Science of the Total Environment

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Biodiesel emissions pro ﬁle in modern diesel vehicles. Part 2: Effect of biodiesel origin on carbonyl, PAH, nitro-PAH and oxy-PAH emissions

Georgios Karavalakis

^a,1

, Vasiliki Boutsika

^b

, Stamoulis Stournas

^a

, Evangelos Bakeas

^b,

⁎

aLaboratory of Fuels Technology and Lubricants, School of Chemical Engineering, National Technical University of Athens, 9 Iroon Polytechniou Str. Zografou Campus, 157 80, Athens, Greece

bLaboratory of Analytical Chemistry, Chemistry Department, National and Kapodistrian University of Athens, Panepistimioupolis, 15771, Athens, Greece

a b s t r a c t a r t i c l e i n f o

Article history:

Received 25 May 2010

Received in revised form 27 August 2010 Accepted 8 November 2010

Available online 3 December 2010

Keywords:

Biodiesel emissions Oxidized biodiesel Carbonyls PAH Driving cycles Vehicles

In the present study, the effects of different biodiesel blends on the unregulated emissions of a Euro 4 compliant passenger car were examined. Two fresh and two oxidized biodiesel fuels of different source materials were blended with an ultra low sulphur automotive diesel fuel at proportions of 10, 20, and 30%

v/v. Emission measurements were conducted on a chassis dynamometer with a constant volume sampling (CVS) technique, over the New European Driving Cycle (NEDC) and the Artemis driving cycles. The experimental results revealed that the addition of biodiesel led to important increases in most carbonyl compounds. Sharp increases were observed with the use of the oxidized biodiesel blends, especially those prepared from used frying oil methyl esters. Similar to carbonyl emissions, most PAH compounds increased with the addition of the oxidized biodiesel blends. It can be assumed that the presence of polymerization products and cyclic acids, along with the degree of unsaturation were the main factors that inﬂuenced carbonyl and PAH emissions proﬁle.

1. Introduction

Diesel engines of motor vehicles make a considerable contribution to both gaseous and particulate air toxics in the urban atmosphere.

These toxic pollutants have been associated with serious adverse health effects, including premature death, respiratory symptoms, impaired lung function and cardiovascular diseases (Geller et al., 2006). Fatty acid methyl esters (FAMEs), collectively referred to as biodiesel, have long been recognized as a means of reducing most harmful exhaust emissions (Durbin et al., 2007; Wu et al., 2009).

Biodiesel is an oxygenated diesel fuel made from vegetable oils and animal fats via transesteriﬁcation. Although biodiesel fuel has similar properties with petroleum diesel, its chemical nature makes it more susceptible to oxidation or autoxidation during long-term storage (Knothe, 2007). On this basis, when biodiesel is used as an automotive fuel it may affect vehicular tailpipe emissions and ultimately urban air quality.

While regulated pollutants with biodiesel are well documented, unregulated emissions such as carbonyl compounds (aldehydes and

ketones) and polycyclic aromatic hydrocarbons (PAHs) of biodiesel fuelled vehicles, lack extensive research. Carbonyls are of critical importance in atmospheric chemistry, while they attract immense attention due to their adverse health effects on humans (Pang et al., 2006). Certain carbonyl compounds, including formaldehyde, acetaldehyde and acrolein are known to be toxic, mutagenic and/or carcinogenic and thus have been identiﬁed as hazardous air pollutants (Grosjean et al., 2001; IARC, 2006). Vehicular carbonyls play a critical role in the tropospheric chemistry, since they are important pre- cursors to free radicals (HOx), ozone and peroxyacylnitrates (PAN) (Bakeas et al., 2003).

Among numerous studies, there are some divergences when considering carbonyl emission results obtained with either pure or blended biodiesel. These differences are mainly dependent on fuel type and quality, engine type and operating conditions, and especially sampling methods. Many studies have shown that biodiesel could increase emissions of carbonyl compounds as a consequence of the oxygen content in the ester molecule (Correa and Arbilla, 2008;

Karavalakis et al., 2009a). In fact, Fontaras et al. (2010) and Karavalakis et al. (2009b)reported substantial increases in carbonyl emissions when using biodiesel blends and concluded that their formation was highly dependent on engine operating conditions and driving cycle. However,Lin et al. (2009) and Peng et al. (2008)found some reductions in carbonyl emissions with the use of biodiesel blends, while Turrio-Baldassarri et al. (2004) found insigniﬁcant differences when comparing diesel fuel and biodiesel. According to Szybist et al. (2007), these reductions were probably due to the

⁎ Corresponding author. Tel.: +30 2107274154; fax: +30 2107274750.

E-mail addresses:gkaraval@cert.ucr.edu(G. Karavalakis),bakeas@chem.uoa.gr (E. Bakeas).

1Present address: University of California Riverside, Bourns College of Engineering, Center for Environmental Research and Technology, 1084 Columbia Ave, Riverside, 92507, USA. Tel.: +1 9517815799; fax: +1 9517815790.

doi:10.1016/j.scitotenv.2010.11.010

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Science of the Total Environment

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decomposition of esters via decarboxylation, which could decrease the probability of forming oxygenated combustion intermediates with respect to diesel combustion.

The widespread use of biodiesel as a transportation fuel has increased the concerns regarding diesel particulate matter (PM) emissions and in particular PAHs. PAHs are a class of complex organic molecules, which include carbon and hydrogen with a fused ring structure containing at least 2 benzene rings (Rhead and Hardy, 2003). PAH emissions are mainly originated from fuel combustion (pyrosynthesis of aromatic compounds), unburned fuel (pyrolysis of fuel fragments) and unburned lubricating oil (Richter and Howard, 2000; Ravindra et al., 2008). The formation of road transport generated PAHs depends on the type of engine, the engine load and speed, fuel formulation, and the effectiveness of the exhaust aftertreatment system (Borras et al., 2009). PAH compounds are widely distributed in the atmosphere and they are well known for their mutagenic and carcinogenic properties (Riddle et al., 2007). 16 PAHs, ranging in size, from naphthalene (2-rings) to benzo[g,h,i]perylene and indeno[1,2,3-c,d]pyrene (6-rings) have been identiﬁed as priority pollutants by the U.S. Environmental Protection Agency (US EPA).

Nitrated polycyclic aromatic hydrocarbons (nitro-PAHs) can be either directly emitted from combustion sources as diesel engines or formed from their parent PAHs by atmospheric OH or NO3 radical initiated reactions (Miet et al., 2009). It should be noted, that the mutagenic and genotoxic effect of diesel PM is mainly attributed to the presence of nitro- PAHs (Heeb et al., 2008). Similar to nitro-PAHs, oxygenated PAH compounds (oxy-PAHs) are mainly emitted from combustion processes and they can also originate from heterogeneous reactions between PAHs and ozone (Tsapakis and Stephanou, 2007). In contrast to their parent PAHs, most of the oxy-PAHs found in diesel particulate phase are directly toxic and mutagenic (Maria del Rosario Sienra, 2006).

The information given in literature regarding the effect of biodiesel on the emissions of PAH compounds is limited and often contradictory. The majority of studies have been conducted on test cell engines, which lack exhaust aftertreatment systems such as those employed in modern passenger cars. Therefore, results are usually retrieved through experiments, which are not necessarily representative of actual driving conditions, making it difficult to assess the actual impact of biodiesel application on diesel vehicle fleet emissions. Although many authors have observed some decrease in PAH emissions with pure and/or blended biodiesels (Correa and Arbilla, 2006; Yang et al., 2007a), some studies reported insignificant differences in PAH emissions or even found some increase with biodiesel (Turrio-Baldassarri et al., 2004; Zhou and Atkinson, 2003).Karavalakis et al. (2009c), Martini et al. (2007) and Yang et al. (2007b), found that PAH emissions with biodiesel were influenced by the cold-start effect and the operating conditions of the driving cycle. Moreover, recent studies conducted byBallesteros et al. (2010) and Karavalakis et al. (2010a), showed that certain PAH compounds may be negatively affected by the biodiesel source

material and quality. The above observations raise additional concerns regarding the impact of biodiesel on PAH emissions and ultimately on air quality because of the European Union's decision to increase biodiesel content in European automotive diesel fuel to 10%

by year 2020.

This work is a contribution to the comprehension on the impact of biodiesel blends of different origin and properties on the unregulated emissions of carbonyl compounds, PAH, nitro-PAH and oxy-PAH of a modern passenger car representative of the current Europeanﬂeet.

Emission measurements were conducted over the New European Driving Cycle (NEDC) and the Artemis driving cycles. The results obtained are useful to determine the effect of biodiesel source material (saturated or unsaturated) and type (fresh or oxidized), along with the inﬂuence of the driving cycle on the toxic pollutants and ultimately on health effect and air quality.

2. Experimental

2.1. Test vehicle, fuels, driving cycles and measurement protocol

The technical speciﬁcations of the Euro 4 compliant vehicle and information regarding detailed chemical analysis of the employed fuels are provided in Part 1 of the present study. Detailed information about the driving cycles used to conduct the measurements and description of the measurement protocol can also be found in Part 1 of the present study.

2.2. Carbonyl compounds analysis

For the determination of the carbonyl compounds (CBCs) in the exhaust gas, samples were collected in 3 L Tedlar bags (SKC-3L). In each configuration, sampling was carried out in duplicate and in two different Tedlar bags. Diluted exhaust gas was drawn from the bags through cartridges at a rate of 150 mL min^{− 1}using a pump. These cartridges contained 2,4-dinitrophenylhydazine (Chromafix-DNPH, Macherey–Nagel) supported on a silica substrate. A total volume of 3 L was pumped through the cartridge. The DNPH reacts in-situ with the carbonyl compounds in the gas phase to yield derivative compounds, which are more stable than their reactive counterparts. The carbonyl- DNPH derivatives were analyzed according to ISO 16000-3 using an Integral 4000 HPLC system (Perkin Elmer) with an ultraviolet–visible detector (λ=360 nm). For the separation of the CBCs a C18column (3μm in particle size, 4.6 mm i.d., and 150 mm in length) was used (Waters, USA). A mixture of CH3CN/H2O was used as mobile phase with gradient elution from 50:50 to 75:25. Theflow rate was maintained at 1.5 mL min^{− 1}and the injection volume was 20μL. The limit of detection was defined using the standard method: 3.3⁎SD, where SD is the standard deviation calculated from reproducible experiments. The recovery efficiency of CBCs varied from 65% to 105% and it was

Table 1

Carbonyl emissions for diesel fuel and UFOME blends over the NEDC and the Artemis driving cycles.

Carbonyls, mg km^{− 1} Diesel UFOME-10 UFOME-20 UFOME-30

NEDC Urban Road Motorway NEDC Urban Road Motorway NEDC Urban Road Motorway NEDC Urban Road Motorway Formaldehyde 0.488 0.497 0.471 0.437 0.566 0.582 0.513 0.469 0.588 0.654 0.583 0.562 0.731 0.778 0.713 0.695 Acetaldehyde 0.491 0.536 0.508 0.455 0.574 0.592 0.52 0.494 0.622 0.697 0.596 0.563 0.714 0.788 0.743 0.699 Acrolein/acetone 0.347 0.406 0.365 0.312 0.453 0.497 0.418 0.386 0.529 0.552 0.511 0.479 0.588 0.595 0.545 0.512 Propionaldehyde 0.358 0.396 0.363 0.325 0.397 0.437 0.394 0.341 0.405 0.448 0.399 0.372 0.437 0.469 0.386 0.367 Crotonaldehyde 0.407 0.438 0.41 0.384 0.408 0.385 0.344 0.312 0.468 0.503 0.472 0.451 0.524 0.574 0.539 0.482 Methacrolein 0.277 0.303 0.285 0.261 0.253 0.296 0.229 0.189 0.273 0.255 0.216 0.202 0.291 0.264 0.232 0.217 2-Butanone 0.159 0.166 0.183 0.142 0.208 0.186 0.155 0.128 0.194 0.172 0.133 0.116 0.22 0.242 0.217 0.194 Butyraldehyde 0.147 0.162 0.151 0.139 0.169 0.198 0.125 0.106 0.183 0.211 0.169 0.148 0.193 0.238 0.214 0.182 Benzaldehyde 0.288 0.324 0.301 0.272 0.285 0.242 0.217 0.187 0.196 0.209 0.175 0.166 0.183 0.154 0.131 0.118 Valeraldehyde 0.127 0.141 0.128 0.101 0.147 0.173 0.126 0.108 0.161 0.192 0.173 0.148 0.207 0.228 0.217 0.195 p-Tolualdehyde 0.187 0.219 0.195 0.162 0.177 0.184 0.143 0.118 0.174 0.189 0.153 0.121 0.213 0.241 0.209 0.183 Hexanaldehyde 0.208 0.244 0.227 0.193 0.19 0.183 0.151 0.124 0.208 0.217 0.185 0.167 0.224 0.238 0.217 0.197

G. Karavalakis et al. / Science of the Total Environment 409 (2011) 738–747 739

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calculated using spiked cartridges. The mean relative standard deviation in recovery efﬁciencies was below 15%.

2.3. PAH, nitro-PAH and oxy-PAH analysis

PAH samples were collected on a glass-fiber filter. Each collected sample was extracted with dichloromethane in ultrasonic bath for 20 min. The extraction was repeated three times using 80 mL of the solvent each time. The extract was then concentrated by rotary evaporation to nearly 2 mL for further cleanup. The cleanup column (inner diameter, 1 cm) was filled with glass wool, 3 g of 6% deactivated silica gel, 2 g of 5% deactivated alumina and 0.5 g of anhydrous sodium sulphate. After loading the sample in the cleanup column, the column was washedfirstly with 20 mL of n-hexane and then with 25 mL of the 30% CH2Cl2 in cyclohexane. Both fractions were reduced to 1 mL under a gentle stream of nitrogen for the analysis. The fraction of CH₂Cl₂/cyclohexane was analyzed for PAHs and their products. A gas chromatograph (Agilent 6890) with a mass spectrometric (MS) detector (Agilent 5975B) was used for the PAHs (Electron Ionization, EI) and nitro-PAHs analysis (Negative Chemical Ionization, NCI). The GC–MS was equipped with a capillary column (HP 5 MS, 30 m × 0.25μm×0.25 mm, Agilent); an auto sampler (Agilent 7673A), with an injection volume of 1μL, pulsed splitless injection. For PAHs analysis the injector temperature was set at 310 °C and the ion source temperature at 280 °C. Oven operating temperature ranged from 55 to 325 °C at 20 °C min^{− 1}. For nitro-PAH and oxy- PAH analyses the injector temperature was set at 280 °C and the ion source temperature at 280 °C. Oven operating temperature ranged from 60 to 200 °C at 20 °C min^{− 1}, and from 200 °C to 300 °C at 5 °C min^{− 1}. The oven remained at this temperature for 10 min. The masses of primary and secondary PAH, nitro-PAH and oxy-PAH ions were determined by using the scan mode for pure standards of these compounds. Quantification was performed by using the selected ion

monitoring (SIM) mode. The efﬁciencies of the studied PAHs ranged from 85.0% to 105.0%. For the studied nitro-PAHs the efﬁciencies ranged from 75.0% to 93.0% and for the oxy-PAHs from 66.5% to 89.5%.

The repeatability of PAHs ranged from 2.7% to 5.4%. For the nitro-PAHs the repeatability ranged from 3.2% to 8.4% and for the oxy-PAHs from 8.7% to 11.1%. The combined uncertainty for the overall emission measurements was calculated to be: PAHs (4.1–8.3%), nitro-PAHs (4.7–12.4%) and oxy-PAHs (10.4%–14.7%).

3. Results and discussion 3.1. Carbonyl emissions

Thirteen carbonyls were identiﬁed and quantiﬁed in the exhaust, including formaldehyde, acetaldehyde, acrolein/acetone, propionaldehyde, crotonaldehyde, methacrolein, 2-butanone, butyraldehyde, benzaldehyde, valeraldehyde, p-tolualdehyde and hexanaldehyde.

The concentration of those carbonyls for all fuel/cycle combinations is listed inTables 1–4. Low molecular-weight carbonyls such as those of formaldehyde, acetaldehyde, acrolein and propionaldehyde were the most abundant compounds in the exhaust. These results compare favourably to those ofDi et al. (2009) and He et al. (2009), showing the predominance of lighter carbonyls in emissions from diesel vehicles. However, heavier compounds such as crotonaldehyde, methacrolein, and butyraldehyde were also present in the exhaust in relatively high concentrations. The addition of biodiesel, independent its origin, provided signiﬁcant increases in emissions of carbonyl compounds over all cycles, which may be attributed to the presence of oxygen in the ester molecule (Correa and Arbilla, 2008). A clear trend of increase in carbonyl emissions was observed with the increase in biodiesel content. In contrast, the use of biodiesel was found to decrease the aromatic benzaldehyde, since aromatics are virtually absent in biodiesel. However, p-tolualdehyde increased with Table 2

Carbonyl emissions for OME blends over the NEDC and the Artemis driving cycles.

Carbonyls, mg km^{− 1} OME-10 OME-20 OME-30

NEDC Urban Road Motorway NEDC Urban Road Motorway NEDC Urban Road Motorway

Formaldehyde 0.537 0.555 0.503 0.463 0.596 0.667 0.571 0.559 0.724 0.751 0.696 0.635

Acetaldehyde 0.577 0.612 0.574 0.526 0.613 0.682 0.624 0.579 0.707 0.771 0.736 0.712

Acrolein/acetone 0.478 0.462 0.417 0.393 0.531 0.515 0.492 0.477 0.568 0.589 0.529 0.492

Propionaldehyde 0.412 0.473 0.428 0.401 0.422 0.466 0.432 0.416 0.436 0.471 0.443 0.414

Crotonaldehyde 0.44 0.484 0.427 0.397 0.47 0.516 0.494 0.422 0.508 0.537 0.496 0.474

Methacrolein 0.233 0.267 0.219 0.185 0.241 0.233 0.218 0.196 0.255 0.262 0.217 0.193

2-Butanone 0.195 0.225 0.183 0.167 0.213 0.257 0.231 0.215 0.209 0.213 0.189 0.168

Butyraldehyde 0.174 0.166 0.127 0.113 0.185 0.218 0.153 0.121 0.197 0.226 0.195 0.174

Benzaldehyde 0.269 0.248 0.215 0.196 0.244 0.232 0.21 0.187 0.205 0.183 0.144 0.131

Valeraldehyde 0.153 0.164 0.137 0.112 0.172 0.196 0.164 0.127 0.188 0.231 0.197 0.178

p-Tolualdehyde 0.195 0.236 0.21 0.188 0.223 0.24 0.217 0.199 0.244 0.267 0.226 0.213

Hexanaldehyde 0.197 0.23 0.236 0.217 0.195 0.242 0.218 0.193 0.185 0.233 0.196 0.178

Table 3

Carbonyl emissions for AFME blends over the NEDC and the Artemis driving cycles.

Carbonyls, mg km^{− 1} AFME-10 AFME-20 AFME-30

Formaldehyde 0.507 0.523 0.494 0.455 0.551 0.57 0.519 0.471 0.627 0.698 0.655 0.624

Acetaldehyde 0.592 0.578 0.519 0.484 0.582 0.608 0.547 0.503 0.646 0.673 0.626 0.595

Crotonaldehyde 0.383 0.396 0.417 0.375 0.395 0.433 0.412 0.382 0.41 0.457 0.426 0.399

Methacrolein 0.253 0.275 0.241 0.203 0.264 0.291 0.273 0.249 0.288 0.308 0.275 0.283

2-Butanone 0.167 0.184 0.155 0.137 0.177 0.193 0.171 0.164 0.198 0.216 0.219 0.203

Butyraldehyde 0.155 0.17 0.162 0.143 0.172 0.191 0.178 0.158 0.153 0.188 0.146 0.122

Benzaldehyde 0.261 0.287 0.242 0.201 0.218 0.237 0.226 0.209 0.186 0.207 0.191 0.167

Valeraldehyde 0.129 0.159 0.142 0.127 0.134 0.127 0.116 0.103 0.16 0.182 0.157 0.136

p-Tolualdehyde 0.193 0.224 0.207 0.186 0.205 0.236 0.215 0.198 0.22 0.275 0.241 0.215

Hexanaldehyde 0.192 0.216 0.184 0.167 0.199 0.227 0.218 0.201 0.213 0.266 0.23 0.192

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biodiesel, an observation which indicates that the formation of this compound may be inﬂuenced by other parameters rather than the aromatic content.

In general, biodiesel fuels do not contain carbonyls in their composition. It is, therefore, reasonable to assume that any carbonyls found in diesel exhaust are combustion products from incomplete oxidation of hydrocarbons (Peng et al., 2008). However, the ester group may be responsible for the higher carbonyl emissions of biodiesel because the decomposition of ester can potentially form carbonyl and oleﬁn by bimolecular hydrogen abstraction (Schwartz et al., 2006). This observation supports the higher carbonyl concentrations of SMEP blends, as this fuel was mainly comprised of unsaturated fatty esters and thus has an increased number of double bonds. The latter are more prone to be decomposed during the combustion process.

Apart from combustion related factors that can inﬂuence the formation of carbonyls, fuel impact may also play an important role on these emissions. In general, aldehydes are not present in diesel fuel and biodiesel but are largely formed in the combustion process from fuel fragments produced in the initial oxidative pyrolysis of the fuel.

These fuel fragments are hydrocarbon radical species that are largely derived from the major constituents of the fuel (Nelson et al., 2008).

The addition of the oxidized biodiesel blends led to signiﬁcant increases in most carbonyl compounds when compared to diesel fuel and the other biodiesel blends. This observation can be supported by the total carbonyl emissions shown inFig. 1. The explanations regarding the adverse impact of the oxidized blends on carbonyl emissions are closely related to effects due to fuel composition, fuel quality and feedstock. All of the above should be considered to be viewed as contributing factors to the carbonyl emission proﬁle for biodiesel. On this basis, both OME and UFOME were severely oxidized fuels. During the oxidation and auto-oxidation process, primary oxidation products are formed, such as peroxides and hydroperoxides which can decompose to ultimately form aldehydes and keto compounds (secondary oxidation products) (Berrios et al., 2010).

Additionally, used frying oil would contain aldehydes and ketones, as well as diglycerides and free fatty acids formed during thermal oxidative processes (cooking) (Guarieiro et al., 2008; Karavalakis et al., 2010b). It is reasonable to assume that these species would retain their linking when transesteriﬁed to methyl esters for use as biodiesel. Therefore, the higher carbonyl concentrations for the oxidized blends can be attributed to the fact that these fuels already contained aldehydes and oxidized fractions in their composition.

The highest levels of formaldehyde and acetaldehyde emissions were observed with the use of the oxidized blends. This phenomenon may be attributed to the presence of short-chain esters that favour formation of the shortest chain aldehydes (namely formaldehyde and acetaldehyde) during combustion (Guarieiro et al., 2008). In the case of OME, these products were probably formed during the ageing

process, whereas in the case of UFOME their formation was favoured by the breakdown of unsaturated fatty acids during the repeated deep frying process. A sharp increase in acrolein emissions was found with the application of the oxidized and AFME blends. Acrolein emissions are mainly originated from oxidation of glycerol, glycer- ides and fatty acids residues present in the biodiesel (Graboski and McCormick, 1998). This result is in agreement with the higher amount of glycerol and glyceride content found in UFOME, OME and AFME as compared to SMEP. It should be noted that the use of the oxidized blends resulted in higher concentrations of heavier carbonyls when compared to those of diesel fuels and the other biodiesel blends.

Concerning the inﬂuence of the driving cycle on carbonyl emissions, some differentiations were observed between the evaluated cycles. Artemis Urban resulted in highest emission levels followed by the NEDC. On the other hand, lower carbonyl emissions were found over Road and Motorway operation. It appeared that the increase in the average speed and load of the cycle and thus the increased combustion efﬁciency led to reduced carbonyl emissions (Sawant et al., 2007). In addition, the higher exhaust temperatures and thus the higher performance of the oxidation catalyst resulted in lower carbonyl emissions (Bikas and Zervas, 2007). Although NEDC is a low- speed low-load cycle, the relatively high carbonyl emissions can be Table 4

Carbonyl emissions for SMEP blends over the NEDC and the Artemis driving cycles.

Carbonyls, mg km^{− 1} SMEP-10 SMEP-20 SMEP-30

Formaldehyde 0.471 0.508 0.481 0.442 0.569 0.61 0.587 0.575 0.604 0.642 0.616 0.579

Acetaldehyde 0.54 0.582 0.553 0.511 0.61 0.643 0.602 0.59 0.676 0.7 0.652 0.613

Crotonaldehyde 0.39 0.431 0.396 0.364 0.41 0.455 0.41 0.387 0.511 0.58 0.53 0.493

Methacrolein 0.281 0.31 0.292 0.266 0.31 0.327 0.303 0.281 0.36 0.409 0.372 0.337

2-Butanone 0.166 0.18 0.172 0.157 0.19 0.215 0.182 0.173 0.194 0.171 0.14 0.101

Butyraldehyde 0.151 0.167 0.161 0.143 0.16 0.169 0.158 0.162 0.172 0.218 0.206 0.188

Benzaldehyde 0.259 0.297 0.246 0.228 0.22 0.261 0.206 0.182 0.147 0.19 0.151 0.136

Valeraldehyde 0.131 0.15 0.127 0.131 0.138 0.155 0.12 0.092 0.15 0.198 0.18 0.145

p-Tolualdehyde 0.159 0.187 0.173 0.141 0.122 0.142 0.121 0.107 0.106 0.138 0.101 0.122

Hexanaldehyde 0.186 0.223 0.205 0.177 0.172 0.201 0.196 0.218 0.157 0.189 0.185 0.161

Fig. 1. Total carbonyl emissions for the tested fuels over the NEDC and the Artemis driving cycles.

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Table 5

Emissions of PAH, nitro-PAH and oxy-PAH for diesel fuel and UFOME blends over the NEDC and Artemis driving cycles.

Diesel UFOME-10 UFOME-20 UFOME-30

NEDC Urban Road Motorway NEDC Urban Road Motorway NEDC Urban Road Motorway NEDC Urban Road Motorway

PAH,μg km^{− 1}

Phenanthrene 9.030 10.677 7.963 5.828 12.851 14.574 8.823 5.471 13.859 15.070 8.809 5.738 15.350 17.251 9.822 6.196

Anthracene 7.725 9.984 4.787 1.498 9.400 14.973 9.944 5.393 12.489 15.217 7.118 4.121 13.754 18.988 8.204 5.976

Fluoranthene 9.350 10.702 5.625 2.360 13.091 16.442 7.494 4.476 14.997 16.923 7.682 5.326 16.310 20.406 9.043 5.932

Pyrene 9.047 9.801 6.059 4.740 9.758 13.131 7.775 4.151 10.272 15.181 8.288 4.753 12.470 14.464 8.720 4.144

Chrysene 9.102 8.210 4.179 2.606 5.756 6.996 3.522 2.566 3.895 7.469 3.118 1.935 3.191 4.217 2.967 1.417

Benzo[a]anthracene 4.341 7.831 3.334 2.258 4.012 7.494 5.213 2.044 3.075 7.015 3.033 1.556 2.564 8.890 2.823 1.881

Benzo[b,k]ﬂuoranthenes 5.911 8.627 4.038 1.540 5.919 9.400 4.912 2.081 5.595 9.773 5.223 2.098 6.972 9.961 5.799 2.399

Benzo[a]pyrene 3.898 4.492 3.633 3.194 3.944 4.575 3.729 3.208 3.605 4.355 3.582 2.984 3.318 3.707 2.961 2.371

Indeno[1,2,3-c,d]pyrene –^a – – – – – – – – – – – 1.675 1.991 1.242 0.847

Dibenzo[a,h]anthracene – – – – – – – – – – – – – – – –

Benzo[g,h,i]perylene – – – – – – – – – – – – 1.347 1.883 1.036 0.669

Nitro-PAH, ng km^{− 1}

3-Nitro-ﬂuoranthene 1.882 3.960 1.639 0.590 1.386 3.800 1.002 0.795 1.403 3.816 0.978 0.679 1.146 3.592 0.960 0.673

1-Nitro-pyrene 4.450 17.177 10.690 7.540 6.320 11.383 7.050 13.520 8.123 13.626 10.021 9.895 8.448 14.693 10.777 8.887

7-Nitro-benzo[a]anthracene 1.448 2.957 1.109 0.856 1.434 2.397 1.720 0.834 1.409 2.402 1.661 0.974 1.478 2.560 1.285 0.811

6-Nitro-benzo[a]pyrene 3.364 5.047 1.589 1.230 1.882 6.437 1.787 1.834 1.889 6.558 2.686 0.376 0.763 8.133 2.574 0.389

Οxy-PAH, ng km^{− 1}

1-Naphthaldehyde – – – – – – – – – – – – – – – –

9-Fluorenone 4.749 9.340 6.363 2.422 5.684 7.959 4.110 2.295 5.677 7.728 4.197 2.444 6.009 8.067 4.380 2.989

Anthraquinone 3.520 6.776 4.604 2.294 6.410 8.356 5.567 3.880 6.954 9.103 6.307 3.273 7.348 9.287 6.080 2.542

9-Phenanthrenecarboxaldehyde 2.440 2.973 1.787 0.859 1.356 2.799 0.868 0.521 1.257 3.017 0.624 0.553 1.209 3.245 0.939 0.750

Benzanthrone 3.821 5.327 3.520 2.923 4.490 6.804 3.498 3.635 5.978 9.751 4.077 4.177 6.993 13.166 5.556 5.169

Benz[a]anthracene-7,12-dione 2.630 4.490 2.350 1.920 3.294 6.171 2.296 1.339 3.017 6.538 2.036 1.484 3.691 6.925 2.002 1.345

a Below the limit of detection.

G.Karavalakisetal./ScienceoftheTotalEnvironment409(2011)738–747

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ascribed to the cold-start effect of the UDC phase and the partial deactivation of the oxidation catalyst.

3.2. PAH, nitro-PAH and oxy-PAH emissions

A total of 12 PAHs, 4 nitro-PAHs and 6 oxy-PAHs were identiﬁed and quantiﬁed in the vehicle's exhaust. Detailed information on these emissions is shown in Tables 5–8. Of the 12 PAHs analyzed, the

compounds of indeno[1,2,3-c,d]pyrene, dibenzo[a,h]anthracene and benzo[g,h,i]perylene were found below the limit of detection. Lower molecular PAHs such as phenanthrene and anthracene and medium molecular weight PAHs such asﬂuoranthene and pyrene were the dominant compounds found in the exhaust for all fuels. The higher levels of light PAHs suggest that these compounds were pyrolysed from incomplete combustion of the fuel (Lea-Langton et al., 2008;

Ravindra et al., 2008). Heavier PAH compounds were also found in the Table 6

Emissions of PAH, nitro-PAH and oxy-PAH for OME blends over the NEDC and Artemis driving cycles.

OME-10 OME-20 OME-30

PAH,μg km^{− 1}

Phenanthrene 11.769 13.672 8.203 5.888 12.548 14.089 9.060 6.254 14.739 15.069 9.538 6.566

Anthracene 9.446 13.324 8.724 4.341 10.479 13.708 8.667 4.149 11.789 15.033 8.528 3.746

Fluoranthene 10.276 15.120 9.001 3.541 7.815 13.573 7.526 2.544 7.803 11.938 3.511 2.703

Pyrene 10.909 12.763 9.327 3.117 10.638 12.306 8.387 2.507 10.528 11.381 7.953 4.669

Chrysene 5.557 7.744 4.534 2.052 5.772 6.687 4.672 1.872 5.476 5.235 2.233 1.278

Benzo[a]anthracene 4.531 7.350 3.778 2.679 4.647 6.934 3.697 2.536 4.433 6.067 3.219 2.050

Benzo[b,k]ﬂuoranthenes 6.106 8.721 5.017 2.315 6.088 8.967 5.151 2.173 6.625 9.198 5.495 2.795

Benzo[a]pyrene 3.886 4.618 3.522 3.103 3.497 4.250 3.437 2.955 3.186 3.609 2.846 2.311

Indeno[1,2,3-c,d]pyrene –^a – – – – – – – – – – –

Dibenzo[a,h]anthracene – – – – – – – – – – – –

Benzo[g,h,i]perylene – – – – – – – – – – – –

3-Nitro-ﬂuoranthene – – – – – – – – – – – –

1-Nitro-pyrene 1.913 5.009 1.053 1.047 1.429 3.907 0.967 0.978 1.402 3.567 0.833 0.884

7-Nitro-benzo[a]anthracene – – – – – – – – – – – –

6-Nitro-benzo[a]pyrene 1.177 2.184 1.244 0.913 1.035 1.883 1.153 0.896 0.984 1.549 1.107 0.890

1-Naphthaldehyde – – – – – – – – – – – –

9-Fluorenone 0.486 1.900 0.785 0.381 0.643 1.827 0.655 0.266 0.764 1.597 0.440 0.204

Anthraquinone 4.811 6.063 3.565 2.168 4.960 6.920 3.329 2.943 5.285 7.500 3.400 2.048

9-Phenanthrenecarboxaldehyde 1.176 2.875 0.802 0.582 1.167 2.906 0.796 0.543 1.169 2.883 0.772 0.547

Benzanthrone 2.745 4.113 2.107 1.651 2.680 4.161 2.068 1.234 2.585 3.901 1.971 1.097

Benz[a]anthracene-7,12-dione 2.718 4.237 2.171 1.298 2.911 4.291 2.166 1.400 3.713 4.343 2.639 1.817

aBelow the limit of detection.

Table 7

Emissions of PAH, nitro-PAH and oxy-PAH for AFME blends over the NEDC and Artemis driving cycles.

AFME-10 AFME-20 AFME-30

PAH,μg km^{− 1}

Phenanthrene 9.665 9.966 7.865 4.847 9.155 10.525 5.886 3.634 9.063 10.128 4.565 2.281

Anthracene 8.315 8.550 6.023 2.277 7.965 9.758 5.394 2.058 7.899 7.448 6.671 2.427

Fluoranthene 9.871 10.338 6.268 3.477 8.958 9.267 5.864 3.140 9.388 8.817 5.290 2.828

Pyrene 9.135 11.327 8.012 5.501 8.247 9.952 5.182 4.126 7.346 9.020 4.280 2.691

Chrysene 4.183 4.939 3.170 2.985 3.794 4.477 3.502 2.772 3.092 3.803 2.786 1.515

Benzo[a]pyrene 2.477 3.142 2.501 2.118 2.132 2.895 2.306 1.966 1.954 2.372 1.607 1.201

1-Nitro-pyrene 3.202 2.060 1.340 1.180 3.330 2.280 1.111 0.624 3.170 2.150 0.907 0.674

1-Naphthaldehyde – – – – – – – – – – – –

9-Fluorenone 2.970 2.751 1.960 1.440 2.550 1.741 1.450 1.150 2.060 2.292 1.550 0.990

Anthraquinone 2.690 5.790 2.314 1.990 2.830 4.980 2.296 2.275 6.559 8.095 2.510 2.850

Benzanthrone 4.880 4.023 2.160 2.730 4.629 3.949 2.080 2.790 4.060 3.330 2.100 2.160

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exhaust, but in lesser amounts than those of light PAHs. The formation of these species might be due to pyrosynthesis of lower molecular weight aromatic compounds to larger PAHs and to the contribution of the lubricant oil (Lim McKenzie et al., 2007).

The application of biodiesel blends produced discordant results, with both increases and decreases in PAH emissions. Light PAHs were the compounds that were adversely affected by the use of biodiesel.

Anthracene emissions were found in higher levels with biodiesel, when compared to diesel fuel. The higher increases were achieved with the use of SMEP, OME and UFOME blends. Some reductions were found for phenanthrene emissions with the use of AFME and SMEP blends. The same observation holds forﬂuoranthene emissions where the addition of biodiesel led to important increases especially over the NEDC and Artemis Urban. Pyrene emissions were found to decrease with the use of AFME blends, but signiﬁcant increases were observed with the SMEP and the oxidized blends. PAH compounds such as those of benzo[a]pyrene, benzo[a]anthracene and chrysene, which are known for their carcinogenic and teratogenic properties, were found at lower levels with biodiesel with respect to diesel fuel. However, some increases in these pollutants were still observed, especially with the use of the oxidized biodiesel blends.

The impact of fuel type and quality was particularly strong on the formation of PAH compounds in the exhaust. As shown inFig. 2(a), the use of oxidized blends led to important increases in total PAH emissions, when compared to the other biodiesel blends. In particular, the application of UFOME blends resulted in higher total PAH emissions than the reference diesel fuel. The average increases for UFOME blends were 11, 27, 21 and 19% over the NEDC, Artemis Urban, Road and Motorway respectively. At this point, it should be mentioned that UFOME-30 was the only fuel which led to the formation of indeno[1,2,3-c,d]pyrene and benzo[g,h,i]perylene. The use of OME blends resulted in average reductions of−28 and −14%

over NEDC and Motorway accordingly, while their effect over Urban and Road cycles can be considered as neutral. As mentioned previously, the combined effect of biodiesel origin and its oxidized nature led to such increases in PAH emissions. It is reasonable to assume that the biodiesel obtained from used frying oils would

contain dimers, trimers, polymerization products, hydroperoxides and cyclic acids originated from breakdown between two fatty acid chains in the same triacylglyceride molecule (via e.g. Diels–Alder reaction) (Ballesteros et al., 2010; Guarieiro et al., 2008; Waynick, 2005). These di-esters can be formed during thermal stressing and they would be present in theﬁnal fuel and thus in the exhaust. This result suggests that PAHs are in some cases, fuel components and their formation is closely related to carbonaceous particulate formation.

The case of AFME and SMEP blends presents some differences when compared to UFOME and OME blends. The PAH emission reduction may be attributed to the presence of excess oxygen in biodiesel and the absence of aromatic and polyaromatic compounds in the fuel (Lin et al., 2006).

Regarding nitro-PAH emissions, the addition of biodiesel led to both increases and decreases with respect to diesel fuel. On the positive side, with the use of AFME, SMEP and OME blends only 1- nitro-pyrene and 6-nitro-benzo[a]pyrene were found in quantiﬁable levels in the particle phase. On the contrary, all the evaluated nitro- PAHs were found above the detection limit with the use of diesel fuel and UFOME blends.Fig. 2(b) shows the total nitro-PAH emissions for all fuel/cycle combinations. It is evident that the differences between blends were particularly noticeable, with UFOME blends having the highest levels of total nitro-PAH emissions, when compared to the other blends. The higher levels of total emissions with UFOME blends can be ascribed to the strong increases of 1-nitro-pyrene emissions with respect to diesel fuel. These increases were on average 71 and 43% over the NEDC and Artemis Motorway respectively. It should be noted that 1-nitro-pyrene is considered to be a tracer for diesel emissions and is a ubiquitous component of PM (Heeb et al., 2008).

Similar to nitro-PAHs, oxygenated PAH levels were found in signiﬁcant lower amounts than their parent PAHs. Of the six oxy-PAH compounds analyzed, only 1-naphthaldehyde was almost undetect- able for all fuels. The prominent oxy-PAHs emitted were anthraquinone, benzanthrone, benz[a]anthracene-7,12-dione, and 9- ﬂuorenone. The relatively high levels of anthraquinone, benzanthrone, and benz[a]anthracene-7,12-dione can be attributed to the fact that these compounds are the most stable fragments of oxidized Table 8

Emissions of PAH, nitro-PAH and oxy-PAH for SMEP blends over the NEDC and Artemis driving cycles.

SMEP-10 SMEP-20 SMEP-30

PAH,μg km^{− 1}

Phenanthrene 9.451 10.904 7.388 5.211 9.606 11.510 7.643 5.062 10.427 12.419 8.131 4.837

Anthracene 8.982 10.432 6.694 3.865 9.346 10.140 6.280 3.531 9.626 10.854 6.565 5.968

Fluoranthene 9.765 11.731 7.207 4.879 9.975 10.851 7.258 4.820 10.248 11.093 5.031 3.432

Pyrene 10.911 12.079 6.321 4.935 11.551 12.246 6.707 4.830 11.873 12.965 6.895 3.723

Chrysene 5.209 6.878 4.798 3.190 4.101 5.726 3.380 2.948 2.875 4.743 2.809 4.740

Benzo[a]pyrene 3.559 3.881 3.148 3.107 3.209 3.225 2.996 2.47 2.822 2.794 2.136 4.461

1-Nitro-pyrene 1.450 3.290 1.090 0.820 1.575 3.617 1.325 1.063 1.830 3.894 1.520 1.209

1-Naphthaldehyde – – – – – – – – – – – –

9-Fluorenone 2.310 1.870 1.360 1.120 1.97 1.97 1.18 0.89 1.55 1.48 1.23 0.97

Anthraquinone 5.923 4.446 2.372 1.850 5.14 3.89 2.87 1.72 3.67 3.85 3.36 1.80

Benzanthrone 3.970 3.933 2.080 2.750 3.61 3.89 1.96 2.22 3.10 3.18 1.33 1.67

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PAHs (Karavalakis et al., 2010b). The general picture shows that the addition of biodiesel independent its origin, resulted in higher anthraquinone emissions than diesel fuel, especially over the NEDC.

The highest increases were 50 and 109% for OME-30 and UFOME-30 respectively. On the other hand, anthraquinone emissions decreased over Artemis operation, with the exception of UFOME application, which led to important increases over all driving conditions. A trend towards lower benzanthrone emissions was observed with most of the biodiesel blends over all cycles. However, UFOME blends sys- tematically led to higher benzanthrone emissions. The maximum increases for UFOME-30 were 83, 147, 58και 77% over NEDC, Artemis Urban, Road and Motorway respectively. The emissions of benz[a]

anthracene-7,12-dione increased with the use of SMEP, OME and UFOME blends when compared to diesel fuel especially over the NEDC. On the positive side, benz[a]anthracene-7,12-dione emissions decreased with the biodiesel blends over Artemis operation.

Fig. 2(c) shows the total oxy-PAH emissions for all fuel/cycle combinations. Total oxy-PAH emissions conﬁrm the negative performance of UFOME blends, where the average increases were 35, 26 and 16% over NEDC, Artemis Urban and Motorway respectively. During Road operation the application of UFOME blends led to marginal differentiations. It should be stressed that with higher UFOME content, total oxy-PAH emissions increased. It is possible that the presence of oxygen and oxidation products in biodiesel resulted in the formation of these micro-contaminants. These observations generate scepticism on the true impact of biodiesel on oxy-PAH emissions because of the speciﬁc toxicity of quinoid and other oxygenated compounds.

The emitted PAHs, nitro-PAHs and oxy-PAHs were clearly in- ﬂuenced by the driving cycle under the present test conditions and showed a strong correlation with the average speed and load of the driving cycle. The exhaust concentrations of all PAHs were quite low over Artemis Road and Motorway cycles when compared to NEDC and Artemis Urban, i.e. simulating driving at low average speeds in city conditions. The observed reductions in PAH emissions can be attributed to the higher average speed and engine load during these driving conditions, which increases exhaust temperatures and thus resulting in better oxidation of these compounds both inside the vehicle catalyst and the sampling system (Bergvall and Westerholm, 2009; Brandenberger et al., 2005). It is also assumed that the Fig. 3. Toxicity equivalent factors (TEFs) for diesel fuels and its blends with different biodiesels.

Fig. 2. (a–c): Emissions of total PAH (a), nitro-PAH (b) and oxy-PAH (c) for all fuel/cycle combinations.