresponding technologies for model-year 2004 and 2007 engines. The 2004 engine was equipped with an exhaust gas recirculation (EGR) system, while the 2007 engine had anEGR system, a crankcase emissions coalescer, and a diesel particulate ﬁlter. The test engines were operated under the transient federal test procedure and samples were collected with a source dilutionsampling system designed to stimulate atmospheric cooling and dilution conditions. The samples wereanalyzed for elemental carbon, organic carbon, and C 1,C2, and C 10through C 33particle-phase and semi- volatile organic compounds. Of the more than 150 organic species analyzed, the largest portion of theemissions from the 2004 engine consisted of formaldehyde, acetaldehyde, and naphthalene and itsderivatives, which were signi ﬁcantly reduced by the 2007 engine and emissions technology. The systems approach in this study simulates the operation of real-world diesel engines, and may provide insight intothe future development of integrated engine technology. The results supply updated information forassessing the impact of diesel engine emissions on the chemical processes, radiative properties, and toxiccomponents of the atmosphere. /C2112009 Elsevier Ltd. All rights reserved. 1. Introduction Increasingly stringent regulations placed upon diesel engine emissions have made it necessary to implement exhaust aftertreat- ment technology to lower the emission levels of criteria pollutants, including nitrogen oxides (NO x) and particulate matter (PM). In 2004, the U.S. Environmental Protection Agency (EPA) implemented emis-sion standards of 0.13 g (KW *h)/C01(0.1 g (bhp*h)/C01) for PM and 3.35 g (KW*h)/C01(2.5 g (bhp*h)/C01)f o rN O xplus non-methane hydrocarbons. More stringent regulations include the recent Euro IV standards of 0.02 g (KW*h)/C01for PM and 3.5 g (KW*h)/C01for NO xand the EPA's 2007 emissions standards of 0.013 g (KW*h)/C01(0.01 g (bhp*h)/C01)f o r PM and 0.27 g (KW*h)/C01(0.2 g (bhp*h)/C01)f o rN O x(phased in by 2010). To meet these regulations, current approaches use engine optimiza-tion strategies for either PM or NO xabatement, and then applySelective Catalytic Reduction (SCR) or Diesel Particulate Filters (DPF)respectively to address the PM eNO xtradeoff. This study re ﬂects the latter approach in the 2007 engine, which includes an EGR system, a crankcase emissions coalescer, and a diesel particulate ﬁlter. The basic function of the DPF is its ability to ﬁlter PM mass emissions with a high level of efﬁ ciency. Diesel PM ﬁltration generally involves the processes of diffusion and interception,coupled with other forces such as thermophoresis and electropho- resis. The ﬁltration efﬁ ciency is known to increase as the PM loading accumulates on the ﬁlter, which comes at the cost of restricted gas ﬂow, corresponding to inhibited performance in the form of engine power and fuel economy ( Eastwood, 2000 ). Therefore, it becomes necessary to regularly remove the PM that has collected on the ﬁlter in order to maintain optimal engine performance. This is typicallyaccomplished through a process known as passive or active regen- eration, in which the PM collected on the ﬁlter is oxidized, usually involving reactions with O 2,N O 2, or hydrocarbons. Besides NO xand PM, diesel emissions also contain a large variety of organic compounds present in both the particle and gas phase*Corresponding author. Tel.: þ1 608 877 3802; fax: þ1 608 873 1550. E-mail address: jerry.z.liu@cummins.com (Z. Gerald Liu). Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv 1352-2310/$ esee front matter /C2112009 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2009.11.036Atmospheric Environment 44 (2010) 1108 e1115 (Eastwood, 2000; Schauer et al., 1999; De Abrantes et al., 2004 ). These organic species are most commonly derived from the diesel fuel, which contains C 10eC25hydrocarbons, and lubrication oil, which contains primarily higher molecular weight hydrocarbons(Schauer et al., 1999; Johnson and Kittelson, 1994; Voss et al., 1997 ). Additionally, the hydrocarbon species present within the diesel fueland lubrication oil may produce thermally-altered byproducts during in-cylinder combustion or during DPF regeneration when temperatures exceed 250 /C14C(Schauer et al.,1999; Rogge et al.,1993 ). This would suggest that diesel engine exhaust may contain a widevariety of organic species, whether present in diesel fuel and lubri- cation oil or produced by thermal alteration. A program known as the Advanced Collaborative Emissions Study (ACES), in collaboration with the U.S. Department of Energy, U.S. Environmental Protection Agency, the California Air Resources Board, the American Petroleum Institute, the Engine Manufacturers Association, the Health Effects Institute, and the Coordinating Research Council, has identi ﬁed more than 650 chemical compounds that may be present in theemissions from diesel engines, the majority of which are part of the complex mixture of organic species. These compounds have been selected according to the risks posed to public health and environ-mental concerns. The emissions reduction achieved by individual aftertreatment components using the same engine and fuel has been assessed and published previously ( Liu et al., 2008a,b,c ). These studies have documented that DPFs signi ﬁcantly reduce the emissions of the major PM constituents, including elemental carbon, organic matter (OM), inorganic ions, and metallic compounds. The same DPF also decreased emissions of many classes of organic compounds: PAHs, aromatics, aldehydes, hopanes, steranes, and n-alkanes. However, real-world diesel engines are integrated systems, and under- standing the net reductions achieved by combustion strategies, aftertreatment components, fuel composition, and other technolo- gies requires a systems approach, as adopted by the present study. This paper evaluated the net effect of the corresponding technolo- gies for model-year 2004 and 2007 engines. The 2004 engine was equipped with an exhaust gas recirculation (EGR) system, while the 2007 engine had an EGR system, a crankcase emissions coalescer, and a diesel particulate ﬁlter. The goals of this study included a detailed understanding of the hydrocarbon speciation from the 2004 engine and an assessment of the emissions reduction between the 2004 and 2007 systems. Using the transient federal test procedure and a source dilution sampling system designed to simulate atmospheric cooling and dilution conditions, samples were analyzed for elemental carbon, organic carbon, and C 1,C2, and C 10through C 33particle-phase and semi-volatile organic compounds. Of the more than 150 organicspecies analyzed, the largest portion of the emissions from the 2004 engine consisted of formaldehyde, acetaldehyde, and naph- thalene and its derivatives, which were signi ﬁcantly reduced by the 2007 engine and emissions technology. The systems approach inthis study represents the operation of real-world diesel engines, and may provide insight into the future development of integrated engine technology. The results supply updated data, applicable to atmospheric models of chemical transport, climate, and recep- tors, which assess the impact of diesel engine emissions on thechemical processes, radiative properties, and toxic components of the atmosphere. 2. Experimental methods2.1. Engine con ﬁgurations For each test engine, the diesel fuel was certi ﬁed for the corre- sponding year. The 2004 fuel was Chevron Phillips Low SulfurCertiﬁed #2 Diesel and the 2007 fuel was Chevron Phillips Ultra- Low Sulfur Certi ﬁed Diesel for which the fuel properties can be found in Table 1 . With the exception of the sulfur content, the two fuels have similar properties. It is expected that the effect of the two fuels on organic compound emissions would be marginal. However, the lower sulfur content of the 2007 fuel would reduce the formation of particle-phase sulfates that contribute to PM mass and number emissions ( Kweon et al., 2003 ). The lubrication oil used for each engine was Valvoline Premium Blue 15W-40 with a sulfatedash content of less than 1%. Two similar 15 L heavy-duty diesel engines, compliant with EPA regulations for the corresponding model-year, were used in order to compare the effects of 2004 and 2007 emissions technology. The 2004 engine was equipped with an EGR system for NO xreduction, which recycles a fraction of the exhaust gas into the engine intaketo dilute the O 2concentration and lower the ﬂame temperature of the working ﬂuid within the cylinder. In contrast, the 2007 engine had an EGR system, DPF, and crankcase emissions coalescer for NO x and PM reduction. The DPF used in the present study consisted oftwo stages. The ﬁrst stage was a diesel oxidation catalyst (DOC), a ceramic ﬂow-through monolith with a precious metal coating designed to aid in the reduction of CO and hydrocarbon (HC) emissions as well as assist with ﬁlter regeneration through the exothermic oxidation of dosed diesel fuel and the conversion of NOto NO 2. The second stage was a catalyzed soot ﬁlter (CSF) designed to reduce PM emissions, lower the soot oxidation temperature, andremove the fuel residuals. Composed of porous cordierite material, the extruded wall-ﬂ ow monolithic CSF had alternating plugs. Diesel particles accumulate on the surface of the ﬁlter media, and as the soot cake layer builds in thickness, ﬁltration efﬁ ciency improves and the restriction increases, which eventually requires regenera- tion. Previous studies have documented that the combined DOC and CSF reduce the emissions of both particle-phase and semi- volatile organic compounds ( Liu et al., 2008a,c ). Crankcase emissions, which include both gaseous and particu- late components resulting from chemical, thermal, and mechanical processes, have become a necessary constituent of the total emis- sions analysis due to their inclusion in the EPA's 2007 regulations. For the 2007 engine, crankcase emissions were ﬁltered using a coalescer as part of an open crankcase ventilation (OCV) system.The coalescer was primarily designed to remove large oil droplets generated mechanically with a secondary function of removing chemically and thermally generated sub-micron oil mist from the gas fumes exiting the crankcase before they are vented to the atmosphere. 2.2. Testing procedure In preparation for testing, the engines were warmed up for 2 h on ISO mode 8, corresponding to 50% load and intermediate speed. For each conﬁ guration, the engine was run over the transientTable 1 Properties for 2004 and 2007 certi ﬁed diesel fuels. Property 2004 Fuel 2007 Fuel API gravity 36.2 36.2Cetane number 47 42Sulfur content (ppm) 308.5 9.2PM content (mg L /C01) 0.1 3.3 Aromatics (LV%) 29.5 32.0 Saturates (LV%) 67.9 63.6 Viscosity @ 40/C14C (cSt) 2.6 2.2 Distillation 5% (/C14C) 202 197 Distillation 95% (/C14C) 333 312Z. Gerald Liu et al. / Atmospheric Environment 44 (2010) 1108 e1115 1109 Federal Test Procedure (FTP) cycle, which has a duration of 20 min. The FTP cycle consists of four stages designed to simulate the highly transient operating conditions that heavy-duty engines often encounter on American road ways. Each con ﬁguration was tested at least three times to determine the measurement variation. Blanksubstrates were exposed during each step of the process to monitor for contamination from dilution air, background conditions, handling, storage, and transportation. Engine exhaust samples were collected with a source dilution sampling (SDS) system which consisted of a constant volume primary dilution tunnel, a secondary micro-diluter, a residence time chamber (RTC), isokinetic sampling probes, multiple sampling trains, and control devices. A detailed discussion of the design, operating principles, and quality assurance and quality control (QA/QC) procedures for the SDS system can be found in the literature (Liu et al., 2008d). In summary, HEPA ﬁlters and activated carbon removed particles and organic compounds, respectively, from thedilution air prior to mixing with the engine exhaust. The dilution air had signi ﬁcantly lower particle number concentrations than the ambient air of the Emissions Laboratory, according to themeasurements from the Scanning Mobility Particle Sizer (SMPS).The mixture of dilution air and engine exhaust was further diluted to simulate atmospheric conditions and sampled with a full-partial- full method to minimize particle loss. A variety of media were used for sample collection including baked quartz ﬁlters, polytetra- ﬂuoroethylene (PTFE) membrane ﬁlters, XAD resin cartridges, polyurethane foam (PUF), thermal desorption (TD) tubes, and dini- trophenylhydrazine (DNPH) cartridges. Upstream of the sample collection media, PM 2.5cyclone separators pre-classi ﬁed the PM inthe exhaust ﬂow.Fig.1 a shows the testing setup and the components of the SDS system, while Fig. 1 b shows the sampling train con ﬁgu- rations and collection media. Before testing, the system was cleanedand the substrates were prepared according to applicable QA/QC practices. 2.3. Analysis methods Samples collected at the Cummins Emission Laboratory were analyzed for chemical species, using a variety of established methods, by the Wisconsin State Laboratory of Hygiene and the Desert Research Institute. Particulate emissions of EC, OC, and particle-phase organics, were captured using quartz and Te ﬂon impregnated glass ﬁber (TIGF). EC and OC in the particulate matter samples were analyzed using a thermal eoptical method described byBirch and Cary (1996). The samples were heated to 870 /C14C under a helium atmosphere to evolve OC, which was then oxidized to CO 2 and reduced to methane before being measured by a ﬂame ioni- zation detector (FID). The sample was then cooled and re-heated in a helium and oxygen atmosphere to remove all carbon from the ﬁlter. The evolved carbon is corrected for pyrolysis carbon formed during the initial analysis stage and the elemental carbon in thesample is quanti ﬁed. OC is reported as the sum of the carbon evolved during the initial stage of the analysis plus the pyrolysiscarbon. Two corrections were made to the resulting OC mass. The measurement from the second ﬁlter was subtracted from the ﬁrst to correct for non-adsorbed gaseous OC emissions ( Kolodziej et al., 2007 ). Also, the OC mass was multiplied by 1.2 to correct for oxygen, hydrogen, and nitrogen constituents of the total organic Fig. 1. Schematic of sample collection apparatus showing (a) the full-partial-full source dilution sampling system and (b) the sampling trains found in the sampling section of the RTC.Z. Gerald Liu et al. / Atmospheric Environment 44 (2010) 1108 e1115 1110 matter (OM) adsorbed onto the EC particles ( Seinfeld and Pandis, 1998 ). Although Turpin and Lim (2001) suggested that a 1.4 factor can be the lowest reasonable estimate and a 1.6 factor appears more accurate for urban aerosol, Bae et al. (2006) concluded that vehicle emissions have an OM-to-OC ratio closer to 1.2, a result of less oxygenated organics in vehicle emissions compared to atmospheric particulate matter. Further analysis for particle-phase compounds was performed on the baked quartz and TIGF ﬁlters. Organic species, including PAHs, PAH derivatives, and aromatics were analyzed using gaschromatography/mass spectrometry (GC/MS) in addition to the gaseous samples collected on XAD resins as described below (Zielinska et al., 2004; Fujita et al., 2007 ). Further, hopanes and steranes, which were collected on quartz ﬁlters, were analyzed using GC/MS. Prior to sampling, the quartz ﬁlters were baked at 550 /C14C for 12 h to ensure that there were no preexisting organics. Gaseous volatile organic compounds (VOCs) including PAHs, aldehydes, and n-alkanes were also analyzed in the present study.Gaseous samples were collected on TD tubes, DNPH cartridges, and XAD resin packed cartridges that were situated behind a TIGF ﬁlter during sample collection. TD tubes and DNPH cartridges were usedfor the collection of gaseous samples to be analyzed for aldehydes and alkanes. TD tubes were analyzed by a modi ﬁed National Institute of Occupational Safety and Health (NIOSH) method 2549in which the organic compounds were quanti ﬁed by GC/MS, after being conditioned to ambient temperature ( Eller, 1994 ). DNPH cartridges were desorbed with acetronitrile and analyzed using high performance liquid chromatography (HPLC), which utilized a NOVA-PAK C18 column and UV detection (Winberry et al., 1999 ). XAD cartridges were extracted with dichloromethane, combined with TIGF ﬁlter extracts, and analyzed for PAHs, and PAH derivatives by GC/MS interfaced with an ion trap operating under electronimpact ionization mode (Varian CP-3800 GC equipped with a CP8400 autosampler and interfaced to a Varian Saturn 2000 Ion Trap), as described in the literature ( Zielinska et al., 2004; Fujita et al., 2007 ). Quanti ﬁcation of the individual compounds was obtained using the selective ion storage technique, monitoring themost characteristic ions of each compound of interest and the cor- responding deuterated internal standard from the Desert Research Institute. Six-point calibration curves for the GC/MS quanti ﬁcation were made for the most abundant and characteristic ion peaks of thecompounds of interest using the deuterated species most closely matched in volatility and retention characteristics as internal stan- dards from the Desert Research Institute ( Zielinska et al., 2004 ). The calibration check (using median calibration standards) was runevery 10 samples to verify the accuracy of analyses. If the relativeaccuracy of measurement, de ﬁned as a percent difference from the standard value, was less than 20%, the instrument was recalibrated.The minimum detection limit (MDL) for PAH and derivatives is 0.02 ng mL/C01, which corresponds to approximately 4 ng sample/C01. Nitro-PAHs were analyzed using negative ion chemical ionization with a triple quadrupole GC/MS/MS system (Varian 1200), which allowed for structural elucidation of unknown compounds with precursor, product and neutral loss scan as well as sensitive analyses of complex mixtures. Quanti ﬁcation of the individual compounds was performed by similar methods as for PAHs, but instead using themultiple ion detection technique. For each analysis, a six- to eight- level calibration was performed for each compound of interest and a calibration check was run every 10 samples as described above. The MDL for nitro-PAH is 0.0005 ng mL/C01, which corresponds to approximately 0.1 ng sample/C01. 3. Results and discussion 3.1. Particulate emissions The experimental design also measured the criteria pollutants, including PM, NO x, CO, total hydrocarbons, as well as O 2,C O 2, etc., but the discussion of these compounds were out of the scope of this paper. EC and OM, the latter of which encompasses the hydro- carbon species in this paper, represent two of the largest constit- uents of the total PM emissions. Both were analyzed in this study and their values are shown in Table 2 . From the 2004 engine, EC emissions comprise approximately 48% of the total PM mass whilethe OM contributes another 44%. However, from the 2007 engine, PM mass emissions contain roughly 21% EC and 35% OM. The remaining portion of the PM mass emissions, 8% for the 2004 engine and 44% for the 2007 engine are composed of inorganic ions, primarily sulfate, ammonia, and nitrate, as well as trace metallic emissions. This implies that the 2007 engine and emissions tech- nology dramatically reduced the emissions of EC and OM, resulting in a larger composition from other sources, such as inorganic ions. Since OM is such a large portion of the total PM mass emissions, it is important to gain a more in depth understanding of the organic species which contribute to the total OM mass. 3.2. Organic emissions As part of the present study, gaseous and particle-phase samples collected from the test engines were analyzed and more than 150 organic compounds were quanti ﬁed. PAHs and aliphatic aldehydes make up the largest portions of the total semi-volatile and particle-phase organic compound mass emissions, of which formaldehyde, acetaldehyde, and naphthalene and its derivatives predominate. Additional compounds, such as alkanes, nitro-PAHs, and hopanes and steranes, were also quanti ﬁed and a summary of the amounts detected and the reductions can be found in Table 2 . When a compound failed to be detected, the symbol “<”indicates the corresponding detection limit. Organic compounds analyzed in the present study included C 1, C2, and C 10through C 33particle-phase and semi-volatile organic compounds. Low molecular weight hydrocarbons such as thosebelow C 22are frequently derived from diesel fuel, while high molecular weight hydrocarbons are present in lubrication oil(Johnson and Kittelson, 1994; Voss et al., 1997 ) although signi ﬁcant overlap exists ( Zielinska et al., 2004 ). The majority of organic hydrocarbons in Table 2 have a low molecular weight and therefore are believed to be from fuel-derived sources and most displayeda reduction of greater than 90%. The fuel-derived portion of the hydrocarbons typically exists in the gas phase at catalyst operating temperatures, while the soluble organic fraction (SOF) portion of11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Number of Carbon atoms2004 Engine 2007 Engine 1.00x10-4 1.00x10-5 1.00x10-6 1.00x10-71.00x10-3g (bhp*h)-1 Fig. 2. Comparison of alkane emissions arranged by carbon number.Z. Gerald Liu et al. / Atmospheric Environment 44 (2010) 1108 e1115 1111 Table 2 Organic species emissions comparison from a 2004 and a 2007 heavy-duty diesel engine. Compound (carbon number) 2004 Enginea2007 Enginea% Reduced Elemental carbon 49 700 /C6 3550 150 /C6 38.2 99.7/C67.2 Organic carbon 37 800 /C6 4360 213 /C6 101 99.4/C611.8 Organic mass 45 300 /C6 5230 256 /C6 121 99.4/C611.8 n-Alkanesn-Undecane (11) <0.01 /C6 2.97 1.04 /C6 1.76 e n-Dodecane (12) <0.01 /C6 0.795 0.279 /C6 0.286 e n-Tridecane (13) 2.25 /C6 0.859 <0.01 /C6 0.186 >99.6/C646.4 n-Tetradecane (14) 10.4 /C6 2.64 <0.01 /C6 0.203 >99.9/C627.3 n-Pentadecane (15) 34.4 /C6 5.52 <0.01 /C6 0.00 >99.9/C616.0 n-Hexadecane (16) 84.6 /C6 13.4 <0.01 /C6 0.00 >99.9/C615.8 n-Heptadecane (17) 96.5 /C6 10.7 <0.01 /C6 0.193 >99.9/C611.3 n-Octadecane (18) 68.8 /C6 12.7 <0.01 /C6 0.413 >99.9/C619.1 n-Nonadecane (19) 52.3 /C6 10.0 <0.01 /C6 1.02 >99.9/C621.1 n-Eicosane (20) 75.0 /C6 7.46 <0.01 /C6 0.931 >99.9/C611.2 n-Heneicosane (21) 68.5 /C6 4.88 <0.01 /C6 0.348 >99.9/C67.6 n-Docosane (22) 48.1 /C6 4.63 <0.01 /C6 0.423 >99.9/C610.5 n-Tricosane (23) 19.3 /C6 8.48 <0.01 /C6 0.00 >99.9/C643.9 n-Tetracosane (24) 0.0127 /C6 2.37 <0.01 /C6 1.07 e Branched alkanesNorpristane (18) 215 /C6 34.6 <0.01 /C6 0.754 >99.9/C616.4 Pristane (19) 89.4 /C6 14.6 <0.01 /C6 0.0725 >99.9/C616.4 Phytane (20) 28.3 /C6 9.02 <0.01 /C6 0.768 >99.9/C634.6 Saturated cycloalkanesDodecylcyclohexane (18) 4.26 /C6 2.67 <0.01 /C6 0.00 >99.8/C662.7 Pentadecylcyclohexane (21) 8.92 /C6 1.91 <0.01 /C6 0.00 >99.9/C621.4 Hexadecylcyclohexane (22) 3.52 /C6 1.85 <0.01 /C6 0.00 >99.7/C652.6 Heptadecylcyclohexane (23) 3.53 /C6 1.05 <0.01 /C6 0.00 >99.7/C629.7 Octadecylcyclohexane (24) 1.02 /C6 1.02 <0.01 /C6 0.00 >99.0/C6100 Nonadecylcyclohexane (25) 0.896 /C6 0.451 <0.01 /C6 0.00 >98.9/C650.3 AromaticsBiphenyl (12) 140 /C6 11.4 47.7 /C6 14.2 65.9/C618.3 2-Methylbiphenyl (13) 13.3 /C6 2.09 54.3 /C6 28.6 e 3-Methylbiphenyl (13) 288 /C6 29.5 152 /C6 64.0 47.2/C632.5 4-Methylbiphenyl (13) 62.5 /C6 5.52 18.8 /C6 5.10 69.9/C617.0 PAHs, POM, and DerivativesNaphthalene (10) 719 /C6 79.6 122 /C6 129 83.0/C629.0 2-Methylnaphthalene (11) 1290 /C6 144 82.7 /C6 52.1 93.6/C615.2 1-Methylnaphthalene (11) 543 /C6 52.5 46.1 /C6 26.1 91.5/C614.5 Dimethylnaphthalenes (12) 1460 /C6 113 89.0 /C6 18.6 93.9/C69.0 Trimethylnaphthalenes (13) 935 /C6 45.9 38.8 /C6 3.95 95.9/C65.3 1-Ethyl-2-methylnaphthalene (13) 115 /C6 14.1 4.25 /C6 1.18 96.3/C613.3 2-Ethyl-1-methylnaphthalene (13) 6.83 /C6 1.59 0.673 /C6 0.193 90.1/C626.1 Anthracene (14) 7.38 /C6 1.00 0.862 /C6 0.385 88.3/C618.8 Phenanthrene (14) 78.6 /C6 11.3 12.3 /C6 3.62 84.4/C619.0 Methylphenanthrenes (15) 85.4 /C6 9.49 3.30 /C6 0.460 96.1/C611.7 Dimethylphenanthrenes (16) 66.9 /C6 5.33 1.17 /C6 0.239 98.3/C68.3 Fluorene (13) 131 /C6 20.6 12.9 /C6 3.54 90.2/C618.4 Methyl ﬂuorenes (14) 0.00 /C6 0.00 10.9 /C6 3.91 e Fluoranthene (16) 4.31 /C6 0.137 1.13 /C6 0.564 73.8/C616.3 Pyrene (16) 11.7 /C6 1.20 0.979 /C6 0.649 91.6/C615.8 Acenaphthalene (12) 30.5 /C6 1.88 2.18 /C6 1.42 92.9/C610.8 Acenaphthene (12) 45.5 /C6 6.55 22.0 /C6 21.1 51.6/C660.8 Chrysene þtriphenylene (18) 1.05 /C6 0.133 0.123 /C6 0.109 88.3/C623.0 Benz[a]anthracene (18) 0.586 /C6 0.0579 0.0632 /C6 0.0698 89.2 /C621.8 Benzo[g,h,i] ﬂuoranthene (18) 0.607 /C6 0.593 0.258 /C6 0.270 57.5/C6142 Benzo[b þkþj]ﬂuoranthene (20) 0.240 /C6 0.0735 0.00776 /C6 0.00715 96.8 /C633.6 Benzo[a]pyrene (20) 0.0797 /C6 0.0378 0.00613 /C6 0.00469 92.3 /C653.3 Benzo[e]pyrene (20) 0.232 /C6 0.0575 0.00374 /C6 0.0983 98.4 /C667.2 Benzo[g,h,i]perylene (22) 0.0724 /C6 0.0240 0.0168 /C6 0.00885 76.8 /C645.4 Nitro-PAHs1-Nitronaphthalene (10) 0.361 /C6 0.0701 0.0858 /C6 0.0198 76.2 /C624.9 2-Nitronaphthalene (10) 0.531 /C6 0.0896 0.0478 /C6 0.00914 91.0 /C618.6 Methylnitronaphthalenes (11) 0.719 /C6 0.110 0.0232 /C6 0.00393 96.8 /C615.8 2-Nitrobiphenyl (12) 0.0228 /C6 0.00974 0.00166 /C6 0.00087 92.7 /C646.5 4-Nitrobiphenyl (12) 0.0103 /C6 0.00644 0.000117 /C6 0.00009 98.9 /C663.4 1-Nitropyrene (16) 0.0550 /C6 0.0154 <0.00025 /C6 0.00 99.5/C628.0 9-Nitroanthracene (14) 0.192 /C6 0.00914 0.0403 /C6 0.00931 79.0 /C69.6 Oxygenated PAHsAcenaphthenequinone (12) 29.1 /C6 2.68 0.945 /C6 1.49 96.8/C614.3 9-Fluorenone (13) 13.9 /C6 2.29 6.54 /C6 1.59 52.9/C627.9 Xanthone (13) 8.75 /C6 3.94 0.386 /C6 0.0908 95.6 /C646.1Z. Gerald Liu et al. / Atmospheric Environment 44 (2010) 1108 e1115 1112 the PM consists primarily of lubrication oil-derived hydrocarbons adsorbed onto carbon particles ( Eastwood, 2000; Farrauto and Voss, 1996 ). It is apparent from Table 2 that the modern engine and emissions technology signi ﬁcantly reduces both low molecular weight and high molecular weight hydrocarbons. Low molecular weight organic species consist primarily of PAHs and n-alkanes. Emissions of alkanes are 751 mg (KW*h)/C01 (560 mg (bhp*h)/C01) and 1.77 mg (KW*h)/C01(1.32 mg (bhp*h)/C01)f r o m the 2004 and 2007 engines, respectively, representing a reductionin excess of 99%. Commonly found in crude oil, from which diesel fuel derives, branched alkanes may serve as diesel exhaust tracers during air quality analysis ( Schauer et al., 1999 ). Branched alkane species in the present study, which included norpristane, pristine,and phytane (C 18eC20), are each reduced to levels below the detection limits of 0.013 mg (KW*h)/C01(0.01 mg (bhp*h)/C01). For the 2004 engine, C 18norpristane has the largest concentration of the alkane species at 288 mg (KW*h)/C01(215 mg (bhp*h)/C01), followed by C 17n-heptadecane at 129.4 mg (KW*h)/C01(96.5 mg (bhp*h)/C01). The reduction of alkanes (also shown in Fig. 2 as arranged by carbon number), like many of the other hydrocarbon species, primarily occurs through gas-phase oxidation, although ﬁltration can make a minor contribution ( Johnson and Kittelson, 1994; Voss et al.,1997; Bagley et al., 1998 ). Most low molecular weight polycyclic aromatic hydrocarbons (2e3 rings) consist of fuel-derived hydrocarbons, typically present in the engine exhaust and in the gas phase ( Zielinska et al., 2004 ). These and other combustion derived PAHs also originate from the crankcase due to leakage around the piston rings and possible absorption into the crankcase oil ( Zielinska et al., 2004; Fujita et al., 2007 ). The concentrations of the tailpipe PAHs from the 2004 engine and combined tailpipe and crankcase PAHs from the 2007 engine are displayed in Fig. 3 , arranged by the number of aromatic rings. The highest PAH emissions from the 2004 engine are observed for methyl and dimethyl substituted naphthalene deriv- atives, both displaying emissions of greater than 1877 mg (KW*h)/C01 (1400 mg (bhp*h)/C01). As can be seen in Fig. 3 , the emission rates of 2 through 6-ring PAHs displayed signi ﬁcant reductions for the 2007 engine, but low initial emission levels of 5 and 6-ring PAHs resultedin larger relative error. Species of particular concern are the 5 ring PAHs such as the highly toxic benzo[a]pyrene. The 2007 engine achieved signi ﬁcant reductions of the majority of these compounds with no observed production of any 5 ring PAHs. Reduction of thesePAH species from the 2007 engine likely proceeded through theoxidation and ﬁltration mechanisms of the DOC and CSF. It has been shown that high fuel sulfur content impairs the performance of theoxidation catalyst ( Eastwood, 2000 ); therefore the ultra-low sulfur content of the 2007 fuel allows for optimal conversion ef ﬁciency and reduced PAH emissions from the oxidation catalyst. Methyl and dimethyl substituted PAHs were reduced at a greater rate than their unsubstituted counterparts. Previous studies have suggested the process of thermal dealkylation as an explanation (Schauer et al., 1999; Tancell et al., 1995 ). The effects of thermal dealkylation should also be seen under high exhaust temperatures during DPFregeneration. Additionally, exposure of PAH compounds to gaseous NO x, such as NO 2, emissions can produce nitro-PAHs. The PAH nitration reactions occur in the atmosphere, during combustion, or withinaftertreatment devices. Airborne nitro-PAHs account for a substantial portion of the total mutagenic properties of ﬁne particles ( Harley et al., 2006 ).Fig. 3 also displays the emission levels of the nitro-PAH compounds from the 2004 and 2007 heavy-duty diesel engines, arranged by the number of aromatic rings. Of the 14 nitro-PAHs analyzed during the present study, the majority are reduced signi ﬁcantly for the 2007 engine, except forTable 2 (continued ) Compound (carbon number) 2004 Enginea2007 Enginea% Reduced Perinaphthanone (13) 29.7 /C6 4.33 1.01 /C6 0.288 96.6/C615.5 Anthraquinone (14) 5.16 /C6 0.886 1.30 /C6 0.506 74.8/C627.0 9-Anthraaldehyde (15) 1.56 /C6 0.829 0.0388 /C6 0.0291 97.5 /C655.0 Benzanthrone (17) 1.89 /C6 0.109 0.0154 /C6 0.00973 99.2 /C66.3 Aliphatic aldehydes Formaldehyde (1) 5160 /C6 2440 <0.01 /C6 58.1 >99.9/C648.4 Acetaldehyde (2) 1480 /C6 783 <0.01 /C6 43.1 >99.9/C655.8 Hopanes 17a(H)-22,29,30-Trisnorhopane (27) 0.430 0.0658 <0.01 0.00 97.7/C615.3 17a(H),21 b(H)-Hopane (30) 1.67 0.0558 0.0109 0.0109 99.3 /C64.0 22S-17 a(H),21 b(H)-29-Homohopane (31) 0.925 0.0309 <0.01 0.00 98.9/C63.3 22R-17 a(H),21 b(H)-29-Homohopane (31) 0.545 0.284 <0.01 0.00 98.2/C652.1 22S-17 a(H),21 b(H)-29,30-Bishomohopane (32) 2.11 1.60 <0.01 0.00 99.5/C675.8 22R-17 a(H),21 b(H)-29,30-Bishomohopane (32) 0.288 0.144 <0.01 0.00 96.5/C650.0 22R-17 a(H),21 b(H)-29,30,31-Trishomohopane (33) 5.33 5.33 <0.01 0.00 e Steranes20S-5 a(H),14 a(H),17 a(H)-Cholestane (27) 5.89 4.87 <0.01 0.00 99.8/C682.7 20R-5 a(H),14 b(H),17 b(H)-Cholestane (27) 0.576 0.0438 <0.01 0.00 98.3/C67.6 20S-5 a(H),14 b(H),17 b(H)-Cholestane (27) 0.749 0.0729 <0.01 0.00 98.7/C69.7 aValues are reported in mg (bhp*h)/C01, uncertainty is given as the standard error of the test results. 23456 Number of rin gs2004 Engine PAHs 2007 Engine PAHs 2004 Engine nitro-PAHs 2007 Engine nitro-PAHs1.00x10-2 1.00x10-3 1.00x10-4 1.00x10-5 1.00x10-6 1.00x10-7 1.00x10-8g (bhp*h)-1 Fig. 3. Comparison of PAH and nitro-PAH emissions arranged by number of aromatic rings.Z. Gerald Liu et al. / Atmospheric Environment 44 (2010) 1108 e1115 1113 a small increase for 9-nitrophenanthrene. The mechanisms of this increase remain unknown; however, it is speculated that the selective nitration of phenanthrene occurs within the DPF or results from sampling artifacts caused by the extended exposure of the sampling substrates to the gaseous NO xin the exhaust stream. High molecular weight organic species in the present study consist of lubrication oil-derived compounds, including saturatedcycloalkanes, hopanes, steranes, and a few 6-ring PAHs; however, some of these compounds partition between the gas and particle phases ( Schauer et al.,1999; Johnson and Kittelson,1994; Fujita et al., 2007 ). Hopanes and steranes are often found in crude oil due to the decomposition of biomass materials ( Zielinska et al., 2004 ). These species, previously used as tracers for diesel PM emissions ( Schauer et al.,1999; Zielinska et al., 2004; Fujita et al., 2007 ), contribute to the SOF in the form of particulate hydrocarbons. Consequently, thereductions seen in Fig. 4 were attributed to particulate ﬁltration mechanisms over the CSF and subsequent combustion during ﬁlter regeneration. The high molecular weight, 6-ring PAH compounds analyzed in the present study were emitted at low levels, with the largest contribution from benzo[g,h,i]perylene. With an emission rate of 0.0971 mg (kW*h)/C01(0.0724 mg (bhp*h)/C01) from the 2004 engine, benzo[g,h,i]perylene was reduced in excess of 75% by the2007 emissions technology. For suf ﬁciently high exhaust tempera- tures, the oxidation catalyst may remove a portion of the SOFemissions before they adsorb onto the primary carbon particles eventually ﬁltered out by the CSF (Eastwood, 2000 ). 3.3. Conclusion The results from this study, along with other studies in the literature, suggest that the 2007 heavy-duty diesel emissionstechnology is capable of reducing elemental carbon, organic carbon, and also the majority of particle-phase and semi-volatileorganic compounds present in diesel engine exhaust. Fig. 5 summarizes the reduction efﬁ ciency of the 2007 technology over the 2004 technology for PAHs, alkanes, and hopanes and steranes, whose details are shown in Table 2 . There is no apparent rela- tionship between carbon number and the reduction efﬁ ciency, suggesting that the reduction efﬁ ciency is more dependent upon each individual compound's molecular structure and subsequentreactivity. The signi ﬁcant reductions observed for formaldehyde, acetaldehyde, PAHs, and nitro-PAHs are of particular importancedue to the carcinogenic and mutagenic properties of these compounds ( Eastwood, 2000; Code of Federal Regulations, 2006 ).The PAHs, which have the less consistent reduction ef ﬁciencies, can be thermally altered, while the hopanes and steranes are mainlydestroyed in both the combustion process and within the after- treatment devices. Acknowledgements The authors acknowledge Dr. Roger McClellan, Prof. John John- son and Prof. Susan Bagley of Michigan Technological University, Prof. David Kittelson of the University of Minnesota, Dr. Doug Lawson of the National Renewable Energy Laboratory, Prof. David Foster of the University of Wisconsin-Madison, Dr. Glynis Lough and Dr. Michael Hays of the U.S. EPA, and Dr. John Wall, Dr. Wayne Eckerle, and Dr. Shirish Shimpi of Cummins Inc. for their valuable discussions. We also acknowledge Thaddeus Swor and Tom Wosi- kowski of Cummins Emission Solutions for their help with sample collection as well as Anna Cunningham and Mark McDaniel of the Desert Research Institute, and Jeff DeMinter, Steve Strebel, and Steve Geis of the Wisconsin State Laboratory of Hygiene for their help with the chemical analysis. Appendix. Supplementary material Supplementary data associated with this article can be found in the online version at doi:10.1016/j.atmosenv.2009.11.036.