Polycyclic aromatic hydrocarbon exhaust emissions from different reformulated diesel fuels and engine operating conditions

Atmospheric Environment 43 (2009) 5944–5952

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Polycyclic aromatic hydrocarbon exhaust emissions from different reformulated diesel fuels and engine operating conditions Esther Borra´s a, *, Luis A. Tortajada-Genaro b, Monica Va´zquez a, Barbara Zielinska c a

´neo (CEAM), Paterna, Valencia, Spain ´n Centro de Estudios Ambientales del Mediterra Fundacio ´gico, Universidad Polite´cnica de Valencia, Valencia, Spain Instituto de Reconocimiento Molecular y Desarrollo Tecnolo c Atmospheric Sciences Center, Desert Research Institute (DRI), Reno, NV, USA b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 February 2009 Received in revised form 13 July 2009 Accepted 14 August 2009

The study of light-duty diesel engine exhaust emissions is important due to their impact on atmospheric chemistry and air pollution. In this study, both the gas and the particulate phase of fuel exhaust were analyzed to investigate the effects of diesel reformulation and engine operating parameters. The research was focused on polycyclic aromatic hydrocarbon (PAH) compounds on particulate phase due to their high toxicity. These were analyzed using a gas chromatography–mass spectrometry (GC–MS) methodology. Although PAH profiles changed for diesel fuels with low-sulfur content and different percentages of aromatic hydrocarbons (5–25%), no significant differences for total PAH concentrations were detected. However, rape oil methyl ester biodiesel showed a greater number of PAH compounds, but in lower concentrations (close to 50%) than the reformulated diesel fuels. In addition, four engine operating conditions were evaluated, and the results showed that, during cold start, higher concentrations were observed for high molecular weight PAHs than during idling cycle and that the acceleration cycles provided higher concentrations than the steady-state conditions. Correlations between particulate PAHs and gas phase products were also observed. The emission of PAH compounds from the incomplete combustion of diesel fuel depended greatly on the source of the fuel and the driving patterns. Ó 2009 Elsevier Ltd. All rights reserved.

Keywords: Polycyclic aromatic hydrocarbons Reformulated diesel Rape oil methyl ester biodiesel Diesel exhaust emissions

1. Introduction Diesel engine exhaust emissions are important sources of ambient air pollution and have been associated with cancer and non-cancer health effects (Scha¨fer and Van Basshuysen, 1995; Yassa and Cecinato, 2005). However, diesel emissions are chemically complex and quite variable, and they included both gas phase and particulate emissions. Diesel exhaust composition depends on several factors, i.e., fuel composition: gasoline, diesel, reformulated diesel and source of biodiesel (Kittelson et al., 2006; Ro¨nkko¨ et al., 2007; Lim et al., 2005); engine type: light-duty and heavy-duty (Miguel et al., 1998; Riddle et al., 2007) and engine operating conditions: driving cycles or individual regimes (Karavalakis et al., 2009; Yang et al., 2007; Kado et al., 2005). Additional factors are the presence of catalysts (Liu et al., 2007) and working conditions such as temperature, pressure and humidity (Ravindra et al., 2008; Zielinska et al., 2004). * Correspondence to: Esther Borra´s Garcı´a, EUPHORE Department, Fundacio´n de Estudios Ambientales del Mediterra´neo (CEAM), C/Charles R. Darwin, 14 46980 Paterna, Valencia, Spain. Tel.: þ34 96 1318227; fax: þ34 96 131 8190. E-mail address: [email protected] (E. Borra´s). 1352-2310/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2009.08.010

Diesel vehicles produce higher particulate emissions than gasoline vehicles, being the polycyclic aromatic hydrocarbons (PAHs), the most relevant organic compound produced (Desantes et al., 2005; Maricq, 2007; Phuleria et al., 2007; Ravindra et al., 2008). Some of these PAH compounds have been shown to exhibit carcinogenic and mutagenic properties (Ning et al., 2003; Kuba´tova´ et al., 2004; Christensen et al., 2005; Huang et al., 2007). Diesel engine PAH particulate emissions are originated from: hightemperature combustion of fossil fuels (pyrosynthesis of aromatic compounds), unburned fuel and lubricating oil. A correlation between fuel chemical composition and PAH emissions has been reported for standard reference materials of diesel particulate exhaust. Lower molecular weight (LMW) PAHs in an unburned diesel fuel have been shown to be the primary contributor to the exhaust emissions of a direct injection diesel engine (Marr et al., 1999). Medium molecular weight (MMW) PAHs such as fluoranthene and pyrene, and high molecular weight (HMW) such as benzo[b]fluoranthene and benzo[a]pyrene are formed during the combustion processes (Turrio-Baldassarri et al., 2003; Bezabeth et al., 2003). However, a better characterization of the PAH exhaust emissions derived from the combustion processes is still needed to evaluate the best strategies to reduce them.

´ s et al. / Atmospheric Environment 43 (2009) 5944–5952 E. Borra

The reformulation of diesel fuels, e.g., by the modification of aromatic compounds or sulfur content and/or by developing alternative fuels, could achieve substantial reductions in PAH exhaust emissions (Geiger et al., 2002; Mi et al., 2000; Karavalakis et al., 2009; Lin et al., 2006; Kado et al., 2005). Abrantes et al. (2004) found that exhaust emissions for light-duty biodiesel vehicle depend on engine operating conditions. On this basis, Lim et al. (2005) measured emissions in idling and standard-cycle tests for heavy-duty buses; Shah et al. (2005) reviewed and discussed exhaust emissions in creep, cruise, cold start and transient regimes for heavy-duty diesel engines and Karavalakis et al. (2009) performed a comparison between light-duty vehicle exhaust emissions and European and Athens driving cycles. Although there are several studies focused on heavy-duty vehicles in relation to PAH exhaust emissions (Correa and Arbilla, 2006; Kado et al., 2005; Westerholm and Li, 1994; Mi et al., 2000), the literature dealing with individual engine operating conditions for light-duty diesel vehicles is limited. Thus, an exhaustive description of light-duty exhaust emissions is a priority task, given the huge number of diesel passenger cars. The aim of the present study is to characterize the PAH exhaust emissions from modified fuels and to establish the effect of engine operating conditions on the PAH content from light-duty diesel engines. For this, we evaluated exhaust emissions concentrations of different types of synthesized diesel fuels – fossil and biodiesel – with different percentages of aromatic hydrocarbons and lowsulfur content. The assayed engine operating conditions were: cold start, idling, steady-state and accelerating. Principal component analysis (PCA) allowed us to extract most of the major trends in the studied factors, resulting in an extensive chemical description. 2. Experimental 2.1. Diesel engines Two different models of light-duty diesel engines were selected in our study as representatives of current diesel engines in European road vehicles, a Renault and a Ford diesel engine. The diesel engine supplied by Renault (Boulogne-Billancourt Cedex, France), model DTI 19.l, was used to collect the exhaust particulate matter from the synthesized diesel fuels – different types of reformulated diesel fuels with different percentages of aromatic hydrocarbons. The diesel engine was mounted on a homemade test bed and placed in a test cell adequately equipped with the necessary instrumentation for the precise control, steady-state conditions and reproducible requirements. Included in this test cell were measuring devices and sensors to control the main working parameters (load, velocity, thermal regime, etc.) and to measure representative variables such as temperature, pressure, fuel and air consumption, torque, fuel-air ratio, efficiency, etc (Barnes and Rudzinski, 2006; Becker, 1999; Wiesen, 2000). The other diesel engine – the Ford model – a light-duty diesel engine model A 90PS Stage 3, Delphi Fuel System, Fixed Geometry Turbo Transit Connect, supplied by the Ford Motor Company (Dearborn MI, USA), was employed to check the differences in exhaust emissions according to the engine operation conditions. The Ford diesel engine was installed in an engine test bed (braking dynamometer), model MP 100 S built by Weinlich Steuerungen (Reilingen, Germany). The test bed consisted of a braking and measurement unit containing the devices for measuring speed and torque, a balance to weigh the fuel tank, a turbine to measure the aspirated air, several thermocouples to measure the air, water and oil temperatures, and sensors to obtain the relative humidity. A Mexa 7170D gas analyzer supplied by Horiba (Kyoto, Japan) was used to directly measure the gaseous exhaust concentrations.

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2.2. Diesel exhaust particles generation Two sets of experiments were performed employing different particulate and gas phase sampling strategies for each engine. 2.2.1. Sampling of particles from different types of diesel fuels Three synthesized fuels with 5%, 15% or 25% aromatic content and a biodiesel (rape oil methyl ester), supplied by Haltermann products (The Dow Chemical, Hamburg, Germany), were selected as representative of the current diesel fuel reformulations (see chemical properties in Table 1). Diesel particulates were collected from the exhaust emission of the Renault diesel engine provided by the Engine Department of the Universidad Polite´cnica of Valencia. Details of the experimental methodology are included in Barnes and Rudzinski (2006), Becker (1999), and Wiesen (2000). A brief description of the steady-state conditions has been included. Engine conditions during particle collection were in the following range: speed 2000 rpm, 3 bar break mean effective pressure, fuel temperature 26–30  C, specific fuel consumption 0.8–0.9 g s1, air mass 71.5–86 kg h1, load at 25% and torque 44–51 Nm. For each diesel fuel, the particles generated were sampled by a particle collection system composed of a home-made cyclone and a filter holder. The cut size of the cyclone was 10 mm. The cyclone eliminated artifacts by extracting large particles from the flow before they passed through the glass fiber filter that was used to collect the particles. The selected probe sampling conditions were a dilution ratio of 3.3, cooling at 22  C with filtered air to avoid condensation processes and 180 s of sampling time. Fig. 1(a) shows all the instrumentation used in these experiments to collect exhaust emission from the Renault diesel engine. The surface morphology of the particles from exhaust samples was investigated by scanning electron microscopy (SEM) with X-ray spectrometer. The spectrometer enabled identification of the part of the sample that corresponded to soot, avoiding the possible existence of artifacts. For this, the analysis of 86 images from exhaust particles was performed, with magnification levels ranging from 750 to 7000. 2.2.2. Sampling of exhaust emissions from different engine operation conditions To study the effect of different engine operating conditions on the exhaust composition, a Ford light-duty diesel engine was used, see Fig. 1 (b). Several engine cycles, such as cold start, idling, accelerating and steady-state conditions, were evaluated as they were considered the four stages in diesel vehicle activity (Chang and Van Gerpen, 1997; Liang et al., 2005). Cold start at 298  5 K Table 1 Fuel characteristics: fossil diesel with different hydrocarbon percentages (1–3) and a methyl ester biodiesel. Diesel 5% Diesel 15% Diesel 25% Biodiesel aromatics aromatics aromatics 819 Density at 15  C (Kg m3) 318 95% V distillation point ( C)  Flash point ( C) 79 Collected quantity at 250  C (%V) 63 Collected quantity at 350  C (%V) 99 Collected quantity at 370  C (%V) 99 Sulfur (%W) 0.004

828 311 85 48 98 98 0.004

822 317 77 55 98 98 0.005

838 341 78 39 99 99 0.003

Residual Aromatic Compounds Benzene (ppm) Toluene (ppm) Ethylbenzene (ppm) p-Xylene (ppm) m-Xylene (ppm) o-Xylene (ppm)

<4 7 6 <4 12 12

<4 9 7 6 10 10

<4 4 8 <4 8 <4

<4 8 7 <4 10 8

´ s et al. / Atmospheric Environment 43 (2009) 5944–5952 E. Borra

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Fig. 1. Diagram of the instrumentation used in the diesel engine experiments. The panel shows the particulate sampling systems. (a) Renault engine tests and (b) Ford engine tests.

was defined as starting-up the engine without a warm-up period and turning it off few seconds later. During idling, the engine speed parameter was around 900  10 rpm, with no load, and engine water, air and oil temperatures stable. Accelerating consisted of increasing the speed from 2000 rpm to 2500 rpm from 2 to 9 times and then immediately returning to 2000 rpm. Finally, an engine was considered to be in steady-state conditions when the speed parameter was 2000 rpm, and the load was constant during the whole test. Table 2 shows average diesel engine conditions. A synthesized fuel (Haltermann), similar to commercial standard diesel, was

selected for the studies on the effect of engine operating conditions on PAH exhaust emissions. Parameters such as speed, torque, oil and water temperature and fuel consumption were monitored. To prevent engine damage, the system was regulated to operate with water and oil temperature less than 100  C. To record the data, the control unit was connected to a computer where all the data was sent every 0.25 s. In this PC we installed a control program created by the Fundacio´n CEAM in Lab View (National Instruments Corporation, Austin, USA). The particles were collected directly on glass microfiber binder free filter GF/A (47 mm diameter, Whatman, Brentford, England)

Table 2 Ford engine experiment conditions. Code

Engine conditions

Injec. time (s)

Speed (rpm)

Torque (Nm)

Total fuel (g)

Specific fuel (g min1)

Rel. humidity (%)

Water. temp ( C)

Oil. temp ( C)

Air. temp ( C)

I.1 I.2 I.3 I.4

Steady-state Steady-state Steady-state Steady-state

240 180 210 255

2006 2012 2000 2000

65 61 63 61

240 173 200 250

60 58 59 58

19 23 16 17

93 92 91 91

101 100 97 100

39 38 34 38

II.1a II.2a II.3 II.4 II.5 II.6 II.7 II.8

Cold start Cold start Idling Idling Idling Idling Idling Idling

Ignition – 5 times Ignition – 3 times 660 480 360 360 720 240

– –

– – 200 240 360 370 360 120

– – 57 60 60 62 60 51

– – 16 21 15 12 9 6

– – 92 93 94 95 95 93

– –

900 900 900 900 900 900

– – 36 56 63 63 51 29

70 91 94 95 94 64

– – 27 30 33 31 33 21

III.1 III.2 III.3 III.4 III.5 III.6 III.7

Steady-state Steady-state Steady-state Acceleration Acceleration Acceleration Acceleration

720 360 240 180 240 225 165

1946 2000 2000 2000 2000 2000 2000

56 64 65 70 66 67 75

150 180 240 190 250 250 190

52 60 61 62 61 63 68

10 18 23 17 12 21 26

94 93 93 91 91 89 90

101 100 100 96 98 93 94

42 40 37 32 34 33 34

Relative standard deviation < 5%. a accumulated experiments.

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from the hot part of the exhaust pipe (see filter holder in Fig. 1), using a constant volume sampler. The air dilution ratio employed was 17.8. The filters were pre-baked at 450  C for 12 h prior to sampling to avoid organic artifacts. These kinds of filters were selected to permit high volume sampling and they are tolerant to high temperatures for PAH sampling, as we demonstrated in Borra´s and Tortajada-Genaro (2007). Blanks filters were also analyzed, but no PAH particulate artifacts were observed. The sampling was only performed after the engine reached constant operating temperatures. Soot exhaust emissions were collected during a time period ranging from 2 to 30 min at 7.7 L min1 (see Table 2). The filters were placed in a desiccator purged with nitrogen for 24 h. After that, the filters were weighed on a precision microbalance to obtain the mass and stored at 4  C. Fig. 1 (b) shows all the instrumentation used in the experiments to directly measure and collect exhaust emission – gas and particle phase – from the Ford diesel engine. 2.3. Chemical analysis of exhaust emissions 2.3.1. Gas phase analysis To study the differences between the different engine cycles, the filtered gas-phase emissions from the Ford diesel engine were determined with the gas analyzer, using different detectors. CO was detected by a non-dispersive infrared detector (NDIR), model

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AIA-721A, with a range of 0–250 mg L1. NOx was measured by a heated chemiluminescence analyzer H-CLD, model CLA-720 MA, in the range 0–1000 mg L1. Total hydrocarbons were determined with a heated flame ionization detector (HFID), model FIA-721HA, in the range 0–1000 mg L1. CO2 was determined by a model AIA722 in the range of 0–10 Vol% with a NDIR detector. O2 was measured by a magneto-pneumatic detector (model MPA-720), in the range 0–25 Vol%. 2.3.2. Particulate PAH analysis To analyze the PAH content of the particulate matter, we used a methodology described in a previous paper (Borra´s and TortajadaGenaro, 2007). For the soot generated in the Renault engine by different diesel fuels, 10 mg of particles were collected on the glass filter of a particulate collecting system composed of a home-made cyclone and a filter holder. The particles were extracted ultrasonically in 15 mL of CH2Cl2 in two 15-minute cycles. For particles from the Ford engine at different engine cycles, PAH compounds were extracted from microfiber filters ultrasonically with two cycles of 15 mL of CH2Cl2 for 15 min. Both kinds of particulate samples were analyzed by triplicate to study their PAH composition. The extracts were concentrated using the rotavapor and dissolved in 1 mL of CH2Cl2. Open-column chromatography (silica gel C60) was employed to fractionate the compounds. The column was eluted with CH2Cl2 and five aliquots

Table 3 Total PAH concentration in ng PAH mg1: priority and tentative compounds in four diesel exhaust particle emissions from Renault engine at steady-state condition. Priority compound

Nomenclature

r.t

Diesel 5%

Diesel 15%

Diesel 25%

Biodiesel

a Naphthalene Acenaphthylene Acenaphthene Fluorene Phenanthrene Anthracene Fluoranthene Pyrene Benzo[a]anthracene Chrysene Benzo[b]fluoranthene Benzo[k]fluoranthene Benzo[a]pyrene Indene[l,2,3-cd]pyrene Benzo[g,h,i]perylene Dibenzo[a,h]anthracene

NaP AcPy AcP Flu PA Ant FL Pyr BaA CHR BbF BkF BaP IND BghiP DBA

5.58 10.87 11.54 13.91 18.74 18.95 25.26 26.40 33.24 33.53 39.11 39.23 40.60 45.80 46.25 46.96

– 0.183  0.009 0.81  0.14 0.46  0.04 1.4  0.3 0.12  0.01 0.2  0.07 0.23  0.11 0.12  0.07 n.d n.d n.d 0.13  0.10 n.d 0.37  0.05 0.30  0.05

– 0.25  0.03 0.73  0.03 0.27  0.03 1.11  0.09 0.06  0.01 0.24  0.01 0.9  0.3 0.24  0.05 n.d n.d n.d 0.23  0.04 n.d n.d n.d

– 0.21  0.03 0.4  0.01 0.26  0.07 2.3  0.3 0.06  0.01 0.14  0.03 0.15  0.05 n.d n.d n.d n.d n.d n.d 0.24  0.06 n.d

– 0.11  0.02 0.46  0.05 0.10  0.01 0.40  0.02 0.06  0.03 0.020  0.003 0.18  0.10 0.018  0.008 0.010  0.007 0.032  0.006 0.018  0.004 0.04  0.01 0.039  0.007 0.07  0.01 0.05  0.01

Tentative compound

Nomenclature

r.t

Diesel 5%

Diesel 15%

Diesel 25%

Biodiesel

b Methylnaphthalene isomer 1-2 Dimethylnaphthalene isomer 1-7 Trimethylnaphthalene isomer 1-2 Methylfluorene isomer 1 Methylfluorene isomer 2 Methylphenanthrene isomer 1/Methylanthracene isomer 1 Methylphenanthrene isomer 2/Methylanthracene isomer 2 Methylphenanthrene isomer 3/Methylanthracene isomer 3 Methylphenanthrene isomer 4/Methylanthracene isomer 4 Methylphenanthrene isomer 5/Methylanthracene isomer 5 Methylphenanthrene isomer 6/Methylanthracene isomer 6 Methylphenanthrene isomer 7/Methylanthracene isomer 7 Dimethylphenanthrene isomer 1/Dimethylanthracene isomer Dimethylphenanthrene isomer 2/Dimethylanthracene isomer Dimethylphenanthrene isomer 3/Dimethylanthracene isomer Dimethylphenanthrene isomer 4/Dimethylanthracene isomer Dimethylphenanthrene isomer 5/Dimethylanthracene isomer Dimethylphenanthrene isomer 6/Dimethylanthracene isomer Benzoanthracene isomer 1 Benzopyrene isomer 1/Benzofluoranthene isomer 1 Benzopyrene isomer 2/Benzofluoranthene isomer 2

Tent_01–Tent_02 Tent_03–Tent_09 Tent_10–Tent_11 Tent_12 Tent_13 Tent_14 Tent_15 Tent_16 Tent_17 Tent_18 Tent_19 Tent_20 Tent_21 Tent_22 Tent_23 Tent_24 Tent_25 Tent_26 Tent_27 Tent_28 Tent_29

– – – 12.53 12.64 19.80 20.06 21.47 21.59 21.66 21.77 22.05 22.15 23.75 24.69 25.84 26.26 27.18 30.32 38.55 40.52

– – – 0.14  0.06 0.8  0.2 0.20  0.08 n.d 0.5  0.2 n.d 0.18  0.06 0.23  0.09 1.2  0.5 0.08  0.03 1.3  0.7 0.03  0.01 0.9  0.3 0.6  0.2 0.06  0.02 0.6  0.2 0.06  0.02 n.d

– – – 0.15  0.02 0.13  0.07 0.18  0.01 n.d 0.27  0.13 0.07  0.02 0.07  0.03 0.38  0.04 0.25  0.08 0.06  0.02 1.11  0.15 n.d 0.24  0.05 0.17  0.03 0.024  0.005 0.17  0.03 0.024  0.005 n.d

– – – 0.2  0.1 0.04  0.01 0.16  0.05 n.d 0.5  0.2 n.d 0.11  0.02 0.20  0.07 n.d 0.03  0.01 1.4  0.6 n.d 0.036  0.007 0.032  0.006 0.010  0.002 0.031  0.006 0.010  0.002 n.d

– – – 0.09  0.02 0.06  0.04 0.3  0.2 0.12  0.6 0.3  0.1 0.3  0.1 0.3  0.2 0.17  0.12 0.047  0.002 0.03  0.01 61 0.05  0.02 0.07  0.02 0.046  0.008 0.017  0.012 0.047  0.008 0.017  0.011 n.d

r.t ¼ retention time (min). Units ¼ ng PAH mg1.

1 2 3 4 5 6

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of 1.4 mL were collected at 0.9 mL min1 flow rate. A fraction collector model CF-1 supplied by Spectra/Chrom (Houston, TX, USA) was used. Each aliquot was dried under a gentle stream of N2. Extracts were dissolved in 150 mL of cyclohexane. One microliter of this solution was injected into the GC–MS system. A 5890 SERIES II Plus gas chromatograph coupled to a HP 5989 mass spectrometer, supplied by Hewlett-Packard Co. (Wilmington, DE, USA), was used. All analyses were conducted on a HP-5MS column of 30 m  0.25 mm I.D  0.25 mm film thickness (Crosslinked 5% Ph Me Siloxane) supplied by J & W Scientific (Albany, NY, USA). The gas chromatograph was held at 90  C for 1 min, ramped at a rate of 10  C min1–120  C, 4  C min1–280  C and then held at 280  C for 10 min. Total chromatogram run time was 54 min. The injection port was held at 280  C and the transfer line from the GC to MS was held at 300  C. Samples were injected in splitless injection mode (t ¼ 0.75 min) using helium as the carrier gas, with a total flow of 50 mL min1 and an on-column flow of 1 mL min1. Electron impact ionization (EI) at 70 eV was used in the analysis. For the qualitative analysis, we used the full-scan mode (SCAN), with the detector recording an m/z range of 50–650. For the quantitative analysis, we used the selected ion monitoring (SIM) mode. The ion source temperature was 200  C and the quadrupole temperature was 100  C. Other MS conditions were 2268 V for the electron multiplier and 5 min for the solvent delay. To quantify the PAH content, a certified standard mixture of 10 ng mL1 in cyclohexane PAH-Mix 9 – Reference Material ref. 20950900CY (16 EPA PAH compounds) – was supplied by Dr Ehrenstorfer (GmbH, Augsburg). The mixture composition was:

low molecular weight (LMW): naphthalene (NaP), acenaphthylene (AcPy), acenaphthene (AcP), fluorene (Flu), phenanthrene (PA), anthracene (Ant); medium molecular weight (MMW): fluoranthene (FL), pyrene (Pyr), benzo[a]anthracene (BaA), chrysene (CHR); and high molecular weight (HMW): benzo[b]fluoranthene (BbF), benzo[k]fluoranthene (BkF), benzo[a]pyrene (BaP), indene[1,2,3c,d]pyrene (IND), benzo[g,h,i]perylene (BghiP), dibenzo[a,h]anthracene (DBA). The error has been provided as the standard deviation, and the retention time within 2s, as compared to standard solutions, was used for the positive identification. The PAH compounds were divided into three categories: priority, tentative or unidentified based on the amount of certainty involved in identifying their structures. The priority compounds (EPA list of 16 PAH compounds) were identified based on a comparison of their mass spectra and their retention time with peaks obtained from the certified standard mixture. The probable structure of tentative compounds was proposed on the basis of their mass spectra and their retention times and included mostly PAHs with methyl and dimethyl groups. The remaining peaks were classified as unidentified compounds. Data analysis was performed with the statistical package SPSS for Windows V 10.0. 3. Results and discussion 3.1. PAH composition from different types of diesel fuels Reformulated diesel fuels – increasing and decreasing a 10% of aromatic hydrocarbon content of standard diesel – and a biodiesel

Table 4 Concentrations in ng PAH mg1 during cold start (II.1–II.2) – idling engine cycles (II.3–II.8) from a Ford engine exhaust. II.2

II.3

II.4

II.5

II.6

II.7

II.8

Priority compounds Nap – AcPy n.d AcP 0.05  0.01 Flu n.d PA 3.5  0.7 Ant 0.24  0.05 FL 51 Pyr 92 BaA 61 CHR 50  10 BbF 1.1  0.2 BkF 69  14 BaP 148  29 IND 15  3 BghiP 23  5 DBA 57  6

II.1

– n.d n.d n.d 92 n.d 13  2 17  3 3.1  0.6 22  4 n.d 60  12 11  2 n.d n.d 81

– 0.21  0.04 0.24  0.05 0.30  0.06 27  5 1.3  0.3 19  4 40  8 2.4  0.5 10  2 n.d 13  3 n.d n.d n.d n.d

– 0.6  0.1 0.30  0.06 0.35  0.07 17  3 2.5  0.5 18  3 36  7 71 16  3 62 70  8 0.04  0.01 n.d 12  2 31  6

– 0.32  0.06 1.1  0.2 0.9  0.2 10  2 n.d 82 36  7 n.d 14  3 n.d 55  11 77  15 n.d 17  3 33  6

– n.d 0.32  0.06 0.5  0.1 61 n.d 15  3 28  6 12  2 31  6 n.d 28  6 14  3 14  3 14  3 36  7

– 0.42  0.08 0.48  0.09 0.38  0.07 71 0.9  0.2 91 19  4 13  3 29  6 37  7 118  23 25  5 n.d 81 12  2

– 0.28  0.05 n.d n.d n.d n.d 61 11  2 n.d 61 n.d 30  6 n.d n.d n.d n.d

Tentative compounds Tent_12 n.d Tent_13 n.d Tent_14 n.d Tent_15 n.d Tent_16 1.6  0.3 Tent_17 3.3  0.7 Tent_18 n.d Tent_19 n.d Tent_20 1.7  0.3 Tent_21 1.6  0.3 Tent_22 n.d Tent_23 2.8  0.6 Tent_24 0.9  0.2 Tent_25 1.1  0.2 Tent_26 0.5  0.1 Tent_27 4.8  0.9 Tent_28 1.4  0.3 Tent 29 188  18

n.d n.d n.d n.d 2.3  0.5 3.7  0.7 n.d n.d 2.7  0.5 2.2  0.4 n.d 3.8  0.8 0.9  0.2 0.6  0.1 0.48  0.09 n.d 5.9  1.1 105  21

n.d n.d n.d n.d 2.7  0.5 4.4  0.9 n.d n.d 3.6  0.7 2.7  0.5 n.d 2.1  0.4 0.46  0.09 0.45  0.09 0.6  0.1 61 3.7  0.7 21  4

n.d n.d n.d n.d 2.2  0.5 2.2  0.5 n.d n.d 1.8  0.3 1.7  0.3 n.d 1.6  0.3 0.30  0.05 0.27  0.05 0.35  0.07 n.d n.d 130  26

n.d n.d n.d n.d 2.2  0.4 4.1  0.8 n.d n.d 2.0  0.4 1.7  0.3 n.d 1.5  0.3 0.46  0.09 0.5  0.1 n.d 2.4  0.5 n.d 39  8

n.d n.d n.d n.d 1.0  0.2 0.7  0.1 n.d n.d 0.9  0.2 0.5  0.1 n.d 0.9  0.2 0.23  0.04 0.16  0.03 n.d 82 n.d 425  85

n.d n.d n.d n.d 0.6  0.1 1.0  0.2 n.d n.d 0.47  0.09 0.5  0.1 n.d 0.37  0.07 0.13  0.02 n.d n.d 51 n.d 107  21

n.d n.d n.d n.d 0.7  0.1 1.2  0.2 n.d n.d 0.8  0.1 1.0  0.2 n.d 1.2  0.2 n.d n.d n.d n.d n.d 50  10

r.t ¼ retention time (min). Units ¼ ng PAH mg1.

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(rape oil methyl ester) were evaluated to assess PAH particulate concentrations in exhaust emissions as a function of fuel chemical composition. The sulfur content was also lower (<0.005%). The fixed operating conditions of the Renault diesel engine – steadystate cycle, 25% load and 2000 rpm – generated exhaust emissions in the medium concentration range for all of the compounds and were representative of the urban driving pattern. The PAH levels are listed in Table 3(a,b). Naphthalene, methyl and dimethylnaphthalenes have been observed in particulate exhaust emissions. Although Ravindra et al. (2008) reported that these light PAHs are mostly found in the gas phase, the presence of these compounds with two or three benzene rings on particles can be considered a positive sampling artifact (Christensen et al., 2005; Zielinska et al., 2004). Several priority PAH compounds (9–12 PAHs) were identified in exhaust emissions using fossil fuels. For the biodiesel exhaust emissions, the 16 EPA PAH compounds were identified; four of them, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, indene[1,2,3-cd]pyrene were present only in these biodiesel exhaust emissions. Tentative compounds were also identified (23–29 PAHs), with methylphenanthrene isomer 2 being specific to the biodiesel emissions. Finally, biodiesel presented a higher number of unidentified PAH compounds (a total of 107) than the rest of the diesel exhaust emissions analyzed (ranging from 35 to 57 PAHs). Our results showed that the PAH compounds identified from the reformulated diesel fuels with different percentages of aromatic hydrocarbons were quite similar. Moreover, the exhaust emissions from biodiesel were characterized by a greater number of compounds. The emission factors of these fuels, expressed in ng PAH mg1 particles, were also evaluated for the Renault engine, see Table 3(a,b). The relative concentrations between the four reformulated diesel fuels were also calculated (Figure S.1 in Supplementary Information). The total concentration of PAHs was similar for the three diesel fuels with different hydrocarbon percentage (3.6  0.6 ng PAH mg1). These results showed that increasing or reducing the diesel fuel aromatic content did not significantly reduce PAH emission. Mi et al. (2000) reported a similar conclusion for heavy-duty engines, where no significant differences were observed when toluene was added to the reformulated fuel. Nevertheless, we detected light trends for some medium molecular weight (MMW) and HMW PAH compounds from reformulated fossil diesel fuels. The concentrations of acenaphthene, benzo[a]anthracene, benzo[a]pyrene and benzo[g,h,i]perylene decreased as the percentage of aromatic hydrocarbons in the fuels increased. Methylphenanthrene isomer 1, dimethylanthracene isomer 1 and dimethylanthracene isomer 4 decreased when the percentage of aromatic hydrocarbons increased. The correlation coefficients ranged between 0.95 and 0.999. In contrast, the use of biodiesel reduced the total concentration of PAHs by around 50% with respect to the reformulated diesels. In this sense, Lin et al. (2006) reported that the diesel exhaust emissions for a palm biodiesel were reduced between 13.2 and 98.8% depending on palm-biodiesel content. However, a high increase in palm biodiesel was found to promote significant problems with the combustion processes, while the optimum palm-biodiesel content provided an emission reduction of 61.2%. Thus, this reduction in exhaust emissions is in the same order as with our rape oil methyl ester biodiesel. In summary, the modification of chemical composition (aromatic content) shows an irrelevant variation in total PAH concentration, although the profiles change. Also, the results confirm that the source of diesel fuel (fossil or biodiesel) significantly affects the formation of PAH particulate content. The exhaust emissions from the biodiesel studied herein presented a larger diversity of compounds, but lower total concentrations than the other diesel exhaust emissions analyzed.

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3.2. Evaluation of exhaust emissions in different engine types The emissions of two representative light-duty engines (a Renault and a Ford diesel engine) was compared in order to check whether the differences in emission results could be attributed to either engine operating conditions or engine design. For the study, we used the same diesel fuel (reformulated diesel with 15% aromatic hydrocarbon component and low-sulfur content) and the same engine operating conditions: steady-state at 2000 rpm, with a load of 50 Nm. The concentrations were similar (<5%) for regulated air pollutants (CO, NOx, CO2, O2). The emission of PAH compounds were slightly different depending on molecular weight. For LMW PAHs, the differences between these two engines were lower than 7  2%. For MMW and HMW PAHs, there were higher differences (12  3% and 19  4%, respectively), with the greatest variance for benzo[b]fluoranthene with an 35%. Nevertheless, both light-duty diesel engines could be assumed to provide similar total exhaust PAH content under the selected operating condition.

Fig. 2. Priority PAH concentrations (ng PAH mg1) in accelerating-steady-state engine regimes from a Ford engine exhaust. (a) Concentrations of low molecular weight PAHs, NaP ¼ gas artifact. (b) concentrations of high molecular weight PAHs.

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Moreover, an analysis of the total variance was performed to check the reproducibility of Ford engine emissions. The two-way ANOVA test was applied to the priority and tentative identified compounds emitted during the four steady-state cycle selected (conditions I.1–I.4). Except benzo[b]fluoranthene and benzo[a]pyrene, PAH compounds were not affected by minimal mechanical variations. This demonstrates that, under fixed engine operating condition, the PAH concentrations are statistically reproducible. 3.3. Evaluation of exhaust emissions in different engine cycles The evaluated engine regimes were: cold start, idling, accelerating and steady-state conditions. For these tests, we used Ford engine and a diesel with an aromatic content similar to US commercial diesel. For the cold start and the idling engine cycles (conditions II.1– II.8), the concentrations of the priority and tentative PAH compounds are shown in Table 4. The values of cold start ranged

0.00005–0.009 mg g1 for LMW, 0.005–0.02 mg g1 for MMW and 0.001–0.15 mg g1 for HMW PAH compounds. The average values of standard PAHs (0.04 mg g1) were lower than those found by Abrantes et al. (2004) (2.4 mg g1). Meanwhile, the values of idling were 0.00035–0.027 mg g1 for LMW, 0.002–0.04 mg g1 for MMW and 0.0057–0.07 mg g1 for HMW. These concentrations were compared to published heavy-duty diesel vehicles emissions (Riddle et al., 2007). Concentrations of phenanthrene, anthracene, fluorantene, pyrene indene[1,2,3]pyrene, and dibenzo[a,h]anthracene were similar. However, benzo[g,h,i]perylene was one order of magnitude higher in our study. The most important contribution of tentative compounds corresponded to an isomer of benzopyrene. In the interpretation of PAH profiles derived from exhaust emissions, multivariate statistical analysis is widely used (Wingfors et al., 2001). Thus, PCA reduced the number of variables or factors, identifying new meaningful underlying variables. The PCA results showed that the first principal component (37% of explained variance)

Fig. 3. Tentative PAH concentrations (ng PAH mg1) in accelerating-steady-state engine regimes from a Ford engine exhaust. (a) Concentrations of low molecular weight PAHs, (b) concentrations of high molecular weight PAHs.

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was correlated with differences between both operation conditions. The second principal component (30% of explained variance) was correlated with fuel consumption. During the cold start cycle (higher power mode), the concentration of all PAHs, was high, especially the compounds with higher molecular weight like chrysene, benzo[k]fluoranthene, benzo[a]pyrene and dibenzo[a,h]anthracene. These conclusions are in agreement with the results reviewed by Ravindra et al. (2008), confirming that cold start operating conditions generate high concentrations of HMW PAHs. In maximum fuel consumption conditions of idling, PAH emissions also became associated with HMW compounds. An analysis of PAH exhaust emissions – sampled at the same temperature – was performed to compare accelerating and steadystate engine conditions (conditions III.1–III.7). Fig. 2 and Fig. 3 show the concentrations for priority and tentative PAH compounds, respectively. A t-test for related samples was made to determine the correlation between the PAH concentrations and both the accelerating and steady-state conditions. For 16 EPA PAH compounds (16 priority PAH compounds of the 45 identified PAHs), the differences between their total concentration and both engine cycles were significant (a ¼ 0.012). The total PAH exhaust emissions produced during acceleration were on average 1.5 times greater than the emissions produced by the steady-state conditions. Moreover, during acceleration cycles, concentrations of HMW PAHs showed an increase of more than twice compared to LMW PAHs. In steady-state, differences in exhaust concentrations have been observed for total LMW PAHs (0.0001–0.007 mg g1) and HMW PAHs (0.003–0.053 mg g1). Riddle et al. (2007) observed these trends for heavy-duty diesel vehicles, although our values were significantly lower. On the other hand, the total LMW PAH concentration in light-duty diesel exhaust reported by Lin et al. (2006) was five times higher than our values, but our values of MMW and HMW PAHs were three times higher than their values. Finally, the most abundant gaseous compounds and particulate PAHs emitted during the combustion process were correlated. Table 5 summarizes the gas exhaust emission concentrations. PCA statistical tests (Figure S.2 in Supplementary Information) were performed, with the first principal component (51% of explained variance) being assigned to the combustion process. Tests showed that total PAH, CO2, total HC and NOx concentrations increase when

Table 5 Exhaust gas emissions data in mg L1 obtained in different diesel engine conditions from a Ford engine. Code

Filter mass (mg)

CO (mg L1)

NOx (mg L1)

I.1 I.2 I.3 I.4

3.3 2.5 3.1 3.3

21 20 22 22

400 360 420 400

II.1 II.2 II.3 II.4 II.5 II.6 II.7 II.8

20.8 7.7 10.3 4.3 2.7 3.1 5.5 2.5

– – 10 21 39 16 12 55

– – 360 320 320 380 360 300

III.1 III.2 III.3 III.4 III.5 III.6 III.7

7.9 9.5 3.9 3.4 2.6 4.5 3.3

27 20 22 22 22 24 24

260 420 400 440 420 360 420

Relative standard deviation < 5%.

Total HC (mg L1)

CO2 (mg L1)

80 85 80 80

O2 (mg L1)

53,000 49,000 52,000 51,000

134,000 139,000 133,000 136,000

– –

– –

95 155 285 125 85 170

50,000 49,000 48,000 52,000 2,68,000 46,000

– – 139,000 137,000 140,000 135,000 140,000 143,000

75 65 80 80 80 215 55

34,000 53,000 54,000 54,000 52,000 55,000 57,000

162,000 133,000 133,000 132,000 135,000 132,000 127,000

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the CO concentration decreases. These results are in agreement with the description of diesel engine combustion processes, and the levels of gaseous compounds are in the expected range. In summary, we demonstrated that the driving pattern has a significant effect on diesel exhaust emissions from light-duty diesel engines. Several studies on heavy-duty vehicle exhaust emissions (Shah et al., 2005; Lim et al., 2005; Kado et al., 2005; Westerholm and Li, 1994), obtained significant correlations between PAH exhaust concentrations at cold start, idle, creep, transient, cruise and steady-state. For light-duty diesel vehicles, Abrantes et al. (2004) only studied PAH exhaust emissions under cold start. Riddle et al. (2007) studied the differences between light-duty gasoline vehicles and heavy-duty diesel trucks under cold start and transient conditions. Karavalakis et al. (2009) compared the exhaust emissions from light-duty diesel engine driving cycles and found important differences for gas and particulate compounds. However, the above study on driving cycles reported this information with respect to the combination of engine operating together but not individually. Therefore, different engine operating conditions have not been previously reported for the same light-duty diesel vehicle exhaust using the same fuel. 4. Conclusions The characterization of PAH compounds from light-duty diesel exhaust emissions, employing alternative biodiesel and some reformulated fuels, opens a new research area for pollution reduction. In this sense, our results confirmed that the emission of PAH compounds from the incomplete combustion of diesel fuel depends greatly on the source of the fuel. In terms of reducing particulate PAHs exhaust emissions, the use of rape oil methyl ester biodiesel is more effective than the modification of aromatic content in a fossil diesel. The effect of driving pattern on diesel exhaust emissions has been also studied. During the cold start cycle, the concentration of all PAH compounds was high, especially the compounds with higher molecular weight. Moreover, the PAH exhaust emissions produced by the acceleration cycle were on average 1.5 times greater than the emissions produced by the steady-state conditions. Since all our experiments simulating on-road conditions were performed under the same conditions – type of engine and chemical composition of diesel –, the present study represents the first time that the effect of engine operating conditions on PAH profiles of exhaust emissions has been evaluated. This kind of information is important for air quality control programs and for the development of new alternative fuels. Thus, the development of control strategies based on the PAH levels observed in our work could be developed to help Governments with future selective regulations. Acknowledgements We gratefully acknowledge the Generalitat Valenciana and the GRACCIE CBS2007-00067 project in the CONSOLIDER-INGENIO 2010 program for supporting this study, the Desert Research Institute (DRI-USA) for testing engine conditions in our facilities and the Engine Department of the Universidad Polite´cnica de Valencia for providing exhaust particles. Appendix. Supplementary material Supplementary data associated with this article can be found in the online version at doi:10.1016/j.atmosenv.2009.08.010.

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